Fachgebiet Angewante Nanophysik, Institut für Physik

Contact: yong.lei@tu-ilmenau.de

yang.xu@tu-ilmenau.de

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

www.tu-ilmenau.de/nanostruk

Vorlesung: Donnerstag (U), 17:00 – 18:30, F 3001 Übung: Donnerstag (G), 17:00 – 18:30, F 3001

Prof. Yong Lei & Dr. Yang Xu, Dr. Huaping Zhao

Techniken der Oberflächenphysik (Techniques of Surface Physics)

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: 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)

Techniques of Surface Physic

---STM and AFM(SPM)

Visible by eyes

10-3 ~~~ mm

Optical microscope

10-6 ~~~ µm

SEM and TEM

10-9 ~~~ nm

m

SPM

The tunneling phenomenon in quantum mechanics: quantum tunneling effect: an electron's penetration of an energy barrier, even though the electron's energy is below the height of barrier.

Scanning Tunneling Microscope

Scanning Tunneling Microscope

• tunneling phenomenon:

The principle of STM

Pro

be

Sample

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 the scanning tunneling microscope (STM).

The manipulation of STM

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

It ~ e-2kd

The structure of STM

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

• STM

The Application of STM

1. Atomic imaging – most important function of STM

Nickel (110)

Platinum (111)

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

Is the atoms or molecules observed by STM is 100%

the real image of atoms or molecules?

those form of the electron cloud of the atom or molecule

2. Manipulation of single atoms and single molecules

Since the invention of STM, people want to "hand" manipulate atoms by

the STM probe to achieve the dream of manipulate atoms. In the early

1990s, IBM's scientists use Xe atoms to write three letters "IBM" on Ni

(110) surface, first demonstrated the possibility of the manipulation of

individual atoms at low-temperature STM.

The dream of directly manipulate atoms

Desorption: Similar to vertical manipulation, but desorption of atom/molecule into surrounding gas phase.

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

Vertical manipulation: Transfer of atoms/molecules between surface and STM tip using electronic/ vibration excitation by tunneling.

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

3. Single-molecular chemical reactions

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

Dissociation: Selective bond breaking within a molecule by tunneling processes.

Schematic of bond formation in Au-PTCDA switch: atom and molecule are both negatively charged and stabilized by repulsive interaction (a). By tunneling out of molecular resonance, PTCDA is neutralized, and electrostatic repulsion is weakened (b). This makes Au atom moving towards molecule and form the bond.

4. Construct molecular-level electronic device

Terbium atom (red) is sandwiched between two organic molecules (grey and blue) to form a single-molecule magnet.

The advantages and disadvantages of STM

• Advantages: allows to detect many features, including roughness, defects, and to determine size and conformation of molecules. Other advantages: • obtain much more details than other microscopes, for a

better understanding of research topics at molecular level. • STM is versatile, it can be used in ultra-high vacuum, air,

water and other liquids, and gas. • can be operated in temperatures from zero Kelvin up to a few

hundred degrees.

• Disadvantages:

• It is difficult to use STM effectively. It is a very specific technique that requires a lot of skill and precision.

• STM requires very stable and clean surfaces, excellent vibration control and sharp tips. STM only can be used to scan conducting samples which are not easily oxidized.

• STM uses highly specialized equipment that is very expensive.

Atomic Force Microscope (AFM)

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

Beam-deflection measurement

The principle of AFM

When a tip is close to sample,

typically two forces operate:

Coulombic and van der Waals

interactions.

Coulombic Interaction: strong, short range

repulsive force between tip and sample.

This repulsion increases as the distance

decreases.

Van der Waals interactions: long range attractive force, which become

obvious at distance of down to 10 nm.

The combination of these interactions

results in a force-distance curve

•As tip is brought towards

the sample, van der Waals

forces cause attraction.

• As tip gets closer to the

sample this attraction

increases.

•However at very small

separations the repulsive

coulombic force becomes

dominant. The repulsive

force causes the cantilever

to bend when the tip is

getting closer to the

surface.

Laser light from a solid-state diode is reflected at back of cantilever and

collected by a position-sensitive detector (2 close-spaced photodiodes).

Angular displacement of cantilever results in 1 photodiode collecting more light

than the other photodiode, producing an output signal, which is proportional to

the deflection of cantilever.

Its high sensitivity and simple operation, and cantilevers do not require

electrical contacts or other special treatments, and can therefore be fabricated

relatively cheaply.

Beam-deflection measurement

The structure of AFM

Feedback circuit

Position Sensing Part

Force Sensing Part

Position Sensing photodetector

The manipulation of AFM

Constant-height scan

• Two scanning processes:

Constant-force scan

• Three AFM imaging modes:

• 1. Contact AFM

• < 0.5 nm probe-surface separation

• 2. Tapping mode AFM

• 0.5-2 nm probe-surface separation

• 3. Non-contact AFM

• 0.1-10 nm probe-surface separation

• 1. Contact AFM

•tip contacts the sample

surface.

