White light-emitting diodes (w-LEDs) have drawn wide attention and extensive study for their low energy consumption, high brightness, long working lifetime, high efficiency, and environmentally friendly features^{[1–5" target="_self" style="display: inline;">–5]}. Currently available commercial w-LEDs are fabricated by the combination of the blue-emitting LED chips and the yellow-emitting phosphors^[6]. However, this approach suffers weaknesses, such as high color temperature, owing to the lack of a red light component. In order to improve the white performance, near ultraviolet (n-UV) (370–420 nm) LED chips coated with blue, green, and red-emitting phosphors is introduced. It provides an excellent color rendering index and low correlated color temperature. Thus, much attention has been concentrated on developing yellow–orange emitting phosphors, which provides more red emission components than yttrium aluminum garnet (YAG) Ce3+^[7]. Several oxynitrides have been developed for this purpose; however, the oxynitrides suffer from harsh preparation conditions, such as the carbothermal method or high-temperature nitrification at high pressure. Thus, it is important to explore novel efficient orange–yellow-emitting phosphors in an oxide host matrix through comparatively simple synthesis approaches for the practice application.

At present, numerous research efforts have been conducted to oxide phosphor with native defects. The native defect luminescent is observed in isolated d10 ion complexes, such as Zn4O compounds, Ca3SnSi2O9, GaN, and BaMoO4^{[8–11" target="_self" style="display: inline;">–11]}. There also exists strong evidence for the observation of photoluminescence (PL) from transition-metal oxides. Examples are β-Ga2O3, SrZrO3, SrTiO3, and Sr2V2O7^{[12–15" target="_self" style="display: inline;">–15]}. The studies attribute the radiative decay process to a distorted octahedral structure, self-trapped excitons, oxygen vacancies, surface states, and charge to transfer via intrinsic defects inside an oxygen octahedron^[16]. Although there is no general consensus in the literature about the nature of the emission, it is a very interesting phenomenon to study these materials acting as the host matrix of LED phosphors, which could provide the energy transfer from the host matrix to activators.

Recently, there is some research focused on germanates as a host of optical materials. As a kind of garnet, germanates have a rich variety of material structure, the same as silicates. It can be expected that rare earth ions in the corresponding germanates’ structure exhibit excellent luminescent characteristics. Li2ZnGeO4:Mn2+ exhibiting a green long persistent luminescence is reported by the Shang group^[17], CaZnGe2O6:Mn2+ gives out a red luminescence as reported by the Che group, the Pan group explored a new type of Zn3Ga2Ge2O10:Cr3+ phosphor, which emits red at 698 nm^[18], and the Xu group investigated Na2CaSn2Ge3O12:Sm3+ as a reddish-orange phosphor^[19]. Among germanates, zinc germanate (Zn2GeO4) is an excellent candidate due to its reasonable conductivity and high stability. Zn2GeO4 has a rhombohedral structure, which is similar to Zn2SiO4, with Zn2+ ions at the tetrahedral sites and Ge at the octahedral sites, and it has a wide band gap of 4.68 eV^[20]. Zn2GeO4 provides a potential application in field emission displays as a novel host matrix, and few literatures focus on its persistent luminescence properties^[21]. Furthermore, Zn2GeO4 as a native defect phosphor is studied by the Qiu group^[22]. Nevertheless, there is only limited study on the PL of Zn2GeO4, and no attempts have been made for energy transfer from the host to activators. In this work, we perform research on Ce3+, Sm3+-doped Zn2GeO4 phosphor, which emits a tunable color due to the effective energy transfer from the host to the activators. The PL properties of the Zn2GeO4 host matrix, Ce3+, Sm3+ single, and co-doped samples are studied. The energy transfer process between Ce3+ and Sm3+ is investigated in detail.

The Zn2GeO4; Zn1.99GeO4:0.01Sm3+; Zn1.96GeO4:0.04Ce3+; Zn1.99−xGeO4:xCe3+, 0.01Sm3+ (x=0.00, 0.01, 0.02, 0.03, 0.04, and 0.05), and Zn1.96−yGeO4:0.04Ce3+, ySm3+ (y=0.005, 0.01, 0.03, 0.05, 0.07, and 0.09) samples are synthesized by the high-temperature solid-state reaction. As raw materials, ZnO (99.99%), GeO2 (99.999%), Sm2O3 (99.99%), and CeO2 (99.99%) are stoichiometrically weighted out. H3BO3 at 5% is employed as flux. After the ingredients are thoroughly mixed and ground in an agate mortar, the mixtures are placed into an alumina crucible. This crucible is heated at 1000°C for 12 h under air atmosphere and slowly cooled to room temperature. The phases of the obtained samples are identified by X-ray powder diffraction (XRD) with Cu Kα [λ=0.15418nm radiation at a scanning step of 0.02° in the 2θ range from 10° to 60°, operated at 36 kV and 30 mA (Rigaku Model D/max-2200)]. The PL and PL excitation (PLE) spectra are measured with a HITACHI F-7000 fluorescence spectrophotometer. The decay curves are recorded on an Edinburgh instruments FLS920 spectrometer. X-ray photoelectron spectra (XPS) are obtained using a PHI5000 Versaprobe-II; the spectrophotometer is American Standard Test Method (ASTM) calibrated and operated under a vacuum (<4×10−8Pa). As the primary excitation source, non-monochromatic Mg Kα (400 W, 1253.6 eV) radiation is used at an angle of 0°C relative to the sample’s surface normal.

