Scanning Tunneling Microscopy for Molecules: Manipulating Electron Transport through the Conduction Gap by Varying the Buffer Layer.

In scanning tunneling microscopy of molecules, an insulating buffer layer is often introduced to reduce interactions between adsorbed molecules and the substrate. We demonstrate that the buffer itself strongly influences the wave function of the tunneling electron at the molecule for tunneling through the molecule's electronic transport gap. We use a theory, which provides a very good agreement with prior experiments on platinum phthalocyanine on a NaCl buffer, to study effects of varying the buffer's composition and thickness and show the importance of the buffer's lattice parameter. Expanding the tunneling electron's wave function using molecular orbitals (MOs) additionally shows how to control the relative weights at the highest occupied MO (HOMO), lowest unoccupied MO (LUMO), and energetically low-lying MOs with few nodal surfaces. Those with significant weight are key for manipulating molecules with tunneling electrons. When used for up-conversion molecular luminescence, in which emitted photons exceed the input tunneling bias, we find that an intricate competition occurs between tunneling through the HOMO or LUMO versus low-lying MOs. The buffer choice provides a substantive handle for controlling such processes. We predict that it can influence the up-conversion efficiency by an order of magnitude.