This document provides an overview of scanning tunneling microscopy (STM) and its components and applications. It discusses the history and development of STM, the principle of electron tunneling that STM utilizes, and how STM works to produce atomic-scale images of surfaces. It also describes various STM components like the scanning tip and piezoelectric scanner. The document outlines different STM modes and how ultra-high vacuum and low-temperature STMs enable more advanced surface imaging and atomic manipulation.

1. Surface Science and Technology (II)
N. Ramkumar
Material Science and Engineering

2. Content
Scanning
tunnelling
microscope
• History
• Introduction
• Working Principle of
STM
• Modes of working of
STM
• Components of a STM
• Applications and
Limitations
Ultra-high vacuum
Scanning tunnelling
microscopy
• Introduction
• Working Process
• Ultrahigh Vacuum Low
Temperature Scanning
Tunneling Microscope
(UHV-LT-STM)
Conclusion References
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4. History
 The development of the family of scanning probe microscopes started with the original invention of the STM in
1981.
 Gerd Binnig and Heinrich Rohrer developed the first working STM while working at IBM Zurich Research
Laboratories in Switzerland.
 This instrument would later win Binnig and Rohrer the Nobel prize in physics in 1986.
 Binnig and Rohrer chose the surface of gold for their first image.
 When the image was displayed on the screen of a television monitor, they saw rows of precisely spaced atoms and
observed broad terraces separated by steps one atom in height.
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5. Introduction
• Scanning tunneling microscope (STM), type of microscope whose principle of operation is based on the quantum
mechanical phenomenon known as tunneling, in which the wavelike properties of electrons permit them to “tunnel”
beyond the surface of a solid into regions of space that are forbidden to them under the rules of classical physics.
• The probability of finding such tunneling electrons decreases exponentially as the distance from the surface increases.
• The STM makes use of this extreme sensitivity to distance.
• The sharp tip of a tungsten needle is positioned a few angstroms from the sample surface.
• A small voltage is applied between the probe tip and the surface, causing electrons to tunnel across the gap.
• As the probe is scanned over the surface, it registers variations in the tunneling current, and this information can be
processed to provide a topographical image of the surface.
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6. Working Principle of STM
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The scanning tunneling microscope (STM) works by scanning a very sharp metal wire
tip over a surface.
By bringing the tip very close to the surface, and by applying an electrical voltage to the
tip or sample, image the surface at an extremely small scale – down to resolving
individual atoms can be obtained.
The STM is based on several principles.
 One is the quantum mechanical effect of tunneling. It is this effect that allows us to
“see” the surface.
 Another principle is the piezoelectric effect. It is this effect that allows us to
precisely scan the tip with angstrom-level control.
 Lastly, a feedback loop is required, which monitors the tunneling current and
coordinates the current and the positioning of the tip.
The STM image of the Cu(111) surface
https://www.nanoscience.com/wpcontent/uploads/2
018/05/Quantum-Corral.jpg

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Tunneling
• Tunneling is a quantum mechanical effect. A tunneling current occurs when electrons move through a barrier
that they classically shouldn’t be able to move through.
• Because of the small probability of an electron being on the other side of the barrier, given enough electrons,
some will indeed move through and appear on the other side.
• When an electron moves through the barrier in this fashion, it is called tunneling.
https://www.nanoscience.com/wp-content/uploads/2018/05/Schematic-of-scanning-tunneling-microscopy.png

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Piezoelectric Effect
• The piezoelectric effect was discovered by Pierre Curie in 1880. The effect is created by squeezing the sides of
certain crystals, such as quartz or barium titanite.
• The result is the creation of opposite charges on the sides.
• The effect can be reversed as well; by applying a voltage across a piezoelectric crystal, it will elongate or
compress.
Feedback Loop
• Electronics are needed to measure the current, scan the tip, and
translate this information into a form that we can use for STM
imaging.
• A feedback loop constantly monitors the tunneling current and makes
adjustments to the tip to maintain a constant tunneling current.
• These adjustments are recorded by the computer and presented as an
image in the STM software. Such a setup is called a constant current
image.
https://www.nanoscience.com/wp-content/uploads/2018/05/Feedback-loop-and-electron-tunneling-for-STM.png

9. Modes of working of STM
Constant current mode:
• Contrast on the image is due to variations in charge density. Feedback electronics adjust the height by a voltage to the
piezoelectric height control mechanism.
• This leads to a height variation and thus the image comes from the tip topography across the sample and gives a constant
charge density surface.
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Constant height mode:
• The voltage and height are both held constant while
the current changes to keep the voltage from
changing. This leads to an image made of current
changes over the surface, which can be related to
charge density.
• It is faster, as the piezoelectric movements require
more time to register the height change in constant
current mode than the current change in constant
height mode.
https://eng.thesaurus.rusnano.com/upload/iblock/a6c/STM-operation-modes_1.jpg

10. Components of a STM
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The components of a STM include scanning tip, piezoelectric controlled
scanner, distance control and scanning unit, vibration isolation system,
and computer.
1. Scanning tip: Electrons tunnel from the scanning tip to the sample,
creating the tunnelling current.
2. Piezoelectric controlled scanner: Piezoelectric crystals expand and
contract very slightly depending on the voltage applied to them and
this principle is used to control the horizontal position x, y, and the
height z of the scanning tip.
The scanning tip of a STM (Centre of the figure)
http://www.hk-phy.org/atomic_world/stm/images/fig07.jpg

