Photoluminescence in ZnO:Co2+ (0.01%–5%) Nanoparticles, Nanowires, Thin Films, and Single Crystals as a Function of Pressure and Temperature: Exploring Electron–Phonon Interactions

详细信息    查看全文

• 作者：Carlos Renero-Lecuna ; Rosa Martín-Rodríguez ; Jesus A. González ; Fernando Rodríguez ; Gloria Almonacid ; Alfredo Segura ; Vicente Mu?oz-Sanjosé ; Daniel R. Gamelin ; Rafael Valiente
• 刊名：Chemistry of Materials
• 出版年：2014
• 出版时间：January 28, 2014
• 年：2014
• 卷：26
• 期：2
• 页码：1100-1107
• 全文大小：422K
• ISSN：1520-5002

文摘

This work investigates the electronic structure and photoluminescence properties of Co²⁺-doped ZnO and their pressure and temperature dependences through high-resolution absorption and emission spectroscopy as a function of Co²⁺ concentration and their structural conformations as a single crystal, thin film, nanowire, and nanoparticle. Absorption and emission spectra of diluted ZnO:Co²⁺ (0.01 mol %) can be related to the ⁴T₁(P) 鈫?⁴A₂(F) transition of CoO₄ (T_d), contrary to MgAl₂O₄:Co²⁺ and ZnAl₂O₄:Co²⁺ spinels in which the red emission is ascribed to the ²E(G) 鈫?⁴A₂(F) transition. We show that the low-temperature emission band consists of a ⁴T₁(P) zero-phonon line and a phonon-sideband, which is described in terms of the phonon density of states within an intermediate coupling scheme (S = 1.35) involving all ZnO lattice phonons. Increasing pressure to the sample shifts the zero-phonon line to higher energy as expected for the ⁴T₁(P) state upon compression. The low-temperature emission quenches above 5 GPa as a consequence of the pressure-induced wurtzite to rock-salt structural phase transition, yielding a change of Co²⁺ coordination from 4-fold T_d to 6-fold O_h . We also show that the optical properties of ZnO:Co²⁺ (T_d) are similar, independent of the structural conformation of the host and the cobalt concentration. The Co²⁺ enters into regular Zn²⁺ sites in low concentration systems (less than 5% of Co²⁺), although some slight shifts and peak broadening appear as the dimensionality of the sample decreases. These structural effects on the optical spectra are also supported by Raman spectroscopy.