Preparation And Surface Photovoltage Properties Study Of ZnO Nanoparticles

With the development of modern information technique, the human societyis undergoing a change that is from the microelectronic time into thephotoelectronic age and the photonic age. As the representatives of wide-bandsemiconductor, ZnO and GaN are defined as the third generation ofphotoelectronic semiconductor materials due to their special properties ofphotoelectric conversion. In comparison with GaN, ZnO has great advantage insome way, for example, lower growth temperature, larger exciton binding energyand easier etching, et al. Therefore, much attention has been paid to ZnOrecently.Due to the three types of fastest-growth directions, various groups ofnanostructure have been grown for ZnO, and this is probably of the richestdiversity of nanostructure among the entire family of oxide semiconductor. Dueto its superior magnetic, photontic, electric properties and sensitive to thesurrounding environment, ZnO nanoparticles have been found to be of wide anddiverse applications in just few years. These applications include the transparentelectrode in display, dye-sensitized solar cell and the photoelectric devices ofheterojunction structure etc. The performance of semiconductor device isdetermined by the electronic structure and charge transfer behavior at thesurface or interface in micro-nano region. So it is necessary to study theinfluence of size and dimension on the electron transfer behavior at the surfaceand interface and understand the influence of surface adsorbates on theproperties of surface charge-transfer behavior in ZnO nanoparticles from bothfundamental science and optoelectronic devices application points of view.In this work, firstly, ZnO nanoparticles of different sizes and dimensionswere prepared. Secondly, the surface photovoltaic properties ofzero-dimensional, one-dimensional and three-dimensional nanostructure wereinvestigated respectively. In this dissertation, the quantum confinement effect ofZnO quantum dots on photogenerated charges was observed for the first timeby means of surface photovoltage spectroscopy and field-induced surfacephotovoltage spectroscopy. Finally, the relationship between the surfacephotovoltage and photoluminescence of the ZnO nanoparticles was elucidatedby the study of the influence of oxygen adsorption on the surface photovoltageas well as photoluminescence. These investigations supply the necessaryexperimental evidence and theory foundation for the application of ZnO-basedfunctional materials. This dissertation includes four sections.In chapter 2, the quantum confinement effect of zero-dimensional ZnOquantum dots on photocarriers was studied. The different sizes of ~3nm, ~8nmand 14nm spherical ZnO nanoparticles were prepared by means of the sol-gelmethod. We laid a strong emphasis on the study of ZnO quantum dots' (~3nm)photovoltage properties. The bounding effect of ZnO nanostructure on thephotocarriers was analyzed and the free exciton states was pointed out. Theinfluence of dc external electric field on the separation and diffusion ofphotogenerated electron-hole pairs was discussed. For ZnO quantum dots thephotogenerated charges exhibit marked quantum confinement properties: Thephotovoltage response intensity is very weak in the absence of external electricfield. However, as an external electric field is applied, the surface photovoltageresponse of confined excitons takes a high symmetry in the changes of theresponse intensity with the strengthening of the two opposed electric fields, andFISPS response band always appears at about 369nm, no matter what bias isapplied. The quantum confinement effect was observed for the first time bymeans of surface photovoltage spectroscopy (SPS) and field-induced surfacephotovoltage spectroscopy (FISPS) in ZnO quantum dots.In chapter 3, the transfer behavior of photogenerated charges inone-dimensional ZnO nanostructure was studied. Firstly, the ZnO nanorods(about 80 nm in length and 14 nm in width) were synthesized by the sol-gelmethod. ZnO nanoneedles and ZnO nanowires were also prepared byhydrothermal technique, respectively. Secondly, the surface photovoltageproperties of one-dimensional ZnO nanorods was studied. In this section, thetwo surface photovoltage response bands were analyzed. The bound excitonstransition behavior was observed in FISPS: An asymmetry in its SPV responseunder the positive and negative electric field is observed. Finally, through thecomparison of photovoltage response of zero-dimensional nanostructure withone-dimensional nanostructure, the influence of dimensions of ZnOnanostructure on the field-induced surface photovoltage response was studied.The result shows that the transfer speed of electrons is different due to theirdiscrepant dimensions of nanostructure. So the photoelectric properties ofone-dimensional nanostructure is superior to that of zero-dimensionalstructure. The one-dimensional nanostructure is beneficial to the application inphotoelectric devices, especially in dye-sensitized solar cells field.In chapter 4, influence of adsorbed oxygen on the surface photovoltageand photoluminescence of ZnO nanorods was investigated. The results of SPSdemonstrate that for ZnO nanorods, the built-in electric field should be a maindriving force for the separation of the photogenerated electron-hole pairs and itsensuing SPV response. The way of photogenerated charges recombination wasalso studied with the aid of PL spectroscopy. It is shown that the two ways ofenergy relaxation in light-excited ZnO nanorods are competitive. When oxygenwas adsorbed at the surface and the built-in electric field is formed, SPVresponse should be the leading one. Nevertheless, when oxygen was absent, theenergy relaxation is mostly carried out by radiative emission. The conclusion ofthis research, that the energy relaxation way of photocarriers can be tuned bychanging the ambience of ZnO nanoparticles, was put forward for the first time.In chapter 5, the ZnO crystals with three-dimensional nanostructure weresynthesized and their surface photovoltage properties were studied. Thethree-dimensional ZnO crystals of flower-shape and bud-shape were preparedseparately by changing the moral ratio of Zn2+ to OH-in solution. The growthprocess was discussed. The surface photovoltage properties of twomorphologies of ZnO were studied. The result shows that different influencesof external electric field on the photovoltage response band of twomorphologies of ZnO was observed due to the morphology difference. Thephotoelectric property of the three-dimensional microball self-assembled bythe two-dimensional structure is superior to that of flower-shaped ZnOself-assembled by one-dimensional structure.