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Transcript of Exploring bulk insulating surfaces and ultra thin insulating films, at the atomic level by UHV-SPM...
TTHHÈÈSSEE
En vue de l'obtention du
DDOOCCTTOORRAATT DDEE LL’’UUNNIIVVEERRSSIITTÉÉ DDEE TTOOUULLOOUUSSEE
Délivré par l'Université Toulouse III - Paul Sabatier Discipline ou spécialité : NANO-PHYSIQUE.
JURY
Louis PORTE, Professeur à l’Université Paul Cézanne Aix-Marseille III. William SACKS, Professeur à l’Université Pierre et Marie Curie Paris VI. Roland CORATGER, Professeur à l’Université Paul Sabatier à Toulouse. Tomaso Zambelli, Habilité à Diriger des Recherches à l’Institut f. Biomedizinische Technik, Zurich. Jacques VIGUÉ, Directeur de recherche CNRS à l'Université Paul Sabatier à Toulouse. Sébastien GAUTHIER, Directeur de recherche CEMES-CNRS à Toulouse.
Ecole doctorale : Sciences de la Matière Unité de recherche : Centre d’Elaboration de Matériaux et d’Etudes Structurales (CEMES),
UPR 8011 CNRS Directeur(s) de Thèse : Sébastien GAUTHIER Rapporteurs : Louis PORTE et William SACKS
Présentée et soutenue par Miguel Angel VENEGAS DE LA CERDA Le 30 Septembre 2008
Titre : Etudes expérimentales de surfaces et de films minces isolants par microscopie à
sonde locale sous ultra vide
UNIVERSITE TOULOUSE III-PAUL SABATIER U.F.R P.C.A
ECOLE DOCTORALE SCIENCES DE LA MATIERE
DOCTORAT DE L’UNIVERSITE DE TOULOUSE III - Paul Sabatier. Discipline : NANO-PHYSIQUE, NANO-COMPOSANT,
NANO-MESURE.
Miguel Angel VENEGAS DE LA CERDA.
Etudes expérimentales de surfaces et de films minces isolants par microscopie à sonde locale sous ultra vide
Thèse dirigée par Sébastien GAUTHIER
Date de la soutenance, le 30 Septembre 2008
Jury :
Louis PORTE, Professeur à l’Université Paul Cézanne Aix-Marseille III,
IM2NP/ L2MP-Marseille, Rapporteur. William SACKS, Professeur à l’Université Pierre et Marie Curie Paris VI,
INSP/STMSG- Paris, Rapporteur. Roland CORATGER, Professeur à l’Université Paul Sabatier,
CEMES/GNS-Toulouse. Tomaso Zambelli, Habilité à Diriger des Recherches à l’Institut f. Biomedizinische Technik,
ETH-Zurich, Switzerland. Jacques VIGUÉ, Directeur de recherche CNRS,
IRSAMC/LCAR-Toulouse. Sébastien GAUTHIER, Directeur de recherche CNRS,
CEMES/GNS-Toulouse.
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Etudes expérimentales de surfaces et de films minces isolants par microscopie à sonde locale sous ultra vide.
Dans ce travail de recherche nous avons réalisé des études expérimentales de surfaces et
de films minces isolants par microscopie à sonde locale sous ultra vide à température ambiante. En particulier nous avons utilisé la microscopie à effet tunnel (STM) et la microscopie à force atomique en mode non-contact (NC-AFM). Nous présentons des résultats qui concernent deux systèmes : la surface isolante KBr(001) et le film mince isolant d'alumine formé par oxydation de la surface (110) d'un cristal de NiAl.
• Dans un premier temps, nous avons modifié la tête du microscope STM/AFM en changeant le dispositif de détection optique des oscillations du cantilever. L'amélioration importante apportée nous a permis de mener une série d'expériences sur la surface de clivage du cristal ionique KBr(001). Nous avons mis en évidence à partir d'images de marches monoatomiques acquises avec la résolution atomique des changements de contraste réversibles déclenchés par le passage de la pointe sur le bord de marche. Ces observations ont été interprétées en terme de déplacements atomiques à l'extrême apex de la pointe entraînant un changement de signe de l'ion terminal, qui détermine le type d'image observée. Cette hypothèse a été confirmée en analysant les courbes expérimentales donnant la force entre la pointe et la surface en fonction de la distance pointe-surface. Cette étude a été suivie de quelques tentatives pour imager des molécules organiques sur cette surface isolante.
• Le système Pd/Al10O13/NiAl(110) à été étudié par microscopie à effet tunnel. La
couche d'oxyde est formée par l'exposition à O2 à une température particulier (~280°C) de la surface (110) d'un cristal de NiAl sous ultravide. Nous avons obtenu des images en résolution atomique qui nous ont permis de remonter à la structure atomique de la couche isolante d'alumine, de stœchiométrie Al10O13, ainsi que de l'un des types de parois de domaine du film isolant. Nous avons également réalisé de mesures de façon à caractériser ses propriétés électriques à une échelle nanométrique. Ce substrat nous a ensuite servi à faire croître des agrégats métalliques de palladium dans différentes conditions. La répartition des îlots ainsi formés n'est pas homogène, certains défauts étant décorés par le palladium. La possibilité d'utiliser ce substrat pour réaliser des jonctions métal-molécule-métal planaires connectables à un système de mesure extérieur par la méthode du nano-stencil développée au CEMES a ensuite été envisagée.
Les perspectives ouvertes par ce travail dans le domaine de l'électronique moléculaire sont
discutées dans la conclusion du manuscrit. mots clés : Microscopie à effet tunnel, microscopie à force atomique, mode non-contact, spectroscopie de force, physique des surfaces, surfaces isolants, ultra vide
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Exploring bulk insulating surfaces and thin insulating films, at the atomic level by UHV-SPM techniques.
In this research work we carried out experimental studies of insulating surfaces and insulating thin films surfaces by scanning probe microscopy techniques under ultra vacuum at room temperature. In particular we used Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy in the non contact mode (NC-AFM). We present experimental results on two systems: the insulating surface KBr (001) and the thin insulating alumina film formed by oxidation of the (110) surface of a NiAl crystal.
• Initially, we modified the STM/AFM head by changing the optical device of the detection of the cantilever oscillations system. This crucial improvement enabled us to carry out a series of experiments on the (001) cleaved surface of the ionic crystal KBr at the atomic level. We have evidence obtained from atomic resolution images, that shows a change in contrast when the tip passes through a step edge. Where we could observe a systematic and reversible change in the contrasts of the image. These observations were interpreted in terms of the atomic displacements of the last extremity tip apex, involving the change of last ion sign. This change of ion determines the type of image observed at atomic resolution. This assumption was confirmed by analyzing the experimental curves giving the force between the tip and the surface according to the tip to surface distance. This study was followed of some attempts to images organic molecules on this insulating surface.
• The Pd/Al10O13/NiAl (110) system was explored by STM and scanning tunneling spectroscopy (STS), where the oxide layer is formed by exposing the NiAl(110) surface to an oxygen atmosphere, while keeping the sample temperature at (~280°C) under ultra-high vacuum. The atomic resolution images obtained enabled us to go down into the atomic structure of the insulating alumina layer, with stoichiometry Al 10O13. In addition, it could be possible to atomically resolve a unit cell of one type of defects formed. We also carried out electrical measurements in order to characterize its electric properties on a nanometer scale. This substrate was then used to growth metal palladium aggregates under various conditions. The distribution of the small formed islands is not homogeneous; certain defects of the alumina film are being decorated by palladium aggregates. The possibility to use this substrate to carry out suitable experiment for interconnections of planar junctions involving a metal-molecule-metal junction onto an external measurement system may probably be considered. This can be held by the nano-stencil technique, developed in the GNS at CEMES
Prospects opened by this work in the field of molecular electronics are discussed in the conclusion part of the manuscript. Keywords : Scanning tunnelling microscopy, atomic force microscopy, non contact mode, force spectroscopy, surface physics, insulating surfaces, ultra high vacuum
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LABORATOIRE DE RATTACHEMENT : Centre d’Elaboration de Matériaux et d’Etudes Structurales (CEMES),
UPR 8011 CNRS 29 rue Jeanne Marvig (BP 94347)
31055 TOULOUSE Cedex 4 FRANCE
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Remerciements
Tout d’abord, je remercie énormément le Conseil des Sciences et Technologies de
l’état d’Aguascalientes (CONCYTEA) pour tout le soutien qu’il m’a apporté. Grâce à cette
bourse de l’état d’Aguascalientes j’ai eu la possibilité de poursuivre mes études à l’étranger
en nanosciences, merci beaucoup.
J’exprime tous mes remerciements aux rapporteurs, Louis PORTE et William SACKS,
de s’être intéressé à ces travaux.
J’exprime tous mes remerciements aux membres du jury, Roland CORATGER,
Tomaso ZAMBELLI et Jacques VIGUÉ pour avoir accepté de faire partie du jury et pour avoir
pris de leur temps pour lire cette thèse.
Je voudrais aussi beaucoup remercier M. Jean-Pierre LAUNAY, directeur du Centre
d’Elaboration des Matériaux et d’Etudes Structurales (CEMES), pour m’avoir aidé à tout
moment et pour cette grande opportunité d’avoir fait mon doctorat au CEMES.
Je suis très reconnaissant envers Christian JOACHIM pour deux raisons:
Tout d’abord pour sa motivation constante, son énergie transmise, ses conseils, son
attention, sa patience, son amitié, et son courage qu’il m’a consacrés avec grande sympathie,
toujours et depuis notre rencontre aux États Unis à Nanotech2003. Merci de m’avoir transmis
ces éléments clés qui m’ont donné la force de poursuivre mes études dans le domaine
fascinant des nano sciences.
En second, car grâce à lui j’ai eu la belle opportunité de venir en France poursuivre
mes études dans un des meilleurs groupes de recherche du monde en Nano Sciences, le GNS.
J’exprime toute ma reconnaissance et mes plus sincères remerciements de manière très
particulière à mon directeur de thèse, M. Sébastien GAUTHIER. Grâce à sa patience, sa
compréhension, son attention, ses orientations, sa motivation, ses conseils, et ses efforts j’ai
pu finir cette thèse. Merci à lui qui dans les moments les plus difficiles et critiques de ce
travail m’a toujours apporté tout son soutien et son aide. Grâce à ses nombreux enseignements
et connaissances, ce travail a acquis la qualité scientifique nécessaire. Merci toujours !
Je remercie spécialement de tout mon amour ma femme Jessy, qui a toujours été avec
moi et m’a toujours soutenu dans les moments les plus durs de cette aventure, à qui je dédie
tous mes efforts. Ainsi qu’à ma famille pour tout son soutien.
Merci à David MARTROU pour ces apports et conseils partagés pendant la partie
expérimentale de cette thèse.
Mille mercis à Haiming GUO pour m’avoir enseigné à travailler l’Ultra Vide d’une
façon toujours très sympa et très attentive.
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Je remercie également Jose ABAD, « Départamento de Física, Centro de Investigación
Optica y Nanofìsica-CIOyN », de l’Université de Murcia en Espagne pour tout ses apports qui
ont été d’une grande aide pour les expériences en AFM.
Je dois exprimer mes remerciements les plus sincères à Olivier GUILLEMET pour ses
conseils de préparation d’échantillons et déposition de molécules qu’il m’a toujours donnés
d’une façon très amicale et sympathique, sa précieuse aide, dans mes études expérimentales
en microscopie de champ proche sous ultra vide, a été de grand aide.
Je remercie vivement Jérôme POLESEL pour les petites conversations très
enrichissantes, son travail de thèse m’a toujours apporté des éléments indispensables pour la
compréhension de la machinerie d’AFM en mode dynamique.
Je dis merci à Henri- Pierre JAQUOT pour ses conseils en chimie qu’il m’a donné
toujours avec grande sympathie.
Je remercie à Christophe DESHAYES pour sa précieuse collaboration dans les images
en SEM des Cantilevers, et à Théophile Noël pour sa collaboration dans les études
préliminaires pour l’implémentation de la Diode super luminescent.
A E. Meyer, L. Nony pour leurs précieuse conseils et orientations pour effectuer le
changement du système optique, je les dis merci beaucoup.
Un grand merci à tous ceux qui m’ont accueilli au CEMES ainsi qu’à tout le personnel
du laboratoire. Je voudrais remercier dans son ensemble le groupe GNS ainsi que le CEMES
pour ce grand accueil et le soutient lors de l’élaboration de ce travail.
Et finalement merci à tout le monde pour les petites aides qu’ils m’ont toujours
offertes.
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Dedicada a Jessy con todo mi amor: por todo su gran apoyo, esfuerzo, paciencia,
comprensión y amor. Que sin su soporte, no hubiéramos sobrevivido en Toulouse.
Millón de gracias Bebe, te amo.
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Contest
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1. General Introduction 19
1.1. Antecedents: the nano-stencil technique……………………….………….………19
1.1.1. Molecular electronics…………………………………………………………19
1.1.2. Connecting a single molecule…………………………………….…..…..…..20
1.1.2.1. The STM metal-vacuum-molecule-substrate junction………........…20
1.1.2.2. Mechanically Breakable Junction………………………….…….....21
1.1.2.3. The CNT–MB–CNT covalent junction approach.……….………….21
1.1.2.4. The 4-ISPM Interconnects approach.………….…….......................22
1.1.3. The Nanostencil technique…………………………………………………....24
1.2. Contributions of this work……………………………………………………....…27
1.2.1. Non – Contact Atomic Force Microscopy…………………………….…...…27
1.2.1.1. NC-AFM history…………………………………...……………......27
1.2.1.2. NC-AFM principle………………………………………………….29
1.2.1.3. Optical beam deflection sensor………………………………..........32
2. Optimization of the AFM beam deflection sensor 35
2.1. Introduction…………………………………………………………………… ...….35
2.2. Sensitivity of the optical beam method………………………………………...….36
2.3. Source of noises………………………………………………………………..……38
2.3.1. The shot noise………………………………………………….………..……39
2.3.2. The Johnson noise……………………………………..…………………...…40
2.3.3. The optical source noise……………………………………………….…...…40
2.3.3.1. Laser noises………………...………………………………………….…40
2.3.4. The cantilever thermal noise………………………………………..….….….41
2.4. Modifications of the RT AFM Omicron head…………………….…….………...42
2.4.1. The superluminescent laser diode……………………………………….....…43
2.4.2. The single mode optical fiber……………………………………………....…44
2.4.3. The focusing optics…………………………………………………….…..…46
2.4.4. New characteristics of the beam deflection sensor…………………….…......47
2.5. Noise spectral analysis………………………………………………………...……48
2.5.1. Estimation of the equivalent input noise of the new beam deflection
system………………….……………………………………………………..52
2.5.2. Estimation of the cantilever temperature at maximum laser power level…….52
Contest
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2.6. Sensitivity measurement and amplitude calibration…………………………..…54
2.7. Conclusions and perspectives………………………………..……………...…..…56
3. NC-AFM study on KBr(001) 59
3.1. Introduction…………………………… …………………….………………….…..59
3.2. Experimental methods……………………………………….………………….….60
3.2.1. Force spectroscopy…………………………………..……..……………..…..61
3.2.1.1.Extraction of the force from ∆f(z) using the Sader-Jarvis formula.............61
3.2.1.2.Description of the technique……………………………………………...64
3.3. Results…………………………………………………………………...……..……66
3.3.1. Two types of atomic resolution images……………………….……...………66
3.3.2. Atomically resolved force curves……………………………...…..…………67
3.3.3. Spontaneous tip polarity reversal………………………………......................70
3.3.4. Tip polarity reversal on a monoatomic step………………...…………….…..71
3.3.5. Potential energy surfaces of the two-level system………….……….…….….73
3.3.6. Tip polarity reversal on two successive monoatomic steps.………………….74
3.3.7. Discussion of the topography results…………………..………………….….78
3.4. Force spectroscopy with the bistable tip……………………..……………...….…79
3.5. General discussion…………………………………………..……………………...82
3.6. Conclusion………………………………………………………….…………….....84
4. The Pd/Al10O13/NiAl(110) system 87
4.1. Introduction…………………………………………………………………… ....…87
4.2. The NiAl(110) intermetallic alloy………………………………………….…..…..88
4.2.1. Preparation procedure…………………………………………….……..……89
4.3. The alumina film formed on NiAl(110)…………………….………………...……90
4.3.1. Preparation procedure………………………………………….……..………90
4.3.2. The oxide film meso scale morphology……………………....……..…….….91
4.3.3. Intermediate scale images…………………………………………..………...93
4.3.4. Structural relation between the alumina unit cell and
the NiAl(110) substrate………………………………………………………..94
4.3.5. Higher Resolution image, with three domain boundaries………….……...….95
4.3.6. Atomic resolution image of the oxide film surface…………...…….……..….97
4.3.7. The atomic model of the A domain………………………….…….…………98
Contest
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4.3.8. Interpretation of the atomic resolution image of domain A…....….…………99
4.3.9. The IA-Anti Phase Domain Boundary atomistic model……….………...….100
4.3.10. Interpretation of the atomic resolution image of the IA – APDB…..…..…...101
4.3.11. Meshing the surface with the repetitive pattern………………………..……101
4.4. Scanning tunneling Spectroscopy measurements…………………………….....103
4.4.1. Introduction……………………………………………………………….....103
4.4.2. Determination of the electronic gap for a 1700 L alumina film……….........104
4.4.3. Apparent thickness of the oxide film as a function of the imaging
bias voltage…………………………………….…………………………...….105
4.4.4. STS measurements in the gap of two different oxides……………...........…108
4.5. Palladium growth…………..……………….……………………………….……110
4.6. Conclusion………………………………………………………..………….…….111
5. Conclusions and perspectives 113
5.1. Conclusions…..………………………………………………….…………………113
5.2. Perspectives ……………………………………………..………….………..……115
5.3. Molecules visualization attempts…………………..…………………….…………115
5.3.1.1.Deposition by sublimation under UHV………….………………...……115
5.3.1.2.Deposition from a chloroform solution…………...……………………..116
5.3.2. Nano manipulation……………………………………...……………...……118
6. References 123
7. Substantial summary in French 131
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Chapter 1 General Introduction
19
CHAPTER 1
General Introduction
Introduction
This chapter is divided into two parts. The first one is dedicated to introduce the reader into
the domain of interest. The different methods that have been recently developed to electrically
connect a molecule between electrodes will be shortly discussed. In particular, we will go
more into detail in the developments made in the GNS group. We will show some
achievements already performed and what are the future goals to be achieved. In the second
part of this chapter, we present an overview of how this thesis has contributed to some of the
goals exposed in the first part. The experimental results that will be developed all along this
thesis are then briefly presented.
1.1 Antecedents: The nano-stencil technique.
1.1.1 Molecular electronics
The constant miniaturization of the electronic components driven by the needs of the
microelectronics industry has shown tremendous progress in the construction of smaller and
smaller electronic devices. A common component normally used as a building block of an
electronic circuit is the transistor, which recently could be considerably reduced in size in the
sub-micrometer range, with a gate thickness below 50 nm and limited to 15 nm
[Stephen2006]. However, this miniaturization of electronic components is starting to be
affected by technological and fundamental limits, from where other miniaturization
alternatives have been proposed. Novel solutions provided by the molecular electronics
Chapter 1 General Introduction
20
community are being developed in order to propose molecules as new candidates for nano-
meter sized building blocks with the goal of integrating them into future electronic
components. To achieve this goal, new challenges need to be tackled. In particular, it is
required to develop methods to connect single molecules to macroscopic conductive
electrodes. In the following, we discuss the most interesting approaches to address a single
molecule being explored nowadays.
1.1.2 Connecting a single molecule.
1.1.2.1 The STM metal-vacuum-molecule-substrate junction.
Figure 1.1: LT-STM image showing the switching of a single naphthalocyanine molecule on
NaCl/Cu(111) by tunnelling current. From [Liljeroth2007].
The most flexible and easy way to electronically access and to contact a single molecule today
in Ultra High Vacuum (UHV) conditions has been rendered possible thanks to the incoming
of Scanning Probe Microscopy (SPM) techniques and surface science developments. In
particular, Scanning Tunnelling Microscopy (STM) can perform experimental studies of
organic molecules deposited on the following substrates: metallic surfaces, insulating films
grown on metallic substrates, semiconducting surfaces and/or insulating films on
semiconducting substrates. A very nice example can be found in [Liljeroth2007] that shows
the experimental study of a single naphthalocyanine molecule (fig. 1.1a) deposited on a
NaCl(100) bi-layer previously grown on top of Cu(111). Interestingly, it was found that this
molecule displays a two-state behavior due to a tautomerization involving the displacement of
two hydrogen atoms from one pair of nitrogen atoms to the other in the central cavity of the
molecule (shown in fig. 1.1c). This activation was produced by a tunnelling current injected
with the STM tip in the red spot of fig. 1.1b. The corresponding STM images show how these
proton transfers affect the LUMO orbital, inducing a transition between a high- and a low
Chapter 1 General Introduction
21
current state. This experiment shows an important progress for novel molecular logic devices
with functional molecules. One major drawback of this STM-based approach is that it is
limited to two electrodes and the geometry of connection cannot be chosen at will.
1.1.2.2 Mechanically Breakable Junctions.
Figure 1.2: The mechanically breakable junction schematic with (a) the bending beam, (b) the counter supports, (c) the notched gold wire, (d) the glue contacts, (e) the piezo element,
and (f) the glass tube containing the molecular solution.
Another approach described in 1997 by M. Reed and J. Tour [Reed1997], shows an
experiment at room temperature with a mechanically controllable break junction (MCB) (Fig.
1.2). In this approach, a notched metal wire is glued onto a flexible substrate and is fractured
due to the bending of the substrate by a piezo element. This mechanism can establish an
adjustable tunnelling gap. A large reduction factor between the piezo elongation and the
electrode separation ensures an inherently stable contact or tunnel junction. The wire contacts
are atomically sharp when broken, as demonstrated in the conductance quantization observed
on the I(V) characteristics of the junction. In the experiments reported here, benzene-1,4-
dithiol molecules were adsorbed from a solution on the surface of the two gold electrodes of
the break junction. This results in the formation of a self-assembled monolayer (SAM) on the
gold electrodes. Characteristic features, detected on the I(V) characteristics of the junction
were attributed to the presence of one or a few molecules positioned between these electrodes.
One major drawback of this approach is that it is not possible to image the junction in order to
establish its precise structure.
1.1.2.3 The CNT–MB–CNT covalent junction approach.
The idea is to use carbon nanotubes as intermediate electrodes to connect a molecule. Carbon
nanotubes have lengths in a range (several thousands of nm) which makes them easy to
Chapter 1 General Introduction
22
connect to metallic electrodes and diameters (1 to 2 nm) that are close in dimensions to
smaller objects, like molecules.
Figure 1.3: A molecular bridge composed of two or three molecules connected
between two single wall carbon nanotubes.
In the work reported by Nuckolls and co-workers, single wall carbon nanotubes (SWNT),
deposited on a substrate are locally cut by oxidation. This cut produces a 10 nm gap and
leaves the terminations of the SWNT with specific ending chemical groups (carboxylic acid
groups). These carboxylic acid groups can then create covalent bonds with the oligoaniline
type molecules of the solution in which this system is subsequently introduced. These
molecules serve as molecular bridge connectors in between the 10 nm gap of the nanotube
[Nuckolls2006]. Note that the gap is not small enough to be occupied only by a single
molecule but by a bridge of two or three of them, furthermore the precise structure of the
junction is not known.
