Energy transfer between Ce3+ and Sm3+ in Zn2GeO4 phosphor with the native defects for light-emitting diodes

Yushuang Qi; Lei Zhao; Wenjuan Bian; Xue Yu; Xuhui Xu; Jianbei Qiu

doi:10.3788/COL201715.081601

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]. 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) ${Ce}^{3 +}$ ^[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 $d^{10}$ ion complexes, such as ${Zn}_{4} O$ compounds, ${Ca}_{3} {SnSi}_{2} O_{9}$ , GaN, and $Ba Mo O_{4}$ ^[8–11]. There also exists strong evidence for the observation of photoluminescence (PL) from transition-metal oxides. Examples are $β - {Ga}_{2} O_{3}$ , $Sr Zr O_{3}$ , $Sr Ti O_{3}$ , and ${Sr}_{2} V_{2} O_{7}$ ^[12–15]. 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. ${Li}_{2} {ZnGeO}_{4}$ : ${Mn}^{2 +}$ exhibiting a green long persistent luminescence is reported by the Shang group^[17], ${CaZnGe}_{2} O_{6} : {Mn}^{2 +}$ gives out a red luminescence as reported by the Che group, the Pan group explored a new type of ${Zn}_{3} {Ga}_{2} {Ge}_{2} O_{10} : {Cr}^{3 +}$ phosphor, which emits red at 698 nm^[18], and the Xu group investigated ${Na}_{2} {CaSn}_{2} {Ge}_{3} O_{12} : {Sm}^{3 +}$ as a reddish-orange phosphor^[19]. Among germanates, zinc germanate ( ${Zn}_{2} {GeO}_{4}$ ) is an excellent candidate due to its reasonable conductivity and high stability. ${Zn}_{2} {GeO}_{4}$ has a rhombohedral structure, which is similar to ${Zn}_{2} {SiO}_{4}$ , with ${Zn}^{2 +}$ ions at the tetrahedral sites and Ge at the octahedral sites, and it has a wide band gap of 4.68 eV^[20]. ${Zn}_{2} {GeO}_{4}$ provides a potential application in field emission displays as a novel host matrix, and few literatures focus on its persistent luminescence properties^[21]. Furthermore, ${Zn}_{2} {GeO}_{4}$ as a native defect phosphor is studied by the Qiu group^[22]. Nevertheless, there is only limited study on the PL of ${Zn}_{2} {GeO}_{4}$ , and no attempts have been made for energy transfer from the host to activators. In this work, we perform research on ${Ce}^{3 +}$ , ${Sm}^{3 +}$ -doped ${Zn}_{2} {GeO}_{4}$ phosphor, which emits a tunable color due to the effective energy transfer from the host to the activators. The PL properties of the ${Zn}_{2} {GeO}_{4}$ host matrix, ${Ce}^{3 +}$ , ${Sm}^{3 +}$ single, and co-doped samples are studied. The energy transfer process between ${Ce}^{3 +}$ and ${Sm}^{3 +}$ is investigated in detail.

