Spectral shifting and NIR down-conversion in Bi3+/Yb3+ co-doped Zn2GeO4

Abstract

We report on spectral modification through NIR down-conversion (DC) photoluminescence (PL) in Yb³⁺-Bi³⁺-co-doped Zn₂GeO₄. Energetic downshifting (DS) of UV-A irradiation occurs via intrinsic luminescence of the high-bandgap semiconductor Zn₂GeO₄ as well as via active Bi³⁺ centres. In parallel, both species act as sensitizers for Yb³⁺, strongly extending its excitation region to ∼500 nm. In the absence of Bi³⁺, band-to-band absorption of Zn₂GeO₄ in the UV region results in PL at ∼475–625 nm. Doping with Yb³⁺ initiates energy transfer from trapped defect states to two neighbouring Yb³⁺ ions in a cooperative DC process, resulting in Yb³⁺-related photoemission at ∼1000 nm. The introduction of Bi³⁺ into Zn₂GeO₄:Yb³⁺ greatly extends the absorption band to the visible blue region. Then, energy transfer also occurs through cooperative DC from Bi³⁺ to Yb³⁺. As a result, a strong increase in the absolute Yb³⁺-related PL intensity is observed. This enables ultra-efficient harvesting of UV-A to visible radiation for energy conversion processes.