Crystalline graphite is routinely used to calibrate scanning tunnelling microscopes because its surface is clean and flat on the atomic scale. However, scanning tunnelling microscopes (STMs) can only detect half of the carbon atoms in the hexagonal surface unit cell. Now Stefan Hembacher and colleagues at the University of Augsburg in Germany and Stanford University in the US have designed a new low-temperature atomic force microscope that can reveal these ‘hidden’ atoms (S Hembacher et al. 2003 Proc. Nat. Acad. Sci. 100 12539).
An STM consists of an oscillating metallic tip that scans the surface of a sample while hovering about 1 nanometre above it. Electrons quantum mechanically tunnel across this tiny gap and the size of the current depends on the size of the gap. Researchers measure this current to build up a picture of the surface. In many materials, all the atoms in the surface contribute to the current, so they can all be imaged. However, graphite contains two kinds of atom in the surface plane: beta atoms which have mobile electrons that contribute to the tunnelling current, and alpha atoms which do not. This means that only the beta atoms can be seen by the STM (figure 1A).
In atomic force microscopy (AFM) the attractive force between the tip and sample is measured, but this method cannot reveal the hidden graphite atoms either because the origins of the tunnelling current in STM and the attractive force in AFM are directly related. However, if the distance between the tip and sample is reduced, these forces become repulsive. Moreover, the electrons belonging to the alpha atoms contribute to these repulsive forces, which allows the alpha atoms to be seen (figure 1B).
Hembacher and co-workers developed an AFM technique with enhanced sensitivity to these short-range forces by reducing the oscillation of the tip and operating the microscope at a temperature of 5K to reduce thermal and electrical noise. They also combined their new technique with an STM in a single instrument so that both tunnelling currents and repulsive forces could be probed at the same time.
“It is nice to see that the theories about the electronic structure of graphite are correct,” Franz Giessibl of Augsburg told PhysicsWeb. “More importantly we can now image matter and see where all the atoms are and where the conductive states are localized.” The team says that the microscope allows them to probe matter in a “gentle fashion”, and could thus be used to study organic and biological materials that are difficult to analyze with traditional imaging techniques.
