Trinity College Dublin Applied Physics Research Group · APRG Report 2014/2015 1 Trinity College...

APRG Report 2014/2015 1

Trinity College Dublin Applied Physics Research Group

School of Physics &

Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN)

APRG Report 2014/2015

Publication Date February 2016

Address: Prof Igor Shvets Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) School of Physics Trinity College Dublin 2 Ireland

Tel: + 353 1 896 1653 Fax: +353 1 671 1759

Website: http://www.tcd.ie/Physics/applied-physics E-mail: [email protected] [email protected]


The Group

From left to right: Prof Igor Shvets, Emma Norton, Dr Brendan O’Dowd, Dr Karsten Fleischer, Friedemann Call (DLR),

David Caffrey, Leo Farrell, Brian Walls, Daragh Mullarkey, Dr Olaf Lübben, Dr Brendan Bulfin, Dr Cormac O’Coileain and

Dr Barry Murphy.

Not pictured: Dr Elisabetta Arca, Andrew Mark Allen, Joseph Dee, Alexander Shvets, Graeme Power, Sebastian

Harenbrock, Ozhet Mauit, Olzat Toktarbaiuly, Bridget Gavin, Killian Walshe and Michael McInerney.


Contents

1. Preface................................................................................................................................................. 5

2. Group Members .................................................................................................................................. 7

2.1 Group Leader ................................................................................................................................ 7

2.2 Postdoctoral Researchers .............................................................................................................. 7

2.3 Postgraduate Students ................................................................................................................... 7

2.4 Research Assistants ....................................................................................................................... 7

2.5 Administration .............................................................................................................................. 7

2.6 Academic Visitors ......................................................................................................................... 8

3. Group Facilities ................................................................................................................................... 9

3.1 CRANN Facilities ....................................................................................................................... 12

4. Selected Example of Experimental Results ...................................................................................... 15

4.1 Disclinations in C60 mono-layers on WO2/W (110) surfaces .................................................... 15

4.2 Fabrication of [001]-oriented tungsten tips for high resolution scanning tunneling microscopy 19

4.3 Rotated domain network in graphene on cubic-SiC(001) ........................................................... 23

4.4 Atomically resolved STM imaging with a diamond tip: simulation and experiment.

Nanotechnology 25 (2014). ............................................................................................................... 25

4.5 Homolytic Cleavage of Molecular Oxygen by Manganese Porphyrins Supported on Ag(111) . 27

4.6 Thermodynamics of CeO2 Thermochemical Fuel Production .................................................... 29

4.7 Stability and capping of magnetite ultra-thin films..................................................................... 31

4.8 Reflectance anisotropy spectroscopy of magnetite (110) surfaces ............................................. 33

4.9 Spin states and glassy magnetism in LaCo1-xNixO3 (0 ≤ x ≤ 0.5) ................................................ 35

4.10 Evidence for Spin glass state of NdCo1-xNixO3 (x =0.3-0.5) ................................................. 37

4.11 Electrical-field-driven metal–insulator transition tuned with self-aligned atomic defects ....... 39

4.12 Magnetic and transport properties of epitaxial thin film MgFe2O4 grown on MgO (100) by

molecular beam epitaxy .................................................................................................................... 41

4.13 Magnetic and transport properties of epitaxial stepped Fe3O4(100) thin films ......................... 43

4.14 Tuning the crystallographic, morphological, optical and electrical properties of ZnO:Al grown

by spray pyrolysis. ............................................................................................................................ 45

4.15 Conducting mechanism in epitaxial p-type Transparent Conducting Oxide Cr2O3:Mg ........... 47

4.16 Spray pyrolysis growth of a high figure of merit, nano-crystalline, p-type transparent

conducting material at low temperature ............................................................................................ 51

4.17 Raman spectra of p-type transparent semiconducting Cr2O3:Mg ............................................ 53

4.18 Band alignment at the interface between Ni-doped Cr2O3 and Al-doped ZnO: implication for

transparent p–n junctions .................................................................................................................. 55


4.19 Synthesis of nanocrystalline Cu deficient CuCrO2 – a high figure of merit p-type transparent

semiconductor ................................................................................................................................... 57

4.20 Nanopatterning and Electrical Tuning of MoS2 Layers with a Subnanometer Helium Ion

Beam ................................................................................................................................................. 59

4.21 Effect of catalyst diameter on vapour-liquid-solid growth of GaAs nanowires ....................... 61

4.22 An analytic approach to modeling the optical response of anisotropic nanoparticle arrays at

surfaces and interfaces ...................................................................................................................... 63

4.23 Optical characterisation of plasmonic nanostructures on planar substrates using second–

harmonic generation .......................................................................................................................... 65

4.24 Unidirectional anisotropy in planar arrays of iron nanowires: A ferromagnetic resonance

study. ................................................................................................................................................. 67

4.25 Magnetic and resonance properties of Fe nanowire arrays on oxidised step-bunched silicon

templates. .......................................................................................................................................... 69

4.26 Induced morphological changes on vicinal MgO (100) subjected to high-temperature

annealing: step formation and surface stability ................................................................................. 71

4.27 Enhanced Shubnikov–De Haas Oscillation in Nitrogen-Doped Graphene ............................... 73

4.28 Transport Gap Opening and High On–Off Current Ratio in Trilayer Graphene with Self-

Aligned Nanodomain Boundaries. .................................................................................................... 75

4.29 Formation of plasmonic nanoparticle arrays – rules and recipes for an ordered growth .......... 77

4.30 Homologous size-extension of hybrid vanadate capsules – Solid state structures, solution

stability and surface deposition ......................................................................................................... 79

5. Commercialisation ............................................................................................................................ 81

5.1 Transformer Monitoring Project ................................................................................................. 81

5.2 Cellix Product Update ................................................................................................................. 83

6. Group Dissemination ........................................................................................................................ 85

6.1 International Alumni Collaboration ............................................................................................ 85

6.2 Peer-Reviewed Publications ....................................................................................................... 87

6.2.1 Surface structures and STM development ........................................................................... 87

6.2.2 Oxides for energy and electronic applications; magnetic oxides ......................................... 87

6.2.3 Transparent conducting Oxides ........................................................................................... 87

6.2.4 Nanostructuring and 2D materials ....................................................................................... 88

6.2.5 Others ................................................................................................................................... 89

6.3 Conference Poster Presentations ................................................................................................. 90

7. Research Students Graduated............................................................................................................ 91

7.1 PhD ............................................................................................................................................. 91

Acknowledgements ............................................................................................................................... 92


1. Preface

Dear Colleagues and Friends,

I am pleased to present the latest report of the Applied Physics Research

Group, School of Physics, Trinity College Dublin (TCD) covering the

period January 2014 to December 2015, (issue date of the report: February

2016).

In terms of research, most of our efforts have been focused on conducting oxides, especially p-type

transparent conducting oxides. These are fascinating materials, much needed for the development of

new transparent electronic devices. Of course, there are many excellent transparent conducting oxides

used by industry at present, such as e.g. zinc oxide doped with Al. The problem is that all these are n-

type conductors. If complementary p-type transparent materials suitable for industrial use were

available, one could construct novel better performing optoelectronic devices such as transparent

transistors. p-type material with the suitable band offset could also improve performance of thin film

solar cells. Research on p-type oxides poses challenging technical and fundamental questions. How to

modify the band structure of oxide to provide hole mobility with relatively low effective mass of current

carriers? The top of valence band in oxides is rather flat. We grow conductive oxide materials using a

range of deposition techniques such as a molecular beam epitaxy, magnetron sputtering, pulsed laser

deposition, spray pyrolysis. We have accomplished some of the best controls over structural properties

of conducting epitaxial oxides and a great degree of understanding of the oxide growth process.

Research is only as good as the people involved and it always gives me great pleasure to recognise the

achievements of my former students and group members. During the period of this report, several of

the team successfully completed their PhDs and moved on to start their careers in research and in

industry: Brendan O’Dowd, Leo Farrell, Brendan Bulfin, Oral Ualibek, and Askar Syrlybekov.

Another former research fellow, Sunil Arora, obtained a professorship position at Panjab University,

one of the best universities in India. We wish Sunil all the best with setting up his research laboratory

specialising in Nanomaterials.

Equally important to the advancements in state of the art in conducting oxides has been our recent

academic alliances which continue to flourish. Alexander Chaika completed his Marie Curie

Advanced Fellowship with us and returned back to the Institute of Condensed Matter Physics of the

Russian Academy of Science. His tenure in Dublin was very productive in terms of new

breakthroughs and publications.


Another outstanding contribution to our portfolio emerged from the work of Askar Syrlybekov, who

completed his PhD dedicated to field induced switching in magnetite, Fe3O4. As part of that project,

the school has expanded its skills in clean room device fabrication.

We always enjoyed robust activity in technology transfer and interaction with industry. This two-year

period was no different. We started a research project with Bell Labs in Ireland, supported under the

SFI Advanced Research Award with Elisabetta Arca.

