Spectral modulation of magnesium-based metal halides via Sn2+/Te4+ doping and applications in anti-counterfeiting

Inorganic metal halides have been drawing significant attention due to their rich configurations and variable emissive properties in the visible and near-infrared region. However, very few reports are available on the all-inorganic metal halides with self-trapped emission in ultraviolet region. Here, the magnesium halides with a large bandgap of over 4.0 eV and alkali metal halides are used to fabricate all inorganic metal halides by ball milling technique. Such “greener” approaches also allowed the reaction at room temperature for preparation of AMgX 3 ·6H 2 O (A = K、Rb、Cs; X = Cl、Br) by combination of MgX 2 and AX, as confirmed by XRD measurement. As a result, the emission of AMgX 3 ·6H 2 O mainly covered the violet and blue region from 396 to 450 nm. After introducing of Sn 2+ and Te 4+ ions into the CsMgBr 3 ·6H 2 O, a typical sample, the emission wavelength of doped ones shifted from 396 nm of original ones to 522 nm for Sn 2+ doped ones, to 569 nm for Te 4+ doped ones. Theoretical calculations showed that the bandgap value was reduced from the original 4.52 eV for CsMgBr 3 ·6H 2 O to an indirect bandgap of 3.95 eV for Sn 2+ doped CsMgBr 3 ·6H 2 O. Combining with the properties of variable-temperature spectra, the Huang-Rhys, and exciton binding energy, the emission mechanism of the product should be attributed to self-trapped exciton (STE) emission. Interestingly, no emission was observed for the Sn 2+ doped samples after exposed moisture. A similar circular quenching-recovery phenomenon is also observed during exposure/de-watering. This material offers a completely new option for hydrochromic materials used for security information.