1. Scanning Tunneling Microscopy and Atomic Force Microscopy
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Scanning probe microscopy
Scanning Tunneling Microscopy and Atomic Force Microscopy

2. Scanning probe microscopy
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Scanning probe microscopy
Scanning Tunneling Microscopy (STM)
- History
- Principle of STM
- Operation modes – constant current mode, constant height mode, conductance mapping, tunneling spectroscopy
- Examples of STM studies – atomic structures, dynamics, STM manipulation
Atomic Force Microscopy (AFM)
- Principle of AFM
- Operation modes – contact, non-contact, intermittent modes
- Variation of AFM – friction force microscopy, conductive probe AFM, electrostatic force microscopy, etc.
- Examples of AFM studies – atomic stick-slip, friction, adhesion properties of surfaces

3. Beginning of Scanning Probe Microscopy
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Scanning probe microscopy
Beginning of Scanning Probe Microscopy
Invention of scanning tunneling microscopy (1982)
Gerd Binnig & Heine Rohrer, IBM Zurich
(nobel prize in 1986)
First STM image of Si (7x7)
Reconstruction on Si (111) surface
Phys Rev Lett (1983)

4. (d: tip-sample separation, K is the constant)
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Scanning probe microscopy
Principle of Scanning Tunneling Microscopy I
STM tip A
I ~ e –2d
(d: tip-sample separation, K is the constant) V
Sample
surface
Tunneling current (I) d
The key process in STM is the quantum tunneling of electrons through a thin potential barrier separating two electrodes. By applying a voltage (V) between the tip and a metallic or semiconducting sample, a current can flow (I) between these electrodes when their distance is reduced to a few atomic diameters.

5. Principle of Scanning Tunneling Microscopy
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Scanning probe microscopy
Principle of Scanning Tunneling Microscopy
Because the density of state of the sample contributes the tunneling current, STM is effective technique for the conductive surface (semiconductor or metallic surface).

6. Scanning probe microscopy
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Scanning probe microscopy
Schematic of Scanning Tunneling Microscopy
The instrument basically consists of a very sharp tip which position is controlled by piezoelectric elements (converting voltage in mechanical deformation)
Figure: Michael Schmid, TU Wien

7. A Imaging modes of Scanning Tunneling Microscopy
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Scanning probe microscopy
Imaging modes of Scanning Tunneling Microscopy
STM tip A
Constant height mode
Feedback off
Constant current mode
Feedback on
STM topographical imaging (constant current mode)
The tip is moved over the surface (x direction), while the current, and consequently the distance between the tip and the sample are kept constant. In order to do so, the vertical (z) position of the tip is adjusted by a feedback loop. Thus reading the z position of the tip, one obtains real-space imaging of the sample surface.

8. Scanning Tunneling Spectroscopy
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Scanning probe microscopy
Scanning Tunneling Spectroscopy
Silicon (100) (2x1) dimer row
reconstruction structure
Tunneling spectroscopy reveals the bandgap of 0.7 eV due to the presence of surface states
(M. Crommie group)

9. STM instrumentation Scanning probe microscopy Beetle type walker
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Scanning probe microscopy
STM instrumentation
                                                          
                                                          
Beetle type walker
Commercial system

10. Examples of STM studies 1. Atomic manipulation (Don Eigler, IBM)
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Scanning probe microscopy
Examples of STM studies
1. Atomic manipulation (Don Eigler, IBM)
A node in the electron standing wave Fe
atoms
Xe atoms on Ni (100) at 8K
assembled by atomic manipulation
Quantum corral (D. Eigler)
Iron on Copper (111)
assembled by atomic manipulation

11. Examples of STM studies (Dynamics of molecules)
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Scanning probe microscopy
Examples of STM studies (Dynamics of molecules)
Water dimers diffuse much faster
than monomer and trimer
Water molecules on Pd(111) surface
T. Mitsui et al. Science (2002)

12. High pressure STM reaction studies
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Scanning probe microscopy
(Somorjai group)
High pressure STM reaction studies

13. Scanning probe microscopy
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Scanning probe microscopy
Examples of STM studies
– Correlating the atomic structure with electronic properties
(n,m) nanotube, if n − m is a multiple of 3, then the nanotube is metallic, otherwise the nanotube is a semiconductor.
STM image and spectroscopy of single walled carbon nanotube
(C. M. Lieber group)

14. Examples of STM studies – revealing periodicity and aperiodicity
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Scanning probe microscopy
Examples of STM studies – revealing periodicity and aperiodicity
STM image of two-fold surface of
Al-Ni-Co decagonal quasicrystal surface
Fibonacci sequence
A progression of numbers which are sums of the previous two terms
f(n+1) = f(n) + f (n-1),
J. Y. Park et al. Science (2005)

15. STM Fabrication and Characterization of Nanodots on Silicon Surfaces
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Scanning probe microscopy
STM Fabrication and Characterization of Nanodots on Silicon Surfaces
This involves field evaporation from either an Al- or Au-coated tungsten STM tip. This has the advantage of allowing imaging of the structures subsequent to fabrication, with the same tip.
Application of a short voltage pulse to a tip held in close proximity to the surface produces nanodots with a probability and dot size which depend on the size and polarity of the pulse.
It has been also demonstrated the modification of existing nanodots, via the application of additional, larger voltage pulses of both polarities.
Left: STM image of Au dots (approx. 10 nm dia. x 1.2 nm ht.) deposited on oxidized Si(100) by application of -8V, 10 msec
pulses to the tip. Right: Same Au dots after modification by application of +10 v pulse (left, dot erased) and –10 v pulse (right, dot enlarged).
J. Y. Park, R. J. Phaneuf, and E. D. Williams, Surf. Sci. 470, L69 (2000).