This brings me to our record in commercialisation, built up over the last 15 years, which has already

generated 3 high tech spin out companies. In order to keep this momentum going, we have started

work on the next technology development project. The chosen topic is energy transmission, and more

specifically, the monitoring of power transformers on distribution networks. This exciting and

challenging area involves the development of smart technology which will facilitate predictive

analytics such as asset lifetime, energy efficiency, energy use patterns, as well as providing multiple

harmonics in the distribution network. This energy intelligence will be streamed at distribution

operator networks’ offices using wireless communications. As part of the aforementioned project, we

have established excellent contacts with key utility companies in the UK and Ireland and really look

forward to what will emerge from the project. This applications driven research is not necessarily

related to our fundamental work. However, the need to stimulate the transfer of innovative new talent

from academia to start new high tech businesses is well recognised worldwide. The energy and

creativity of our PhD graduates make a large contribution to what is now termed the “Irish high tech

ecosphere” and it is crucial that this trend continues.

The doors of the School of Physics are open, metaphorically and literally, to new ways of networking

and communicating and I warmly welcome you to contact us with your ideas or projects.

I look forward to forging exciting new avenues in research and innovation, in 2016, in participation

with new and old friends, peers, sponsors and collaborators, in the national and international wider

physics community.

Last but not least, may I also take this opportunity to thank the national and international funding

agencies whose generous support has allowed me and my team continue our research.

Regards,

Igor Shvets


2. Group Members

2.1 Group Leader

Prof Igor Shvets, Chair of Applied Physics

2.2 Postdoctoral Researchers

Elisabetta Arca [email protected]

Alexander Chaika [email protected]

Karsten Fleischer [email protected]

Olaf Luebben [email protected]

Barry Murphy [email protected]

Brendan O’Dowd [email protected]

Cormac O’Coileain [email protected]

2.3 Postgraduate Students

Leo Farrell Ozhet Mauit

Killian Walshe Michael McInerney

Daragh Mullarkey David Caffrey

Ozhet Toktarbaiuly Askar Syrlybekov

Brian Walls Emma Norton

2.4 Research Assistants

Mark Allen Joseph Dee

Alexander Shvets Graeme Power

2.5 Administration

Bridget Gavin


2.6 Academic Visitors

Prof. Han-Chun Wu, School of Physics, Beijing Institute of Technology (BIT), China

Dr. Sergey Bozhko, Institute of Solid State Physics, Russian Academy of Science, Russia

Dr. Sunil Kumar Arora, Centre for Nano Science and Nano Technology (CNSNT), Chandigarh, India

Dr .Alexander Chaika, Institute of Solid State Physics, Russian Academy of Science, Russia

Dr. Yury Vygranenko, Instituto Politécnico de Lisboa, Portugal

Dr. Friedemann Call, Institute of Solar Research, German Aerospace Centre (DLR), Germany


3. Group Facilities

Home built Ultra High Vacuum Scanning Tunnelling Microscope comprising:

MBE and Preparation chamber

STM chamber

0.1 T in-plane magnetic

LEED and AES optics

Resistive heater to 900 K for annealing

Electron beam heater to 2500 K for annealing

Triple-source e-beam evaporator

Cold-cathode ion source for Ar+ ion etching

Residual gas analyser

2 variable leak valves for gas processes

Load-lock

Home Built Ultra High Vacuum Scanning Tunnelling Microscope comprising

MBE and Preparation chamber

Magneto-optical Kerr effect chamber

STM chamber

LEED and AES optics

2 resistive heaters to 900 K for annealing

Electron beam heater to 2500 K for annealing

Single source e-beam evaporator

Mini high temperature effusion cell

Dual filament ion source for Ar+ ion etching


Load-lock

Commercial Low Temperature Ultra-High Vacuum Scanning Tunnelling Microscope (CreaTec)

Preparation chamber

Main manipulator

Liquid nitrogen cooling

E-beam heater for annealing

Load-lock chamber

STM chamber (with cryostat and superconducting magnet)

LEED optics

4 pocket e-beam evaporator

Knudsen cell for the evaporation of molecules

Ion source for Ar+ etching


Molecular Beam Epitaxy system (DCA M600)

Load chamber

Deposition chamber

Large volume cryopanel

RHEED

Residual Gas Analyser


Single pocket e-gun

Multi-heart e-gun

3 effusion cells

Substrate manipulator

Deposition rate monitors (crystal monitors)

Oxygen plasma source

Magnetron Sputtering system

Load chamber

Growth chamber

Three 2” magnetron guns

1.5” magnetron gun

Oxygen compatible sample heater to 900 K

DC and RF power supplies

Multi gas lines (Ar, O2, etc) with MFC controller

Atomic Terrace Low Angle Shadowing System (ATLAS) – three systems available

Load chamber

Growth chamber

Quartz crystal monitor

Ion gauge

10 cc high temperature Knudsen cell

Automated shutter

XPS/UPS – Omicron Multi-ProbeXP, a UHV system with dual wavelength x-ray source and

separate preparation chamber

Atomic Force Microscope (NT-MDT SolverPro)

High Resolution X-Ray Diffractometer (Bruker D8 Advanced)

Physical Properties Measurement System equipped with a 14 Tesla superconducting magnet

(Quantum Design)

Vibrating Sample Magnetometer / Alternating Gradient Field Magnetometer (Princeton Corp.

Model 2900 MicroMag)

UV-VIS Spectrophotometer with integrating sphere (Perkins Elmer 650S)

Reflection Anisotropy Spectroscopy (RAS) system for optical characterisation of planar arrays of

nanostructures

Home built 4 probe transport and magnetotransport measurement tool with 2T electromagnet for

Resistance v Temperature, Magnetoresistance v Temperature, DC/AC Hall measurements, and

Seebeck measurements


High temperature tube furnace for the annealing of samples to > 1200 °C for periods of up to 12

hours

Computational facilities, including:

Head node: Processor type: Dual Core AMD Opteron 875 Clock speed: 3.00 GHz Total number of cores: 16 Interconnect: Infiniband RAM: 24 GB OS: Scientific Linux

Nodes: Processor type: Quad-Core AMD Opteron 2352 Clock speed: 2.10 GHz Total number of cores: 32 RAM: 64 GB Interconnect: Infiniband Number of nodes: 4 RAM per node: 16 OS: Scientific Linux


3.1 CRANN Facilities

Class 100 Clean Room

Lithography Area

Spin resist

UV Mask Aligner

Laser Mask Writer

Solvent Wet Bench

Dry Plasma Etcher

Microscope

Acid Wet Bench

Class 1000 Clean Room

Deposition / Metrology Area

Temescal Evaporation System

LPCVD Furnace

Dicing Saw

Advanced Microscopy Laboratory

Zeiss Orion Plus – Helium Ion Microscope

Resolution below 0.75 nm

Elemental analysis

He-beam lithography

FEI Titan – Transmission Electron Microscope

STEM capabilities

A Gatan Tridiem Energy Filtering (EFTEM) system for Electron Energy Loss

Spectroscopy (EELS)

An Energy Dispersive X-ray (EDX) elemental analysis system

Alignments at 80 kV suited for the study of carbon based materials

Zeiss Auriga – Focused Ion Beam (FIB) with Cobra ion column

Ion imaging resolution of 2.4 nm

Electron imaging resolution 1 nm

Sample preparation for TEM lamella

Sequential cross sectioning for three dimensional image construction

A reactive gas injection system for reactive ion etching and Pt/SiO2/W deposition

Nano-manipulators dedicated to electrical measurements and TEM sample

preparation

Electron/Ion beam lithography, (Raith Elphy Quantum)

FEI Strata 235 – Focused Ion Beam

Electron/Ion beam lithography (Raith Elphy Quantum)

Transmission Electron Microscope sample preparation

Energy Dispersive X-ray (EDX) elemental analysis system (Silicon Drift Detector)

Nano-manipulators for in-situ TEM sample preparation


A gas injection system which allows reactive ion etching or the deposition of metals

such as platinum

Zeiss Ultra Plus – Scanning Electron Microscope

Imaging resolution of 1nm

Scanning TEM imaging [STEM] to a resolution of 0.8 nm

Accelerating voltages between 100V and 30kV

Charge neutralization system suitable for imaging non-coated insulating materials

EDX elemental analysis, imaging and mapping [Oxford Instruments INCA system]

Extensive electron detection system including:

Energy Selected Backscattered detector

Angular selected backscatter detector (for atomic number or Bragg scattering

contrast

Secondary Electron detector

Zeiss Electron Beam Lithography SEM – Supura 40

Raith Elphy Quantum software and beam control system for electron beam

lithography

Four micromanipulators with a low current measurement system for high precision

electrical measurements

Zeiss Electron Beam Lithography SEM – EVO 50

Raith Elphy Quantum software and beam control system for electron beam

lithography

High repeatability stage with large stage movements


4. Selected Example of Experimental Results

4.1 Disclinations in C60 mono-layers on WO2/W (110) surfaces

1,3S.I. Bozhko, 2V.Taupin, 2M.Lebyodkin, 2C.Fressengeas,1E.A.Levchenko, 3K.Radikan, 1V.N.Semenov and 3I.V.Shvets

1Institute of Solid State Physics, RAS, Russia 2Laboratoire d’Etude des Microstructures et de Mécanique des Matériaux (LEM3)

Université de Lorraine/CNRS, France 3Centre for Research on Adaptive Nanostructures and Nanodevices

(CRANN), School of Physics, Trinity College Dublin, Dublin 2, Ireland

Abstract: A scanning tunneling microscopy study of a planar close packed C60 hexagonal monolayer

on a WO2/W (110) surface reveals the existence of extended C60 domains taking two preferred

orientations at angle with an underlying periodic groove structure in the substrate. The disorientation

between two C60 domains is accommodated in a tilt boundary region by a linear array of molecular

structural units identified as disclination dipoles, i.e., rotational defects in the hexagonal structure of

the C60 planar monolayer. A field theory of Volterra’s crystal defects (disclinations and dislocations) is

used to construct maps of the elastic energy, elastic strains and stresses induced by the rotational

defects over the monolayer. Using realistic elastic constants for the hexagonal fullerene monolayer, the

predicted regions of high elastic compressive energy are found to overlap with the regions where the

orbital structure of the fullerene molecules is visible, i.e. where their molecular rotation is stopped.

