Pentacene snapshot

28 August 2009 - AFM

You can no longer trust the Swiss to look after your money without interference from the tax collector but as always you can trust them to build high-precision instruments like clocks or lets say a high-definition atomic force microscope. Gross et al. (IBM) have produced intriguing ATM images of the molecule pentacene with unprecedented resolution (DOI).

In the images not only the carbon-carbon bonds are clearly visible but also the carbon-hydrogen bonds. Other notable features: the molecule actually resembles the ball-and-stick model that students of chemistry are supposed to ignore in favor of a space-filling model, the extremities appear to stand out and surprise: the molecules appear to cast a shadow?

For the experiment pentacene is deposited on a copper surface with a 2 atom thick layer of sodium chloride. To the end of the gold ATM tip is attached a carbon monoxide molecule (picked up along the way) and the tip is lowered to 4 Angstrom above the surface and the force exerted on it (at 4 K) is measured. Typically an attractive force of 110 picoNewton is measured inside one of the arene rings and this force decreases to 60-90 pN when the tip hovers above one of the carbon atoms. The researches assign the observed atomic contrast to Pauli repulsion (do not bother to read the pretty worthless Wikipedia article on this topic), the other forces (van der Waals force) and electrostatic forces are attractive but only create background noise, only explaining the observed dark halo.

Interestingly the result is presented as a new technique for molecular structure elucidation like NMR or crystallography.