Size dependent optical properties in ZnO nanosheets
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Size dependent optical properties in ZnO nanosheets Brahim El Filali and Aaron I. Díaz Cano, UPIITA-Instituto Politécnico Nacional, México D.F.07738, México. ABSTRACT Photoluminescence (PL), scanning electronic microscopy (SEM) and Raman scattering have been studied in crystalline ZnO nanosheets with different sizes after the thermal annealing at 400 °C for 2 hours in ambient air. ZnO nanosheets were created by the electrochemical (anodization) method using the variation of the etching durations with obtained ZnO nanosheet sizes from the range 40-360 nm. Earlier it was shown using the X ray diffraction (XRD) method that thermal annealing performed the ZnO oxidation and crystallization with the creation of the wurtzite crystal lattice. Four PL bands are revealed in PL spectra with the PL peaks at 1.60, 2.08, 2.50 and 3.10 eV. Size decreasing of ZnO nanosheets stimulates tremendous changes of ZnO optical parameters. It is shown that decreasing the ZnO nanosheet sizes is accompanied by the intensity increase of a set of Raman peaks and the surface defect related PL bands. The reasons of emission transformation and the nature of optical transitions have been discussed as well. INTRODUCTION There is a great interest in growing the ZnO nanosystems using the anodization technology developed early for the PSi production [1-3]. Nano crystalline ZnO – wide band gap semiconductors with the direct band gap (3.37eV) and high exciton binding energy (60meV) at room temperature, promises the numerous applications in optical and electronic devices [4] such as room temperature white light-emitting diodes [5] or ultraviolet nanolasers [6], field emission cathodes [7] or field – effect transistors [8]. However, even high-quality ZnO samples contain a variety of point defects contributing to the visible luminescence [9]. Actually it is not clear the relation between the radiative defects appeared and the size of ZnO nanosystems. In this work the dependence of optical properties of ZnO nanosheets versus their sizes has been studied. EXPERIMENTAL DETAILS The electrochemical anodization of Zn foils was performed in a two electrode system including the Zn electrodes exposed in the electrolyte. The pieces of a Zn foil (Aldrich 99.99% purity, 0.25 mm thickness) with a radius of 6 mm were treated in acetone and ethanol for 15 min with ultrasonic cleaning and then washed with deionized water. The electrolyte composition was a 1:10 volume mixture of HF and deionized water without surfactants. The applied voltage was 5 V and the reaction time ranges from 1 min to 10 min (Table 1). Then these samples were annealed at 400 °C during 2 hours in ambient air. The morphology of ZnO films has been studied using the scanning electron microscopy (SEM) (Quanta 3D FEG-FEI). Raman scattering spectra were measured in Jobin-Yvon LabRAM HR 800UV micro-Raman system using an excitation by a solid state LED with a light wavelength of 785 nm [10,11]. PL spectra were measured at the excitation by a He-Cd laser with
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a wavelength of 325 nm and a beam power of 80
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