Remote Control Effect of Li⁺, Na⁺, K⁺ Ions on the Super Energy Transfer Process in ZnMoO₄:Eu³⁺, Bi³⁺ Phosphors

Abstract

Luminescent properties are affected by lattice environment of luminescence centers. The lattice environment of emission centers can be effectively changed due to the diversity of lattice environment in multiple site structure. But how precisely control the doped ions enter into different sites is still very difficult. Here we proposed an example to demonstrate how to control the doped ions into the target site for the first time. Alkali metal ions doped ZnMoO₄:Bi³⁺, Eu³⁺ phosphors were prepared by the conventional high temperature solid state reaction method. The influence of alkali metal ions as charge compensators and remote control devices were respectively observed. Li⁺ and K⁺ ions occupy the Zn(2) sites, which impede Eu and Bi enter the adjacent Zn(2) sites. However, Na⁺ ions lie in Zn(1) sites, which greatly promoted the Bi and Eu into the adjacent Zn(2) sites. The Bi³⁺ and Eu³⁺ ions which lie in the immediate vicinity Zn(2) sites set off intense exchange interaction due to their short relative distance. This mechanism provides a mode how to use remote control device to enhance the energy transfer efficiency which expected to be used to design efficient luminescent materials.

Introduction

Phosphor-converted white light emitting diodes (pcWLEDs) are treated as next generation lighting source^1,2. Currently, the most common pcWLEDs employ blue InGaN LED coated with yellow Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) phosphor^3,4. It is un-optimized for indoor use due to emission spectrum deficient in the red spectral region. To enhance red emission and raise color rendering index, a blend of YAG:Ce³⁺ and red emitting phosphor is generally utilized⁵. However, the current commercial red phosphor like Y₂O₂S:Eu³⁺ present chemical instability and low absorption in near ultraviolet (UV) region⁶. Hence high efficient and stable red phosphors that can be excited in near UV region should be developed.

The luminescence property of phosphor is known to be strongly affected by crystal lattice environment of the host. At present, adjusting the lattice environment of activators is a hotspot for solid state lighting⁷. In order to improve the luminescence efficiency, site occupation of activators in host lattice has been investigated from different perspectives^5,7,8,9. Peng⁵ found the site occupancy preference of Mn⁴⁺ in Sr₄Al₁₄O₂₅ is at the Al(4) and Al(5) higher covalent sites rather than the Al(6) site. They believe the high fluorescence intensity and thermal stability are due to the special environment of Mn⁴⁺ centers. Wang⁹ has improved the thermal stability of CaAlSiN₃:Eu²⁺ phosphors through neighboring-cation substitution. Tsai⁷ has enhanced the luminescent behavior of Eu²⁺ doped CaAlSiN₃ phosphors through adjusting the lattice environment of Eu²⁺ ions. And the lattice environment of Eu²⁺ doped CaAlSiN₃ phosphor has been modified through cation substitution to induce charge variation and a rearrangement of neighboring nitride clusters.

Bi³⁺ is a common activator and there are lots of reports on Eu³⁺ red luminescence enhancement via energy transfer from Bi³⁺ to Eu³⁺ in a variety of hosts^10,11. A free Bi³⁺ ion has 6s² electronic configuration and the absorption band of phosphors doped with Bi³⁺ might be extended to near UV region due to the influence of the host lattice on the outermost electrons^12,13. Lili Wang¹⁴ has summarized and established the relationships between the positions of energy levels of Bi³⁺ and environmental factor h_e of host materials in dozens of compounds. Recently, Bi³⁺ and Eu³⁺ co-doped ZnMoO₄ phosphors have been synthesized through solid state reaction and their properties have been discussed¹². It has been found that there exists a super energy transfer process from Bi³⁺ to Eu³⁺ due to the special S-shaped cluster in ZnMoO₄. There are three kinds of Zn sites in ZnMoO₄ and every six nearest Zn atoms form an S-shaped cluster. According to the quantitative relationship between energy levels of Bi³⁺ and host lattice environment established before¹⁴, the positions of ¹S₀→³P₁ transition of Bi³⁺ in three different Zn sites were predicted and then it was concluded that Bi³⁺ ions prefer to occupy Zn(2) sites. Thus the distance between Bi³⁺ and Eu³⁺ ions can be adjusted through their total concentrations. When their total molar concentration is larger than one sixth of that of Zn sites, Bi³⁺ and Eu³⁺ will locate in two adjacent Zn(2) sites and therefore extreme efficient energy transfer occurred¹².

