Chaoshu Shi

The electronic structure and spectral properties of ZnO and its defects

The electronic structure of ZnO and its defects, which include intrinsic point defects and their complexes, have been calculated using full-potential linear Muffin-tin orbital method. According to our calculation data, the positions of the defect state levels have been determined in the energy band of ZnO. Based on the results above, we analysis the mechanism of the absorption and emission spectra of ZnO and discuss the effects of the electronic structure of complete ZnO and its defects on the spectral properties.

Potential white-light long-lasting phosphor: Dy3+-doped aluminate

The white-light long-lasting phosphor CaAl2O4:Dy3+ was prepared and investigated. The white-light afterglow spectra under the irradiation of 254 or 365nm are comprised of the blue light emission and the yellow light emission, originating from the transitions of F9∕24→H15∕26, F9∕24→H13∕26 in the 4f9 configuration of Dy3+. The afterglow can last 32min for the best sample with Dy-doped concentration of 2at.%. The decay curve and the thermoluminescence curve show to be a second-order process. Thermoluminescence curves exhibit a complicated structure in the range of 230–450K with the peaks at 244, 280, 310, and 346K. The two thermoluminescence bands peaking above room temperature have corresponding traps with the depths of 0.54 and 0.61eV, which are responsible for the afterglow emission at room temperature. This work provides a promising approach for the development of white-light long-lasting phosphor.

Spectral properties and thermoluminescence of PbWO4 crystals annealed in different atmospheres

Abstract Photoluminescence (PL) and their excitation spectra, cathodoluminescence (CL) and thermoluminescence (TL) of PbWO 4 crystals annealed in oxygen, air and vacuum have been investigated. The results indicate that the green luminescence is not caused by (WO 3 +F) centers but is probably related to (WO 4 +O i ). Assignment of the excitation states is also discussed. Some structural defects related to the oxygen vacancy ( V 0 ) are the main mechanism inducing the fluorescence excitation band peaking at 378 nm and forming shallow traps corresponding to a TL peak at 134 K. The deeper traps shown by thermoluminescence at ca. 250 K originate from local oxygen excess defects.

Visible quantum cutting in BaF2:Gd, Eu via downconversion

The visible quantum cutting in BaF2: Gd, Eu via Downconversion has been observed. In the quantum cutting process, one VUV photon absorbed by Gd3+ can be split into two visible photons emitting by Eu3+ through cross-relaxation between Gd3+ and Eu3+. According to the calculation from the emission spectra under different wavelength excitation, we can obtain the two-step energy transfer process with a visible quantum efficiency up to 194%

Annealing effect for surface morphology and luminescence of ZnO film on silicon

Abstract The atomic force microscopy (AFM), the X-ray diffraction with glancing input angle (GXRD) and the cathodoluminescence (CL) spectra of a ZnO film on Si, annealed at different temperatures, were measured. The results showed that, the crystal quality of the film improved with increasing the annealing temperature, while the hexagonal phase of the ZnO film transformed into a kind of mixture phase including a hexagonal and a trigonal phase, after annealing at 800 °C for 1 h. The CL spectrum also shows the intrinsic emission bands of ZnO and Zn2SiO4, in which the ZnO is the main source of the spectrum. Increasing the temperature continuously up to 950 °C changed the main source of samples luminescence from the emission of ZnO to the emission of zinc silicate. This indicates a creation of new ternary compound Zn2SiO4 in the film.

Investigation on the origin of the blue emission in titanium doped sapphire: Is F+ color center the blue emission center?

Titanium doped sapphire (Ti:Al2O3) shows a 420 nm blue emission band whose corresponding excitation band lies at 244 nm. With the aim of ascertaining the origin of this blue emission, using synchrotron radiation we measured the vacuum ultraviolet to ultraviolet absorption spectrum of titanium doped sapphire. It is suggested that the blue emission is due to a center other than a trivalent Ti3+ ion or a tetravalent Ti4+ ion, most probably an F+‐type color center formed in the crystal.