Such overlapping is consistent with the idea that apparent stillness of the C60 molecules is due to lattice

compression.

Well-ordered close-packed high-density hexagonal molecular layers were reported in several

experiments on the deposition of C60 onto the surface of metals, semiconductors, oxides and substrates

covered by buffer layers. The structure and properties of the C60 monolayers is determined by a

competition between the C60-C60 planar interactions and the substrate-monolayer interactions. Indeed,

the formation of a well-ordered planar hexagonal lattice suggests the presence of a strong intermolecular

interaction, but the C60-substrate interactions may influence properties of the monolayer, such as the

shape or the orientation of the fullerene islands. In the present paper, we report a Scanning Tunneling

Microscopy (STM) study and theoretical interpretations of a two-domain structure in monolayer C60

films deposited onto WO2/W (110) surfaces, with a molecule-molecule separation close to 1 nm.

The growth of the monolayer film starts at terrace step edges, and form C60 islands with planar close-

packed hexagonal lattice (Fig. 1a,b).

Figure 1: a),b) 16×16nm2 STM images of C60 films of two preferred orientations. The grooves of WO2/W (110)

(indicated by green broken lines) appear in the STM images of the C60 films as a periodic structure of dim molecules highlighted by white circles.


The orientation of the hexagonal lattice with respect to the direction of the nano-rows of the WO2/W

(110) surface is 30.6°+ 2° (Fig. 1a) in 80% of the film, but the orientation is different in about 20% of

the film, with α being equal to 40.7°+ 2° (Fig.1b). This is indication that the C60-substrate interaction

plays a significant role in the distribution of the absorption energy and arrangement of the molecules.

The existence of two preferred orientations results in tilt boundaries separating differently oriented

domains (Fig.1c) and in a planar polycrystalline structure of the C60 monolayer. The tilt angle between

the domains is 10°+ 4°, a value considered as a small angle for grain boundaries in three-dimensional

solids. Close examination of the tilt boundary area in Fig.1c reveals that the misorientation between

adjacent domains is accommodated by a linear array of Molecular Structural Units (MSUs) where the

rotational defect localizes. In MSUs, the hexagonal symmetry of the molecular lattice is broken: the

elementary honeycomb pattern of the lattice is either opened by a positive wedge angle of value +(15°+

2°), or constricted by a negative wedge angle –(15°+ 2°). We propose to associate the MSUs with a

localized patch of continuous wedge disclination density, with line and Frank vectors normal to the

monolayer. In doing so, our modeling paradigm is to account for the rotational incompatibility of the

molecular lattice by focusing on densities of crystal defects defined continuously at intermolecular

scale, rather than the molecules themselves.

Figure 2: A. Wedge disclination density field superimposed on the bi-crystal fullerene planar monolayer. The six

MSUs along the tilt boundary are highlighted by dotted lines. Inset (1) shows the representation of one MSU with

edge dislocations. Inset (2) shows the measure of Frank vectors for the representation of the same MSU with a

wedge disclination dipole. B. In-plane tensile stress fields ),( 2211 TT superimposed on the bi-crystal fullerene

planar monolayer. Bright fullerenes displaying their orbital structure (blue spots) are seen to overlap with high-

compression regions around negative disclinations.

As can be seen in Figs.2a,b, the largest dilatations/contractions values predicted by the theory are

localized at negative/positive disclination sites respectively. Remarkably, the areas where the orbital

structure of the fullerene molecules is visible, i.e. where their molecular rotation is stopped, coincide

with regions of high compressive stresses, amounting to 500MPa, around negative disclinations. Such


overlapping suggests that stillness of the C60 molecules is due to compression induced by the defected

structure of the lattice. This is supported by the fact that the average distance between still molecules

determined from STM data is about 0.2Å smaller than the intermolecular distance between rotating

molecules, which results in a similar estimate of the overstress.

This work is published in: Bozhko, S. I.; Taupin, V.; Lebyodkin, M.; et al., Disclinations in C-60

molecular layers on WO2/W(110) surfaces. PHYSICAL REVIEW B Volume: 90 Issue: 21 Article

Number: 214106


4.2 Fabrication of [001]-oriented tungsten tips for high resolution

scanning tunneling microscopy

A. N. Chaika1,2,* N. N Orlova1, V. N. Semenov1, E. Yu. Postnova1, S. A. Krasnikov2,

M. G. Lazarev1, S. V. Chekmazov1, V. Yu. Aristov1,3, V. G. Glebovsky1, S. I. Bozhko1,

I. V. Shvets2 1Institute of Solid State Physics RAS, Chernogolovka, Moscow district 142432, Russia

2CRANN, School of Physics, Trinity College Dublin, Dublin 2, Ireland

3HASYLAB at DESY, D-22607 Hamburg, Germany

Abstract: The structure of the [001]-oriented single crystalline tungsten probes sharpened in ultra-

high vacuum using electron beam heating and ion sputtering has been studied using scanning and

transmission electron microscopy. The electron microscopy data prove reproducible fabrication of the

single-apex tips with nanoscale pyramids grained by the {011} planes at the apexes. These sharp, [001]-

oriented tungsten tips have been successfully utilized in high resolution scanning tunneling microscopy

imaging of HOPG(0001), SiC(001) and graphene/SiC(001) surfaces. The electron microscopy

characterization performed before and after the high resolution STM experiments provide direct

correlation between the tip structure and picoscale spatial resolution achieved in the experiments.

The atomic structure of a scanning tunneling microscopy (STM) probe is crucial for enhancement

of the spatial resolution and reliable interpretation of experimental data. Shortly after the invention of

STM it was realized that only sharp tips having single atom at the apex can provide stable and reliable

atomically resolved imaging. Although several-atom-terminated tips can be utilized, the ultimate

resolution can be reached only with sharp tips collecting most of the tunneling current through the

electron orbitals of a single atom closest to the surface.

Recently, we have demonstrated that oriented single crystalline tungsten tips can produce high-

quality atomically resolved images of surfaces with complicated atomic structure. Furthermore, even

orbital contribution of the front tungsten atom at the tip apex can be controlled in precise distance-

dependent STM experiments to achieve ultimate, picometer-scale lateral resolution. However, all STM

data presented in our previous papers were obtained with W[001] tips, which apex structure had not

been characterized by electron microscopy before or after the experiments. This could leave a room for

contention and speculation about the origin of the observed subatomic features and the actual structure

of the tips’ apexes responsible for picoscale resolution.


In this work, we have conducted complete step-by-step characterization of the W[001] tips by

scanning (SEM) and transmission (TEM) electron microscopy, from chemical etching to ultra-high

Figure 1. (a-d) TEM images of the W[001] tip apex after electron beam heating at 1000°C and ion sputtering. (a)

Bright-field TEM image and (b) electron diffraction pattern taken from the apex. (c and d) Dark-field TEM images

of the apex taken with the C and D diffraction spots on panel (b). (e-h) 25×25 Å2 STM images of the SiC(001)-

c(2×2) (e) and SiC(001)-(3×2) (f) surfaces. (g and h) The corresponding cross-sections 1-2 and 3-4 taken from the

images in panels (e) and (f).

vacuum (UHV) sharpening. Moreover, for one of the W[001] tips the structure of the apex has been

characterized by TEM before and after STM experiments with picometer, orbital resolution. The

electron microscopy studies proved that high resolution STM data were obtained using the W[001] tip

having a nanoscale pyramid grained by the {011} planes at the apex which is not substantially modified

during STM experiments thus providing a correlation between the tip structure and the spatial resolution

obtained in STM experiments.

Figure 1(a-d) shows TEM data obtained from one of the W[001] tips prepared using standard

procedure involving electrochemical etching, electron beam heating in UHV and co-axial Ar+-

sputtering. Bright-field TEM image in Fig. 1(a) shows the nanometer-sized pyramid at the apexes

grained by the {011} planes. Electron diffraction pattern taken from the apex [Fig. 1(b)] exhibits

characteristic diffraction reflexes, which correspond to the {011} and {001} crystallographic planes

forming the pyramid. This is further illustrated by the dark-field TEM images taken from two different

diffraction spots [marked by C and D in Fig. 1(b)]. The dark-field image in Fig. 1(c) reveals that the

angle at the tip apex is close to 90° that corresponds to the [001]-oriented tungsten apex grained by the

{011} planes.