•The photo detector monitors

the changing of cantilever

deflection and the force is

calculated using Hooke’s law:

•The feedback circuit adjusts the probe height to maintain a constant

force and deflection on the cantilever, i.e. deflection setpoint.

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

Constant-force scan

• 2. Tapping mode AFM (Intermittent contact )

•cantilever oscillates at (or

slightly below) its resonant

frequency. Oscillation

amplitude ranges in 20-100

nm. Tip lightly touch

(“taps”) on sample surface

during scanning.

•Oscillation decreases when tip is closer to surface. Hence changes of

oscillation amplitude are used for measuring tip/surface separation.

Feedback circuit adjusts probe height to maintain a constant amplitude

of oscillation. i.e. the amplitude setpoint.

• Resonant frequency of cantilever dependent on tip/surface separation.

Tapping mode in air:

A small piezoelectric crystal on AFM tip holder makes the cantilever oscillate up and down.

Tapping mode in liquids:

Tapping mode operation in liquids is a very useful tool for biologists.

• 3. Non-contact AFM

• cantilever oscillate near sample

surface, but does not contact it.

Oscillation is at slightly above

resonant frequency. Van der Waals

force decrease resonant frequency.

This decrease of resonant

frequency causes oscillation

amplitude to decrease.

•Normally adsorbed fluidic layer is much thicker than the dominant

region of van der Waals force - probe is either out of range of van der

Waals force, or becomes trapped in fluidic 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 distort image • cause damage to soft samples.

Tapping Mode

- High lateral resolution (1 to 5 nm). - Lower forces, less damage to soft samples. - Almost no lateral forces.

• Slower scan speed than contact mode

Non-contact Mode

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

- Can get atomic resolution in UHV environment

• Slower scan speed than tapping and contact modes • Usually only applicable in extremely hydrophobic samples with a minimium fluidic layer.

Advantages and Disadvantages of AFM Modes

Electrostatic force microscope (EFM)

Variety of AFM

EFM is a dynamic non-contact atomic force microscopy where the

electrostatic force is probed. This force arises due to the attraction

or repulsion of separated charges. It is a long-range force and can

be detected 100 nm or more from the sample.

A conductive tip and sample separated with a distance z. A bias

voltage between tip and sample is applied forming a capacitor C.

The capacitance depends on the geometry of the tip and sample.

The total energy stored in the capacitor is U = ½ C⋅ΔV2.

The z component of the force (the force along the axis connecting

the tip and sample) is thus:

Felectrostatic = ½ C/z ΔV2

Magnetic force microscope (MFM)

MFM is a variety of AFM in which a sharp magnetized tip scans a

magnetic sample; the tip-sample magnetic interactions are detected

and used to reconstruct the magnetic structure of the sample

surface. Many kinds of magnetic interactions are measured by MFM.

MFM scanning often uses non-contact AFM mode.

Tip curvature radius: no coating AFM tip

10 nm; coating AFM tip 35-50 nm.

Coating layer: normally two kinds of

metals, sometimes just one metal.

High resolution AFM tip:

Tip curvature radius: 1 nm.

• 1. Imaging

The application of AFM

AFM 3D image of a detail of the free surface of an artificial pattern

The figure illustrates 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 scanning range 45x45 μm

AFM image of human plasma sample (fibrinogen)

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

Illustration of an AFM tip measuring the force to move a cobalt atom on a crystalline surface. (Credit: Image courtesy of IBM) http://www.youtube.com/watch?v=BUq2bQkL1zo

Single-molecule force microscope uses a target molecule at AFM tip end to probe surface molecules with a strong attractive force. Force measurements are mapped as an image.

• 3. As a nanoscale tool

bending, cutting and extracting soft materials (Polymers, DNA, nanotubes), at nano-scale

grab and hold a nanoparticle in a position

Manipulating nanotube on Si substrate. AFM tip creates Greek letter "theta" from a 2.5 μm long nanotube

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

Advantages :

1) high-resolution 3-D surface images

2) not require special sample treatments (no sample's destruction)

3) Usually not require vacuum (operate both in air and liquid);

4) could be used for organic materials

Disadvantages:

1) image size is much smaller than that of electron microscopes;

2) slow scanning rate, unlike an electron microscope which

does it in almost real-time.

3) tip convolution -- not true sample topography

4) expensive tips

The advantage and disadvantage of AFM

Tip convolution----Tip Related Artifacts

protrusions (dots) appear

wider, depressions (pores) appear narrower than the reality.

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

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

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

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.

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.

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.

Thank you and have a nice day!