NEGF-DFT: a first principles formalism for modeling molecular electronics

Using molecules as functional units for electronic device application[1] is an interesting perspective and a possible goal of nano-electronics. Work in this field has clearly demonstrated that many of the important molecular device characteristics relate specifically to a strong coupling between the atomic and the electronic degrees of freedom. However, from a theoretical point of view, the accurate prediction of the properties of atomic and molecular scale devices – including the true I-V curves with as few adjustable parameters as possible – still represents a formidable challenge, despite the advances and wide-spread application of large scale density functional theory (DFT) based ab initio modeling over the last two decades. Recall that most of the previous DFT-based ab initio condensed matter simulations[2,3] solve only two kinds of problems: (i) finite systems such as isolated molecules, as in quantum chemistry; (ii) periodic systems consisting of supercells, as in solid state physics. However, a molecular electronic device is neither finite nor periodic: it typically has open boundaries which are connected to long and different electrodes extending to electron reservoirs far away, where the external bias potentials are applied. In other words, calculations of finite or periodic systems do not have the correct boundary conditions for quantum transport. Therefore, new formalisms for electronic analysis are required to carry out first principles transport modeling for molecular electronics. In this work, we breifly outline a formalism that combines DFT with the Keldysh non-equilibrium Green’s functions (NEGF) so that nonequilibrium quantum transport properties can be predicted from atomistic approach without any phenomenological parameters[4,5]. We then apply our NEGF-DFT approach to investigate a C60 molecular junction connected to Au electrodes. The rest of the paper is organised as follows. In Sec.2 we present the NEGFDFT formalism. In Sec. 3 we present the transport properties of the Au-C60-Au tunnel junction. Sec. 4 is reserved for a short summary and outlook of future work. 2 NEGF-DFT Formalism