University of Exeter Shield

Theoretical Physics
Simulations of STM Imaging

This page contains the following:

1. What is STM?

Scanning Tunnelling Microscopy (STM) is a microscopy technique which allows you to see individual atoms on a surface. It works by scanning an atomic scale tip across the surface of the material. A tiny tunnelling current passes between the tip and the surface. This tunnelling current is highly dependant on the density of states. There are then two ways of operating the microscope:
  1. Constant Current Mode. The tip is allowed to move up and down so that the current passing between the tip and the surface is always constant. Thus as the tip scans over different atom types and surface heights it moves up and down to keep the current constant. The microscope measures this and ends up with a `contour map' of the surface.
  2. Constant Distance Mode. The tip is kept at a constant distance from the surface, and the microscope then measures the change in current as the tip moves across the surface.

2. How do we simulate it?

At the moment our work in this area is in the early stages of development. We are able to accurately model the charge and wave function spatial distribution in our systems, and from this construct various contour maps. Current work is based on a benzene molecule resting on the surface of graphite, however this will be expanded in the future. We have two MPhys students, J. Hogan and R. MacKie developing the modelling techniques. The following are some preliminary test images produced by C. Ewels:

A Charge density plot of Benzene on Graphite (40K)

A wave function plot of the same system (41K)

NB: The carbon atoms on the charge density plot appear as darker dots than the surrounding regions. This is because we use pseudo-potentials to simulate the core electrons that are not involved in bonding. This means that they do not appear in the total charge density, and so appear darker. Since the hydrogen atoms do not have any core states they do not show the same effect.

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