Graphene Nanoribbons And Silicon Carbide Nano With The First Principle

After the first successfully synthesizing of semiconducting-oxide nanobelts in 2001, the nanobelts have caught special attention because of their distinguished performance in electronics, optics, catalysis and piezoelectricity, especially their fundamental potential applications in nano-devices. Over the past years, the other metal, metal-oxide or non-metal nanobelts have also been successfully synthesized by various methods. At the same time, many new and powerful experimental techniques, such as scanning tunneling microscopy (STM), atomic force microscopy (AFM), and high resolution transmission electron microscopy (HRTEM) were used to study the properties of nanobelts. In addition, with the development of the quantum mechanics theory and the computer technology as well as the increasing power of computer performance, the use of computer simulation to study the structure and properties of low-dimensional materials are complementarily useful.Graphene nanoribbon (GNR) and silicon carbon nanoribbon (SiCNR) have become the focus of low-dimensional materials research owing to their novel physical, chemical, and biological properties as well as the potential applications in nano-devices. The use of computer simulation to study their structural and electronic properties is of not only in understanding low-dimensional material's fundamental physical phenomenon, but also the promising applications such as interconnects and functional building blocks for novel electrical and optical nano-devices. Under the generalized gradient approximation (GGA), the electronic properties are studied for both GNRs and SiCNRs with zigzag shaped edge or armchair shaped edge by using the first-principles projector-augmented wave (PAW) potential within the density function theory (DFT) framework. In detailed, the band structures, density of states and charge density contours are given in chapter 3 for the GNRs in different width with zigzag shaped edge (2≤Nz≤24) or armchair shaped edge (3≤Na≤41). The metallic and ferromagnetic characters due to appearance of the edge effects and the corresponding flatband at the Fermi level for the GNRs with zigzag shaped edge (ZGNRs) are resulted from the unpaired free electrons belong to edge C atoms connected to H atoms. For the GNRs with armchair shaped edge (AGNRs), however, most of these free electrons combine together and overlap upon the edge dimer C-C bonds, so the strength of the edge dimer C-C bonds is stronger than that of the inner C-C bonds. With less unpaired free electrons, so the edge effects and the corresponding flatband at the Fermi level are absent and lead to the semiconducting and nonmagnetic characters of the AGNRs. For a certain width, the effect of the free electrons decreases for C atoms from edge to inner successively, so that the contributions to the sharp peak at the Fermi level in the density of states and the charge difference between the up-spin and down-spin and thus ferromagnetism decrease for C atoms from edge to inner successively for the ZGNR.The band structures, density of states and charge density contours are given in chapter 4 for the SiCNRs in different width with zigzag edge (Nz=2-24) or armchair edge (Na=3-20). The SiCNR with zigzag shaped edge (ZSiCNR) is also metallic and ferromagnetic except for the thinner ribbons (Nz=2-4) with small direct band gaps. The projected density of states (PDOS) onto individual atom shows that a sharp peak appeared at the Fermi level for broader ZSiCNR comes from the edge C and Si atoms with H terminations. Similar to AGNR, the SiCNR with armchair shaped edge (ASiCNR) is semiconducting and nonmagnetic. The direct band gaps of ASiCNR exhibit sawtoothlike periodic oscillation features and quench to a constant value of 2.359eV as width Na increases. The charge density contours analysis shows the valence charges are strongly accumulated around C atom, reflecting a significant electron transfer from Si atom to C atom and thus an ionic binding feature for C-Si bond. The dangling bonds give rise to extra band in SiCNR with bare edges and change the electronic properties of SiCNR.