Fachgebiet 3D-Nanostrukturierung, Institut für Physik

Contact: yong.lei@tu-ilmenau.de

yang.xu@tu-ilmenau.de; liying.liang@tu-ilmenau.de

Office: Heisenbergbau (Gebäude V) 202, Unterpörlitzer Straße 38 (tel: 3748)

www.tu-ilmenau.de/nanostruk

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

Prof. Yong Lei & Dr. Yang Xu (& Liying Liang)

Techniken der Oberflächenphysik (Techniques of Surface Physics)

Contents of Class 7

(how to characterize surfaces)

STM and AFM

(how to characterize morphologies, chemical analysis, and

crystalline structures)

SEM and TEM

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)

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

the surface of the UTAM.

CdS replicated mask

Alumina

CdS nanodots

UTAM surface nano-patterning technique

(how to characterize surfaces)

STM and AFM

SPM

Scanning Tunneling Microscopy (STM)

Atomic Force Microscopy(AFM)

Scanning Probe Microscopy (SPM)

Gerd Binnig Heinrich Rohrer

(born 20 July 1947) German physicist (born June 6, 1933) Swiss physicist

They shared half of the 1986 Nobel Prize in Physics with for the design of STM (the other half of the Prize was awarded to Ernst Ruska).

Scanning Tunneling Microscopy

• STM

The principle of STM

Pro

be

Sample

http://www.youtube.com/watch?NR=1&v=lR9-O_uwomc&feature=endscreen

It ~ e-2kd

The structure of STM

http://www.youtube.com/watch?v=47UgMpXFVj4

The manipulation of STM

Constant current image (topography) of an atomic layer iron on W(001) with defects and atoms.

The Application of STM

• 1. Atomic Microscope

Nickel (110)

Platinum (111)

High performance STM image showing atomic resolution on Si(111) 7nm x 7nm

cobalt sulfide "nanoflower" structure synthesized on a Au(111) surface 9nm x 9nm

• 2. Manipulation of single atoms or single molecules

Desorption: Similar to vertical manipulation, but desorb individual absorbate directly into surrounding gas phase.

Lateral manipulation: Transfer of atoms/molecules along surface using attractive/ repulsive forces between tip and absorbate.

Vertical manipulation: reversible transfer of atoms/molecules between surface & STM tip using additional electronic/ vibrational excitation of absorbate.

positioned 48 iron atoms into a circular ring in order to "corral" surface state electrons and force them into "quantum" states.

• 3. Single-molecular chemical reactions

Synthesis: Selective bond formation between two molecular units using lateral manipulation, followed by electronic/vibrational excitation.

Dissociation: Selective bond breaking within a molecule

The advantages and disadvantages of STM

• Advantages: 3D profile of a surface, to examine roughness, surface defects and determining molecules such as size and conformation.

Other advantages of STM include:

• much more details than many other microscopes, better understand on a molecular level.

• Versatile. STM can be used in ultra high vacuum, air, water and other liquids and gasses.

• STM can be operated in temperatures as low as zero Kelvin up to a few hundred degrees.

• Disadvantages:

3 major downsides to using STMs:

• Less effectiveness. STM is a very specific technique that requires a lot of skill and precision.

• STM require very stable and clean surfaces, excellent vibration control and sharp tips. And STM only can be used to scan good conductor samples (no easy surface oxidized)

• STMs use highly specialized equipment that is fragile and expensive.

Atomic Force Microscope (AFM)

• 1986 --- Binnig, Quate and Gerber invented the first atomic force microscope

The principle of AFM

•When tip closes to sample,

mainly 2 forces operate.

•Typically forces contributing to

the movement of AFM cantilever

are coulombic and van der Waals

interactions.

Coulombic force: This strong ,short range

repulsive force is from electrostatic repulsion

by the electron clouds of tip and sample.

This force increases as the separation

decreases.

•Van der Waals force: longer range attractive force, which is felt at separations of up to

10 nm or more. As tip gets closer to the sample, this attraction increases.

The combination of the 2 forces. The

repulsive force causes cantilever to bend as tip

is very close to surface.

