Opto-Electronic Properties and Stability of Artificial Zinc Oxide Molecules
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Opto-Electronic Properties and Stability of Artificial Zinc Oxide Molecules Liudmila A. Pozhar1 and Gail J. Brown2 1 Center for Materials for Information Technology, University of Alabama, P.O. Box 870209, Tuscaloosa, AL, 35487-0209 2 Materials and Manufacturing Directorate, Air Force Research Laboratory, 3005 Hobson Way, Bldg. 651, Wright-Patterson AFB, OH, 45433-7707 ABSTRACT The Hartree-Fock (HF), restricted open shell HF (ROHF), and multiconfiguration selfconsistent field (CI/CASSCF/MCSCF) approximations are used to study computationally the electronic properties of zinc oxide artificial molecules whose structure and composition have been derived from those of the symmetry elements of the wurtzite bulk lattice of zinc oxide. Such molecules may provide realistic models for small ZnO quantum dots (QDs) synthesized in “vacuum” or quantum confinement (such as that of well-defined nanopore arrays of silica and alumina membranes) using variety of methods in particular, supercritical fluid deposition. The computational direct optical transition energy (OTE) of the confined molecule appears to be several times smaller than that of the corresponding vacuum cluster. The charge and spin density distributions of these molecules (CDDs and SDDs, respectively) differ significantly, revealing dramatic effects of quantum confinement on electronic properties of Zn-O clusters. The obtained results suggest that manipulations with the electronic properties of the confined clusters by sophisticated design of their quantum confinement may provide means for synthesis of Zn-O – based electronic materials that combine a wide, tunable band gap with large, tunable exciton binding energy.
INTRODUCTION Among metal oxides, ZnO is of a great interest as an opto-electronic material due to a combination of its wide band gap and large exciton binding energy at room temperature. In recent years, a range of ZnO nanostructures have been synthesized experimentally using a variety of physical and chemical processes [1,2]. Opto-electronic properties of these nanostructures depend significantly on the shape and composition of the synthesized nanoclusters defined by a particular synthesis method. Smaller nanoclusters exhibit a blueshifted photoluminescence (PL) peaks, as compared to that of the bulk wurtzite ZnO lattice. Some of such ZnO QDs in the size range from 6 nm to 9.3 nm grown electrochemically [3] demonstrate the band gap PL of 360 nm (or 3.444 eV). Even smaller, 3 nm ZnO clusters have been synthesized experimentally by implantation [4]. The absorption peak of these nanoclusters is at about 290 nm (or 4.2753 eV). These experimental results confirm a tendency known for the majority of atomic clusters, namely, that the OTE of such clusters increases with a decrease in the clusters size, and that the smallest OTE is that of the corresponding bulk lattice (if such a lattice exists). Experimental interest to ZnO QDs [5] has been followed by a number of computational studies that use half-heuristic and ab initio, HF- and density-function
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