The optoelectronic properties of metal oxides, especially zinc oxide (ZnO) have been studied with respect to their semi-conduction, light emission and photo-catalytic properties. ZnO has been demonstrated to function as efficient light-emitting diodes and laser diodes in the UV-visible range, which ZnO p-n homojunctions have been obtained by the synthesis of p-type ZnO thin films. Metal oxides, in particular ZnO and indium oxide (In2O3), in their pure form, have been obtained having nanostructural morphology. ZnO and In2O3 have a binding energy which is relatively higher than that of typical semiconductor materials, and therefore have potential applicability in electronic devices. ZnO is also a promising material for optoelectronic applications because of its wide band gap (3.37 eV) and large exciton binding energy (60 meV), which is considerably greater than conventional semiconductor materials such as silicon (Si, 15 meV), germanium (Ge, 4.2 meV), Zinc sulfide (ZnS, 20 meV), gallium nitride (GaN, 21 meV), gallium arsenide (GaAs, 4.9 meV) and indium arsenide (InAs, 2.11 meV). In addition, In2O3 is a promising material for optoelectronic applications because of its direct band gap around 3.6 eV.
Although metal oxides, including ZnO are predicted to be useful in a variety of applications such as in solar cells, sensors and photocatalysis, their practical realization has been largely limited by the need for economically viable synthetic processes that are capable of producing free-standing varied morphology materials in good yield that are required for their incorporation in the fabrication of such devices.
With the advent of carbon nanotubes (CNT) and their use, albeit in a limited way, in electronic device fabrication, attempts to utilize metal oxides, including ZnO and In2O3 in a similar manner has been made. Such attempts include efforts to synthesize ZnO materials, with varied nanostructural morphology, by utilizing a number of methods such as metal organic chemical vapor deposition (MOCVD), chemical vapor deposition (CVD), physical vapor deposition (PVD), thermal evaporation. Such attempts also include efforts to synthesize In2O3 materials with varied nanostructural morphology by electrodeposition, heating of indium grains without catalyst, reduction of In2O3 by hydrogen, and heating of indium phosphide (InP) coated with a gold (Au) layer have been used. However these methods mostly require substrates upon which the metal oxide material is grown, and produce materials in relatively small yields. The prior art methods are therefore not optimal for producing free-standing materials in gram quantities.