Lanthanide and Heavy Metal Free Long White Persistent Luminescence from Ti Doped Li–Hackmanite: A Versatile, Low‐Cost Material

Abstract

Persistent luminescence (PeL) materials are used in everyday glow‐in‐the‐dark applications and they show high potential for, e.g., medical imaging, night‐vision surveillance, and enhancement of solar cells. However, the best performing materials contain rare earths and/or other heavy metal and expensive elements such as Ga and Ge, increasing the production costs. Here, (Li,Na)8Al6Si6O24(Cl,S)2:Ti, a heavy‐metal‐ and rare‐earth‐free low‐cost material is presented. It can give white PeL that stays 7 h above the 0.3 mcd m−2 limit and is observable for more than 100 h with a spectrometer. This is a record‐long duration for white PeL and visible PeL without rare earths. The material has great potential to be applied in white light emitting devices (LEDs) combined with self‐sustained night vision using only a single phosphor. The material also exhibits PeL in aqueous suspensions and is capable of showing easily detectable photoluminescence even in nanomolar concentrations, indicating potential for use as a diagnostic marker. Because it is excitable with sunlight, this material is expected to additionally be well‐suited for outdoor applications.

Synthesis and Features of Luminescent Bromo- and Iodohectorite Nanoclay Materials

“…It can thus be assumed that a titanium-related center is responsible for the blue emission in Br-Hec and I-Hec. According to previous literature, this blue emission can be attributed to TiO 6 entities as reported for Benitoite (BaTiSi 3 O 9 ) [43] or to Ti 3+ -V O (oxygen vacancy) pairs [23,24,47,48]. Moreover, the doping of the X-Hec materials with Ti 3+ (x Ti = 0.1 mol %) increases the intensity of the main emission ( Figure A6), without significant changes to its band position confirming titanium as the luminescent center.…”

“…The multisite origin for emission is also witnessed in the decrease of emission band width [42] with increasing excitation wavelength. Similar blue-green emission has previously been observed for other silicates such as fluorohectorite [23], chlorohectorite [24], montmorillonite [43], kaolinite [44], pyrophyllite [45], topaz (Al2SiO4(F,OH)2) [46], synthetic hackmanite (Na8Al6Si6O24(Cl,S)2) [47,48], benitoite (BaTiSi3O9) [49], and SiO2 [50,51]. The literature presents mainly two different explanations for the origin of this blue-green emission.…”

“…300 nm are related to the e − (O 2− (2p))→Ti IV charge transfer, which can occur through two possible molecular transitions: t 1u (π b )→t 2g (π*) or t 2u (π)→t 2g (π*), being typically observed at ca. 280 nm [23,24,42,48,59]. Lastly, the energy gaps between the valence and the conduction bands are identified at ca.…”

“…One proposition categorizes this emission as photoelectric response as well as inner center and recombination type of luminescence [52], attributing the blue emission to triplet-singlet transition in twofold coordinated Si centers [53], due to oxygen deficit and irradiation effects [50]. The second proposition associates the emission with luminescent centers containing trivalent titanium paired with an oxide vacancy, which may exist either as matrix element or as an impurity [23,24,[46][47][48][49].…”

Counting the Photons: Determining the Absolute Storage Capacity of Persistent Phosphors

“…The performance of persistent phosphors is often determined on the basis of luminance decay curves, which are usually measured for as long as the emission can be perceived by the dark-adapted eye [20]. Although this approach provides a relatively easy way to compare different persistent phosphors, the method also entails some problems.…”