MgZnO Nanocrystallites: Photoluminescence and Phonon Properties
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MgZnO Nanocrystallites: Photoluminescence and Phonon Properties John L. Morrison, Xiang-Bai Chen, Jesse Huso, Heather Hoeck, James Mitchell, Leah Bergman. [email protected]. Department of Physics University of Idaho, Moscow ID Tsvetanka Zheleva, Army Research Lab, Adelphi, MD 20783-1197. [email protected]. Abstract We report on the ultraviolet photoluminescence (UV-PL), Raman and structural properties of wurtzite MgxZn1-xO nanopowders of average size ~ 30 nm that were synthesized via the thermal decomposition method. For the studied composition range of 0.26 ≥ x ≥ 0 , the room temperature UV-PL was found to be tuned by ~ 0.23 eV towards the UV-spectral range, and the PL emission was established to be due to an excitonic-type recombination mechanism. The first-order LO Raman mode was found to exhibit a blueshift of ~ 33 cm-1. The LO-mode of the nanopowders is discussed in terms of a mixed A1-E1 symmetry phonon known as a quasi-LO mode. The observed 30 cm-1 blueshift indicates that the E1 is the principle symmetry component in the Raman scattering of the MgxZn1-xO nanopowders. Introduction ZnO and MgxZn1-xO are promising emerging materials capable of luminescence in the ultraviolet (UV) spectral range [1-7]. ZnO has the hexagonal wurtzite structure of bandgap ~ 3.4 eV while MgO has the NaCl cubic structure of bandgap ~ 7.5 eV [8-9]. The alloy system of MgxZn1-xO has been recently realized and it has been demonstrated that up to approximately 35% of Mg concentration the alloy has the wurtzite structure; above that concentration a phase transition into the cubic structure takes place [4-5]. The MgxZn1-xO alloy system may provide a new optically tunable family of wide bandgap materials that can be used in UV luminescent applications as well as a potential conjugate material for the group III-nitrides. Most of the research efforts to date have concentrated on the growth and study of MgxZn1-xO epitaxial films, some of which exhibited strong UV-PL emission [1-7], while significantly less effort has extended into the issue of the alloy in its polycrystalline and powder forms. The synthesis of MgxZn1-xO microcrystalline films via the sol-gel as well as the synthesis of nanocrystalline films via electrophoresis deposition methods was previously reported [10-11]. In contrast our research focuses on the novel synthesis of MgxZn1-xO nanopowders via thermal decomposition as well as on their UV-PL and Raman properties. Experimental Approach For the powder synthesis we mixed in an aqueous solution the desired mol fraction of 99.997% pure magnesium acetate tetrahydrate, [Mg(CH3COO)2], and 99.999% pure zinc acetate dihydrate, [Zn(CH3COO)2]. A small amount of the mixed solution was dropped onto a silicon wafer and heated relatively slowly in ambient conditions to ~90o C until all the water evaporated. The sample was then heated up to 540o C for one hour where decomposition of the precursors occurred to form the alloy, then slowly cooled to room temperature over the course of an hour. Figure 1 presen
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