First Principles Studies On Nano-and Surface Systems

In the recent decades, we witnessed tremendous progresses in nanoscience and surface science. All these developments not only extended the scope of conventional subjects such as condensed matter physics, quantum chemistry, and functional ma-terial, but also possess great impacts on information technology, clear energy, envi-ronmental science and so on. On the other hand, with the increasing complexity and the considerable quantum effects in nano-scale systems, first-principles calculations became more and more important in research activities. Theoretical calculations can not only be employed to explain the experimental observation, to reveal the underly-ing physics, but also to guide of experiments, even to perform ideal experiments via constructing appropriate models.All the works in this thesis can be divided into three parts:the study of the effects of chemical modification of edged carbon atoms in armchair edged graphene nanorib-bons (AGNRs); the study of electronic transport properties of graphene nanoribbons (GNR) based nano-systems by using the non-equilibrium Green's function technique combined with density functional theory; the development of computational proce-dures to simulate inelastic electron tunneling spectra in surface-adsorbate systems.At the beginning, we briefly introduced the background of the first-principles electronic structure calculation and some methods/approximations widely used in the nowadays research. People started the journey of describing real systems by using quantum mechanics since the birth of quantum theory in the beginning of twentieth century, from this point of view, the whole electronic structure theory is an outcome of the process of people looking for a method affordable and in the mean time can give acceptable results to describe the real world. In Chapter 1, along with the thread of approximations from the most rudimental level to the advanced, we first introduced the three basic approximations widely adopted in electronic structure calculations, i.e. non-relativistic approximation, Born-Oppenheimer approximation, and the inde-pendent particle approximation; Hartree-Fock approximation and self-consistent field method as the cornerstones of the modern electronic structure theory are then intro-duced; Next, we gave a brief description on density functional theory and the non-equilibrium Green's function technique combined with density functional theory; In the end, a short introduction for software packages used in this thesis is presented.In chapter 2, we depicted the rapid developing interdisciplinary subjects named nanoscience and surface science. On the one hand, the development of scanning probe techniques enables us to observe objects at the chemical limit, such as single atom or single molecule, and more important, scanning probe techniques allow us to con-struct and control various microscopic systems, even allow us to control chemical re-actions; Molecular junction is another great progress that allows us to perform single atomic/molecular electric measurements, and has great potential in the future micro-circuits. On the other hand, the emerging of new materials provides platforms for peculiar physics, for instance, the rise of graphene attracted tremendous research inter-ests both in basic physics and various applied subjects. Due to the requirement of the detailed physics of surface-adsorbate systems in the research of heterogenous cataly-sis, epitaxial growth, life science and molecular electronics, there are many challenging open questions in surface science.Graphene related materials are expected to have great potential in the next gen-eration micro electric devices. As a kind of derived material, GNRs can be prepared from graphene. The electronic properties of GNRs are sensitive to its chirality, width, chemical environment, and many other issues. All the hydrogen saturated AGNRs are semiconductors. In chapter 3, we studied the effect on the electronic properties of edge chemical modification of AGNRs. We found that chemical modification not only changes the electronic structures of AGNRs, such as energy gap engineering and semiconductor-metal transition, but also changes the family behavior in gap-width re-lation. Furthermore, AGNR is expected to be a kind of molecular device, our study on the electronic structures of AGNRs upon chemical modification by NO2 and NH2, which is widely used in molecular electronics as anchoring groups, shows that the modification does not alter the electronic structure qualitatively, which implies that we can integrate AGNR functional units into molecular circuits without focusing on much extra work. There are mainly two kinds of research in the area of molecular electronics. First is the direct investigation of the electron transport properties of molecular or nano systems, which can be achieved via various experimental or theoretical approaches. Activities such as device design can also be classified into this field, indeed, many kinds of molecular devices such as molecular rectifier, molecular switch, quantum dot, and negative differential resistance (NDR) device have been designed or proposed the-oretically. The other is the application of electron transport properties to realize other purpose, such as chemical identification. In chapter 4, we designed a quantum dot and a NDR device based on GNRs by using theoretical calculations, the functionality are robust for both devices.In chapter 5, we discussed our own implementation for the simulation of inelastic electron tunneling spectra (IETS) in surface-adsorbate systems. Our method is based on Tersoff-Hamann aprroximation and finite difference method; We benchmarked our code in two test systems:CO adsorbed on Cu(100) and acetylene adsorbed on Cu(100), both cases give reliable/robust results and excellent performance. Then we applied it in two practical programs:cis-2-butene adsorbed on Pd(110) surface and melamine tautomers adsorbed on Cu(100) surface, all these test cases and practical applications are remarkable demonstrations of the fact that IETS is a power tool in surface sci-ence, especially in the chemical identification of adsorbate species, configuration, and conformation.