Cold bluish white and blue emissions in Cu+-doped zinc phosphate glasses

Abstract Reducing agent-free zinc phosphate glasses doped with cooper ions (Cu+) were synthesized by melt-quenching technique. The physical properties of these glasses such as density, molar volume and refractive index were studied. The X-ray diffraction (XRD) patterns for all prepared glasses, confirm their amorphous structure. Raman spectroscopy suggest that the phosphate network is composed by tetrahedral PO4 units, since the vibrations of P–O–P, PO2 and P O, observed in the spectra, belong to the PO4 units. All glasses display absorptions bands in the ultraviolet and near infrared regions, associated with the absorption edge and contribution of the Cu+:1Ag → 1T2g + 3T2g (280 nm) and Cu2+: 2Eg → 2Tg (890 nm) transitions, respectively. The optical band gap values determined from the absorption spectra assuming indirect optical band gap, are reduced from 4.83 to 2.95 eV for Cu+ content in the range of 0–0.3 mol%. For Cu+ concentration higher than 0.3 mol%, band gap values increase up to 3.17 eV. Such last behaviour was attributed to the reduction of the amount of Cu+ species and/or Moss-Burstein effect. The emission spectra of the Cu+-doped zinc phosphate glasses upon 272 and 290 nm excitation, displayed overall bluish white light and blue emission, depending the excitation wavelength and Cu+ contents. A maximum photoluminescence quantum yield value of 12.5% with an inherent uncertainty up to 3.75%, was reached under 290 nm excitation. The emission decay time profiles were well fitted to a bi-exponential function from which the average lifetimes values resulted to be in the interval of 26.8–16.7 μs, which are shortened for Cu+ contents higher than 0.3 mol%. The structural and photoluminescence characteristic makes of the reducing agent-free Cu+-doped zinc phosphate glasses potentially attractive for blue phosphors and WLEDs applications.