Fabrication And Characterization Of ZnO Nanosheets,Nanotwin And Mn-doped ZnO

ZnO nanostructures have widely potential applications for UV laser, field effect transistors, chemical and biological sensing, light-emitting devices, piezotronics, transparent and flexible electronics, solar cells and spintronic. This dissertation focuses on the controlled preparation and property characterization of ZnO nanosheets and doped ZnO nanorods, which are grown via a vapor transport and condensation method. We first explore how to control the dimension and defects of ZnO nanostructures by either of metal and metal alloy catalysts, then investigate the variation of optical and magnetic properties of ZnO nanorods by Mn doping. The dissertation is divided into five chapters which are outlined as following.In chapter one, we first introduce the background of our research, then review the recent progress of ZnO. We finally outline the main contents of our study.In chapter two, we put our interest into the synthesis, characterization and device application of10nm thickness ZnO nanosheets, which were grown by using Ag as catalyst. Photoluminescence (PL) shows that the bound exciton emission dominates the optical property of ZnO nanosheets. Field effect transistors (FETs) based on the ZnO nanosheets were fabricated by e-beam lithography. It is found that, under the condition of oxygen molecules adsorption, the FETs works in the enhanced mode with field-effect mobility of254cm2/Vs, on/off current ratio of109and carrier concentration of～1020cm. On the contrary, without the oxygen molecules adsorption, the device shows weak gate modification with field-effect mobility of250cm2/Vs, on/off current ratio of5and carrier concentration of～1018cm3. The results demonstrate that, although the ZnO nanosheets can form the space-confined two dimensional electron gases naturally, the excess electrons originated from the abundant defects in the ZnO nanosheets make the electron density too large to be controlled by the gate voltage. However, the oxygen molecules adsorption on the surface of ZnO nanosheets can decrease the electron density and induce the surface depletion, resulting in a good device performance by the gate control. Lastly, we suggest possible methods to further optimize the ZnO nanosheets FETs.In chapter three, the ZnO bicrystalline nanosheets have been synthesized by using AgxAu1-x alloy catalyst. HRTEM image and electron diffraction pattern reveal that there is a twin boundary in the bicrystalline and the twin is the type [2-1-10]/{01-13}. A series of control experiments show that, the ZnO bicrystalline nanosheets cannot formed with the pure Au or Ag catalyst as well as without catalyst; in the case of low supersaturation of Zn vapor, no ZnO bicrystalline nanosheet occurs even with the AgxAu1-x alloy catalyst. Hence, both the high supersaturation of Zn vapor and AgxAu1-x alloy catalyst are prerequisites for the formation of ZnO bicrystalline nanosheets. Moreover, we find that the density of ZnO bicrytalline nanosheets can be controlled through varying the Ag/Au ratio of alloy catalyst:the richer the Ag content, the more density the bicrytalline nanosheets. Compared to the previous reports of bicrytalline produced under high temperature, the bicrytalline nanostructure can be synthesized under mild condition in our case, implying that the alloy catalyst can decrease the twin formation energy of wurtzite ZnO.In chapter four, we prepare the Mn doped ZnO nanorods with different doping concentrations and investigate the optical and magnetic properties of the nanorods. The room temperature PL shows that, with the increase of Mn concentration, the intensity of deep level (DL) emission of the doped nanorods increases first and then decreases. The tendency is not consistent with the variation of the oxygen vacancy concentration obtained from XPS characterization. However, XPS valence band spectra of different Mn doped nanorods illustrate Mn2+ions inducing new mid-gap density states near about2.2eV below the conductive band. The reason of variation of DL emission of the doped nanorods is attributed to the mid-gap states, which can provide new nonradiation channels for the excited exciton. Moreover, both blue-shift and broadening of the near band emission of ZnO nanorods can be ascribed to Mn doping. Specially, the Zn0.924Mn0.076O nanorods show an abnormal magnetic behavior, i.e., the paramagnetism at low temperature and the ferromagnetism at RT, which is probably attributed to the carrier induced RKKY mechanism. In addition, by careful analysis of the paramagnetic data, we consider that most of Mn2+ion of the Zno.924Mn0.076O nanorods existed as Mn2+-Mn2+antiferrimagnetic pairs while the rest existed as the isolated paramagnetic Mn2+ion.In chapter five, we investigate the Mn-related Raman scattering behavior of Mn-doped ZnO nanorods. Multi-phonon Raman scattering (MRS) of the Mn-related mode with LO phonon overtone up to n=8is clearly observed in Zn0.924Mn0.076O nanorods excited by the laser with wavelength of514.5nm. Through the characterization of Raman spectra of doped nanorods excited by a series of different wavelength lasers, the resonant effect of MRS can be further identified. The behavior is considered as a real electron state transition related with Mn2+/3+photoionization process:one of3d electrons of Mn2+ion is excited to the conduction band of ZnO, resulting in a localized bound exciton; when excited exciton back to the ground state, the energy of excited state dissipates via emitting a photon and multi-phonon. Moreover, based on the results of varying temperature of Raman spectra, we find that the cross section of multi-phonon scattering obeys the temperature dependence of T-1/2. Lastly, the possible coupling mechanism between electron, phonon and spin induced by a Mn2+/3+photoionization transition, the role of Jahn-Teller effect in the multi-phonon scattering process and double-exchange of Mn2+-Mn2+anti-ferrimagnetic pairs are also discussed.