Energy transfer between Pr3+ and Mn2+ in SrB4O7:Pr,Mn

The luminescence of Pr3+ or Mn2+ singly doped, as well as Pr3+ and Mn2+ codoped, SrB4O7 powder microcrystalline samples were investigated using synchrotron radiation. The photon cascade emission originating from the S01 level was observed in the SrB4O7:Pr3+ (0.1mol%) sample upon 206nm excitation. There are desirable spectral overlaps in the region of 330–430nm between the emission spectra of the SrB4O7:Pr3+ sample and the excitation spectra of the SrB4O7:Mn2+ sample monitoring the Mn2+ luminescence at 640nm. These spectral overlaps are in favor of the energy transfer from Pr3+ to Mn2+, converting the first step photon from Pr3+:S01 in the unpractical ultraviolet or near-ultraviolet regions into the red Mn2+ emission. A comparison of the emission spectra of the SrB4O7:Pr3+, Mn2+ sample with that of the SrB4O7:Pr3+ sample revealed the existence and the efficiency of the proposed energy transfer between Pr3+ and Mn2+, suggesting an promising vacuum ultraviolet phosphor based on Pr3+ and Mn2+ combination with...

Luminescence properties of CaZr(PO4)2:RE (RE = Eu3+, Tb3+, Tm3+) under x-ray and VUV–UV excitation

Novel phosphors of Eu3+, Tb3+ and Tm3+ doped CaZr(PO4)2 were synthesized by solid-state reactions and their x-ray and vacuum ultraviolet–ultraviolet (VUV–UV) spectroscopic properties were investigated. The bands near 147 or 185 nm in the VUV excitation spectra of those doped samples are attributed to the PO4 absorption and O to Zr charge transfer transition in the host. The excitation and emission spectra indicate that the phosphors can be effectively excited by x-ray or 147 nm and 172 nm and exhibit a satisfactory red, green and blue light performance, respectively. Considering the high luminescent intensity, excellent colour purity and chemical stability, CaZr(PO4)2:RE (RE = Eu3+ Tb3+, Tm3+) are attractive red, green and blue emitting x-ray and plasma display panel phosphors.

Luminescent properties of Gd2SiO5 powder doped with Eu3+ under VUV?UV excitation

The luminescent properties of Gd2SiO5 powder crystals doped with Eu3+ were investigated using synchrotron radiation and a VUV laser (157.6?nm) as excitation sources. The excitation spectra in the range of 160?330?nm monitoring the red emission from Eu3+ ions reveal bands corresponding to intraconfigurational 4f?4f transitions of Gd3+ and charge transfer states (CTS) of Eu3+?O2?, as well as interband transitions of the Gd2SiO5 host, indicating an efficient energy transfer process from the host or directly from Gd3+ to Eu3+ ions. The inspection of emission lines from Eu3+ ions suggests that there are three inequivalent sites for Eu3+ in this host. The excitation band around 215?nm and the unique emission features associated with this excitation band were attributed to one of the inequivalent Eu3+ sites, which exhibits an unusually weak coupling with host lattice. The energy levels of Eu3+ in the Gd2SiO5 host were tentatively assigned according to the laser excited emission spectra.

Vacuum ultraviolet spectroscopic properties of rare earth (RE=Ce,Tb,Eu,Tm,Sm)-doped hexagonal KCaGd(PO4)2 phosphate

Hexagonal KCaGd(PO4)2:RE3+ (RE=Ce,Tb,Eu,Tm,Sm) were synthesized by coprecipitation method and their vacuum ultraviolet–ultraviolet (VUV-UV) spectroscopic properties were investigated. The bands at about 165nm in the VUV excitation spectra are attributed to the host lattice absorptions. For Ce3+-doped samples, the bands at 207, 256, 275, and 320nm are assigned to the 4f-5d transitions of Ce3+ in KCaGd(PO4)2. For Tb3+-doped sample, the bands at 203 and 222nm are related to the 4f-5d spin-allowed transitions. For Eu3+-doped sample, the O2−–Eu3+ charge-transfer band (CTB) at 229nm is observed, and the fine emission spectrum of Eu3+ indicates that Eu3+ ions prefer to occupy Gd3+ or Ca2+ sites in the host lattice. For Tm3+- and Sm3+-doped samples, the O2−–Tm3+ and O2−–Sm3+ CTBs are observed to be at 176 and 186nm, respectively. From the standpoints of the absorption band, color purity, and luminescent intensity, Tb3+-doped KCaGd(PO4)2 is a potential candidate for 172nm excited green plasma display phosphors.