Effect of bonding and antibonding character of electronic states on their tunneling spectra

Abstract : We argue that the bonding or antibonding character of electronic states of a substrate can influence significantly the observed scanning tunneling microscopy (STM) spectra. In particular, the antibonding states formed by nonorthogonal orbitals of the substrate present much stronger extension in the vacuum compared to corresponding bonding combinations and are therefore much more visible in STM experiments. A clear example is provided by black phosphorus (BP), which is one of the most interesting two-dimensional layered materials nowadays. The pronounced asymmetry in its conductance spectra, with significant increase at positive voltage, can be attributed to the antibonding nature of the conduction band compared to the bonding valence band, despite the same pz -orbital character. Furthermore, this asymmetry can be at the origin of different broadenings of frontier molecular orbitals observed experimentally for molecules on BP.