Tunneling is the weird phenomenon that results in particles having less energy than their potential energy - ie they have negative kinetic energy. That's strange I hear you say, and indeed it is. As with all these things classical mechanics says it's impossible, so you need to look to the murky world of quantum theory to understand it.
A tip is scanned over a surface at a distance of a few atomic diameters in a point-by-point and line-by-line fashion. At each point the tunneling current between the tip and the surface is measured. The tunneling current decreases exponentially with increasing distance and thus, through the use of a feedback loop, the vertical position of the tip can be adjusted to a constant distance from the surface.
The amount of these adjustments is recorded and defines a grid of values which can be displayed as a grayscale image.
Instead of assigning the values to a color we can also use them to deform the grid in the direction perpendicular to the surface.
Now we can bring back the grayscale and paint each square according to an average of the four defining grid points.
graph taken from: Physical Chemistry, Peter Atkins, 6th Ed, OUP (1998)
This page has been created by Peter White, June 2001