The next approaches have the ambition to avoid the drawbacks of these methods by
combining the visualization and manipulation capabilities of STM or AFM with a planar
architecture.
1.1.2.4 The 4-ISPM Interconnects approach.
Another strategy currently being developed is based on the facilities provided by the
commercial UHV four independent scanning probes microscope (4-ISPM), from Omicron
Nanotechnology [Omicron2008a], in combination with recent surface science progress
[Yang2007].
Chapter 1 General Introduction
23
The 4-ISPM (figure 1.4a) is integrated in a common stage and equipped with a scanning
electron microscope. The entire equipment is a sophisticated analytical instrument, designed
for local and non-destructive four point contact for electrical measurements and in situ
function tests of mesoscopic devices.
Using the four tips of this device to directly connect a single molecule is not possible, due to
the size of the tips. One needs to use intermediate electrodes. An approach based on the
manipulation of gold islands on MoS2 was recently proposed [Yang2007]. It is illustrated in
figure 1.4b. Four islands are used to connect a single molecule. It is important that the islands
have a small height, to allow STM or AFM imaging in the inter-electrode area. This is the
case for the gold islands that are grown on MoS2. In addition, it turns out that these islands
can be manipulated so that the relative position of the four electrodes can be adapted to the
molecule to be connected.
This strategy faces two major challenges:
Addressing the islands will require building ultra sharp tips.
Finding metal/substrate systems where the substrate has the desired electronic
properties (it should ideally have a large band gap) and the metal grows in a 2D mode to get
large islands with a small height is a difficult task because, generally, for thermodynamic
reasons, metals tend to grow 3D on these low energy surfaces (note that MoS2 is a
semiconductor, but with a rather low gap of ~1.2 eV [Lauritsen2004]).
a) b)
Figure 1.4: a) The four STM tips stage, with the sample in the center and b) atomistic view of
the proposed interconnection.
Chapter 1 General Introduction
24
The next approach is similar to the present one, except that fixed electrodes are used instead
of four mobile STM tips. It develops the idea of using intermediate scale electrodes. For this
reason, it suffers from the same limitations concerning the metal/substrate system.
1.1.3 The Nanostencil technique.
The nanostencil technique consists in using a stencil deposition mask embedded in an AFM
cantilever to make nanoelectrodes (figure 1.5a). As shown in figure 1.5b, a membrane is
managed in the side of the hollow tip of a cantilever by Focused Ion Beam (FIB). Different
patterns can then be drilled by FIB (figure 1.5c to f).
Figure 1.5: (a) Schematic view of the geometrical configuration for the AFM nanostencil technique. (b) Scanning electron microscopy image showing a recessed box in the rear face of
AFM tip, which was first thinned down to 80 nm by FIB. (c)-(f) different patterns drilled in the AFM tip wall. From [Guo2007].
Such a cantilever can be used in a static or dynamic way. In the static nanostencil technique,
the pattern defined by the aperture in the tip is simply replicated on the surface, with a
determined deformation, due to the geometry (figure 1.5a) and in certain cases to surface
diffusion. In the dynamic nanostencil technique, the sample is moved in a predefined way
during the deposition through the cantilever aperture, in order to draw the desired pattern on
the substrate surface. A very important specificity of this technique is that the surface can be
imaged by AFM before and after the fabrication of the device, with the tip that was used for
Chapter 1 General Introduction
25
the fabrication. This is crucial to achieve nanometer scale alignment between two stencil
levels.
One major drawback of this technique is that the aperture in the cantilever tends to clog
rapidly. The only way to limit this phenomenon is to minimize the amount of deposited
material, hence the size of the deposited pattern. This is done by combining two stencil
lithography steps. The first one uses a static micro stencil mask built in a thin silicon nitride
membrane to fabricate micro-electrodes (figure 1.6a). This is done on a special post in the
preparation chamber. The deposited micro electrodes connect a small area (figure 1.6d and e),
where the subsequent stenciling step can be performed, to micro pads (figure 1.6c) which will
be connected to the I(V) measuring device outside the UHV chamber. This last connection is
established by a movable array of conducting microcantilevers.
Figure 1.6: (a) Optical microscopy image of a static stencil silicon nitride membrane with
microelectrodes patterns. (b) 3D view of the microelectrodes protected by the shadow of the AFM cantilever. (c) Contact AFM image of Au microelectrodes deposited on a SiO2 substrate through a static stencil mask, size: 73µm × 39µm. (d) and (e) A star and two wires have been
deposited in the region delimited by the micro-electrodes. From [Guo2007].
Figure 1.6 shows the schematic view of the setup developed by modifying a variable
temperature AFM/STM Omicron head [Omicron2008b]. The sample is placed on an X-Y
piezo-actuated table from Piezosystem Jena (Germany) equipped with capacitive sensors to
linearize the displacement in a 100 x 100 µm2 area. This table allows to accurately position
the cantilever with respect to the microelectrodes for dynamic stencil deposition. Note that
this table is also useful for large scale AFM imaging, beyond the range of the piezo ceramics
of the omicron head (5 x 5 µm2). We would like to mention that we had the opportunity to
collaborate in this project with a personal contribution that consisted in the conception and
construction of the electronic interface used to move this table.
Metal evaporation for deposition is performed from an effusion cell by collimating the
evaporated beam onto the FIB-drilled AFM cantilever by a series of three diaphragms with
Chapter 1 General Introduction
26
decreasing diameters. In addition, a micro-comb made with an array of ten metallic
cantilevers (xyz µPS), can be brought in contact with the micro-pads of the micro-electrodes
thanks to a X-Y-Z micro positioning stage in order to electrically connect the nanodevice to
external measuring instruments [Ondarçuhu2000].
Figure 1.7: The nanostencil machine. On the left, a picture of the X-Y piezo actuated table, mounted on the VT AFM head. On the right, a schematic diagram of the experimental setup.
The substrate is mounted on the X-Y nano-positioning stage; the collimated metal vapor beam is guided from the evaporator to the substrate through the apertures in the AFM cantilever. The X-Y-Z micro positioner (xyz µPS) is used to position the microcantilever array with the
help of an optical microscope.
The size of the patterns that can be fabricated by the dynamic nanostencil method are
presently limited to a few 10 nm. That is the reason why, as for the preceding method
described in 1.1.2.4 intermediate electrodes have to be used, as illustrated in figure 1.8.
Figure 1.8: This figure suggests how a molecule could be electrically addressed using Pd
clusters grown on an alumina/NiAl(110) substrate.
Chapter 1 General Introduction
27
This figure is built on an STM image of Pd islands grown on a thin alumina layer on
NiAl(110), a system that will be studied in chapter 4.
1.2 Contributions of this work
This work reported in this thesis participated in the development of the nanostencil project
along two directions:
• Improvement of the AFM
It is very important to visualize all the steps of the fabrication of the device by imaging. The
only technique able to image single molecule on insulating substrate is AFM in the so-called
Frequency Modulation (FM-AFM) or Non-Contact mode (NC-AFM). This technique is
briefly presented in the next part of this chapter.
• Investigation of Pd/alumina/NiAl(110)
As already mentioned, the nanostencil technique requires finding a suitable metal/insulator
system. The ideal system would exhibit a two-dimensional, epitaxial growth of a crystalline
metallic deposit. Unfortunately, the growth mode for most metal/insulator system is 3D,
mainly because the surface free energy of insulators is usually much lower than that of metals.
Nevertheless, one can hope to achieve 2D growth even in these conditions if kinetic
limitations come into play. We have chosen to investigate the alumina/NiAl(110), which is
made by oxidation of NiAl(110), because it was reported that Pd grows on this surface in the
form of flat crystalline islands [Yoshitake2006, Hanssen1999], which, as already suggested
(figure 1.8) could be used as intermediate scale electrodes to electrically address a molecule.
The results which have been obtained from STM and STS measurement on this insulating
layer as well as preliminary studies of Pd deposition on it are reported in chapter 4: "The
Pd/Al10O13/NiAl(110) system".
1.2.1 Non–Contact Atomic Force Microscopy
1.2.1.1 NC-AFM history
Since its invention in 1986 [Bining1986], Atomic Force Microscopy has developed beyond
all expectations. It is nowadays used in many scientific and technological domains to
characterize the surface of a large variety of materials in different environments, ranging from
Chapter 1 General Introduction
28
ultra high vacuum (UHV) to liquids. This success relies heavily on the availability of
microfabricated force sensors, which convert the force felt by a sharp tip positioned in the
vicinity of the sample into a displacement, which can be measured by different techniques.
These sensors have generally a diving board, cantilever beam geometry. Today, a large
number of different cantilever types, differing by their resonant frequency f0, stiffness k, and
quality factor Q are commercially available (figure 1.9).
AFM was originally used in the static mode, where the quasi-static deflection of a low k
cantilever (typically 0.1 N/m) is used to get a topography of the surface. While this mode can
produce nice images displaying the atomic periodicity of crystalline surfaces, it is not capable
of true atomic resolution except under very special circumstances [Giessibl1992]. The low k
necessary to improve the force sensitivity makes the static mode prone to jump-to-contact
instabilities and the tip-substrate interaction is generally quite strong, due to long-range van
der Waals forces that press the tip against the substrate.
a) b)
Figure 1.9: A typical NC-AFM cantilever chip, from Nano science Instruments, a) The entire chip top view and cross section views, in b) a zoom on the cantilever, top view and side view.
In dynamic modes, which were developed to limit this tip-surface interaction, the cantilever is
vibrated near its resonance frequency. There are two basic methods for dynamic operation:
amplitude-modulation (AM-AFM) and frequency-modulation (FM-AFM) or NC-AFM. In
AM-AFM [Martin1987], the cantilever is driven at a fixed, near-resonance frequency. The
changes in the amplitude and the phase of the oscillator while the tip scans over the sample
surface are then used as imaging signals. Designed originally to use long-range forces
(electrical or magnetic forces) for intermediate scale imaging (10 nm resolution), this mode
was later used at closer distance, to reach the repulsive tip-sample interaction regime. This
"Tapping Mode" [Zhong1993] is now used routinely for most ambient conditions AFM
investigations. It allows to get true atomic resolution [Erlandsson1997], but is limited by its
Chapter 1 General Introduction
29
inherent slowness: the time scale for amplitude changes in AM-AFM, given by τ~2Q/ω0, is
proportional to Q. But the performance of cantilevers, limited by their thermal noise, is
inversely proportional to Q. Increasing Q in AM-AFM improves the signal-to-noise ratio, but
in the same time, leads to prohibitively long acquisition times.
The solution to this problem was proposed by Albrecht and co-workers [Albrecht1991] in the
founding paper of FM-AFM. In this method, the cantilever is embedded in a positive
feedback loop that oscillates at the cantilever resonance frequency while another loop
maintains its oscillation amplitude at a pre-set value. In contrast to the AM method, where the
frequency is externally fixed, the resonance frequency of the cantilever in the FM method
varies under the influence of the tip-sample forces. The time scale for these variations is only
limited by the time scale of the oscillation itself (~1/f0). The FM technique is thus potentially
much faster than the AM method.
True atomic resolution was first obtained in NC-AFM by Giessibl [Giessibl1995], and
Kitamura and Iwatsuki [Kitamura1995] in 1995. It has now been achieved on a wide variety
of surfaces (metals, covalent or ionic semiconductors, covalent or ionic insulators) and the
technique, after been used in UHV for a long time, is now adapted to ambient conditions and
to liquids, especially for applications in biology [Fukuma2007].
1.2.1.2 NC-AFM principle
NC-AFM is compared to STM in figure 1.10. In STM, one uses the tunneling current I to
keep the tip-surface distance at a constant value while scanning the surface with the tip. This
is achieved by a feedback loop in which the tunneling current is compared with a preset value
I0, and the resulting error signal I - I0 is used to act on the tip position D via a Proportional-
Integral (P-I) corrector and a piezoceramic actuator (figure 1.10a). The excellent resolution of
STM stems from the exponential dependence of the tunneling current on the distance.
(a)
(b)
Figure 1.10: Comparison between (a) STM and (b) NC-AFM principles.
Chapter 1 General Introduction
30
At this level of description, NC-AFM works in a similar way. Instead of using the I(D)
characteristics of the tunneling junction, one relies on the ωres(D) curve of an oscillator that
uses the cantilever as its frequency determining element. The resonant frequency of the
oscillator ωres depends on the force exerted by the substrate on the tip that in turn depends on
the tip-substrate distance, as illustrated by the curve in figure 1.10b. ωres(D) is compared to a
preset value ωres0 and the resulting error signal ωres - ωres
0 is used to act on the tip position D.
The complexity of NC-AFM comes from the way this oscillator is implemented. In the
following, we built it step by step.
The cantilever can be considered as a one degree of freedom harmonic oscillator, described by
a harmonic transfer function )(ωCl (figure 1.11) that writes:
)(
1
)(
)()(
020
220
ωω
ωωωω
ωω
Q
jk
F
zCl
+−== (1.1)
where 0ω is the resonance frequency of the cantilever, k its stiffness and Q its quality factor.
Figure 1.11: Block representing the harmonic transfer function of the cantilever
The block diagram of figure 1.12 shows how the NC-AFM oscillator is built. The cantilever is
inserted in a positive feedback loop including a block of harmonic transfer function )(ωG .
Figure 1.12: Block diagram of the NC-AFM oscillator
The global transfer function reads then:
)()(1
)(
)(
)()(
ωωω
ωωω
GCl
Cl
F
zT
−==
This system will oscillate at ω = ωc if: 0)()(1 =− ωω GCl , that is:
0)()(
1)()(
=Φ+Φ=
cGcCl
cc GCl
ωωωω
Chapter 1 General Introduction
31
where ClΦ and GΦ are the arguments of )(ωCl and )(ωG . Using the second condition and
equation (1.1), one obtains:
( ) 01
1arctan
20
20
=
−+Φ
ωωω
ωωQcG
and:
1)]([tan4
1
)](tan[2 2200 +
Φ+
Φ=
cGcGc QQ ω
ωω
ωω (1.2)
This relation shows that, in general, ωc, the resonant frequency of the oscillator differs from
ω0, the resonant frequency of the cantilever. The phase of the controller G fixes ωc. This
oscillator is a phase-controlled oscillator [Dürig1997]. In addition, for an arbitrary phase
setting GΦ , ωc depends on the quality factor Q. This means that if the tip-substrate force is
non-conservative, but becomes dissipative, "topographic" information, which is supposed to
be carried by the conservative force will be mixed with "dissipation" information carried by
the non-conservative component of the tip-substrate force. Relation (1.2) shows that the only
setting which can decouple the "topographic" and "dissipation" channels is 2/π=ΦG . In this
case, ωc = ω0, the resonant frequency of the oscillator coincides with the resonant frequency
of the cantilever.
But with this phase setting, the system described by the diagram of figure 1.12 is not stable; it
is then necessary to control the oscillation amplitude, as shown in figure 1.13.
Figure 1.13: Implementation of the amplitude controlled positive feedback NC-AFM oscillator
Two feedback loops are involved. The first one, in blue, includes a Phase Locked Loop (PLL)
based frequency demodulator whose role is first to measure the oscillation frequency and
Chapter 1 General Introduction
32
second to synthesize a sinusoidal carrier at the oscillation frequency and with a phase
satisfying the 2/π=ΦG condition mentioned previously. The second loop, in red, includes
an amplitude demodulator. The oscillation amplitude A is measured and compared to a preset
value A0. The error signal A - A0 is filtered by a P-I corrector and then multiplied with the
carrier produced by the PLL to produce the excitation force necessary to maintain the
oscillation amplitude at A0. From this excitation force, it is straightforward to extract the
energy which is dissipated to maintain the oscillation amplitude at A0. It includes two parts,
the first one that corresponds to the energy dissipated in the cantilever, due to the finite value
of the Q factor and the second one that corresponds to the non-conservative component of the
tip-substrate interaction force.
Combining this oscillator with the distance regulation loop presented in figure 1.10b gives
finally the diagram for the complete control system shown in figure 1.14. It is seen that two
images are simultaneously produced: A "topographic image" built from the piezo position D
necessary to maintain the frequency shift at ∆fc and a "dissipation" image built from the
excitation force necessary to maintain the amplitude at A0.
Figure 1.14: Complete diagram of the NC-AFM
1.2.1.3 Optical beam deflection sensor
Several methods have been proposed and developed to detect the cantilever deflection: the
original electron tunneling method [Binnig1986], optical interferometry [Rugar1989],
piezoelectrical detection [Giessibl2002], piezoresistive detection [Tortonese1993] or optical
Chapter 1 General Introduction
33
beam deflection [Meyer1988]. Our setup is based on the Optical Beam Deflection (OBD)
method, illustrated in fig. 1.15.
Figure 1.15:.The optical beam deflection method.
A light source (laser or light-emitting diode) is focused by an optical system of lenses onto the
back of a cantilever. The reflected light goes to a photo-detector sensor that is split into four
quadrants. The small displacements of the extremity of the cantilever are magnified by an
optical lever effect. By combining the electrical signals from the four quadrants of the
photodiode, it is possible to measure the vertical deflection of the cantilever, but also its
lateral torsion, which is related to the friction of the tip on the substrate.
At the beginning of this thesis, the RT Omicron AFM head was equipped with a light emitting
diode (LED). Previous experiments had convinced us that the performance of this head was
severely limited by this light source, mainly because of its large size, leading to the
impossibility to properly focus it on the cantilever. It was then decided to replace the LED by
a superluminescent laser diode. The way this modification was performed and the
characterization of the resulting improvement in the performance of the AFM head are
reported in detail in chapter 2: "Optimization of the AFM beam deflection sensor".
Following this instrumentation work, experiments were carried on a reference surface for NC-
AFM: KBr(001). The initial goal was to test the new setup, but the work went much farther.
The main result is the demonstration that the atomic contrast observed in the dissipation
images of KBr(001) is related to an adhesion hysteresis phenomenon, which involves a two-
level system localized near the apex of the tip. This demonstration is based on the observation
of a reversible change in the polarity of a particular tip when crossing monoatomic steps
while imaging KBr(001) with atomic resolution, coupled with the observation of bistable
Chapter 1 General Introduction
34
∆f(z) curves with the same tip. These studies are described in chapter 3: "NC-AFM study on
KBr(001)".
These experiments were followed by different attempts to adsorb and image molecules on the
same surface. Preliminary observations, which are far from being understood, but which
could suggest other, more precise and controlled experiments are briefly discussed in the last
chapter: "Conclusions and perspectives".
Chapter 2: Optimization of the AFM beam deflection sensor
35
CHAPTER 2
Optimization of the AFM beam deflection sensor
2.1 Introduction
In this chapter, we describe the modifications that were made on the RT STM/AFM head
from Omicron [Omicron1997] during this thesis. The experiments performed with the
optimized setup are described in chapter 3: "NC-AFM study of KBr(001) ".
In 2.1, we derive an analytical expression giving the sensitivity (in Vm-1) of the beam
deflection sensor in terms of the different parameters of the device. This expression is useful
to discuss how the sensor can be optimized. But it is not enough to optimize the sensitivity.
Noise should also be considered, as what we are interested in is to improve the signal to noise
ratio. The different sources that contribute to the noise of the instrument are discussed in 2.2.
The modifications of the RT STM/AFM head are described in 2.3. Measurements of the noise
spectrum of the cantilever displacement were performed in order to characterize quantitatively
the new optical beam deflection system. They are presented and analyzed in 2.4. We show
that, once the stiffness of the lever is known (part 2.5), this analysis provides a non-
destructive method to measure the sensitivity of the instrument that is useful to calibrate the
oscillation amplitude of the cantilever, which is an essential experimental parameter for NC-
AFM.
Chapter 2: Optimization of the AFM beam deflection sensor
36
2.2. Sensitivity of the optical beam method
Fig. 2.1: a) Optical beam deflection set-up, b) front view of the photo-detectors and scheme of the pre-amplification electronics.
The basic set-up of the Optical Beam Deflection (OBD) method is illustrated in fig. 2.1,
[Sarid1991, Fukuma2005]. A light source (laser or light-emitting diode) is focused by an
optical system of lenses onto the back of a cantilever. The reflected light goes to a photo-
detector sensor that is split in two parts, PD1 and PD2. l is the length of the lever, S the
distance from the end of the lever to the photo detectors, d × d the surface of the optical spot
on the photo-detector, P the optical power at the entrance of the system, and α the power
attenuation coefficient due to imperfections in the optical path (figure 2.1a). The efficiency of
the photo-detector is given by its responsivity η (in A/W), which is the conversion coefficient
between the incident optical power (in W) and the generated electrical output current (in A).
The photo-induced currents i1 and i2 produced by PD1 and PD2 are converted into voltages V1
and V2 by two transimpedance amplifiers whose gain G is given by the resistor RIV, IVRG −= .
Finally a voltage 21 VVV −=∆ is produced by a difference amplifier (figure 2.1b).
The deflection of the lever under the influence of a downward force F is given by
[Sarid1991]:
( )lyEI
Fyyz 3
6)(
2
−=
Where y is the coordinate along the length of the cantilever, E the Young’s modulus, and I the
area moment of inertia. The angle at the extremity of the lever is then given by:
EI
Fl
dy
dz
ly2
2
−===
θ
Chapter 2: Optimization of the AFM beam deflection sensor
37
The spring constant of the cantilever, defined as zFk /−= depends on the Young’s modulus
according to 3/3klEI = . With these expressions, the angular deflection of the optical beam
θ2 can be related to the cantilever linear deflection z by lz /32 =θ . The linear displacement
of the reflected optical beam in the photo-detector plane reads then zzlSd β==∆ )/3( ,
where the amplification factor lS /3=β can easily reach 1000, with the typical values
l=0.1 mm and S= 20 mm.
When 0=z , the optical spot is centered on the photo-detector. Then P1+P2=αP with
P1=P2=αP/2. In general, when the cantilever is deflected:
( )
−+=
−∆+=
)(
61
2
21
21 dsdl
SzP
dsd
dPP
αα and ( )
−−=
−∆−=
)(
61
2
21
22 dsdl
SzP
dsd
dPP
αα
Then the power imbalance 21 PPP −=∆ reads ldPSzP /6α=∆ , considering that ds << d (ds
is typically of the order of 20 µm, while d is usually in the millimeter range). Finally, the
cantilever deflection z is detected at the output of the preamplifier by:
zld
SzPRVVV IV σηα =−=−=∆ 6
21 (2.1)
This expression of the sensitivity σ (in Vm-1) suggests that in order to increase the
displacement sensitivity of the technique, d should be as small as possible. But usually, this
quantity cannot be chosen at will, as shown by Sarid [Sarid 1991]. The light beam should be
focused on the cantilever, whose width is in the range of 20 µm. The spot on the photodiode is
then affected by diffraction effects. For the purpose of estimation, one can use the Airy's
formula, which gives the diameter q of the spot produced at a distance R from a screen with a
circular aperture of diameter a illuminated by light of wavelength λ: aRq /22.1 λ= . For a
light beam spot on the cantilever of diameter a, the far field diffraction limited spot size on the
photo-detector is given approximately by: aSd /λ= . Then:
zl
aPRV IV λ
ηα 6−=∆
Note that this new expression is independent of the distance S from the lever to the photo-
detector.
All the parameters that influence the displacement sensitivity of the method appear in this
expression. To increase the sensitivity, one can:
• Increase the light source power P. Current sources have powers in the mW range. A
limitation is the heating of the cantilever by the absorbed light.