Another W[001] tip with qualitatively the same pyramid at the apex was utilized for high

resolution STM studies of the SiC(001) reconstructions and graphene/SiC(001) and graphite imaging

with subatomic resolution. As an example, Figs. 1(e) and 1(f) show typical atomically resolved STM

images of the c(2×2) and (3×2) reconstructions of the SiC(001) surface. The cross-sections in Figs. 1(g)

and 1(h) demonstrate that vertical resolution of the order of several picometers can be obtained routinely

with the [001]-oriented single crystalline tungsten tips.

Atomic resolution was easily achieved on HOPG(0001) with the same W[001] tip. Furthermore,

the STM images measured at small tunnelling gaps [Fig. 2(b-d)] revealed qualitatively the same

subatomic features as reported in our previous works. The images shown in Fig. 2(b-d) were measured

with the W[001] tip at unchanged tip state, fixed bias voltage and various tunneling currents. They


reveal the transformation of spherically symmetric atomic features to multiple subatomic features

reproducing the shape of the tungsten atom d-electron orbitals with increasing tunneling current

(decreasing tip-surface distance), as schematically shown in Fig. 2(a). The difference in the gap

resistances responsible for the transition from spherically symmetric atomic features to two-fold and

four-fold split subatomic features in Figs. 2(b-d) is in agreement with our previous studies. TEM images

of this W[001] tip taken after STM experiments on HOPG(0001) demonstrated only minor changes of

the apex pyramid. These minor modifications of the apex were, presumably, related to additional ion

sputtering applied to the tip prior to the STM experiments. The TEM studies proved safe tip to sample

approach without a transfer of material from the sample to the tip and enhanced stability of the W[001]

tip structure during high resolution STM experiments demanding very small tip-sample distances,

where subatomic resolution can be achieved.

Figure 2: A schematic model of the STM experiment on HOPG(0001) with the [001]-oriented W tip (a). STM images

(6 × 6 Å2) measured with the W[001] tip at a sample bias voltage of -50 mV and tunneling currents of 0.15 nA (b),

0.2 nA (c) and 0.4 nA (d). The [100] and [010] crystallographic directions of the tungsten tip in the experiment

coincide with the x and y axes of the STM scanner.

This work is published in: A.N. Chaika, N.N Orlova, V.N. Semenov, E.Yu. Postnova, S.A. Krasnikov,

M. G. Lazarev, S. V. Chekmazov, V. Yu. Aristov, V. G. Glebovsky, S. I. Bozhko, I.V. Shvets,

Fabrication of [001]-oriented tungsten tips for high resolution scanning tunneling microscopy.

Scientific Reports, Volume 4, January 2014, Pages 3742(1-6).


4.3 Rotated domain network in graphene on cubic-SiC(001)

Alexander N Chaika1,2, Olga V Molodtsova3, Alexei A Zakharov4, Dmitry Marchenko5, Jaime

Sánchez-Barriga5, Andrei Varykhalov5, Sergey V Babenkov5, Marc Portail6, Marcin Zielinski7, Barry

E Murphy2, Sergey A Krasnikov2, Olaf Lübben2, Igor V Shvets2

and Victor Y Aristov1,3

1 ISSP, RAS, Chernogolovka, Moscow District, 142432, Russian Federation

2 CRANN, School of Physics, Trinity College Dublin, Dublin 2, Ireland 3 HASYLAB at DESY, D-22607 Hamburg, Germany

4 MAX-lab, Lund University, Box 118, SE-22100 Lund, Sweden 5 Helmholtz-Zentrum Berlin für Materialien und Energie, D-12489 Berlin, Germany

6 CNRS-CRHEA, Rue Bernard Grégory, 06560 Valbonne, France 7 NOVASiC, Savoie Technolac, Arche Bat 4, BP267, 73375 Le Bourget du Lac, France

Abstract: The atomic structure of the cubic-SiC(001) surface during ultra-high vacuum graphene

synthesis has been studied using scanning tunneling microscopy (STM) and low-energy electron

diffraction (LEED). Atomically resolved STM studies prove the synthesis of a uniform, millimeter-scale

graphene overlayer consisting of nanodomains rotated by ±13.5° relative to the (110)-directed

boundaries. The preferential directions of the domain boundaries coincide with the directions of carbon

atomic chains on the SiC(001)-c(2 × 2) reconstruction, fabricated prior to graphene synthesis. The

presented data show the correlation between the atomic structures of the SiC(001)-c(2 × 2) surface and

the graphene/SiC(001) rotated domain network and pave the way for optimizing large-area graphene

synthesis on low-cost cubic-SiC(001)/Si(001) wafers.

Its unique electronic properties make graphene a very appealing material for future applications.

However, to be considered as a potential candidate to replace silicon in electronics, graphene should be

controllably grown on large-area insulating substrates compatible with existing lithographic

technology. Synthesis on hexagonal silicon-carbide (α-SiC) substrates is one of the more promising

methods for graphene fabrication on insulating substrates. It is known that even uniform multilayers of

graphene on carbon-terminated hexagonal SiC(000-1) substrates possess the physical properties and

electronic spectra of a free-standing graphene monolayer. However, the graphene synthesized on high-

cost wafers cut from bulk α-SiC single crystals cannot be considered as a viable candidate for industrial

mass production.

It has been shown recently that synthesis on low-cost, large-diameter cubic-SiC(001)/Si(001) wafers

can represent a realistic way to mass-produce graphene layers suitable for electronic applications.

However, the first papers on graphene/SiC(001) brought no information about the graphene overlayer’s

continuity on the millimetre-scale, crucial for potential technological applications, and provided

contradictory information about the atomic structure and electron spectrum of graphene on SiC(001).

Here we present the results of the first comprehensive STM and LEED studies of the atomic structure

of the SiC(001) surface carried out during all stages of ultra-high vacuum (UHV) synthesis of graphene

on cubic-SiC(001).

STM and LEED data taken from different samples and surface regions prove the millimeter-scale

continuity of the graphene layers on SiC(001), which consist of rotated nanodomains with four

preferential orientations connected through the [110]- and [1-10]-directed boundaries. Atomically

resolved STM studies of different graphene domains within the network show all the features typical

of quasi-freestanding graphene (i.e., rippling, random bond length distribution, high flexibility of the

topmost layer, interference patterns near defects and boundaries).

It is crucial to note that the continuity of the domain network is not broken by the APD defects, which

would otherwise be considered as a potential obstacle for the growth of uniform, continuous graphene

coverage on cubic-SiC. However, the presence of domain boundaries can modify the electronic


properties of graphene. Therefore, an increase in the domain size or greater control over the boundary

directions can be considered as the next steps for improving graphene quality on cubic-SiC(001).

It is suggested from the presented STM studies that the graphene domain size can be increased by

minimizing the flashing time of the silicon-terminated SiC(001)-c(4×2) reconstruction, because

continuous annealing could produce a lot of -directed carbon chains, which can become grain

boundaries after the graphene synthesis. Subsequently, using vicinal SiC(001) substrates could achieve

a preferential orientation of the carbon chains on the SiC(001)-c(2×2) reconstruction and improve the

quality of the graphene/SiC(001) by aligning the graphene nanoribbons and grain boundaries along the

one of two equivalent directions.

Figure 1: (a-d) Atomically resolved STM images of the 3×2 (a), c(4×2) (b) and c(2×2) (c) reconstructions of the

SiC(001) surface and graphene/SiC(001) system (d). Inset in panels (a-c) are typical two-domain LEED patterns of the corresponding surface atomic structures.

Figure 2: (a,b) 18 × 11 nm2 atomically resolved STM images of the vertical (a) and horizontal (b) nanoribbons.

Inset in panel (b) shows an FFT pattern with two 27º-rotated systems of spots. (c-e) Models explaining the origin of the 24 diffraction spots in the LEED patterns of graphene/SiC(001). The four different coloured hexagons, red, blue, green and brown represent the four domain orientations, indicated by similarly-coloured arrows in (a) and (b). Inset in panel (e) shows a LEED pattern taken from graphene/SiC(001) samples at Ep=65 eV, demonstrating 1×1 substrate spots (highlighted by yellow arrows) along with 12 double-split graphene monolayer spots, indicated by one dotted arrow for each orientation.

This work is published in: A.N. Chaika, et al., Nanotechnology 25, 135605 (2014)


4.4 Atomically resolved STM imaging with a diamond tip:

simulation and experiment. Nanotechnology 25 (2014).

