First-Principles Investigation On New Functional Molecular Electronic Devices

In recent years, people have successfully fabricated advance nanomaterials, such as graphene and graphene nanoribbons (GNRs), h-boron nitride (BN)-sheets and BN nanoibbons (BNNRs). These carbon (C)-or BN-based nanomaterials have been paid much attention due to their wide range of applications including mechanical, thermal and electrical and optical properties. And their quantum transport have become one of the hot topics in the materials science and condense matter physics. The thesis presents the first-principles (or ab initio) calculations for electronic structures and transport in molecular devices constructed by C-or BN-based nanomaterials. The analysis on the density of states (DOS), transmis-sion coefficients and molecular projected self-consistent Hamiltonian eigenstates (MPSH) gives an insight into the observed results for the system. The findings here are interesting and may be helpful for experimental fabrication of nanoscaled devices.The thesis is divided into six chapters as follow:In chapter1, we introduce the fabrication and basic properties of C-or BN-based nanomaterials, the research background on various molecular devices with interesting physical properties. In chapter2, the computational theory and method used in the thesis are introduced, including density functional theory (DFT) and nonequilibrium Green’s function (NEGF) method as well as the ab initio software package Atomistix ToolKit (ATK).In chapter3, we present the ab initio calculations for spin-dependent electron transport in a molecular device constructed by carbon chains, phenyl ring, and zigzag-edged GNR (ZGNR) electrodes, where the ZGNRs are modulated by ex-ternal magnetic field. The coexistence of switching, negative differential resistance (NDR) and dual spin-filtering effects in the model device is observed. Interest-ingly, a two-state molecular conformational switch can be realized by changing the orientation of phenyl ring, where the majority-spin current modulation (ON/OFF ratio) is170-479. Moreover, the device can generate and controll a full dual spin-polarized current through either the source-drain voltage or magnetic configuration of the electrodes and shows perfect dual spin-filtering effect. The selective spin current is due to symmetry match between two ZGNR electrodes spin channel and the carbon chain’s spin selection. In addtion, the obvious NDR behavior has been observed in our model.In chapter4, we investigate the electronic transport for a molecular device constructed by a phenylene ethynylene oligomer (OPE) molecular with different side groups embedding in a carbon chain which sandwiched between two ZGNR. electrodes. Using the ab initio calculations, the unusual dual conductance, neg-ative differential resistance (NDR) behavior, and obvious rectifying performance are observed in such proposed molecular device. The analysis of the molecular projected self-consistent Hamiltonian (MPSH) of the frontier molecular orbitals and transmission coefficients under various external voltage biases gives an in-side view of the observed results. It suggests that the dual conductance behavior and rectifying performance are due to the asymmetry distribution of the frontier molecular orbitals as well as the corresponding coupling between the molecule and electrodes. However, the NDR behavior comes from the conduction orbital being suppressed at certain bias. Interestingly, the conduction properties can be tuned by introducing side groups to the molecule.In chapter5, by ab initio calculations, we theoretically investigate the spin-dependent electronic transport of a Z-shaped junction constructed by a few-nanometer-long armchair-edged boron nitride nanoribbon (ABNNR) sandwiched between two semi-infinite zigzag-edged BNNR (ZBNNR) electrodes with different hydrogen-passivated edge treatments. The calculations conclude that the short ABNNR ex-hibits metallic behavior. And the transport property of this ZBNNR/ABNNR/ZBNNR junction is sensitively to the edge passivation. Interestingly, the spin-filtering and rectification effects are observed in the junctions without any edge passivation or with boron-edge passivation. The transmission spin-polarization approaches100%in both cases. The analysis on the projected density of states and spatial distri-bution of MPSH gives an insight into the observed results for the system. These results suggest the spintronics with rectification property can be synthesized from a BN sheet by properly tailoring and edge passivation, which is helpful to experi-mental fabricate BNNR-based nanodevices.In chapter6, we firstly summary our work, and then prospect the future investigation on this topic.