Fachgebiet 3D-Nanostrukturierung, Institut für Physik

Contact: yong.lei@tu-ilmenau.de; fabian.grote@tu-ilmenau.de

Office: Heliosbau 1102, Prof. Schmidt-Straße 26 (tel: 3748)

www.tu-ilmenau.de/nanostruk

Vorlesung: Mittwochs (U), 9 – 10:30, C 108 Übung: Mittwochs (G), 9 – 10:30, C 108

Yong Lei & Fabian Grote

Techniken der Oberflächenphysik

(Technique of Surface Physics)

• Objects are contacted via their surface.

• Chemical reactions: Catalysis, electrodes of batteries

• Friction and Lubrication

• Nanotechnology is Surface Physics

Surface Physics - Why?

Main contents of this course

Surfaces become more important for smaller objects

Almost all aspects of physical properties are related to their

surfaces (nano-surfaces):

Optical properties (band-gap, defect emissions)

Sensing properties (gas, chemical and bio-sensors)

Field-emission properties

Devices (super-capacitors, sensors, optical …)

Class 1 (an introduction)

A general introduction of fundamentals of surface physics and

their most important points

(how to fabricate the surfaces especially within nano-sized range)

(how to characterize the surfaces)

(what the main properties of surfaces)

(what’s the main applications of surfaces)

Kai M. Siegbahn (Swedish) Nobel Prize 1981 Physics Developing the method of Electron Spectroscopy for Chemical Analysis, now usually described as X-ray photoelectron spectroscopy (XPS)

G. Binnig (German) & H.

Rohrer (Swiss)

Nobel Prize 1986 Physics

Designing of the scanning

tunneling microscope (STM) →

SPM systems

Nobel Prizes with research related to surface physics and structures:

Gerhard Ertl (German)

Nobel Prize 2007 Chemistry

for his studies of chemical

processes on solid surfaces

Albert Fert (French) & Peter

Grünberg (German)

Nobel Prize 2007 Physics

Interfaces - Giant

magnetoresistance effect (GMR)

which is a breakthrough in gigabyte

hard disk drives.

7

Konstantin Novoselov & Andre Geim (Russian)

Nobel Prize 2010 Physics

for groundbreaking experiments regarding the two-dimensional

graphene

For carbon nanotubes – CNT

(by Ijima in 1991) and the

equally important discovery

of inorganic fullerene

structures (by Tenne)

1996: Curl, Kroto, Smalley

1985 or1986: fullerenes (C60,

bucky balls);

2010: Geim, Novoselov 2005-

2007: 2D graphene

The allotropes of carbon:

hardest naturally occurring

substance, diamond

one of the softest known

substances, graphite.

Allotropes of carbon: a) diamond; b)

graphite; c) lonsdaleite; d–f) fullerenes

(C60, C540, C70); g) amorphous carbon; h)

carbon nanotube.

from http://en.wikipedia.org/wiki/Carbon.

Most important structural aspects of nanostructures:

Surface

Extremely large surface area (very large surface/volume

ratio): when the dimensions decrease from micron level to

nano level, the surface area increases by 3 orders in

magnitude. This will lead to much improved and enhanced

physical properties (sensing, optical, catalysis ...):

Cube – Cubic structures – divided into 8 pieces – surface area

2 times(doubled)

Cube – Cubic structures – divided into 1000 pieces – surface

area 10 times

Surface charge properties of structures are the major point of functions of

sensing devices.

The main reason of the high interest in the use of nanostructures is the

large surface-to-volume ratio, so that more surface atoms to participate in

the surface reactions

The electronic, chemical, and optical processes on metal oxides concerning the

sensing, which is benefit from reduction in size to the nano range (Kolmakov, Annu

Rev Mater Res 2004)

Surface plasmon resonance: plasmons propagate in x- and y-directions along

metal-dielectric interface (distances ~ tens to hundreds microns), and decay in z-

direction. The interaction between surface-confined EM wave and surface layer

leads to shifts in the plasmon resonance condition.

Localized surface plasmons: light interacts with particles much smaller size than

incident light wavelength. This leads to a plasmon that oscillates around

nanoparticles with a LSPR frequency. The shape, size, material, and local

dielectric properties—all of which determine the LSPR wavelength.

Schematic of (a) a surface plasmon resonance and (b) a localized surface

plasmon resonance. (Katherine A., Annu. Rev. Phys. Chem. 2007. 58, 267.)

Characterization of surfaces

An appropriate characterization will play a crucial role in determining

various surface structures and their properties (especially for nano-

surfaces.

