Scanning tunnelling spectroscopy. STM, STS, and the densities of states includes effect of tunnel...
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Transcript of Scanning tunnelling spectroscopy. STM, STS, and the densities of states includes effect of tunnel...
![Page 1: Scanning tunnelling spectroscopy. STM, STS, and the densities of states includes effect of tunnel barrier shape.](https://reader035.fdocuments.in/reader035/viewer/2022081602/5517e4c9550346c1568b468e/html5/thumbnails/1.jpg)
Scanning tunnelling spectroscopy
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STM, STS, and the densities of states
EFermi
EFermi
Energy
xSample Tip
sample tip
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 Spectroscopy:how 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
dI/dV of Ag(111) at different biasesK. Morgenstern et al., Phys. Rev. B 71 (2005) 155413
Display dI/dV vs. V in points of interest
Overgaag et al., ACS Nano, in press (2008)
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Overgaag et al., ACS Nano, in press (2008)
Spectroscopy on isolated PbSe quantum dots
Spectroscopy on PbSe and CdSe quantum dots
Spectroscopy on PbSe quantum dots dimers, trimers,and larger aggregates
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Stable STM is needed - STS is extremely sensitive to noise!
Cool sample, make sure you have good vibrational damping and a good tip.
Experimental considerations in STS
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
0'sinsin
dttt
2sinsin
ABdttBtA
0cossin
dttt
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
Reference
Phaseshift
Input Output Mixer
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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
EF=EC-kTln(rC/rD)
C: Conduction bandV: Valence bandA: Additional tip induced charge
EF=kTln(rV/rA)-EV
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STS on a nanowire
a
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K. W. Hipps,Handbook ofApplied Solid State Spectroscopy
The role of thesecond derivativein STS
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The role of the second derivative in STS
Feature enhancementby taking the derivative
B.C. Stipe. et. al., Science 280, 1732 (1998)
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Vibration excitation of the molecule occurs when tunneling electrons have enough energy to excite a quantized vibrational level Inelastic tunneling channel
Single Molecule Vibrational Spectroscopy
B.C. Stipe. et. al., Science 280, 1732 (1998)
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Further improvements and problems
Bandbending.
Changes in tip will induce changesin tunneling spectra!!!
Safe way: compare with ab-initio theory
Suppres exp. rise in current
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Full calculation with sample + tip!!
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Tip influences sample...