Spectroscopy evaluation of crystalline and amorphous Cd2V2O7 as blue phosphors

Abstract

Cd2V2O7 compounds in crystalline and amorphous phases were fabricated by the melt-quenching process. Characterizations such as X-ray diffraction, Raman spectroscopy and photoluminescence were performed. X-ray diffraction patterns of the crystalline sample showed peaks associated with a pure Cd2V2O7 monoclinic structure, in agreement with the Raman vibrational spectrum. In the case of the amorphous sample, X-ray diffraction patterns only exhibited a broad band, typical of a glassy structure, whereas its Raman spectrum displayed two broad vibrational modes centered at 350 and 850cm−1, attributed to stretching vibrations of VO3 groups. In spite of the structural differences, both samples presented similar photoluminescence features, consisting of a wide band in the 375–525nm range with two peaks at 411 and 432nm, associated respectively with the 3T2 → 1A1 and 3T1 → 1A1 electronic transitions in the VO4 tetrahedron, under 340nm excitation. Thus, blue light emission with CIE1931 chromaticity coordinates x~ 0.200 and y~ 0.145, and color purity of 62–63%, is achieved from the crystalline and amorphous Cd2V2O7 compounds. The emission decay time profiles were well fitted to a bi-exponential function from which the calculated average lifetime values resulted to be 112 ± 13 and 99 ± 4ns for the crystalline and amorphous Cd2V2O7 samples, respectively. Theoretical calculations based on the density of electronic states revealed that the photoluminescence arises through charge transference processes from 3d orbitals of four-fold coordinated vanadiums to 2p orbitals of three-fold coordinated oxygens in the VO4 tetrahedron, being the basic unit of Cd2V2O7 in crystalline and amorphous phases.