Polaron and Phonon Properties in Proton Intercalated Amorphous Tungsten Oxide Thin Films

Authors

M. Saenger, University of Nebraska - Lincoln
T. Hoing, Flabeg GmbH & Co. KG, 93437 Furth im Wald, Germany
Brian W. Robertson, University of Nebraska - LincolnFollow
R. Billa, University of Nebraska - Lincoln
Tino Hofmann, University of Nebraska - LincolnFollow
Eva Schubert, University of Nebraska - LincolnFollow
Mathias Schubert, University of Nebraska - LincolnFollow

Date of this Version

2008

Comments

Published in PHYSICAL REVIEW B 78, 245205 (2008). Copyright ©2008 The American Physical Society. Used by permission.

Abstract

We report on the evolution of the polaron and phonon mode properties in amorphous tungsten oxide thin films measured by spectroscopic ellipsometry in the infrared to ultraviolet spectral regions as a function of the intercalated proton density. A parametric physical model dielectric function is presented, which excellently describes the ellipsometry data over a large intercalated charge-density range. Upon increased amounts of intercalated charge we observe a strong increase in the polaron absorption in the visible spectral range, a decrease in the infrared W–O bond polarity, and an increase in the W=O bond polarity. Our findings support the oxygen-extraction model as the polaron formation mechanism in tungsten oxide in agreement with previous theoretical works based on first-principles pseudopotential calculations. We discuss and suggest polaron formation by oxygen-related defect generation as origin for the coloration mechanism in tungsten oxide. We further discuss possible evidence for very large effective mass of the polarons within the insulator-to-metal transition regime.