16. Atomic Force Microscopy
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Scanning probe microscopy
Atomic Force Microscopy

17. History of Atomic Force Microscopy
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Scanning probe microscopy
History of Atomic Force Microscopy
Invention of atomic force microscopy (1985)
Binnig, Quate, Gerber at IBM and Stanford
Binnig et al. PRL (1985)

18. Principle of Atomic Force Microscopy
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Scanning probe microscopy
Principle of Atomic Force Microscopy
Position sensitive
Photodiode array
Laser beam
Bending
Torsion
LOAD
FRICTION
Forces:
Van der Waals force
electrostatic force
Magnetic force
Chemical force
Pauli repulsive force
When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law.
This deflection is characterized by sensing the reflected laser light from the backside of cantilever with the position sensitive photodiode.
Because force signal (including Van der Waals force, electrostatic force, Pauli repulsive force) is measured, various samples including insulator can be imaged in AFM.

19. Constant height and force mode AFM
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Scanning probe microscopy
Constant height and force mode AFM
laser
detection
cantilever
Constant height mode
(force feedback off)
Constant force mode
(force feedback on)
AFM topographical imaging (constant force mode)
The tip is moved over the surface (x direction), while the force, and consequently the distance between the tip and the sample are kept constant. In order to do so, the vertical (z) position of the tip is adjusted by a feedback loop. Thus reading the z position of the tip, one obtains real-space imaging of the sample surface.

20. AFM instrumentation Scanning probe microscopy micromotor Beetle type
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Scanning probe microscopy
AFM instrumentation
micromotor
photodiode
Beetle type
walker
cantilever

21. Cantilevers in atomic force microscopy
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Scanning probe microscopy
Cantilevers in atomic force microscopy
Tip
Coating
(TiN)
back
(Au)
for laser
reflection
Cantilevers can be seen as springs. the extension of springs can be described by Hooke's Law F = - k * s. This means: The force F you need to extend the spring depends in linear manner on the range s by which you extend it. Derived from Hooke's law, you can allocate a spring constant k to any spring.
Damping spring of wheel in the car : N/m, spring in the ball point pencil : 1000 N/m, spring constant of commercial cantilever :0.01 – 100 N/m

22. Imaging mode in atomic force microscopy
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Scanning probe microscopy
Imaging mode in atomic force microscopy
Feedback : lever deflection
the feedback system adjusts the height of the cantilever base to keep this deflection constant as the tip moves over the surface
(friction force microscopy, conductive probe AFM)
Feedback : oscillation amplitude
The cantilever oscillates and the tip makes repulsive contact with the surface of the sample at the lowest point of the oscillation (Tapping mode AFM)
Feedback : oscillation amplitude
the cantilever oscillates close to the sample surface, but without making contact with the surface.
Electrostatic / magnetic force microscopy
Feedback : lever deflection
the tip does not leave the surface at all during the oscillation cycle. (interfacial force microscopy)

23. Friction force microscopy
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Scanning probe microscopy
Friction force microscopy
AFM topographical and friction images of C16 silane self-assembled monolayer on silicon surface revealing lower friction of molecule layers

24. EEW508
Scanning probe microscopy
Measurement of adhesion force between tip and sample with force-distance curve
At the point A, the tensile load is the same with the adhesion force (FAB corresponds to the adhesion force)

25. Force Volume Mapping Scanning probe microscopy
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Scanning probe microscopy
Force Volume Mapping
Three dimensional mapping the adhesion force and Young’s modulus
Adhesion force (nN) 35 30 25 20 15 10 5
18nN
28nN
CdSe tetrapod
topography
Adhesion mapping

26. AFM images of various materials
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Scanning probe microscopy
AFM images of various materials
Contact mode AFM topography (left), friction (right) images of graphite surface
Contact mode topographical (up) and friction images (bottom) of polymer
Contact mode friction image (left) and its line profile of mica surface which show atomic stick-slip process

27. AFM Scanning probe microscopy
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Scanning probe microscopy
Nanoscale material properties is different from macroscopic properties – for example, friction
Friction at the
Macroscopic scale
Single
asperity
Real contact
Friction at the
single asperity
AFM

28. Perspective of SPM Scanning probe microscopy
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Scanning probe microscopy
Perspective of SPM
1981 : First STM results in the lab
1985 : Invention of AFM in Stanford (Quate group)
1986 : Nobel Prize for Rusk , Binnig & Rohrer
: First commercial instruments (Park Scientific Instrumentation from Stanford, Digital Instrumentation from Paul Hansma)
: first year > STM papers published
2005 : Over STM and AFM papers published
Scanning Probe Microscopy is one of major tools to characterize and control nanoscale objects

29. Scanning tunneling microscopy (STM):
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Scanning probe microscopy
Summary
Scanning tunneling microscopy (STM):
tunneling current between the sharp tip and conductive surface is detected and used to acquire STM images.
Atomic force microscopy (AFM):
Force between the cantilever and the surface is measured and used for AFM imaging
Both insulating and conductive materials can be imaged in AFM