Yttrium oxyhydrides for photochromic applications: Correlating composition and optical response

It has been recently demonstrated that yttrium oxyhydride (YHO) films can exhibit reversible photochromic properties when exposed to illumination at ambient conditions. This switchable optical property enables their utilization in many technological applications, such as smart windows, sensors, goggles, and medical devices. However, how the composition of the films affects their optical properties is not fully clear and therefore demands an investigation. In this paper, the composition of YHO films manufactured by reactive magnetron sputtering under different conditions is deduced in a ternary diagram from time-of-flight elastic recoil detection analysis. The results suggest that stable compounds are formed with a specific chemical formula—$\mathrm{Y}{\mathrm{H}}_{2\text{−}\delta }{\mathrm{O}}_{\delta }$. In addition, optical and electrical properties of the films are investigated, and a correlation with their compositions is established. The corresponding photochromic response is found in a specific oxygen concentration range ($0.45<\delta <1.5$) with maximum and minimum of magnitude on the lower and higher border, respectively.

Figure 1

Composition deduced from TOF-ERDA of the homogeneous sample after 30 min of annealing at different temperatures.

Figure 2

Composition as deduced from TOF-ERDA and represented in a ternary diagram of Y, H, and O. Red triangles and blue diamonds mark transparent and opaque films, respectively. The dashed line represents compositions corresponding to the chemical formula $\mathrm{Y}{\mathrm{H}}_{2\text{−}\delta }{\mathrm{O}}_{\delta }$ suggested in this paper, whereas the black arrow shows the composition for pure $\mathrm{Y}{\mathrm{H}}_2$.

Figure 3

Transmission coefficient (averaged over 500–900 nm) (a) and direct optical band gap (b) plotted as a function of the oxygen to yttrium ratio δ. The dashed lines represent the fit to the data using an exponential decay and growth for transmittance and band gap, respectively. The dotted lines show the threshold value of δ where the transition from the metallic opaque to the insulator transparent state is observed.

Figure 4

Transmission spectra (a) of the sample GI-2 measured before illumination and after 30 min of illumination. Time-resolved changing of the transmittance (b) under blue laser ($\lambda =375\mathrm{nm}$) illumination and bleaching.

Figure 5

Photochromic response (averaged from 600–800 nm) after 30 min of blue laser ($\lambda =375\mathrm{nm}$) illumination plotted as a function of the oxygen to yttrium ratio, δ.