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Radiative/Nonradiative Recombination Affected by Defects and Electron–Phone Coupling in CdWO4 Nanorods

Zhang, Cuiling, Guo, Donglin, Xu, Weina, Hu, Chenguo, Chen, Yanxue
The Journal of Physical Chemistry C 2016 v.120 no.22 pp. 12218-12225
absorption, cadmium, emissions factor, nanorods, photoluminescence, temperature
Tungstates are important photoluminescence (PL) materials owing to their unique luminescence center. However, radiative or nonradiative recombination affected by defects and electron-phone coupling have not been well understood. In this paper, we have synthesized CdWO₄ nanorods and studied its temperature-dependent PL spectrum from 20 to 300 K. Theoretical calculations demonstrate that Cd vacancy (VCd) and O vacancies (VO⁰, VO¹⁺, and VO²⁺) induce extra levels in the band gap, by which the VCd, VO⁰, and VO¹⁺ defects mainly contribute to the absorption in 0–4 eV region, while VO²⁺ causes the emission bands peaked at 490 nm in PL spectrum. Because of the broken symmetry of octahedron two possible types of lowest unoccupied molecular orbital (LUMO) appear and the transitions from the each LUMO to the highest occupied molecular orbital (HOMO) contribute to the emission at about 410 and 436 nm. The emission intensity of the peaks decreases with an increase in temperature due to the thermal quenching by nonradiative recombination. This work insights into the understanding of physical nature of emission and presents a detailed analysis of the electron transition behaviors in the wolframite-type monoclinic CdWO₄, which can be used not only to explain the photoluminescence mechanism of CdWO₄, but also for the structure design to obtain better emission properties in tungstates.