V. Grushko1*, O. Lübben2, A. N. Chaika2,3**, N. Novikov1, E. Mitskevich1, A. Chepugov1, O.

Lysenko1, B. E. Murphy2, S. A. Krasnikov2, I. V. Shvets 1V. Bakul Institute for Superhard Materials, Kiev, 04074, Ukraine, CRANN, School of Physics, Trinity

College, Dublin 2, Ireland, Institute of Solid State Physics RAS, Chernogolovka, Moscow district

142432, Russia

Abstract: The spatial resolution of a scanning tunneling microscope (STM) can be enhanced using

light element-terminated probes with spatially localized electron orbitals at the apex atom. Conductive

diamond probes can provide carbon atomic orbitals suitable for STM imaging with sub-Ångström

lateral resolution and high apex stability crucial for the small tunneling gaps necessary for high-

resolution experiments. Here we demonstrate that high spatial resolution can be achieved in STM

experiments with single-crystal diamond tips, which are generally only considered for use as probes

for atomic force microscopy. The results of STM experiments with a heavily boron-doped, diamond

probe on a graphite surface; density functional theory calculations of the tip and surface electronic

structure; and first-principles tunneling current calculations demonstrate that the highest spatial

resolution can be achieved with diamond tips at tip–sample distances of 3–5 Å when frontier p-orbitals

of the tip provide their maximum contribution to the tunneling current. At the same time, atomic

resolution is feasible even at extremely small gaps with very high noise in the tunneling current.

The electronic structure of the tip apex atom plays a key role in scanning tunneling microscopy (STM)

experiments. Light element-terminated probes with spatially localized atomic orbitals at the apex which

have a minimal number of electron states involved in the tunneling can provide enhanced spatial

resolution in STM experiments and simplify the interpretation of atomically resolved STM data.

However, the methods proposed for the fabrication of light atom-terminated probes so far could not

routinely produce stable tips with a controlled atomic and electronic structure at the apex. In this work

we show that oriented heavily boron-doped diamond tips from synthetic single crystals can be

considered as very promising probes for high-resolution STM studies. The conductive diamond probes

can provide carbon atomic orbitals suitable for STM imaging with picoscale spatial resolution and high

apex stability at small tunneling gaps necessary for high-resolution experiments.

For a detailed understanding of the tunneling parameters optimal for high resolution imaging with

diamond probes, we analyzed the interaction between the -oriented diamond tip and the graphite

surface and the influence of this interaction on the tunneling current and electronic structure of the tip

and surface atoms. We have calculated the density of electron states (DOS) of the interacting atoms and

the contribution of different atomic orbitals of the diamond tip apex atom using density functional

theory (DFT) and the non-perturbative approach. Our STM experiments and theoretical calculations

demonstrate that high spatial resolution can be achieved with conductive diamond probes both at

relatively large tunneling gaps, when the p-orbitals of the tip provide the maximum contribution to the

tunneling current; and small gaps when the PDOS of the tip and surface atoms are substantially modified

and the noise in the tunneling current is very high due to an increased tip – sample interaction.

Figure 1(a-d) displays a comparison of the spatial resolution achieved with a boron-doped [111]-

oriented diamond and [001]-oriented tungsten probes. The images taken with the diamond and W[001]

probes [Figs. 1(a) and 1(b)] reveal two sublattices corresponding to non-equivalent α and β atoms in

the honeycomb lattice. However, the hollow sites are substantially deeper and individual surface atoms

are better resolved in the image measured with the boron-doped diamond probe [see the cross-sections

on Figs. 1(c) and 1(d)]. This is related to different spatial distribution of the carbon p-orbitals and

tungsten d-orbitals at the apexes of the STM tips. In particular, carbon atomic orbitals are further

protruded in the z-direction and more localized in the x-y plane than tungsten d-orbitals. DFT

calculations revealed a minor difference in the DOS corresponding to the α and β-atoms of the graphite


surface [Fig. 1(e,f)]. The density of electron states at EF is larger for β atoms by ~25% [Fig. 1(f)] and

the difference decreases for the DOS integrated over a wider range of the electron energies. This

difference in the total DOS of the α and β-atoms is responsible for the two non-equivalent sublattices

in Figs. 1(a) and 1(b). The observed height difference in the STM images correlates well with the minor

difference in DOS associated with the non-equivalent atoms [Fig. 1(f)]. The smaller height difference

in the STM image measured at Vb=-0.4 V with the W[001] tip [Fig. 1(b) and 1(d)] agrees with the

decreasing difference in the DOS integrated over the corresponding energy range.

Figures 1(g) and 1(h) show a comparison of the experimental HOPG(0001) STM image resolved using

the diamond tip [Fig. 1(h)] and the calculated charge density map corresponding to the surface electron

states near EF [Fig. 1(g)]. Both images demonstrate different contrast on α and β atoms. The DOS [Fig.

1(e,f)] and the charge density map [Fig. 1(g)] were calculated at a tip-sample distance of 4.5 Å where

the orbital structure of the tip and surface atoms is not substantially modified by the interaction and

relaxations of the tip and surface atoms are minimal. The excellent agreement between the experimental

and theoretical images [Figs. 1(g) and 1(h)] proves that the highest resolution images revealing the true

honeycomb lattice of the graphite surface were measured with the diamond probe operated at tunneling

gaps of 3.5–4.5 Å where the tip did not strongly interact with the surface. This result shows the

advantages of the oriented single crystal diamond probes having the p-orbitals at the apex: their structure

is stable and well defined while high lateral and vertical resolution can be achieved at larger tunneling

gaps comparing to typical tip-sample distances used in experiments with transition metal tips. This

allows STM imaging of the surface electronic structure with sub-Ångström lateral and vertical

resolution without modifying the surface DOS by tip-sample interaction.

Figure 1. (a,b) 18×9 Å2 atomically resolved STM images of HOPG(0001) measured with a diamond tip at Vb = -50 mV and I = 0.1 nA (b) and a W[001] tip at Vb = -0.4 V and I = 0.18 nA (b). (c,d) Cross-sections 1-2 (c) and 3-4 (d) of the images in panels (a) and (b), respectively. (e,f) Total density of electron states associated with the α and β atoms of a graphite (0001) surface. (g,h) Comparison of the 9×9 Å2 calculated electron density distribution map in the energy range from EF-0.2 eV to EF (g) and the experimental STM image measured on HOPG(0001) with the diamond probe at Vb = -50 mV and I = 0.8 nA (h). Both images show non-equivalence of the α and β atoms in accordance with the density of states shown in panels (e) and (f).

This work is published in: V. Grushko, et al. Nanotechnology, Volume 25, Issue 2, January 2014, Pages 025706(1-11).


4.5 Homolytic Cleavage of Molecular Oxygen by Manganese

Porphyrins Supported on Ag(111)

Barry E. Murphy1, Sergey A. Krasnikov1,2, Natalia N. Sergeeva3, Attilio A. Cafolla2, Alexei B.

Preobrajenski4, Alexander N. Chaika1, Olaf Lübben1 and Igor V. Shvets1

1 CRANN, School of Physics, Trinity College Dublin, Dublin 2, Ireland;

2 School of Physical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland; 3 School of Chemistry, University of Leeds, Leeds LS2 9JT, UK; 4 MAX-lab, Lund University, Box 118, S-22100 Lund, Sweden.

Abstract: Oxygen binding and cleavage are important for both molecular recognition and catalysis.

Mn-based porphyrins in particular are used as catalysts for the epoxidation of alkanes, and in this study

the homolytic cleavage of O2 by a surface-supported monolayer of Mn porphyrins on Ag(111) is

demonstrated by scanning tunneling microscopy, X-ray absorption and X-ray photoemission. As

deposited, {5,10,15,20-tetraphenylporphyrinato}Mn(III)Cl (MnClTPP) has a flat orientation with its

macrocycle parallel to the substrate and the axial Cl-ligand pointing upward, away from the substrate.

The adsorption of MnClTPP on Ag(111) is accompanied by a reduction of the Mn oxidation state from

Mn(III) to Mn(II) due to charge transfer between the substrate and the molecule. Annealing the

Mn(II)ClTPP layer up to 510 K causes the chlorine ligands to desorb from the porphyrins while leaving

the monolayer intact. The Mn(II)TPP is stabilized by the surface acting as an axial ligand for the metal

centre. Exposure of the Mn(II)TPP/Ag(111) system to molecular oxygen results in the dissociation of

O2, and forms pairs of Mn(III)OTPP molecules on the surface. Annealing at 445 K reduces the

Mn(III)OTPP complex back to Mn(II)TPP/Ag(111). The activation energies for Cl and O removal were

found to be 0.35 ± 0.02 eV and 0.26 ± 0.03 eV, respectively.

Control over molecules on the atomic scale is routine in nature, for without it hemoglobin could not

transport oxygen and almost all other biological processes would be impossible. However, such precise

manipulation of matter on the smallest scale is still some way off for humanity. Considerable research

has been focused on this issue for the past 50 years and with the advent of scanning probe microscopy

and other highly local techniques, great strides have been made in the fields of atomic-scale

manipulation, molecular electronics and molecular structure determination.

3d transition metal (TM) porphyrins have featured widely in recent research due to their rich

coordination chemistry making them promising candidates for a large number of applications such as

catalysis, nonlinear optics, enzyme models, sensors and molecular electronics. Learning from nature,

many biomimetic systems based on 3d TM porphyrins have been studied, and adapted for use in

oxidation reactions. In particular, manganese porphyrin complexes have been shown to selectively

catalyze the halogenation of C–H bonds and are often used as catalysts for the chemical transformation

of alkenes into epoxides. Given the ability of porphyrins to bind and release gases and to act as an active

center in catalytic reactions in biological systems, porphyrin-based films on surfaces are extremely

appealing as chemical and gas sensors as well as nanoporous catalytic materials.