However, there are defects because of charge imbalance in Bi³⁺, Eu³⁺ co-doped ZnMoO₄ phosphor. Additionally, concentration quenching occurred when the total concentration of Bi³⁺ and Eu³⁺ exceed one sixth of Zn sites. To solve the problem of unbalance charge as well as to explore the reasons for luminescence quenching, ZnMoO₄:Bi³⁺, Eu³⁺, M⁺ (M = Li, Na, K) phosphors were prepared and their properties were investigated. We found that only Na⁺ can strongly enhance the luminescence intensity of Eu³⁺. Based on the previous work as well as the preferred coordination number of Li, Na and K, site occupation of Bi³⁺ and Eu³⁺ was further adjusted and controlled. Thus site occupation preference of Bi³⁺ and Eu³⁺ and energy transfer between them in ZnMoO₄ were discussed in detail. Moreover, the red emission of Eu³⁺ was further increased after co-doping with charge compensator Na⁺ ions.

Results

Crystal structure

Figure 1 showed the results of the X-ray diffraction of the product after calcination at 700 °C for 3 h. All the diffraction peaks could be indexed to the triclinic wolframite ZnMoO₄ phase (JCPDS 35–0765) with space group P-1 and there was no formation of impurity phases. This indicated that the obtained samples were single-phased and the co-doped Eu³⁺, Bi³⁺ and alkali metal ions did not lead to any significant change in the host structure.

Figure 1

XRD patterns of the ZnMoO₄, Zn_0.95Eu_0.05MoO₄, Zn_0.90Eu_0.05Bi_0.05MoO₄ and Zn_0.80Eu_0.05Bi_0.05M_0.10MoO₄ (M = Li⁺, Na⁺, K⁺) phosphors were sintered at 700 °C for 3 h.

As can be seen from Fig. 2, every six Zn-O polyhedra form an S-shaped cluster. The completely centro-symmetric S-shaped cluster has three kinds of Zn sites. Each Zn(1) combine with five O ions to form Zn(1)O₅ hexahedral. In the center of S-shaped clusters, each two Zn(2)O₆ polyhedron connected by sharing edges and their relatively distance is only 3.2202 Å. Zn(2) and Zn(3) ions have six coordination number in an approximately octahedral coordination environment. In our previous work, the host lattice environment of luminescence centres strongly affects the luminescence properties. Covalence of chemical bond, coordination number of central ions and site symmetry appear to be important factors for the luminescence properties of Bi³⁺ ion¹⁵. The environment factor of three kinds of Zn sites were marked in corresponding position in the diagram. The five coordinated Zn(1) site has largest environmental factor value. The environment factor of Zn(2) and Zn(3) sites are very similar with only 5% difference. However, only Na⁺ ions can form five-coordinate structure. Therefore, Bi³⁺, Eu³⁺, Li⁺ and K⁺ ions preferentially occupy Zn(2) sites while only Na⁺ can lie in Zn(1) sites.

Figure 2

Information of S-shaped clusters in ZnMoO₄ crystal structure.

(a) perspective view and (b) side view.

The valence state of bismuth in doped samples

When Bi was introduced into phosphors via higher temperature solid state reaction method, it could readily transform into different valence states. In order to ensure the accuracy of the experiments, it is highly essential to identify the valence of bismuth in the samples. Therefore, we examined a representative sample by XPS. The selected spectrum is shown in Fig. 3, which correspond to Zn_0.80Eu_0.05Bi_0.05Na_0.10MoO₄ sample. The XPS of which shows two characteristic Bi³⁺ peaks at around 159.0 and 164.3 eV due to ⁴f_7/2 and ⁴f_5/2, respectively. It match well with those of α-Bi₂O₃^16,17. It means the dominance of Bi³⁺ in Bi doped ZnMoO₄ samples.

