Zn_2−aGeO₄:aRE and Zn₂Ge_1−aO₄:aRE (RE = Ce³⁺, Eu³⁺, Tb³⁺, Dy³⁺): 4f–4f and 5d–4f transition luminescence of rare earth ions under different substitution

Abstract

Generally, luminescent properties of rare earth ions doped host can be tuned by controlling the host composition, that is, when substituted for different cations of host, the rare earths ions can present different characteristics. In this research, Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺ can show two different emission bands due to the 5d–4f transitions of Ce³⁺ ions. Moreover, the CIE chromaticity coordinates and the luminescence photographs of Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺ exhibit different characteristics. The emission bands appear blueshift and redshift due to the combination of crystal field splitting and nephelauxetic effect. However, for Zn_2−aGeO₄:aR and Zn₂Ge_1−aO₄:aR (R = Eu³⁺, Tb³⁺, Dy³⁺), the 4f–4f characteristic transitions of Tb³⁺, Eu³⁺ and Dy³⁺ are observed, and the emission and excitation peaks are not influenced by the crystal field.

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1 Introduction

Recently, tuning luminescence appearance has been widely investigated, that is, to tune the spectral position and shape and improve the thermal stability and the luminescence efficiency of phosphors.^1,2 Many strategies can be used, such as chemical unit cosubstitution, neighboring-cation substitution, cation-size-mismatch, nanosegregation and neighbor-cation control, mixing of nanophases, site-preferential occupancy, and so on.^3–9 Herein, most of the strategies are the control of substituted cations, they can exhibit different luminescence when the doping ions substitute for the cations of different position. For example, the superposition of emission spectra that cover the entire visible spectrum can be obtained due to that Ce³⁺ ions substitute for two different sites of Ca²⁺ sites (eight- and seven-coordination) in Ca₄F₂Si₂O₇ phosphor.¹⁰ (Ca_{0.993−x−y}Mg_xSr_y)₉Y(PO₄)₇:0.007Eu²⁺ can show a continuous blueshift emission with increasing Mg²⁺ concentration and a redshift with increasing Sr²⁺ level.¹¹ Therefore, the effect of substitution on luminescence is worth being explored.

There are some reports focused on the material Zn₂GeO₄ due to its excellent properties, such as the simplicity of the crystal structure, high stability, decomposition of water, negative thermal expansion and electroluminescence.^12–16 Zn₂GeO₄ with a wide band gap (4.4 eV) has potential application in photocatalysts, optoelectronic devices and displays.^17–24 Zn₂GeO₄:Eu³⁺ nanocrystals show a white light emission, which displays a potential application for white light-emitting diodes (white LEDs).²⁵ Cr³⁺, Yb³⁺, Er³⁺ doped zinc gallogermanate nanoparticles exhibit an excellent superlong near-infrared persistent luminescence.²⁶ With respect to the above, we select Zn₂GeO₄ as host, and attempt to investigate the effect of rare earths ions on luminescence properties. We select Eu³⁺, Tb³⁺, Dy³⁺ based on 4f–4f transition and crystal field sensitive Ce³⁺ (5d–4f transition) as doping ions, and Zn_2−aGeO₄:aRE³⁺ and Zn₂Ge_1−xO₄:aRE³⁺ (RE = Ce³⁺, Eu³⁺, Tb³⁺, Dy³⁺) were synthesized using a high temperature solid-state method. There is a blueshift and redshift can be obtained when Ce³⁺ doped in Zn₂GeO₄, due to that positions of the 5d levels are greatly influenced by the crystal field. We find that the emission and excitation peaks are not influenced, however, the relative emission and excitation intensities of phosphors can be tuned when Eu³⁺, Tb³⁺, Dy³⁺ substitute for Zn²⁺ and Ge⁴⁺ in Zn₂GeO₄.

2 Experimental

2.1 Sample preparation

A series of phosphors with composition Zn_2−aGeO₄:aRE³⁺ and Zn₂Ge_1−aO₄:aRE³⁺ (RE = Ce³⁺, Eu³⁺, Tb³⁺, Dy³⁺) were synthesized by a high-temperature solid-state method. The starting materials, including reagent grade ZnO, GeO₂ (99.99%), CeO₂ (99.99%), Eu₂O₃ (99.99%), Tb₄O₇ (99.99%), Dy₂O₃ (99.99%), were weighed in stoichiometric proportion, mixed homogeneously and ground by an agate mortar. The resultant mixtures are heated up to 850 °C held for 2 h in air, and then cooled down to room temperature naturally with the furnace. After intermediately ground to improve the homogeneity, the mixtures were sintered at 1350 °C for 2.5 h in air to get the final phosphors.