In the present work, scanning tunnelling microscopy (STM), X-ray absorption (XA), X-ray

photoemission spectroscopy (XPS) and density functional theory (DFT) calculations have been

employed to study the self-assembly and central ligand transformation of {5,10,15,20-

tetraphenylporphyrinato}Mn(III)Cl (MnClTPP) on the Ag(111) surface. The results of this work

demonstrate the physical manipulation of an axial bond through the application of heat, charge transfer

from the substrate to the molecule, and the stabilising effect of the underlying Ag(111) surface on the

oxidation state of the central TM ion and the porphyrin itself. The reversible oxidation and reduction of

the central ion by gaseous O2 demonstrated here paves the way for future studies of the applicability of

MnClTPP in gas sensing or catalytic applications.

Figure 1. STM images of 1ML of MnOTPP after various oxygen exposures: (A) 15 min (1800 L) with ∼15% of the molecules oxidized; (B) 30 min (3600 L), ∼40% oxidized; (C) 90 min (10800 L), ∼90% oxidized. (D) Statistical distribution of the probability that an oxidized molecule has one or more nearest neighbors that are also oxidized (blue), compared to a binomial (random) distribution for the case of uncorrelated oxidation (red). Images with ∼8% oxidized molecules were used for the statistical analysis. (E) Dependence of the fraction of oxidized molecules on oxygen exposure. (F) STM image of a region of the oxidized Ag(111) surface free of molecules after several oxidation–reduction cycles.

This work is published in: B.E. Murphy, ACS Nano 8 (5), 5190–5198 (2014)


4.6 Thermodynamics of CeO2 Thermochemical Fuel Production

B. Bulfin, F. Call, M. Lange, O. Lübben, C. Sattler, R. Pitz-Paal, I. V. Shvets

CRANN, School of Physics, Trinity College Dublin, Dublin 2, Ireland

Abstract: In this work the thermodynamics of thermochemical fuel production using a CeO2 redox cycle

are studied. The need to reduce the oxygen partial pressure in order to improve efficiency is

investigated, with both sweep gas and vacuum pumping considered as methods of achieving this. At

ambient pressure the cycles can be maximized with respect to the temperature swing and the minimum

oxygen partial pressure. For reduction at 1500 °C the maximum efficiency was found to be 4.5%, which

is significantly lower than the values found in previous studies. In addition isothermal operation had

very low efficiency (less than 2%) under all of the conditions considered. If the system is operated at

lower than ambient pressure, the pumping efficiency will depend on the pressure. From an investigation

of commercially available pumps the pressure dependence was given an analytical expression. The

results showed the cycles have an optimal operating pressure and that using sweep gas, as well as

pumping, only reduced the overall efficiency. Recovering heat from both the high temperature ceria

and the oxidation reaction, and using it as process heat, was also considered. With 60% of this heat

being recovered, the peak efficiency for the 1500 °C pumped cycle increased to 11%. Finally the

practicality of the cycles, in terms of the quantity of ceria required to maintain continuous operation,

are considered, and some suggestions for improving the cycle are given.

In recent months, climate change has made many news headlines, largely due to the publition of a three

part report which provided a review of scientific bases, impacts, and vulnerability, and possible

mitigation plans for climate change. It was commissioned by the Unite Nations, put together by the

Intergovernmental Panel on Climate Change, and was the fifth report of its kind. It was based on some

12,000 peer reviewed publications. Although often sensationalized by the media and policy makers, it

presents strong evidence that there will be some serious implications for society as a result of climate

change.

In this work, a thermodynamic analysis for

the solar driven production of fuel using ceria

is presented which could offer a sustainable

alternative to current fuels such as petroleum

derived from fossil fuels. We provide an

analysis of the efficiency of such cycles with

a realistic look at the different constraints

affecting the efficiency. This includes an

analysis on different methods used to reduce

the oxygen partial pressure during reduction

and a full analysis of the oxidation reaction.

This allows the efficiency to be maximized

by selecting the cycle parameters.


Figure 1: Schematic of the reactor

showing the processes involved in reduction, their heat costs, the flow of the nitrogen gas and the cycle products.

The effect of performing the

reduction at reduced pressures

was investigated for pumps

which have an efficiency that

decreases with decreasing

operating pressure. The results

show the cycles have an optimal

operating pressure and that using

sweep gas as well as pumping

only reduced the efficiency.

Decreasing the pressure may

offer other benefits. It improves the gas phase transport properties, which should improve the reduction

kinetics. The optimal efficiency, without solid state heat recovery, for a cycle operating at 1500 °C was

found to be 7.5%.

a) b)

Figure 2: a) Plot of the ηfuel vs – log(P) for a range of values of Trd (1400, 1500, and 1600 °C), with Tox = 1000 °C, for a pumped reactor. b) Plot of ηfuel vs – log(P) for a range of values of Trd (1400, 1500, and 1600 °C), for a pumped

reactor with solid state heat recovery from the cycled ceria. Here it was assumed that 60 % of the cerias sensible heat could be recovered.

This work is published in: B. Bulfin, F. Call, M. Lange, O. Lübben, C. Sattler, R. Pitz-Paal I. V.

Shvet, Energy and Fuels, 29 (2), pp 1001–1009 (2015)


4.7 Stability and capping of magnetite ultra-thin films.

K. Fleischer, O. Mauit, and I. V. Shvets

CRANN, School of Physics, Trinity College Dublin, Dublin 2, Ireland

Abstract: In Ultrathin films of Fe3O4 have been grown epitaxially on nearly lattice matched MgO(001).

The stability of 4 nm thick films in ambient air and under annealing in an oxygen atmosphere at 200°C

has been studied. By magneto optical and Raman measurements, we can confirm the presence of the

Fe3O4 phase and the formation of a maghemite top layer passivating the Fe3O4 thin film. In a second

step, we are able to demonstrate that this top layer oxidation in ambient air can be prevented by a 2 nm

thick magnesium ferrite passivation layer, while a thicker 20 nm MgO layer prevents oxidation even at

elevated temperatures.

Magnetite is a material used for its ferromagnetic properties in magnetic data storage and ferrofluids.

Its basic electrical, optical, and crystallographic properties at room temperature are well characterised.

For applications of magnetite, in-plane devices with controlled stoichiometry and crystallinity are

typically required. The control over the magnetite properties in thin film form and reliable

measurements of thin film properties are complicated due to top layer oxidation of the films upon air

exposure. While Fe3O4 is stable at room temperature, the top layers of thin film samples can be further

oxidized forming maghemite (γ-Fe2O3). Therefore, understanding the issues of oxidation of ultra thin

films of magnetite and finding ways of preserving the film stoichiometry under ambient air is important.

The oxidisation of the ultra thin films with high crystalline quality was investigated by Raman

spectroscopy and spectroscopic magneto optical Kerr effect. We demonstrated that these optical

techniques can be used to quantitatively characterise the oxidisation process in films as thin as 3 nm,

and we can reliably distinguish the different iron oxides phases. In particular Raman spectroscopy has

been proven to be able to measure the overlying oxide layer thickness, while characteristic signatures

in the spectroscopic MOKE signal allowed us to identify the unwanted surface oxide.

We have demonstrated that uncapped ultra thin Fe3O4 layers are not stable in ambient air but are surface

oxidized and transformed into γ-Fe2O3. The initial oxidation after even just one day of exposure to air

is limited to approximately 1 nm. On the timescale of months, the γ-Fe2O3 layer gets thicker. Annealing

in an oxygen atmosphere accelerates this process. The introduction of an even thinner MgFe2O4 capping

layer can prevent the initial oxidation, while caps of 20 nm of MgO fully protect the Fe3O4 thin films.

Best results have been achieved by a combination of thin MgFe2O4 cap and the MgO cap. While

protecting samples with MgO is not a problem for optical measurements, it poses a problem in electrical

measurements, or if the Fe3O4 is to be used as conductive or spin injecting buffer layer. In this case,

MgFe2O4 capping can be used as an alternative, as long as samples are processed within a couple of

days after the growth.


Figure 2: (a) Polar MOKE measurements of an capped 3 nm and uncapped 50nm thick Fe3O4 layer on MgO(001)

at a magnetic field of 250mT. The optical model qualitatively describes the differences between the 3 nm and 50nm thick samples due to the changes on overall reflection of the layer stack. (b) Effect of air exposure and O2 annealing on the MOKE of an uncapped 4nm thick sample at 125 mT.

