Transcript 0909 2014 STM

Basic Imaging Modes
1. Contact mode AFM
2. Lateral Force Microscopy ( LFM)
3. Scanning tunneling microscopy
Review last
ecture
Position sensitive
Photo-detector
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
Position sensitive photodetector
Force Detection system
x1
Hook’ law: F= - kx
k: Spring constant of the cantilever
x2
Materials, and dimension of the
cantilever
k increases with lever thickness, decreases with lever length
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.
Atomic Force Microscopy
Force mapping
Very rough surface
Tip crashed or sample destroyed
Smarter imaging ---Need a Feedback system
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 )
Review last lecture
Youtube:
AFM Principle - How AFM Works
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.
Measure lateral twist of the cantilever
Cantilever Movements and optical deflection detection
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.
Contact imaging mode
Advantages:
Disadvantages:
Scanning Tunneling Microscopy (STM)
– Tunneling current ( pA ~ nA) between tip and
sample is exponentially dependent on their
separation ( <= 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
Scanning Tunneling Microscopy
V
Feel
I
Tunneling Current
D: distance between tip and sample
Flat surface
Electronic brain
Current image
Resolution
Tip sharpness
How to make a tip
Smart way!
Current measurement
Feedback system
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.
If two conductors are so close that their leak out electron wavefunctions
overlap. The electron wavefunctions at the Femi level K is given by:
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.
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.
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.
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
where 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.
Electronic states in Tip
Electronic states in sample
Unoccupied
Occupied
unoccupie
d
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.
Si ( 100 ) surface
Change tip bias
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