The structure of AFM

Feedback System

Position Sensing Part

Force Sensing Part

Position Sensing photodetctor

Three primary imaging modes:

• 1. Contact AFM

< 0.5 nm probe-surface separation

• 2. Tapping mode AFM (Intermittent contact )

0.5-2 nm probe-surface separation

• 3. Non-contact AFM

0.1-10 nm probe-surface separation

• 1. Contact AFM

•In contact mode the tip contacts

the sample surface.

•The detector monitors the

changing cantilever deflection and

the force is calculated using

Hooke’s law:

•The feedback circuit adjusts the probe height to try and maintain a

constant force and deflection on the cantilever. This is known as the

deflection setpoint.

F = − k x (F = force, k = spring constant, x = cantilever deflection)

• 2. Tapping mode AFM

•In tapping mode cantilever

oscillates at or slightly below its

resonant frequency. The

amplitude of oscillation typically

ranges from 20 to 100 nm. Tip

slightly “taps” on sample surface

during scanning.

•The oscillation is also damped when the tip is closer to the surface.

Hence changes in the oscillation amplitude can be used to detect the

distance between the tip and the surface. The feedback circuit adjusts

the probe height to try and maintain a constant amplitude of oscillation

i.e. the amplitude setpoint.

• 3. Non-contact AFM

•In non-contact mode

cantilever oscillates near

sample surface, but does not

contact it. The oscillation is at

slightly above the resonant

frequency. Van der Waals

forces decrease the resonant

frequency.

•In ambient conditions , the adsorbed fluid layer is often much thicker than the region where van der Waals forces are significant. So the probe is either out of range of van der Waals force, or becomes trapped in the fluid layer. Therefore non-contact mode AFM works best under ultra-high vacuum conditions.

The Properties of the different operation modes in AFM.

Advantage Disadvantage

Contact Mode

- High scan speeds - “Atomic resolution” possible - Easier scanning of rough

samples (with large changes in vertical topography).

• Lateral forces can distort the image • Combination of these forces reduces spatial resolution and can cause damage to soft samples.

Tapping Mode

- Higher lateral resolution (1 to 5 nm). - Lower forces and less damage to soft samples in air.

• Slower scan speed than in contact mode

Non-

contact Mode

- Both normal and lateral forces are minimised, so good for very soft samples

- Can get atomic resolution in a UHV environment

• Slower scan speed • Lower lateral resolution, limited by tip-sample separation. •Usually only applicable in extremely hydrophobic samples.

Advantages and Disadvantages of AFM Modes

• 1. Imaging

The application of AFM

AFM 3D image The figure is 800 nm wide and 10 nm high Pd/Fe/Pd thin film dots.

NCAFM image of the Ge/Si(105) surface, 4.2 nm x 4.2 nm

PMMA spheres scaning range 45x45 μm

AFM image of human plasma cell

• 2. Measuring forces (and mechanical properties) at the nanoscale

An AFM tip measuring force, move a cobalt atom on a crystalline surface. The ability to measure the exact force and to move individual atom is one of the keys to design and construct small structures. (Credit: Image courtesy of IBM) http://www.youtube.com/watch?v=BUq2bQkL1zo

• 3. As a nanoscale tool

Bending, cutting and extracting soft materials (Polymers, DNA, nanotubes) under high-resolution image controlling

grabbing and holding a nanoparticle in position

Manipulation of a nanotube on a silicon substrate. The AFM tip is used to create the Greek letter "theta" from a 2.5 micronmeter long nanotube

A single nanotube (in red) originally on an insulating substrate (SiO2, in green) is manipulated in a few steps onto a tungsten film (in blue), and finally is stretched across an insulating tungsten oxide barrier (in yellow).

Advantages :

1) True and high-resolution 3D surface images;

2) not require special sample treatments;

3) not require a vacuum (can be in both air and liquid);

4) could be used for organic materials.

Disadvantages:

1) imaging feature size much smaller than electron microscopes;

2) slow in scanning an image, unlike an electron microscope which does it in almost real-time.

3) not true sample topography, but the interaction of the probe with the sample surface

4) expensive tips

The advantage and disadvantage of AFM

Tip convolution----Tip-Related Artifacts

dots appear wider, pores narrower than the real size.

Radius of tip end determine the resolution of the scan

http://www.youtube.com/watch?v=fivhcWYEtkQ

Thank you and have a nice day!