Pressure and resonance effects in scanning tunneling microscopy of molecular adsorbates

We have investigated an empirical tight-binding model of a tunnel junction which contains a molecule in the gap. Tunnel conduction is enhanced if the molecule has a molecular orbital (MO) with an eigenenergy close to the Fermi energy (E_F) of the metal. A special case arises when the gap contains an insulator in which a molecule (with a resonant MO) is located equidistant from the metal electrodes. The transmission through this double barrier can approach unity, independent of the thickness of the gap. A resonant MO enhances tunnel conduction even when the geometry is far from optimum. There have been many reports of molecular adsorbates which appear to act as conductors when imaged with the scanning tunneling microscope (STM). Images of DNA constitute the largest group, but electrochemical studies of nucleic acids indicate that MO's are about an electronvolt from E_F for typical metals, so that resonant tunneling alone will not account for the STM images. However, the pressure in the STM tunnel gap may approach a few gigapascals. The ultraviolet absorption of DNA is red-shifted by ∼0.5 eV at these pressures, indicating that increased intermolecular interaction has broadened and shifted MO's by at least this much. The STM may operate by adjusting the gap transducer so that an MO is moved by the amount required to establish the set-point current through the resonant tunneling enhancement effect. The image contrast is a complicated function of the mechanical and electronic properties of the tip, substrate, and adsorbate molecules, and usually cannot be interpreted in terms of height.