II. EXPERIMENTAL

Calcium carbonate (CaCO₃, 99%), tungsten oxide (WO₃, 99%), terbium oxide (Tb₄O₇, 99.99%), and ytterbium oxide (Yb₂O₃, 99.99%) were used as raw materials. All of them were sufficiently commixed in agate mortar with appropriate amount of ethanol. Then the powders were sintered at 550 °C and 1000 °C for 3 h each in air.

Crystal phase of the obtained materials was determined by powder x-ray diffraction (XRD, Rigaku D/MAX 2400 x-ray diffractometer with Ni-altered Cu K_α radiation). Emission and excitation measurements were performed using a 450 W Xe lamp as the excitation source. Mean decay lifetime measurements were performed using uF900 as the excitation source. All measurements were carried out at room temperature.

III. RESULTS AND DISCUSSION

The experimental and calculated results of the XRD refinement of Ca_0.89WO₄: 0.01Tb³⁺, 0.1Yb³⁺ are plotted in Figure 1. The inset exhibits the XRD patterns of Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺ (0 ≤ x ≤ 0.25) phosphors. The observed peaks of Ca_0.89WO₄: 0.01Tb³⁺, 0.1Yb³⁺ satisfy the reflection condition, R _p = 5.48% and R _wp = 4.77%, which indicate that the refinement result is reliable. By analyzing the result, it is found that Ca_0.89WO₄: 0.01Tb³⁺, 0.1Yb³⁺ is of scheelite structure with I4₁/a space group and with lattice constants a = 5.2386(3) Å, b = 5.2386(3) Å, and c = 11.3723(2) Å, which are in good agreement with those of CaWO₄ (JCPDS Card No. 77-2233). Therefore, the crystal structure of CaWO₄ is not changed obviously by doping 1% Tb³⁺ and 10% Yb³⁺. For Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺ (x = 0, 0.01, 0.05, and 0.2) samples, the XRD patterns are similar and could also be indexed to single phase (as shown in the inset of Fig. 1). However, when the doping concentration of Yb³⁺ reaches 25%, an impurity phase assigned as Yb₂WO₆ is observed at 2θ = 24.08°, 25.6°, 26.72°, and 29.9°. As a result, the suitable concentration range of Yb³⁺ in Ca_0.99WO₄: 0.01Tb³⁺ is acquired at 0–20%.

FIG. 1.

x-Ray diffraction (XRD) refinement of Ca_0.89WO₄: 0.01Tb³⁺, 0.1Yb³⁺ sample; the inset is the XRD patterns of Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺ (0 ≤ x ≤ 0.25) phosphors.

Luminescent measurements are recorded to give convincing evidence for the existence of Tb³⁺-Yb³⁺ energy transfer (ET). Figure 2 illustrates the excitation spectrum of Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺ (x = 0.2; dotted line) monitored at 992 nm (²F_5/2 → ²F_7/2 of Yb³⁺) and the emission spectrum of Tb³⁺ in Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺ (x = 0; solid line) excited by 254 nm. An intense absorption band is observed around 480–530 nm in the excitation spectrum, and an intense emission peak at 489 nm ascribing to the ⁵D₄ → ⁷F₆ transition of Tb³⁺ is exhibited in the emission spectrum. The overlap of the excitation and emission spectra suggests that the ET from ⁵D₄ level of Tb³⁺ to ²F_5/2 level of Yb³⁺ in CaWO₄: Tb³⁺, Yb³⁺ phosphor exists.Reference Zhang, Yang, Jiang and Ji¹⁵

FIG. 2.

Excitation spectrum of Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺ (x = 0.2; dotted line) monitored at 992 nm (²F_5/2 → ²F_7/2 of Yb³⁺) and emission spectrum of Tb³⁺ in Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺ (x = 0; solid line) excited by 254 nm.

The emission spectra of Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺ (0 ≤ x ≤ 0.20) in the VIS region from 520 to 650 nm excited by 489 nm are shown in Fig. 3(a). Under 489 nm excitation, emission peaks at around 545, 587, 622, 653, 673, and 681 nm are clearly observed, which could be assigned to characteristic ⁵D₄ → ⁷F_j (j = 5, 4, 3, 2, 1, 0) transitions of Tb³⁺.Reference Su, Li and Li¹⁷ The emission spectra of Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺ (0 ≤ x ≤ 0.20) in the NIR region from 960 to 1060 nm are presented in Fig. 3(b) (λ _ex = 489 nm). The emission band from 960 to 1040 nm could be attributed to the transition of Yb³⁺ from the high Stark level of the ⁵F₂ multiplet to the low Stark level of the ⁷F₂ multiplet. Similar explanation was reported in the references.Reference Wang, Xie and Zhang^16, Reference Chen, Wang, Yu and Ma¹⁸ From the comparison of the spectra, it is found that the emission intensities of Tb³⁺ rapidly decreased with the increase of Yb³⁺ concentration. Meanwhile, it is noticed that the luminescence intensities of the NIR emission enhance with increasing Yb³⁺ concentration. This is another evidence for the cooperative ET from Tb³⁺ to Yb³⁺. However, the decrease in NIR emission intensities is also observed when Yb³⁺ concentration is more than 5% because of concentration quenching.

