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

Abstract

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 3.1μm to 6.9μm 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 0.50–0.75μm and 2–3μm cubic and needle-like structures when the substrate is not rotating during deposition. NIR transmission spectra reveal narrow absorption lines at 1134nm and 1218nm caused by 4f–4f absorption of divalent and trivalent Tm, respectively. Photoluminescence excitation and UV–VIS transmission spectra show broad bands between 275nm and 700nm, caused by the Tm2+ 4f13→4f125d1 transitions. Excitation into these bands results in 2F5/2→2F7/2 line emission by Tm2+ at 1134nm. 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.