Practical Preparation Methods For ZnO Micro/Nano-Materials

As a wide band-gap semiconductor,ZnO has attracted intense attention due to its applications in electronic and photoelectronic devices.With the development of nanotechnology in recent years,micro/nano-structural ZnO has become increasingly important.This dissertation focuses on the study of various practical,simple and low-cost growth technologies for micro/nano-structural ZnO.Magnetron sputtering, Sol-Gel and solution combustion methods have been applied to prepare ZnO in different morphologies and structures.Furthermore,ZnO properties have been characterized and the growth mechanisms are discussed.Firstly,nanostructure ZnO materials were controllably deposited by radio frequency magnetron sputtering method,with optimized Ar/O₂ ratio and sputtering power,choosing suitable buffer layers.The analysis shows the c-axis orientated ZnO nano grains were prepared preferably on(0002)plane due to the lowest surface energy, resulting in the largest possibility of nucleation.Meanwhile,with consideration of structural model,the role of gas pressure and substrate temperature on the preparation of compacted ZnO nanorod arrays is discussed.By analyzing the results of TEM, XPS,Raman and so on,a mechanism of nanograin modification is proposed.Since the energy required by adjusting grains inα-axis is lower than that for enlarging grains,lower temperature annealing treatment will preferably adjust grain's orientation more ordered alongα-axis.Higher temperature thermal treatment stimulates grain size increase as well as grain arrangements by providing more kinetic energy.As the result,annealing treatment performed in oxygen atmosphere causes decreasing of oxygen vacancy.Secondly,Sol-Gel synthesis was performed to realize one-and two-dimension growth of ZnO by adjusting the pH value of solutions.In the ZnO nanorods synthesized in neutral sol solution,luminescence spectra compose of a near band emission originating from donor-accepter pair(DAP)and a broad visible emission band ranging from green to yellow.Subsequent annealing treatment not only improves emission intensity of the nanorods,but also results in the green emission due to the formation of oxygen vacancy(V₀⁺).While in acid sol solution(pH=6)ZnO nanorods with uniform size were prepared,whose near band emission is characterized by a neutral bond exciton.By altering sol solution's acidity from 4,5 to 6,ZnO lateral growth caused by OH^-ligand reaction mechanism was realized with the synthesis of 2-D platelets.With increasing sol solution acidity,more ordered hexagonal shape, higher yield and stronger emission intensity of platelets can be achieved.Different from nanorod ZnO materials,ZnO nano-platelets emit yellow luminescence centered at around 580 nm.Based on the above analysis,a synthesis mechanism of ZnO low dimension structure material was proposed.During the hydrolysis in different pH value sol solutions,ZnO growth is determined by intrinsically orientated growth as well as OH^-ligand reaction.In neutral sol solution,the prismatic ZnO nanorods develop along the[0001]direction;while in acid sol solution,the H⁺ ions in higher concentration preferably react with OH^-ligands at the crystal surface,inhibiting growth along the[0001]direction,making the lateral growth more preferable and resulting in the appearance of platelets.Finally,a solution combustion synthesis of pure and indium-incorporated ZnO powder was performed by using metal nitrates as oxidizer agents and carbohydrazide as fuel.The surface morphologies,crystal structure and optical properties of various ZnO powders were characterized.The powders synthesized by this method are spongy clusters consisting of platelet-shaped nanocrystals with wurtzite structure.As the In³⁺concentration increases in reactant solutions,the as-synthesized ZnO powders show increase of grain boundaries,decrease of grain size and deterioration of crystal quality,as well,lower photoemission intensity due to increase of non-radiative electron transition.In addition,comparing to the oxygen vacancy related green emission from pure ZnO,the introduction of In impurity in ZnO causes the yellow emission at about 580 nm due to the forming of interstitial oxygen(V_i^-),whose intensity gradually decreases with the increasing of In concentration in reactants. Post-growth thermal treatments improve the material quality,resulting in well-ordered hexagonal-shaped particles,larger size grains and higher photoemission intensity. Thermal treatment also causes quenching of yellow band emission and the appearance of a green emission band in indium-incorporated ZnOs,which are related to the interstitial oxygen out-diffusion and the formation of indium-oxygen vacancy complexes In^3+-V₀+,respectively.The green emission in indium incorporated ZnO powders red-shifts to 550 nm relative to that at 540 nm in pure ZnO powder,whose intensity decreases with the In concentration increasing.With considering above experimental results,emission mechanism in visible light region of ZnO micro/nano powders complies with the formation energy of oxygen vacancy(V₀⁺)model.Given a certain exciting energy,with the increase of In³⁺concentration,the Fermi level energy increases and then the formation energy of oxygen vacancy(V₀⁺)increases,resulting into the lower intensity of the green emission。...