Study On The Preparation And Properties Of ZnMgO Nanostructures

Nowadays, much effort has been made to fabricated nanoscale materials, and thecontrollable fabrication and adjustment of the properties of one-dimensional zinc oxide (ZnO)nanostructures have caused much attention due to their variety of novel properties andapplications in light-emitting diodes, sensors, solar cells and field-effect transistors. Adiversity of nanostructured ZnO materials such as nanowires, nanobelts, nanowalls andnanorods have been synthesized. To optimize the electrical and optical properties of ZnO, thedoped ZnO nanostructures with various elements has been used widely. Among these elements,the Mg-doping in ZnO is preferred because the band gap of ZnO can be modulated within acertain range from3.37to7.7eV as MgO has a larger band gap (7.7eV) than ZnO. The ionicradius of Mg2+(0.057nm) is very close to Zn2+(0.06nm), and the replacement of Zn by Mgdoes not give rise to significant changes in lattice constants. The adjustable bandgap of ZnMgOnanostructures has opened up a new research space for the development of new semiconductordevices based on bandgap engineering.At present, Mg-doped ZnO nanostructures have been obtained with many methods, suchas pulsed laser deposition (PLD), molecular beam epitaxy (MBE), sol-gel deposition,hydrothermal synthesis and chemical vapor deposition (CVD) etc.These deposition methods allhave their advantages and disadvantages.Today, the preparation technology of Mg-doped ZnO nanostructure is relatively mature,and the structure, bandgap and luminescence properties of ZnMgO has been carried out anextensive and deep research. However, there are still many problems need to be revealed, forexample, how to contronl the change of the Mg doped content and the bandgap varies ofZnMgO with the change of Mg content.In this dissertation, the Mg doped ZnO nanostructures were prepared by chemical vapordeposition method and hydrothermal method by changing the conditions of the preparationprocess and the doping concentration. Then, the structure, components, band gaps andluminescence characteristics of the samples were analyzed detaily. The main results are asfollows:(1) Mg-doped ZnO nanorods have been successfully fabricated on Si (111) substrates with different growth time from30to90min by CVD technique. XRD showed that thenanorods grown with60min processed high c-axis preferred orientation. With the growthtime increasing, the length and diameter of the nanorods increased, and the nanorods showednon-oriented distribution. The nanorods had the best crystallinity when the growth time was90min. The growth progress of Mg-doped ZnO nanorods included two steps: the growth ofthin film and then the growth of nanorods. The PL spectra exhibited that the UV emissionpeak of the samples showed an obvious blue shift from388to376nm, indicating theenlarging of the band gap owing to the substituting of Mg in Zn site.(2) ZnMgO nanosheets have been successfully fabricated on Si (111) substrates withdifferent growth temperature from850℃to1050℃by CVD technique. The measurementresults showed the size and the thinckness of nanosheets become lager, and the needle-likestructure at the top become coarser and some disappear, and the crystalline quality of samplesdecrease with the increasing growth temperature. The reason may be that the activity of Znatoms and Mg atoms increased with higher temperature, which results in a more deposition ofZnMgO and an increase in defects in the sample. The PL spectra exhibited that the intensityof UV emission peak of the samples becomes weaker, it related to the crystalline quality ofthe samples with the temperature increasing, and which decreasing the luminescent propertiesof the samples in the ultraviolet light-emitting region.(3) Mg-doped ZnO nanorods have been successfully fabricated on Si (111) substrates byvaring the Zn/Mg molar ratio (50:1,45:5,40:10) using CVD technique at the growthtemperature of750℃with the growth time60min. With the increasing Mg content, thediameter of nanorods become smaller, and the thickness of nanorods become uniform, and thenanorods are smooth without protrusions, the preferred orientation of the samples becomeworse. When the Zn/Mg molar ratio of reactants was50:1, ZnMgO nanosheets have the bestcrystalline quality, and the crystalline quality starts to deteriorate with the Mg content ofreaction increasing, which may be caused by defects as more Mg ions incorporated into ZnO.The UV emission peak showed an obvious blue shift from382to376nm.(4) ZnMgO nano-particles have been successfully fabricated by using hydrothermalmethod with the raw materials of zinc acetate (Zn (CH3COO)22H2O), magnesium acetate(Mg (CH3COO)24H2O) and NaOH. By changing the incorporation of Mg, we found the morphology of nanoparticles become ruguler. With the increase of Mg incorporation, thecluster spheres composed of nanoparticles change into the hexagonal cylinder structuregradually, when the sample is pure ZnO or less Mg incorporation in ZnO, ZnMgOnanoparticles have a good crystal quality relatively, there are fewer defects, under thiscondition, the intensity of visible light in the sample is weaker than the peak intensity of theultraviolet light emission, it is considered that the crystalline quality of ZnMgOnanostructures can be improved with a small amount of Mg incorporated into ZnO. The UVemission peaks show a clear blue shift with Mg content increase.(5) Mg-doped ZnO nanorods have been successfully fabricated on Si (111) substrates byusing two-step process. ZnO seed layer were firstly fabricated on a Si substrate by thechemical vapor deposition method, and then ZnMgO nanorods with different Mg contentwere prepared by hydrothermal method under certain conditions. The results show that theMg doped ZnO nanorods possess uniform size, perfect alignment and crystallinity. The PLspectra exhibited that the UV emission peak of the simples show an obvious blue shift from396to386nm, indicating the enlarging of the band gap owing to the substituting of Mg in Znsite with the increasing of Mg content.