Scanning tunnelling spectroscopy

r STM, STS, and the densities of states 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 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)

Spectroscopy on PbSe quantum dots dimers, trimers, 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)

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.

Lock-in amplifier 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 Phase shift f 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) EF=kTln(rV/rA)-EV C: Conduction band V: Valence band A: Additional tip induced charge

STS on a nanowire a

The role of the second derivative in STS K. W. Hipps, Handbook of Applied Solid State Spectroscopy

The role of the second derivative in STS Feature enhancement by taking the derivative B.C. Stipe. et. al., Science 280, 1732 (1998)

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)

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

Full calculation with sample + tip!!

Tip influences sample...