Density functional theory calculation of the electronic structure ofBa0.5Sr0.5TiO

Abstract

First-principles quantum-mechanical techniques, based on density functional theory ͑B3LYP level͒ were employed to study the electronic structure of ordered and deformed asymmetric models for Ba 0.5 Sr 0.5 TiO 3 . Electronic properties are analyzed and the relevance of the present theoretical and experimental results on the photoluminescence behavior is discussed. The presence of localized electronic levels in the band gap, due to the symmetry break, would be responsible for the visible photoluminescence of the amorphous state at room temperature. Thin films were synthesized following a soft chemical processing. Their structure was confirmed by x-ray data and the corresponding photoluminescence properties measured.

Experimental and theoretical studies on the enhanced photoluminescence activity of zinc sulfide with a capping agent

“…Theoretical results have shown that the symmetry break process (an effect of order-disorder) in the structure of various semiconductors is a condition necessary for the existence of intermediary energy levels within the forbidden bandgap. 45 As shown in Figs. 5(a)-5(f), the evolution of the PL spectra was obviously different.…”

“…In the case of capped ZnS, the recombination of electrons and holes gives rise to a strong green light emission and in the case of uncapped ZnS, to blue light emission. In our model, the wide-band model, 45 the most important events occur before excitation, i.e., before the photon arrives. The deep and shallow defects generated by the complex clusters give rise to localized states in the bandgap and inhomogeneous charge distribution in the cell, allowing electrons to become trapped.…”

Influence of Structural Disorder on the Photoluminescence Emission of PZT Powders

“…14,15 The PL at room temperature occurs due to structural disorder of the system. Therefore, if the system is totally disordered, then the PL does not exist; moreover, a totally ordered system does not exhibit PL.…”

“…Therefore, if the system is totally disordered, then the PL does not exist; moreover, a totally ordered system does not exhibit PL. 12,14 The analysis of structural disorder in these PZT powders helps to understand the origin of PL emission. Figure 3 presents X-ray diffractograms of the PZT powders annealed at 300°C for 4, 8 and 16 h and annealed at 700°C.…”

“…PL emission occurs due to interaction of the TiO 5 -TiO 6 pair. 14 The results for Ti k-edge XANES are shown in Figure 4. The XANES spectrum for PZT annealed at 700°C reveals that the local structure around titanium atoms is characteristic of the structurally ordered PZT with only TiO 6 units, in good agreement with XRD and PL data.…”