Scanning tunnelling microscopyand

electronic structure

Avdelningen för synkrotronljusfysik, Fysiska institutionen, Lunds universitetSpektroskopi och materiens kvantmekaniska beskrivning FYS T20, VT 2008

What is scanning tunnelling microscopy?

Michael Schmid, TU Wien A. Emundts, Forschungszentrum Jülich,http://www.fz-juelich.de/video/emundts/

Ag(111), 145 Å x 145 Å

Atomic resolution with scanning tunnelling microscopy

Moving atoms

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

The experimental geometry

V- +

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

V-+

The experimental geometry

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

d

dm

eI22

Tunnel current

: sample work function I ~ 50 pA – 5 nA

Working d: around 5 Å

What does STM have to do withthe electronic structure?

Everything!

At very low biases STM images the electron densityat the Fermi level.

At higher biases STM images the integrated electron densitybetween the Fermi level and the applied voltage.

Actually, STM images represent a convolution of theelectronic states of the sample and the tip.

Potential

Simplified potentialwith energy level

Wave function

Potential and wave function

sample region

tip region

Tunnelling can occur!

EFermi EFermi

Energy

xSample Tip

EFermi

EFermi

Energy

xSample Tip

-eV

-eV

EFermi

EFermi

Energy

xSample Tip

No bias

Negative bias

Positive bias

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

Solutionsfor sampleelectrons

sample tip

Gottlieb and Wesoloski,Nanotechnology 17 (2006) R57

Pk

3x10-3

210-1-2-3E

k - (eV)

Pt for different times t

t = 10 ps t = 1 ps t = 0.1 ps

Topographic vs electronic contrast

STM image of the TiO2(110) surface byDiebold et al., Tulane University.The (001) direction is along the oxygen andtitanium rows.Brighter areas correspond to a more retractedtip.

Charge density plot of the TiO2(110) surface, U. Diebold,Surf. Sci. Rep. 48 (2003) 53). The contour lines correspondto a progression of charge density, very much like the contourlines on a topographic map.

Interpret the STM image in terms of the oxygen and titaniumrows of the sample(a) in a purely topographic interpretation(b) using the charge density plot.

EFermiEFermi

Energy

xSample Tip

Real (inhomogeneous) density of states

EFermi

EFermi

Energy

xSample Tip

very smalltunnelling current!

EFermi

EFermi

Energy

xSample Tip

STM on phthalocyanines

Central atom not observedBias: +0.5 V

Central atom observedBias: +0.1 V

Lu et al., J. Phys. Chem B 101 (1997) 5391

Metal d orbitals

Give an interpretation of the differencein terms of what you’ve learnt!