Zn2GeO4:Cr,Mn phosphor for visible to NIR luminescence-based applications – a theoretical and experimental investigation

Abstract

Zn₂GeO₄ (ZGeO) phosphor exhibits a wide bandgap energy, making it highly suitable for luminescence-based applications spanning the entire electromagnetic spectrum, from ultraviolet (UV) to near-infrared (NIR) wavelengths. In this work, nominally undoped and Cr-doped ZGeO (ZGeO:Cr) were prepared by solid-state reaction. X-ray diffraction and Raman spectroscopy confirmed the monophasic willemite crystalline structure of the ZGeO and ZGeO:Cr samples, and X-ray photoelectron spectroscopy corroborated the identification of Zn, Ge, O, and Cr elements. X-ray photoemission indicates an insulator character for ZGeO and ZGeO:Cr. The oxide host revealed a direct bandgap energy of 4.77 eV, assessed by room temperature absorption, in line with the density functional theory (DFT) calculations that predicted 4.8 eV at the Γ point of the first Brillouin zone. Cr⁴⁺ was found to occupy distorted tetracoordinated Ge⁴⁺ sites with C₁ symmetry, in agreement with the measured unfolded ³A₂ → ³T₁, ³T₂ intraionic absorption. Er³⁺ and Mn²⁺ trace impurities occupy distorted Zn²⁺ sites, also with C₁ symmetry. A Mn²⁺–O²⁻ charge transfer state, placed 0.8 eV below the conduction band minimum, was identified by absorption measurements. In addition, as calculated by DFT, Cr³⁺ impurities exhibit lower energy formation when placed in distorted interstitial octahedral Zn–Ge and Zn–Zn rings with C₃ and S₆ symmetry, respectively. The identified site locations were found to be compatible with the measured unfolded ⁴A₂ → ⁴T₂, ⁴T₁ intraionic absorption, with the ions subject to intermediate/low crystalline field strengths. The emission of the samples is dominated by structureless broad bands spanning from the visible to mid-infrared. A broad bluish-white emission results from an overlap of emitting centers related to both intrinsic defects (generating a blue emission) and trace impurities of Mn²⁺ (generating a green emission). Besides, Cr³⁺ and Cr⁴⁺ were found to coexist in the oxide host, and their multiple-site occupation is responsible for the observed broad emission bands in the NIR-I (700–950 nm) and NIR-II (1000–1700 nm) spectral regions, opening the way to the exploitation of the NIR luminescence for light-based devices.