Figure 3

XPS spectra of Zn_0.80Eu_0.05Bi_0.05Na_0.10MoO₄ phosphor.

Photoluminescence properties

In order to investigate the relationship between doping concentrations and luminescence properties, the variation of emission intensity in Zn_0.90−yEu_yBi_0.10MoO₄ phosphors with the increasing Eu³⁺ concentration needs to study. Figure 4(a) showed the excitation spectra of the phosphors monitored at the ⁵D₀→⁷F₂ transition emission (616–619 nm) of Eu³⁺. In the PLE spectra, the broad band within the range 250–340 nm could be ascribed to the O²⁻→Mo⁶⁺ and O²⁻→Eu³⁺ charge transfer band (CTB) transition. The sharp lines were due to the intra configurational 4f-4f transitions of Eu³⁺ ions, which could be assigned to ⁷F₀-⁵D₄ (363 nm), ⁷F₀-⁵L₇ (385 nm), ⁷F₀-⁵L₆ (395 nm), ⁷F₀-⁵D₃ (417 nm), ⁷F₀-⁵D₂ (466 nm) and ⁷F₀-⁵D₁ (537 nm), respectively. In addition, a strong excitation band peaking at about 354 nm in Zn_0.80Eu_0.10Bi_0.10MoO₄ phosphor was observed due to the ¹S₀→³P₁ transition of Bi³⁺. When the total molar concentration was beyond 1/6, Bi³⁺ and Eu³⁺ began to sit two adjacent Zn(2) sites. According to Fig. 4(a), when the Eu³⁺ content was more than 0.0667, the Bi³⁺ ions ¹S₀-³P₁ excitation band peaking at about 331 nm decreased suddenly and the peak position of excitation band shifted from 355 nm to 331 nm obviously. The optimal doping concentration of Eu³⁺ was 0.0667. Moreover, the new super energy transfer from Bi³⁺ to Eu³⁺ emerged due to their short distance. According to the inset in Fig. 4(b), the PL intensity increased with increasing Eu³⁺ concentration within the range from 0.03 to 0.0667. However, when the doping concentration of Eu³⁺ was beyond 0.0667, the emission intensity decreased with increasing Eu³⁺ concentration. The concentration quenching occurred due to the short distance between Eu³⁺ ions. This indicated that the over high concentration of Eu³⁺ can result in Eu³⁺ ions occupy two adjacent Zn(2) sites in S-shaped cluster structure.

Figure 4

PLE (a) and PL (b) spectra of Zn_0.90−xEu_xBi_0.10MoO₄ (x = 0.03–0.22) phosphors at room temperature.

