Of Silicon Nanoribbons And Fluorine Saturated Aluminum Nitride Nanobelts First-principles Study

After the first successfully synthesizing of semiconducting-oxide nanobelts in2001, many other materials nanoribbons are synthesized hrough various methods, such as graphene nanoribbons, BN nanoribbons, SiC nanoribbons, ZnO nanoribbons and Si nanoribbons, etc. The nanoribbons have caught special attention because of their distinguished performance in electronics, optics, catalysis and piezoelectricity, especially their fundamental potential applications in nano-devices. Among them, the graphite nanoribbons are studied extensively and deeply. At the same time, many new and powerful experimental techniques, such as atomic force microscopy (AFM), scanning tunneling microscopy (STM), and high resolution transmission electron microscopy (HRTEM) were used to study the properties of nanoribbons. In addition, with the increasing power of computer performance, the computer technology and as well as the development of the quantum mechanics theory the use of computer simulation to study the structure and properties of low-dimensional materials are complementarily useful.However, graphene cannot fit easily into the silicon-based electronic industry. Hence,"graphenium" microprocessors are unlikely to appear in the near future as it is hard to see chip makers re-tooling to use carbon instead of silicon. As a consequence, substantial theoretical efforts have recently focused on the silicon analogue, i.e., grapheme-like silicon,"silicene". Although silicon and carbon belong to the fourth group element, it has novel physical, chemical and biological characteristics and the potential applications in nano-electronics, which has become another focus following the graphite nanoribbons.The use of computer simulation to study their structural and electromagnetic properties is of not only in understanding low-dimensional material's properties as well as fundamental physical phenomenon, but also the promising applications such as interconnects and functional building blocks for novel electrical and optical nano-devices. In this paper, based on the generalized gradient approximation (GGA), the structures and the electronic properties are studied for both zigzag and armchair shaped edges silicon nanoribbons (ZSiNRs and ASiNRs) as well as the effects of the dangling bonds, vacancy, a single C chain and double C chains on the electromagnetic properties of SiNR, as well as the electromagnetic properties of the F-terminated A1N nanoribbon 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 chapter3for the ZSiNRs and ASiNRs. The results show that the length of the Si-H bond is always1.50A, but the edge Si-Si bonds are shorter than the inner ones with identical orientation, implying a contraction relaxation of edge Si atoms. An edge state appears at the Fermi level EF in broader ZSiNRs, but does not appear in all ASiNRs due to their dimer Si-Si bond at edge. With increasing width of ASiNRs, the direct band gaps exhibit not only an oscillation behavior, but also a periodic feature ofâ–³3n>â–³3n+1>â–³3n+2for a certain integer n. The charge density contours analysis shows that the Si-H bond is an ionic bond due to a relative larger electronegativity of H atom. However, all kinds of the Si-Si bonds display a typical covalent bonding feature, although their strength depends on not only the bond orientation but also the bond position. That is, the larger deviation of the Si-Si bond orientation from the nanoribbon axis as well as the closer of the Si-Si bond to the nanoribbon edge, the stronger strength of the Si-Si bond. Besides the contraction of the nanoribbon is mainly in its width direction especially near edge, the addition contribution from the terminated H atoms may be the other reason.Combined with three spin configurations, the effects of the dangling bonds on the electronic and magnetic properties of ZSiNRs and ASiNRs are given in chapter4systemically. The dangling bonds at one edge or both edges make ZSiNRs a transformation from ferromagnetic state of the perfect ZSiNRs to antiferromagnetic state. However, the dangling bonds at one edge and both edges make ASiNR a transfer from nonmagnetic semiconductor of the perfect ASiNRs to ferromagnetic and antiferromagnetic metals, respectively. Furthermore, the magnetic moment of the ferromagnetic state increases for the perfect, bare one edge and bare both edges successively for either ZSiNRs or ASiNRs.The electronic and magnetic properties are studied for H-terminated ZSiNRs with a single C chain as well as double C chains, which is given in chapter5. The results show that when ZSiNRs decorated with a single C chain, at X point, the gap of the two bands across the Fermi level decreases with the C chain from edge to the center of the ribbon successively, and the higher one comes mainly from the Si atom near the C chain, and the lower one from the Si atom far away from the C chain. Whether ZSiNRs decorated with the single C chain at different positions or the double C chains at the two edges of the ZSiNRs, the C chain is almost close to a straight one which results in a transverse contraction near C chain and thus the ribbon width. The C-Si and Si-H bonds are typical ionic bond and the C-H is a covalence bond. Both a single C chain decorated and double C chains ZSiNRs are always metallic independent of the position of the single C chain in the ribbon or the ribbon width for the double C chains. Whether a perfect ZSiNRs or a single C chain decorated ZSiNRs, the ferromagnetic state is preferred over the antiferromagnetic state while double C chains decorated one has an AFM ground state and the defect states contributed from double C chains are composed of two degenerated bands across the Fermi level. A single C chain or double C chains decorated ZSiNRs is more stable than the perfect one. Furthermore, the electronic and magnetic properties of a ZSiNRs can be modulated in detail by a single C chain decoration at different positions. All these results have been explained satisfactory from the electronegativity difference and the bound force to the electrons because of the atom radius difference between the elements.The investigation of the structural and electronic properties for ASiNRs with a mono vacancy or a divacancy are given in chapter6. We find that either a monovacancy or a parallel oriented divacancy change a direct semiconductor ASiNRs to an indirect one, while a slanting oriented divacancy change it to a metallic character. However, neither a monovacancy nor a divacancy can change the nonmagnetic character of the ASiNRs even the metallic ASiNRs with a slanting oriented divacancy. Furthermore, the optimized vacancy structure and the electronic properties are independent of the vacancy positions relative to the edge of the nanoribbon.Electronic properties are studied for the F-terminated A1N nanoribbons with both zigzag and armchair edges (ZA1NNR and AA1NNR) in chapter7. The results show that both the ZA1NNR and AA1NNR are all semiconducting and nonmagnetic, and the indirect band gap of the ZA1NNR and the direct band gap of the AA1NNR ones decrease monotonically with increasing the ribbon width, In contrast, the F-terminated A1N nanoribbons have narrower band gaps than those of the H-terminated ones when the ribbons have the same band width. The density of states (DOS) and local density of states (LDOS) analyses show that the top of the valence band for the F-terminated ribbons is mainly contributed by N atoms, while at the side of the conduction band, the total DOS is mainly contributed by A1atoms. The charge density contours analyses show that Al-F bond is an ionic bond due to a much stronger electronegativity of F atom than that of Al atom, while N-F bond is a covalent bonding feature because of the combined action of the stronger electronegativity and the smaller covalent radius.Summaries of the full text are given in chapter8.