Structural Metastability and Quantum Confinement in Zn1–xCoxO Nanoparticles

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• 作者：G. Almonacid ; R. Martín-Rodríguez ; C. Renero-Lecuna ; J. Pellicer-Porres ; S. Agouram ; R. Valiente ; J. González ; F. Rodríguez ; L. Nataf ; D. R. Gamelin ; A. Segura
• 刊名：Nano Letters
• 出版年：2016
• 出版时间：August 10, 2016
• 年：2016
• 卷：16
• 期：8
• 页码：5204-5212
• 全文大小：668K
• 年卷期：0
• ISSN：1530-6992

文摘

This paper investigates the electronic structure of wurtzite (W) and rock-salt (RS) Zn_1–xCo_xO nanoparticles (NPs) by means of optical measurements under pressure (up to 25 GPa), X-ray absorption, and transmission electron microscopy. W-NPs were chemically synthesized at ambient conditions and RS-NPs were obtained by pressure-induced transformation of W-NPs. In contrast to the abrupt phase transition in W–Zn_1–xCo_xO as thin film or single crystal, occurring sharply at about 9 GPa, spectroscopic signatures of tetrahedral Co²⁺ are observed in NPs from ambient pressure to about 17 GPa. Above this pressure, several changes in the absorption spectrum reveal a gradual and irreversible W-to-RS phase transition: (i) the fundamental band-to-band edge shifts to higher photon energies; (ii) the charge-transfer absorption band virtually disappears (or overlaps the fundamental edge); and (iii) the intensity of the crystal-field absorption peaks of Co²⁺ around 2 eV decreases by an order of magnitude and shifts to 2.5 eV. After incomplete phase transition pressure cycles, the absorption edge of nontransformed W-NPs at ambient pressure exhibits a blue shift of 0.22 eV. This extra shift is interpreted in terms of quantum confinement effects. The observed gradual phase transition and metastability are related to the NP size distribution: the larger the NP, the lower the W-to-RS transition pressure.