Chapter 2: Optimization of the AFM beam deflection sensor
38
• Increase α, the light transmission coefficient of the system. This includes minimizing
reflections at the different interfaces, optical absorption by the different components
and using reflective (metallized) cantilevers.
• Use an efficient photodiode, with a as high as possible value for the responsivity
η. There is not much to gain here, as most of the Si PIN photodiodes used for AFM
have comparable responsivities, which, depending on the wavelength, vary between
0.4 and 0.7 A/W.
• Maximize a, the size of the light spot on the cantilever to minimize the diffraction
which tends to enlarge the size of the spot on the photo-detector. Of course a should
stay below w, the width of the cantilever.
• Minimize l, the length of the cantilever. The length of typical commercial cantilevers
varies between 100 µm and 200 µm. This choice is related to the desired resonance
frequency of the cantilever.
• Minimize λ, the wavelength of the light used. Usual diodes used in AFM cover a
range of wavelengths between about 600 nm and 1000 nm.
• Increase the gain RIV of the preamplifier.
Sensitivity is of course an important parameter for the instrument, but it cannot be used
without considering in the same time the noise that will inevitably perturb the measurements.
Optimizing the sensitivity can lead to an increase of the noise. In the following, we consider
the main sources of noise that affect the measurements in NC-AFM.
2.3. Sources of noise
Two types of noise can be distinguished in NC-AFM, the thermal noise of the cantilever, and
the noise of the deflection sensor. The thermal noise of the cantilever is a fundamental noise
arising from the thermal fluctuations of the cantilever. It is intrinsic, and does not depend on
the type of deflection sensor used. The most important noises present in the OBD method
according to [Fukuma2005] are the Johnson noise of the conversion resistors of the
preamplifier, the shot noise of the photo-detectors and the noise of the optical source. In the
following, we discuss first the noise of the OBD sensor, then the thermal noise of the
cantilever. To get the expressions that will be used to interpret the experiments discussed in
the following of this chapter, we need first to introduce the Omicron preamplifier, which in
Chapter 2: Optimization of the AFM beam deflection sensor
39
contrast to the basic preamplifier shown in figure 2.1 uses a four-quadrants photodiode (figure
2.2).
Figure 2.2: The Omicron Pre-Amplifier electronic circuit with the four-quadrants photo-detector used in the AFM/STM RT-UHV head.
Each of these four quadrants has its own I-V converter. Three different signals can be
extracted from different combinations of their outputs A, B, C and D:
• The friction signal, related to the lateral displacement of the optical spot on the
photodiode, given by 5× [(A+B) – (C+D)].
• The average (A+B+C+D )/4, which reads:
4/IVtotal PRV ηα= (2.2)
• The topography signal, 5×[(B+D)-(A+C)] expressed from (2.1) as:
zld
SPRV IVFN
65ηα= (2.3)
or:
zVzVld
SV totaltotalFN µ== 120
(2.4)
where totalVµ is the displacement sensitivity.
2.3.1 The shot noise
The shot noise affecting an electrical current is related to the discreteness of the electron
charge. It appears each time the discrete nature of electrons has to be taken into account in the
description of the phenomenon. For instance, it affects the tunneling current across a barrier,
but not the current in a good conductor. In the case of photo-detection, the electrons
Chapter 2: Optimization of the AFM beam deflection sensor
40
contributing to the photocurrent are generated by discrete events, described as the absorption
of a photon of the incident light.
The current shot noise Power Spectral Density (PSD) of the photocurrent I is expressed
as ( ) eIfSshotI 2= , where e is the magnitude of the electron charge [Schottky1918]. Each
quadrant of the photo-detector generates its own noise. The corresponding voltage noise after
the transimpedance amplifiers of the circuit presented in figure 2.2 is given by:
( ) PeRRIIIIefS IVIVDCBAshotV ηα22 2)(2 =+++=
The corresponding noise voltage at the output of the preamplifier is then given by:
( ) totalIVIVIVshotV eVRPeRPeRfS
FN
2222 2005025 ==×= ηαηα (2.5)
2.3.2 The Johnson noise
Johnson noise [Johnson1928, Nyquist1928] is the electronic noise generated by the thermal
agitation of the charged carriers inside an electrical conductor at equilibrium. It is a
consequence of the fluctuation-dissipation theorem [Callen1951]. To each of the converting
resistors RIV is associated a voltage PSD:
IVBjohnson
V TRkfS 4)( = (2.6)
where kBT is thermal energy. The corresponding noise voltage at the output of the
preamplifier is then given by:
( ) IVBjohnson
VBjohnson
VBjohnson
VBjohnson
VAJohnson
V TRkSSSSfSFN
4005)( 2 =×+++= (2.7)
2.3.3 The optical source noise
Light emitting diodes (LED) and lasers are the two most common light sources used in AFM.
Lasers are affected by specific noise, which we briefly describe in the following. LED have
also their specific noise and suffer from a major drawback: the size of their equivalent source
is usually quite large, making them difficult to focus. This is the main reason for the head
modification that is described in part 2.4.
2.3.3.1. Laser noises
It is well known that the beam intensity, the beam direction, and the profile of the beam of a
laser fluctuate. The main source of these fluctuations is mode hopping which corresponds to
transitions between different modes of the laser resonator. These hops are generally provoked
by external disturbances, such as a variation of the temperature or, in the case of optical
Chapter 2: Optimization of the AFM beam deflection sensor
41
feedback, when some light is reflected back into the laser cavity. It has been shown
[Ojima1986] that this noise can be largely reduced by modulating the laser diode current at
high frequency, which causes the diode to "average" over its accessible modes. This trick is
commonly used in videodisc players [Ojima1986] and has been adapted to laser diodes used
for AFM [Kassies2004, Fukuma2005]. It has the other advantage of eliminating the
interferences between the light reflected by the cantilever and the light scattered by the
sample, which are commonly observed in AFM, because it reduces drastically the coherence
length of the laser. Another type of source, which presents the same advantages, is the
superluminescent diode (SLD), which will be introduced in 2.4.1.
Let us note that independently of these considerations, the noise generated by the intensity
fluctuations is generally negligible in AFM because what is measured is the difference
(B+D)-(A+C): most of the laser intensity fluctuations are eliminated as a common mode noise
by the preamplifier. Furthermore, the noise induced by beam direction and profile fluctuations
can be diminished by using a single mode optical fiber to couple the laser diode to the
focusing optics of the OBD sensor.
2.3.4 The cantilever thermal noise
The energy of a system in contact with a thermostat at finite temperature T fluctuates, by an
amount which is very small for a macroscopic object, but that becomes significant for a
microscopic and relatively soft object such as an AFM cantilever. To a very good
approximation, a cantilever can be considered as a one degree of freedom harmonic oscillator.
As stated by the equipartition theorem, the average energy for each quadratic term in its
Hamiltonian should be given by TkB2/1 , where Bk is the Boltzmann constant. The mean
value of the potential energy is then given by:
Tkkz B2
1
2
1 2 =
from which the variance of the displacement z, which characterizes the fluctuations of the
cantilever can be extracted:
k
Tkz B=2 (2.8)
This variance can be related to )( fSThermalz , the power spectral density of the displacement z,
by:
∫∞
=0
2 )( dffSz Thermalz (2.9)
Chapter 2: Optimization of the AFM beam deflection sensor
42
It is usual to express these displacement fluctuations in terms of the input of the force-
displacement harmonic transfer function of the cantilever:
)()()(2
fSfCfS ThermalForce
Thermalz = (2.10)
Where C(f) is the free cantilever harmonic transfer function and )( fSThermalForce is the power
spectral density of the force that has to be applied to the ideal, noiseless cantilever to produce
the displacement fluctuations characterized by )( fSThermalz . C(f) is readily derived from
Newton's equation: )(tFzzkzm +−−= &&& γ . Were γ is a viscosity, by injecting
)2exp()( tfjfzz π= and )2exp()()( tfjfFtF π= :
Qffjff
kffC
oo
o
+−= 22
2
)( (2.11)
where mkf /2/10 π= is the resonance frequency of the cantilever and γπ /2 0fmQ = is its
quality factor. Combining (2.8-11), we get:
∫∞
+−==
0
2
22
22 )( dffS
Qffjff
kf
k
Tkz Thermal
Forceoo
oB
It turns out that )( fSThermalForce is independent of frequency. The integral can then be evaluated
giving finally:
TkQf
TkkS B
o
BthermalForce γ
π4
2 == (2.12)
The thermal force fluctuations depend only on the dissipation. This is another expression of
the fluctuation-dissipation theorem and the expression (2.12) is analogous to the expression
(2.6) we already used for the voltage noise in a resistor, where R has been replaced by γ.
Finally, the contribution of the cantilever thermal noise at the output of the preamplifier reads,
from (2.4, 2.10, 2.11, 2.12):
[ ]2
22222
32222
)()(
2)( total
oo
oBtotal
thermalForce
ThermalV V
ffffQk
QfTkVSfCS
FN +−==
πµµ (2.13)
2.4. Modifications of the RT AFM Omicron head
The RT Omicron AFM head was used before this thesis, in particular during the PhD work of
Jérôme Polesel Maris [Polesel2005] to investigate different surfaces (Al2O3(0001), TiO2(110),
Chapter 2: Optimization of the AFM beam deflection sensor
43
KBr(100),...). During these experiments, a number of problems were identified, which lead to
the conclusion that this set-up was far from state-of-the-art, rendering impossible certain
experiments. In particular:
• It was impossible to work below a frequency shift |∆f|≈10 Hz
• It was impossible to work with an amplitude below A0≈5 nm.
• It was difficult to get atomic resolution on KBr(001).
• It was impossible to see the thermal noise peak on the frequency spectrum of
the cantilever displacement.
All these observations point to a bad signal-to-noise ratio. Discussions with E. Meyer and L.
Nony (working at this period as a post-doctoral fellow) from the Institute of Physics in Basel
convinced us that these problems originate from the deflection sensor and in particular from
the LED used in this system. The main reason seemed to be that this diode is difficult to
focus. The size of its spot is much larger than the typical width of a cantilever and most of the
light power is lost at this level. It was then decided to replace the LED by a laser. We
gratefully acknowledge the help and advices from E. Meyer and L. Nony during this
modification, which is described in the following. Some of the work reported here was done
by Théophile Noël (student at ISEN-Lille) during his stay in CEMES from June to September
2006.
2.4.1 The superluminescent laser diode
Figure 2.3: Schematic of the AFM setup
Chapter 2: Optimization of the AFM beam deflection sensor
44
The superluminescent diode is a special type of laser diode that combines the high output
power and brightness of a laser diode with the low coherence of a LED [SLDshort]. It is
easier to focus than a LED due to its smaller equivalent source size and the low coherence
length eliminates completely the interference effects between the light reflected by the
cantilever and the light reflected by the sample that generally affect the approach curves in
AFM. We choose a diode from SuperLumDiodes, Ltd. [SuperLum] (ref: SLD-26-HP) with
the characteristics summarized in table 2.1.
Output power at the end of the monomode fiber 5 mW
Maximum SLD current 155 mA
Spectral center 681 nm
Spectral bandwidth, FWHM 9.6 nm
Table 2.1: SLD characteristics
The output of the diode is connected to a single mode optical fiber equipped with a FC/APC
connector. The diode is also equipped with an internal photodiode, which is used by the
power driver (ref: Pilot-4AC) to regulate its optical power and with a thermistor and a
thermoelectric cooler to regulate the temperature of the diode. The diode is pressed on a heat
sink to avoid excessive heating.
The LED that was installed on the AFM head before the modifications had a power of
1.2 mW. The SLD diode we chose is more powerful and is expected to dissipate more. In
addition, SLDs are very sensitive to heating and would not support the outgassing procedure
that has to be applied to the vacuum chamber to reach the UHV regime, which consists in
heating the chamber at about 120°C for days. It was then necessary to let the SLD outside the
vacuum chamber and to guide the light with an optical fiber.
2.4.2 The single mode optical fiber
Commercial single mode optical fibers are generally protected by a polymer coating. To avoid
outgassing problems, we ordered special gold-coated fibers from Fiberguides Industries
(Stirling, NJ, USA). It was then necessary to equip the fiber with a FC/APC connector in
order to connect it to the laser. We asked to a specialized company to do it, but unfortunately,
after many trials and despite specialized equipment, we had to conclude that it was impossible
to get a connector with a transmission better than a few percents on this fiber. The 4 µm size
Chapter 2: Optimization of the AFM beam deflection sensor
45
of the core of these fibers is quite small, meaning that it should be centered in the connector
with a better than 1 µm precision to obtain a sufficient transmission coefficient. We then
decided to use a commercial, acrylate-coated fiber. It has been in use for more than one year
without any significant effect on the vacuum level in the chamber.
The fiber cable has been purchased from Thorlabs (item P2-620A.FC-5). It is equipped with 2
FC/APC connectors. When connected on the SLD, the transmission was always better than
50%, reaching sometimes 80%, a figure that is not attainable without industrial assembling
capabilities. The single mode optical fiber (SM600 from Thorlabs) has a 4 µm core, a 125 µm
cladding and a 245 µm acrylate coating. The operating wavelength is 622/680nm and the
numerical aperture 0.12.
The following steps were followed to mount the fiber on the vacuum chamber and the
microscope:
- After removing one of the connectors, the fiber external protective jacket is stripped
off over a 1 m length.
- This section of the fiber is then introduced into a 0.5 mm hole managed in the center
of a CF40 flange that is subsequently filled with an epoxy glue.
- The acrylate coating is then stripped off over a 0.1 m length at the extremity of the
fiber.
- The extremity of the fiber is cleaved with a fiber cleaver (XL410 from Thorlabs) in
order to minimize the optical reflection at the end of the fiber and get a perfect circular optical
spot shape.
- The fiber extremity is glued with a cyanoacrylate glue in a 2.5 mm diameter zirconia
ferrule, which will be positioned afterwards in the optical assembly of the AFM (figure 2.4b).
This procedure is rather tedious and delicate, but the final result is satisfying. The system has
been performing satisfactorily during more than one year.
Chapter 2: Optimization of the AFM beam deflection sensor
46
2.4.3 The focusing optics
(a)
(b)
Figure 2.4: (a) Path of the optical beam in the optical block of the AFM head. (b) Optical focusing set-up.
The optical system to focus the beam on the cantilever has been designed with the free
software Oslo edu Edition 6.2.4 [Oslo]. It was chosen, for the sake of simplicity, to replace
only the LED, without changing the optical block of the AFM, in particular, the two moveable
mirrors (fig. 2.4a). The only degree of freedom was then the distance between the extremity
of the fiber and the first mirror (figure 2.4a), which was used to adjust the focus on the
cantilever. The constraints in the design of the optical system were the distances between the
first mirror, the cantilever, the second mirror and the photodiode, which are imposed, the size
of the spot on the cantilever, which should be smaller than its width (25 µm) and the size of
the spot on the photodiode, which should be smaller than the size of its active area. The main
problem was to minimize the spot diameter on the cantilever, which is enlarged by diffraction
and spherical aberration. The optimal solution is built with two planar convex lenses, this
arrangement being known to minimize spherical aberration. With suitable lenses
(EPCX45230 and EPCX45355 in BK7 with a MgF2 anti-reflective coating from Edmund
Optics [Edmund], with focal lengths 10 mm and 15 mm), diffraction contributes to less than
15 µm and spherical aberration to less than 10 µm to the size of the optical beam on the
cantilever. The minimal size of the spot is then 18 µm.
Chapter 2: Optimization of the AFM beam deflection sensor
47
The dimensions of the mechanical system shown in figure 2.4b are such that it can replace the
LED without any other modification on the head. The distance between the whole system and
the mirror 1 (figure 2.4a) can be roughly adjusted. The distance between the fiber extremity
and the first lens can be finely tuned with a screw, in order to adjust easily the spot diameter
on the cantilever.
2.4.4 New characteristics of the beam deflection sensor.
The first tests after these modifications showed a tremendous improvement in the
performance of the head. In particular:
• It is now possible to work at a frequency shift |∆f|≈1 Hz
• It is now possible to work with an amplitude below A0≈1 nm.
• We now get atomic resolution on KBr(001) routinely.
• It is easy to see the thermal noise peak on the frequency spectrum of the
cantilever displacement.
These improvements originate in two combined factors. αP, the optical power incident on the
photodiode is much higher than before the modifications, increasing from approximately 20
µW for the LED to more than 1 mW for the SLD. This is not only due to the more powerful
light source (5 mW for the SLD instead of 1.2 mW for the LED), but also to the much better
focusing on the cantilever. This gain by a factor of 50 gives a √50∼7 improvement in the S/N
ratio if one considers that the measurements are shot noise limited (as will be demonstrated in
the following). But this evaluation is an underestimate, since, due to the bad focusing with the
LED, much scattered light was detected by the photodiode. The power of 20 µW mentioned
above includes optical power that is not useful for the cantilever displacement measurement.
Overall, we estimate that the S/N ratio improved by at least a factor of 10.
In the following, we discuss in more detail the measurements of the noise made in order to
better characterize the new set-up.
Chapter 2: Optimization of the AFM beam deflection sensor
48
2.5 Noise spectral analysis
Figure 2.5: Noise measurement setup
In order to characterize the new optical beam deflection system, measurements of the noise
spectrum of the cantilever displacement, given by VFN (2.4), were performed. In our analysis,
the expression of the total PSD considers the addition of the thermal noise, the shot noise and
the Johnson noise power spectral densities:
)()()()( fSfSfSfS JohnsonV
ShotV
ThermalVV FNFNFNFN
++= (2.14)
Each of these terms has a different dependence in the laser power P, which can be adjusted
with the SLD driver and measured from the Vtotal signal. One can rewrite eq. (2.14) in terms of
the Vtotal, using (2.5, 7 and 13):
IVBtotalIVtotalthermalForceV RTkVeRVfSfCS
FN400200)()( 222 ++= µ (2.15)
It is seen that each of these terms has a different dependence in Vtotal. This behavior will be
used to separate the different contributions to the noise cantilever displacement spectrum.
Figure 2.5 shows the measurement set-up. The spectrum analyzer (Hewlett Packard
HP2582A) we used is limited to the 0 to 25 kHz frequency range. It was then necessary to
shift the frequency range of the signal of interest in this range. This was done by heterodyning
the signal with a multiplier and a waveform generator. The VFN output signal of the pre-
amplifier and the output of the wave generator were connected to the inputs of the multiplier
circuit. The PSD of the output of the multiplier is obtained from the spectrum analyzer.
The following measurements were performed in UHV, with a NCH cantilever from
NanoSensors [Nanosensors2000] with the following nominal characteristics: n+-Si, 0.01-
0.025 ohm.m, thickness 2.5-4.5 µm, width 20-40 µm, length 125 µm, tip height 10-15 µm,
Chapter 2: Optimization of the AFM beam deflection sensor
49
fo=280-265 kHz, k=25-50 N/m. The gain of the preamplifier was determined by RIV =
90.9 kohm, with a bandwidth of 400 kHz.
Two types of spectrum were measured while varying the laser power P:
• Around the resonance frequency of the cantilever, on a 500 Hz frequency range to
characterize the thermal noise contribution. Each spectrum corresponds to the average
of 256 spectra.
• Far from the resonance peak around a frequency of 170 kHz on a 25 kHz frequency
range, to extract the shot noise and Johnson noise contributions. Each spectrum
corresponds to the average of 64 spectra.
Figure 2.6: Background noise as a function of Vtotal.
Around 170 kHz, the spectra are flat. The average of each of these spectra varies linearly with
Vtotal as shown by the linear adjustment displayed in fig. 2.6. This behavior is a clear signature
of the shot noise. This linear dependence is rather specific and it is difficult to imagine other
sources of noise, which would have not been considered in our analysis, giving such a linear
dependence in the laser power. For this reason, we will use the data of fig. 2.6 to calibrate our
measuring set-up. The signal at the input of the spectrum analyzer VHP is related to VFN by VHP
= δ VFN. δ depends on the gain of the multiplier and the amplitude of the waveform generator
signal. The PSD given by the analyzer reads then SVHP = δ 2 SVFN (2.16). From (2.15), the slope
B of the linear adjustment in fig. 2.6 should be given by 2200 δ⋅⋅= eRB IV , from which δ 2 =
6.15.
Using this value to evaluate the Johnson noise term gives =2400 δIVBTRk 8.9 10-12 V2Hz-1
(with T=300 K), much smaller than the measured value A = 4.97 10-12 V2Hz-1 (fig. 2.6). This
is not surprising as many other sources of white noise are present in the system. The resistors
are not ideal, and most electronic components generate white noise. Furthermore, measuring
Chapter 2: Optimization of the AFM beam deflection sensor
50
the thermal noise of a resistor of relatively low value (RIV = 90.9 kohm) requires special
arrangements that we did not implement for these experiments.
(a)
(b)
(c)
(d)
(e)
(f)
Figure 2.7: Spectra in the vicinity of the cantilever resonance for different values of totalV .
Open squares: experimental data, red lines: analytical adjustments.
Note that for these spectra, 01.0/2 ≈= kTkz B nm.
The spectra in the vicinity of the cantilever resonance are plotted in figure 2.7 for different
values of the laser power, as measured by Vtot. Considering that the shot and Johnson noise
background (fig. 2.6) should be negligible compared to the thermal noise in this frequency
range, an adjustment of these spectra was performed using the following analytical
expression:
Chapter 2: Optimization of the AFM beam deflection sensor
51
[ ]22222
3222
)()(
2)()(
ffffQk
QfTkfSfC
oo
oBthermalForce
+−=
πξξ (2.17)
where T=300 K, k=30 N/m, and f0, Q and ξ have been chosen in order to reproduce as well as
possible the experimental peak position, width and amplitude.
The spectra are very well described by the analytical expression (2.17) except in (e) and (f)
where the neglect of the white noise background becomes perceptible. Then, from equations
(2.13, 16 and 17): 2222totalVµδξ = . This dependency in 2
totalV is confirmed in figure 2.8, from
which we extract 1522 1056.1=µδ m-2 and 715 1059.115.6/1056.1 ==µ m-1. The sensitivity
is then given by: totaltotal VV 9.15=µ in mV.nm-1.
It can be checked that this value has the good order of magnitude for our system. From
equation (2.4): ldS/120=µ . d can be estimated from the Airy's formula: aSd /22.1 λ=
where a is the size of the beam spot on the cantilever. As seen in 2.2, a=18 µm when the
system is perfectly focused. Then taking l = 125 µm (NCH cantilever) and λ=680 nm :
71008.222.1/120 == la λµ m-1, which compares quite well with the measured value, taking
into account the crudeness of this evaluation.
Figure 2.8: Dependence of ξ 2 on Vtotal: Black squares: experimental data, blue crosses: fit with ξ 2= 2151056.1 totalV .
Chapter 2: Optimization of the AFM beam deflection sensor
52
2.5.1. Estimation of the equivalent input noise of the new beam deflection system.
To characterize a sensor, it is customary to express its noise as an equivalent input noise, that
is the noise that has to be applied to the input of an ideal noiseless sensor to produce the
measured output noise. This figure-of-merit facilitates the comparison between different types
of sensors. In our case, this equivalent input noise should be expressed in m2Hz-1, since we
deal with a displacement sensor. Figure 2.7 shows that our system is dominated at high laser
power by the shot noise: at 52.3=totalV V, the shot noise contributes to 92% of the total noise.
