Presentation is loading. Please wait.

Presentation is loading. Please wait.

Scanning tunnelling spectroscopy. STM, STS, and the densities of states includes effect of tunnel barrier shape.

Similar presentations


Presentation on theme: "Scanning tunnelling spectroscopy. STM, STS, and the densities of states includes effect of tunnel barrier shape."— Presentation transcript:

1 Scanning tunnelling spectroscopy

2 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, d 2 I/d 2 V Measure curves in each point of an image, display di/dV maps at specific voltages dI/dV of Ag(111) at different biases K. Morgenstern et al., Phys. Rev. B 71 (2005) Display dI/dV vs. V in points of interest Overgaag et al., ACS Nano, in press (2008)

5 Spectroscopy on isolated PbSe quantum dots Spectroscopy on PbSe and CdSe quantum dots Spectroscopy on PbSe quantum dots dimers, trimers, and larger aggregates

6 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.

7 Lock-in amplifier Orthogonality relationships for sinusoidal functions: Out of phase

8 Lock-in amplifier e.g. I(V) curve measured over 10 seconds (e.g.1kHz):

9 Lock-in amplifier From material by R. Scholten – University of Melbourne

10 Lock-in amplifier – noise reduction

11 Derivative with lock-in

12

13 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 InputOutput Mixer

14 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)

15 Spectroscopy to determine doping GaAs(110) n-type GaAs overgrowth p-type E F =E C -kTln(r C /r D ) C: Conduction band V: Valence band A: Additional tip induced charge E F =kTln(r V /r A )-E V

16 STS on a nanowire a

17

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

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

20 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)

21 Further improvements and problems Bandbending. Changes in tip will induce changes in tunneling spectra!!! Safe way: compare with ab-initio theory Suppres exp. rise in current

22 Full calculation with sample + tip!!

23 Tip influences sample...


Download ppt "Scanning tunnelling spectroscopy. STM, STS, and the densities of states includes effect of tunnel barrier shape."

Similar presentations


Ads by Google