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

2. What is scanning tunnelling microscopy?
A. Emundts, Forschungszentrum Jülich,
Michael Schmid, TU Wien

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

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

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

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

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

8. 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.

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

10. 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

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

13. 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.

15. 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!