Dynamics of the photochromic effect of WO3

摘要

We have studied the photochromic effect of tungsten oxide thin films, i.e. their coloration by exposure to ultraviolet light, in order to investigate the basic mechanisms of the chromogenic behavior of this material. We have measured the absorptivity of our samples by means of optically excited surface plasmons which enabled us for the first time to resolve the photochromism of layers 10 nm thick on a scale as short as seconds. In contrast to the widely accepted double-charge-injection model, we found that in the case of photochromism, oxygen is exchanged between the sample and the surrounding atmosphere. On the basis of this model, a simple nonlinear rate equation is presented which accounts both for the temporal behavior and for the atmosphere dependence of the absorbance of the films during and after ultraviolet light exposure.