title: Structural Metastability and Quantum Confinement in Zn1-xCoxO Nanoparticles.
abstract: This paper investigates the electronic structure of wurtzite (W) and rock-salt (RS) Zn1-xCoxO 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-Zn1-xCoxO as thin film or single crystal, occurring sharply at about 9 GPa, spectroscopic signatures of tetrahedral Co(2+) 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 Co(2+) 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.
authors.affiliation: ICMUV, MALTA-CONSOLIDER Team, Departamento de Física Aplicada, Universitat de Valencia , E-46100 Burjassot (Valencia), Spain.
authors.forename: G
authors.initials: G
authors.lastname: Almonacid
authors.affiliation: Departamento de Física Aplicada, MALTA-CONSOLIDER Team, Universidad de Cantabria - IDIVAL , Santander, E-39005, Spain.|Departamento de Química e Ingeniería de Procesos y Recursos, ETSIIyT, Universidad de Cantabria - IDIVAL , Santander, E-39005 Spain.
authors.forename: R
authors.initials: R
authors.lastname: Martín-Rodríguez
authors.affiliation: Departamento de Física Aplicada, MALTA-CONSOLIDER Team, Universidad de Cantabria - IDIVAL , Santander, E-39005, Spain.|Departamento de Física Aplicada, Universitat de Valencia , E-46100 Burjassot (Valencia), Spain.
authors.forename: C
authors.initials: C
authors.lastname: Renero-Lecuna
authors.affiliation: ICMUV, MALTA-CONSOLIDER Team, Departamento de Física Aplicada, Universitat de Valencia , E-46100 Burjassot (Valencia), Spain.
authors.forename: J
authors.initials: J
authors.lastname: Pellicer-Porres
authors.affiliation: Departamento de Física Aplicada, Universitat de Valencia , E-46100 Burjassot (Valencia), Spain.
authors.forename: S
authors.initials: S
authors.lastname: Agouram
authors.affiliation: Departamento de Física Aplicada, MALTA-CONSOLIDER Team, Universidad de Cantabria - IDIVAL , Santander, E-39005, Spain.
authors.forename: R
authors.initials: R
authors.lastname: Valiente
authors.affiliation: DCITIMAC, MALTA-CONSOLIDER Team, Universidad de Cantabria , Santander, E-39005, Spain.
authors.forename: J
authors.initials: J
authors.lastname: González
authors.affiliation: DCITIMAC, MALTA-CONSOLIDER Team, Universidad de Cantabria , Santander, E-39005, Spain.
authors.forename: F
authors.initials: F
authors.lastname: Rodríguez
authors.affiliation: ODE Beamline, Synchrotron Soleil, L'Orme des Merisiers, BP48 Saint Aubin, F-91192 Gif-sur-Yvette Cedex, France.
authors.forename: L
authors.initials: L
authors.lastname: Nataf
authors.affiliation: Department of Chemistry, University of Washington , Seattle, Washington 98195-1700, United States.
authors.forename: D R
authors.initials: DR
authors.lastname: Gamelin
authors.affiliation: ICMUV, MALTA-CONSOLIDER Team, Departamento de Física Aplicada, Universitat de Valencia , E-46100 Burjassot (Valencia), Spain.
authors.forename: A
authors.initials: A
authors.lastname: Segura
keywords: Zn1−xCoxO nanoparticles; absorption; metastability; phase transition