Effect of radiative energy transfer and direct excitation on the up-conversion and down-shifting emission properties of Er3+-doped Zn3(VO4)2

Abstract

Er3+-doped Zn3(VO4)2 samples were synthetized by solid-state reaction process at 800 °C. The doping level was varied from 1.0 to 4.0 mol%. All samples were analyzed by different techniques such as X-ray diffraction, Raman spectroscopy, optical absorption and photoluminescent spectroscopy. X-ray diffraction patterns revealed a majority presence of Zn3(VO4)2, with residues of Zn2V2O7, Zn4V2O9, ZnO, and ErVO4. Raman spectroscopy displayed vibrational modes associated with the Zn3(VO4)2 compound. Additionally, vibrational modes with lower intensity related to Zn2V2O7 and ErVO4 compounds were also present, in concordance with X-ray diffraction patterns. UV–Vis optical absorption spectra showed four bands located at 2.23, 2.37, 2.53 and 2.74 eV which are due to the Er3+: 4S3/2, 2H11/2, 4F7/2 and 4F5/2 electronic transitions, respectively. The band gap energy (Eg) values were found around 2.75 eV. The visible down-shifting emission spectra of the Zn3(VO4)2 sample upon 350 nm excitation, revealed a broadband from 400 to 800 nm due to the VO43− tetrahedron. In presence of Er3+, the emission band exhibited sinks, which match well with absorption of the Er3+: 4F7/2, 2H11/2, 4S3/2 and 4F9/2 levels, indicating the existence of radiative energy transfer from the host to Er3+. This energy transfer gave rise to the NIR Er3+: 4I13/2 → 4I15/2 emission. Non-radiative energy transfer was ruled out by the no shortening of the decay time profiles. The up-conversion emission spectra recorded upon 980 nm diode laser excitation, showed the characteristics Er3+ emission bands in the green and red region, which linearly grow with the Er3+ content. Such process is dominated by an excited state absorption mechanism. The NIR down-shifting emission spectra upon 980 nm excitation, displayed the NIR Er3+ emission band attributed to the Er3+:4I13/2 → 4I15/2 transition. The Stark splitting exhibited by the NIR emission attained upon direct and indirect excitations, revealed the existence of two Er3+ sites, which are activated depending on the excitation mechanism.