Let us recall that the thermal noise of the cantilever is not a noise of the sensor but is used
there as a known noise source to characterize it. To calculate the equivalent input noise, we
just need to divide the measured shot noise (in V2/Hz) by the square of the sensitivity totalVµ
in V2m-2:
27
27221027.3
)1059.1(
200200 −===total
IV
total
totalIVz
V
eR
V
VeRS
µ m2Hz-1
for the maximum optical power, corresponding to totalV =3.52 V (figure 2.7a) or Sz1/2 =
57 fm.Hz-1/2.
This experiment was performed with a non-coated cantilever, for which totalV was limited to
about 4 V. With a NCHR cantilever (coated with Al), we could achieve the equivalent of a 3-
fold increase in totalV (we had to reduce the gain of the preamplifier to avoid saturation at
10V). In this situation, the equivalent input noise becomes: Sz1/2 = 57/√3 = 22 fm.Hz-1/2.
This figure can be compared with the best value for a beam deflection system obtained so far,
reported by Fukuma and Jarvis [Fukuma2006] for a NCHR cantilever in air where
Sz1/2 = 11 fm.Hz-1/2.
2.5.2. Estimation of the cantilever temperature at maximum laser power level.
Figure 2.9: Dependence of f0 and Q in Vtotal. f0: Black squares: experimental data, red line: fit with f0= -6.77 Vtotal.+ 271200.5, Q: red circles and black line.
Chapter 2: Optimization of the AFM beam deflection sensor
53
Figure 2.9 reveals that f0 decreases linearly when the optical power increases. This
observation can be related to the heating of the cantilever by the optical beam. In the
following, we use these data to estimate the temperature increase of the cantilever at
maximum optical power.
The resonant frequency of the cantilever can be calculated from:
( )ct mm
kf
24.02
10 +
=π
(2.18)
Where mt is the mass of the tip and mc is the mass of the cantilever [Sarid1991]. The stiffness
k can be calculated from the dimensions of the cantilever and the Young's modulus of silicon
E = 1.69 1011 Nm-2. As already mentioned at the beginning of the chapter, k can be expressed
as 3/3 lEIk = . The area moment of inertia of a rectangular lever is related to its width w and
thickness t by 12/3wtI = [Sarid1991]. Then:
33 4/ lEwtk = (2.19)
showing that the resonance frequency depends on the temperature via (1) the change in the
dimension of the cantilever and (2) the temperature dependence of the Young's modulus of
silicon. A detailed study of the temperature dependence for comparable cantilevers
[Gysin2004] showed recently that the variations of the resonant frequency are largely
dominated by the variations of the Young's modulus. As explained in [Gysin2004], the
temperature dependence of the Young's modulus of Si can be estimated from the temperature
dependence of its bulk modulus B, using E=3B. Then:
5
0
00
0
0 10770.15.271200
5.2712005.2711762
)0(
)0()52.3(22 −×−=−=−=∆=∆=∆=∆
Vf
VfVf
f
f
k
k
E
E
B
B
Figure 2.10: Si Bulk modulus as a function of temperature. Black squares: experimental values from [Nandanpawar1978], red line: fit with B=995.6 - 0.058 T Gpa.
Chapter 2: Optimization of the AFM beam deflection sensor
54
From the temperature dependence of B, plotted in figure 2.10: )(058.0 GPaTB ∆−=∆ .
Combining these two relations, we get:
KKB
T 3058.0
9783109.5
058.0
3003109.5 55
≈−
××−=−
×××−=∆−−
This crude estimate shows that the temperature increase of the cantilever is easily detectable,
but is negligible for room temperature experiments. Nevertheless, it shows that the upper limit
for the laser power P is not very far from our experimental conditions.
The decrease of Q with increasing temperature displayed in figure 2.9 is more difficult to
interpret. It is related to internal friction phenomena, which depend on several distinct
mechanisms and are highly material specific.
2.6. Sensitivity measurement and amplitude calibration.
In the preceding paragraph, we have used a value k = 30 N/m, somewhat arbitrarily, to extract
the sensitivity. In fact, the noise measurements can be considered either as a way to calibrate
the amplitude of oscillation, once the stiffness is known or alternatively as a way to calibrate
the stiffness, once the amplitude is known. The amplitude can be determined by using the
cantilever in the contact mode, using the known calibration of the z piezoelectric ceramics to
calibrate the cantilever displacement under a z increment, but this technique damages the tip.
Our strategy is to avoid destructive measurements. It is then necessary to use a non-
destructive method to measure the stiffness.
Several methods have been proposed to determine k [Burnham2003]:
• The first group of methods uses known analytical formulas, which relate k to the
dimensions of the cantilever and the elastic moduli of the cantilever material
[Cleveland1992]. These methods are better for cantilevers of simple shape made from
well-characterized material, for instance our cantilevers (diving board geometry and
Si).
• The second group of methods in based on the measurement of the static deflection
under the application of a force, via a calibrated cantilever [Cumpson2002] or by the
addition of known masses [Cleveland1992]. These methods are very tedious and can
be destructive.
• The third group of methods uses the thermal noise in vacuum [Hutter1992] or in a
medium of known viscosity and density [Sader1999].
Chapter 2: Optimization of the AFM beam deflection sensor
55
We chose the simplest method, which belongs to the first group, where we calculate the value
of k from (2.19): 33 4/ lEwtk = . 3 cantilevers were examined by SEM, two NCH cantilevers
extracted from the same wafer and one NCHR cantilever (same type as NCH, but with a 20
nm Al coating) from another wafer. These SEM observations were performed by Christophe
Deshayes in CEMES. The images and the measurement of the dimensions of the first NCH
cantilever are shown in fig. 2.11. In fig. 2.11a, the length has been measured between the
attachment point of the cantilever to its support and the apex of the tip. This convention is
somewhat arbitrary, but is consistent with approximating the shape of the cantilever by a
rectangle. In fig. 2.11b, as the section of the cantilever is trapezoidal, the width is calculated
as the mean between the widths of the two faces of the cantilever. In fig. 2.11c, the cantilever
had to be aligned rather precisely to get a precise value for its thickness. Applying relation
(2.19) with the dimensions reported in fig. 2.11 gives k=30.3 N/m.
(a)
(b)
(c)
(d)
Figure 2.11: SEM images of a NCH cantilever.
To check that this value is reasonable, the resonant frequency of the cantilever can be
calculated from equation (2.18). The tip can be modeled as a square-base pyramid of volume
V=Ah/2, where h=15.6 µm is the height of the tip and A=(12.9 µm)2/2 the area of its base
(fig. 2.11d). We get mc = 1 pg and f0 = 305 kHz. This evaluation gives an error below 10%
Chapter 2: Optimization of the AFM beam deflection sensor
56
since the resonance frequency of this cantilever was measured at 280 kHz. This corresponds
to an error below 20% on the value of k, in agreement with the Nanosensors Product Guide
[Nanosensors2000].
The same measurements were performed for the other two cantilevers. The results are
summarized in the following table:
l (µm) w (µm) t (µm) k (N.m-1)
NCH1 127.5 26.7 3.82 30.3
NCH2 130.8 29.6 3.74 28.5
NCHR 121.2 25.0 3.80 32.5
Table 2.2: Results of the measurements for 3 cantilevers
The contribution of the Al coating was neglected in the calculation of k for the NCHR
cantilever as it is expected to lead to a minor (<1%) correction, estimated from a calculation
of effective Young's modulus for a bi-layer cantilever [Roark2002]. It is seen that the value of
k does not vary significantly from one cantilever to the other. The cantilever used for the
noise measurements discussed in 2.4 was taken from the same wafer than NCH1 and 2. This
is why we took k=30 N/m for the quantitative evaluation of the results of this study.
Note that this method for measuring k could be improved by a better description of the shape
of the cantilever, which could be done by numerical methods.
2.7. Conclusions and perspectives.
The two main achievements reported in this chapter are the improvement of our RT AFM
head and the development of the noise measurements, which happen to constitute a useful and
practical method to measure the oscillation amplitude of the cantilever.
We have shown that the main factors in the head performance improvement are the increase
of the optical power incident on the photodiode and the better focusing of the optical beam.
There is now a limited room for further improvements. The laser power cannot be much
increased because of the heating of the cantilever. The focusing and quality of the optics can
certainly be further improved, but the gain there will be modest. Another improvement that is
planned is to replace the actual photodiode by a smaller one, of smaller capacitance. This will
decrease the background noise generated by the preamplifier.
Chapter 2: Optimization of the AFM beam deflection sensor
57
We have developed a non-destructive and simple method to evaluate the oscillation amplitude
from the noise spectra. Its main limitations in terms of precision are in the measure of k where
a precision of 20% is estimated. Increasing this precision to 10% is certainly feasible by a
better modelization of the cantilever by numerical methods. It is important to measure as
precisely as possible the experimental parameters to allow quantitative measurements in NC-
AFM.
Chapter 2: Optimization of the AFM beam deflection sensor
58
Chapter 3 NC-AFM study on KBr(001)
59
CHAPTER 3
NC-AFM study on KBr(001)
3.1 Introduction
The surfaces of ionic insulators, such as alkali halides [Bennewitz2002a] or CaF2
[Reichling2002] have played a major role in the development of atomic resolution NC-AFM.
They are easily prepared by cleavage and atomic resolution images are routinely obtained. In
addition, the interpretation of their atomic contrast is now well understood. This is
particularly true for the KBr(001) surface, which was extensively investigated both
experimentally [Hoffmann2004, Hoffmann2002] and theoretically [Pakarinen2006] by a
number of groups and can, for these reasons, be considered as one of the reference surfaces
for NC-AFM studies.
The starting point of the experiments described in this chapter was the need to validate the
improvements made on our AFM head, described in chapter 2. We then naturally choose
KBr(100) for the reasons outlined above and also because this substrate was investigated
previously with the AFM head before its modifications, during the thesis of J. Polesel-Maris
[Polesel2005]. While performing these experiments, new results were obtained, which can all
be related to changes in the atomic structure of the tip apex under the influence of tip-
substrate interactions.
The experimental methods needed to perform reliable measurements in NC-AFM are
described in part 3.2. The observation of a systematic and reversible change in the atomic
contrast when the tip crosses a monoatomic step of the KBr(001) surface is presented in part
3.3. These experiments are complemented by force and dissipation spectroscopy
measurements performed with the same tip, reported in part 3.4. These results are discussed in
part 3.5 before concluding the chapter.
Chapter 3 NC-AFM study on KBr(001)
60
3.2. Experimental methods
The KBr crystals were cleaved in air, quickly transferred to the UHV system, and finally
heated at 400 K for 1 hour to remove the charges produced during the cleavage process. This
preparation method produces an atomically well-ordered surface with (001) terraces separated
by monoatomic steps, as shown in figure 3.1. Under standard UHV conditions (pressure in the
10-10 Torr range), the surface remains clean for a few days.
(a)
(b)
Figure 3.1: (a) Constant-frequency detuning image of the KBr(001) surface. ∆f=-50 Hz,
App=7.7 nm, (γ=-1.3 fN m1/2). (b) Profile along the white line drawn on (a) showing 3 monoatomic steps of height 0.33 nm. Arrows point to atomic kinks on the steps. Parameters
of the cantilever: f0=269 800 Hz, k≈30 N/m, Q≈25 000.
We use NCH silicon cantilevers provided by NanoSensors (Neuchatel, Switzerland). The
work discussed here was performed with a cantilever that was characterized by thermal noise
measurements from which the values of the resonance frequency f0, the spring constant k and
the quality factor Q were deduced (see the legend of figure 3.1). These values are used to
calibrate the dissipation scale and to calculate the value of the "normalized frequency shift" γ
= kA3/2∆f/f0 introduced by Giessibl [Giessibl1997] as a useful parameter to compare images
obtained under different experimental conditions.
Residual long-range electrostatic forces were compensated by applying a bias voltage
between the plate supporting the sample and the tip as described in [Guggisberg2000]: the
Chapter 3 NC-AFM study on KBr(001)
61
distance regulation feedback loop is disabled and the frequency shift is recorded as a function
of the bias voltage (figure 3.2). The maximum of the parabola (which corresponds to the
minimal force) gives then the voltage that has to be applied to compensate the contact
potential difference between the tip and the sample.
Figure 3.2: ∆f(V) curve on KBr(100). f0=269 800 Hz, App = 6.1 nm. The red curve is a parabolic fit giving the position of the maximum at V=-0.78 V.
Great care was taken to set the time constants of the different feedback loops of the control
system. Improper settings of these loops can induce artifacts, especially when measuring
dissipation signals [Nony2006]. The phase in the AGC loop was set to its optimal value,
where the f∆ channel is decoupled from the dissipation channel by minimizing the
dissipation signal.
3.2.1. Force spectroscopy
3.2.1.1. Extraction of the force from ∆f(z) using the Sader-Jarvis formula
The frequency shift f∆ can be related to the tip-substrate interaction force by the following
relation, first derived by Giessibl [Giessibl1997]:
[ ]∫ ++−=∆ 0
1
0
00int0
0
)2sin()2sin(1(f
dttftfAbFkA
f
f
f ππ (3.1)
For a cantilever of resonance frequency 0f oscillating with an amplitude A. b is the distance of
closest approach to the surface. This expression, derived using the Hamilton-Jacobi approach,
Chapter 3 NC-AFM study on KBr(001)
62
is valid for any amplitude of oscillation provided that 1/ 0 <<∆ ff , a condition that is always
met in our measurements.
To determine the interaction force from the observed frequency shift, eq. (3.1) must be
inverted. This inversion is simple only in the small amplitude limit. In this case, intF can be
linearized:
[ ] ( ) )2sin()2sin(1( 0int
int0int tfAz
FAbFtfAbF
Abz
ππ+=∂
∂++≈++
and eq. (3.1) reduces to:
Abzz
F
kf
f
+=∂∂−=∆ int
0 21
The frequency shift is proportional to the gradient of the interaction force, which plays the
role of an effective additional stiffness for the cantilever harmonic oscillator.
In the general case, when the amplitude A is comparable or larger than the scale on which intF
varies significantly, the inversion is more challenging. An analytical expression, valid only in
this large amplitude limit has been derived by Durig [Durig1999]. Numerical schemes for
arbitrary amplitude have been proposed by Durig [Durig2000] and Giessibl [Giessibl2001].
An accurate analytical approximation, valid for arbitrary amplitudes, has been derived by
Sader and Jarvis [Sader2004]. It consists in injecting the interaction force, expressed in terms
of its Laplace transform in eq (3.1) and approximating the resulting Bessel function by a Padé
approximant. The resulting formula reads:
( ) ( )( )
( )( )
zdf
zf
zd
d
zz
A
f
zf
zz
AkzF
z
′
′∆′−′
−′∆
−′+= ∫
∞
0
2
3
0
2
1
int28
12π
(3.2)
Three remarks are in order:
• To get the value of the force at a distance z from the surface requires integrating from
z to infinity. This means that, in principle, the ( )zf∆ curve must reach the region
where the interaction force becomes negligible, i.e. ( ) 0≈∆ zf . But due to the
denominators in ( )zz −′ in the integrand, and to the fact that for large amplitude it is
the second term in the integrand that dominates, this condition is not very stringent.
Chapter 3 NC-AFM study on KBr(001)
63
• This expression involves the derivative of ( )zf∆ . Differentiating an experimental
curve generally generates noise. This noise can be reduced by a suitable smoothing
procedure of the initial data.
• The precision of the force extraction is affected by the precision on the values of k and
A, demonstrating the importance of the precise measurement of these quantities to
make quantitative force measurements, as stressed in chapter 2.
Figure 3.3 shows a simple example of application of the Sader-Jarvis (SJ) formula (3.2),
starting from the following analytical expression for a Morse-van der Waals tip-substrate
force:
( ) [ ] [ ]{ })(exp)(2exp26 2int σκσκκ −−−−−+−= zzE
z
HRzF b (3.3)
where the first term on the right side corresponds the van der Waals force, with a tip radius R
and a Hamaker constant H and the second term corresponds to the Morse contribution,
described by the parameters κ, Eb and σ. Expression (3.3) is plotted in figure 3.3a (black
squares), with H=1 eV, R=10 nm, and the parameters extracted from [Giessibl2003], which
have been optimized to describe a Si-Si bond, κ=15.5 nm-1, Eb=2.15 eV and σ=0.235 nm.
The corresponding frequency shift for a cantilever oscillating at f0=270 kHz with an
amplitude A=5 nm can be calculated analytically from eq. (3.1) [Nony2006] and is plotted in
figure 3.3a (black line).
(a) (b)
Figure 3.3: Sader and Jarvis formula applied to the simple case of a calculated Morse-van der Waals tip-substrate force. (a) Black square: force, continuous black line:
frequency shift calculated using (3.1), red line: force calculated using the SJ expression (3.2) from the frequency shift. (b) Zoom of (a) for the forces.
Chapter 3 NC-AFM study on KBr(001)
64
The SJ formula was then applied to this frequency shift curve by numerical integration. The
resulting force curve, plotted in figure 3.3a (red line) is very close to the original one,
showing the efficiency of the SJ formula.
3.2.1.2. Description of the technique
The procedure to get ∆f(z) and D(z) curves consists in choosing the position on the surface
where the data should be taken, disabling the distance regulating feedback loop and making
the distance vary according to a pre-defined pattern while monitoring the ∆f and D signals.
We generally use patterns of the type illustrated in figure 3.4a. In this example, the tip is first
retracted by 2nm (black line), then approached by about 2.4 nm (red line) and this sequence is
repeated in time-reversed order (green and blue lines). ∆f is recorded as a function of time and
can be plotted as a function of z, as shown in figure 3.4b.
(a)
(b)
(c)
(d)
Figure 3.4: (a ) z(t) pattern (right scale) and corresponding ∆f(t) curve (left scale). (b) ∆f(z) curves extracted from (a). (c) Zoom of (b). (d) Corrected ∆f(z) curves. App=7 nm, f0=271,900
Hz, ∆f=-144 Hz.
We adopted this type of pattern essentially because it allows identifying some of the
numerous artifacts that can spoil the data in NC-AFM. Because the second half of the data
corresponds to a time-reversed sequence displacement, all the artifacts related to the different
Chapter 3 NC-AFM study on KBr(001)
65
response times of the apparatus can be avoided or identified. These include an improper
setting of the feedback loops parameters and the history-dependent response of the z
piezoelectric ceramics. In the zoom shown in figure 3.4c, one can see that the four ∆f(z)
curves do not coincide, especially in the 0-0.3 nm z domain. The black and green curves,
measured when the tip retracts from the substrate are on the right of the red and blue curves
measured when approaching the tip toward the substrate. Because this effect is systematic, it
cannot be the consequence of thermal drifts. It is more important for larger z excursions. A
systematic study of spectra acquired in different conditions convinced us that it is related to
the response of the z piezoelectric ceramics, which is affected by creep. It is well know that
the creep of PZT ceramics follows a logarithmic law. More precisely, the response of a PZT
to a voltage step is given by:
+=
00 1)( t
tLogLtL γ , where L0 is the elongation at time t0
after the application of the voltage [Jung2000]. We applied this relation to correct the data of
figure 3.4 by considering the voltage ramp as a succession of voltage steps and adjusting the
parameter γ to make the four curves of figure 3.4c coincide. The result, shown in figure 3.4d
is satisfying: The separation between the forward and backward curves, which was of the
order of 0.05 nm before correction has been reduced to about 0.01 nm, which is comparable
to the noise level in these experiments. This correction procedure has been applied to the data
when necessary.
∆f(z) and D(z) curves can be also affected by piezo creep and thermal drifts in the x-y plane.
In this case the location on which the spectrum is measured becomes uncertain. This effect
can be minimized by waiting for the stabilization of the instrument and choosing positions in
close proximity to the scanning line to avoid large piezo excursions.
Chapter 3 NC-AFM study on KBr(001)
66
3.3. Results
3.3.1 Two types of atomic resolution images
The constant-frequency shift atomic resolution images we obtained on the KBr (001) surface
can be classified in two types, displayed in figure 3.5.
Figure 3.5: Constant-frequency detuning images of KBr (001). Size: 3 nm x 3 nm (a) ∆f=-340
Hz, Ap-p=5 nm (γ=-4.7 fN m1/2), (b) ∆f=-230 Hz, Ap-p= 7 nm (γ=-5.3 fN m1/2), and e) a front view of the KBr bulk crystal atomistic model.
Both show a square lattice with a period of 0.47 +/-0.02 nm close to a/√2=0.466 nm, which
corresponds to the interatomic distance between two ions of the same species, where a=0.66
nm is the bulk lattice parameter of KBr (figure 3.5e). Similar images have been reported and
compared to calculated images in [Pakarinen2006, Hoffmann2004]. Detailed comparisons
between experimental data and image calculations have established that the atomic contrast
formation on ionic surfaces is generally dominated by the short-range electrostatic interaction
between a polar ionic tip and the substrate. The sign of the tip apex ion determines two types
of images, which differ by specific features of the contrast as observed in most experimental
studies [Foster2002, Hoffmann2002, Pakarinen2006]. Of special importance in this
interpretation is the role played by the relaxations of the substrate and tip ions under the
influence of the tip-substrate interaction. Ions displaced in the direction perpendicular to the
surface extend the range of the electrostatic interaction, hereby enhancing the atomic contrast
[Livshits1999]. This effect is even more pronounced on defects, as analyzed, for instance, for
NaCl steps in reference [Bennewitz2000].
Chapter 3 NC-AFM study on KBr(001)
67
These works strongly suggest that the image of figure 3.5a was obtained with a Br--
terminating tip, while the image of figure 3.5b was obtained with a K+-terminating tip. In
figure 3.5a, the Br- tip is attracted by the K+ ions, which therefore appear as white bumps, and
repelled by the Br- ions, which appear as dark disks. The characteristic features which connect
neighboring K+ ions, seen on the profile of figure 3.5c as secondary maxima, result from the
bending of the tip due to the strong repulsive force it feels when it is above Br- ions
[Pakarinen2006]. In figure 3.5b, the K+ tip is attracted by the Br- ions, which therefore appear
as white bumps. The profile in figure 3.5d shows a slight asymmetry, not seen in the
calculated images [Pakarinen2006], which is most likely due to a non-ideal tip. The observed
corrugations are quite high, above 0.1 nm. This could be related to a relatively flat or round
tip which, due to increased long-range forces, allows stable imaging closer to the surface,
where the stronger short-range interaction is expected to produce higher corrugations
[Bennewitz2002b].
3.3.2 Atomically resolved force curves
Figure 3.6 shows an atomic resolution image with a weak "K+ tip" contrast (corrugation
≈ 0.03 nm). Spectra have been taken either on Br ions (black dots) or on K ions (white dots).
Figure 3.6: 3 x 3 nm image of KBr(001). App=7 nm, f0=271900 Hz, ∆f=-144 Hz.
These 14 spectra are displayed in figure 3.7a and b, and show the raw data. It is seen that the
two types of spectra are highly reproducible. The approach and retract curves are non
distinguishable. But the spectra differ significantly between the two types of ions showing an
atomic contrast. As expected, the difference is concentrated in close proximity to the surface,
where the chemical interaction responsible for the contrast is supposed to play a role. The
average curves for the two sites have been inverted using the SJ formula.
Chapter 3 NC-AFM study on KBr(001)
68
(a) (b)
(c) (d)
Figure 3.7:(a) Spectra obtained on the points labeled in figure 3.6. Black lines: spectra on Br, red lines: spectra on K. (b) Zoom of (a) and average curves (shifted downward by -80 Hz for clarity). (c) Average of the approach and retract curves. The green curve is the spectrum on K shifted to the left by 0.03 nm to correct for the atomic corrugation. (d) Force-distance
curves on the two sites. App=7 nm, f0=271,900 Hz, ∆f=-144 Hz.
