| GaN basedâ…¢-â…¤nitrides have attracted great attention recently,as driven by the commercial success in their application in optoelectronic devices,such as blue light-emitting-diode(LED),ultraviolet(UV)photo-detector and short wavelength laser diode.In contrast to the intensive theoretical and experimental efforts made on optical and electrical properties of nitrides,kinetic process in growth of nitrides films remains less explored.Zinc oxide has been recognized as one of the most important semiconductor materials in scientific research and technological applications,owing to its direct wide band gap (3.37eV) a large exciton binding energy(60 meV).Due to the hexagonal wurtzite structure and polar surfaces,a diverse group of nanostructures,including nanowires, nanorings,nanobelts,nanorods and nanotubes,have been fabricated by different methods such as wet-chemical method,electrochemical deposition,templating method, radio-frequency magnetron sputtering deposition and metal-organic chemical vapor deposition.Tin as an important catalyst has been broadly used in ZnO films and nanostructures,which can obviously affect the nanostructure and optical,electronic and piezoelectric properties.In this report,the tin has unusual effects on the formation of the bicrystalline ZnO nanowires and the diode-shaped ZnO nanocrystals.Otherwise,we also have novel porous ZnO nanowalls prepared by a two-step growth method with a carbon-thermal evaporation.The structure,elemental composition and morphology of the ZnO nanostructures were determined by X-ray diffraction(XRD),Fourier transformed infrared spectroscopy (FTIR),X-ray photoelectron energy spectroscopy(XPS),scanning electronic microscope(SEM) and high-resolution transmission electronic microscope(HRTEM). All the results are as follows:1.Novel bicrystalline ZnO nanowiresTwo types of novel bicrystalline ZnO nanowires have been synthesized by a thermal evaporation method.The morphology and microstructure of the nanowires have been extensively investigated.One type of the nanowires has a twin boundary extending down its entire length.The other type is those nanowires with twin crystal-single crystal junction.The catalyst particles have uneven thickness which might be caused by remelt of droplets during the growth process of the catalyst particles.At the initial nucleation stage,the Sn-Zn alloy droplets on substrates have irregular shapes,inner twins and defects.The twin defects in the Sn-Zn alloy droplets in the initial growth process are proposed for interpreting the growth of these two kinds of bicrystalline nanowires.According to the feature of hexagonal system,it is difficult to form ZnO twin crystal. And as a significant defect structure,twin boundaries could generate positive energy and are expected to have important effects on optical,electronic,mechanical,and chemical properties and twins can affect their subsequent performance in new technologies.So those new types of ZnO nanowires in our work may exhibite high potential for fabricating novel nano electronic and optical devices with enhanced performance.2.Novel diode-shaped ZnO nanocrystalsThe diode-shaped ZnO nanostructure was fabricated through a simple thermal evaporation process.The diode-shaped ZnO nanostructures are high yield,which can be divided into two configurations:the one is with two nanowires on the two ends of the nanorod;the other is with one nanowire on one of the ends of the nanorod,where the other end is flat,According to the X-ray diffraction pattern and HRTEM images,the whole structure is a perfect single crystal along the[0001]growth direction with the length of a few 10 mm.According to the characteristic of the ZnO wurtzite structure, the Zn-terminated(0001) surface is catalytically active,while the O-terminated(0001) is catalytically inert.Therefore,Zn atoms were enriched on the Zn-terminated(0001) surface to form screw dislocation and lead to the fast growth of nanowire by self-catalyst process.If the surfaces on the two ends of the nanorod were Zn-terminated (0001),the configuration with two nanowires was formed.If one surface is Zn-terminated(0001) and the other is O-terminated(0001),the configuration with one nanowire was formed.Based on our XRD and electron microscopy study,the new effect of Sn in the growing process is discussed.Most of the Q1D nanostructures have been produced by the metal-catalyst mechanism,which have some obvious characteristics: the remains of metal droplets on the tips or the roots of wire and rod;the catalyst initiates and guides the growth.Especially,tin as an important catalyst has been broadly used.However,in this report,there are no catalyst particles on the ends of nanowires or nanorods.The tin has unusual effects on the formation of the diode-shaped ZnO nanocrystals:the Sn configuration-limiting prevent Zn three-dimensional nucleation and solvent-analogous effects to increase the probability of collision between Sn and Zn atoms and accelerate the move to the growth front.The screw dislocation and polar orientation effects on the growth process were also discussed.The diode-shaped ZnO nanostructures not only provided valuable models in understanding crystal growth mechanisms in nanometer scale,but also exhibited high potential for fabricating novel electronic and optical nanodevices with enhanced performance.3.Porous ZnO Nanowalls with Granular and Fibrous StructuresNovel porous ZnO nanowalls were prepared by a two-step growth method with a carbon-thermal evaporation,where the as-grown Au films were first deposited on Si (111) substrates by magnetron sputtering and then the porous ZnO nanowalls were grown on the substrates.The morphology and microstructure of the nanowalls have been extensively investigated.One type of the porous structures is the granular morphology.The other type is the fibrous morphology.The thickness of the Au films, namely the size and the spacing of the Au particles are proposed for interpreting the growth of these two kinds of porous structures,such as the length and spacing of pores. Compared with many research activities on various one-dimensional ZnO nanomaterial growth,fewer studies have been reported on the growth of ZnO nanowall structures, which have potential applications in photo catalysts,biosensors,solar cells and other fields demanding high surface-to-volume ratios.Carbon thermal reduction and MOCVD technique are used to synthesize this nanostructure.In most cases for ZnO nanowalls synthesis,sapphire substrates were needed because the epitaxy relation for the nanowall growth is necessary.To produce nanomaterial with low cost,and large production,which especially do not require perfect epitaxy growth,therefore,exploring a simple and cheap method,is meaningful.Our group has proposed a simple and efficient method to fabricate porous ZnO nanowalls.In addition,it is more important that we have successfully synthesized the porous ZnO nanowalls for the first time. Porous ZnO nanostructures have a high surface-to-volume ratio as well as other excellent inherent properties,they have widely application,especially as nanosieve filters,catalyst supports,masks,and gas sensors.4.ZnO flower-like structures and ZnO comb-like nanobeltsZnO flower-like structures is composed by many short columns.Those columns are of high crystallinity.The tips of columns are narrow while the bottoms are bigger.The different diameters are caused by the less of O2 gass and the Sn solvent-analogous effects to increase the probability of collision between Sn and Zn atoms and accelerate the move to the growth front.ZnO comb-like nanobelts are synthesized by chemical vapor deposition(CVD). X-ray diffraction(XRD) reveals that the ZnO comb-like nanobelts are of high crystallinity with wurtzite structure.The products are comb-shaped morphology through the observation using scanning electron microcope(SEM).The formation of the comb-like microbelts follows a three-step process:the comb-stems are formed by a fast vapor-solid(VS) growth mechanism,and the comb-teeth are grown by self-catalyst growth parallel to the(0001) polar surface.
|
PDF Full Text Request