FIG. 3.

Emission spectra of Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺ (0 ≤ x ≤ 0.20) in VIS region from 520 to 650 nm (a) and NIR region from 960 to 1060 nm (b) (λ _ex = 489 nm).

The lifetimes of phosphors could reflect the ET among luminescent centers. In Fig. 4, the luminescent decay curves of Tb^{3 + 5}D₄ → ⁷F₅ transition at 545 nm are plotted for Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺ (0 ≤ x ≤ 0.20) under 489 nm excitation. The decay curve of a singly doped (Ca_0.99Tb_0.01)WO₄ phosphor fits well to a single exponential function with a decay time of 602.94 μs. The lifetimes of the other samples are determined to be 595.16, 520.93, 461.83, and 359.6 μs for x = 0.01, 0.05, 0.1, and 0.2, respectively, in Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺. Obviously, the decay times decrease rapidly with increasing Yb³⁺ doping concentration in the range of 0–20 mol%. The decline in the lifetimes can be well explained by the introduction of extra decay pathways because the codopant Yb³⁺: ET from Tb³⁺ to Yb³⁺ enhances the Tb³⁺ decay rate. This result is in good agreement with that obtained from Figs. 2 and 3. Hence, the ET from Tb³⁺ to Yb³⁺ in Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺ (0 ≤ x ≤ 0.20) is proved.

FIG. 4.

Luminescent decay curves of Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺ (0 ≤ x ≤ 0.20) monitored at 545 nm by 489 nm excitation; the inset is the energy level graph about the QC process from Tb³⁺ to Yb³⁺.

It is noteworthy that the excitation band in Fig. 2 could not be ascribed to the characteristic transition of Yb³⁺. If the overlap between donor characteristic emission and acceptor characteristic absorption is absent, the second-order downconversion may become the dominant relaxation process, in which one donor excites two acceptors simultaneously.Reference Vergeer, Vlugt, Kox, Den Hertog, van der Eerden and Meijerink⁹ For the resonance ET in the second-order downconversion, it should be fulfilled that the sum of the absorption energy of the two acceptors equals the emission energy of the one donor. Therefore, the excitation spectrum, emission spectra, and decay lifetimes of Ca_0.99−xWO₄: 0.01Tb³⁺, xYb³⁺ (0 ≤ x ≤ 0.20) indicate that NIR QC occurs under excitation at the ⁵D₄ level of Tb³⁺ as the two NIR photons could be emitted by absorbing one VIS photon due to the cooperative ET from Tb³⁺ to Yb³⁺. The energy level graph about the QC process from Tb³⁺ to Yb³⁺ is illustrated in the inset of Fig. 4.

The ET efficiency (η _ETE) and QE (η _QE) could be determined from the luminescent decay curves. The η _ETE is defined as the ratio of Tb³⁺ ions that depopulate by ET to Yb³⁺ ions over the total number of Tb³⁺ ions excited. The total η _QE is defined as the ratio of the number of photons emitted (visible and infrared) to the number of photon absorbed, assuming that all excited Yb³⁺ ions decay radiatively. This assumption leads to an upper limit of the QE, which is lower than the actual efficiency due to concentration quenching. By dividing the integrated intensity of the decay curves of Yb_xCa_0.99−xTb_0.01WO₄ phosphors to the integrated intensity of Ca_0.99Tb_0.01WO₄ curve, we can obtain η _ETE as a function of the Yb³⁺ concentration: ,Reference Wegh, Donker, Oskam and Meijerink^1, Reference Vergeer, Vlugt, Kox, Den Hertog, van der Eerden and Meijerink⁹ where τ denotes the decay lifetime and x represents the Yb³⁺ concentration. The relation between η _QE and η _ETE is linear and defined as ,Reference Wegh, Donker, Oskam and Meijerink^1, Reference Vergeer, Vlugt, Kox, Den Hertog, van der Eerden and Meijerink⁹ where the nonradiate energy loss is ignored and η _Tb is set to 1. Using the above two equations, the values of η _ETE and η _QE are obtained. Figure 5 shows the decay lifetimes and η _QE of Yb_xCa_0.99−xTb_0.01WO₄ phosphors as a function of Yb³⁺ concentration. It is found that the maximum value of η _QE reaches 140.4% when the doping concentration of Yb³⁺ reaches 20%. This means that the depopulation of Tb^{3 + 5}D₄ level proceeds about 40 out of 100 times by excitation of two Yb³⁺ ions to the ²F_5/2 level. As mentioned in the introduction, this NIR QC phosphor with high QE could be considered as a promising candidate for Si-based solar cells.

FIG. 5.

Decay lifetimes and η _QE of Yb_xCa_0.99−xTb_0.01WO₄ phosphors as a function of Yb³⁺ concentration (0 ≤ x ≤ 0.20).