Towards Tm2+ in Sialon Thin Films Using the Gradient Deposition Method

There are different material candidates for efficient industry compatible luminescence solar concentrators to be used in electricity generating glass applications. Among them, divalent thulium (Tm2+) doping has been shown to have a high potential. It has been reported to have 65% absorption of the solar spectral range when in di-halide hosts, while showing no self-absorption and a relatively neutral color [1]. These characteristics are possible because Tm2+ has a strong absorption from the UV up to about 900 nm. This absorption is generated by intense 4f13 → 4f125d1 electronic transitions and a single emission line at 1140 nm, credited to the 2F5/2 → 2F7/2 transition of the 4f13 configuration. However, this specific oxidation state of thulium is rather anomalous: so far it was only stabilized on di-halide crystals. These crystals are very hygroscopic in nature, therefore not sufficiently attractive for scaling up in industry standard thin film growth techniques. Thus, it is necessary to research on the possible extension in the list of materials able to host such a promising optically active element. However, the intrinsic complex nature of the lanthanide's electronic structure has prevented a detailed understanding of the valence stability of its elements. Therefore, designing industry compatible hosts ensuring that such exotic ion is stable and capable of efficiently emitting NIR light is a challenging task.In this work, we show the progress made so far towards unraveling the conditions for stabilization of the Tm2+ in industry compatible thin film materials – more specifically, thin film materials of the SiAlON family. I report recent results on Tm doped SiAlON thin films grown by combinatorial magnetron reactive sputtering. The gradient characterization method [2], developed at the TU Delft's luminescence materials research group, was used in order to relate position dependent film composition – determined by energy dispersive x-ray spectroscopy – with local transmission, photoluminescence emission and excitation data. This singular approach makes it possible to retrieve a wide composition range in a smaller number of different films. These results are discussed upon the valence stability model of lanthanides of Dorenbos [3], and the model of impurity ionization adopted in solid state statistics.References[1] O. M. ten Kate, K. W. Kramer, and E. van der Kolk, Efficient luminescent solar concentrators based on self-absorption free, Tm2+ doped halides, Sol. Energy Mater. Sol. Cells, vol. 140, pp. 115–120, Sep. 2015.[2] E. P. J. Merkx and E. van der Kolk, Method for the Detailed Characterization of Cosputtered Inorganic Luminescent Material Libraries, ACS Comb. Sci., vol. 20, no. 11, pp. 595–601, Nov. 2018.[3] P. Dorenbos, Valence stability of lanthanide ions in inorganic compounds, Chem. Mater., vol. 17, no. 25, pp. 6452–6456, Dec. 2005.