The ${Zn}_{2} {GeO}_{4}$ ; ${Zn}_{1.99} {GeO}_{4} : 0.01 {Sm}^{3 +}$ ; ${Zn}_{1.96} {GeO}_{4} : 0.04 {Ce}^{3 +}$ ; ${Zn}_{1.99 - x} {GeO}_{4} : {x Ce}^{3 +}$ , $0.01 {Sm}^{3 +}$ ( $x = 0.00$ , 0.01, 0.02, 0.03, 0.04, and 0.05), and ${Zn}_{1.96 - y} {GeO}_{4} : 0.04 {Ce}^{3 +}$ , ${y Sm}^{3 +}$ ( $y = 0.005$ , 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%), ${GeO}_{2}$ (99.999%), ${Sm}_{2} O_{3}$ (99.99%), and ${CeO}_{2}$ (99.99%) are stoichiometrically weighted out. $H_{3} {BO}_{3}$ 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 $K α$ [ $λ = 0.15418 nm$ radiation at a scanning step of 0.02° in the $2 θ$ 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 ( $< 4 \times 10^{- 8} Pa$ ). As the primary excitation source, non-monochromatic Mg $K α$ (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 ${Sm}^{3 +}$ and ${Ce}^{3 +}$ -doped ${Zn}_{2} {GeO}_{4}$ samples. All the diffraction peaks detected can be indexed to the pure phase of ${Zn}_{2} {GeO}_{4}$ (JCDPS No.11-0687). No obvious shifting of peaks or second phase is observed at the current doping level, indicating that ${Sm}^{3 +}$ and ${Ce}^{3 +}$ ions are completely dissolved in the ${Zn}_{2} {GeO}_{4}$ host matrix. Based on the effective ionic radii^[23], ${Sm}^{3 +}$ ( $r = 0.0958 nm$ , $CN = 6$ ) and ${Ce}^{3 +}$ ( $r = 0.095 nm$ , $CN = 6$ ) ions are proposed to occupy the ${Zn}^{2 +}$ ( $r = 0.074 nm$ , $CN = 6$ ) sites rather than ${Ge}^{4 +}$ ( $r = 0.053 nm$ , $CN = 6$ ) sites, due to the fact that the sites of ${Ge}^{4 +}$ are too small for ${Ce}^{3 +}$ or ${Sm}^{3 +}$ to occupy. Due to the nonequivalent substitution, an excess of a positive charge in the lattice must be compensated. Two ${Ce}^{3 +}$ ions replace three ${Zn}^{2 +}$ ions to balance the charge of the phosphor, which create two positive defects and one negative defect. As the hole traps, Zn vacancies ( $V_{Z n}^{''}$ ) are formed by $2 {Ce}^{3 +} \overset{3 {Zn}^{2 +}}{\to} 2 {Ce}_{Zn}^{•} + V_{Z n}^{''}, 2 {Sm}^{3 +} \overset{3 {Zn}^{2 +}}{\to} 2 {Sm}_{Zn}^{•} + V_{Z n}^{''} .$

Figure 1.(a) XRD patterns of ${Zn}_{1.99} {GeO}_{4} : 0.01 {Sm}^{3 +}$ , ${Zn}_{1.96} {GeO}_{4} : 0.04 {Ce}^{3 +}$ , ${Zn}_{1.95} {GeO}_{4} : 0.04 {Ce}^{3 +}$ , $0.01 {Sm}^{3 +}$ , and the JCPDS card of ${Zn}_{2} {GeO}_{4}$ (No.11-0687), (b) XPS of ${Zn}_{1.96} {GeO}_{4} : 0.04 {Ce}^{3 +}$ , 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 ${Zn}^{2 +}$ and ${Ge}^{4 +}$ when ${Ce}^{3 +}$ 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 ${Zn}_{1.96} {GeO}_{4} : 0.04 {Ce}^{3 +}$ is employed to elucidate the chemical state of ${Ce}^{3 +}$ 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 ${Ce}^{3 +}$ ions.

