Generation of tunable wavelength lights in core-shell CaWO4 microspheres via co-doping with Na+ and Ln3+ (Ln = Tb, Sm, Dy, Eu)

Microemulsion‐Mediated Synthesis and Properties of Uniform Ln:CaWO4 (Ln = Eu, Dy) Nanophosphors with Multicolor Luminescence for Optical and CT Imaging

“…As observed, the value of <τ>, which in the case of the undoped sample (7.527 μs) is similar to that previously reported for CaWO 4 microparticles (8.46 μs), [37] progressively decrease as the Eu doping level increases, which is as expected from the ET process. Consequently, the value of η obtained from these <τ> values increases (from 29.09 % for the 2 % Eu-doped phosphor to 36.44 % for the 10 % Eu-doped sample).…”

“…The decay curves obtained for the I(t) = I 01 exp(-t/τ 1 ) + I 02 exp(-t/τ 2 ) (1) where I(t) is the luminescence intensity, t is the time after excitation, and τ i (i = 1, 2) is the decay time of the i-component, with initial intensity I 01 . Such biexponential behavior has been previously observed for this system [35,37] and other lanthanidebased nanophosphors [38] and accounts for the contribution of the Eu 3+ ions located close to the surface of the nanoparticles (short time component) whose luminescence is influenced by OH species or surface defects that act as luminescence quench- [38] The fitting parameters are presented in Table 4, which shows that the short-time component is a minority in all cases and that the lifetime values tend to decrease as the Eu doping level increases. This variation is also manifested by the average lifetime values, <τ>, calculated from Equation (2): (2) where t f represents the time required for the luminescence signal to reach the background.…”

“…[37] As observed, the weak and narrow bands in the 320-380 nm range (the most intense one appearing at 352 nm), due to the direct excitation of the Dy 3+ ground state to higher levels of the 4f-manifold, [37] are much less intense than the broad band, with a maximum at <250 nm, due to the energy transfer from the tungstate groups to the Dy 3+ cations. Upon excitation at λ ex = 250 nm, the characteristic emissions of the Dy 3+ ions due to the electronic transitions from the excited 5 F 9/2 level to the 6 H 15/2 (from 470 to 490 nm) and 6 H 13/2 (572 nm) levels, [37] are detected for all Dy-doped samples (Figure 8b), along with the broad emission due to the CaWO 4 matrix (430 nm), whose intensity decreases with respect to that of the undoped sample, due to the ET process. As in the case of the Eu-containing phosphors, the most favorable excitation path for this phosphor is also through the ET band, as confirmed by the emission spectra obtained after excitation at 250 nm and 352 nm (Figure 8d).…”

Lanthanide Luminescence in Solids

“…One such method to produce white light uses downconversion in core-shell materials. This was achieved by employing Y 2 O 3 : Eu 3+ core-LnPO 4 (Ln ¼ Ce, Tb) shell micron-size particles [153]; by the use of nanoparticles comprising LaF 3 doped with Eu 3+ as core; with Tb 3+ as first shell; with Tm 3+ as second shell; and then with LaF 3 passive coating [154]; or by coreshell CaWO 4 microspheres co-doped with Na + and Ln 3+ (Ln ¼ Tb, Sm, Dy, Eu) [155]. The use of several lanthanide ions for white light generation can be avoided if use is made of CT emission in conjunction with the emission of one lanthanide ion.…”

Synthesis, characterization, and enhanced luminescence of CaWO4:Eu3+/SBA-15 composites

“…ions belonging to 5 D 0 ? 7 F J (J = 0, 1, 2, 3, 4) [44]. The electronic dipole transition of 5 D 0 ?…”