Emission Diversity of ZnO Nanocrystals with Different Growth Temperatures
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Emission Diversity of ZnO Nanocrystals with Different Growth Temperatures E. Velázquez Lozada1*, T. Torchynska2, G. Camacho González3 1 SEPI – ESIME – INSTITUTO POLITECNICO NACIONAL, México D. F. 07738, México., 2 ESFM – INSTITUTO POLITECNICO NACIONAL, México D. F. 07738, México. 3 ESIME – INSTITUTO POLITECNICO NACIONAL, México D. F. 07738, México.
ABSTRACT Scanning electronic microscopy (SEM), X ray diffraction (XRD) and photoluminescence (PL) have been applied to the study of structural and optical properties of ZnO nanocrystals prepared by the ultrasonic spray pyrolysis (USP) at different temperatures. The variation of temperatures and times at the growth of ZnO films permits modifying the ZnO phase from the amorphous to crystalline, to change the size of ZnO nanocrystals (NCs), as well as to vary their photoluminescence spectra. The study has revealed three types of PL bands in ZnO NCs: defect related emission, the near-band-edge (NBE) PL, related to the LO phonon replica of free exciton (FE) recombination, and its second-order diffraction peaks. The PL bands, related to the LO phonon replica of FE, and its second-order diffraction in the room temperature Pl spectrum testify on the high quality of ZnO films prepared by the USP technology.
INTRODUCTION Nanocrystalline Zinc oxide (ZnO) with wide band gap energy nearly 3.37 eV, high exciton binding energy (60 meV at 300K) and easy way of nanostructure preparation has attracted great attention during the last two decades [1]. In addition to exceptional exciton properties, ZnO possesses a number of deep levels that emit in the whole visible range and, hence, can provide intrinsic “white” light emission. ZnO nanostructures are being investigated as promising candidates for different optoelectronic applications, such as: the non-linear optical devices [2], light-emitting devices [3-6], transparent electrodes for solar cells [7] and laser diodes [8], as well as for the excellent field emitters [9], electrochemical sensors and toxic gas sensors [10]. The control of the ZnO defect structure in these nanostructures is a necessary step in order to improve the device quality. Since the structural imperfection and defects generally deteriorate the exciton related recombination process, it is necessary to grow the high quality films for efficient light-emitting applications. The ultrasonic spray pyrolysis (USP) method offers many advantages such as easy compositional modifications; easy introducing the various functional groups, relatively low annealing temperatures and the possibility of coating deposition on a large area substrate. It will be interesting to study the optical emission of USP produced ZnO NCs doped by Ag in order to identify the best regimes for obtaining the bright emitting nanosystems.
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EXPERIMENTAL DETAILS ZnO:Ag thin solid films were prepared by the USP technique (Fig. 1) on the surface of soda-lime glass substrate for the two substrate temperatures (400 and 450 oC) and different deposition times (Table 1). Table 1. Technological regimes and NC para
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