1. Probe Microscopes
Designer: S.H.Kazemi
IASBS University, Spring 2017

2. Resolution AFM STM SEM OM SAM
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Resolution
Here are some of the techniques we will examine and a comparison of their lateral resolution capabilities.
1 cm
1 mm
1 µm
1 nm
1 Å
AFM
STM
SEM OM
SAM
Lateral Resolution. Optical Microscopy is usually what one thinks of when studying microscopy. It is prevalent and widely available. Its resolution is limited by the diffraction of light, which is limited to about 1/2 a wavelength of the light being diffracted through the optical lensing elements of the microscope. While this is several hundred nanometers, in practice it is challenging to get good optical resolution down to the 1 µm range.
The next most widely available technique would be that of Scanning Electron Microscopy. Here we are limited by the diffraction of electron waves. Since these are in the Ångstrom range, there are instruments with resolution of a few nanometers quite widely deployed. Recent reports have described a new instrument able to observe atoms (a few Ångstroms in size) but this is not available for use yet. Each instrument achieves high resolution by focusing the electron beam as tightly as possible. The minimum spot size achievable by focusing the electron beam defines the resolution of a particular instrument.
Scanning Auger Microscopy (SAM) is closely related to SEM, except that the scattering in the sample leads to a “smearing” of the incident beam, and broadens the effective spot size. Its resolution general lags behind that for SEM.
Probe Microscope techniques like STM and AFM have resolution that is controlled by the sharpness of the probe tip. This can be in the range of a few nanometers, but the differing contrast mechanisms mean that STM can easily achieve atomic resolution while atomic resolution for AFM is difficult to achieve.
Interference Microscopy is a newer technique that employs optical wavelength light. Its lateral resolution is controlled exactly like that of optical microscopy and it has the same limitations. (Its virtues come elsewhere.)
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3. Types of Microscope Smaller Scale? Scanning Probe Microscope
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Types of Microscope
Smaller
Scale?
Optical Microscope
Electron Microscope
date of invention
1644
1931
Operation
In air, liquid and vacuum
in vacuum
Resolution
Horizontal (x,y)
Vertical (z)
Magnification
0.5μm NA
1-2x10³
2nm
10-10⁶
Sample preparation
Not much
Sample should be frozen and dried
Sample conditions
Cannot be completely transparent
Surface should be conductive
Scanning Probe Microscope
1981 nm
0.01nm
5x10²-10⁸
none
Surface cannot be too rough 3
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4. Scanning Tunneling Microscope (STM)
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Scanning Tunneling Microscope (STM)
Basic principle is tunneling.
Tunneling current flows between tip and sample when separated by less than 100nm.
The tunneling current gives us atomic information about the surface as the tip scans.
Scanning tunneling microscopy is a technique developed in the eighties and provides images of solid surfaces with unprecedented resolution. The operation of a scanning tunneling microscope (STM) is based on the so-called tunneling current, which starts to flow when a sharp tip approaches a conducting surface at a distance of approximately one nanometer. The tip is mounted on a piezoelectric tube, which allows tiny movements by applying a voltage at its electrodes. Thereby, the electronics of the STM system control the tip position in such a way that the tunneling current and, hence, the tip-surface distance is kept constant, while at the same time scanning a small area of the sample surface. This movement is recorded and can be displayed as an image of the surface topography. Under ideal circumstances, the individual atoms of a surface can be resolved and displayed.
Viewing the GIF file by flash player 4
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Operation of STM
Measure the tunneling current between a conducting tip and conducting sample when a DC bias voltage is applied. Resolution in z (perpendicular to surface) is ~0.01 nm and in x/y is ~0.1 nm, resulting in atomic-resolution images. 5
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6. Operation of STM The (barrier) is several nm Constant height
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Operation of STM
tip
Scan direction
The (barrier) is several nm
Lower current
Constant height
Higher current
Atomic surface
I (nA)
x (nm)
The current profile duplicates the atomic surface 6 6
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7. Making a sharp tip By electrochemical etching method
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Making a sharp tip
By electrochemical etching method
Consider an Au tip prepared in 0.8 M KCN from a gold wire (Au).
The following electrochemical redox reaction takes place: 4Au(s)+8KCN(aq)+O2(g)+2H2O→ 4Au(CN)2−(aq)+4OH−(aq)+8K+(aq)
By applying different voltages, some part of tip were washed with the distilled water 7
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STM
It should be noted, however, that STM images not only display the geometric structure of the surface, but also depend on the electronic density of states of the sample, as well as on special tip-sample interaction mechanisms which are not fully understood yet.
