Sol-gel synthesis of highly luminescent magnesium oxide nanocrystallites

An attempt has been made to prepare MgO nano-crystallites through decomposition of sal-gel derived magnesium oxalate dihydrate in air, oxygen and nitrogen ambient at 500, 600, 800, and 1000 degrees C for 2 h each and study them with regard to their phase, average crystallite size, morphology, and photoluminescence (PL) behaviour. They are shown to possess f.c.c. (NaCl-type) structure with lattice parameter a similar to 4.211 angstrom, average crystallite size in the range of 3.0-73.5 nm, <1 1 1> preferred orientation at decomposition temperature of 500 degrees C (in nitrogen and oxygen ambient), and a distorted rod-like morphology. The PL peaks observed around 395 and 440 nm have been assigned to T-2(1u)-->(2)A(1g) and B-3(1u)-->(1)A(g) transitions associated with the relaxation of excited states of F+ - and F-2(2+)-centres, respectively. Further, the emission band intensity is found to depend on decomposition temperature and gas ambient, crystallite size, and their morphology. However, in nitrogen ambient above 800 degrees C, several other PL peaks observed at 491.8, 501.8, 503.5, 509.3, 561.5, and 563.0 nm arise due to aggregates of F centres and/or extra energy levels created in the energy band gap by nitrogen incorporation. A mechanism for nitrogen trapping in MgO has been suggested. Further, emission intensity depends on both colour centres and surface states. (C) 2010 Elsevier B.V. All rights reserved.