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Hybrid-basis modeling of electron transport through molecules on silicon

Following considerable recent progress in experimental techniques and theoretical modeling of molecular electronics, the promise of molecular devices has attracted the interest of a lot of physicists, chemists, and engineers towards electron transport in such nanoscale systems. The conducting properties of molecular devices have been theoretically studied by using both first-principles and semi-empirical methods. In this work, we develop a formalism to couple two different basis functions in order to accurately model both molecules and contacts under non-equilibrium condition. As an example, we use EHT parameters optimized to describe bulk silicon, and couple it with an ab-initio basis set, 6-3lg(d), to simulate the contact surface atoms and the molecules grown on silicon. Such a coupling is achieved by matching the surface Green's function in real space in both basis sets. Moreover, we use this hybrid-basis formalism to couple the contacts with a density-functional treatment of the molecule to simulate scanning tunneling spectroscopy (STS) measurements of C60 on a Si substrate. Our simulated conductance-voltage curves can be used to explain the different STS results observed experimentally due to the different types of bonding between C60 and silicon. As a final application of our hybrid-basis modeling technique, we describe STM measurements of organic molecules grown on silicon. Our simulations exhibit a prominent negative-differential resistance (NDR) in such molecular I-Vs due to the interaction between the molecular levels and the silicon band-edge. We are able to use these results to qualitatively interpret experimental observations of room-temperature negative differential resistance.