Growth Of Bi,Co-doped CdWO₄ Crystals By Bridgman Method And Their Optical Spectra

Bi,Co-doped glasses and fibers have attracted great interest owing to their promising applications as active mediums of superbroadband NIR optical lasers and amplifiers.At present, most research concern bulk glasses and optical fibers. However, surprisingly little attention has been given to crystals.Usually, it is much easier to interpret the spectroscopic properties and understanding the nature of the active centers in crystal hosts than glasses or fibers, because of the ordered and rigid crystal lattice which can improve the efficiency of luminescence center.In the previous publications, the emission has been attributed to electronic transition of Bi⁵⁺, Bi⁺, Bi clusters and so on,but the origin is still controversial.In this paper, we focused on the Bridgman method of Bi,Co-doped CdWO₄ crystals and their optical spectra .A preliminary exploration was made about the valence state of Bi that was responsible for the emission.In chapter one,we discussed the application of main group metal ions and transition metal ions in laser materials and their latest research progress,especially about the near-infrared luminescence materials.Finally,we conclude the discovery of Bi with diverse valence state in different research fields.In chapter two,we studied preparation and characterization of CdWO₄.We introduced the Bridgman method and the structure of CdWO₄ single crystal and some measurements related to our research.In chapter three, the properties of broadband near-infrared luminescence in Bi-doped CdWO₄ single crystals were discussed.The emission spectra of various parts of crystal were investigated under excited by 808 nm and 980 nm. The weak emission band at 1396¹550 nm (centered at 1504 nm) and strong band at 1037¹274 nm(centered at 1078 nm)were observed, which their lifetime were 238μs and 294μs, respectively. The emission intensity at 1504 nm increased as the growth direction; while the intensity at 1078 nm reduced as the growth direction.The mechanism for the band emission were discussed from the obtained spectra. The emission band at 1078 nm was probable related to Bi ion, while the weak band at 1504 nm was probable related the defects of the crystal.In chapter four,the absorption spectra,emission spectra and X-Ray photoelectron spectroscopy (XPS) of various parts of as-grown Bi:CdWO₄ crystal were investigated.The four emission peaks at 470, 528, 1078 and weak 1504 nm were observed under the excitation at 311, 373, 808, and 980 nm lights. According to the X-Ray photoelectron spectroscopy of the Bi:CdWO₄ single crystals, Bi₂O₃(Bi³⁺) and NaBiO₃(Bi⁵⁺) samples, we inferred that Bi3+ and Bi⁵⁺ existed in Bi:CdWO₄ crystals simultaneously. The fluorescence emission of visible bands at 470 and 528 nm resulted from [WO₆] group in Bi:CdWO₄ and Bi³⁺ ions doped in the lattice, while the fluorescence emission of the near infrared band at 1078nm due to the Bi5+ luminescence. The analytic result of XPS was consistent with the change of the fluorescence intensity. The emission intensity at 1078nm reduced and the content of Bi⁵⁺ ion decreased gradually; while the intensity at 528nm enhanced and the number of Bi³⁺ increased along the growing direction.In chapter five,we studied the emission spectroscopic properties of Co-doped CdWO₄ single crystal which prepared by Bridgman method,and confirmed the intrinsic luminescence of CdWO₄.Three absorption peaks at 518nm,564nm and 655 nm, which are attributed to the overlapping of ⁴T₁(F)→⁴A₂(F) and ⁴T₁(F)→⁴T₁(P) of Co²⁺ octahedrons, and a wide band centered at 1863 nm,which is attributed to ⁴T₁(F)→⁴T₂(F) were observed.The fluorescence emission of visible bands at 480 and 778nm nm resulted from recombination of electron-hole pairs of [WO₆] group and ⁴T₁(P)→⁴T₁(F) in Co:CdWO₄ and Co²⁺ ions doped in the lattice,respectively.Finally, we concluded all the results that have been obtained in the paper,we realized the insufficiencies of our work, and anticipate the prospect of broadband emission materials.