Unravelling the UV luminescence of Bi-doped LiYGeO4: a journey from first principles to temperature-dependent photoluminescence

Abstract

Materials exhibiting persistent luminescence (PersL) have garnered attention due to their unique ability to emit light for extended periods after the excitation stops. LiYGeO₄ has arisen as a promising host for PersL due to its notable defect abundance. When doped with Bi, it can provide a long-lasting ultraviolet emission, which may find interesting applications in areas such as photodynamic therapy or self-sustained photocatalysis. In the present paper, undoped and Bi-doped LiYGeO₄ (0.5 mol%) samples were synthesized by solid-state reaction. X-ray diffraction confirmed the presence of the LiYGeO₄ phase, alongside residual yttrium germanate phases. Additionally, we calculated its previously undocumented band structure using hybrid density functional theory to provide new theoretical insights. These calculations indicate that LiYGeO₄ has a direct bandgap at the Γ point, further supported by the absorption data. Elemental analyses allowed quantification of the samples, identifying the loss of Li during the synthesis. Room temperature (RT) photoluminescence (PL) showed a strong emission band peaked around 350–360 nm. Despite the extensive research conducted on LiYGeO₄:Bi, the mechanisms underlying the PersL phenomenon remain unclear. To address this, we conducted temperature-dependent PL from 17 K to RT, using a 325 nm photon excitation. We propose that, at RT, the observed emission arises from the overlap of the ³P⁽¹⁾₁ → ¹S₀ and ³P⁽²⁾₁ → ¹S₀ Bi³⁺ intraionic transitions, sublevels of the ³P₁ state, which is completely split under the local crystal field generated by the ion environment. However, at cryogenic temperatures, the ³P₀ → ¹S₀ transition is the dominant one. Moreover, PersL emission was achieved for nearly 7 h, with 250 nm photon excitation for 10 min, arising from a contribution of the overlapped ³P⁽¹⁾₁ → ¹S₀ and ³P⁽²⁾₁ → ¹S₀ transitions.