It is widely reported in the literature that the co-doping of alkali ions in the host lattice can enhance the luminescence significantly due to the strongly affects on the crystal structure. In this paper, Li⁺, Na⁺ and K⁺ ions were added to act as the remote control device to adjust the position and crystal structure environment of Bi³⁺ and Eu³⁺ ions in Zn_0.90Eu_0.05Bi_0.05MoO₄ phosphors. Because alkali metal ions can modify the local symmetry and the surroundings near the rare earth ions, which can significantly affect the photoluminescence properties. The PL and PLE spectra of Zn_0.90Eu_0.05Bi_0.05MoO₄, Zn_0.80Eu_0.05Bi_0.05Li_0.10MoO₄, Zn_0.80Eu_0.05Bi_0.05Na_0.10MoO₄ and Zn_0.80Eu_0.05Bi_0.05K_0.10MoO₄ phosphors at room temperature are shown in Fig. 5. The photoluminescence excitation (PLE) spectra monitored at the ⁵D₀-⁷F₂ transition emission (616–619 nm) of Eu³⁺ were shown in Fig. 5(a). From Fig. 5(a), the excitation spectra of samples are obviously affected by doping ions. Especially, the introduction of Na⁺ ions remarkably enhanced the excitation band intensity in the ultraviolet and near ultraviolet region. According to Fig. 5(b,c,d), the shape and positions were similar in the PL spectra for all samples. When excited at about 350 nm, the PL intensity of Zn_0.80Eu_0.05Bi_0.05Na_0.10MoO₄ phosphor enhanced three times compared to the Zn_0.90Eu_0.05Bi_0.05MoO₄ phosphor. The order of emission intensity for the Eu³⁺ ions with the three alkali metal ions is Na≫Li>K. However, there is no contribution on luminescence enhancement for Li⁺ ions. Furthermore, the introduction of K⁺ can significantly reduce the emission intensity of Eu³⁺. This reflects the impact of the alkali metal ions as remote control device on the energy transfer from Bi to Eu. From Fig. 5(c), when excited at about 394 nm to excite the inner 4f electron transition, the emission spectrum simultaneously affected by energy transfer from Bi to Eu ions due to the extending of Bi³⁺ ions absorption band. The order of emission intensity for the Eu³⁺ ions with the three alkali metal ions is Na≫Li>K.

Figure 5

Excitation (a) and emission (b,c,d) spectra of Zn_0.90Eu_0.05Bi_0.05MoO₄ and Zn_0.80Eu_0.05Bi_0.05M_0.10MoO₄ (M = Li⁺, Na⁺ and K⁺) phosphors.

Without the influence of Bi³⁺ excitation band, the emission spectra which were excited at about 465nm reflect the influence of alkali metal ions as charge compensators on the luminescent center of Eu³⁺ ions. From Fig. 5(d) it can be seen that the influence of doping ions on the fluorescence intensity was different from Fig. 5(b,c). The introduction of Li⁺, Na⁺ and K⁺ ions influenced the luminous intensity of Eu³⁺ ions obviously. The order of emission intensity for the Eu³⁺ ions with the three alkali metal ions is Li>Na>K. This reflects the effect of introduce alkali metal ions as charge compensators. However, from Fig. 5(a,b), only Na⁺ ions can strongly enhance the emission intensity which was different with the effect of charge compensation. Li⁺ and K⁺ ions have hindered the energy transfer from Bi to Eu ions. This shows that there is a special enhancement mechanism different with charge compensation. For further investigating the enhancement mechanism, the doping ions were fixed at Na⁺ ions.

The excitation spectra of Zn_0.80−xEu_0.05Bi_0.05Na_xMoO₄ phosphors under the excitation wavelength of 614 nm are shown in Fig. 6. From the picture, it can be seen that the introduction of Na⁺ into the host can significantly enhance the luminescence intensity. Therefore, to maintain the charge balance with Na⁺ ions will be more advantageous to improve the energy transfer. However, it was found that the position of excitation band blue-shifted clearly, which indicates that the lattice environment of Bi³⁺ ions has been changed obviously. This is affected by the charge compensation agent. Figure 7 gave the most probable remote control mechanisms.

Figure 6

PLE spectra of Zn_0.80−xEu_0.05Bi_0.05Na_xMoO₄ phosphors with various concentration of Na⁺ ions.

Figure 7

The schematic diagram of luminescence enhancement mechanisms with remote control device.