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3.Distance control and scanning unit:
Position control using piezoelectric means is extremely fine, so a
coarse control is needed to position the tip close enough to the sample
before the piezoelectric control can take over.
4.Vibration isolation system:
STM deals with extremely fine position measurements so the isolation
of any vibrations is very important.
5.Computer:
The computer records the tunneling current and controls the voltage to
the piezoelectric tubes to produce a 3-dimensional map of the sample
surface.
http://www.hk-phy.org/atomic_world/stm/images/fig08.gif
Components of a STM

12. Applications and Limitations
• Most of STM applications are in nanotechnology and biology.
• STM images are not direct surface images of the sample as in the case of optical microscopy rather it is measure of
the local density of states of a material at it surface as a function of lateral (x-y) position on the sample surface and
energy.
• To characterization of surface structure on atomic scale. Produce topographic map of the surface.
• STM only for conducting materials. Image interpretation also difficult if more than one type of atom on surface.
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13. Ultra-high vacuum
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14. Introduction
• STM is the basis of a powerful "surface structure" apparatus that incorporates an STM into a UHV chamber that also
contains complementary facilities for AES, XPS, LEED, FTIR, and TPD.
• They used a "single-tube piezo type" STM with a sample holder that has low (LHe) temperature capabilities.
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https://koelgroup.princeton.edu/sites/koelgroup/files/styles/pwds_media_large_no_crop/public/media/untitled-1.jpg?itok=b6nBGMMN

15. Working Process
• Scanning tunneling microscopy (STM) provides key information on
the structure and electronic properties of the surfaces of materials.
• UHV STM is a versatile instrument that combines STM with other
ways to probe or measure the sample.
• UHV conditions allow clean surfaces to be prepared.
• A side chamber enables deposition of metals onto the sample within
the same vacuum system. The STM itself is a four-probe system.
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https://fmross.mit.edu/sites/default/files/images/STM-Interior_0.png
Photograph of the interior of the STM chamber showing the geometry of the
sample and four probes

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Image 1: One probe contacting a flake of a 2D material and a second
scanning probe, imaged using the SEM within the STM chamber
Image 2: STM topography of native defects in few-layer MoS2,
recorded at constant current and at room temperature. The most
common defect is the single S vacancy; the defects that show a larger
wave function extent may be charged states of the same defect
https://fmross.mit.edu/sites/default/files/images/STM-MoS2-defects.png
https://fmross.mit.edu/sites/default/files/images/STM-TwoProbes_0.png

17. Ultrahigh Vacuum Low Temperature Scanning Tunneling
Microscope (UHV-LT-STM)
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• Placing the microscope into an Ultra High Vacuum
environment allows for controlled surface preparation.
• Operation in air or other "dirty" environments complicates
the interpretation of atom-sized features. In a clean
environment, however, both the surface and the atoms placed
upon it can be carefully controlled.
• Operation at Cryogenic Temperatures reduces electronic
noise and eliminates some smearing between separate energy
levels.
• At small enough temperatures, these different levels can be
independently probed.
http://research.physics.berkeley.edu/zettl/projects/images/lowtempstm.jpg

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• In addition, low temperatures freeze adsobed atoms and molecules in place on a surface.
• Using the microscope tip, Atomic Manipulation can move the otherwise frozen adsorbates. If stable over long periods of
time, unique structures may be built up on an atom-by-atom basis.
Bundle of carbon nanotubes
[Image by Low temperature UHVSTM]
Au (111)
[Image by Low temperature UHVSTM]
http://research.physics.berkeley.edu/zettl/projects/images/stmtube.gif
http://research.physics.berkeley.edu/zettl/projects/images/stmgold.jpg

19. Conclusion
• In STM, it will be possible to see in real time the actions of the experiments at atomic scales, understanding their
interactions much better and helping the development of nanotechnology.
• Unfortunately most materials are not electrically conductive on their surface. Even metals like aluminum are covered
with non-conductive oxides. A newer microscope, the atomic force microscope, or AFM, overcomes this limitation by
measuring the force between the tip and the sample, rather than the electrical current.
• The STM can be used not only in ultra-high vacuum but also in air, water, and various other liquid or gas ambient,
and at temperatures ranging from near zero kelvin to a few hundred degrees Celsius.
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20. References
• Nanoscience Instruments | Techniques | Scanning Tunneling Microscopy | https://www.nanoscience.com/techniques/ scanning-
tunneling-microscopy/.
• Encyclopædia Britannica, Inc | Science | Physics | Scanning Tunneling Microscopy | https://www.britannica.com/
technology/scanning-tunneling-microscope | Calvin F. Quate, Applied Physics and Electrical Engineering, Stanford University,
California.
• Atomic world | Scanning Tunneling Microscope(STM) | Principle of STM | http://www.hk-phy.org/atomic_world/stm
/stm03_e.html.
• Princeton University | Koel Research Group | UHV-STM | https://koelgroup.princeton.edu/facilities/uhv-stm.
• Massachusetts Institute of Technology Cambridge | Frances M. Ross Research Group|UHV Scanning Tunneling Microscopy |
https://fmross.mit.edu/projects/stm-fib.
• Zettl Research Group | Research Project | Low Temperature High Magnetic Field Ultra High Vacuum |
http://research.physics.berkeley.edu/zettl/projects/stm.html.
• Scanning Tunneling Microscope, An Najah National University, Faculty of Science, Physics Department Prof: Gassan Saffarini
Prepared by: Balsam Ata 2012.
• Glossary of Nano technology related terms | STM operation modes| https://eng.thesaurus.rusnano.com/wiki/article14
154#:~:text=Depending%20on%20variation%20of%20these,z%20and%20V%20are%20held.
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