Nature Nanotech. 7, 227–231 (2012)

In the past few years, advances in scanning and force microscopies have enabled scientists to view individual molecules with unprecedented clarity. As well as providing stunning pictures of molecular features few thought it would ever be possible to 'see', the techniques have already been used to identify a natural product.

Credit: © 2012 NPG

Now, Fabian Mohn and colleagues at IBM Research in Zürich have probed within a molecule and revealed its charge distribution. They used a combination of scanning tunnelling microscopy (STM), atomic force microscopy and Kelvin probe force microscopy (KPFM) to study naphthalocyanine. This molecule, which can be electrically switched between tautomers that differ in the position of hydrogen atoms in the central ring, was placed on an insulating sodium chloride layer on a copper surface.

On examining naphthalocyanine using STM, Mohn and colleagues found that increasing the bias voltage meant they could see an asymmetry in the centre of the molecule, reflecting the position of the hydrogen atoms (pictured, top left). Moving to using KPFM with a copper probe tip, they found extra charge density in the regions of the molecule perpendicular to the hydrogen atoms (pictured, bottom left). Mohn and colleagues electrically switched the position of the hydrogen atoms and saw the charge density swap to the other regions.

To obtain even more resolution, they used a CO-functionalized tip. The images obtained (such as that pictured on the right) reveal the location of the charge density more precisely, showing that the outermost aromatic ring and the four outer nitrogen atoms take the lion's share.