Computational Study of Electron Transport and Dynamics in Nano-Devices

Abstract

Devices for nanoscale electronics are proposed. First principles computations are used to evaluate the performance of devices. A nanoscale electronic device is made up of a molecule, as a performing unit, sandwiched by electrodes (and sometimes linkers to bind them all). Understanding the effects of each component of a molecular device is at the heart of designing a useful device. Firstly, the effects of electrodes on the characteristics of an electronic device are studied. Transport characteristics of Au, Ru, and carbon nanotube electrodes are investigated using the nonequilibrium Green function (NEGF) method combined with a density functional theory (DFT). By systematic modification of the device region, the effect of the electrode materials on the electron transport is extracted. The band structure and surface density of states of an electrode material, independent of the choice of other device components, have unique influences on the transmission curve. Carbon nanotube electrodes can offer unusual nonlinear current−voltage characteristics due to semiconducting nature. Secondly, it is shown that simple materials can be combined to play a role of a spin valve, where conductance is controlled by magnetization of electrodes. Graphene is a promising material for spintronics due to its outstanding spin transport property. Its maximally exposed 2pz orbitals allow tuning of electronic structure toward better functionality in device applications. Using the fact that the positions of carbon atoms are commensurate with those of Ni atoms on the substrate, a graphene spin-valve device is proposed based on the epitaxial graphene grown on the Ni (111) surface. Its transport properties are explored with NEGF theory combined DFT. The device has magnetoresistance (110%) due to the strong spin-dependent interaction between the Ni surface and the epitaxial graphene sheet.Thirdly, a DNA sequencing device based on graphene nanoribbon (GNR) is proposed and scrutinized. An ultrafast DNA sequencing can be achieved by electrically distinguishing nucleobases on GNR. Once a nucleobase sit on GNR utilizing π–π interaction, Transmission dips appear in regions characteristic of a nucleobase. Analyzing the molecular orbitals and the features of dips in conductance for our GNR-based sequencing device, it is proven that the Fano resonance is responsible for the characteristic dips of each nucleobase. On the other hand, - interaction is essential in that it reduces a noise in measurements, a major problem suffered by many nanopore-based sequencing devices. Consequently, the complexes of a DNA base bound to graphitic systems are studied. Considering naphthalene as the simplest graphitic system, DNA base-naphthalene complexes are scrutinized up to coupled cluster theory with singles, doubles and perturbative triples excitations [CCSD(T)] at the complete basis set (CBS) limit. The stacked configurations are the most stable where dispersion is a principal cause of binding. We compared the CCSD(T)/CBS results with several density functional methods applicable to periodic systems. The predicted values are 18-24 kcal/mol, large enough to hold a nucleobase tightly on GNR, although many-body effects on screening and energy need to be further considered.