Defect Driven Emission from ZnO Nano Rods Synthesized by Fast Microwave Irradiation Method for Optoelectronic Applications

Abstract

Because of its large direct band gap of 3.37 eV and high exciton binding energy (~60 meV), which can lead to efficient excitonic emission at room temperature and above, ZnO nanostructures in the würtzite polymorph are an ideal choice for electronic and optoelectronic applications. Some of the important parameters in this regard are free carrier concentration, doping compensation, minority carrier lifetime, and luminescence efficiency, which are directly or indirectly related to the defects that, in turn, depend on the method of synthesis. We report the synthesis of undoped ZnO nanorods through microwave irradiation of an aqueous solution of zinc acetate dehydrate [Zn(CH 3 COO) 2 . 2H 2 O] and KOH, with zinc acetate dihydrate acting as both the precursor to ZnO and as a self-capping agent. Upon exposure of the solution to microwaves in a domestic oven, ZnO nanorods 1.5 μm -3 μm and 80 nm in diameter are formed in minutes. The ZnO structures have been characterised in detail by X-ray diffraction (XRD), selective area electron diffraction (SAED) and high-resolution scanning and transmission microscopy, which reveal that each nanorod is single-crystalline. Optical characteristics of the nanorods were investigated through photoluminescence (PL) and cathodoluminescence (CL). These measurements reveal that defect state-induced emission is prominent, with a broad greenish yellow emission. CL measurements made on a number of individual nanorods at different accelerating voltages for the electrons show CL intensity increases with increasing accelerating voltage. A red shift is observed in the CL spectra as the accelerating voltage is raised, implying that emission due to oxygen vacancies dominates under these conditions and that interstitial sites can be controlled with the accelerating voltage of the electron beam. Timeresolved fluorescence (TRFL) measurements yield a life time (τ) of 9.9 picoseconds, indicating that ZnO nanorods synthesized by the present process are excellent candidates for optoelectronic devices. INTRODUCTIONZinc oxide (ZnO) is a multifunctional, direct, wide band gap (E g =3.37 eV) semiconductor with a large free-exciton binding energy (~60 meV), rendering excitonic emission processes possible at or above room temperature[1,2,3]. Thus, ZnO is a promising material for UV and blue light emitting devices with several fundamental advantages over GaN, its chief competitor. It is more radiation-resistant than GaN, comparatively inexpensive, and biocompatible [4,5]. Defects in ZnO play a major role in tailoring its optical, piezoelectric, mechanical, electronic and optoelectronic properties [3]. The desired properties of ZnO nanostructures can be tuned with either native defects or foreign atom incorporated defects (doping). Intrinsic point defects in ZnO nanorods give rise to luminescence over a broad range in the visible spectrum. This enables tunable resonant emission in the visible spectral range with a pure ZnO system. Native defect engineering is favorable over doping because foreign atoms usually affe...

Interfacial Grown Zinc Oxide Origami Structure for Broadband Visible Light Driven Photocatalysts

“…It indicates a lifetime shorter than 75 ps (below the instrument response function of ≈75 ps) due to the ultrathin ZnO nanosheets. Compared to the 191 ps measured on ZnO nanorod with diameter of 80 nm, [59] in our case the ultrathin ZnO nanosheets in the origami structure are much thinner (<10 nm), so that the carriers can migrate much faster to the surface, and recombine through the IDLs in case of the lifetime measurements using ZnO powders, therefore a much shorter lifetime is expected. In case of the MB-degradation tests, the carriers which reach the surface can generate radicals in the aqueous solution for the MB-degradation.…”