Nanodiamond-induced modifications of Eu-doped phosphate glasses toward photonic applications: A synergistic physico-chemical approach

Phosphate glasses were melted with fixed Eu2O3 content and varying amounts of nanodiamond (ND) powder seeking to obtain insights into the physico-chemical aspects underpinning optical properties while tuning the light-emitting properties towards photonic applications. A synergistic quantitative study incorporating X-ray diffraction, optical transmission, photoluminescence (PL) spectroscopy, Raman scattering, and differential scanning calorimetry (DSC) was carried out. The emitted light color was characterized via its CIE coordinates. The data from various techniques were correlated and discussed in the framework of energy band theory, along with the carbon-induced Eu3+ → Eu2+ reduction concomitant with non-bridging oxygen bond modifications. The optical band gap is calculated from the transmission spectra via Tauc plots to narrow from 4.1 eV to 3.3 eV with increasing carbon concentration from ND (CND ∼ 0.14-1.4 mol%). The optical band gap values appear proportional to the integral intensities of the Raman-active bands of the glass. The Eu2+ absorption band of the 4f7 → 4f65d transition is deduced from the absorption spectra of the glass and utilized to explain the optical band gap narrowing concentration-trend with 5d levels introduced below the conduction band edge. A novel method for quantifying the PL emission spectra is suggested and correlated to the CND concentration in the melt and to the optical band gap. Further, DSC was used to corroborate the data with the Raman and optical studies. The study contributes to the fundamental understanding and design of tunable light-emitting Eu-doped phosphate glasses for photonic applications.