The purity of all the prepared samples is systematically checked by XRD. Figure 1(a) shows the typical XRD patterns of Sm3+ and Ce3+-doped Zn2GeO4 samples. All the diffraction peaks detected can be indexed to the pure phase of Zn2GeO4 (JCDPS No.11-0687). No obvious shifting of peaks or second phase is observed at the current doping level, indicating that Sm3+ and Ce3+ ions are completely dissolved in the Zn2GeO4 host matrix. Based on the effective ionic radii^[23], Sm3+ (r=0.0958nm, CN=6) and Ce3+ (r=0.095nm, CN=6) ions are proposed to occupy the Zn2+ (r=0.074nm, CN=6) sites rather than Ge4+ (r=0.053nm, CN=6) sites, due to the fact that the sites of Ge4+ are too small for Ce3+ or Sm3+ to occupy. Due to the nonequivalent substitution, an excess of a positive charge in the lattice must be compensated. Two Ce3+ ions replace three Zn2+ ions to balance the charge of the phosphor, which create two positive defects and one negative defect. As the hole traps, Zn vacancies (VZn′′) are formed by $$\begin{gathered} 2Ce3+→3Zn2+2CeZn•+VZn′′, \\ 2Sm3+→3Zn2+2SmZn•+VZn′′. \end{gathered}$$

Fig. 1. (a) XRD patterns of Zn1.99GeO4:0.01Sm3+, Zn1.96GeO4:0.04Ce3+, Zn1.95GeO4:0.04Ce3+, 0.01Sm3+, and the JCPDS card of Zn2GeO4 (No.11-0687), (b) XPS of Zn1.96GeO4:0.04Ce3+, and the inset is the enlargement of the XPS spectrum from 860 to 900 eV.

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Figure 1(b) shows the XPS analyses of Zn2+ and Ge4+ when Ce3+ ions are doped in the phosphors. It indicates the presence of a relatively large amount of Zn, Ge, O, and Ce. In particular, the XPS of Zn1.96GeO4:0.04Ce3+ is employed to elucidate the chemical state of Ce3+ ions. The enlargement of Fig. 1(b) shown in the inset can be well fitted with a distribution of a peak centered at 885.4 eV, which is attributed to Ce3+ ions.