These results show that one can obtain an atomic contrast not only on the images but also on
the force curves, even at room temperature. Two results extracted from the literature, which
can be used to compare with our results are displayed in figure 3.8. In a are shown
the ∆f and the forces curves obtained on KBr(001) at low temperature (7 K) [Hoffmann2002].
The overall shape and the order of magnitude of the forces are comparable to our results. Note
nevertheless that the van der Waals contribution, evaluated by fitting an analytical expression
on the data obtained far from the surface, was subtracted from the data, in order to isolate the
short-range contribution to the force. It was not possible to apply the same procedure to our
data because the chosen z range does not extend far enough form the surface to allow a
precise determination of the van der Waals contribution. In b are shown the force curves
Chapter 3 NC-AFM study on KBr(001)
69
obtained at room temperature on NaCl(001) [Schiermeisen2006]. This substrate is expected to
behave qualitatively as KBr(001).
(a)
(b)
Figure 3.8: (a) ∆f and force curves obtained on KBr(001) at 7 K by [Hoffmann2002]. The black line corresponds to the topographic maximum (presumably a Br ion) and the grey line corresponds to the topographic minimum (presumably a K ion). (b) Force curves obtained on
NaCl(001) at RT by [Schiermeisen2006].
An important difference between the low temperature (LT) data (fig. 3.8a) and the room
temperature (RT) data (fig. 3.7d and 3.8b) is that the repulsive part of the force curves is
much steeper at LT. In addition, the RT curves present deviations from the expected smooth
behavior that have been interpreted in [Schiermeisen2006] in terms of mechanical relaxation
of the tip apex under the influence of the tip-substrate interaction.
As recently proposed by [Ghamesi2008], real tips generally do not have the perfect structure
that has been used in most of the calculations done so far. Realistic tips can take different
structures that can be close in energy so that the structure can evolve during the approach of
the tip toward the surface, especially at RT, where the barriers separating these different
energy minima are more easily crossed. This type of interpretation could account for the
differences mentioned in the preceding paragraph.
In the following, we detail another example, which shows that indeed, the structure of the tip
can change during imaging.
Chapter 3 NC-AFM study on KBr(001)
70
3.3.3 Spontaneous tip polarity reversal
Figure 3.9: Observation of a spontaneous tip polarity change. (a) z, (b) ∆f and (c) dissipation (W) image. Image size: 8 nm x 6.2 nm; ∆f=-210 Hz, Ap-p=7 nm (γ=-4.8 fN m1/2). The contrast
was enhanced in the Br- part of the image (a) for clarity. (d), (e), and (f) Profiles corresponding to the lines drawn on the images.
Figure 3.9 shows images in which the contrast abruptly changed while imaging the surface.
This phenomenon is routinely observed when imaging ionic crystal surfaces and is attributed
to a change of the tip polarity [Reichling2002, Hoffmann2002]. The discussion of figure 3.5
shows that in this particular instance, the tip switches from a K+ termination in the upper part
of the images to a Br- termination in the lower part. This switching event happened
spontaneously since it was not induced by a deliberate change in the experimental conditions
or by the presence of a defect in the immediate proximity of the tip. It is inherently stochastic
and may be induced, for instance, by an accidental fluctuation of the tip-surface distance. The
change of the structure of the tip modifies the tip-substrate interaction potential, meaning that
it should be accompanied by a frequency jump. In the constant-frequency detuning mode, this
jump is rapidly corrected by a change in the tip-surface distance. The profiles displayed in
figure 3.9 show that the K+-tip is approximately 0.3 nm closer to the surface than the Br—tip
(figure 3.9d), while the average frequency detuning keeps the same value on the two parts of
the image, except for a small residual oscillation due to imperfect distance regulation, as
presented in figure 3.9e. The change of the structure of the tip also induces a lateral
displacement of the apex of the tip, as seen in figure 3.9, where the atomic rows on the two
sides of the transition are not in the position expected from the crystallographic structure of
KBr. Due to the crystalline nature of the sample, it is not possible to quantify this
displacement up to a lattice period of the surface from this image. Finally, it is observed that
atomic resolution also appears in the damping image with the K+-tip, with an increase in the
Chapter 3 NC-AFM study on KBr(001)
71
dissipation level of approximately 20 meV/cycle relative to the Br--tip. Note that the
dissipation is minimal on the Br ions, as shown by the profiles in figure 3.9. Atomic
resolution damping images were reported, for instance, in [Loppacher2000].
3.3.4 Tip polarity reversal on a monoatomic step
During the same experiment, we sometimes observed a systematic and reversible change of
the polarity of the tip apex when imaging monoatomic steps.
Figure 3.10: (a) z, (b) ∆f and (c) W image of a monoatomic step with a Br--terminating tip. Image size: 18 nm x 6.4 nm; ∆f=-120 Hz, Ap-p=7.1 nm (γ=-2.8 fN m1/2). (d), (e), (f) Profiles
corresponding to the lines drawn on the images.
Figure 3.10 shows an example where a step is imaged "normally", with a Br- tip, while figure
3.11 shows an example where the tip changes its polarity on the step. Notice that the absolute
value of the frequency shift was increased between figure 3.10 and 3.11, meaning that the tip
is closer to the surface in figure 3.11. The results of figure 3.10 are comparable to images
taken on NaCl steps [Bennewitz2000]. The height of the step measured on the topographic
profile [fig. 3.10d] is around 0.33 nm, in agreement with the lattice parameter of KBr. In
addition, one recovers the normal structure where the K+ <110> rows are laterally shifted by
half the nearest K+-K+ distance when crossing the step, as expected. The step induces a small
dip on the frequency detuning profile [figure 3.10e] and a small peak on the damping profile
[figure 3.10f].
Chapter 3 NC-AFM study on KBr(001)
72
Figure 3.11: Observation of tip polarity switching on a step edge. (a) Forward z, (b) backward z, (e) ∆f and (f) W image. Image size: 10.3 nm x 6.9 nm; ∆f=-210 Hz, Ap-p=7 nm
(γ=-4.8 fN m1/2). (c), (d), (g) and (h) Profiles corresponding to the lines drawn on the images.
In figure 3.11, the monoatomic step separates a lower terrace on the left from a higher one on
the right. The tip polarity changes from Br- on the left terrace to K+ on the right one. In
contrast to the tip switching analyzed in figure 3.9, this change is systematic. It is also
reversible, the tip commuting from Br- to K+ during the forward scan (from left to right) and
from K+ to Br- during the reverse, backward scan. Notice that the forwards scanned
topographic image [figure 3.11a] is extremely similar to the backwards scanned topographic
image [figure 3.11b]. This observation demonstrates that the scanning speed is low enough to
prevent any distortion of the image and that the tip switching is not induced by a too slow tip-
substrate distance regulation when crossing the step. We also checked that the frequency
detuning and damping images are not affected by the scanning direction and more generally
that the four images do not depend on the angle between the scanning direction and the step
edge.
Here again, similarly to figure 3.9, the relative position of the Br- ions imaged on the lower
terrace and the K+ ions imaged on the upper terrace is not the position expected from the
Chapter 3 NC-AFM study on KBr(001)
73
crystallographic arrangement of KBr. Finally, notice that the steps are decorated by triangle-
like structures, which are not observed in the "normal" imaging case of figure 3.9. They will
be discussed later in this chapter.
Interestingly, in the topographic profile of figure 3.11c and d, the tip height increases by only
0.03 nm at the step, instead of 0.33 nm in the case of normal step imaging as in figure 3.10.
But, as argued for the spontaneous tip polarity reversal of figure 3.9, the change in the
structure of the tip modifies the tip-substrate interaction potential and induces a change in the
tip height. The measured step height in figures 3.11c and d should then be corrected by this
amount. This leads to the conclusion that the distance between the K+-tip and the upper
terrace is smaller by 0.33-0.03=0.3 nm than the distance between the Br--tip and the lower
terrace. This is precisely what is observed in figure 3.9d. In addition, a similar contrast in
dissipation is observed when the tip is K+, with a minimum on the Br ions and the dissipation
profile indicates that for a given tip polarity, the dissipation level shown in figure 3.11h is the
same as in figure 3.9f. These similarities between figures 3.9 and 3.11 strongly suggest that
the same change in the tip structure is at the origin of both the spontaneous tip polarity change
in figure 3.9 and the deterministic step-induced tip polarity change in figure 3.11.
3.3.5 Potential energy surfaces of the two-level system
Two different mechanisms can be invoked to explain a reversal of the tip polarity. Ions can be
transferred during an accidental contact between the tip and the surface. Indeed, it is believed
that the ionic tips invoked in the interpretation of images of ionic substrates are formed by the
transfer of a cluster from the substrate during the first stage of imaging [Reichling2002]. A
second possibility is that the structure of the tip apex evolves without any material transfer
from the substrate. The repetitive and reversible character of the step-induced tip switching
observed in figure 3.11 strongly favors the second explanation since a transfer of material
between the tip and the substrate would leave visible defects on the surface, which are not
observed. The two states of the tip correspond then to two local minima in a potential energy
surface, as shown schematically in figure 3.12. Such two-level systems (TLS) have been
invoked in the context of DFM to relate the energy dissipated between the tip and the surface
to adhesion hysteresis [Sasaki2000, Kantorovich2004] and to interpret abrupt jumps in ∆f(D)
approach curves on KBr (001) in terms of structural changes of the tip apex [Hoffmann2007].
In both cases, these changes are related to the deformation of the potential energy surface of
the total tip-substrate system as the tip approaches the substrate. The step-induced switching
Chapter 3 NC-AFM study on KBr(001)
74
of figure 3.11 can be explain in a similar way if one assumes that the double well potential is
deformed by the interaction of the tip with the monoatomic step according to the scheme of
figure 3.12. In the dual well potential of figure 3.12, one well corresponds to a tip terminating
with a K+ ion while the other well represents a tip terminating with a Br- ion.
Figure 3.12: Qualitative sketch of the deformation of the potential energy curve of the tip-substrate system as a function of the tip position relative to a monoatomic step.
In states A and D, far from the step, the potential energy surface of the system is the same, but
in A the tip is in the Br- state, while in D it is in the K+ state. These two states are separated by
a barrier that hinders the tip switching at RT. We consider in figure 3.12 that the K+ tip is
more stable than the Br- tip, but this assumption is arbitrary. Close to the step, the Br-→K+
transition happens when the potential energy surface takes the shape shown in state C; the K+
state is stabilized relative to the Br- state; the inter-well barrier is low enough to allow the
transition. In contrast, the K+→Br- transition is associated with state B, where the Br- state is
stabilized. This model, which is is the minimal way to explain the observations of figure 3.11,
requires C to be on the right of B. But, as already mentioned, no significant difference can be
observed between the forward and the backward images in figure 3.11-a and 3.11-b and in the
profiles of figure 3.11-c and 3.11-d. This observation implies that the locations of B and C are
too close to be distinguished in the images of figure 3.11. The tip polarity reversal should then
happen on a very short length scale, well below 1 nm.
3.3.6 Tip polarity reversal on two successive monoatomic steps
More insight can be gained by considering the images of two successive steps like in the
upper part of figure 3.13. In this area, on the middle terrace, the state of the tip depends on the
direction of scanning. It is K+ for the forward scan and Br- for the backward scan. In contrast,
Chapter 3 NC-AFM study on KBr(001)
75
in the lower part of the images, the left step is no more in the image frame and the images
become similar to the images of figure 3.11.
Figure 3.13: Observation of tip polarity switching on two successive step edges. (a) Forward z, (b) backward z, (c) ∆f and (d) W image. Image size: 20 nm x 13 nm, ∆f=-210 Hz, Ap-p=7 nm
(γ=-4.8 fN m1/2).
These observations can be rationalized with the help of figure 3.14a, which generalizes the
drawing of the potential energy curves of the TLS (figure 3.12) to the two steps cases.
Figure 3.14: (a) Deformation of the potential energy curve of the tip-substrate system in the two steps case of figure 3.13. (b) Profiles corresponding to the lines drawn on the images of figure 3.13. Continuous line: forward z profile (left scale), dash line: backward z profile (left
scale) and dotted line damping profile (right scale).
Chapter 3 NC-AFM study on KBr(001)
76
At the beginning of the forward scan, the tip is in the Br- state A. It crosses the L step exactly
as in the single step case, to reach the K+ state Df on the middle terrace. Then something new
happens: the system has to switch two times on the R step to end in the K+ state G at the end
of the scan. This is unavoidable, if one wants to remain consistent with the potential energy
curves proposed in figure 3.14. The tip has to switch from the K+ state on the middle terrace
to the Br- state close to the step edge, before being switched back to the K+ state by the R step.
This last polarity reversal is similar to the single step case. The only difference with the single
step case for this forward scan is the switching event from Df to E in figure 3.14a. A closer
look at figure 3.13a shows indeed that the R step is bordered on its left side by a fluctuating
line [labelled α in figure 3.13a], which as will be confirmed later by the analysis of the
topographic forward profile of figure 3.14b is precisely the locus of this K+→Br- transition.
The backward scan can be followed in a similar way. The tip starts on the right terrace in the
K+ state G, it crosses the R step as in the single step case, to reach the Br- state Db on the
middle terrace; then, the system has to switch two times to reach the initial state A. The new
switching event, specific to the two steps case is from Db to C. Here again, this transition can
be localized in figure 3.13b. It appears as a fluctuating line near the top of the L step [β in
figure 3.13b].
The location of these switching events can be determined by a detailed examination of the
profiles shown in figure 3.14b. On the left of the forward topographic profile the tip, in its Br-
state begins to climb the L step until it switches to the K+ state on the dashed line I. The tip
height z then decreases, but as in figure 3.11a and c, this decrease is compensated by the step
climbing. This Br→K+ transition produces a sharp peak in the topographic profile which can
also be observed in the profile of figure 3.11c. At the right of the middle terrace, on the
dashed line III, the profile shows an abrupt upwards jump that is located on the fluctuating
line α mentioned previously. Its value of approximately 0.3 nm is consistent with a K+→Br-
transition as was suggested by the sequence of figure 3.14a. The tip is then in its Br- state and
the profile becomes similar to the profile observed previously on the L step: it shows the same
sharp peak (dashed line IV) that we attribute to the Br-→K+ transition. On the backward
profile, the tip describes the same peak on the R step (corresponding now to a K+→Br-
transition on the dashed line IV), but stays in the Br- state for imaging the middle terrace.
Here again, the profile has the same shape as in figure 3.11d. Then, at the left side of the
middle terrace, on the dashed line II [corresponding to the fluctuating line β in figure 3.13b],
the tip height decreases abruptly by an amount of approximately 0.3 nm, which is consistent
Chapter 3 NC-AFM study on KBr(001)
77
with a Br- to K+ polarity reversal. The tip is then in its K+ state and describes a profile similar
to the profiles observed previously, switching back to the Br- state on the line I.
In figure 3.14b, the gray lines are qualitative extrapolations between the trajectories of the K+
tip between the middle and upper terrace (R step) for the forward profile and of the Br- tip
between the middle and lower terrace (L step) for the backward profile. The resulting
completed profiles allow us to position the switching events relative to the steps.
A detailed analysis of the images shows that the position and the height of the peaks which
are generated by the Br-→K+ transition during the forward topographic scan and by the
K+→Br- transition during the backward topographic scan depend on the position along the
step edge. This observation can be understood if one assumes that the transition probability is
modulated by the atomic structure of the step edge. This modulation is at the origin of the
triangular-like patterns mentioned previously, as shown in figure 3.15. The right apex of the
triangle belongs to the Br- ions lattice of the upper terrace (disks in figure 3.15) while the left
two apices (arrows in figure 3.15) correspond to the transition peaks. These peaks have no
clear relation with the ions positions of the substrate. There are two peaks in a period along
the step edge, suggesting that they are related to the bridge sites between the two types of ions
that constitute the step edge.
Figure 3.15: Detail of the triangular-like patterns (black triangles). The position of the Br-
ions of the upper terrace is indicated by disks.
Finally, notice the perfect correlation between the state of the tip and the level of dissipation
shown in figure 3.13d and 3.11b. The dissipation level is higher when the tip is in the K+
state, even for a short time as for instance between lines I and II on the profiles of figure
3.11b, confirming our analysis of these profiles.
Chapter 3 NC-AFM study on KBr(001)
78
3.3.7 Discussion of the topography results
The topographic profile of figure 3.9d shows that the tip-substrate distance decreases by 0.3
nm when the tip switches from Br- to K+. A naive interpretation would be to assume that the
switch results from the diffusion of a K+ ion between a site at the apex and a site on a side of
the tip, leading to its shortening by an amount of the order of 0.28 nm, the ionic diameter of
K+. But this is not exact, since the data of figure 3.9 being obtained in the constant-frequency
detuning mode, the change in the tip-substrate interaction force resulting from this structural
modification has to be taken into account. Nevertheless, this 0.3 nm distance jump suggests
that the tip polarity reversal is associated with a minor change of the tip apex structure,
involving the rearrangement of only a few ions.
The tip switching also induces a lateral displacement of the image, corresponding to the
displacement of the outermost atoms of the tip, which dominate the imaging process. In figure
3.13, the same object -the middle terrace- is imaged with the two types of tip. The two images
of this object are separated by less than 1 nm, in the direction perpendicular as well as in the
direction parallel to the steps (the kink on the left step edge can be taken as a reference). This
observation confirms our previous conclusion that the tip switching should involve very few
atoms.
As already mentioned, the transition from B to C in figure 3.9 happens on a very short length
scale. This indicates that the interactions involved in the tip switching vary significantly on
this scale. This observation, together with the fact that the images showing tip switching are
in the atomic resolution regime, suggests that the forces responsible for switching are the
short-range electrostatic forces between the polar tip apex and the ions of the surface. It is
likely that relaxation effects, which are more pronounced on the step edges, play a role by
extending the interaction range in their immediate vicinity [Livshits1999]. This would
produce the highly localized interaction necessary to account for the sharpness of the
transition from B to C.
The observation of the fluctuating lines in figure 3.13 indicates that the probability of the
transitions from Df to E and Db to C varies from 0 to 1 on a length scale that is given by the
amplitude of the fluctuations of these lines, which corresponds to a few Angstroms. These
transitions have a probabilistic character, in contrast to the B to C transition that is
deterministic at the observed scale. In the regions covered by the fluctuating lines, the
potential energy surface is deformed in such a way that the probability to cross the inter-well
barrier is finite, but less than unity.
Chapter 3 NC-AFM study on KBr(001)
79
These examples give an idea of the structural modifications that could be at the origin of the
switching phenomenon we observe. It is not possible to propose a more specific model,
without the help of calculations of the activation barriers involved, which should satisfy the
conditions mentioned in discussing the deformations of the potential energy surface in the
vicinity of a step (figures 3.9 and 3.13) and in the previous paragraph.
In the following, we show that this bistability is also observable in the force-distance
measurements performed with the same tip.
3.4. Force spectroscopy with the bistable tip
Figure 3.16a shows an image of a monoatomic step, which displays the tip switching from a
Br- to a K+ tip discussed previously. The tip stays Br- on a few scan lines, as seen on the
profile in figure 3.16b.
(a)
(b)
Figure 3.16 (a) Image of a monoatomic step with the bistable tip ∆f = -230 Hz, A=3.2 nm. Size:10 nm x 7.8 nm. (b) Profiles corresponding to the lines drawn in figure a.
The ∆f spectra obtained at the places indicated in figure 3.16a are displayed in figure 3.17.
Figure 3.17: ∆f spectra obtained at the places indicated in figure 3.16a. The colors
corresponds to that indicated in Fig. 3.16a.
Chapter 3 NC-AFM study on KBr(001)
80
The tip was retracted and approached from the imaging set point twice, with a z amplitude of
1.14 nm. The 7 spectra demonstrate that the approach and retract curves are very
reproducible. No tip irreversible evolution could be detected during the acquisition of these
data.
3 types of spectra can be distinguished: s1 and s4, which are clearly obtained with a Br tip, s3
and s7 which are obtained with a K+ tip and s2, s5 and s6 obtained in proximity to the step,
meaning that it is not possible to specify unambiguously from the image the type of tip at the
beginning of the spectrum.
(a)
(b)
Figure 3.18: (a) Average ∆f spectra obtained at the places indicated in figure 3.16a. The colors corresponds to that indicated in Fig. 3.16a. (b) s1 and s4 have been shifted by 0.27
nm toward the right, s7 has been shifted by 0.03 nm toward the right.
s1 and s4 present the usual, monotonous decrease of the frequency when approaching the
substrate. The average of the corresponding 4 spectra for s1 and s4 is plotted in figure 3.18a.
In contrast, s3 and s7 present a strongly anomalous non-monotonous behavior. The
corresponding averages are also plotted in figure 3.18a. When the tip approaches the surface,
the resonant frequency decreases, as usual in response to attractive forces. Around z=2.5 nm
(D in fig. 3.18b), the frequency reaches a minimum and grows again, suggesting the onset of a
repulsive interaction. ∆f then rises to a maximum (B in fig. 3.18b) and decreases again to
reach A.
There are 3 points of intersection of s3 and s7 with ∆f=-230 Hz, the frequency shift used for
the image of figure 3.16a (figure 3.18). The middle point C, near z=0.2 nm cannot be used for
imaging because around this point the frequency shift increases when the tip gets closer to the
surface, leading to an instability. The two remaining points could be used for imaging. It turns
out that A is used for imaging with the K+ tip, as demonstrated by the location of s3 and s7 in
the image of figure 3.16a. The other point (E) is not used for the distance regulation in this
case.
Chapter 3 NC-AFM study on KBr(001)
81
The clue to the understanding of this experiment is that A and E are separated by a z distance
of approximately 0.27 nm. This z change corresponds clearly to the switching from a Br- tip
to a K+ tip measured previously. In addition, as shown in figure 3.18b, the s1 and s4 ∆f curves
are very similar to s3 and s7 when they are shifted by this quantity toward the right on the z
axis. These observations suggest that the point E in figure 3.18b is the regulation point for the
Br- tip in the image of figure 3.16a. In other words, the tip switching can be related to a
unique ∆f(z) curve presenting two stable operating points, A for the K+ tip and E for the Br-
tip. When working with the K+ tip, the tip structure switches back and forth between a K+
termination and a Br- termination during each oscillation cycle, while the Br- tip stays
unchanged.
Note that from the point of view of the control system these two operating points are
equivalent. What is measured is the frequency shift. That the tip switches or not does not
matter. The intuitive idea that the tip would switch from K+ to Br- during its first retraction
from the surface and stay Br- afterwards is wrong!
The s3 and s7 curves are separated by 0.03 nm. This separation, which is lower than the
corrugation observed in figure 3.16a (around 0.07 nm according to the profile shown in figure
3.16b), is probably related to the fact that the locations of s3 and s7 relative to the atomic
contrast are different. The shape of s6 strongly suggests that this curve corresponds to a K+
tip, even if this assumption is not clearly supported by the image of figure 3.16. The
separation between s6 and s1 is of the order of magnitude of the corrugation observed on the
profile of fig. 3.16b.
Figure 3.19:Force (right scale) and dissipation (left scale) curves corresponding to s1 (black), s3 (green) and s6 (magenta).
