Density of States imaging
As long as measured in STM current is determined by the tunneling processes through tip-sample surface gap its value depends not only on the barrier height but on the electron density of states also. Accordingly obtained in STM images are not simply images of sample surface relief (topography), these images can be hardly affected by the density of electronic states distribution over the sample surface. Good example of Local Density of States (LDOS) influence on the STM image is well-known image of highly oriented pyrolitic graphite (HOPG) atomic lattice. Only half atoms are visible in STM. Similar case is image of GaAs atomic lattice.
LDOS determining can also help to distinguish chemical nature of the surface atoms. LDOS acquisition is provided simultaneously with the STM images obtaining. During scanning the Bias Voltage is modulated on the value dU, the modulation period is chosen to be much shorter than the time constant of the feedback loop in the STM.
Suitable modulation of tunnel current dI is measured, divided by dU and presented as LDOS image. On Example the topography and LDOS image of HOPG sample are presented.
References
- G. Binnig and H. Rohrer: Surf. Sci. 126 (1983) 236. 2. Rep. Prog. Phys. 55, 1165-1240 (1992).
