Emission modification in ZnO nanosheets at thermal annealing
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Emission modification in ZnO nanosheets at thermal annealing Aaron I. Diaz Cano*1, Brahim El Filali1, Tetyana V. Torchynska2 and Jose L. Casas Espinola2 1 UPIITA – Instituto Politécnico Nacional, México D. F. 07738, México. 2 ESFM- Instituto Politécnico Nacional, México D. F. 07738, México.
ABSTRACT Photoluminescence (PL) and its temperature dependences, as well as the X ray diffraction (XRD), have been studied in the freshly prepared amorphous phase ZnO nanosheets, obtained by the electrochemical (anodization) method, and in the crystalline annealed ZnO nanosheets. The freshly prepared samples have been divided in two groups. One of these groups has been annealed at 400 °C for 2 hours in ambient air. Defect related PL bands with the peaks at 2.102.13, 2.42-2.46 and 2.65-2.69 eV are detected in amorphous state. Appreciable changes in the size of nanosheets as a function of thermal treatments have been revealed. XRD study has shown that annealing stimulates the Zn oxidation and the creation of ZnO with a wurtzite crystal lattice. In crystalline ZnO seven PL bands appeared with the PL peaks 1.46, 1.58, 2.02, 2.43, 2.70, 2.93 and 3.16 eV at 10K. The reasons of emission transformation and the nature of optical transitions related to the studied PL bands have been discussed. It is shown that the anodization method permits by a controllable way to obtain the wide range ZnO emission that is interesting for the future applications in room temperature “white” light-emitting diodes. INTRODUCTION ZnO nanostructures have attracted considerable attention due to their promising applications in electronic, photoelectronic and sensing devices, mainly due to the potential for engineering the properties not obtained in the bulk materials. The past few years have witnessed a much progress in the synthesis of ZnO nanostructures that promises their applications in optical and electronic devices [1-4], ZnO nanowall and nanowire arrays can be used as room temperature white lightemitting diodes and ultraviolet nanolasers [5], ZnO nanoneedles possess excellent field emission performance [6], ZnO nanorods were used for high performance field–effect transistors [7]. ZnO nanostructures have attracted attention as well owing their possible applications in the low voltage and short-wavelength (368 nm) electro-optical devices, transparent ultraviolet (UV) protection films, gas sensors, and even spintronic devices [8]. Various methods to synthesis of ZnO nanostructures have been employed such as sol–gel methods [9, 10], spray pyrolysis [11, 12], metal–organic chemical vapor deposition [13, 14], rf sputtering [15, 16], thermal evaporation [17], pulsed laser deposition [18], chemical vapor deposition [19], catalysis driven molecular beam epitaxy [20] and sonochemical method [21]. Electrochemical (anodization) method has been recognized as one of the most simple, economic, non-vacuum and low temperature effective method to prepare the nanomaterials. ZnO nanosheets prepared by this method are of particular interest because of their high porosity and
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