Specific features of the structure of ZnO nanocrystals grown in pores of Y 2 O 3 spherical matrices
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TERIALS AND CERAMICS
Specific Features of the Structure of ZnO Nanocrystals Grown in Pores of Y2O3 Spherical Matrices N. A. Dulinaa, Yu. V. Yermolayevaa, V. N. Baumera, A. V. Tolmacheva, E. A. Kudrenkob, and G. A. Emel’chenkob a
b
Institute for Single Crystals, National Academy of Sciences of Ukraine, Kharkov, 61103 Ukraine Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia e-mail: [email protected] Received July 2, 2014
A two-phase spherical Y2O3/ZnO nanocomposite particle with a diameter variable in the series of 70, 130, 180, 250, and 400 nm (variance ≤15%) has been obtained; the first phase is a mesoporous Y2O3 matrix, while the second is a crystalline ZnО phase located in the size-limited pore volume. Specific features of ZnO nanocrystals formed on the high-curvature surface of pores with excess surface energy are studied. It is shown that the thermally activated coarsening of ZnO crystallites accompanied by an increase in the nanophase structural quality occurs during Y2O3/ZnO annealing in the temperature range t = 600–800°C. It is established that the wurtzite lattice is strained on the surfaces of small Y2O3 pores. DOI: 10.1134/S106377451502008X
INTRODUCTION Nanoscale zinc oxide is interesting for application in many UV optoelectronic devices (new effective laser media operating at room temperature, optical sensors, and crystal phosphors [1–4]) because of their wide band gap Eg = 3.37 eV and high free-exciton binding energy (60 meV). It is known that the UV luminescence intensity of zinc oxide in the range of bound excitons at a wavelength λ ≈ 380 nm [5] is determined by the quality of its crystal structure [6]. A general strategy of increasing the ZnO structural quality implies the activation of mass transfer by increasing the annealing temperature. At the same time, oxygen evaporation can be observed in microcrystalline ZnO powders and films with an increase in the annealing temperature even above 300°C (zinc oxide crystallization point), which induces intrinsic donor defects (oxygen and interstitial zinc vacancies) [7–9]. The exciton recombination from these defects leads to the enhancement of the green luminescence impurity band at λ = 500–510 nm [10, 11]. The violation of ZnO stoichiometry with respect to the anionic sublattice upon thermal annealing was excluded in several studies (examples are technologies of zinc oxide treatment in oxygen radicals at plasma discharge [12] or ZnO formation by the thermolysis of metal-organic precursors [13–15]). However, these methods are technologically difficult to implement. As a result, an alternative approach to ZnO formation was proposed in [16, 17]: nanocrystals are grown
in mesopores of monosize nano- and submicron Y2O3 spheres [18–22] with the formation of Y2O3 matrix– ZnO filler nanocomposites. The zinc oxide nanophase formation in a pore facilitates the slow removal of gaseous products of the zinc nitrate thermolysis reaction (O2 and NO2); thus, the conditions for retaining oxygen during the Z
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