| Since2004, graphene has become a hot topic in condensed matter physics, chemistry, photonics, biology and energy and other fields. Meanwhile, two-dimensional graphene-like materials also become the research frontier. In particular, the successful preparation of boron nitride and silicene has greatly inspired the enthusiasm of scientific research workers. Low-dimensional materials with its unique chemical structure and corresponding optoelectronic and other properties, will have broad application prospects in nano-photonics, clean energy, bio-medicine.Computer simulation is the communication bridge between theory and experiment, the high efficiency and shortly research cycle making it become the third largest tools beyond theoretical analysis and experimental research. In terms of material-designing and properties prediction, first-principles calculations based on density functional theory is often considered as the most effective way to study nanomaterials for structural stability, electronic structure, magnetic properties, optical absorption and excitation and chemical reaction processes. First-principles calculation was performed to explore the atomic adsorption and diffusion processes on the two-dimensional and one-dimensional system, which will impact the electronic properties of the systems in turn. The materials involved in our work include two-dimensional (2D) graphene, boron nitride, silicene, silicane and their corresponding one-dimensional (1D) nanoribbons.The thesis is organized as follows:Chapter I gives a brief introduction of research background and motivation. Chapter II introduces the theoretical fundamentals used in our research work. Chapters III to VI describe in detail and summarize the work done during my Ph.D degree studies. The main content and results in this dissertation are listed as follows:1. Energetic and structural properties of gold atom (Au) and gold dimer (Au dimer) adsorbed on pristine and defective graphene (Gra) and boron nitride monolayer (BN) are investigated using density functional theory. The electron transfering as well as band structure changing are also given in the results. We considered dozens of initial configurations to get the most stable adsorption structure. After optimization, Au atom prefers stabling at atop site to the other site when it adsorbed on graphen, whereas Au dimer like bridge site. Interestingly, Au/Au-dimer prefers a top sites over other hollow or bridge sites on pristine and alternatively defective BN. Through comparing the perfect and defective substrate, it is easy to find that the doped defects can trap Au/Au-dimer effectively. From the point of view of energy, BN-NC is the best surface among the six tested. There are some changes took place for band structure after atomic adsorption, including both band translation and new bands appearing at Fermi energy, which is very obvious in BN system. In the following, we obtained DOS and PDOS of Au/Au-dimer on BN-NC, founding the reason of the strengthen interaction between Au and substrate is that charge transfer occurring between them. The contribution mainly comes from the hybridization of Au-5d,6s, C-2p, N-2p and B-2p. In this way, we got the similarities and differences between the two systems and obtained that pristine or defective BN can no longer be treated as inert substrates.2. Although BN monolayer has good thermal and chemical stability, but the band gap hinders their application in the electronic devices. Doping or modification of edges is an effective way of tailoring the electronic properties of nanoribbons. In this contribution, we performed first-principles calculations within density functional theory (DFT)to investigate the adsorption and diffusion of Au adatoms on BNNRs with zigzag or armchair edges. When the Au atoms adsorbed on BN monolayer, they can only stable at BT (atop at B atom) and NT (atop at N atom) two positions after full optimization. The B site configuration is marginally favored over the N site configuration on the BN sheet by about0.206eV, and both of them are significantly more favorable than the hollow site configuration. Meanwhile, the boron atoms move slightly upward by0.182A°but the original structure of BN is preserved. In addition, we found that Au diffusion from the B site to the neighboring N site encounters the lowest barrier of0.007eV when it on the BN sheet. This implies that Au adatom prefers to diffuse along the path B-N-B when it sites on BN sheet. It is clear that the energetically preferable site for the Au adatom on the BNNRs is not right above a B atom but has a small deviation. Au adatom can easily segregate from inner region toward both edges of A-BNNRs and to the B edge of Z-BNNRs. The longitudinal diffusion along subedges is energetically favorable. For A-BNNRs, the energy barrier is only0.111eV but negligible for Z-BNNRs. In contrast, the longitudinal diffusion of Au atom along edge has to overcome higher energy barriers,0.345-0.507eV. The electronic structure calculations indicate the wide-band-gap features are preserved in the Au/BNNRs as the Au adatoms concentration is low. When increasing the Au concentration, the band gap for Z-BNNRs is closed, yet, for A-BNNRs, narrowing the band gap of the system with more localized bands emerging within the band gap. These results are wished to be validated by further experiments. Thus, by Au doping can realize the purpose of tailoring the electronic properties of nanoribbons. Further analysis revealed that the band gap closure in Au-doped Z-BNNRs is related to the interaction between the Au atomic chain and the edge B atoms.3. First-principles calculations were performed to explore the intriguing electronic and magnetic properties of silicane and hybrid silicane-silicene nanoribbons. There have been reported four kinds of silicane configurations, in which the chair structure is the most energetic favorable one and followed by the boat one. At the beginning, we optimized the two configurations of silicane and found that the chair-silicane is more stable than the boat one by about38meV/atom. This is agreement with Lew et al. and Ding et al. very well. Yet, we still display the band structures of boat-silicane and the results tell us that both of them are wide gap semiconductor with spin degenerate. The corresponding valance band maximum (VBM) and conduction band minimum (CBM) are at the G point and K point for chair one, whereas, both VBM and CBM are at G point for boat one. Like the carbon counterparts, all the Silicane-NRs can be classified into armchair and zigzag ones according to the edge-shape and the width across the ribbon can be classified by the number of dimer lines (NA) or zigzag chains (Nz). We can see that the band gaps decrease with width increasing and gas of ASilicane-NRs are larger than that of ZSilicane-NRs with the same width conditions. Corresponding boat-NRs resembles the above results with no spin-spilt band structures. In the absence of an electric-field, the ZSilicene-NRs terminated with mono-hydrogen display a semiconducting feature at the ground AFM state, while the metstable ferromagnetic state is metallic. From the application point of view, the ferromagnetic semiconducting characteristic is highly desired in the ZSilicene-NRs. So we consider hybrid Silicane-NRs with Silicene-NRs side by side and the former side abbreviated as N while the latter one abbreviated as M, thus the hybrid system denoted as A/Z-N/M-NRs. For A-N/M-NRs with N+M=9, the band gaps present oscillatory behavior and can be classified into three families with M=3n,3n+1and3n+2(where n is a positive integer), respectively. Yet in Z-N/M-NRs, the zigzag chain at the interface can be either fully hydrogenated or half hydrogenated, which will lead to a very different results.(1) The interface half hydrogenated:The ground state consist of ferromagnetic ordering along the zigzag chains of the ZSilicene-NRs part and descending from central region to edge area which mainly come from Pz oribtals. The imbalance between the spin up and spin down states results in a net magnetic moment of1.000μB per unit along the ribbon and the gaps can be tuned by M.(2) interface fully hydrogenated:the states between the ZSilicene-NRs edge and the Silicane-Silicene interface have opposite spins and approximately zero magnetic moment in the unit cell. However, because of the chemical potential difference between the ZSilicene-NRs edge and the interface, the bands nearest the Fermi level are not completely degenerate. Gaps between VBM and CBM also varying with M and the boat configurations give the similar results as chair ones. Thus, the hybrid systems have rich electronic properties for gap engineering and for applications as a spin filter. |
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