Scanning Tunneling Microscopy and Atomic Force Microscopy

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Scanning Tunneling Microscopy and Atomic Force
Microscopy
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)
• - History
• - 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

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Beginning of Scanning Probe Microscopy
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)
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Scanning probe microscopy
Principle of Scanning Tunneling Microscopy
I
STM tip
I e 2?d (d tip-sample separation, K is the
constant)
A
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.
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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).
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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
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Scanning probe microscopy
Imaging modes of Scanning Tunneling Microscopy
STM tip
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.
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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)
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Scanning probe microscopy
STM instrumentation
Beetle type walker
Commercial system
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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
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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)
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Scanning probe microscopy
(Somorjai group)
High pressure STM reaction studies
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Scanning probe microscopy
Examples of STM studies Correlating the
atomic structure with electronic properties
STM image and spectroscopy of single walled
carbon nanotube (C. M. Lieber group)
(n,m) nanotube, if n - m is a multiple of 3, then
the nanotube is metallic, otherwise the nanotube
is a semiconductor.
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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(n1) f(n)
f (n-1),
J. Y. Park et al. Science (2005)
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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).
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Scanning probe microscopy
Atomic Force Microscopy
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History of Atomic Force Microscopy
Scanning probe microscopy

• Invention of atomic force microscopy (1985)
• Binnig, Quate, Gerber at IBM and Stanford

Binnig et al. PRL (1985)
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Scanning probe microscopy
Principle of Atomic Force Microscopy
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.
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Scanning probe microscopy
Constant height and force mode AFM
laser detection
cantilever
Constant force mode (force feedback on)
Constant height mode (force feedback off)
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.
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Scanning probe microscopy
AFM instrumentation
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Scanning probe microscopy
Cantilevers in atomic force microscopy
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 10000 N/m, spring in the ball point pencil
1000 N/m, spring constant of commercial
cantilever 0.01 100 N/m
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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)
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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
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Measurement of adhesion force between tip and
sample with force-distance curve
Scanning probe microscopy
At the point A, the tensile load is the same with
the adhesion force (FAB corresponds to the
adhesion force)
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Force Volume Mapping
Scanning probe microscopy

• Three dimensional mapping the adhesion force and
  Youngs modulus

CdSe tetrapod
Adhesion mapping
topography
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AFM images of various materials
Scanning probe microscopy
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
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Nanoscale material properties is different from
macroscopic properties for example, friction
Scanning probe microscopy
Single asperity
Real contact
AFM
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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 gt 1000 STM papers published
• 2005 Over 2000 STM and 6500 AFM papers
  published
• Scanning Probe Microscopy is one of major tools
  to characterize and control nanoscale objects

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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