Luminescence detection of phase transitions in crystals and nanoparticle inclusions

P.D. Townsendª, B. Yang^b and Y. Wang^c

^ª Science and Technology, University of Sussex, Brighton, BN1 9QH, UK, e–mail: p.d.townsend@sussex.ac.uk

^b Physics Department, Beijing Normal University, Beijing, 100875, China, e–mail: yangbr@bnu.edu.cn

^c School of Materials Science and Technology, China University of Geosciences, Beijing, 100083, China, e–mail: wyfemail@gmail.com

Recibido el 12 de octubre de 2007
Aceptado el 9 de agosto de 2008

Abstract

Luminescence measurements are extremely sensitive to variations in structural environment and thus have the potential to probe distortions of fluorescence sites. Changes can be monitored via luminescence efficiency, emission spectra or excited state lifetimes and these factors are influenced by the local neighbourhood around the emission site, and therefore by structure, composition, pressure and temperature. A rarely exploited approach for condensed matter has been to use the changes in luminescence responses during heating or cooling of a material to provide a rapid survey to detect the presence of phase transitions. One can often differentiate between bulk and surface effects by contrasting results from radioluminescence for bulk responses, and cathodoluminescence or photoluminescence for surface effects. One expects that discontinuous changes in optical parameters occur during temperature changes through phase transitions of insulating materials. In practice, optical signals also exist from surface states of fullerenes and high temperature superconductors etc which identify the presence of structural or superconducting transitions. Numerous examples are cited which match standard documented transitions. Interestingly, many examples show the host signals are strongly sensitive to impurity phase transitions from inclusions such as nanoparticles of water, N₂, O₂ or CO₂. Recent luminescence data reveal many examples of new transitions, hysteresis and irreversible changes. The signals equally respond to relaxations of a structure and surprisingly indicate that in some materials, such as SrTiO₃ or ZnO, ion implantation of the surface triggers relaxations and phase changes throughout the bulk of the material. Luminescence routes to detect phase transitions are powerful tools but have a tiny literature and so the subject is ideal for rapid exploitation and development.

Keywords: Phase transitions; nanoparticle inclusions; luminescence.

Resumen

La espectroscopia de luminiscencia es extremadamente sensible a las variaciones en la estructura de los materiales, de tal forma que puede ser utilizada para investigar distorsiones en sitios fluorescentes. Dichos cambios pueden ser monitoreados a través de la eficiencia de la luminiscencia, espectros de emisión o tiempos de vida de estados excitados; estos factores son afectados por el medio que rodea el sitio de emisión, y por lo tanto su estructura, composición, presión y temperatura. Un método que ha sido poco explotado en estudios de la materia condensada es usar la respuesta luminiscente durante el calentamiento o enfriamiento de un material a manera de inspección rápida para detectar la presencia de transiciones de fase. Usualmente es posible discernir entre los efectos del material en volumen y la superficie contrastando los resultados que se obtienen de la radioluminiscencia en el caso del sustrato y la catodoluminiscencia o fotoluminiscencia para los efectos de superficie. Es de esperarse que ocurran cambios discontinuos en los parámetros ópticos durante variaciones de temperatura a través de transiciones de fase en materiales aislantes. De hecho, también existen señales ópticas de estados superficiales de fulerenos y superconductores bajo altas temperaturas, entre otros, las cuales identifican la presencia de transiciones estructurales o de la superconductividad. Numerosos ejemplos muestran que las señales ópticas del material son altamente sensibles a transiciones de fase de impurezas como nanopartículas de agua, N₂, O₂ o CO₂. Varios resultados recientes de luminiscencia revelan muchos ejemplos de nuevas transiciones, histeresis y cambios irreversibles. Las senales responden igualmente a relajaciones de alguna estructura e indican de manera sorprendente que en algunos materiales como SrTiO³ o ZnO, la implantación iónica de la superficie activa relajaciones y cambios de fase hacia todo el volumen. Los métodos de luminiscencia para detectar transiciones de fase son herramientas poderosas pero poseen una bibliografía incipiente, de manera que el tema es ideal para ser explotado y desarrollado rápidamente.

Descriptores: Transiciones de fase; inclusiones nanométricas; luminiscencia.

PACS: 64.60.–I; 64.70.Nd; 78.60.–G; 71.55.Jv

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