This work is published in: K. Fleischer, O. Mauit, and I. V. Shvets, Applied Physics Letters

104, 192401 (2014)

0.0

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MgO

Fe3O4 (4nm)

(d)

(c)

(b)

-Fe2O3R

am

an inte

nsity (

cps)

Uncapped

2 days

2 months

O2 anneal

Fe3O4

(a)

MgO (20nm)

MgO

Fe3O4 (4nm)

-Fe2O3

MgO capped

2 days

2 months

O2 anneal

Fe3O4

MgFe2O4 (2nm)

MgO

Fe3O4 (4nm)

Ram

an inte

nsity (

cps)

Raman shift (cm-1)

MgFe2O4 capped

2 days

2 months

O2 anneal

MgO (20nm)

MgFe2O4 (2nm)

MgO

Fe3O4 (4nm)

Raman shift (cm-1)

Full cap

2 days

2 months

O2 anneal

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5-1.8

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-1.4

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3nm Fe3O4 on MgO(100), capped

po

lar

MO

KE

at

25

0m

T (

mra

d)

photon energy (eV)

Model: MOKE ~ xy/

fit of 50nm data

Model of 3nm

using same xy

(a)

MgO

Fe3O4 (4nm)

2 days

2 months

O2 anneal

photon energy (eV)

aging

(125 mT)

Figure 1: Raman spectra of a

differently capped Fe3O4 4 nm thin

film on MgO(001). Measurements

were taken after two days of air

exposure (solid lines), after 2 months

of air exposure (dashed) after

annealing for 20 min in oxygen

(dotted) at 200°C. (a) shows data for

the uncapped Fe3O4, (b) the same

film capped by 2 nm MgFe2O4, (c) 20

nm MgO, and (d) 2 nm MgFe2O4 and

20 nm MgO. By deconvolution of the

line shape of the measured Raman

mode we can quantify the

maghemite content.


4.8 Reflectance anisotropy spectroscopy of magnetite (110)

surfaces

K. Fleischer,1,* R. Verre,1,2 O. Mauit,3 R. G. S. Sofin,4 L. Farrell,3 C. Byrne,1 C. M. Smith,1

J. F. McGilp,1 and I. V. Shvets1,3

1 School of Physics, Trinity College Dublin, Dublin 2, Ireland 2 Department of Applied Physics, Chalmers University of Technology, 412 96 Goteborg, Sweden

3 Centre for Research on Adaptive Nanostructures and Nanodevices, Trinity College Dublin, Dublin 2, Ireland 4 Department of Physics, College of Science Sultan Qaboos University, Muscat, Oman

Abstract: Reflectance anisotropy spectroscopy (RAS) has been used to measure the optical

anisotropies of bulk and thin-film Fe3O4 (110) surfaces. The spectra indicate that small shifts

in energy of the optical transitions, associated with anisotropic strain or electric field gradients

caused by the (110) surface termination or a native oxide layer, are responsible for the strong

signal observed. The RAS response was then measured as a function of temperature. A distinct

change in the RAS line-shape amplitude was observed in the spectral range from 0.8 to 1.6 eV

for temperatures below the Verwey transition of the crystal. Finally, thin-film magnetite was

grown by molecular beam epitaxy on MgO(110) substrates. Changes in the RAS spectra were

found for different film thickness, suggesting that RAS can be used to monitor the growth of

magnetite (110) films in situ. The thickness dependence of the RAS is discussed in terms of

various models for the origin of the RAS signal.

Magnetite (Fe3O4) is a ferromagnetic oxide known for millennia. In recent years it has received

renewed attention due to its potential use in magnetic tunnel junctions. Nevertheless many aspects of

the materials fundamental electronic structure are still not fully understood, most prominently the

Vervey transition at 125K where the resistance of the material suddenly increases due to either a

structural phase transition or charge ordering.

Fe3O4 bulk samples, as well as epitaxial thin films have been investigated by reflectance anisotropy

spectroscopy, Raman spectroscopy and Magneto-Optical Kerr spectroscopy. On (110) surfaces we

demonstrated that RAS is sensitive to measure changes in the electronic structure of the material at

the Vervey transition and how the findings relate to current models of the phase transition. The

anisotropic and magnetic contributions were successfully separated, with the validity of the approach

being demonstrated by comparing the MOKE response with previous published data. The RAS

spectra resemble the derivative of the dielectric function of the samples, indicating that small shifts in

energy of the optical transitions, such as those associated with anisotropic strain, are responsible for

the anisotropic response. The changes in the RAS response across the Verwey transition have been

measured and could be useful in clarifying the electronic structure of the orbital ordering and charge

ordering if ab initio calculations of the RAS spectra of the room-temperature and low-temperature

phases become available. Finally, the RAS response of Fe3O4 (110) thin films grown on a MgO(110)

substrate has been measured. Significant changes in the spectral response occur as the film thickness

is varied between 15 and 75 nm. The results suggest that RAS can be used as a nondestructive simple

optical method to monitor the film growth in situ. All samples have been measured in ambient

conditions and the measured reflectance anisotropy originates either from a surface anisotropy of a B-

site terminated (110) surface, or an inherent anisotropy introduced by the surface oxidization of the

(110) surface.


This work is published in: K. Fleischer et.al., Phys. Rev. B 89, 195118 (2014)

1.0 1.5 2.0

-20

0

20

40

60

125 K

100 K

(a)

r/

r (1

0-3)

photon energy (eV)

245 K

15

20110 120 130

110 120 130

-5

0

5

[email protected]

sensor temperature (K)

1k

10k

Tv

Resis

tance (

)

Tv

RvT optical cryostat

[email protected]

(b)

RvT in CCR

r/

r (1

0-3)

sensor temperature (K)

RvT optical cryostat

5k

10k

15k

Tv

1.0 1.5 2.0 2.5

sa

mp

le te

mp

era

ture

(K

)

(c)

photon energy (eV)

-7.5

-5.5

-3.5

-1.5

0.50

2.5

4.5

r/

r (1

0-3)

100

120

140

160

180

200

220

240

Tv

(a) Real part of the RAS spectrum of bulk magnetite

measured between 240 and 100 K in 5-K steps. The

lower the temperature, the higher the RAS

amplitude. The circles highlight more sudden

changes in specific spectral ranges upon the Verwey

transition.

(b) Shows RAS transients taken at 1.4 and 0.8 eV

during a sample warmup with 2 K/min and

simultaneous resistance measurement as reference.

The true sample temperature was estimated by

offsetting the sensor temperature by the measured

different in Tv of each transient with the reference

measurement in the shielded electrical cryostat

(CCR).

(c) Difference between RAS spectra measured at

two adjacent temperature steps. A significantly

larger change in the RAS signal is measured just

above the Verwey temperature. Difference spectra

have been color coded for better visibility of the

effect.


4.9 Spin states and glassy magnetism in LaCo1-xNixO3 (0 ≤ x ≤ 0.5)

Vinod Kumara*, Rajesh Kumara, D.K.Shuklab , S.K.Arorac, I.V.Shvetsc, Kiran Singhd and Ravi

Kumare

aDepartment of Physics, National Institute of Technology, Hamirpur (H.P) – 177 005, India b Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany

cCRANN, School of Physics, Trinity College Dublin, Dublin 2, Republic of Ireland dTata Institute of Fundamental Research, Homi Bhaba Road, Colaba Mumbai-400005, India

eCentre for Materials Science & Engineering, National Institute of Technology, Hamirpur (H.P) 177005, India

Abstract: We investigated the effect of Ni substitution on electronic structure and magnetic properties

of perovskites LaCoO3 in the substitution range 0 ≤ x ≤ 0.5. A homovalent +3 state and spin state

transition of Co+3 has been observed upon Ni substitution in x-ray absorption measurements at the Co

and the Ni K-edges. Thermally driven spin state transition has been found to disappear with Ni

substitution. A change in nature of magnetic interactions from antiferromagnetic to ferromagnetic and

spin glass behaviour with substitution is observed in dc and ac magnetization measurements. Ni

substitution has been found to lower the average effective magnetic moment which has been ascribed

to the decrease in Co/Ni ratio. Changes in fine structure and magnetic properties due to Ni substitution

have been explained through the stabilization of intermediate spin state of Co+3 by the lattice expansion

induced changes in crystal field. The Jahn-Teller distortion is assumed to be suppressed in the expanded

lattice and possibility of antiferro-orbital ordering has been proposed for the ferromagnetic super-

exchange interactions Co+3(IS)-O-Co+3(IS).

The rare earth LaCoO3 has been studied extensively and is an established non-magnetic semiconductor.

Its magnetic behaviour around 100 K has been understood in terms of spin state transition of Co ion

from high spin (HS) to low spin (LS) ground state with decrease in temperature. However above ~100

K, Co ion is found in intermediate spin (IS) state with t52g e11g configuration showing JahneTeller (JT)

effect, which is responsible for its semiconducting behaviour. Spin state transition in Co can be

controlled by substitution either at the lanthanum or cobalt cation site and is responsible for a variety

of interesting physical phenomena like disorder induced metal insulator transitions, ferromagnetism,

negative giant magneto resistance (GMR) and reentrant spin glass behavior in LaNixCo1_xO3. But the

origin of magnetic interactions, especially the appearance of ferromagnetism in LaNixCo1_xO3 lacks

clear understanding as compared to manganites. To understand the origin of ferromagnetic component

observed in the compounds containing both Ni and Co we have chosen to study the effect of Ni

substitution on magnetic properties of LaCoO3.

