Scanning tunnelling microscopy

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Presentation transcript:

Scanning tunnelling microscopy and electronic structure Avdelningen för synkrotronljusfysik, Fysiska institutionen, Lunds universitet Spektroskopi och materiens kvantmekaniska beskrivning FYST20, VT 2009

What is scanning tunnelling microscopy? A. Emundts, Forschungszentrum Jülich, http://www.fz-juelich.de/video/emundts/ Michael Schmid, TU Wien

Atomic resolution with scanning tunnelling microscopy Ag(111), 145 Å x 145 Å

Moving atoms Diffusing Pt atoms on Pt(110) T. R. Linderoth, University of Aarhus

The experimental geometry Image from: Blanco et al., Prof. Surf. Sci. 81 (2006) 403

The experimental geometry Image from: Blanco et al., Prof. Surf. Sci. 81 (2006) 403

Tunnel current f: sample work function I ~ 50 pA – 5 nA Working d: around 5 Å

What does STM have to do with the electronic structure? Everything! At very low biases STM images the electron density at the Fermi level. At higher biases STM images the integrated electron density between the Fermi level and the applied voltage. Actually, STM images represent a convolution of the electronic states of the sample and the tip.

Potential and wave function sample region tip region Simplified potential with energy level Wave function Tunnelling can occur!

Tunnel currents for homogeneous density of states No net current Positive net current Negative net current Conventions: - bias applied to sample - current measured on tip

Mathematical treatment sample tip Solutions for sample electrons Gottlieb and Wesoloski, Nanotechnology 17 (2006) R57

Topographic vs electronic contrast Charge density plot of the TiO2(110) surface, U. Diebold, Surf. Sci. Rep. 48 (2003) 53). The contour lines correspond to a progression of charge density, very much like the contour lines on a topographic map. STM image of the TiO2(110) surface by Diebold et al., Tulane University. The (001) direction is along the oxygen and titanium rows. Brighter areas correspond to a more retracted tip. Interpret the STM image in terms of the oxygen and titanium rows of the sample (a) in a purely topographic interpretation (b) using the charge density plot.

STM on phthalocyanines Metal d orbitals Central atom not observed Bias: +0.5 V Central atom observed Bias: +0.1 V Lu et al., J. Phys. Chem B 101 (1997) 5391 Give an interpretation of the difference in terms of what you’ve learnt!