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...