Although the STM itself does not need vacuum to operate (it works in air as well as under liquids), ultrahigh vacuum is required to avoid contamination of the samples from the surrounding medium. 8
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STM
A problem in investigating metal surfaces is the fact that these surfaces appear very flat to an STM, i.e., the apparent height of individual atoms (corrugation) is 1/100 to 1/10 of an atomic diameter.
Therefore, for resolving individual atoms the distance between the tip and the sample must be kept constant within 1/100 of an atomic diameter or better (approx nm!!). This demands not only very high rigidity of the STM itself, but the STM must be efficiently decoupled from environmental vibrations. 9
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10. Scanning Tunneling Microscope (STM)
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Scanning Tunneling Microscope (STM)
STM images are usually displayed as grey scale images with protrusions shown white and depressions black. However, new instruments employ colors from dark brown to yellow to add more possibility to height distinction.
In a few cases image processing has been used for contrast enhancement to display both the atomic corrugation and a larger height range such as different layers of atoms. 10
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11. Quantum Tunneling Tunneling current is proportional to exp (-2Ka)
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Quantum Tunneling
From boundary conditions, solving the quantum mechanics equations lead to the following transmission equation which shows the possibility of tunneling phenomena,
Thus for large Ka
Tunneling current is proportional to exp (-2Ka)
The thickness a of the energy barrier can be found by measuring the tunneling current! 11
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What is tunneling?
The probability that the electron will exist outside the barrier in the vacuum is non zero.
If these leak-out waves overlap and a small bias voltage is applied between the tip and the sample, a tunneling current flows.
The magnitude of this tunneling current does not give the nuclear position directly, but is directly proportional to the electron density of the sample at a point. 12
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13. What does piezo-electric mean?
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What does piezo-electric mean?
In 1880 Pierre Curie discovered that by applying a pressure to certain crystals he could induce a potential across the crystal.
The STM reverses this process. Thus, by applying a voltage across a piezoelectric crystal, it will elongate or compress.
A typical piezoelectric material used in an STM is Lead Zirconium Titanate 13
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STM modes of operation
There are several modes of operation which the most important are:
1- Constant current mode (mapping the piezo motion)
2- Constant height mode (mapping the tip current variations)
3- Contact STM mode
4- Ac mode
Of course, the first and second mode are more operational and well-known especially for conducting substrate, however the insulating material can be visualized by STM although its mechanism is unknown. 14
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STM: some examples
Here are some examples of using STM in both surface science studies and advanced technological applications.
Quantum corral:
Fe atoms adsorb on a copper (111) surface forming a "quantum corral“ in a very low temperature (4K). Actually, the image shows the contour of the local density of electron states. The corral is about 14.3 nm in diameter.
A small bias voltage (mV to 3 V) is applied between a atomically sharp tip and the sample. If the distance between the tip and the sample is large no current flow. However, when the tip is brought very close ( 10 Å) without physical contact, a current (pA to nA) flows across the gap between the tip and the sample. Such current is called tunneling current which is the result of the overlapping wavefunctions between the tip atom and surface atom, electrons can tunnel across the vacuum barrier separating the tip and sample in the presence of small bias voltage.
The magnitude of tunneling current is extremely sensitivity to the gap distance between the tip and sample, the local density of electronic states of the sample and the local barrier height. The density of eletronic states is the amount of electrons exit at specific energy. As we measure the current with the tip moving across the surface, atomic information of the surface can be mapped out. 15
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16. STM image of Si(100) surface
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STM image of Si(100) surface
The images reflect the spatial distribution of the occupied π-bonding state is between the dimer atoms, while the unoccupied π-antibonding states localized away the dimer.
The bottom pictures depict the model of the reconstruction of the bulk-terminated (1x 1) lattices into a (2 x 1) via dimerization, the large dots represent atoms on top layer, while small dots are atoms of second layer. 16
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17. Metrological Applications
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Metrological Applications
This is the STM image of and individual turn mark on a diamond-turned Al substrate to be used for subsequent magnetic film deposition for a high capacity hard disc drive. The image obtained by scanning electron microscope (SEM) is shown to the right for comparison. The high spatial resolution of STM provides an important complement to the SEM.
The microtopography and nanotopography of a surface is crucial in many applications, such as for high precision optical components and disk drive surface roughness of machined or ground surfaces in area where such a finish is crucial. 17
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Manipulation of Atoms
Here, Fe atoms are placed on Cu(111) surface at very low T (4K), Fe atoms are first physisorbed on the Cu, then the tip is placed directly over a physisorbed atom and lowered to increase the attractive force by increasing the tunneling current.
Then, Fe was dragged by the tip, moves across the surface to a desired position. finally, the tip was withdrawn by lowering the tunneling current. 18
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19. Atomic Force Microscopy (AFM)
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Atomic Force Microscopy (AFM)
AFM is performed by scanning a sharp tip on the end of a flexible cantilever across the sample while maintaining a small force.