Three broadly approved aspects of characterization are

1. Morphology

2. Crystalline structure

3. Chemical analysis

SEM: Scanning Electron Microscope; STM/AFM: Scanning Tunneling

Microscope/Atomic Force Microscope

TEM: Analytical Transmission Electron Microscope

X-Ray: X-ray Morphology; IP: Image Processing; LM: Lightweight

Morphology; RBS: Rutherford Backscattering Spectrometry (Kelsall et

al., Nanoscale science and technology. 2005)

TEM: Analytical Transmission Electron Microscopy; AES: Auger

Electron Spectrometer; XRD: X-ray Diffraction; RBS: Rutherford

Backscattering Spectrometry; XPS: X-ray Photoelectron Spectrometer;

(Kelsall et al., Nanoscale science and technology. 2005)

SEM: Scanning Electron Microscopy; ATEM: Analytical Transmission

Electron Microscopy;

AEM: Auger Electron Microscopy. XRD: X-ray Diffraction; LEED: Low-

energy electron diffraction; RBS: Rutherford Backscattering

Spectrometry (Kelsall et al., Nanoscale science and technology. 2005)

Surface patterns in nature

Structural color – function of surface patterns

1 µm

butterfly

peacock

packing of melanin cylinders (provided by L Chi)

Surface patterns and structures (artificial)

and their applications in diverse (micro-electronic) devices

From Intel Homepage, Public Relations

Dual-core CPU

feature-size 45 nm

Surface Nano-Patterning

Fabrication of surface nanostructures

Memory devices with high integration density;

Field emission devices;

Sensors with high sensitivity;

Optical devices with tunable properties

What is an excellent surface nano-patterning technique?

1. Ability to prepare surface patterns within the nanosized range;

2. Well-defined surface nano-patterns;

3. Large pattern area – high throughput;

4. A general process – applicable;

5. Low cost. Perfect ?

Electron-beam lithography

Excellent structural controlling

Low throughput

High equipment costs

Imprint technologies

High throughput

Wear

Structures with low

aspect ratio

Self assembly Low costs

High throughput

Limited class of materials

Low structural controlling

Some surface nano-patterning techniques

in fabricating ordered surface nanostructures

Alternative method that combines these advantages and is applicable

for a broad range of surface nanostructures ?

UTAM (ultra-thin alumina mask) surface nano-patterning:

Template-based surface nano-fabrications

Porous Alumina Membranes (PAMs)

Interesting and useful features:

• highly ordered pore arrays +

large area

• Nanometer-sized pores

• High aspect ratio

• size controllable (10 – 400 nm)

Configuration diagram of the PAMs

Porous Alumina Membranes (PAMs)

(a) (b)

Regular arrays of short (a) and long Ni nanowires (b) after the removal of PAM, the

diameter is about 90 nm, the length is about 800-1000 nm (a) and 3-4 μm (b), respectively.

thus the aspect ratio of the nanowires are about 10 (a) and 40 (b), respectively.

Motivation

Use ultra-thin ordered porous alumina as evaporation or etching masks, and

transfer the regularity of the pore arrays to the nanostructure arrays on

substrates.

UTAM surface nano-patterning technique

Fabrication of Highly Ordered Nanoparticle Arrays Using Ultra-thin

Alumina Mask (UTAM)

Fabricating ultra-thin alumina masks (UTAM) on Al foils and then

mounting them onto the surface of silicon wafers

Al foil

First alumina layer

Al foil Al foil

Second alumina layer

Al foil

Ultra-thin alumina mask

Si wafer

Ultra-thin alumina mask

Fabrication process

Fabrication of the nanodot arrays

Ultra-thin alumina mask

Si wafer

Ultra-thin alumina mask

Si wafer

Ultra-thin alumina mask Nanoparticle array

Si wafer Si wafer

Highly ordered CdS nanodot arrays, UTAMs and CdS top layer on

the surface of the UTAM.

CdS replicated mask

Alumina

CdS nanodots

Nanodots (top view, Pd) Nanoholes (top view, Si)

Tuning of the shapes and sizes of UTAM-prepared nanostructures

To control the structural parameters (shape, size and spacing) is

very important

Controllable sizes and shapes:

The pore diameters of the UTAMs can be adjusted from about 10 to

400 nm to yield nanoparticles of corresponding size.

Nanometer-sized discs, hemispheres, hemi-ellipsoids, and

conics (by changing the aspect ratio of the pores of the UTAMs,

and the amount of material deposited through the UTAMs).