Figure 2 shows the PLE and PL spectra of ${Zn}_{2} {GeO}_{4}$ , ${Sm}^{3 +}$ , ${Ce}^{3 +}$ single-doped, and co-doped ${Zn}_{2} {GeO}_{4}$ samples at room temperature, respectively. As shown in Fig. 2(a), a broadband with a maximum at 446 nm is observed in the ${Zn}_{2} {GeO}_{4}$ 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 ${Zn}_{2} {GeO}_{4}$ derives from the recombination of the donor–acceptor, as discussed in a previous report^[19]. That is, the electrons trapped on $V_{O}^{•}$ and $Z n_{i}^{•}$ are recombined with holes trapped on $V_{Ge}$ and $V_{Zn}^{'}$ directly or through the conduction band. Figure 2(b) presents the PLE and PL spectra of ${Sm}^{3 +}$ -doped ${Zn}_{2} {GeO}_{4}$ . It is found that ${Sm}^{3 +}$ -doped ${Zn}_{2} {GeO}_{4}$ exhibits similar PL properties as the ${Zn}_{2} {GeO}_{4}$ host matrix under the excitation at 267 nm. The characteristic emission of ${Sm}^{3 +}$ is not obvious with the excitation of 267 nm, indicating that the energy transfer between the ${Zn}_{2} {GeO}_{4}$ host matrix and ${Sm}^{3 +}$ 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 ${Sm}^{3 +}$ are not observed under the excitation of 365 nm, indicating that both the host and ${Sm}^{3 +}$ ions could not be excited effectively in this host matrix. As shown in Fig. 2(c), the emission spectrum of ${Zn}_{2} {GeO}_{4} : {Ce}^{3 +}$ 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 ${Zn}_{2} {GeO}_{4}$ sample. The PLE spectra of ${Zn}_{2} {GeO}_{4} : {Ce}^{3 +}$ 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 ${Zn}_{2} {GeO}_{4}$ could be excited under 267 nm, as discussed above. Therefore, the emission located at 425 nm in ${Zn}_{2} {GeO}_{4} : {Ce}^{3 +}$ originated from both the ${Zn}_{2} {GeO}_{4}$ host and ${Ce}^{3 +}$ 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 ${Ce}^{3 +}$ ions ( $5 d \to 4 f$ ). Therefore, it is safe to say that the ${Ce}^{3 +}$ ions could be excited by the energy aborted by the host matrix as a luminescent center. The PL spectrum of ${Zn}_{2} {GeO}_{4} : {Ce}^{3 +}$ , ${Sm}^{3 +}$ 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 ${Sm}^{3 +}$ ions ( ${}^{4}{G_{5 / 2}} \to {{}^{6}H}_{7 / 2}$ , ${}^{4}{G_{5 / 2}} \to {{}^{6}H}_{5 / 2}$ ), respectively^[24]. The PLE spectrum of ${Zn}_{2} {GeO}_{4} : {Ce}^{3 +}$ , ${Sm}^{3 +}$ is obviously different from that of ${Zn}_{2} {GeO}_{4} : {Sm}^{3 +}$ when the emission of 617 nm is monitored. The PLE spectrum of ${Zn}_{2} {GeO}_{4} : {Ce}^{3 +}$ , ${Sm}^{3 +}$ , consists of two bands located at 267 and 365 nm, which is ascribed to the characteristic excitation of the host matrix and ${Ce}^{3 +}$ ions, respectively, as discussed above. Therefore, ${Ce}^{3 +}$ acting as a bridge, conspicuously promotes the energy transfer from the host matrix ${Zn}_{2} {GeO}_{4}$ to ${Sm}^{3 +}$ , for ${Sm}^{3 +}$ ions exhibiting an insignificant intensity under the excitation of 267 nm. Besides, it is found that the emission of ${Ce}^{3 +}$ is almost insignificant, verifying that ${Sm}^{3 +}$ could be excited efficiently by the characteristic excitation transition of ${Ce}^{3 +}$ (365 nm). Thus, the energy transfer between ${Ce}^{3 +}$ to ${Sm}^{3 +}$ could be expected.

Figure 2.(a) PLE and PL spectra of ${Zn}_{2} {GeO}_{4}$ , (b) ${Zn}_{1.99} {GeO}_{4} : 0.01 {Sm}^{3 +}$ , (c) ${Zn}_{1.96} {GeO}_{4} : 0.04 {Ce}^{3 +}$ , and (d) ${Zn}_{1.95} {GeO}_{4} : 0.04 {Ce}^{3 +}$ , $0.01 {Sm}^{3 +}$ 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 ${Zn}_{2} {GeO}_{4} : {Sm}^{3 +}$ (black) and the PL of ${Zn}_{2} {GeO}_{4} : {Ce}^{3 +}$ (red). The characteristics the PLE spectrum of ${Sm}^{3 +}$ exhibits the broad band absorption in the range of 200-430 nm, while the emission of ${Ce}^{3 +}$ ions, located at the 350–650 nm range, is attributed to the $5 d \to 4 f$ transition^[26]. The spectral overlap of the ${Sm}^{3 +}$ excitation and ${Ce}^{3 +}$ emission indicates that the energy transfer process from ${Ce}^{3 +}$ to ${Sm}^{3 +}$ occurs.

Figure 3.(Color online) PLE spectrum of ${Zn}_{1.99} {GeO}_{4} : 0.01 {Sm}^{3 +}$ (black) and PL spectrum of ${Zn}_{1.96} {GeO}_{4} : 0.04 {Ce}^{3 +}$ (red).

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To further demonstrate the energy transfer from ${Ce}^{3 +}$ to ${Sm}^{3 +}$ , the PL spectra of ${Zn}_{1.99 - x} {GeO}_{4} : {x Ce}^{3 +}$ , $0.01 {Sm}^{3 +}$ ( $x = 0.00$ , 0.01, 0.02, 0.03, 0.04, and 0.05) are shown in Fig. 4. The emission intensities of ${Sm}^{3 +}$ increase remarkably with an increasing concentration of ${Ce}^{3 +}$ and reaches the maximum at $x = 0.04$ ; meanwhile, the emission intensity of ${Ce}^{3 +}$ at 446 nm is observed as almost insignificant. The characteristic emissions of ${Ce}^{3 +}$ are not observed, which suggests the effective energy transfer.

