Ultrathin ZnO Nanowires: Synthesis, Characterization, Optical And Photocurrent Characteristics

One-dimensional nanomaterials have attracted a great deal of interest because of unique optical,electrical, acoustic, mechanical properties and technological applications compared with their bulk phasecounterparts. ZnO is an important electronic and photonic II-VI semiconductor with a bandgap of3.37eVand exciton binding energy of60meV.With the researches of ZnO, it was found that when the nanowires are contact with the electrodes, theSchottky barrier (SB) is formed. Then the current transport properties of the nanowires mainly becontrolled by the potential barrier, rather than by the resistance of nanowires. So sometimes, there are somelimitation of the nanodevices in application. These problems are caused by the larger diameter of thenanowires. The diameter of the nanowires is usually more than100nm in the studies, which is greater thanthe value of the potential barrier’s width (20nm) that caused by the surface state. This leads to theexistence of the depletion layer on the surface of the nanowires. However, there is a high-conductivity layerwhich is not depleted completely in the center of the nanowires. A decrease of the nanowire diameter leadsto complete depletion at a critical diameter with unchanged surface barrier height for electrons in theconduction band. At this time, the potential barrier can not be formed. Because of the disappear of thepotential barrier and high-conductivity layer, the current transport properties of the nanowires is controlledby the resistance of nanowires. Therefore it is necessary to synthesis the ultrathin nanowires.Many synthetic methodologies for1-D ZnO nanostructrures have been developed, but it is also achallenge to tailor the morphology and size of the nanowires or nanrods, especially the systhesis ofnanowires below10nm. And there are also some problems in the fabrication of nanodevices, such as, highcost, low sensitivity, complex process controls. Therefore more low cost, simplicity and efficient methodsshould be explored in the fabrication of nanodevices.So, we carried out the following works. First, we synthesized the ultrathin ZnO nanowires by differentmethods. And nano-electrode was prepared by using electron beam lithography (EBL) technology. On thebasis of the above work, we built the nanodevices based on the ZnO ultrathin nanowires. Then itsphotoelectric properties were studied.In our work, we first took advantage of wet chemical methods (solvothermal and homogeneousprecipitation route) to achieve the synthesis of ultrathin ZnO nanowires. By changing the reactionparameters, as a result, a fine size tuning of ZnO nanowires (from4nm to10nm) with high quality is conveniently achieved by a modified solvothermal route. And we also use the homogeneous precipitationtechnique to prepare ZnO ultrathin nanorods. The diameter of the nanorods is about15nm. This method isnot only convenient but also can solve the low production yield in the synthesis of ultrathinnanowires/nanorods by solvothermal method. The as-prepared ultrathin ZnO nanowires were investigatedusing scanning electron microscopy (SEM), transmission electron microscopy (TEM) and x-ray diffraction(XRD). Optical properties of the ultrathin ZnO nanowires were also studied; the strong UV excitonicemission and weak deep-level emission reveal their very limited structural defects and narrow sizedistribution, which is interesting and important for facilitating their special applications for photoelectricnanodevices in modern science and technology.The second section is the fabrication of electrode with nanometer-scale gaps by electron-beamlithography (EBL). And finally the nanoelectrode was achieved, which the distance of the gaps was about600nm,300nm and100nm respectively. It is important to reduce the influence of proximity effect in theprocess of EBL. Then the nanoelectrodes were characterized by SEM and AFM. It shows that the surface ofthe nanoelectrodes that fabricated by EBL are smooth and few residues of Au. The IV curve obtained whenthe electrodes were empty, in other words the electrodes were not assembled by nanowires, shows that thecurrent is generally in the range of10-14to10-15A. It means that the fabrication of nanoelectrodes issuccessful and could be used in the next work.Finally, we put ultrathin ZnO nanowires and electrodes into nanodevices by two ways. Then thephotoelectric properties were studied. As a result, the photocurrent under the UV illumination wasincreased two orders of magnitude contrast the current in the dark.