Dissipation curves taken simultaneously with the data of figure 3.16a are presented in figure
3.19 as well as the force curves derived from the ∆f spectra by the Sader-Jarvis method. Note
that this derivation was done only for the purpose of qualitative evaluation, because, as can be
Chapter 3 NC-AFM study on KBr(001)
82
seen in figures 3.17 and 3.18, the data were not measured far enough from the surface to reach
the region where the tip-substrate interaction could be neglected. A clear correlation between
these two sets of curves can be observed. The dissipation is negligible for the s1 curves, when
no switch is active, while the dissipation signal increases for the s3 and s6 curves, when the
tip switching is activated at each oscillation cycle. Interestingly, the dissipation levels for s3
and s6 near the imaging operation point are comparable to what is observed in fig. 3.9f, 3.11h
and 3.14b for the mean dissipation level (≈30 meV/cycle) as well as for the contrast in
dissipation atomic (≈20 meV/cycle). We can then relate the origin of the atomic contrast in
the dissipation image to an atomic site-sensitive tip switching effect.
3.5. General discussion
Let's first summarize the results obtained so far:
• We have shown that in certain imaging conditions, the polarity of the tip can change
systematically and reversibly when crossing a monoatomic step. This was interpreted
in terms of a TLS.
• The experimental observations suggest that the change of the structure concerns a very
small region of the apex of the tip, possibly a few atoms.
• The force and dissipation curve obtained with the same tip show that the tip explore its
two states at each oscillation cyle when it is K+ but not when its is Br-
• A contrast is observed on the dissipation images for the K+ tip in correlation with the
exploration of these two states.
An example of a specific model for a TLS near the apex of a KBr tip was recently proposed in
[Hoffmann2007]. One of the corners of a cubic KBr cluster constitutes the tip. The TLS is
associated with the displacement of one the ions of a KBr "molecule" adsorbed on one of
(001) facets in the immediate vicinity of the tip apex in such a way that the tip polarity is
reversed from one state to the other. That such a tip can indeed form can be appreciated from
the simulations of contact between an ionic tip and the surface that have been reported for LiF
[Shluger1997] or NaCl [Lantz2006]. In these works, it is shown that the contact is followed
by the stretching of a string of ions from the surface as the tip separates from the substrate.
Chapter 3 NC-AFM study on KBr(001)
83
We believe that the breaking of such a string could produce a tip with a few unstable adsorbed
molecules that could be a good candidate to explain our observations.
Figure 3.20: From [Hoffmann2007]
More recently, a theoretical study showed that the potential energy surfaces of realistic Si tips
exhibit many energetically close local minima that correspond to different structures
[Ghasemi2008], with the consequence that these tips easily deform in a reversible manner
under the influence of the tip-substrate interactions, causing an increase in the tip-substrate
dissipation.
Analogous phenomena were already observed on force curves (see figure 3.8b and
[Schiermeisen2006]), and invoked to explain the observation of atomic contrast on dissipation
images [Sasaki2000, Kantorovich2004] by the so-called adhesion hysteresis mechanism. A
work is performed by the tip at each oscillation cycle if the distance at which the switch
occurs is not on the average the same when the tip approaches or retracts from the surface.
The energy balance that should be respected shows that the work is given by:
∫−= dzzFW )(
Figure 3.21 shows where the dissipation cycle could be involved in our measurements. We
consider that the Br- tip is described by the right part of the ∆f curve extrapolated by the dash
red line in the left part of the graph and that the K+ tip corresponds to the dash black curve.
Chapter 3 NC-AFM study on KBr(001)
84
Figure 3.21: Tentative positioning of the hysteresis cycle on the ∆f experimental curves. The
dash red (resp. black) line corresponds to the Br- (resp. K+) tip.
If the operating point is A, the tip, which has a Br- ion at its apex when far from the substrate,
switches to a K+ terminated tip from the point labeled 1 on the red curve to the point labeled 2
on the black curve in figure 3.21, presumably under the influence of a repulsive force, as
indicated by the increase of ∆f in this part of the red curve. When the tip retracts it switches
back from the black to the red curve near 3. Note that only the part of this cycle that is
described counter clockwise, that is 1->2->3->1 will dissipate energy [Sasaki2000,
Kantorovich2004]. This is consistent with figure 3.19, where it can be seen that for the s3 and
s6 curves, the dissipation starts to increase near the maximum of the force curves, which
corresponds approximately to point 3 in figure 3.21. Note also that the distances at which the
tip switches are determined by the thermal crossing of an energy barrier, which is a random
event. The switching probability becomes important only in the neighborhood of point 3 and
the measured ∆f curve is an average between the two extrapolated ∆f curves corresponding to
the two states of the tip in this distance range.
3.6. Conclusion
The main result of this chapter is the demonstration that the atomic contrast observed in the
dissipation images of KBr(001) is related to an adhesion hysteresis phenomenon, which
involves a TLS localized near the apex of the tip. No direct experimental evidence for the
contribution of a TLS as the main source of dissipation has yet been obtained, because
accessing to an individual dissipating event is difficult due to its stochastic nature and the
Chapter 3 NC-AFM study on KBr(001)
85
high frequency of the cantilever oscillation. Nevertheless, the detailed investigation of the tip
switching on the monoatomic steps of KBr, coupled with the observation of bistable ∆f curves
provide a consistent picture which fully confirms the adhesion hysteresis mechanism.
We believe that this type of phenomenon is quite general and that it should be possible to
reproduce the observations that were made in this chapter with other tips. More precise
experiments are under way to provide a better basis to a modelization of the phenomenon.
Chapter 3 NC-AFM study on KBr(001)
86
Chapter 4. The Pd/Al10O13/NiAl(110) system
87
CHAPTER 4
The Pd/Al10O13/NiAl(110) system
4.1 Introduction
Aluminum oxide films grown from aluminum rich metallic alloys play an important role in
technology and surface science. For the automotive industry as supports in heterogeneous
catalysis [Khan2006], for the electronic industry in metal-insulator-metal architectures
oriented toward the development of electron-emitting devices [Yoshitake2006] and for STM
studies of adsorbed single molecules [Mikaelian2006], among many other applications.
Nowadays, the film formed by oxidation of the NiAl(110) surface is one of the most
intensively studied surface oxide.
One particular attractive characteristics of this system is that, if the film is thin enough, it can
be studied by STM and STS. Numerous STM studies have led to significant advances in the
understanding of the structural and electronic properties of this complex system.
We have explored this system for the following reasons: firstly, the oxide film grown on
NiAl(110) provides a suitable support for the deposition of organic molecules [Qiu2004].
Secondly, for very low rate of metal deposition, metal growth on this surface exhibits a
pseudo two-dimensional growth mode [Yoshitake2006]. These characteristics make this
system an attractive candidate that can be explored with aims to construct metal-molecule-
Chapter 4. The Pd/Al10O13/NiAl(110) system
88
metal junctions with the AFM-nano-stencil experiments [Guo2007]. Thirdly, one can perform
comparative experimental studies by STM and by NC-AFM.
In this chapter, we show the experimental results obtained by STM and STS techniques on the
Pd/Al10O13/NiAl(110) system. In section 4.2 we describe the NiAl(110) surface at the atomic
level and the preparation procedure to atomically clean the crystal under UHV conditions.
Section 4.3 is dedicated to the aluminum oxide film. We describe the preparation procedure,
the formation of the oxide film, and the structural features extracted from the STM images,
from the mesoscopic scale down to the atomic level. STS measurements on the oxide film are
detailed in section 4.4. First, the influence of the oxide film electronic band gap on the STM
images is discussed. Then, we describe our measurements for two different alumina film
fabrication procedures. Finally, the last part of the chapter, section 4.5, is devoted to the
palladium growth experiments on alumina. In this section, we discuss the formation of
palladium particles and their morphology on top of the alumina film for two coverage. These
results are then compared with those found in recent published papers.
4.2 The NiAl(110) intermetallic alloy
NiAl is an ordered binary intermetallic alloy that has been studied by many groups
[Davis1985, Lui1989, Hansen2001a, Jaeger1991]. The NiAl crystal has the cesium chloride
(CsCl) structure, cubic centered type with nickel atoms in every corner enclosing one
aluminum atom in the center and with a lattice parameter of 2.887 Å.
Figure 4.1a shows the NiAl(110) surface top view atomistic model of a 3x3 super cell, where
the pink balls are the aluminum atoms and the blue balls the nickel atoms. The (110) surface
unit cell dimensions are a1=2.887 Å and a2=4.083 Å, as shown in figure 4.1a. This surface is
affected by a relaxation of the last atomic layer, the aluminum atoms are displaced toward the
vacuum and the nickel atoms are displaced toward the bulk producing a ripple of 0.22 Å
[Davis1985], as shown in the side view of the atomistic model in figure 4.1b. An atomic
resolution STM image of a 4 nm by 4 nm area is displayed in figure 4.1c. It gives the
characteristic directions of the NiAl(110) surface, which will be used in the following since
all the images presented in this chapter have been obtained on the same sample. According to
[Hansen2001a], aluminum atoms appear higher, with white contrast while nickel atoms
appear lower, with black contrast. Finally, on figure 4.1d, we show a tunneling spectrum,
which exhibits the characteristic behavior of a metallic substrate.
Chapter 4. The Pd/Al10O13/NiAl(110) system
89
a)
c)
b)
d)
Figure 4.1: Atomistic model of NiAl(110) surface. (a) Top view. (b) Side view showing the relaxation. Atomic resolution STM image of a 4 nm x 4 nm area,
V=-0.6 V, I=0.09 nA. d) Tunnel spectrum.
4.2.1. Preparation procedure
The first step to prepare a clean NiAl surface is to remove the contaminated surface layer by
Ar+ sputtering. Normally the energy used is 1.5 keV but sometimes it is required to increase
this parameter, with caution, because sputtering at 5 keV produces dislocations in the crystal.
We have found that sputtering the sample at 1.5 keV to 3 keV during 40 minutes with an
emission current of 3 µA gives satisfactory results.
The next step is to anneal the NiAl(110) crystal. The key for a good preparation is to use the
right temperature of annealing, which is around 1100 °C [Song2005a]. Figure 4.2 shows the
progression from an insufficiently annealed surface to a surface that presents large terraces
separated by monoatomic steps. Four preparations at ~ 750 ºC, ~ 806 °C, ~ 970 °C, and
~ 1150 °C show that annealing with temperatures around 1000 to 1100 °C is necessary to heal
Chapter 4. The Pd/Al10O13/NiAl(110) system
90
the defects created by ion bombardment. At least 10 to 20 sputtering-annealing such cycles
are needed to perfectly clean a new sample.
54nm 52nm 55nm
a) ~ 750 ºC b) ~ 806 °C c) ~ 970 °C d) 3D view at ~ 1150 °C
02468
1012
1 31 61 91 121
151
181
211
241
Distance [nm]
Hei
ght [
Å]
e)
Figure 4.2: (a) to (d) shows a series of annealing temperatures from which it is possible to follow the formation of the NiAl(110) terraces as well as the disappearance of the white balls
present in the first image. e) Profile of image d.
The mean terrace width in figure 4.2d is 60 nm and the monoatomic step height is measured
at ~2 Å as expected from the NiAl structure (figure 4.1).
To clean organic contamination, it is sometimes useful to enter oxygen during the annealing
process at a pressure of 1x10-7 Torr for 10 minutes. But this procedure shortens the lifetime of
the filament of the electron bombardment heater.
When starting from an oxidized, but otherwise clean sample, ion bombardment is usually not
required, an annealing step is sufficient to get a clean well-ordered surface.
4.3. The alumina film formed on NiAl(110)
4.3.1. Preparation procedure
The preparation procedure can be achieved by a two steps process [Jaeger1991] or by a single
step oxidation [Lay2005], both of them can oxidize the surface at 1200 L dose, where
1L=1x10-6 Torr s:
Chapter 4. The Pd/Al10O13/NiAl(110) system
91
• The two step process: In the preparation procedure reported in [Jaeger1991], the
sample is first oxidized, and the resulting film is annealed. Once the NiAl(110) crystal
has been cleaned, oxygen is introduced into the preparation chamber at a pressure of
1x10-6 Torr during 20 minutes for a 1200 L dose, while keeping the sample
temperature at ~280 °C. The second step consists in annealing the sample at
temperatures around ~695 °C to ~927 °C [Lykhach2005, Jaeger1991] to induce the
crystallization of the oxide film. Alumina layers produced by this method have a
thickness of about 5 Å. This method is the most used nowadays, and is the one chosen
for our experiments. We will describe it in more details in further sections.
• The one step process: The alumina thin film can also be formed directly at
crystallization temperatures, around ~797 °C [Lay2005], where the formation of the
alumina is in competition with the decomposition of the film. This process requires a
lower oxygen partial pressure in the chamber, which can produce a better oxide film
[Yoshitake2006, Yoshitake2004, Lay2002]. However, longer oxidation times are
required for the same dose of 1200 L. For this preparation it is required to find the
equilibrium temperature point between three important kinetic processes: oxidation,
crystallization and decomposition. This method can produce alumina films with a
thickness greater than ~8.5 Å [Yoshitake2006].
To summarize, the complete preparation procedure of the alumina film comprises typically
the following steps. The NiAl(110) surface is cleaned by one or several cycles of argon
sputtering at 1.5 to 3 keV, with an ion current of 3 µA, during 40 minutes, followed by
annealing at 1100 °C during 20 minutes. Once the NiAl(110) surface is clean and well
structured, we keep it at a temperature that can go from 155 °C to 600 °C to oxidize it with a
1200 L oxygen dose at a pressure of 1x10-6 Torr, during 20 minutes. Finally, an annealing at
800 °C for five minutes is performed to crystallize the oxide film.
This fabrication method leads to the formation of a well crystallized alumina layer that
decorates the NiAl(110) terraces all over the surface. In the following, we describe the general
structural features of the alumina film, starting from the mesoscopic scale.
4.3.2. The oxide film meso scale morphology
Figure 4.3 shows a 200 nm x 140 nm STM image, obtained at a sample bias voltage of +2.88
V and a tunnelling current of 8.2 pA. Four terraces can be distinguished, separated by steps.
Chapter 4. The Pd/Al10O13/NiAl(110) system
92
The single steps present an apparent height of about ~ 2.9 Å, while double steps, marked with
black arrows, show an apparent height of about ~ 6.2 Å.
The surface displays flat defectless areas, separated by characteristic lines. These lines have
an apparent height around 1.5 Å and are organized in approximately periodic networks.
According to the bibliography [Müller1990, Libuda1994], they have two distinct origins:
• Due to the two fold symmetry of the NiAl(110) surface, the alumina film grows in
two reflection domains (RD), designated as domain A and domain B, which are
separated by Reflection Domains Boundaries (RDB). RDB show no characteristic
order.
• The alumina film unit cell is commensurate with the NiAl(110) unit cell in a
direction which is close to the NiAl[ ]011 direction, because of a "row matching"
phenomenon, which will be explained in section 4.3.4. This commensurability
generates stress along the [ ]011 direction and leads to the formation of line
defects, known as Antiphase Domains Boundaries (APDB), which separate two
domains of the same type. They exhibit two different morphologies: (1) straight
shape, denoted as IA or IB, and (2) zigzag shape, denoted as IIA or IIB
[Kulawik2003].
Figure 4.3: STM image of a sample prepared by the two steps process consisting in a double (2 x 1200 L) oxidation at ~285 °C followed by crystallization at ~800 °C during 10 minutes.
200 nm x 140 nm, +2.88 V, 8.2 pA.
Chapter 4. The Pd/Al10O13/NiAl(110) system
93
In the image of figure 4.3, the lower and upper terraces, mainly covered by domain B, exhibit
APDBs, while the A and B domains that coexist on the large intermediate terrace are
separated by a RDB. An area labeled as DD (for Different Domains) seems to result from a
bad crystallization of the alumina, probably due to the presence of impurities during the
oxidation process.
4.3.3. Intermediate scale images
The formation of the oxide film on top of the NiAl(110) crystal can take place as well at a
slightly lower oxidation temperature, around ~155 °C. Figure 4.4 shows a STM image of the
alumina film when imaging at a relatively high voltage, outside the band gap of the oxide
film, when the unoccupied states of the oxide participate in the tunneling current. At a sample
bias voltage of +4.2 V and a tunneling current of 10 pA the main structural features of the
oxide film can be imaged.
Figure 4.4: STM image of a double 1200L oxidation (2x1200L) at ~155°C, subsequent annealing at 812°C and 850°C during 10 minutes for each one. Scanning zone of 28.5 nm x
19 nm, + 4.2 V of bias voltage and 10 pA for the tunneling current.
Chapter 4. The Pd/Al10O13/NiAl(110) system
94
In Figure 4.4, the STM tip was positioned in a region of the surface that was mainly covered
by the B domain. The image shows an area of 28.5 nm x 19 nm, where well formed straight
IB-APDB are present. They exhibit a height of about ~0.8 Å with a width of ~2 nm (see the
profile in figure 4.4). In addition, the surface presents a network of dark straight lines, parallel
to the IB-APDB. These features are typical characteristics when imaging in this voltage range
[Nilius2004]. A unit cell is positioned on the image of figure 4.4. One of its sides is parallel to
the dark lines and its size in the perpendicular direction corresponds to 2 lines (as it can be
distinguished in the upper right corner of the image, the period of the dark lines network
corresponds in fact to two lines).
Figure 4.5 shows a higher resolution STM image of a domain boundary (RDB) separating a A
and a B domain. A careful examination of the atomic features which are discernible in this
image allows to distinguish the periodicities of the alumina film and to position the unit cells
for the two A and B domains.
Figure 4.5: 17 nm x 20 nm STM image at -2.11 V, 15 pA. The alumina film preparation is the same as for figure 4.4 (2x1200 L at ~155°C and annealed at 812°C and 850°C).
In the following, we would like to schematically describe the alumina unit cell and the
orientation of the two reflection domains with respect to the NiAl(110) support.
4.3.4. Structural relation between the alumina unit cell and the NiAl(110) substrate
Some characteristics of the alumina film are summarized in figure 4.5, which shows the
alumina unit cells of the A and B domains and their orientation with respect to the substrate as
well as the different types of domain boundaries.
Chapter 4. The Pd/Al10O13/NiAl(110) system
95
Figure 4.5: (a) Orientation of the alumina unit cells of A and B domains relative to the NiAl(110) substrate and orientation of the domain boundaries. (b) Illustration of the near
coincidence between the diagonal of the unit cell and a characteristic distance of the NiAl(110) substrate.
The straight APDB (IA or IB) run parallel to the short side of the oxide unit cell (b1), while
the zigzagged APDB (IIA or IIB) are parallel to its diagonal. A RDB is present along the
[ ]011 direction, but these boundaries are also observed in different directions. The alumina
unit cell has a rhomboidal shape, which is almost rectangular, with dimensions b1=10.55 Å,
b2=17.88 Å and angle α=88.6°, as shown in figure 4.5a. It covers 16 NiAl(110) units cells.
The long side of the unit cell makes an angle +/-θ with respect to the [ ]011 direction, as
indicated in figure 4.5. Values of θ found in the literature range from ~24.1° to ~24.74°.
Figure 4.5b shows that the length of the diagonal of the unit cell nearly coincides with a
characteristic distance of the NiAl(110) substrate. Indeed, the alumina film is commensurate
to the substrate in this direction. This commensurability generates stress along this direction
and leads to the formation of the APDB.
We now proceed to go into more detail by increasing the resolution of the STM images in
order to access the atomic structure of the film.
4.3.5. Higher Resolution image, with three domain boundaries
After reducing gradually the sample bias voltage and the tunneling current, atomic features
started to appear. Figure 4.6 shows a higher resolution STM image of a 18 nm by 20 nm area
of the alumina film. The sample bias voltage is +150 mV, and the tunneling current 4 pA.
This image shows weakly the atomic characteristics of the film. In this part of the surface, the
alumina film exhibits three IA-APDB that separate four A domains. These APDB are
Chapter 4. The Pd/Al10O13/NiAl(110) system
96
composed of straight parts, which belong to the IA types and kinks, which reveal small
sections of IIA-APDB. For these tunneling parameters of imaging, the APDB appear darker
than the A domains, in contrast to the image presented in figure 4.4, which was obtained at
+4.2 V, where the IB/IIB-APDB presented double lines with a brighter contrast. This
inversion of the contrast as a function of the bias was systematically observed in our
experiments.
In this image (fig.4.6) the drift of the STM was minimal and the imaging conditions were
stable. As a result, it was possible to position and to measure the unit cells in the A domain
and in the IA domain boundaries. The six rectangles that have been drawn in figure 4.6
include a row of 4 A domain unit cells, labeled A (blue color), limited by two IA-APDB unit
cells, labeled IA (black color). Their dimensions are ~17.7 Å x 10.6 Å for the A domain and ~
20.46 Å x ~10.6 Å for IA domain boundary.
Figure 4.6: High resolution image, 18 nm x 20 nm area at 150 mV and 4 pA. Same preparation conditions as for figure 4.4.
If we look carefully, this image can give us the atomic structure of the IA and IIA-APDB,
however from now on we will focus only on the A-RD and IA-APDB unit cells. Let’s take a
look at the structure with a better atomic resolution image, obtained by lowering again the
applied bias voltage and reducing the scanning area.
Chapter 4. The Pd/Al10O13/NiAl(110) system
97
4.3.6. Atomic resolution image of the oxide film surface.
We decreased again the applied bias voltage to +50 mV while keeping the same tunneling
current of 4 pA. For these tunneling conditions the atomic features of the oxide film could be
imaged more precisely.
The image presented in figure 4.7 shows a 10 nm x 10 nm surface area. An alumina A domain
goes from the lower left part to the upper right part of the image, in the middle of two IA anti
phase domain boundaries. Two repetitive patterns can be identified, especially over the bright
contrast zone. They correspond to the repetition of the unit cells of the A-RD and the IA-
APDB, indicated with rectangles and labeled as “A” and “IA” respectively. Note that the
corners of these cells are located on a dark spot surrounded by a clear circle, which can be
used as a guide to identify the unit cells.
Figure 4.7: Atomic resolution image of a 10 x10 nm surface at +50 mV and 4 pA. Same preparation conditions as for figure 4.4.
As will be discussed in the following, this type of image has been used to determine the
atomic structure of the film [Kresse2005, Schmid2006], by considering that the position of
the oxygen atoms is approximately given by the white bumps that are observable in the
image. Before this discussion, we present the resulting model in the following section.
Chapter 4. The Pd/Al10O13/NiAl(110) system
98
4.3.7. The atomic model of the A domain
The bulk alumina chemical formula is Al2O3. Alumina can exist in a number of crystalline
phases, labeled for instance κ, γ, δ, θ and α. The most stable phase is α-Al 2O3 (sapphire),
which crystallizes in the corundum structure. In the α-Al 2O3, the oxygen atoms are arranged
in hexagonal closed-packed planes and the aluminum atoms are placed in 2/3 of the
octahedral interstitial sites between the oxygen planes [kresse2005]. The oxide film formed
on top of NiAl (110) has been previously described considering the gamma and kappa phases
of the bulk alumina structure in order to propose some structural models [Jaeger1991,
Stierle2004]. However, it was found that the film structure differs from all the other alumina
phase structures, as revealed in [kresse2005] by STM experiments coupled to Density
Functional Theory (DFT) calculations.