Figure 4.PL spectra of ${Zn}_{1.99 - x} {GeO}_{4} : {x Ce}^{3 +}$ , $0.01 {Sm}^{3 +}$ ( $x = 0.00$ , 0.01, 0.02, 0.03, 0.04, and 0.05) samples.

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In order to illustrate the energy transfer from ${Ce}^{3 +}$ and ${Sm}^{3 +}$ , the decay curves of ${Zn}_{1.96 - y} {GeO}_{4} : 0.04 {Ce}^{3 +}$ , ${y Sm}^{3 +}$ ( $y = 0.00$ , 0.005, 0.03, and 0.07) are measured, as shown in Fig. 5 ( $λ_{e x} = 365 nm$ , $λ_{e m} = 446 nm$ ). 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]: $I = A_{1} \exp (- t / τ_{1}) + A_{2} \exp (- t / τ_{2}),$ (1)where $I$ is the luminescence intensity, $A_{1}$ and $A_{2}$ are fitting parameters, $t$ is the time, $τ_{1}$ and $τ_{2}$ are rapid and slow lifetimes for exponential components, respectively. The values of $A_{1}$ , $A_{2}$ , $τ_{1}$ , and $τ_{2}$ are obtained, as shown in Table 1. Based on these parameters, the average decay time $τ$ of ${Ce}^{3 +}$ can be calculated by the following equation^[27]: $τ = \frac{A_{1} τ_{1}^{2} + A_{2} τ_{2}^{2}}{A_{1} τ_{1} + A_{2} τ_{2}},$ (2)

Table 1. Decay Kinetics for Zn1.96−yGeO4:0.04Ce3+, ySm3+ (y=0.00, 0.005, 0.03, and 0.07) Phosphors

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Table 1. Decay Kinetics for Zn1.96−yGeO4:0.04Ce3+, ySm3+ (y=0.00, 0.005, 0.03, and 0.07) Phosphors

τ1 (ns) A1 τ2 (ns) A2
1.4559 1142.543 13.2079 11.988
1.2594 1178.256 13.1723 9.651
1.2763 1147.682 11.5937 8.7932
1.422 1243.268 11.2764 9.143

Figure 5.Decay curves of ${Zn}_{1.96 - y} {GeO}_{4} : 0.04 {Ce}^{3 +}$ , ${y Sm}^{3 +}$ ( $y = 0.00$ , 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 ${Zn}_{1.96 - y} {GeO}_{4} : 0.04 {Ce}^{3 +}$ , ${y Sm}^{3 +}$ , $y = 0.00$ , 0.005, 0.03, and 0.07, respectively. The decay time of ${Ce}^{3 +}$ decreases with the increased concentration of ${Sm}^{3 +}$ , which strongly confirms the existence of the energy transfer process from ${Ce}^{3 +}$ to ${Sm}^{3 +}$ in the ${Zn}_{2} {GeO}_{4}$ host.

The efficiency of the energy transfer from ${Ce}^{3 +}$ to ${Sm}^{3 +}$ can be estimated according to the following equation: $η = 1 - τ_{s} / τ_{s 0},$ (3)where $η$ means the energy transfer efficiency, $τ_{s}$ and $τ_{s 0}$ are on behalf of the lifetimes of ${Ce}^{3 +}$ in the absence and the presence of ${Sm}^{3 +}$ , 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 ${Sm}^{3 +}$ .

The CIE chromaticity diagram of ${Zn}_{2} {GeO}_{4} : {Ce}^{3 +}$ and ${Zn}_{1.93} {GeO}_{4} : 0.04 {Ce}^{3 +}$ , $0.03 {Sm}^{3 +}$ 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 ${Zn}_{1.96} {GeO}_{4} : 0.04 {Ce}^{3 +}$ and ${Zn}_{1.93} {GeO}_{4} : 0.04 {Ce}^{3 +}$ , $0.03 {Sm}^{3 +}$ samples, which indicates that the emitting color of these samples changes from blue to red–orange, accordingly.

