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

resumo

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 Gamma 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.

palavras-chave

SUPER BROAD-BAND; ZINC GERMANATE; PHOTOLUMINESCENCE PROPERTIES; PHOTOCATALYTIC ACTIVITY; CR4+-DOPED CA2GEO4; OPTICAL-PROPERTIES; SPECTROSCOPY; EMISSION; ION; ENERGY

categoria

Materials Science; Physics

autores

Batista, MS; Dias, MP; Candeias, MB; Marques, G; Gouveia, JD; Girao, AV; Costa, FM; Leitao, JP; Rino, L; Deuermeier, J; Fortunato, E; Martins, R; Pimentel, A; Rodrigues, J; Monteiro, T; Pereira, SO

Grupos

G3 - Materiais Eletroquímicos, Interfaces e Revestimentos
G6 - Materiais Virtuais e Inteligência Artificial

Projectos

Collaboratory for Emerging Technologies, CoLab (EMERGING TECHNOLOGIES)

CICECO - Aveiro Institute of Materials (UIDB/50011/2020)

CICECO - Aveiro Institute of Materials (UIDP/50011/2020)

Associated Laboratory CICECO-Aveiro Institute of Materials (LA/P/0006/2020)

agradecimentos

This study has the support of the Institute for Nanostructures, Nanomodelling and Nanofabrication (i3N), through projects UIDB/50025/2020-2023 (DOI: 10.54499/UIDB/50025/2020) & UIDP/50025/2020-2023 (DOI: 10.54499/UIDP/50025/2020) & LA/P/0037/2020 (DOI: 10.54499/LA/P/0037/2020), as well as IonProGO (2022.05329.PTDC), DOI: 10.54499/2022.05329, 2023.00054.RESTART, LIGHEART (2022.08597.PTDC), CO2RED (PTDC/EQU-EPQ/2195/2021), and POEMS (101217446) projects, all funded by the Foundation for Science and Technology (FCT). J. Rodrigues acknowledges FCT support via grant CEECINSTLA/00005/2022. J. D. Gouveia acknowledges the FCT grant 2023.06511.CEECIND, in the scope of the Individual Call to Scientific Employment Stimulus - 6th Edition, M. S. Batista thanks i3N and FCT for the PhD grant (UI/BD/152567/2022). G. Marques acknowledges the financial support from Project "Agenda ILLIANCE" [C644919832-00000035 | Project no. 46], financed by PRR - Plano de Recuperac & atilde;o e Resiliencia under the Next Generation EU from the European Union. A. V. Gir & atilde;o also acknowledges the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020, financed by national funds through the FCT/MEC (PIDDAC). The authors thank MSc. Celeste Azevedo and Doctor Rosario Soares (CICECO - Aveiro Institute of Materials), for the DR and XRD experiments, respectively.