Remote control mechanism

The introduction of alkali metal ions can not only lower the charge imbalance but also influence the position of Eu³⁺ and Bi³⁺ ions. According to the previous work, Bi³⁺ and Eu³⁺ ions could occupy preferentially the Zn(2) site¹². However, because of the charge imbalance when Bi³⁺ or Eu³⁺ seated in the Zn(2) site, alkaline ions will occupy Zn(1) or Zn(2) site which is the nearest dopant neighbor. As reported in literatures, Zn(2) and Zn(3) have six oxygen ligands, while Zn(1) has only five coordination number^18,19,20 which can provide more compact space to charge compensation with small ionic radius. Moreover, compared with Zn(2) and Zn(3) site, Zn(1) site has the largest environment factor h_e¹². As far as we know, in alkaline ions (Li⁺, Na⁺ and K⁺), only Na⁺ ions can form five-coordinated polyhedra²¹. Therefore, Li⁺ and K⁺ ions could only occupy Zn(2) site which is nearest Eu³⁺ or Bi³⁺ ions. It is not conductive to shorten the relatively distance between Eu³⁺ and Bi³⁺ ions. Thence, the introduction of Li⁺ and K⁺ can obstruct the energy transfer from Bi³⁺ to Eu³⁺ and then reduce the emission intensity of Eu³⁺ ions. Table 1 lists the ionic radius under different coordination numbers. It can be seen that only Na⁺ ions can form five coordination sites. When Na⁺ ions were added as charge compensators, Zn(1) site was more suitable to be occupied due to the similar ionic radius between Na⁺ (1.14 Å) and Zn²⁺ (0.82 Å) for five coordination. In summary, when Na⁺ was co-doped as charge compensator, it can seat into Zn(1) site and shorten the relative distance between Bi³⁺ and Eu³⁺. The possible mechanism was shown in schematic diagram in Fig. 7.

Table 1 The ionic radius list with different coordination number²¹.

Conclusion

In summary, a series of Eu³⁺, Bi³⁺ and alkali metal ions co-doped ZnMoO₄ phosphors were prepared by the solid-state method in air atmosphere. Li⁺, Na⁺ and K⁺ ions acted as the remote control device were added to improve the luminescence intensities. The super energy transfer process from Bi³⁺ to Eu³⁺ cannot appear only when the total concentration of Bi³⁺ and Eu³⁺ ion is very high. Without alkali metal ions, Eu³⁺ and Bi³⁺ ions lie in Zn(2) sites. The introduced Li⁺ and K⁺ ions seat into Zn(2) site which against the energy transfer from Bi³⁺ to Eu³⁺. However, Na⁺ ions lie in Zn(1) site and then help Bi³⁺ and Eu³⁺ ions to seat into the adjacent Zn(2) sites. Compared with Li⁺ and K⁺ ions occupied Zn(2) sites, Na⁺ ions which preferentially occupied Zn(1) site can get closer to the distance between Bi³⁺ and Eu³⁺ ions. The results indicated that Na⁺ ions provided intensity of energy transfer from Bi³⁺ to Eu³⁺. This mechanism provides a mode how to use remote control device to enhance the energy transfer efficiency which expected to be used to design efficient luminescent materials.

Experiment

Materials and Synthesis

A series of ZnMoO₄:1/6Eu³⁺, 1/6Bi³⁺, xM⁺ (M = Li, Na, K) and ZnMoO₄:0.05Eu³⁺, 0.05Bi³⁺, 0.1M⁺ (M = Li, Na, K) phosphors with various concentrations and alkali metal ions were synthesized through the solid state reaction method in air atmosphere. The raw materials were ZnO (99%), Eu₂O₃ (99.99%), Bi₂O₃ (99.9%), Li₂CO₃ (99%), Na₂CO₃ (99%), K₂CO₃ (99%) and MoO₃ (99%). The starting materials were weighed according to the stoichiometric ratio and well mixed in agate mortar. The mixtures were put into alumina crucible and calcined in muffle furnace at 700 ^oC for 3 h and then the white powder phosphor was obtained. All samples were ground into a powder with an agate mortar and pestle for further analysis.

Materials Characterization

The crystal structure of the samples was determined by the Bruker D8 Advance X-ray diffractometer (Cu Kα₁ radiation, λ = 0.15406 nm) with radiation at a 0.02^o (2θ) /0.05 s scanning step. The photoluminescence excitation (PLE) and emission (PL) spectra were recorded with a Hitachi F-4600 spectrophotometer equipped with a 150 W xenon lamp as an excitation source. All the measurements were performed at room temperature.