Figure 6.CIE chromaticity diagram of ${Zn}_{1.96} {GeO}_{4} : 0.04 {Ce}^{3 +}$ and ${Zn}_{1.93} {GeO}_{4} : 0.04 {Ce}^{3 +}$ , $0.03 {Sm}^{3 +}$ 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 $R_{c}$ for the energy transfer from ${Ce}^{3 +}$ to ${Sm}^{3 +}$ ions can be calculated through the concentration quenching method. According to the equation proposed by Blasse^[28], $R_{c} = 2 {(\frac{3 V}{4 π X_{c} N})}^{1 / 3},$ (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: $V = 0.5632 {nm}^{3}$ and $N = 6$ . $X_{c}$ is the critical concentration (the total concentration of sensitizer of ${Ce}^{3 +}$ ions and activator ions of ${Sm}^{3 +}$ ), where the emission of ${Zn}_{1.96 - y} {GeO}_{4} : 0.04 {Ce}^{3 +}$ , ${y Sm}^{3 +}$ phosphors reaches the maximum. Herein, $X_{c} = 0.07$ is the sum of ${Ce}^{3 +}$ concentration with 0.04 and the critical concentration of ${Sm}^{3 +}$ with 0.03. Accordingly, the critical energy transfer distance for ${Ce}^{3 +}$ and ${Sm}^{3 +}$ in the ${Zn}_{2} {GeO}_{4}$ 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 ${Ce}^{3 +}$ to ${Sm}^{3 +}$ in the ${Zn}_{2} {GeO}_{4}$ host.

According to Dexter’s energy transfer formula of multi-polar interaction and Reisfeld’s approximation^[29], $(η_{s 0} / η_{s}) \propto C^{n / 3}$ , where $η_{s 0}$ and $η_{s}$ are the luminescence quantum efficiencies of the sensitizer ( ${Ce}^{3 +}$ ) in the absence and presence of the activator ( ${Sm}^{3 +}$ ), respectively. The relation $(η_{s 0} / η_{s}) \propto C^{n / 3}$ can be obtained, and the value $η_{s 0} / η_{s}$ is an approximation calculated by the $I_{s 0} / I_{s}$ ^[30]. C is the sum of the content of ${Ce}^{3 +}$ and ${Sm}^{3 +}$ . $n = 6$ , 8, and 10, corresponding to the dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions. The relations between $I_{s 0} / I_{s}$ and $C^{n / 3}$ 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 ${Ce}^{3 +}$ to ${Sm}^{3 +}$ 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.

Figure 7.Dependence of $I_{s 0} / I_{s}$ of ${Ce}^{3 +}$ on $C^{6 / 3}$ , $C^{8 / 3}$ , and $C^{10 / 3}$ .

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In conclusion, ${Ce}^{3 +}$ , ${Sm}^{3 +}$ -doped ${Zn}_{2} {GeO}_{4}$ phosphors are prepared by a traditional solid-state reaction in this work. The efficient energy transfer process between ${Ce}^{3 +}$ and ${Sm}^{3 +}$ is investigated by the decay curves. The emission color changes from blue in ${Zn}_{2} {GeO}_{4} : {Ce}^{3 +}$ to orange–red in ${Zn}_{2} {GeO}_{4} : {Ce}^{3 +}$ , ${Sm}^{3 +}$ via the efficient energy transfer from ${Ce}^{3 +}$ to ${Sm}^{3 +}$ . Besides, ${Ce}^{3 +}$ acting as a bridge, can promote the energy transfer from ${Zn}_{2} {GeO}_{4}$ to ${Sm}^{3 +}$ inconspicuously. The results indicate that ${Zn}_{2} {GeO}_{4} : {Ce}^{3 +}$ , ${Sm}^{3 +}$ phosphor provides a potential application as an efficient orange–red phosphor for LEDs.

Category: Materials

Received: Apr. 6, 2017

Accepted: May. 18, 2017

Published Online: Jul. 20, 2018

The Author Email: Xue Yu (yuyu6593@126.com)

DOI:10.3788/COL201715.081601

Table 1. Decay Kinetics for Zn1.96−yGeO4:0.04Ce3+, ySm3+ (y=0.00, 0.005, 0.03, and 0.07) Phosphors

Table 1. Decay Kinetics for Zn1.96−yGeO4:0.04Ce3+, ySm3+ (y=0.00, 0.005, 0.03, and 0.07) Phosphors