Scanning Probe microscopy (AFM and STM) head point AFM: Configuration of AFM Parts of AFM system and Principle of AFM Three Modes of AFM AFM Instrument Advantage and disadvantage STM Schematic Diagram AFM and STM https://www.linkedin.com/in/preeti-choudhary-266414182/ https://www.instagram.com/chaudharypreeti1997/ https://www.facebook.com/profile.php?id=100013419194533 https://twitter.com/preetic27018281 Please like, share, comment and follow. stay connected If any query then contact: chaudharypreeti1997@gmail.com Thanking-You Preeti Choudhary

1. Scanning Probe microscopy
(AFM and STM)
Preeti Choudhary
chaudharypreeti1997@gmail.com
MSc (Applied Physics)

2. AFM
• Atomic-force microscopy (AFM) or scanning-force
microscopy (SFM) is a very high-resolution type of scanning
probe microscopy (SPM), with demonstrated resolution on the
order of fractions of a nm, more than 1000 times better than
the optical diffraction unit.
• Used to measure a roughness of a sample surface at a high
resolution, to distinguish a sample based on its mechanical
properties (for example, hardness and roughness) and, in
addition, to perform a microfabrication of a sample (for
example, an atomic manipulation)

3. Configurations of AFM
An AFM typically consists of the following features:
(1) Cantilever ,
(2) Support(Configured to support cantilever.),
(3) Piezoelectric element(Configured to oscillate cantilever at its eigen frequency.),
(4) Tip (Fixed to open end of a cantilever, work as a probe of AFM,
(5) Detector (Configured to detect the deflection and motion of the cantilever.),
(6) Sample (Will be measure by AFM),
(7) xyz-drive, (Moving a Sample (6) and Sample (8) Stage to be displaced in x, y, and z directions
with respect to a tip apex(4)), and
(8) Stage.

4. Parts of AFM system
• 1. Laser – deflected off
cantilever
• 2. Mirror –reflects laser beam
to photodetector
• 3. Photodetector –dual element
photodiode that measures
differences in light intensity and
converts to voltage
• 4. Amplifier
• 5. Register
• 6. Sample
• 7. Probe –tip that scans sample
made of Si
• 8. Cantilever –moves as
scanned over sample and
deflects laser beam

5. Energy U and force F between tip and sample as a function of their distance z. The force is
the derivative (= slope) of the energy. It is attractive at large distances (van der Waals force,
non-contact mode), but it becomes highly repulsive when the electron clouds of tip and
sample overlap (Pauli repulsion, contact mode).
In AFM the force is kept constant, while in STM the current is kept constant.
Principle of AFM
r
V(r)
Non-contact
mode
Contact mode
Figure 3.16. Potential energy between tip and
sample as a function of the distance between them.
The potential is attractive when they are far apart
(non-contact), but it will become strongly
repulsive when they are close together (contact).
F
U
repulsive attractive
z

6. Three Modes of AFM
Contact Mode
Non-Contact Mode
Tapping (Intermittent
contact) Mode

7. 40 m
AFM Cantilever and Tip
To obtain an extra sharp AFM tip one can attach a carbon nanotube
to a regular, micromachined silicon tip.

8. AFM:
Instrument

9. Contact Mode
• Measures repulsion between tip and sample
• Force of tip against sample remains constant
• Feedback regulation keeps cantilever deflection
constant
• Voltage required indicates height of sample
• Problems: excessive tracking forces applied by probe
to sample

10. Non-Contact Mode
• Measures attractive forces between tip and sample
• Tip doesn’t touch sample
• Van derWaals forces between tip and sample
detected
• Problems: Can’t use with samples in fluid
• Used to analyze semiconductors
• Doesn’t degrade or interfere with sample- better for
soft samples

11. Tapping (Intermittent-Contact) Mode
• Tip vertically oscillates between contacting sample
surface and lifting of at frequency of 50,000 to
500,000 cycles/sec.
• Oscillation amplitude reduced as probe contacts
surface due to loss of energy caused by tip
contacting surface
• Advantages: overcomes problems associated with
friction, adhesion, electrostatic forces
• More effective for larger scan sizes

12. What are the limitations of AFM?
• AFM imaging is not ideally sharp

13. Advantages and Disadvantages of
AFM
• Easy sample preparation
• Accurate height information
• Works in vacuum, air, and liquids
• Living systems can be studied
• Limited vertical range
• Limited magnification range
• Data not independent of tip
• Tip or sample can be damaged

14. The Future of Atomic Force
Microscopy
• Sharper tips by improved microfabrication processes: tip –
sample interaction tends to distort or destroy soft biological
molecules
• Atomic or angstrom resolution images of live cell surfaces:
development of more flexible cantilever springs and less
damaging and nonsticky probes needed

15. STM

16. STM
• A scanning tunneling microscope (STM) is an instrument for
imaging surfaces at the atomic level. Its development in 1981 earned
its inventors, Gerd Binnig and Heinrich Rohrer (at IBM Zurich),
the Nobel Prize in Physics in 1986.
• For an STM, good resolution is considered to be 0.1 nm lateral
resolution and 0.01 nm depth resolution.
• With this resolution, individual atoms within materials are routinely
imaged and manipulated. The STM can be used not only in ultra-
high vacuum but also in air, water, and various other liquid or gas
ambients, and at temperatures ranging from near zero kelvin to a few
hundred degrees Celsius

17. Conti..
• The STM is based on the concept of quantum tunneling. When a
conducting tip is brought very near to the surface to be examined,
a bias (voltage difference) applied between the two can allow electrons
to tunnel through the vacuum between them. The resulting tunneling
current is a function of tip position, applied voltage, and the local
density of states (LDOS) of the sample.
• Information is acquired by monitoring the current as the tip's position
scans across the surface, and is usually displayed in image form. STM
can be a challenging technique, as it requires extremely clean and
stable surfaces, sharp tips, excellent vibration control and sophisticated
electronics

18. STM ….Conti..
• The resolution of an image is limited by the radius of
curvature of the scanning tip of the STM.
• Additionally, image artifacts can occur if the tip has two
tips at the end rather than a single atom; this leads to
“double-tip imaging,” a situation in which both tips
contribute to the tunneling.
• The tip is often made of tungsten or platinum-iridium,
though gold is also used.

19. Schematic Diagram

20. Scanning Tunneling Microscope (STM)
feedback
regulator
high voltage
amplifier
z
x
y
I
probing tip
sample
xyz-Piezo-Scanner
Negative feedback keeps the current constant (pA-nA) by moving the tip up and down.
Contours of constant current are recorded which correspond to constant charge density.

21. Technology Required for a STM
• Sharp, clean tip
(Etching, ion bombardment, field desorption by pulsing)
• Piezo-electric scanner
(Tube scanner, xyz scanner)
• Coarse approach
(Micrometer screws, stick-slip motors)
• Vibrational damping
(Spring suspension with eddy current damping, viton stack)
• Feed-back electronics
(Amplify the current difference, negative feedback to the z-
piezo)

22. STM versus AFM
STM is particularly useful for probing
electrons at surfaces, for example the
electron waves in quantum corrals or the
energy levels of the electrons in dangling
bonds and surface molecules.
AFM is needed for insulating samples.
Since most polymers and biomolecules
are insulating, the probe of choice for
soft matter is often AFM. This image
shows DNA on mica, an insulator.

23. Thanking-You
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