Figure 4.8: Atomistic model of the alumina unit cell, from reference [kresse2005]
The room temperature STM images reported in this work presented square and triangular
features given by the arrangement of the bright contrast detected, and it was attributed to the
oxygen surface atoms positions. In other experiments [Kulawik2003], it was suggested that
the aluminum atoms appear as bright protrusions. This contrast change depends on the bias
voltage and the tip condition. An initial model was built from these positions and
subsequently refined by DFT calculations. The final result is shown in figure 4.8. Square and
Oxygen atoms
Aluminum atoms
Chapter 4. The Pd/Al10O13/NiAl(110) system
99
triangular features are marked in green color. The oxygen atoms keep a tetrahedral or
pyramidal coordination with the oxygen tip pointing toward the substrate as shown by the
white lines outside the unit cell in figure 4.8. These building blocks are responsible for the
triangular and square features in STM images. It was proposed that the existence of square
pyramids can be understood as truncated octahedra, knowing that oxygen octahedra with a
central metal atom are the most important building blocks in metal oxides structures.
Figure 4.8 shows that at the surface, the unit cell is composed of 28 oxygen atoms, indicated
with orange color and red contour. Slightly below this oxygen plane are 24 Al atoms, with
light blue color and contoured with dark blue. The oxide film also contains another oxygen
layer with 24 atoms in the unit cell and then the aluminum interface layer with one atom by
the substrate unit cell, that is 16 atoms. The interface Al atoms have a strong preferential
position above the Ni atoms of the substrate.
The arrangement of the atoms in the oxide film is close to p2gg planar symmetry. The
alumina unit cell covers 16 NiAl(110) unit cells so that the stoichiometry of the film in the
reflection domain is (NiAl)16 Al16O24 Al24O28, or (Al6O7)interface /(Al4O6)surface or simply as
Al 10O13, which is different from the usual Al2O3 bulk stoichiometry originally proposed.
While the composition of the surface layer corresponds to Al2O3, the interface layer is more
Al rich. This is related to the fact that the interface Al atoms are strongly bound to the Ni
substrate.
4.3.8. Interpretation of the atomic resolution image of domain A
Figure 4.9 shows a 3nm x 3nm zoom on the A domain of figure 4.7. The corrugation is of the
order of 0.9 Å in the unit cell. Assuming that the bright features correspond to the oxygen
atom, it was possible to approximately position 28 oxygen atoms, indicated with red circles in
figure 4.9, in the alumina unit cell. These atoms were then gathered in order to identify the
square and triangular features that appear in the model of figure 4.8. It is seen that all the
features of the atomic model are found in the image, with relative position that are in
agreement with the model. These data confirm the analysis of [Kresse2005].
We now introduce the structural model for a IA-APDB.
Chapter 4. The Pd/Al10O13/NiAl(110) system
100
Figure 4.9: 3 nm x 3 nm zoom of image 4.7 on two alumina unit cells of the A domain showing atomic resolution. Bias voltage +50 mV and tunneling current 4 pA.
4.3.9 The IA-Anti Phase Domain Boundary atomistic model
Figure 4.10 shows the atomic structure of the anti phase domain boundary of type IA,
obtained by LT and RT STM experiments and refined with DFT calculations [Schmid2006].
The cell is the same as that of the A-RD previously described but now it is split in the middle
Figure 4.10: Atomistic model of the IA-APDB from [Schmid2006]
Oxygen atoms
Aluminum atoms
New O xygen atoms
New aluminum atoms
Missing Oxygen atoms
Chapter 4. The Pd/Al10O13/NiAl(110) system
101
of the unit cell (in the b2 side marked with a white dashed single line in the A-RD previously
shown in figure 4.8). The two pieces are moved apart by 3 Å, this is indicated here by the
double white dashed line. This separation provides an additional space that allows a reduction
of the compressive stress in the alumina film. Nevertheless, new atoms are inserted in this gap
as shown with yellow and light blue colors in figure 4.10. A partial rearrangement of the
interface aluminum atoms is also necessary to stabilize the structure. Two oxygen atoms are
missing at the positions marked by the red dash circle lines. This oxygen deficiency behaves
like an electron donor. It produces a favorable adsorption site for electronegative species
[Schmid2006].
4.3.10. Interpretation of the atomic resolution image of the IA – APDB
Figure 4.11 shows a zoom on the IA domain boundary of figure 4.7. As for figure 4.9, it was
possible to position the O atoms by assuming that they correspond to bright contrast. The
resulting atomic arrangement confirms the atomic model of figure 4.10.
Figure 4.11: Zoom of image 4.7 on an IA-APDB. Bias voltage +50 mV and tunneling current 4 pA.
4.3.11. Meshing the surface with the repetitive pattern.
Finally, we would like to present in figure 4.12, the image already shown in figure 4.7 but
with a mesh of unit cells. We have superimposed the unit cells along one column that goes
from one IA-APDB to the other. This construction shows that it is possible to arrive at a
Chapter 4. The Pd/Al10O13/NiAl(110) system
102
consistent interpretation of the atomic structure of a complex surface like the alumina film on
NiAl(110).
Figure 4.12: Image already shown in fig. 4.13, with the models of the A domain and the IA domain boundary superimposed.
The second part of this chapter will be focused on the electronic properties of the alumina
film that can be extracted from STS measurements.
Chapter 4. The Pd/Al10O13/NiAl(110) system
103
4.4. Scanning tunneling Spectroscopy measurements
4.4.1. Introduction
Even though many studies have been reported on the alumina film on NiAl(110) by several
surface science experimental techniques, like angular resolved ultraviolet or X-ray
photoemission spectroscopy, low energy electron diffraction, ion scattering spectroscopy, and
high resolution electron energy loss spectroscopy, among others, we have not found much
information regarding experimental results using Scanning Tunnelling Spectroscopy (STS)
measurements to characterize a specific rate of NiAl(110) oxidation, and then compare these
results to another different oxidation rate. Important information can be obtained from STS
measurements and from the comparative results obtained. For that, it is important to briefly
introduce the electronic characteristics of the alumina film explored by STM.
The alumina film presents a well-defined two dimensional band structure due to its small
thickness [Jaeger1994]. The gap between occupied and unoccupied states in the oxide layer
was determined by X-ray absorption spectroscopy, and core and valence photoelectron
spectroscopy to be around ~6.7 eV, slightly smaller than the gap of bulk alumina, which is ~8
eV. The valence band (VB), which derives mainly from the O2p orbitals, localized at -4.5 eV
below the Fermi level. The conduction band (CB), which is dominated by the Al3s levels, is
localized around +2.2 eV above the Fermi level. This experimental study was carried out on a
600L alumina film specimen [Andersson1999].
In contrast, a NiAl(110) sample oxidized with a dose of 900 L studied by STM in
[Hansen2001b] presented an onset of tunneling near the CB side that started at a bias voltage
of ~ +1.3 V instead of +2.2 V. The origin of the discrepancy could be due to the presence of
defects in the film.
Finally, calculations of the density of states for a perfect oxide film and for a I type of APDB
were reported in [Schmid2006]. For the perfect film, the VB starts around ~-3.6 eV, while the
CB starts at ~ +2.95 eV, resulting in a total gap of ~6.6 eV. For the domain boundary, the VB
and the CB are a shifted towards negative values, the VB at ~-4V and the CB at ~+2eV,
giving a reduced total gap of ~6 eV. Three unoccupied states appear along the domain
boundary at 2.3 eV, 2.9 eV and 3.9 eV, in good agreement with the experimental results of
Nilius et al [Nilius2004] who found states at 2.5 eV, 3.0 eV and 4.5 eV by STS.
More generally, the literature shows that the electronic properties of the alumina film are
influenced by defects in the domain boundary, but also in the domains between two
Chapter 4. The Pd/Al10O13/NiAl(110) system
104
boundaries. The presence of these defects is likely to depend on the preparation conditions in
a manner that has not yet been precisely investigated.
In the following, we use these references to discuss our measurements of the gap on a 1700 L
alumina film. Then, we show how the electronic structure of this film influences the thickness
measurement when the imaging bias voltage varies. Finally, we briefly compare the STS
measurements performed on a double 1200 L (2x1200 L) and a 7500 L film.
4.4.2. Determination of the electronic gap for a 1700 L alumina film.
Figure 4.14: 57 nm x 57 nm image of a 1700 L oxidation dose, at a sample bias voltage of +2.3 V and a tunneling current of 5 pA. The NiAl(110) was cleaned by heating at 1100°C for 20 minutes. Then, the 1700 L oxidation was performed at 1x10-6 Torr during 28
minutes, with a sample temperature of 650°C, followed by a final annealing at 800°C during 10 minutes.
The image presented in figure 4.14 shows the oxide film formed with a 1700 L oxygen dose.
APDB appear on the two terraces, separated by a monoatomic NiAl(110) step. While the
upper terrace shows IA and IIA domain boundaries, the lower one exhibits only IIA APDB.
The STS measurements were performed in a voltage range of -5 V to 5 V, in order to span the
entire gap. The I-V and differential conductance curves are shown in figure 4.15. The
tunneling set point (+3 V, 3 pA) is marked with a pink filled square on the blue curve.
Chapter 4. The Pd/Al10O13/NiAl(110) system
105
-50
-30
-10
10
30
50
70
90
110
5.00
3.99
2.98
1.97
0.96
-0.0
5
-1.0
6
-2.0
7
-3.0
8
-4.0
9
Volts[V][eV]
tunn
elin
g cu
rren
t [p
A]
-0.5
4.5
9.5
14.5
19.5
24.5
29.5
34.5
39.5D
iffer
entia
l con
duct
ance
[pA
/V]
EF CBVB
Gap ~ 6.97eV
~ - 3.88eV ~ + 3.08eV
0eV
+3.1
+3.6 9
+4.3
+3.99
+4.8 0
Figure 4.15: I(V) and dI/dV(V) curves on the alumina film of fig. 4.14.
From these curves we can extract the following data:
• The onsets of the valence and conduction bands can be roughly estimated at ~-
3.88 eV and ~3.08 eV, giving an apparent total gap of ~6.97 eV. Obviously this is
only a simple approximation where we consider that the bands begin at the first
important onset in the dI/dV curve. With this limitation in mind, these values are in
good agreement with the calculation of Schmidt et al [Schmid2006] discussed
previously.
• Interestingly, some states are detected near the conduction band edge, at +3.1 eV,
+3.6 eV, +3.99 eV, +4.3 eV and +4.8 eV. Here again, these values are in good
agreement with the calculations of Schmidt et al [Schmid2006], where they obtained
characteristic states after the CB onset at +3.2 eV, +3.6 eV, +4.2 eV and +4.8 eV,
except that we detect an extra feature around +3.99 eV.
This two-dimensional electronic band gap produces an apparent thickness of the film on the
STM images that strongly depends on the sample bias voltage, as described in the following.
4.4.3. Apparent thickness of the oxide film as a function of the imaging bias voltage
The inset of fig. 4.16 shows a 140 nm x 200 nm image of a 1200 L film obtained at +3.2 V
and 10 pA where a pinhole, that is a small area where the NiAl surface is not covered by the
oxide was formed. Pinholes are known to appear during the crystallization of the amorphous
oxide, as reported in [McCarty2001]. The profiles extracted from images taken at different
Chapter 4. The Pd/Al10O13/NiAl(110) system
106
bias voltages from -2.2 V to 4.2 V, along the green line indicated in the image are presented
in fig. 4.16. This line starts from the NiAl clean terrace, crosses a NiAl monoatomic step and
continues on the upper terrace, which is covered by the alumina bilayer. To measure the
alumina film thickness, it is then necessary to subtract the height of a monoatomic NiAl step,
which is approximately 2 Å.
Figure 4.16: Apparent height of the alumina film. The inset shows the 200 nm x 140 nm image of a 1200 L alumina film where these measurements were performed. This image
shows a pinhole in the alumina film where the clean NiAl(110) is present. The profiles were obtained on the horizontal green line. Tunneling parameters: +3.2 V at 10 pA.
We find that the apparent film thickness is of the order of 1 Å from -2.2 to +0.4 V and that it
begins to increase between +1.2 V and +2.2 V to reach a maximum of more than 6 Å at
+4.2 V. The apparent height of the film as a function of the applied bias voltage has been
discussed in [Hansen2001b]. It was found that from -10 V to around +1.5 V, the film presents
a very small or even null height, while from +1.5 V to +4.2 V its thickness starts to increase to
reach a maximum height of 3.5 Å. No further increment was observed from +4.2 V to +10 V.
These results are in qualitative agreement with ours, except that we find significantly higher
thickness. We think that this discrepancy is related to the different work functions of the NiAl
and the alumina-covered NiAl surfaces. The work function is lower on the alumina covered
surface [Song2005], meaning that the current decreases slower than on NiAl. One expects
then to measure a higher apparent height of the alumina film when the tip-substrate distance
increases, that is when the tunneling current is smaller. We performed these experiments with
a 10 pA current while Hansen et al [Hansen2001b] used a 600 pA current.
The small apparent height of the film measured when the bias voltage is in the gap of the
oxide is expected because the density of states of the film is very low at these energies. The
Chapter 4. The Pd/Al10O13/NiAl(110) system
107
increase of the apparent thickness, which begins around +1.5 V and saturates near +3 V can
be clearly related with the onset of the conduction band of the oxide. The fact that this
increase starts well before the conduction band edge indicates that defects participate in this
phenomenon. In contrast, it is surprising that the valence band of the oxide does not seem to
influence the measurement of the alumina film height when the bias voltage is below the VB
edge, that is ~-4 eV. Two explanations have been proposed:
• In [Hansen2001b], a model based on the decomposition of the tunneling current in two
additive contributions, one from the NiAl substrate, and the other from the oxide film
is proposed. No physical justification for the additive assumption is given. We think
that this model is wrong.
• Subsequently, Iwasaki et al [Iwasaki2002] proposed another explanation. A one-
dimensional calculation of the tunneling current shows that the combined effect of the
lowering of the work function of the sample by the alumina film [Song2005] and of its
high dielectric constant leads to a strong asymmetry of the I(V) curve, as suggested in
figure 4.17. This asymmetry could explain that the alumina valence band does not
contribute significantly to the apparent height of the film.
Figure 4.17 (extracted from [Iwasaki2002]): The one dimensional model for electron tunneling between a W tip and the alumina/NiAl(110). a) For positive sample bias (+4.2 V), b) negative sample bias (-4 V). The potential energy diagrams for tunneling between a W tip
and the bare NiAl(110) are also plotted (broken lines). The tip-NiAl distance is 0.7 nm.
In the following, we present STS data obtained in the gap of two samples that were prepared
differently.
Chapter 4. The Pd/Al10O13/NiAl(110) system
108
4.4.4. STS measurements in the gap of two different oxides
Figure 4.18-a shows a high-resolution STM image at -1.55 V and 8 pA. The specimen was
oxidized twice with a 1200 L dose at ~285°C and subsequent annealed at ~800°C during ten
minutes. Both reflection domains are shown, with I and II APDB. The two domains are
separated by a reflection boundary (RDB).
b) Figure 4.18: a) 50 nm x 50 nm image of a doubly oxidized sample at -1.55 V and 8 pA. b) STS
measurements on different parts of the oxide.
Three sets of I(V) and dI/dV curves have been measured on the A and B domain, on the RDB
and on the I APDB in the bias range from -1.8 V to +2 V. The overall shape of the average
curves does not depend on the location on the surfaces. Peaks appear on the conductance
curves at different positions that depend on the location. This type of peaks was
systematically observed on different samples, prepared with different experimental
conditions. We always observe an influence of the domain boundaries on the conductance
spectra, meaning that at least some of these peaks are related to the presence of defects.
Unfortunately, it was not possible to establish a clear correlation between the peaks presence
and positions and the preparation procedure.
a)
Chapter 4. The Pd/Al10O13/NiAl(110) system
109
To illustrate this point, we present the measurements made on an oxide film prepared with a
higher partial oxygen pressure. The image of a sample oxidized at a pressure of 4.85 x 10-6
Torr during 25 minutes at 280 °C and annealed 800 °C during 6 min is shown in figure 4.19a.
Two large terraces separated by a monoatomic NiAl step are covered by mainly the A
domain. The averages of the STS measurements obtained on this image are shown in figure
4.19b.
STS average measurements on the RD
-0.05
0
0.05
0.1
-2-1
.8-1
.6-1
.4-1
.2 -1-0
.7-0
.5-0
.3-0
.10.
09 0.3
0.51
0.72
0.93
1.14
1.34
1.55
1.76
1.97
eV
-0.05
0
0.05
0.1
It dI/dV
nA nA/V
EF
-1.74+1.76
+1.2-1.16
a) b)
Figure 4.19: (a) Image of a sample oxidized at 7500 L and 280 °C (+1.86 V, 8 pA), and (b) corresponding STS measurements.
The spectrum of figure 4.19b is similar to that of figure 4.18b taken on a RD but with
different peak positions. This evolution could be related to the higher oxygen dose used to
prepare the sample of fig. 4.19, but this assumption should be completed by other
experiments.
Chapter 4. The Pd/Al10O13/NiAl(110) system
110
4.5 Palladium growth
Figure 4.22 shows images of Pd deposits for two different coverages. As investigated in
previous works [Hanssen1999, Napetschnig2007], two types of clusters can be seen. The
largest ones appear on the domains of the oxide films while the others decorate either the
domain boundaries (fig. 4.22a) or the upper edge of NiAl steps (fig. 4.22b). The growth in the
domain is considered as resulting from a homogeneous nucleation, while the growth on the
domain boundaries is likely to be related to a nucleation of the Pd islands on the reactive sites
that were mentioned in paragraph 4.3.9, where the atomic model of the APDB was presented.
The decoration of the upper step edge could be related to a barrier preventing the diffusing Pd
atoms to go down on the lower terrace.
Figure 4.22: (a) 35 nm x35nm image at -2.3 V, 10 pA, of a 0.09ML Pd deposition, at room temperature on a 1200L oxide film, at ~250°C and subsequent annealing at 800°C. (b) 50 nm
x 50 nm, +3 V, 2.6 pA image. 0.58ML of Pd deposited on a 1200L at~ 650°C, annealed at 812°C for 10 minutes.
We did not investigate the growth of Pd in detail. Our purpose was to look if this system
could be a good candidate for electrically addressing molecules in a planar configuration with
the nanostencil experiment described in chapter 1. Figure 4.22 shows that the islands which
grow either on the domain boundary or on the step edges are separated by quite small gaps,
which could be suitable to connect a molecule, as suggested in figure 1.8. A precise
estimation of the gap size is difficult due to the finite resolution of the tip. Their height is also
difficult to measure, because it depends strongly on the imaging voltage, in a way that is not
well understood [Napetschnig2007]. Following this reference, and our own evaluations based
on the calibration of the Pd deposition, we estimate a height of the order of 1 nm, which is
Chapter 4. The Pd/Al10O13/NiAl(110) system
111
low enough for allowing the STM visualization of a molecule connected between two islands.
A problem is that the small lateral size of these islands makes them not easily compatible with
state-of-the-art nanostencil capabilities. Exploring in detail the growth conditions could allow
growing larger islands.
An important point that should be investigated to go farther in this direction is the alumina
film quality at the scale of the microelectrodes that will be deposited by static stencil. The
leakage induced by defects should be low enough to allow measurements of the current in the
connected molecule. Note that it is possible to increase the alumina film thickness by the one
step process as reported by Yoshitake and coworkers [Yoshitake2006] (figure 4.23).
Figure 4.23 Thickness variation of the alumina film during the oxidation process for the
single step oxidation. From [Yoshitake2006].
4.6 Conclusion
The structural and electronic properties of the alumina film grown on NiAl(110) by oxidation
were investigated by STM and STS. Most of the results already published were confirmed, in
particular the atomic model for the perfect domain and for the anti phase domain boundaries
of type I. The film presents a large gap, even on the domain boundaries, which make it
suitable as a substrate to decouple molecules from the metallic substrate. Pd deposition leads
to flat islands that decorate the NiAl steps and the alumina domain boundaries, leading to
inter island gap sizes of molecular dimensions. Further work is needed before deciding to use
Chapter 4. The Pd/Al10O13/NiAl(110) system
112
this system for the nanostencil experiments. First, ways to grow larger (but not thicker)
islands have to be found; second, the electrical properties at the scale of the microelectrodes
have to be investigated.
Chapter 5. Conclusions and perspectives
113
Chapter 5
Conclusions and perspectives
5.1 Conclusions
The main contributions of this work can be summarized as follows:
• Our RT AFM head was considerably improved by changing its light source. This
improvement was characterized by careful noise measurements, which in addition
provide a non-destructive method to measure the oscillation amplitude of the
cantilever, which is an important experimental parameter in NC-AFM. This
instrumentation work was essential for a better mastering of this complex instrument
and for establishing the basis for quantitative measurements. It opened the way to the
experiments reported in the following.
• This improved head was used to investigate the cleaved KBr(001) surface, which is a
reference surface for NC-AFM. Atomic resolution is now obtained routinely on this
surface. The methodology to get reliable ∆f(z) curves and to extract force curves from
them was developed. A set of experiments was performed with a tip that presented an
interesting behavior. On topographic, constant ∆f atomic resolution images, the atomic
contrast was observed to change in a deterministic and reversible manner when the tip
crosses monoatomic steps: the images formed on the lower and the upper terraces
present the same atomic periodicity, but with important differences in the basis of the
lattice. In addition, atomic contrast in dissipation is observed on the upper terrace but
not on the lower one. These observations published in [Venegas2008] were interpreted
as a reversible evolution of the structure of the tip, resulting in a change of the sign of
Chapter 5. Conclusions and perspectives
114
the tip terminating ion, in reference to the now accepted imaging mechanism on ionic
surfaces. ∆f(z) curves, obtained with the same tip, present two distinct behaviors: One
the lower terrace, where the dissipation is negligible, the ∆f(z) curve presents the
standard, monotonous, globally attractive character. On the upper terrace, where an
atomic contrast in dissipation is observed, the ∆f(z) curve is such that two imaging
operation points are accessible: the curve is bistable. Our observations establish a
direct link between this bistability, the switching behaviour of the tip on the
monoatomic steps and the appearance of atomic contrast in the dissipations images on
the upper terrace. These results strengthen the adhesion hysteresis hypothesis that was
proposed as the main source of dissipation in NC-AFM [Kantorovich2004]. In
addition they show that the tip can be a relatively "soft" object, generating dissipation,
as recently established by modelling of Si tips [Ghasemi2008].
• Different attempts were made to observe indigo molecules on this KBr(001) surface.
Deposition by UHV sublimation or from a solution were used. These experiments are
not sufficiently advanced to justify a detailed report. Some preliminary observations
are discussed in the "perspectives" part of this chapter.
• The structural and electronic properties of the alumina film grown on NiAl(110) by
oxidation were investigated by STM and STS. Most of the results already published
were confirmed, in particular the atomic model for the perfect domain and for the anti
phase domain boundaries of type I. The film presents a large gap, even on the domain
boundaries, which make it suitable as a substrate to decouple molecules from the
metallic substrate. Pd deposition leads to flat islands that decorate the NiAl steps and
the alumina domain boundaries, leading to inter island gap sizes of molecular
dimensions. Further work is needed before deciding to use this system for the
nanostencil experiments, as suggested in fig. 1.8. First, ways to grow larger (but not
thicker) islands have to be found; second, the electrical properties at the scale of the
microelectrodes have to be investigated.
Chapter 5. Conclusions and perspectives
115
5.2 Perspectives
One of the most exciting capabilities provided by the NC-AFM technique is the visualization
of organic molecules on insulating surfaces. As previously described in chapter 1, this is one
of the critical steps involved in the nanostencil technique. For that it is required to deposit
molecules on a surface. Here we have explored two methods of molecules deposition. The
first one consisted in deposition of indigo molecules by conventional sublimation from the
molecular solid under UHV. The second one consisted in the deposition of a drop of indigo in
a chloroform solution at ambient atmosphere. We briefly describe our findings in the
following.