Typical tip radii are on the order of 1nm to 10nm.
AFM has two major modes, tapping mode and contact mode.
In scanning mode, constant cantilever deflection is maintained.
In tapping mode, the cantilever is oscillated at its resonance frequency. 19
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20. Atomic Force Microscopy (AFM) Schematic representation
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Atomic Force Microscopy (AFM)
Schematic representation
Cantilever with a sharp tip
Laser beam deflection system (introduced by Meyer and Amer)
Detector and Feedback electronics
Piezoelectric tube
Image display system 20
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How does AFM work?
Measure the forces between the sharp tip & sample surface
Short-range forces
Chemical forces: ionic bonds, covalent bonds, metallic bonds
Repulsion forces:
Pauli repulsion, ionic repulsion
Long-range forces
Van der Waals forces
Capillary forces
Magnetic forces
Electrostatic force 21
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22. How are forces measured?
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How are forces measured?
Hooke’s law: F= -ks
Laser Beam Deflection Method 22
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23. Fabrication of Cantilever
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Fabrication of Cantilever
Made from Si₃N₄ or Si
As soft as possible to achieve high sensitivity
Spring constant < equivalent spring constant between atoms of sample in order not to dragging the atoms out of its atomic site 23
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24. Fabrication of Tip Made from Si₃N₄ or Si As sharp as possible
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Fabrication of Tip
Made from Si₃N₄ or Si
As sharp as possible
The radius of curvature of the tip does not influence the height of a feature but destroy the lateral resolution 24
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25. Modes of Imaging Constant Height
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Modes of Imaging
Constant Height
Cantilever is "dragged" across the surface of the sample
Tip is free to move up and down
Force between tip and sample surface is measured directly using the deflection of the cantilever
No need to wait for the response of feedback system, scan in high speed
No signal error
* If surface is rough, can cause damage to tip and surface 25
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26. force between tip and sample surface remain constant
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Modes of Imaging
Constant Force
Move the cantilever up and down using the piezoelectric tube
so that the position of laser beam is unchanged
force between tip and sample surface remain constant
Suited for almost every surface
* Scan slowly, need to wait for the response of feedback system
* Sensitive to random noise, has signal error 26
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27. Constant Force Modes Contact Mode (<0.5nm tip-surface separation)
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Constant Force Modes
Contact Mode (<0.5nm tip-surface separation)
Tapping Mode (0.5-2nm tip-surface separation)
Non-contact Mode (2-10nm tip-surface separation) 27
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28. * Force can damage or deform soft samples
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Contact Mode
Tip almost touches the surface
Force on the tip is repulsive
Force between the tip and the surface is kept constant during scanning by maintaining a constant deflection
Better resolution than tapping mode and non-contact mode
Fast scanning
Good for rough surface
* Force can damage or deform soft samples 28
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Tapping Mode
Cantilever is driven to oscillated up and down at its resonant frequency
Probe slightly taps on the surface during scanning, contacting the surface at the bottom of its swing
Adjust the height of cantilever by the piezoelectric tube to maintain a constant oscillation amplitude
i.e. constant force between tip and surface is maintained
High resolution for the samples that are easily damaged (biological sample)
* Slower scanning speed needed 29
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30. Non-contact Mode Tip does not contact the surface
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Non-contact Mode
Tip does not contact the surface
Similar to tapping mode, cantilever is oscillated at its resonant frequency
Adjust height of cantilever to keep constant oscillation amplitude, constant force between tip and surface
Prevent tip from sticking to the surface (Note: all samples unless in a controlled UHV or environmental chamber have some liquid adsorbed on the surface).
Low force exerted on surface
No damage to tip and surface
* Lower resolution
* Slower speed 30
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31. Some Applications Biological Science : Live cell
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Some Applications
Biological Science : Live cell
Human Lung Cancer Cell
Scan size: 60 µm 31
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Applications
Data Storage : Help in design hard disk drive at nanoscale level
Semiconductor characterization
Materials Science
Polymer Science
DVD
Surface topography of ZnO film
Image of polymer blend 32
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33. Some high quality AFM images
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Some high quality AFM images
Bio-degradable polymer nano fiber, poly caprolactam 33
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34. Some high quality AFM images
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Some high quality AFM images
Random network of single walled carbon nanotubes(SWNTs) 34
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35. Some high quality AFM images
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Some high quality AFM images
GaN on SiC substrate 35
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36. Some high quality AFM images
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Some high quality AFM images
Nano-honeycomb pattern on TiO2 surface 36
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37. Some high quality AFM images
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Some high quality AFM images
High resolution AFM topography of gold surface 37
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38. End of probe Microscopy
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End of probe Microscopy 38
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