scanning tunnelling spectroscopy. stm, sts, and the densities of states includes effect of tunnel...
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Scanning tunnelling spectroscopy
STM, STS, and the densities of states
EFermi
EFermi
Energy
xSample Tip
sample tip
includes effect of tunnel barrier shape
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!
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)
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
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.
Lock-in amplifier
0'sinsin
dttt
2sinsin
ABdttBtA
0cossin
dttt
Orthogonality relationships for sinusoidal functions:
Out of phase
Lock-in amplifier
e.g. I(V) curve measured over 10 seconds
(e.g.1kHz):
Lock-in amplifier
From material by R. Scholten – University of Melbourne
Lock-in amplifier – noise reduction
Derivative with lock-in
Derivative with lock-in
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
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)
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
STS on a nanowire
a
K. W. Hipps,Handbook ofApplied Solid State Spectroscopy
The role of thesecond derivativein STS
The role of the second derivative in STS
Feature enhancementby taking the derivative
B.C. Stipe. et. al., Science 280, 1732 (1998)
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)
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
Full calculation with sample + tip!!
Tip influences sample...