Highly ordered nano-disc arrays. Pore diameter, cell size and thickness of the

UTAM are about 80, 105, and 160 nm, respectively. The aspect ratio of the

apertures of the UTAM is about 1:2. The average height and size of the nano-discs

are approximately 1.5 and 80 nm, respectively.

Highly ordered nano-disc arrays

AFM Section Analysis of the nano-discs, the average height and size of

the nano-discs are approximately 1.5 and 80 nm, respectively.

Highly ordered nano-hemisphere arrays. Pore diameter, cell size and thickness of

the UTAM are about 80, 105, and 240 nm, respectively. The aspect ratio of the

apertures of the UTAM is about 1:3. The average height and base diameter of the

nano-hemispheres are approximately 35-40 and 75 nm, respectively.

Highly ordered nano-hemisphere arrays

AFM Section Analysis of the nano-hemisphere. To accurately reflect the shape

of the nanoparticles, we used the same dimension scale for the horizontal and

vertical coordinates. The average height and base diameter of the nano-

hemispheres are approximately 35-40 and 75 nm, respectively.

Ordered nano-hemiellipsoid arrays. Pore diameter, cell size and thickness of the

UTAM are about 80, 105, and 310 nm, respectively. The aspect ratio of the

apertures of the UTAM is about 1:4. The average height and base diameter of the

nano-hemiellipsoids are approximately 50-55 and 65 nm, respectively.

Highly ordered nano-hemiellipsoid arrays

AFM Section Analysis of the nano-hemiellipsoids. To accurately reflect the shape

of the nanoparticles, we used the same dimension scale for the horizontal and

vertical coordinates. The average height and base diameter of the nano-

hemiellipsoids are approximately 50-55 and 65 nm, respectively.

Ordered nano-conic arrays. Pore diameter, cell size and thickness of the UTAM

used in the fabrication process are about 80, 105, and 650 nm, respectively. The

aspect ratio of the apertures of the UTAM is about 1:8. The average height and

base diameter of the nano-conics are approximately 55-60 and 60 nm, respectively.

Highly ordered nano-conic arrays

AFM Section Analysis of the nano-conics. To accurately reflect the shape of the

nanoparticles, we used the same dimension scale for the horizontal and vertical

coordinates. The average height and base diameter of the nano-conics are

approximately 55-60 and 60 nm, respectively.

Schematic outline of the shape and size adjustment of nanoparticles by changing

the aspect ratio of the apertures of the UTAMs and the amount of material deposited

through the UTAMs. (Y Lei, et al., Chem. Mater., 17, 580, 2005.)

Closure-effect Shadowing-effect

Aspect ratio and deposition duration

Attractive features of the UTAM surface nano-patterning:

Large pattern area (> 1cm2) and high throughput;

high density of the surface nanostructures (1010 - 1012 cm-2);

a general process to prepare different patterns (semiconductors,

metals);

well-defined nanostructures;

low equipment costs.

Tunable properties of UTAM-prepared surface patterns

based on the adjustment of the structural parameters

1. Ordered arrays of metal/semiconductor core-shell nanodots

with tunable core-shell structures and optical properties.

(Y Lei, et al., J. Am. Chem. Soc., 127, 1487, 2005).

2. Ordered CdS nanodot arrays with adjustable luminescence

properties.

(Y Lei, et al., Appl. Phys. Lett., 86, 103106, 2005).

3. Ordered arrayed metal oxide nanodots with the similar size,

shape, crystalline structure and orientation. This work is a step

towards the goal of achieving iso-nanoparticle arrays and full

property tuning.

(Y Lei, et.al., Nanotechnology, 16, 1892, 2005).

4. Large-scale ordered carbon nanotube (CNT) arrays initiated

from highly ordered metallic arrays on silicon substrates.

(Y Lei, et al., Chem. Mater., 16, 2757, 2004).

Device applications of UTAM-prepared surface patterns

1. Metal–insulator–semiconductor (MIS) memory device based on

ordered Ge nanodot arrays.

(Z Chen, Y Lei, et al., J. Cry. Grow., 268, 560, 2004).

2. Fe/Pt multi-layer nanodot arrays with interesting magnetic

properties (J Ellrich, Y Lei, H Hahn, patent application, 2008).

3. Other possible device applications (Lei Y, et al., Adv. Eng. Mater., 9, 343,

2007).

A challenging technical point

for UTAM technique to realize quantum-sized surface structures (below 10-

20 nm)

Minimum pore diameter of UTAMs is about 10 nm → impossible to

synthesize surface structures smaller than 10 nm;

the arrangement regularity and monodispersity of the pores are poor

when the pore diameter is smaller than 20 nm;

Prevents the fabrication of surface structures within or close to the

quantum-sized range (below 10-20 nm) using the UTAM patterning

technique

largely limits the investigation of the quantum confinement effect using

the UTAM surface nano-patterning process.

