Manipulating Morphology,Optical And Electrical Properties Of Re-growth ZnO Nanowires By Thin Interfacial Layer

ZnO nanowires(NWs)possess the wide direct bandgap(?3.4 eV),the large exciton binding energy(-60 meV),the strong exciton oscillation strength(Rabi splitting-200 meV),the high-quality crystal structures and the well-defined geometries,making it much suitable as excellent short-wave length(opto)electronic materials.Till now,a varities of photonic and(opto)electronic devices,such as resonant cavities,waveguides,light emitting diode and nano lasers,have been proposed.It has been well known that the good optical and electrical properties,which can be controled or modulated by the detailed morphologies and structures of ZnO NWs,are essential prerequisite for above applicaitons.However,mastery over the morphologies and properties of ZnO NWs is still a grand challenge,although a great deal of reseaches have been carried out.In this context,we propose a novel re-growth strategy of chemical vapor deposition(CVD)via coating thin interficial layer on the initial ZnO NWs.We then focus on how the interficial layer can manipulate the growth and morphologies of ZnO NWs,the enhancement of the properties of ZnO NWs and what is the underlying related mechanism.The contents of the dissertation are summaried briefly as follows.In chapter one,we firstly introduce the basic knowledge of the synthesis methods and growth mechanism of ZnO nanomatirals.Then we summarize the current status of sveral issues,including the doping strategy for ZnO nanomatirals,the modulation of morphology and properties of ZnO nanomatirals.Lastly,the motivation and content of our research are presented.In chapter two,we report manipulating and enhancing the radial growth rate by up to two orders of magnitude(from ca.0.7 A/s to ca.70 A/s)for ZnO NWs through a simple few-layer TiOx-assisted re-growth method.It is found that,unlike most other impurities,the Ti element tends to "float" on the outer surface of the re-grown NWs,thus the superior intrinsic electrical and luminescent properties are naturally guaranteed.As compared with the control NWs,these TiOx-assisted re-grown ZnO NWs show a 5-and 10-fold intensified yet undisturbed near-band-edge(NBE)emission at room temperature and 83 K,respectively.At the same time,both the conductivity(ca.104 S/m)and the electron concentration(ca.1019 cm-3)increase by one order of magnitude with almost unimpaired electron mobility.In order to understand the key role of the TiOx layer,a possible growth mechanism is proposed based on the Ti-Zn cation exchange in the vapor-solid reaction.In chapter three,we have successfully grown Al-doped ZnO nanowires(NWs)with a unique core-shell structure and a ?-doping profile at the interface through the combination of chemical vapor deposition re-growth and few-layer AlxOy atomic layer deposition.In contrast to the conventional heavy doping,in which the near-band-edge(NBE)luminescence degrades greatlly,the interfacially Al-doped NWs exhibit an over 20-fold intensified NBE emission and a ca.6-fold decreased deep-level(DL)emission comparing with the homogenous ZnO NWs at room temperature.The excellent performance benefits from both the suppressed nonradiative decay and the enhanced radiative recombination rates.Further experiments and simulation reveal that the former is due to the weakened EPC stemming from the screening effect of the high-concentration electrons,whereas the latter can be attributed to the confined and intensified optical field in the Al-doped interfacial layer due to the Purcell effect.This finding can facilitate the designs and applications of ZnO devices in nano-photonics and nano-optoelectronics.In chapter four,we find that the ZnO homojunction NWs can be readily formed by raising the concentration of oxygen gas in the base ambience before the CVD process starts.A possible growth schematic is proposed based on the self-catalyzed vapor-liquid-solid mechanism.Additionlly,by combined the controlled experiments and the classical nucleation theory,we interpret how the diameter and the densities of ZnO NWs can be tuned by varying the incubating time of precursor in the reaction chamber.Furthermore,the relationship between the diameter and the length of NWs is also explained by a simply diffusion model.More importantly,by means of first-principles calculation,the energy for the substituted doping and the surface energies for different facets of doped ZnO NW have been calculated.The results can be successfully used to understand the differnt distributions of dopants and the morphological evolution of NWs in TiOx and AlxOy-assisted re-grown ZnO NWs.In chapter five,we present a brief outlook and suggest several topics for the future studies on ZnO nanomatrials.