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. Cr 4+ was found to occupy distorted tetracoordinated Ge 4+ sites with C 1 symmetry, in agreement with the measured unfolded 3 A 2 → 3 T 1, 3 T 2 intraionic absorption. Er 3+ and Mn 2+ trace impurities occupy distorted Zn 2+ sites, also with C 1 symmetry. A Mn 2+ -O 2-charge transfer state, placed 0.8 eV below the conduction band minimum, was identified by absorption measurements. In addition, as calculated by DFT, Cr 3+ impurities exhibit lower energy formation when placed in distorted interstitial octahedral Zn-Ge and Zn-Zn rings with C 3 and S 6 symmetry, respectively. The identified site locations were found to be compatible with the measured unfolded 4 A 2 → 4 T 2, 4 T 1 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 Mn 2+ (generating a green emission). Besides, Cr 3+ and Cr 4+ 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.