Well-controlled pore-opening process to the barrier layer of UTAMs

realizing pore-opening and surface nanostructures within the quantum-

sized range

An alumina barrier layer between the pore bottom and the aluminum foil of as-

prepared PAMs. It has a hemispherical and scalloped geometry. Using acidic

etching solutions, the barrier layer can be thinned and finally removed.

UTAMs used in the pore-opening process were prepared using 0.3 M modulated

H2SO4 solutions (glycol/water: 3:2) under 25 V at 4 oC,

cell-size 60 nm, pore-diameter 20 nm, barrier layer thickness 20 nm

Pore-opening process was carried out using a 5 wt% H3PO4 solution at 30 oC

Before the etching, UTAMs were covered by a protecting PMMA layer on the top

so that the H3PO4 solution only etch on the bottom surface.

Before etching in 5 wt% H3PO4 solution (30 oC)

18.76

[nm]

0.00200.00 nm 500.00 x 500.00 nm

o min 1:1

(a)

After 8 min etching in 5 wt% H3PO4 solution (30 oC)

20.00

[nm]

0.00200.00 nm 500.00 x 500.00 nm

10 mins surface

(b)

After 18 min etching in 5 wt% H3PO4 solution (30 oC)

The pore diameter is about 10 nm

46.16

[nm]

0.00200.00 nm 500.00 x 500.00 nm

2-18mins edge

(d)

After 15 min etching in 5wt% H3PO4 solution (30 oC)

The pore diameter is about 5 nm

24.08

[nm]

0.00200.00 nm 500.00 x 500.00 nm

10min 1:1

(c)

After 24 min etching in 5 wt% H3PO4 solution (30 oC)

The pore diameter is about 17 nm

40.98

[nm]

0.00200.00 nm 500.00 x 500.00 nm

2-24mins

(e)

After 30 min etching in 5 wt% H3PO4 solution (30 oC)

The pore diameter is about 22 nm

54.12

[nm]

0.00200.00 nm 500.00 x 500.00 nm

h20 30mins

(f)

UTAMs with pore-openings smaller than 20 nm can be used to

fabricate ordered arrays of quantum dots

Using an UTAM with pore-openings ~ 17 nm (24 min etching time), large-

scale (~ 2 cm2) ordered arrays of Au quantum-dots were prepared on a Si

wafer.

Based on the histogram and its Gaussian fit curve of the measured

diameters of the Au nanodots, the average diameter of the nanodots is

about 17.02 nm.

(a) (b)

Barrier layer 5 nm

10 nm 17 nm

Quantum dot array

Small 2010, 6 (5), 695-699.

UTAM surface nano-patterning

Three-Dimensional Surface Nano-Patterning: Concepts, Challenges

and Applications (motivations)

Multifunctional surface nano-structures

From Intel Homepage, Public Relations

Dual-core CPU

feature-size 45 nm

Three-Dimensional Surface Nano-Patterning: Concepts, Challenges

and Applications (motivations)

Multifunctional surface nano-structures

An efficient evolution from 2-D to 3-D surface nano-patterning:

Change from nanodots or nanorings to nanowires or nanotubes

One of the most attractive advantages of nano-

materials (extremely large surface area) is

missing in the existing 2-D surface nano-patterns

Large contacting influence from the substrate →

very large signal noises → degrades device

performance

Only way to increase the device density is to

decrease the pattern size

nanodots

nanorings

Three-Dimensional Surface Nano-Patterning: Concepts, Challenges

and Applications (motivations)

Multifunctional surface nano-structures

An efficient evolution from 2-D to 3-D surface nano-patterning:

Change from nanodots or nanorings to nanowires or nanotubes

A much larger surface area

Much lower contacting influence from the

substrate

Possible to increase the device density in the

lateral direction

nanowires

nanotubes

From 2D to 3D surface patterns using templates

From 2D to 3D surface patterns using templates

Large-scale free-standing metallic nanowires for 3D surface patterns: (Left): top view of

nanowire array of an area of about 775 μm2. (Right): high regularity of nanowire arrays.

Schematic of the addressing system (only shows an array of 3 × 3)

Addressing System for 3-D surface nano-patterns

with nano-scale resolution

3D Surface Nano-Patterning: Addressing

nanowire ‘1A’

Thank you and have a nice day!