The resolution of a scanning probe microscope can be improved by attaching a small molecule to the tip of a probe and this has previously allowed individual atoms and bonds within adsorbed organic molecules to be resolved. Researchers at IBM's Zurich Research Laboratory, Universidade de Santiago de Compostela and CEMES-CNRS in Toulouse have now shown that an atomic force microscope (AFM) with a carbon monoxide molecule on its tip can also be used to discriminate between different types of chemical bonds (Science 337, 1326–1329; 2012).

Credit: © IBM Research, Zurich

Leo Gross and colleagues imaged C60 molecules and polycyclic aromatic hydrocarbons adsorbed on a copper surface and found that carbon–carbon bonds within the molecules had different contrasts and apparent lengths, features that could be correlated with their bond orders. The images are formed by measuring the change in resonant frequency, or frequency shift, of the oscillating cantilever on which the tip is mounted: repulsive forces create a positive frequency shift, attractive forces a negative shift. Bonds with a higher bond order, which have a greater electron density, appeared brighter due to a stronger Pauli repulsion between the tip and the sample. The shorter length of such bonds could also be distinguished because the CO molecules at the apex of the tip tilt during imaging, which amplifies the apparent differences in length.

The AFM image (pictured), which measures approximately 20 Å across, shows a polycyclic aromatic hydrocarbon composed of 13 fused benzene rings. The bonds of the ring at the centre of the planar molecule are of greater bond order than the bonds connecting the central ring to the outer rings, and accordingly appear brighter and have a shorter apparent length. The bonds can be distinguished despite the fact that they differ in length by only 0.03 Å. (The bonds at the periphery of the molecule are subject to a variety of effects, which obscure the bond-order differences.)