Studies On Physical Properties Of One Dimensional ZnO Nanostructures

Owing to the numerous nanostructures and the excellent physical and chemical properties, zincoxide （ZnO） nanostructures have attracted extensive attention from both experimental and theoreticalsides. Among them, one-dimensional ZnO nanostructures quickly become the focus of the presentstudies due to strong quantum confinement effects, surface effects and the important application inbuilding the future electronic and optoelectronic nanodevices. In this thesis, using the first-principlescalculations on the basis of density functional theory, we investigate systematically the modulation ofchemical doping and external electric field on the structure, electronic structures and magnetism ofone-dimensional ZnO nanostructures, which provides important theory guidance for realizing thenanoelectronic and optoelectronic nanodevices of one-dimensional ZnO nanostructures. Moreover, wehave also studied the band gap modulation of MoS₂nanotubes by applying the electric field and strain.Our main results are summarized as follows:Using density functional theory calculations, we have systematically studied the structuralstability, energy band structrue and d⁰magnetism of zigzag ZnO nanoribbons with a single C atomsubstituting one O atom. We find that the formation energy of C dopant depends strongly on theposition: the doped C atom close to O terminated edge is most favorable energetically forH-passivated zigzag ZnO nanoribbons, whereas the doped C atom prefers to substitute the O atomnear Zn terminated edge for unpassivated zigzag ZnO nanoribbons. These features are qualitativelyexplained using a simple capacitor model, which is proposed according to the polar property of thezigzag ZnO nanoribbon along the direction of the width. At the same time, the studies show that the Cdoped ZnO nanoribbons have d⁰magnetism, and the magnetization of the ZnO nanoribbons mainlyfocus on the C atom, also the C has the different magnetization in different doping site, which isbecause that average Zn-C bond length is different in different doping site, so the nature of C-Zn bondis different. Moreover, we also find the electronic structure of the C doped ZnO nanoribbon isdifferent with different C doping site, even we can get the semiconductor-half metal-metal transitionin C doped ZnO nanoribbons. The C doped ZnO nanoribbons with tunable electronic structure andmagnetic properties might have important application in promising spintronics device.We have investigated extensively the structures and electronic properties of the single walledZnO nanotubes （NTs） under the transverse electric field using the density functional theory. Under thetransverse electric field, the circular cross-sections of ZnO NTs are deformed to elliptic along the direction of the electric field. We found that the radial deformation is mainly due to the change ofbond angles induced by the different change of bond length in different site under the electric field.Tube aspect ratio strongly denpends on the diameter and the transverse electric field strength, but it isinsensitive to the tube chirality. The band gap for the ZnO NTs reduces monotonically with increasingthe electric field strength, the change of the band gap increases with increasing the diameters of thenanotubes, but it is insensitive to the tube chirality. The function between the band gap of ZnOnanotube and the electric field was established. We also found that the coupling effect of the electricfield and the radial deformation weaken the reduction of the band gap. These results might haveimportant implications in understanding the electromechanical coupling effect of the ZnO NTs andutilizing them as building blocks for the future optoelectronic nanodevice.We have systematically studied the electronic properties of the various ZnO nanowires （NWs）and faceted NTs under a transverse electric field using the density functional theory. The band gaps ofthe ZnO NWs and NTs reduce gradually with increasing the electric field strength, and the variation isfaster with increasing the diameters and decreasing the wall thickness for the NWs and NTs,especially, leading the semiconductor-metal transition when the electric field strength is strongenough, which is related with the variation of the dipole moment under the electric field, at the sametime, we can also explain the reduction of the bandgap under the electric field according to the tilt ofthe energy level in different potential region. Moreover, the effective masses of the electron and holefor the ZnO NWs and NTs can be modulated by the electric field. These studies provide usefulguidance for designing future microelectronic and optoelectronic nanodevices based on the ZnOnanostructures.Using density functional theory calculations, we systematically investigated the structures andthe electronic properties of the MoS₂nanotubes under the transverse electric field and the variationsof the band gaps for the MoS₂nanotubes under the strain. Under the transverse electric field, the bandgaps of the naotubes with different chiralities reduce monotonically with increasing the electric fieldstrength, eventually leading the semiconductor-metal transition; the circular cross-sections of theMoS₂nanotubes with different chiralities are deformed to elliptic under the transverse electric field,and the aspect ratios depend on the diameters of nanotubes and the electric field strength. At the sametime, we find that the radial deformation under the electric field strengthen the decreasing of the bandgap induced by the electric field. Under radial compressive strain, the variations of the band gaps forthe nanotubes with different chiralities are small. Under the tensile axial strain, the band gaps of thezigzag and armchair nanotubes decrease linearly; while under the compressive axial strain, the changes of the band gaps for zigzag and armchair nanotubes are different. These studies provideimportant guidance for designing future optoelectronic nanodevices based on the MoS₂nanotubes.