Basic Imaging Modes

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Basic Imaging Modes

1. Contact mode AFM
2. Lateral Force Microscopy ( LFM)
3. Scanning tunneling microscopy

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Review last lecture
Position sensitive Photo-detector
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Summarize Main components of AFM
AFM tip/cantilever assembly Force detection
system Electronic Feedback system (Electronic
Brain) Scanner ( precise position control
system) Vibration damping system
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Position sensitive photodetector
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Force Detection system
Hook law F - kx
k Spring constant of the cantilever Materials,
and dimension of the cantilever
k increases with lever thickness, decreases with
lever length
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Contact Mode The original AFM mode, providing
topographic imaging and a gateway to advanced
techniques. Contact mode is the basis for all
AFM techniques in which the probe tip is in
constant physical contact with the sample
surface. While the tip scans along the surface,
the sample topography induces a vertical
deflection of the cantilever. A feedback loop
maintains this deflection at a preset load force
and uses the feedback response to generate a
topographic image. Contact Mode is suitable for
materials science, biological applications and
basic research. It also serves as a basis for
further SPM techniques that require direct
tip-sample contact.
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Atomic Force Microscopy
Force mapping
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Smarter imaging ---Need a Feedback system
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Piezoelectric Scanners (Scanning Mechanism )
Piezoelectric effect piezoelectric crystals
The electrical polarization produces is
proportional to the stress and the direction. The
polarization changes if the stress changes from
compression to tension
Reverse piezoelectric effect
Materials lead zirconate titanates
( Pb(Ti, Zr)O3) PZT type )
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Review last lecture
Youtube
AFM Principle - How AFM Works
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Lateral Force Microscope ( LFM) Or Friction
Force Microscope ( FFM)
Langmuir-Blodgett single-layer thin film made of
a mixture of behenic acid (BA) and diphenyl
bis-(octadecylamino)phosphonium bromide (DPOP).
Both topographic (left) and LFM (right) images
were acquired simultaneously.
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Measure lateral twist of the cantilever
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Cantilever Movements and optical deflection
detection
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• Lateral Force Microscopy (LFM) is derived from
  Contact Mode imaging. In Contact Mode, the
  vertical bending of the cantilever probe is
  measured as it scans across the surface. By also
  measuring the lateral bending of the cantilever,
  information regarding the surface friction
  characteristics of a sample can be determined.
• Lateral forces can arise from changes in the
  frictional coefficient of a region on the sample
  surface or from onsets of changes in height. LFM
  is therefore useful for measuring lack of
  homogeneity in surface materials and producing
  images with enhanced edges of topographic
  features.

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Contact imaging mode
Advantages Disadvantages
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Scanning Tunneling Microscopy (STM)

• Tunneling current ( pA nA) between tip and
  sample is exponentially dependent on their
  separation ( lt 10 angstroms), the local density
  of electronic states of the sample and the local
  barrier height. The density of electronic states
  is the amount of electrons exit at specific
  energy.
• Topographic image formed by feedback loop which
  maintains a constant tunneling current during
  scanning
• Typically limited to conductive and
  semiconductive surfaces

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Scanning Tunneling Microscopy
V
Feel
I
Tunneling Current
D distance between tip and sample
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Flat surface
Current image
Electronic brain
Resolution
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Tip sharpness How to make a tip
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Smart way!
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Current measurement Feedback system
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Tunneling Current
wavelike properties of electrons in quantum
mechanics. There is still a non-zero probability
that it may traverse the forbidden region and
reappear on the other side of the barrier.
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If two conductors are so close that their leak
out electron wavefunctions overlap. The electron
wavefunctions at the Femi level K is given
by                               
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m is mass of electron, ?  is the local
tunneling barrier height or the average work
function of the tip and sample. When a small
voltage, V is applied between the tip and the
sample, the overlapped electron wavefunction
permits quantum mechanical tunneling and a
current, I will flow across the vacuum gap.
At low voltage and temperature
d is the distance between tip and sample. If the
distance increased by 1 Angstrom, the current
flow decreased by an order of magnitude, so the
sensitivity to vertical distance is terribly
high.
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Caution
STM does NOT probe the nuclear position directly,
but rather it is a probe of the electron
density, so STM images do not always show the
position of the atoms, and it depends on the
nature of the surface and the magnitude and sign
of the tunneling current.
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Local barrier height Equation (2) obviously shows
the current is exponentially depends on both gap
distance and the local barrier height Change of
current might be due to corrugation of the
surface or to the locally varying local barrier
height. The two effects can be separated by the
relationship.                                  
     
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Local Density of States (LDOS) Density of States
(DOS) represents the amount of electrons exist
at specific values of energy. The tunneling
conductance,   (or I/V ) is proportional to the
LDOS.                                      wh
ere r(r, E) is the local density of states of the
sample. Keeping the gap distance constant,
measure the current change with respect to the
bias voltage can probe the LDOS of the sample.
Moreover, changing the polarity of bias voltage
can get local occupied and unoccupied states.
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Electronic states in Tip
Electronic states in sample
Unoccupied
unoccupied
Occupied
Occupied
When the tip is negatively biased, electrons
tunnel from the occupied states of the tip to
the unoccupied states of the sample. If the tip
is positively biased, electrons tunnel from the
occupied states of sample to the unoccupied
states of the tip.
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Si ( 100 ) surface Change tip bias
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