Structural, thermal and optical investigations of Dy3+-doped B2O3–WO3–ZnO–Li2O–Na2O glasses for warm white light emitting applications

Abstract

Optically transparent Dy3+-doped borate glasses with the chemical composition (60-x) B2O3-10 WO3-10 ZnO-10 Li2O-10 Na2O-x Dy2O3 (x=0.1, 0.25, 0.5, 0.75, 1.0, and 1.5mol %) have been synthesized by melt quenching technique and were characterized by X-ray diffraction (XRD), Scanning Electron Microscopy and Energy Dispersive X-ray Analysis (SEM-EDX), Attenuated Total reflectance-Fourier transform Infrared (ATR-FTIR) spectroscopy, Raman spectroscopy, Thermo-gravimetric analysis (TGA), Differential scanning calorimetry (DSC), optical absorption and luminescence techniques. The amorphous nature of the prepared glasses has been confirmed through XRD and SEM measurements, and the EDX spectra show all the elements present in the respective glasses. The vibrational features of various functional groups like stretching vibrations of BO linkages in BO4 tetrahedra, asymmetric stretching vibrations of BO bond in trigonal BO3 units, stretching vibrations of W–O–W in WO4 or WO6 units, vibrations of Zn–O bonds from ZnO4 groups, bending modes of ZnO4 units, and vibrational modes of BO3 and BO4 due to alkali borates were identified by ATR-FTIR and Raman spectroscopy. For the prepared glasses; weight loss, and the glass transition (Tg), onset crystallization (Tx), crystallization (Tc) and melting (Tm) temperatures were determined from TGA and DSC measurements, respectively. Following the obtained Tg, Tx, Tc, and Tm values, the thermal stability (∆T) and Hruby's values (HR) were determined, the calculated ∆T values increased in the temperature range 131–143°C and HR values varied in the range 0.766–1.014 with Dy3+ concentration increment from 0.1 to 1.5mol %, and the increasing stability of the glasses shows that they are thermally resistant. The optical properties of the titled glasses have been explored from the absorption, photoluminescence excitation (PLE) and photoluminescence (PL) spectra. For the 0.5mol % Dy3+-doped glass, to obtain the information concerning the nature of the ligand field environment around the Dy3+ ions, the Judd-Ofelt (JO) intensity parameters Ωλ (λ=2, 4 and 6) were evaluated from the measured oscillator strengths (f) of various absorption bands. Further, the radiative parameters such as radiative transition probabilities (AR), branching ratios (βR), and the radiative lifetimes (τR) of 4F9/2→6H15/2, 4F9/2→6H13/2, 4F9/2→6H11/2 and 4F9/2→6H9/2 main emission transitions were calculated by using JO parameters. The photoluminescence properties were examined under ultraviolet (UV)/near UV (NUV) and blue excitations. Luminescence spectra measured for different concentrations of Dy3+-doped glasses by exciting the glasses at 350, 364, 386, 400, 425 and 452nm wavelengths show concentration quenching beyond 0.5mol% and the luminescence quenching has been explained by using cross-relaxation (CR) channels and resonant energy transfer (RET). For the 0.5mol% Dy3+-doped glass, the measured decay lifetime and the evaluated quantum efficiency values are ~211.8μs and 68.1%, respectively. Following the luminescence spectra, the yellow-to-blue (Y/B) luminescence intensity ratios, color chromaticity coordinates (x, y) and correlated color temperatures (CCT) of the glasses have also been estimated to evaluate the white light generation with respect to Dy3+ ion concentration. The color coordinates and CCT values for 0.5mol% Dy3+-doped glass (optimum concentration) represent the warm white light region for all the selected UV and blue excitation wavelengths. The obtained results indicate that the optimized 0.5mol% Dy3+-doped glass may be useful for the warm white light emitting applications as well as for solid state yellow laser emission and luminescent display devices.