Basic Imaging Modes

1. Contact mode AFM2. Lateral Force Microscopy ( LFM)3. Scanning tunneling microscopy

Position sensitive Photo-detector

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AFM tip/cantilever assemblyForce detection systemElectronic Feedback system (Electronic Brain)Scanner ( precise position control system)Vibration damping system

Summarize: Main components of AFM

Position sensitive photodetector

x2

x1 Hook’ law: F= - kx

Force Detection system

k: Spring constant of the cantilever

Materials, and dimension of thecantilever

k increases with lever thickness, decreases with lever length

Contact ModeThe 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

( Pb(Ti, Zr)O3) PZT type )Materials: lead zirconate titanates

Reverse piezoelectric effect

The electrical polarization produces is proportional to the stress and the direction.The polarization changes if the stress changes from compression to tension

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.

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.

Advantages:

Disadvantages:

Contact imaging mode

Scanning Tunneling Microscopy (STM)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

ITunneling Current

FeelV

D: distance between tip and sample

Resolution

Flat surface

Electronic brain Current image

Tip sharpnessHow 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 wavefunctionsoverlap. 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.

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.

Caution

Local barrier heightEquation (2) obviously shows the current is exponentially depends on both gap distance and the local barrier heightChange 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, 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.

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.

Unoccupied

Electronic states in Tip

Occupied

Electronic states in sample

Occupied

unoccupied

Si ( 100 ) surface

Change tip bias

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