Scanning Probe Techniques

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Scanning tunnelling spectroscopy
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STM, STS, and the densities of states
includes effect of tunnel barrier shape
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Spectroscopy what can we learn?
dI/dV reflects major features of local density
of states of the sample ( tip) near the fermi
level. We can learn about the electronic
properties (valence states) of the surface very
locally!
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Scanning Tunnelling Spectroscopyhow to do it
Switch off feedback loop and measure one of the
following curves I(V) , I(z) , dI/dV , dI/dz ,
d2I/d2V
Measure curves in each point of an image, display
di/dV maps at specific voltages
Display dI/dV vs. V in points of interest
dI/dV of Ag(111) at different biases K.
Morgenstern et al., Phys. Rev. B 71 (2005) 155413
Overgaag et al., ACS Nano, in press (2008)
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Spectroscopy on PbSe and CdSe quantum dots
Spectroscopy on PbSe quantum dots dimers,
trimers, and larger aggregates
Spectroscopy on isolated PbSe quantum dots
Overgaag et al., ACS Nano, in press (2008)
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Experimental considerations in STS
Stable STM is needed - STS is extremely
sensitive to noise! Cool sample, make sure you
have good vibrational damping and a good tip.
Instead of measuring I(V) and taking the
derivative measure dI/dV right away! Use
Lock-in amplifier.
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Lock-in amplifier
Orthogonality relationships for sinusoidal
functions
Out of phase
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Lock-in amplifier
e.g. I(V) curve measured over 10 seconds
(e.g.1kHz)
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Lock-in amplifier
From material by R. Scholten University of
Melbourne
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Lock-in amplifier noise reduction
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Derivative with lock-in
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Derivative with lock-in
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Phase control

• Reference has phase control
• Can vary from 0 to 360
• Arbitrary input signal phase
• Tune reference phase to give maximum DC output

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dI/dV spectroscopy in nanowires

• Modulate tunneling voltage with a 50mV 1kHz
  sinus signal
• Clean up current measure dI/dV with lock-in
  amplifier
• Now keep STM tip fixed and vary voltage to obtain
  dI/dV vs V
• Density of states at specific positions on the
  surface with atom resolution!

GaAs(110)
The bright feature at (II) is a Ga vacancy (we
are imaging As atoms)
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Spectroscopy to determine doping
GaAs(110) n-type
GaAs overgrowth p-type
EFkTln(rV/rA)-EV
EFEC-kTln(rC/rD)
C Conduction band V Valence band A Additional
tip induced charge
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STS on a nanowire
a
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The role of the second derivative in STS
K. W. Hipps, Handbook of Applied Solid State
Spectroscopy
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The role of the second derivative in STS
Feature enhancement by taking the derivative
B.C. Stipe. et. al., Science 280, 1732 (1998)
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Single Molecule Vibrational Spectroscopy
Vibration excitation of the molecule occurs when
tunneling electrons have enough energy to excite
a quantized vibrational level
Inelastic tunneling channel
B.C. Stipe. et. al., Science 280, 1732 (1998)
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Further improvements and problems
Suppres exp. rise in current
Bandbending.
Changes in tip will induce changes in tunneling
spectra!!! Safe way compare with ab-initio
theory
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Full calculation with sample tip!!
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Tip influences sample...