| Copper is a widely used commercial metal due to its availability and outstanding properties such as good strength, excellent malleability and superior corrosion resistance. There are many qualities that maintain Cu as the material of choice for making interconnects in electron device, for examples, Cu has excellent electrical conductivity, thermal conductivity, lower resistivity and higher melting point. As the advance of the nano science and technology one-dimensional (ID) copper nanowires (CuNWs) are of particular interest, which have been expected to apply in ultra-large-scale integration (ULSI) circuits and nano-electromechanical systems (NEMS) as well as others nanodevices. Therefore, in this paper, by using first-principles projector-augmented wave potential within density-functional theory, we have systematically investigated the stabilities, electronic properties and oxidative activity of CuNWs with different structures as well as the properties of CuNWs encapsulated in nanotubes. The main conclusions can be summarized as follows.(1) We systematically investigated the changes in the electronic structure of Cu in going from three-dimensional (3D) to two-dimensional (2D) to one-dimensional (1D) structure. The interatomic distance and the binding energy per bond shows significant drop as dimensionality decreases. The band structure and the density of states (DOS) show a gradual sharpening of the DOS bandwidth and an increase in its peak value with the lowering of dimensions. Meanwhile, the peak of the DOS shifts to the Fermi level and the interaction between adjacent Cu atoms becomes stronger as Cu undergo a change of dimensionality from3D to2D to1D.(2) The relaxed structures and electronic properties have been investigated for CuNWs in [100],[110] and [111] crystallographic directions with different cross sections. For all studied CuNWs, the relaxed structures show a "round corner" phenomenon and the relaxation amount of the surface Cu atoms increases with increasing CuNWs size. The atoms on the same line originally are not on the same line after relaxation, especially the surface atoms; i.e., so-called rumple phenomenon exits. The binding energy per bond shows significant increase as the size of the nanowire increases. The [110] crystallographic wire is more stable than the others and easily synthesize in experiment, in agreement with the experimental result. The enhanced interactions appear between the surface atoms as well as the surface atoms and their first nearest neighbor atoms; we term this phenomenon the "skin effect", which enhances the mechanical properties of the nanowire compared to bulk. Finaly, the electronic transport properties enhances with the increase diameter of CuNWs.(3) The equilibrium structure and electronic properties of22free-standing CuNWs having different cross sections with1-14Cu atoms per unit cell have been systematically investigated. For each wire the equilibrium lattice constant was obtained. The binding energy increases with increasing atom number per unit cell in different structures. As for the polygonal structures of a fixed cross section, the preferred structures should be the staggered ones which contain a linear chain along the wire axis passes through the center of the polygons, where each chain atom is just located at a point equidistant from the planes of polygons. All the nanowires are metallic. Finally, the density of charge revealed delocalized metallic bonding and an enhanced interaction appears between the atoms for all of the CuNWs compared with that in Cu bulk crystal.(4) In this study, we have investigated the interaction of atomic or molecular oxygen with the CuNWs via total-energy calculations using first-principles calculations,(a) The energetics, mechanical, and electronic properties of the contaminated CuNWs have been presented. The impurities (O atom or O2molecule) form stable and strong bonds with the monatomic Cu chain. Upon elongation, the nanowires contaminated with atomic impurities usually break from the remote Cu-Cu bond. The electron transferring from neighboring Cu atoms to O atom shows some degree of ionic character of the Cu-O bond. Our findings indicate that the Cu chains can be easily formed in the presence of molecular or atomic oxygen.(b) The structural stability and electronic properties of a single oxygen atom adsorbed on the surface of foursquare Cu nanowires for a wide range of adsorption sites have been systematically investigated. We find that the oxygen atom can adsorb exothermically or spontaneously on the surface of CuNWs. We found that the long bridge site at the edge of the CuNWs is the most stable site for oxygen adsorption, which is always slightly energetically favorable than the hollow site on the surface. The O-Cu chemical bond shows some degree of mixed ionic/covalence character from a detailed investigation of the different charge density and the projected density of states. In addition, the main factors affecting the preferred adsorption site of the oxygen adsorbed CuNW systems are analyzed from the local geometrical configurations and electronic properties. (5) The structural and electronic properties of nanocables systems formed by CuNWs encapsulated in inorganic nanotubes have been systemically studied.(a) When the four squares CuNWs in [100] crystallographic directions with different cross sections encapsulated into the armchair (8,8) GaNNT, the initial shapes are preserved without any visible changes for the Cu5@(8,8) and Cu9@(8,8) combined systems, but a quadraticlike cross-section shape is formed for the outer nanotube of the Cu13@(8,8) combined system due to the stronger attraction between nanowire and nanotube. The extremely small (larger) formation energies indicating that the interactions between nanowire and nanotube are very weak (stronger) for the Cus@(8,8) and Cu9@(8,8)(Cu13@(8,8)) combined systems (system). The electrons of Ga and N atoms in outer GaN sheath affect the electron conductance of the encapsulated metallic nanowire in the Cu13@(8,8) combined system. But in the Cu5@(8,8) and Cu9@(8,8) combined systems, the conduction electrons are distributed only on the copper atoms, charge transport will occur only in the inner copper nanowire, which are effectively insulated by the outer GaN nanotube.(b) As for the Cum@(n,0) combined systems of helical CuNWs encapsulated in a series of zigzag (n,0) BeONTs, the initial cylindrical shapes are preserved without any visible changes. The most stable combined systems are Cu6@(10,0) and Cug@(11,0) with an optimal tube-wire distance of about2.8A and a simple superposition of the band structures of their components near the Fermi level. A quantum conductance of3G0is obtained for both Cu6and Cu8nanowires in either free-standing state or filled into BeONTs. Both the projected densities of states (PDOS) and charge density analyses also show their conduction electrons are localized on the inner CuNW, therefore, the electronic transport will occur in the inner CuNW and the outer BeONT only function as an insulating sheath. So the combined systems of nanocables is top-priority in the ULSI circuits and MEMS devices that demand steady transport of electrons... |
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