First-principles analysis of the STM image heights of styrene on Si(100)

We report on theoretical investigations of scanning tunneling spectroscopy (STM) image heights on Si(100). Calculations are performed using density functional theory (DFT) within the Keldysh nonequilibrium Green's function (NEGF) formalism. The nonequilibrium potential drop between Si(100) and a STM tip is determined self-consistently. This potential drop is found to play an important role in the calculated image height characteristics of adsorbed hydrocarbons by lowering the vacuum barrier and shifting molecular levels. Numerical data collected for image heights of styrene against a hydrogen passivated Si(100) background are found to agree quantitatively with the corresponding experimental results. We also present a comparison between results obtained by the NEGF-DFT formalism and the Tersoff-Hamann approximation, showing that nonequilibrium analysis can be important in the study of STM image heights of molecules.