Deposition of luminescent NaCl:Tm2+ thin films with a Tm concentration gradient using RF magnetron sputtering

Luminescent thin films were deposited using magnetron sputtering of a NaCl single crystal and Tm-metal. By using a combinatorial approach, a single film with a thickness ranging from 3.1 μm to 6.9 μm and a Tm to Na ratio varying from 0.05 to 0.26 was obtained. XRD shows the formation of the simple cubic NaCl structure and SEM images display a mix of 0.50-0.75 μm and 2-3 μm cubic and needle-like structures when the substrate is not rotating during deposition. NIR transmission spectra reveal narrow absorption lines at 1134 nm and 1218 nm caused by 4f-4f absorption of divalent and trivalent Tm, respectively. Photoluminescence excitation and UV-VIS transmission spectra show broad bands between 275 nm and 700 nm, caused by the Tm2+ 4f13 → 4f125d1 transitions. Excitation into these bands results in 2F5/2 → 2F7/2 line emission by Tm2+ at 1134 nm. The broad absorption range covering the entire UV and VIS part of the solar spectrum and the absence of self-absorption of the sharp emission line makes NaCl:Tm2+ a promising material for luminescent solar concentrators as thin films on glass provided light scattering can be minimised.