Zinc Oxide And Its Nanostructures

Zinc oxide (ZnO) has three key advantages. First, it is a semiconductor, with a direct wide band gap of 3.37 eV and a large exciton binding energy (60 meV). It is an important functional oxide, exhibiting near-ultraviolet emission and transparent conductivity. Secondly, because of its noncentral symmetry, ZnO is piezoelectric, which is a key property in building electromechanical coupled sensors and transducers. Finally, ZnO is bio-safe and biocompatible, and can be used for biomedical applications without coating. With these three unique characteristics, ZnO could be one of the most important nanomaterials in future research and applications. In this thesis, first-principles density-functional theory (DFT) calculations are systematically performed to study the various ground-state properties, such as piezoelectric properties, elastic properties, structural features, energy properties, etc., of ZnO and especially its nanostructures. The main contents are listed below:(1) Study on Piezoelectricity of two-dimensional ZnO nanofilms. Size-dependent piezoelectricity of ZnO nanofilms is calculated and analyzed by using DFT based Berry phase polarization computational method. It is found that the effective piezoelectric constant of the ZnO nanofilms increases with increasing thickness, and becomes higher than that of bulk ZnO when the number of Zn-O double layers increases to 10 (2.4 nm in thickness). And the piezoelectric response can be 11% higher than the bulk value when the number of the Zn-O double layers is increased to 12. Because the quantum confinement effect is raised in the low-dimensional nanostructures, for the nanofilms of several nanometers in thickness, the electrons are strongly confined within the slab. The spontaneous polarizations existing between the Zn-O double layers will be accumulated with the increasing film thickness, which may result in higher change rates of polarizations for very thin nanofilms. However, with the film thickness continually increasing, after a possible critical point, the accumulation will not dominate the piezoelectric property. Then the surface effect will trail off, and the piezoelectric constant will approach to the bulk value. In addition, our result coincides with the experimental result that the effective piezoelectric coefficient of ZnO nanobelt with the (0001) top surface could be higher than that of the bulk ZnO.(2) Study on Piezoelectricity of one-dimensional ZnO nanowires. Size-dependent piezoelectricity in [0001]-oriented ZnO nanowires (0.4~3.0 nm in diameter) is investigated using DFT calculations. The effective piezoelectric constant of ZnO nanowires is found to increase with increasing diameter, but the values are much smaller than that of bulk ZnO, at least in the limited size studied in the present calculations. Both the structure reconstruction and quantum confinement in ZnO nanowire are considered to be the main contributions to this size effect. Energy bands calculations show that with increasing diameter, the band gap of the ZnO nanowires decreases from 2.8 to 1.1 eV.(3) Calculations on the elastic constants of 2D ZnO nanofilms and 1D ZnO nanowires. The effective elastic constants of ZnO nanofilms and nanowires are calculated and analyzed. Obvious size effects are also found in the limited sizes for both the two cases. For the nanofilms with less than 6 Zn-O double layers, their effective elastic constants are much lower than the corresponding bulk value. While the film thickness continues increasing, the obtained result becomes almost invariable and approaches to the bulk value. For the [0001]-oriented ZnO nanowires, their corresponding effective elastic constants are also much lower than the bulk value when the diameter is very small. However, with increasing diameter, the elastic constant increases almost linearly to approach the bulk value. In our limited computational scale, the obtained effective elastic constant of the nanowires increases to around 88% of the corresponding bulk value when the diameter is about 2.4 nm. This size effect is mainly due to the increased surface/volume ratio in nanoscale. This work is still ongoing when completing the present thesis.(4) Study on the structures and properties of zero-dimensional ZnO nanoclusters. Through structural optimizations and energy levels analysis on zero-dimensional ZnO nanoclusters with hexagonal prism configurations, we found that the structural relaxations led to zinc atoms moving toward the center of the cluster, whereas oxygen atoms moving outward. This is important to know when attempting to passivate dangling bonds at the surfaces with surfactants, since they will then be bonded first of all to oxygen atoms. In addition, the shape-driven phase transition from the four-coordinate wurtzite to the six-coordinate rocksalt structure is found in a ZnO cluster with 48 atoms, which implies that cutting out a stoichiometric cluster with more favorable structure, the dangling bonds should be as few as possible.(5) Calculations on electro-optic tensors of II-VI semiconducting materials. The electro-optic coefficients of the II-VI semiconducting compounds with wurtzite and zinc-blende structures are calculated by using DFT perturbation theory. Especially, the electro-optic tensors and the nonlinear optical constants of ZnO with different strains are also obtained. It is shown that among the II-VI compounds ZnO has the highest elastic constant, piezoelectric constant, and electro-optic coefficient. The piezoelectric constants of the II-VI compounds with zinc-blende structure are almost one order smaller than that of the materials with wurtzite structure. With increasing strains from -1% to 1%, both the electro-optic coefficient and the absolute value of the nonlinear optical constant of ZnO decrease almost linearly by 9.5% and 8.2%, respectively.