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Calculation of Cu[110] nanowires relaxation structure and electronic properties of the first principle

Author: Sun
Tutor: ZhangJianMin
School: Shaanxi Normal University
Course: Atomic and Molecular Physics
Keywords: Cu[110] nanowires binding energy electronic structure first-principles study
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Type: Master's thesis
Year: 2013
Downloads: 23
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Abstract


Among the low-dimensional systems, nanowires (NWs) are a major topic of interest for their potential applications in biosensors, nanoelectromechanical systems, and miniaturized optic-electronic devices. Over the last decade, metallic NWs are among attractive one-dimensional nanostructures that have generated a lot of scientific interest in theory and possible use for molecular electronic devices. Cu NWs, as typical representative metal NWs, have played a vital role in micro-/nano-scaled electronic devices, and provided ideal model systems to experimentally investigate the novel physical phenomena such as quantized conductance and size effects. The structural and functional properties of the Cu NWs have been the focus of a broad range of both experimental and theoretical works due to the high surface to volume ratio compared to the corresponding bulk or surface systems.Although Cu nanowires of different types of geometries have been fabricated with the experimental developments, little research on the size-dependent relaxed structures and electronic properties of Cu NWs of various shapes and orientation have been reported in detail. Since the [110] oriented Cu NWS appear to be the most promising ones due to their spatial separation of carriers, incurrent work, two types of geometrical structures of Cu [110] NWS of different diameters, formed by stacking of atomic polygons with rectangular and hexagonal cross sections perpendicular to the wire axis are presented. We combine first-principles binding energies with these two models to describe the more stable structure of Cu [110] nanowires.Under GGA, the relaxed structural and electronic properties have been investigated for Cu (110)nanowires with rectangular or hexagonal cross-sections of the first three diameter using the first-principles PAW potential within DFT. The following conclusions are obtained:(1) For all six-sized nanowires, the relaxed structures still have symmetry and with increasing initial distance of the atoms away from the central axis of the nanowires, the relaxation amount has an increasing trend. Furthermore, the relaxation direction changes from in larger inner relaxation for apex atoms and smaller inner relaxation for the side center atoms in rectangular nanowires while inner relaxation for apex atoms and outward relaxation for the side center atoms in hexagonal nanowires showing there is a "round corner" phenomenon.Third, the atoms on the same line originally are not on the same line after relaxation, the so called rumple phenomenon exists.(2) The calculated total energies combined with phenomenological models were used for microscopic description of nanowire binding energies. The obtained binding energies are fitted by appropriate equations of state to determine the energies of broken bonds at surfaces and edges of various Cu [110] NWs. According to the stability and energetics analysis, the hexagonal structures are energetically more stable than the rectangular ones. Electronic band structure calculations show that the hexagonal wires exhibit metallic behavior distinctly. Therefore the [110] oriented Cu NWs is energetically more favorable for hexagonal cross sections perpendicular to the wire axis agrees well with the experimental result.(3) In the band structure of six-sized nanowires, the d bands are occupied as expected from the Cu bulk, this is attributed to the valence electron configuration of3d104s1for Cu atom. The states near the Fermi level are nearly free electron (NFE) like states with a parabola shape and a large energy range. Number of bands which cross the Fermi level is crucial for the quantum ballistic conductance and the stability of nanowire.Under ideal conditions, the conductance is determined by the number of bands crossing the Fermi level. According to the band structure of six-sized nanowires,we can conclude that hexagonal cross sections of Cu [110] NWs is more conductive and stable.(4) The vanishing of the neighbor atoms outside the nanowire, on the one hand, accompanies the vanishing of their electrons which are originally shared with the surface atoms, so an enhanced interaction presents between the surface atoms as well as the surface atoms and their first nearest neighbor atoms. We term this phenomenon "skin effect", which enhances the mechanical and the electronic transport properties of the nanowire compared to bulk. Furthermore, similar to the relaxed atomic structures, the charge density contours of the(110) plane still have the C2symmetry for all nanowires considered. Many surface phenomena are related to the surface charge redistribution.

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