5.2.1. Molecules visualization attempts
5.2.1.1 Deposition by sublimation under UHV
a) b) c)
Figure 5.1: Structure observed on the top of an indigo terrace. a) A larger scale image, 82.4 nm x 47.7 nm, of the terrace edge, height of 9.29 Å, ∆f = -180 Hz, amplitude = 4.4 nm, b)
the repetitive pattern found with some defects in the upper right corner of the terrace, ∆f=-225 Hz, amplitude = 3.3 nm, c) this image was obtained with a very low oscillation amplitude
A0 = 1.4 nm in a zoomed area over the terrace, ∆f=-433 Hz.
Indigo molecules were evaporated under UHV from an alumina crucible on KBr(001) at room
temperature. While imaging, we found a large terrace with a height of about 10 Å, shown in
figure 5.1a. Note that the shape of the terrace edge is quite different from that presented by a
KBr(001) step edge. Figure 5.1b shows an image obtained on this terrace. A periodic lattice,
which seems to be hexagonal can be distinguished. The length of the side of the unit cell is
measured to be 4.5 nm. This length has no obvious relation with the size of the indigo
molecule or with a length appearing in the unit cell of the indigo crystal, which is known.
Nevertheless we assume that this object is a crystalline indigo island where the arrangement
of the molecules is different from the arrangement in the bulk because of its small thickness
(of the order of 1 nm). This assumption is partially supported by the image of a defect on this
Chapter 5. Conclusions and perspectives
116
island presented in figure 5.1c where small objects with a shape and a size compatible with
single indigo molecules are distinguishable, as demonstrated by the superposition of the
atomic model of the molecule on the image.
5.2.1.2 Deposition from a chloroform solution
Figure 5.2: The KBr(001) surface after deposition of a drop of indigo in a chloroform solution. A0 = 4.5 nm, 200 nm x 200 nm.
We prepared a solution with chloroform and the indigo powder. Then we put a drop of
solution onto a clean KBr(001) in ambient atmosphere. After that the sample was rapidly
transferred to UHV where an annealing step was performed at ~150°C for one hour.
Figure 5.2 shows a 200 nm x 200 nm image of the sample with an amplitude = 4.5 nm and
∆f = -27 Hz. We can observe that the molecules decorate the step edges, as exhibited by the
brighter contrast areas in the image. In addition, it is possible to distinguish small balls in
certain locations on the surface.
To understand better, we performed two others experiments. The first one consisted in the
deposition of only one drop of chloroform, as shown in figure 5.3a. For the second one, we
deposited again a drop of the solution containing the indigo molecules (figure 5.3b). For each
preparation we followed the same surface treatment under UHV.
40nm
Chapter 5. Conclusions and perspectives
117
(a)
4.0nm
(b)
Figure 5.3: a) After deposition of one drop of chloroform, ∆f=-189Hz, amplitude is 5nm. The inset shows an NC-AFM atomic resolution image of 3.9 nm x 4.1 nm of one defect found on the surface, image obtained at ∆f=-204Hz and same amplitude. b) After deposition of one
drop of indigo solution, amplitude 2.7 nm, ∆f = -100 Hz.
Figure 5.3a shows a high resolution image, after deposition of the chloroform drop. Here we
can clearly see three darker objects. It is difficult to identify them. The inset shown in figure
5.3a shows a zoom on one of these defects with atomic resolution, with a K+ terminated tip.
As we never observed this type of objects on clean KBr(001) surfaces, we suppose that the
chloroform creates these defects.
Figure 5.3b shows an atomic resolution image of a 20 nm x 20 nm area of the sample after
deposition of the solution containing the molecules, the tip has a K+ termination. Now we
found six nano-objects exhibiting a brighter contrast in form of elongated lobes that seem to
be in pairs. The cross shape formed around the nano-object is probably due to the convolution
of the object and the shape of the tip. It is important to say that we could find a different nano
object than those shown in figure 5.3b that will be shown and described in the next image (see
figure 5.4).
Chapter 5. Conclusions and perspectives
118
(a)
(b)
Figure 5.4. a, b) Two images of the KBr(001) surface after deposition of a drop of indigo in a
chloroform solution. A0 = 2.7 nm.
A zoom on each of the different nano-objects found is now presented in figure 5.4. In figure
5.4a, the object exhibits two lobes that are connected at the lower right side end and around
the middle of them. The right lobe seems to be higher than the left one and its dimensions are
very similar to that of the indigo molecule, ~ 1.3 nm in length by 0.37 nm in width, the
measured height is 1.15 Å. The entire width of both lobes is 0.94 nm which is very small
compared with two indigo molecules coupled by their hydrogen bonds, around ~1.19 nm.
This difference suggests that one of these lobes is produced by the same effect of tip shape
convolution. Figure 5.4b shows a single nano object with ~ 1.3 nm in length and ~ 0.52 nm in
width, and height of about ~ 0.78 Å. Here we can appreciate the same tip convolution with a
cross shape around the object.
However all these arguments are only suppositions and further experiments are required in
order to better understand what are the objects present in the last figures. However after the
chloroform deposition, it seems to be that some defects are formed that could perhaps provide
suitable absorption sites for molecules.
5.2.2. Nano manipulation
Small objects like atoms or molecules can be manipulated in two ways: in a lateral or in a
vertical way. Lateral manipulation consists in pushing, pulling or dragging the object without
separating it from the surface [Oyabu2005, Sugimoto2007, Sugimoto2005, Hirth2006].
Vertical manipulation implies to pick the object with the tip and to put it in another location
[Oyabu2003, Morita2]
Chapter 5. Conclusions and perspectives
119
Defects formed on the surface like vacancies can also be manipulated by lateral methods as
reported in [Hirth2006] on CaF2(111).
Here we could experience the same phenomenon. After deposition of a drop of chloroform,
while imaging with atomic resolution at ∆f=-157Hz, in a down direction and amplitude=
5 nm, we found a defect that we could manipulate. This first image exhibiting the
manipulation is shown in figure 5.5a.
(a)
(b)
(c)
Figure 5.5: Lateral manipulations exhibited after the KBr(001) surface with a drop of chloroform. amplitude is 5 nm, 40 nm/s, 10 nm x10 nm in a) the manipulation of a lagoon, b) the manipulated lagoon now static, when going up no manipulation was expressed, only when
scanning in the down direction c) multiple manipulation of three lagoons, note that they continue their respective directions after collision.
In this image, we started to manipulate the defect by laterally controlling the scanning area,
we moved the scanning cursor so we can keep the defect in motion. Then while going up we
decreased the tip to surface distance by setting up ∆f = -86 Hz so an image of the defect can
be obtained, see fig.5.5b. We keep moving this defect until we found two other defects that
were manipulated at the same time. Interestingly we found that while moving the three
defects at the same time they followed a specific direction, and even though the first defect
found crosses the other two defects, all of them continued with their own direction. It is
important to say that this lateral manipulation was exhibited only when scanning in the down
direction.
Vertical manipulation could be also experienced, after deposition of the indigo solution,
where an object was picked up by the tip, inducing an increase in the dissipation signal. Then
by approaching to the surface and scanning over a defect, it appeared that the object was left
on the defect, with a simultaneous reduction of the dissipation signal to its original value.
These experiments need to be explored in order to reproduce what we found, so that a better
reference can be given. If chloroform creates defects then they can be used as anchor sites to
Chapter 5. Conclusions and perspectives
120
deposit molecules on this insulating surface. On the other hand manipulations are very
important for the nanostencil project that requires in some part of the construction procedure
to take a single molecule and to place it in a specific location. This system
Indigo+Chloroform/KBr(001) could be a good candidate to practice vertical and lateral
manipulations in order to define the correct protocols to manipulate a single molecule that
could be exploited in future nanostencil experiments.
.
121
122
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Chapitre 1 Introduction,
- le Project de nanostencil et l’électronique moléculaire.
La miniaturisation constante des composants électroniques, conduit par des besoins de
l'industrie de la microélectronique, a montré un énorme progrès dans la construction de
dispositifs électroniques qui sont de plus en plus petits. Un composant commun
normalement utilisé comme un bloc constitutif d'un circuit électronique c’est le transistor,
qui a été considérablement réduit en taille. Cependant, cette miniaturisation des
composants électroniques commence à être affectée par des limites technologiques et
fondamentales. C’est pourquoi que d’autres alternatives ont été proposées par la
communauté scientifique de l'électronique moléculaire. Ces idées proposent l’utilisation
des molécules comme des blocs actifs et fonctionnels dans les circuits électroniques en
vue de les intégrer dans un dispositif électronique dans le futur. Pour atteindre cet objectif
il faut résoudre la problématique de connexion électrique entre une molécule unique et des
électrodes métalliques. Des approches intéressantes qui sont explorées aujourd’hui, sont :
� La jonction métal-vide-molécule-métal proportionnée par la
microscopie à effet tunnel (STM).
� Les Jonctions mécaniquement cassables.
� La Jonction covalente Nano tube de carbone-Molécule-Nano tube de
carbone.
� L’utilisation des îlots métalliques connectés par quatre pointes de STM
indépendants.
� La méthode de nanostencil, (cette approche sera expliquée dans le
prochain paragraphe).
o La méthode de nanostencil utilise une sonde locale ; notamment un cantilever
de Microscopie de force atomique (AFM), qui est percé et utilisé comme
masque dynamique pour collimer un faisceau des atomes métalliques. Dans
une architecture planaire, différentes techniques de déposition de métal sous
ultra vide sont requise. La première qui se sert d’un micro stencil statique pour
la construction de plots métalliques ayant une taille micrométrique, la
deuxième consiste à faire la croissance métallique pour déposer sur la surface
des petits agrégats métalliques ayant les dimensions en taille de l’ordre d’une
molécule. En suit, le dépôt métallique par nanostencil dynamique pour relier le
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micro électrode avec l’îlot métallique. Finalement, les microélectrodes son
connectes vers l’extérieur de l’environnement ultravide pour pouvoir faire des
mesures électriques.
Ce travail de recherche apporté dans cette thèse à participé au développement du
projet de nanostencil dans deux directions:
• L’amélioration de l'AFM : Il est très important de visualiser toutes les étapes de la
fabrication du dispositif construit par la méthode de nanostencil à partir des images
d’AFM. La seule technique capable d'image à la molécule unique sur un substrat isolant
est la microscopie de force atomique (AFM) dans le mode de modulation en fréquence
(FM-AFM) aussi appelle non contact (NC-AFM). Cette technique sera brièvement
présentée dans la deuxième partie du chapitre d’introduction.
• La recherche effectuée dans le système Pd/Alumina/NiAl (110) : Comme nous
avons déjà mentionné, la technique de nanostencil exige de trouver un système physique
de métal/isolant approprié pour pouvoir faire une croissance bidimensionnelle et epitaxial
d'un dépôt métallique cristallin. Malheureusement, le mode de croissance pour la plupart
de systèmes métal/isolant est de 3D, principalement parce que l'énergie libre des isolants
est habituellement beaucoup plus inférieure à celle des métaux. Néanmoins, on peut
espérer réaliser la croissance en deux dimensions, même dans ces conditions, si des
limitations cinétiques sont présentes. Nous avons choisi d'étudier l'alumine sur la surface
NiAl (110), qui est formé par oxydation de NiAl (110), parce que le palladium peut faire
la croissance d'îlots cristallines bien facettes et avec un surface plate [Yoshitake2006,
Hanssen1999]. Comme est suggéré dans la figure 1.8 dans le chapitre 1, ces îlots
pourraient être employés comme électrodes intermédiaires pour adresser électriquement
une molécule. Les résultats des mesures électriques obtenus sur cette couche d'oxyde par
microscopie à effet tunnel (STM) et par spectroscopie à effet tunnel (STS) ainsi comment
les études préliminaires du dépôt de palladium sont présentes dans le chapitre 4 : " ; Les
system Pd/Al10O13/NiAl (110) ".
Dans la deuxième partie de ce chapitre nous présentons un bref historique de la microscopie à
force atomique, les différents modes de fonctionnement de cette technique (le mode de
contact et le mode de contact intermittent (Tapping)) sont mentionnés. Le mode de non-
contact sera plus détaillé parce que c’est la technique utilisée dans cette thèse.
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Chapitre 2, Optimisation du capteur de déflection d’AFM
Dans ce chapitre nous présentons les améliorations de la tête AFM qui ont été
effectuées pendant le parcours de cette thèse. Nous commençons pour décrire la méthode
de levier optique et sa sensibilité, qui est la méthode appliquée pour la détection de la
flexion du cantilever. Ensuit nous faisons une brève description de la source de bruits dont
nous parlons de bruits qui apportent plus de contribution :
o Le bruit de grenaille
o Le bruit Johnson
o La source de bruit optique
� Le bruit du laser
o Le bruit thermique du cantilever
La modification de la tête omicron AFM comprend le remplacement de la diode
infrarouge par une diode laser super luminescente. Nous commençons par la description
de La diode laser super luminescente. Cette diode à été connectée vers l’intérieure de
l’ultra vide par une fibre optique de mode simple. Celle-là arrive à un système de
focalisation optique pour amener le faisceau laser jusqu’au la partie derrière du cantilever.
Les nouvelles caractéristiques du capteur de detection du levier optique son présentées,
celles-ci nous ont permit d’effectuer d’analyses spectrales du bruit existant dans le
système, pour caractériser le system de detection. Nous avons ainsi effectué des
estimations de bruit équivalent à l'entrée du nouveau système de detection du levier
optique et des estimations de la température du cantilever au niveau maximum de
puissance du laser. Enfin, nous avons mesuré la sensibilité et calibrage d'amplitude de
l’oscillation du cantilever.
Chapitre 3, Etude de la surface KBr(001) par AFM de non contact.
Nous avons exploré la surface isolant KBr(001) à l’échelle atomique. D’abord pour
tester les nouvelles améliorations effectuées dans la tête, mais nos résultats sont allés plus
loin. Des comportements intéressants d’un point en particulier lors de l’interaction avec la
surface, ont été étudies et publiés [venegas2008].
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La manière de préparer la surface est par clivage mécanique à l’aire et tout de suit la
surface est rentrée dans l’ultra vide pour poursuivre un dégaussage à une température de
150°C pendant une heure.
Nous avons effectue de la spectroscopie de force qui consiste en approchant la ponte
vibrante (oscillateur harmonique dissipé et actionné) nous avons eu la possibilité
d’enregistrer le signal de décalage en fréquence par rapport à la distance. Nous avons
utilisé cette information pour faire l’extraction de la force d’interaction à partir du
décalage en fréquence avec la formule de Sader-Jarvis.
Les résultats obtenus:
o Deux types d’images en résolution atomique. La terminaison de la pointe joue
un rôle capital pour le type de contraste montré dans une image de NC-AFM.
Lors les premiers approches de la pointe sur la surface, la pointe prend des
petites morceaux de KBr qui donnent la possibilité à la pointe d’avoir deux
types de terminassions possibles: une terminaison en K+ et une autre en Br-.
En dépendant du type de terminaison à l’extrême apex de la pointe, de
différents contrastes sont observés. Par exemple, si la pointe est en terminaison
K+ la pointe sera attirée par les ions de Br- sur la surface de sorte que la
machine rétractera la pointe. Ce réaction de la machine, produise un contraste
blanc sur l’image à l’écran, de manière que les images en résolution atomique
montrent les ions de Br- avec un contraste haut tandis que les ions K+ serons
montrés en contraste noir, parce que la pointe sera repoussée par les ions K+ et
la machine va approcher la sonde local.
o Courbes de force déterminées au niveau Atomique. Nous avons fait quelques
mesures pour extraire des courbes de force dans les différents types d’ions et
avec une pointe en terminaison K+. Nous avons observé une petite différence
dans la force.
o Inversion de polarité spontanée à l’extrême apex de la pointe : La polarité de la
pointé changeait d’une façon spontanée dans quelques expériences. Nous
avons obtenu des images en résolution atomique qui montrent ce changement
d’ion en faisant l’inversion de contraste dans l’image.
o Inversion de polarité de la pointe provoque par une marche monoatomique.
Nous avons noté que cette changement de polarité pouvait être induit par la
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interaction d’une marche monoatomique. Avec une pointe qui a été
fonctionnalisé d’une façon très particulière. Les images montrent la terrace à
gauche avec une pointe en terminaison Br- et la terrace de droit avec une
pointe en terminaison K+.
o De ce fait, Nous avons proposé donc un model schématique pour expliquer
l’énergie de potentiel de surface de un système à deux niveaux.
o Aussi, cette inversion de polarité de la pointe à été observé sur deux marches
monoatomiques successives.
- Ainsi, nous avons effectué de l’spectroscopie de force avec une pointe bistable d’où
nous avons remarqué la différence entre une pointe K+ et Br-.
Chapitre 4, Le système Pd/Al10O13/NiAl(110)
L’alliage intermétallique NiAl(110), description du Crystal à une échelle atomique, la
cellule unitaire, le paramètre de maille et la relaxation de la surface sous l’environnement
ultra vide. Ensuite nous présentons la procédure de préparation de cette surface pour avoir
un surface atomiquement propre, en faisant de cycles des bombardements ioniques à 1.5
keV et recuits par bombardement électronique jusqu’au atteindre des températures de
l’ordre de ~ 1100°C.
Le film mince isolant d’alumina formé sur la surface propre de NiAl (110) est faite par
oxydation direct de l’échantillon à une température spécifique. Il y a deux méthodes de
préparation :
� Le processus de deux étapes qui consiste en oxyder la surface et puis
après faire un recuit pour cristalliser la coche d’oxyde.
� Le processus à une seule étape, qui consiste en faire l’oxydation à
température de cristallisation
Morphologie du film d’oxyde à une échelle mesoscopic, le processus de cristallisation
produise deux différents domaines de réflexion, de paroir domaine apparaissent et laissent un
réseau caractéristiques de lignes de default.
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o Nous présentons des image à échelles intermédiaires pour commencer à décrire
la cellule unitaire de l’oxyde et pouvoir décrire la relation structural entre la
cellule unitaire d’alumina et le substrat de NiAl (110)
o Image de haut résolution avec trois paroirs domaines qui nous permettent de
visualiser la formation de paroirs domaines. Deux types de défauts sont
produits désignés comme I et II.
o Les Images en résolution atomique de la surface d’oxyde est en accord avec le
Model atomique connu, l’interprétation de l’image en résolution atomique du
domaine de réflexion est décrit. Une comparassions est fait aussi pour Le
modèle atomistique de la Frontière antiphase de domaine du type IA avec son
image en résolution atomique. Finalement nous faisons un maillage de la
surface avec le modèle atomique répétitive.
o Mesures de spectroscopie à effet tunnel. Nous commençons à parler des
propriétés électroniques de la couche d’oxyde en décriant le gap qui présent cet
oxyde et les énergies, où ils apparaissent les bandes de conduction et de
valence. Après nous présentons les mesures électriques par STS sur la alumina
prépare a 1700L (1L=1x10-6Torr s). Ici nous présentons une discussion de
l’hauteur apparente de l’épaisseur de la couche d’oxyde en fonction de la
tension d’imagerie. Ensuit nous montrons de mesures électriques dans le gap
par STS deux différents oxydes. Finalement on discute quelques expériences
par rapport à la croissance cristalline de palladium sous ultra vide.
Chapitre 5, Conclusions et perspectives
- Conclusions
o La tête RT-AFM a été considérablement améliorée en changeant sa source
lumineuse. Cette amélioration a été caractérisée par les mesures de bruit
soigneusement faites, une méthode non destructive pour mesurer l'amplitude
d'oscillation de cantilever a été développée. Ce paramètre expérimental est très
important pour la microscopie de force atomique en mode de non contact (
NC-AFM). Ce travail d'instrumentation était essentiel pour une meilleure
maîtrise de cet instrument qui est très complexe, et pour établir une base pour
des mesures quantitatives. Il a ouvert la voie aux expériences rapportées dans
le suivant.
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o Cet amélioré de la tête AFM a été utilisé pour étudier la surfaces KBr (001),
qui est une surface de référence pour NC-AFM. Maintenant, la résolution
atomique de cette surface est obtenue facilement. Une méthodologie fiable
pour obtenir des courbes de décalage en fréquence V.S. distance pointe-
surface, df(z), pour obtenir les courbes de force a été développée. Un ensemble
d'expériences a été exécuté avec une pointe qui a présenté un comportement
intéressant. Sur des images atomiques topographiques et df constantes on a
observé le changement de contraste atomique façon déterministe et réversible
lorsque la pointe croisait sur des marches monoatomiques: les images obtenus
formé sur les terrasses inférieures et les terrasses supérieures présentent la
même périodicité atomique, mais avec des différences importantes dans la base
de la structure cristalline. En outre, on observe le contraste atomique dans la
dissipation sur la terrasse supérieure mais pas sur l’inférieure. Ces
observations, publies dans [Venegas2008] ont été interprétées pendant qu'une
évolution réversible de la structure du bout de la point, ayant pour résultat un
changement du signe du bout terminant de la point, autrement dite de
changement d'ion terminal, ayant comment référence le mécanisme de
formation d’image sur les surfaces ioniques. Les courbes df(z), obtenues avec
la même point présentent deux comportements distincts : Un sur la terrasse
inférieure, où la dissipation est négligeable, et la courbe df(z) présente le même
caractéristique standard, monotone, et globalement attractive. Sur la terrasse
supérieure, où on observe un contraste atomique dans la dissipation, la courbe
df(z) est telle que deux points de fonctionnement de formation d’image sont
accessibles : la courbe est bistable. Nos observations établissent un lien direct
entre cette bi stabilité, le comportement de commutation du bout sur les
marches monoatomiques et l'aspect du contraste atomique dans les images de
dissipations sur la terrasse supérieure. Ces résultats renforcent l'hypothèse
d'hystérésis d'adhérence qui a été proposée comme source principale de
dissipation dans NC-AFM [Kantorovich2004].
o Différentes tentatives ont été faites d'observer des molécules d'indigo sur cette
surface de KBr(001). Le dépôt par sublimation d'UHV ou d'une solution ont
été employés. Ces expériences ne sont pas suffisamment avancées pour
justifier un rapport détaillé. Quelques observations préliminaires sont discutées
dans la partie de " perspectives" a la fin de ce chapitre.
French summary
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o Les propriétés structurales et électroniques du film d'alumine forme à ma
surface de cristal NiAl(110) par oxydation ont été étudiées par STM et STS. La
plupart des résultats déjà publies ont été confirmées, en particulier le modèle
atomique pour le domaine parfait et pour la frontière antiphase de domaine du
type I. Le film présente un gap, même sur les frontières de domaine. Cette
propriété lui rend approprié comme substrat pour découpler des molécules du
substrat métallique. Le dépôt de palladium qui forme des ilots plates et facettés
décorent les borde de marches et frontières antiphase de domaine de l'alumine,
ceux-ci sont formes aux petites tailles et avec des espaces entre eux avec des
dimensions moléculaires.
- Perspectives
o Tentatives pour Visualiser des molécules: nous avons explore deux dépositions
de molécules :
� Déposition par sublimation sous ultra vide
� Déposition de molécules à partir d’une solution de chloroforme
o Nous avons expérimente de la nanomanipulation aussi :
• Manipulation horizontal: présente après d’avoir faite le dépôt
d’un goute de chloroforme
• Manipulation vertical: présente après d’avoir faite le dépôt
d’indigo+chloroforme.