Figure 2 shows the PLE and PL spectra of Zn2GeO4, Sm3+, Ce3+ single-doped, and co-doped Zn2GeO4 samples at room temperature, respectively. As shown in Fig. 2(a), a broadband with a maximum at 446 nm is observed in the Zn2GeO4 host matrix, while an evidently symmetric band located around 267 nm is detected when the emission of 446 nm is monitored. The bluish PL of Zn2GeO4 derives from the recombination of the donor–acceptor, as discussed in a previous report^[19]. That is, the electrons trapped on VO• and Zni• are recombined with holes trapped on VGe and VZn′ directly or through the conduction band. Figure 2(b) presents the PLE and PL spectra of Sm3+-doped Zn2GeO4. It is found that Sm3+-doped Zn2GeO4 exhibits similar PL properties as the Zn2GeO4 host matrix under the excitation at 267 nm. The characteristic emission of Sm3+ is not obvious with the excitation of 267 nm, indicating that the energy transfer between the Zn2GeO4 host matrix and Sm3+ is not effective. When monitoring the emission at 617 nm, a weak peak located at 267 nm is detected in the PLE spectrum. Both the PL of the host and Sm3+ are not observed under the excitation of 365 nm, indicating that both the host and Sm3+ ions could not be excited effectively in this host matrix. As shown in Fig. 2(c), the emission spectrum of Zn2GeO4:Ce3+ under the excitation of 267 nm is recorded, and the emission band located at 425 nm is observed, which is similar to the spectrum of the Zn2GeO4 sample. The PLE spectra of Zn2GeO4:Ce3+ exhibits a broadband at 200–450 nm with a maximum at 365 nm that is monitored at the emission of 446 nm, which exhibits asymmetry to some extent. However, consider the fact that the Zn2GeO4 could be excited under 267 nm, as discussed above. Therefore, the emission located at 425 nm in Zn2GeO4:Ce3+ originated from both the Zn2GeO4 host and Ce3+ ions. Besides, an intense blue emission in the range of 370–650 nm with the maximum peak located at 446 nm is detected under the excitation of 365 nm, which could be ascribed to the transitions of Ce3+ ions (5d→4f). Therefore, it is safe to say that the Ce3+ ions could be excited by the energy aborted by the host matrix as a luminescent center. The PL spectrum of Zn2GeO4:Ce3+, Sm3+ under the excitation at 267 nm is shown in Fig. 2(d) (blue line). It exhibits an obvious wide peak at 446 nm and two obscure spikes at 567 and 617 nm, ascribed to Sm3+ ions (G5/24→H67/2, G5/24→H65/2), respectively^[24]. The PLE spectrum of Zn2GeO4:Ce3+, Sm3+ is obviously different from that of Zn2GeO4:Sm3+ when the emission of 617 nm is monitored. The PLE spectrum of Zn2GeO4:Ce3+, Sm3+, consists of two bands located at 267 and 365 nm, which is ascribed to the characteristic excitation of the host matrix and Ce3+ ions, respectively, as discussed above. Therefore, Ce3+ acting as a bridge, conspicuously promotes the energy transfer from the host matrix Zn2GeO4 to Sm3+, for Sm3+ ions exhibiting an insignificant intensity under the excitation of 267 nm. Besides, it is found that the emission of Ce3+ is almost insignificant, verifying that Sm3+ could be excited efficiently by the characteristic excitation transition of Ce3+ (365 nm). Thus, the energy transfer between Ce3+ to Sm3+ could be expected.

Fig. 2. (a) PLE and PL spectra of Zn2GeO4, (b) Zn1.99GeO4:0.01Sm3+, (c) Zn1.96GeO4:0.04Ce3+, and (d) Zn1.95GeO4:0.04Ce3+, 0.01Sm3+ samples, respectively.

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According to Dexter’s theory^[25], an efficient energy transfer requires a partial overlap between the excitation spectrum of the activator and the emission spectrum of the sensitizer. Figure 3 depicts the PLE of Zn2GeO4:Sm3+ (black) and the PL of Zn2GeO4:Ce3+ (red). The characteristics the PLE spectrum of Sm3+ exhibits the broad band absorption in the range of 200-430 nm, while the emission of Ce3+ ions, located at the 350–650 nm range, is attributed to the 5d→4f transition^[26]. The spectral overlap of the Sm3+ excitation and Ce3+ emission indicates that the energy transfer process from Ce3+ to Sm3+ occurs.

Fig. 3. (Color online) PLE spectrum of Zn1.99GeO4:0.01Sm3+ (black) and PL spectrum of Zn1.96GeO4:0.04Ce3+ (red).

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To further demonstrate the energy transfer from Ce3+ to Sm3+, the PL spectra of Zn1.99−xGeO4:xCe3+, 0.01Sm3+ (x=0.00, 0.01, 0.02, 0.03, 0.04, and 0.05) are shown in Fig. 4. The emission intensities of Sm3+ increase remarkably with an increasing concentration of Ce3+ and reaches the maximum at x=0.04; meanwhile, the emission intensity of Ce3+ at 446 nm is observed as almost insignificant. The characteristic emissions of Ce3+ are not observed, which suggests the effective energy transfer.

Fig. 4. PL spectra of Zn1.99−xGeO4:xCe3+, 0.01Sm3+ (x=0.00, 0.01, 0.02, 0.03, 0.04, and 0.05) samples.

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In order to illustrate the energy transfer from Ce3+ and Sm3+, the decay curves of Zn1.96−yGeO4:0.04Ce3+, ySm3+ (y=0.00, 0.005, 0.03, and 0.07) are measured, as shown in Fig. 5 (λex=365nm, λem=446nm). It is found that all of the decay curves can be well fitted by the second-order exponential decay mode as the following equation^[2]: I=A1exp(−t/τ1)+A2exp(−t/τ2),(1)where I is the luminescence intensity, A1 and A2 are fitting parameters, t is the time, τ1 and τ2 are rapid and slow lifetimes for exponential components, respectively. The values of A1, A2, τ1, and τ2 are obtained, as shown in Table 1. Based on these parameters, the average decay time τ of Ce3+ can be calculated by the following equation^[27]: τ=A1τ12+A2τ22A1τ1+A2τ2,(2)

Fig. 5. Decay curves of Zn1.96−yGeO4:0.04Ce3+, ySm3+ (y=0.00, 0.005, 0.03, and 0.07) monitored the 446 nm emission under 365 nm excitation.

