The atomic force microscope (AFM) was invented five years after the scanning tunnelling microscope (STM), but it has since become the most widely used form of scanning probe microscopy. However, the STM has remained the pre-eminent instrument for imaging and manipulating structures on the atomic scale. Now, researchers at IBM's Zurich Research Laboratory and Utrecht University have shown that an AFM operated in non-contact mode can surpass the imaging capabilities of its elder sibling by resolving the atoms within a molecule adsorbed on a surface (Science 325, 1110–1114; 2009).
Leo Gross and colleagues examined molecules of pentacene (C22H14) adsorbed on a copper(111) surface and on a sodium chloride film. With the STM, resolving atoms within an adsorbed molecule is difficult because the tunnelling current is sensitive to the local electron density of states near the Fermi level, and this density of states extends over the entire pentacene molecule. However, by adding a carbon monoxide molecule to the tip of an AFM, and by probing the short-range chemical forces, Gross and colleagues were able to resolve the atomic positions and bonds inside the pentacene molecules.
The AFM image above, which measures 27.5 Å across, clearly shows the five fused benzene rings in the pentacene molecule. To take the image Gross and colleagues scanned the AFM, which was oscillating with an amplitude of 0.2 Å, at a constant height above the surface and measured how the force on the tip changed with position.
By comparing the experimental data with the results of density functional theory calculations, the researchers found that Pauli repulsion forces are the origin of the atomic resolution.
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Vaughan, O. A closer look at the atoms in a molecule. Nature Nanotech 4, 620 (2009). https://doi.org/10.1038/nnano.2009.283