2 r STM, STS, and the densities of states includes effect of tunnel barrier shape
3 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!
4 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/d2VMeasure curves in each point of an image,display di/dV maps at specific voltagesDisplay dI/dV vs. V in points of interestdI/dV of Ag(111) at different biasesK. Morgenstern et al., Phys. Rev. B 71 (2005)Overgaag et al., ACS Nano, in press (2008)
5 Spectroscopy on PbSe quantum dots dimers, trimers, Spectroscopy on PbSe and CdSe quantum dotsSpectroscopy on PbSe quantum dots dimers, trimers,and larger aggregatesSpectroscopy on isolated PbSe quantum dotsOvergaag et al., ACS Nano, in press (2008)
6 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.
7 Lock-in amplifierOrthogonality relationships for sinusoidal functions:Out of phase
8 Lock-in amplifiere.g. I(V) curve measured over 10 seconds(e.g.1kHz):
9 Lock-in amplifierFrom material by R. Scholten – University of Melbourne
13 Phase control Reference has phase control Can vary from 0 to 360° Arbitrary input signal phaseTune reference phase to give maximum DC outputReferencePhaseshift fInputOutputMixer
14 dI/dV spectroscopy in nanowires Modulate tunneling voltage with a 50mV ~1kHz sinus signalClean up current & measure dI/dV with lock-in amplifierNow keep STM tip fixed and vary voltage to obtain dI/dV vs VDensity 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)
15 Spectroscopy to determine doping GaAs(110) n-typeGaAs overgrowth p-typeEF=EC-kTln(rC/rD)EF=kTln(rV/rA)-EVC: Conduction bandV: Valence bandA: Additional tip induced charge
18 The role of the second derivative in STS K. W. Hipps, Handbook of Applied Solid State Spectroscopy
19 The role of the second derivative in STS Feature enhancementby taking the derivativeB.C. Stipe. et. al., Science 280, 1732 (1998)
20 Single Molecule Vibrational Spectroscopy Vibration excitation of the molecule occurs when tunneling electrons have enough energy to excite a quantized vibrational levelInelastic tunneling channelB.C. Stipe. et. al., Science 280, 1732 (1998)
21 Further improvements and problems Suppres exp. rise in currentBandbending.Changes in tip will induce changesin tunneling spectra!!!Safe way: compare with ab-initio theory