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As shown in Fig. 5, the average decay times (τ) are determined to be 2.48, 2.20, 1.95, and 1.83 ns for Zn1.96−yGeO4:0.04Ce3+, ySm3+, y=0.00, 0.005, 0.03, and 0.07, respectively. The decay time of Ce3+ decreases with the increased concentration of Sm3+, which strongly confirms the existence of the energy transfer process from Ce3+ to Sm3+ in the Zn2GeO4 host.

The efficiency of the energy transfer from Ce3+ to Sm3+ can be estimated according to the following equation: η=1−τs/τs0,(3)where η means the energy transfer efficiency, τs and τs0 are on behalf of the lifetimes of Ce3+ in the absence and the presence of Sm3+, respectively. As depicted in the inset of Fig. 5, the efficiency of the energy transfer ascends gradually from 11.3% to 26.2% with the increment of the concentration of Sm3+.

The CIE chromaticity diagram of Zn2GeO4:Ce3+ and Zn1.93GeO4:0.04Ce3+, 0.03Sm3+ phosphors under 365 nm are measured and presented in Fig. 6. The CIE coordinates shifted from (0.1709, 0.1891) to (0.604, 0.3528) in Zn1.96GeO4:0.04Ce3+ and Zn1.93GeO4:0.04Ce3+, 0.03Sm3+ samples, which indicates that the emitting color of these samples changes from blue to red–orange, accordingly.

Fig. 6. CIE chromaticity diagram of Zn1.96GeO4:0.04Ce3+ and Zn1.93GeO4:0.04Ce3+, 0.03Sm3+ phosphors.

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In general, it is known that if the energy transfer belongs to the exchange interaction, the critical distance between the sensitizer and activator should be shorter than 0.3–0.4 nm. The critical distance Rc for the energy transfer from Ce3+ to Sm3+ ions can be calculated through the concentration quenching method. According to the equation proposed by Blasse^[28], Rc=2(3V4πXcN)1/3,(4)where V is the volume of the unit cell, and N is the number of the center cations in the unit cell. The crystallographic data and the above calculation are given as follows: V=0.5632nm3 and N=6. Xc is the critical concentration (the total concentration of sensitizer of Ce3+ ions and activator ions of Sm3+), where the emission of Zn1.96−yGeO4:0.04Ce3+, ySm3+ phosphors reaches the maximum. Herein, Xc=0.07 is the sum of Ce3+ concentration with 0.04 and the critical concentration of Sm3+ with 0.03. Accordingly, the critical energy transfer distance for Ce3+ and Sm3+ in the Zn2GeO4 host is calculated to be 1.3682 nm. The value is much larger than 0.4 nm, which shows that the electric multi-polar interaction rather than the exchange interaction is responsible for the energy transfer from Ce3+ to Sm3+ in the Zn2GeO4 host.

According to Dexter’s energy transfer formula of multi-polar interaction and Reisfeld’s approximation^[29], (ηs0/ηs)∝Cn/3, where ηs0 and ηs are the luminescence quantum efficiencies of the sensitizer (Ce3+) in the absence and presence of the activator (Sm3+), respectively. The relation (ηs0/ηs)∝Cn/3 can be obtained, and the value ηs0/ηs is an approximation calculated by the Is0/Is^[30]. C is the sum of the content of Ce3+ and Sm3+. n=6, 8, and 10, corresponding to the dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions. The relations between Is0/Is and Cn/3 when n=6, 8, 10 are illustrated in Fig. 7. It is obvious that n=6 fits well with the liner relationship, illustrating that the energy transfer from Ce3+ to Sm3+ is realized mainly through the dipole–dipole interaction. The error bars in Fig. 7 represent the standard deviation, and their values are less than 0.3.

Fig. 7. Dependence of Is0/Is of Ce3+ on C6/3, C8/3, and C10/3.

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In conclusion, Ce3+, Sm3+-doped Zn2GeO4 phosphors are prepared by a traditional solid-state reaction in this work. The efficient energy transfer process between Ce3+ and Sm3+ is investigated by the decay curves. The emission color changes from blue in Zn2GeO4:Ce3+ to orange–red in Zn2GeO4:Ce3+, Sm3+ via the efficient energy transfer from Ce3+ to Sm3+. Besides, Ce3+ acting as a bridge, can promote the energy transfer from Zn2GeO4 to Sm3+ inconspicuously. The results indicate that Zn2GeO4:Ce3+, Sm3+ phosphor provides a potential application as an efficient orange–red phosphor for LEDs.