First - Principles Study Of Cu Nanowires And Cu_n @ (8,8) SiCNT Nanowires

The properties of nanomaterials with high surface to volume ratio, such as nanorods, nanotubes and nanowires, have been the focus of a broad range of both experimental and theoritical works as these low dimensional materials exhibit quite distinctive characteristic compared to the corresponding bulk. Cu nanowires, as a typical representative of the metal nanowires, with some novel physical phenomena and properties, such as conductivity and quantum size effects, etc.; are extremely important application research value, and can be used as an ideal model system to study one-dimensional materials. Copper nanowires exposed to air, prone to oxidation reactions lose their properties, and therefore need to construct nano-cable structare to protect it. In this paper, by using the projector-augmented-wave (PAW) potential approach to the density-functional theory (DFT) within the generalized-gradient approximation (GGA) implemented in Vienna ab initio simulation package (VASP), the relaxed structures and electronic properties have been systematically investigated for Cu nanowires with different cross section as well as the electronic properties of Cuw@(8,8)SiCNT nanocables. The main conclusions can be summarized as follows:(1) For all five-size nanowires we studied, the relaxed structures still have the tetragonal symmetry, the relaxation direction changes from in larger inner relaxation for apex atoms and smaller inner relaxation for the side center atoms, which is the phenomenon called "round conner." Furthermore, with increasing initial distance of the atoms away from the central axis of the nanowires, the relaxation amount has an increasing trend In addition, although the majority of atoms are inward relaxation, but the direction of relaxation is different. Specifically, the line of symmetry atoms remain after relaxation in a straight line, the other atoms of a straight line in the original is not on a straight line, the so called "rumple" phenomenon exists. Analysis of the distribution of DOS, the total atomic density is mainly contributed by the d orbital, and, above the Fermi level, the total density of states values are very small, and very similar; but below the Fermi level, in the total density of states curve followed by the coordination number of atoms in order of decreasing, showing a depressed area in the low-level, but raise in high energy level:Therefore, we can conclude:the smaller the coordination number of atoms, the Fermi level Hereinafter, the number of electrons of low energy level occupy less area, and the more the number of electrons in the area occupied by the high energy level. As the effects of the surface of the coordination number of surface atoms is smaller than the corresponding system to produce the material in bulk coordination number, it caused a redistribution of surface electron charge. Thus, between the surface and its nearest atoms in between your atom has a significantly enhanced attractive interaction, called "skin effect." This effect is the strength, toughness and modulus of elasticity and other physical properties of nanomaterials1D stronger than the corresponding bulk material properties.(2) Systematic study of the electronic properties of the structure of the nanocable composed of different diameter square Cu nanowires along [001] orientation encapsulated in SiCNT. The initial shapes are preserved without any visible changes after optimization for the Cu5@(8,8)SiCNT and Cu9@(8,8)SiCNT combined systems, but a quadratic-like cross-section shape is formed for the outer nanotube of the Cui3@(8,8)SiCNT combined system due to the stronger interaction between nanowire and nanotube. The formation energies obtained are negative,this means that the formation processes of these three systems are exothermic and could exist stably, and compared with Cu13@(8,8)SiCNT combined system, the smaller formation energies of Cu5@(8,8)SiCNT and Cu9@(8,8)SiCNT combined systems indicating a weak interactions between nanowire and nanotube. Though analyzing the projected densities of states and the charge densities of three systems, we can find:the interaction between Cu atom of inner nanowire and the Si atom, especially between Cu and C atoms, are very strong, and the electrons for Si and C atoms in outer SiC sheath affect the electron conductance of the encapsulated metallic nanowire in the Cu13@(8,8)SiCNT combined system. But in the Cu5@(8,8)SiCNT and Cu9@(8,8)SiCNT combined systems, the Si and C atoms in outer SiC sheath have no effect for the electron conductance, there are no overlap of the charges between inner Cu nanowire and outer SiC nanotube, so electron transport will occur only through the inner Cu nanowires and the outer inert SiC nanotube only function as insulating cable sheaths. Considering the minimization of the formation energy and wire-tube interaction in the nanocable, and the maximal metal filling ration in nanotube, we know that theCu9@(8,8)SiCNT combined system is the ideal nanocable, is top-priority in the ultra-large-scale integration circuits (ULSIC) and nano-electromechanical systems (NEMS) devices that demand steady transport of electrons.