Development of a multi-space constrained density functional theory for non-equilibrium quantum transport simulations

Abstract

In this thesis, we develop a novel method to properly describe the working mechanism of nanoscale devices which has been unable to deal with within first-principles. The thesis starts by reporting novel methods regarding to (near-) equilibrium quantum transport simulation based on matrix Green''s function formalism (MGF) combined with DFT. In the first part, we suggest the employment of MGF method to effectively describe metal-organic interface problems, especially on electronic interaction and energy level alignment. Based on this, we devise a way to achieve a reliable metal-organic contact with orientation-independent charge injection. The second part is on the selection of optimal basis sets for the application of the tunneling devices. For the complicated biosensor systems, we next derive rules of thumb for the selection of basis sets in quantum transport calculations. Then, we develop a novel first-principles electronic structure calculation method for the junction systems subjected to non-equilibrium conditions: multispace-constrained density functional theory (ms-cDFT). Based on this, we observe and report a device characteristics of 2D layered materials-based vertical junctions: a vertically-stacked graphene-hexagonal boron nitride-graphene tunneling device. By following variational principles, the working mechanism of various devices with respect to different atomistic details can be correctly described. The new method can be generally utilized in studying the various applications such as bias-driven solvent dynamics of electrochemical system, bias-dependent catalytic activity, and molecular sensor.