First Principles Quantitative Modeling of Molecular Devices.

摘要

In this thesis, we report theoretical investigations of nonlinear and nonequilibrium quantum electronic transport properties of molecular transport junctions from atomistic first principles. The aim is to seek not only qualitative but also quantitative understanding of the corresponding experimental data. At present, the challenges to quantitative theoretical work in molecular electronics include two most important questions: (i) what is the proper atomic model for the experimental devices? (ii) how to accurately determine quantum transport properties without any phenomenological parameters? Our research is centered on these questions. We have systematically calculated atomic structures of the molecular transport junctions by performing total energy structural relaxation using density functional theory (DFT). Our quantum transport calculations were carried out by implementing DFT within the framework of Keldysh non-equilibrium Green's functions (NEGF). The calculated data are directly compared with the corresponding experimental measurements. Our general conclusion is that quantitative comparison with experimental data can be made if the device contacts are correctly determined.;We investigated the role of contact formation and its resulting structures to quantum transport in several molecular wires and show that interface contacts critically control charge conduction. It was found, for Au/BDT/Au junctions, the H atom in -SH groups energetically prefers to be non-dissociative after the contact formation, which was supported by comparison between computed and measured break-down forces and bonding energies. The H-non-dissociated (HND) junctions give equilibrium conductances from0.054G0 (equilibrium structure) to 0.020G0 (stretched structure) which is within a factor of 2-5 of the measured data. On the other hand, for all H-dissociated contact structures - which were the assumed structures in the literature, the conductance is at least more than an order of magnitude larger that the experimental value. The HND-model significantly narrows down the theory/experiment discrepancy. Finally, a by-product of this work is a comprehensive pseudopotential and atomic orbital basis set database that has been carefully calibrated and can be used by the DFT community at large.;We calculated properties of nonequilibrium spin injection from Ni contacts to octane-thiolate films which form a molecular spintronic system. The first principles results allow us to establish a clear physical picture of how spins are injected from the Ni contacts through the Ni-molecule linkage to the molecule, why tunnel magnetoresistance is rapidly reduced by the applied bias in an asymmetric manner, and to what extent ab initio transport theory can make quantitative comparisons to the corresponding experimental data. We found that extremely careful sampling of the two-dimensional Brillouin zone of the Ni surface is crucial for accurate results in such a spintronic system.