X-ray absorption near edge spectroscopy (XANES) at the K-edge of Co and Ni in LaCo1_xNixO3,

confirmed homovalent +3 valence state for all the cations (La, Ni, Co) for all the compositions. Presence

of the Co and Ni, pre-edge features in the K-edge XAS spectra is sensitive to the crystal field around

an absorbing atom and help to understand the varying spin configurations while the valence state of

the atom remains unchanged. A pre-edge structure in Co K-edge, lying below 7715 eV has been zoomed

and shown in Fig.1. This pre-edge structure has been assigned to Co+3 1s-3d transitions and was

attributed to following two contributions. A low energy feature ‘A’ has been attributed to transitions to

the t2g orbitals, while feature ‘B’ at a higher energy was attributed to transitions to the eg orbitals. The

relative change of spectral weight of the features ‘A’ and ‘B’ with Ni substitution in the present study

is a signature of t2g to eg electron transfer i.e. a transition to an excited spin state, predominantly an IS

state. A weak pre-edge feature ‘a’ has also been observed in Ni K-edge spectra showing insignificant

variation with increasing Ni content. In La1_xNdxNiO3 and LaNi1_xFexO3 perovskites where Ni ion is in

+3 formal valence state, pre-edge is suggested as an indication of mixed ground state (3d7-3d8 L

configuration, where L represents a ligand hole).


Figure 1: XANES pre-edge spectra for Co K-edge of

LaCo1_xNixO3 (0.3 ≤ x ≤ 0.5). The arrows indicate the shift in the spectral weight with substitution.

Figure 2: Temperature dependence of the real and imaginary (inset on right side) components of the ac

susceptibility for LaCo1_xNixO3 (x = 0.3, 0.5), measured at 1.3, 13, 133, and 1333 Hz. Inset on left side shows best fit to power law.

To understand the detailed magnetic behaviour we performed dc- and ac-magnetization studies as a

function of temperature (T) in Ni substituted samples. For pure LaCoO3, the T dependent magnetization

below 100 K exhibits a sharp decrease of magnetization (under zero field cooled, ZFC conditions) down

to 23 K. which is attributed to spin state transition of Co+3 from HS state to LS state. Further absence of

saturation in magnetization hysteresis loop even at low temperature is a signature of canted AFM

behaviour. This indicates that all Co ions are not in low spin state but a fraction of them may exist in

excited spin state, which is consistent with our XAS results. In contrast to this, for x > 0 (Ni substituted

samples) the magnetization increases with decreasing T indicating that the thermally driven spin state

transition has disappeared. We also notice that the T dependence of field cooled (FC) magnetization

becomes less pronounced with increasing ‘x’ tends to be temperature independent for x ≥ 0.2. This

along with the peak broadening in ZFC curve, indicate the appearance of FM correlations with

increasing ‘x’. Our analysis of magnetic measurements shows that the strength of the FM interactions

increases with increase in Ni concentration, Presence of FM interactions in low T regime is related to

the fact that with Ni substitution, lattice expansion is observed. Which leads to a decrease in crystal

field (cf), favoring a transition of Co+3 ion from LS to IS, consistent with XAS results. In summary,

Ni substitution gives rise to simultaneous presence of short-range FM (between Co+3 (IS) ions) and

AFM (between Ni+3-Co+3 and Ni+3-O-Ni+3) interactions at low temperature.

This work is published in: Kumar, Vinod; Kumar, Rajesh; Shukla, D. K.; et al. MATERIALS

CHEMISTRY AND PHYSICS Volume: 147 Issue: 3 Pages: 617-622 Published: OCT 15 2014


4.10 Evidence for Spin glass state of NdCo1-xNixO3 (x =0.3-0.5)

Vinod Kumar1a), Rajesh Kumar1, Kiran Singh2*, S. K. Arora3, I. V. Shvets3

and Ravi Kumar4#

1Department of Physics, National Institute of Technology, Hamirpur, Himachal Pradesh 177 005, India 2Tata Institute of Fundamental Research, Homi Bhaba Road, Colaba Mumbai-400005, India

3CRANN, School of Physics, Trinity College Dublin, Dublin 2, Republic of Ireland 4Centre for Materials Science and Engineering, National Institute of Technology, Hamirpur, Himachal Pradesh

177 005, India

Abstract: Low-temperature magnetic properties of single phase NdCo1-xNixO3(x = 0.3-0.5) have been

studied using ac and dc magnetic susceptibility measurements. Nickel substituted samples have been

found to exhibit a different magnetic state at low temperature as compared to pristine NdCoO3. The

temperature dependent dc magnetization M (T) revealed the presence of a sharp cusp occurring at

characteristic temperatures TP, for x = 0.3, 0.4, 0.5. Below TP, clear effect of magnetic field can be seen

in M (T) curves and TP decreases with increasing magnetic field as well as Ni substitution content. The

isothermal magnetization measurements at low temperatures shows small unsaturated hysteresis loop

at lowest temperature (10 K). The ac susceptibility results show a clear frequency dependent feature.

These results are analyzed to distinguish superparamagnetic and spin glass behavior by using Néel-

Arrhennius, Vogel-Fulcher law and power law fitting. This analysis ruled out the superparamagnet like

state and suggests the presence of significant inter-cluster interactions, giving rise to spin-glass like

cooperative freezing.

Magnetic oxides with perovskite crystal structure have proven to be a fertile research area for physicists,

solid-state chemists, and materials scientists, due to the fascinating array of superconducting, magnetic,

and electronic properties they exhibit. Perovskite related cobalt and nickel oxides have attracted intense

interest because of the existence of unique property of spin-state transition and the peculiar magnetic

ground state of substituted cobaltites such as glassy ferromagnetism and giant magnetoresistance

(GMR) around the metal-insulator transition. Recently, we reported that NdCo1−xNixO3 samples

prepared by solid state reaction method exhibits single phase behaviour with orthorhombic Pbnm

symmetry. We found a composition dependent crossover from antiferromagnetic (AFM) to

ferromagnetic (FM) interactions at low temperatures. Low temperature FM component in substituted

samples has been attributed to the stabilization of Co+3 ions in intermediate-spin (IS) state. Further, the

temperature dependence of the ac magnetic susceptibility and zero field cooled (ZFC) magnetization

showed a characteristic maximum which is the signature of blocking/freezing process of the

superparamagnetic/spin glass systems. In order to clarify the origin of the spin glass behaviour we

performed a detailed study of the magnetic behavior of NdCo1−xNixO3 samples.

A series of single phase NdCo1-xNixO3 (0 ≤ x ≤ 0.5) samples prepared by conventional solid state

reaction method were used in this study and the temperature dependent magnetization with zero field

cooling (ZFC) and field cooled cooling (FCC) were performed at different magnetic fields using

physical properties measurement system (PPMS) of Quantum design. The ac-susceptibility (χ΄) was

measured in an ac field of 1 Oe at frequencies of 1.3,13, 133, and 1333Hz, using SQUID

(Superconducting Quantum Interference Device) magnetometer (Quantum Design).

Figure 1 shows the dc magnetization data for NdCo1-xNixO3 samples collected in ZFC and FC modes,

at three different magnetic fields, 0.5, 1 and 5 kOe. We find a broad peak/maxima around a

characteristic temperature (Tp) which is called Tp. A clear bifurcation of ZFC and FC curves can be seen

at Tirr (thermomagnetic irreversibility temperature). Furthermore, with increasing magnetic field, Tp and

Tirr shifted to lower temperatures and ZFC peaks become broader and the bifurcation between ZFC and

FC curves also decreases. At 5 kOe magnetic field, this peak is completely smeared out and the

bifurcation of ZFC and FC curves also disappears, i.e. showing no thermomagnetic irreversibility.


These different features observed in NdCo1-xNixO3(x = 0.3, 0.4, 0.5), indicate the presence of a

spin/cluster-glass or a super-paramagnetic state at low temperature.

Figure 1: Temperature dependence of magnetization at different magnetic fields for NdCo1-xNixO3 (0.3 ≤ x ≤ 0.5). The solid symbols represent the data in ZFC mode and open symbols in FC mode.. Figure 2 Temperature dependence of the in-phase and out-of-phase (insets) components of the ac susceptibility for NdCo1-xNixO3 (0.3 ≤ x ≤ 0.5), measured at 1.3, 13, 133, and 1333 Hz.

Further evidence of such type of magnetic state can be found by studying the isothermal magnetization

measurements performed at different temperatures (not shown). We find that above 10K, magnetization

is almost linear with respect to the applied field. On the other hand at temperature below Tp i.e. at 10

K, a clear hysteresis loop appears at lower fields and a linear behaviour is seen at high fields. This

suggests the presence of a glassy state at low fields, which is due to the competing FM/AFM

interactions. Since at high fields the frozen moments tend to orient with the applied field, the glassy

state disappears. The M-H curves at 10 K show non zero remnant magnetization, but there is no

saturation even at higher magnetic field. It suggests the presence of a non-saturating (AFM) component.

The existence of such non-ferromagnetic component along with the ferromagnetic component is in

agreement with the cluster model of other disordered cobaltites,

We find evidence of spin glass state from the temperature dependence of the real, χ'(T) and imaginary,

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