Structure And Properties Of MgO Nanotube Cluster

MgO is applied as an important material. Bulk MgO is known as an inert material with a high melting point, as a typical wideband-gap insulator. When MgO is used as a substrate or nanomaterials in nanotechnology, its properties need to be better understood, especially in the transition process: from bulk to cluster. This has been prompted numerous studies of MgO clusters. The geometries obtained in theory are all in agreement with the results in experiment, which with the stoichiometry of (MgO)_3n were suggested to have tubular structures. And found a number of isomer geometries for (MgO)_n cluster. However, measurements of geometric structure have not been possible. A report that all MgO clusters have been suggested to have cubic nanocrystal structures and not nanotube strcutures, for the IR spectra for MgO small clusters match those of the larger clusters and the bulk. In this paper, we investigate structural transition and electronic properties with increasing diameter and/or length in MgO nanotube clusters, using the density functional theory at B3LYP/6-31G(d) level; the IR and Raman spectra, nonlinear optical properties of MgO nanotube clusters are studied in detail. The rest of the paper is organized as follows:1. Size dependent structural and electronic properties of MgO nanotube clusters.The atomic and electronic structure is discussed. Geometries and bond lengths, binding energies, the degree of ionicity in the bonds and the electronic (HOMO-LUMO) gaps are analyzed as a function of diameter and length of the nanotube cluster. The size dependence of bond length presents anisotropic property. 3MR is the most stable species over all MgO nanotube isomers. The averaged atomic charge increases with the layers of MgO nanotube clusters. A mixed covalent and ionic bonding always exists in MgO nanotube clusters. Though bulkMgO is known as an inert material, it's possible that MgO nanotube clusters have properties of semiconductor.2. Study on the Electronic Properties of MgO Double Nanotube Clusters by Density Function Theory.The averaged binding energy, energy gap, APT atomic charge and total charge density of the MgO nanotube clusters were calculated with B3LYP method at 6-31G(d) 1 eVel. The calculations show that the averaged binding energy decreases approximately linearly with increasing coordination. Structural stability increasing in company with length of MgO nanotube clusters, and the most stable structure is 3MR species. The charge transfer increasing with length of MgO double nanotube clusters, the averaged atomic charge of 2DMR, 3MR and 3DMR converge to 1.298, 1.270 and 1.306, respectively. Mixed covalent and ionic bonding property exists in MgO nanoclusters.3. DFT Calculations of Vibrational Spectra and nonlinear optical properties for MgO Nanotube Clusters.The IR spectra are similar to those in the corresponding MgO cluster and bulk materials which have different structures. The strongest peaks of the IR spectra are located in the range from 650 to 750 cm-1, which is the stretch motion mainly dure to Mg-0 bonds which along to the axis of nanotube. The Raman spectra are very sensitive to structural variations in MgO clusters. There are distinct difference in Raman spectra for 2DMR, 3MR and 3DMR, such as the range of the strongest peaks. And the red-shifted of vibratioanl frequency is observed as increasing cluster length. The Raman spectra are well suited for an experimental determination of the slightly different structures in MgO cluster. The total dipole moment, the static dipole polarizability, the polarizability anisotropy invariant, the average polarizability per atom and the first hyperpolarizabilities are calculated.The total dipole moment and the first hyperpolarizabilities oscillate between zero and a constant when the layer is grown for the layer dependence of symmetry in MgO nanotube clusters. The present results for small MgO nanotube clusters would lead to the unique identification of the stable structures of MgO cluster.