A study of the photochromic effect in oxygen containing rare-earth metal hydride thin films and multilayer structures
Abstract: In this work, we have studied the photochromic response of several oxygen containing rare-earth hydride thin films (REHO, RE = Y, Nd, Gd, and Dy). Their chemical composition was characterized by an iterative multi-method approach based on ion beams, while the photochromic effect was measured by means of optical spectrometry. We report photochromic responses for YHO, NdHO, GdHO and DyHO of several thicknesses but averaged chemical compositions described by the formula REH2-δOδ; in the range of 0.45 < δ < 1.5 (δ being the [O]/[RE] ratio). Possible side-effects of the ambient conditions on the photochromic effect in YHO thin films were investigated by comparing the optical properties (photochromic response and bleaching) of YHO films capped with two different diffusion barrier layers (Al2O3 and Si3N4) to their respective uncapped sample. The ambient atmosphere was found to play no significant role in the photochromic effect. In sequence, identical YHO thin films were prepared on three different (transparent) substrates (i.e., soda lime glass, CaF2, and Al2O3 as buffer layer). The effect of substrate induced stress in the YHO thin films was investigated in details and no significant correlation between the substrate and photochromic effect was observed. Finally, isotope labeling in double layers of YHO and YDO was done to investigate possible diffusion of hydrogen/deuterium within the REHO layers. No diffusion of hydrogen/deuterium could be observed in the double layered structure during a one-week period and one illumination/bleaching cycle.
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