2.2 Characterization

The phase formation of samples were characterized by X-ray powder diffraction (XRD) performed on a Bruker D8 X-ray diffractometer with Ni-filtered Cu Kα radiation (λ = 0.15405 nm), operating at 40 mA, 40 kV. The XRD data for Rietveld structure analysis were collected over the 2θ ranges from 10° to 80° in a step-mode with step length were 0.02°. To analyze the chemical composition of phosphors, electron-dispersive X-ray (EDX) data were collected by a Nova NanoSEM 650 with an accelerating voltage of 10 kV. Furthermore, the scanning electron micrograph (SEM) was used to collect images for investigating particle morphology. Room-temperature photoluminescence spectra of samples were recorded with a Hitachi F-4600 fluorescence spectrophotometer using a 450 W Xe lamp as the excitation source, with a scanning wavelength from 200 to 700 nm, scanning at 240 nm min⁻¹. The diffuse reflection (DR) spectra were measured by a Hitachi U4100 UV-VIS-NIR spectroscopy using BaSO₄ as a reference, with a scanning wavelength from 200 to 700 nm. Luminescence decay curves of samples were collected by a Horiba FL-4600 fluorescence spectrophotometer using a Xe lamp as the excitation source. Luminescence decay curves of Ce³⁺ were measured using a 265 nm nano-LED as the excitation source. The Commission International de I'Eclairage (CIE) coordinates for all samples were recorded with a PMS-80 UV-VIS-NEAR IR spectra analysis system.

3 Results and discussion

3.1 Phase formation and structure

The XRD patterns of Zn_2−aGeO₄:aRE and Zn₂Ge_1−aO₄:aRE (RE = Ce³⁺, Eu³⁺, Tb³⁺, Dy³⁺) are measured, and similar diffraction patterns are observed for each sample. As a representative, we only select the results of Zn_1.99GeO₄:0.01RE and Zn₂Ge_0.99O₄:0.01RE. The XRD patterns of Zn_1.99GeO₄:0.01Ce³⁺ and Zn₂Ge_0.99O₄:0.01Ce³⁺ are used to perform the Rietveld refinement with General Structure Analysis System (GASA) software,²⁷ as shown in Fig. 1a and b, respectively. All of samples are verified to comprise a single phase and they are consistent with the standard file of Zn₂GeO₄ (JCPDS no. 11-0687). Zn₂GeO₄ shares a hexagonal structure with space group R3̄, a = b = 14.231 Å, c = 9.53 Å, α = β = 90°, γ = 120° and V = 1671.4 ų, and the structure of Zn₂GeO₄ viewed from [001] is shown in the inset of Fig. 1a. There are three crystallographic positions of cations in the unit cell: two independent 4-fold coordinated Zn²⁺ sites and a 4-fold coordinated Ge⁴⁺ site in the crystal structure of Zn₂GeO₄, as shown in the inset of Fig. 1b. Fig. 2 illustrates that the representative XRD patterns of rare earth ions R (R = Eu³⁺, Tb³⁺, Dy³⁺) substitute for Zn²⁺ and Ge⁴⁺ in Zn₂GeO₄, respectively, as well as the standard pattern of JCPDS no. 11-0687. It is apparent that all the diffraction peaks are well assigned to the pure phase of Zn₂GeO₄ without any impurity.

Fig. 1 X-ray Rietveld refinement for Zn_1.99GeO₄:0.01Ce³⁺ (a) and Zn₂Ge_0.99O₄:0.01Ce³⁺ (b), the inset shows the crystal structure of Zn₂GeO₄.

Fig. 2 (a) The XRD patterns of Zn_1.99GeO₄:0.01Eu³⁺, Zn_1.99GeO₄:0.01Tb³⁺, Zn_1.99GeO₄:0.01Dy³⁺, and the standard PDF card (JCPDS no. 11-0687). (b) The XRD patterns of Zn₂Ge_0.99O₄:0.01Eu³⁺, Zn₂Ge_0.99O₄:0.01Tb³⁺ and Zn₂Ge_0.99O₄:0.01Dy³⁺, and the standard PDF card (JCPDS no. 11-0687).

Table 1 shows the cell parameters and volumes of Zn_1.99GeO₄:0.01RE and Zn₂Ge_0.99O₄:0.01RE (RE = Ce³⁺, Eu³⁺, Tb³⁺, Dy³⁺). It is seen that the cell parameters and volumes of Zn_1.99GeO₄:0.01RE and Zn₂Ge_0.99O₄:0.01RE are larger than that of Zn₂GeO₄, due to that the radii of RE are larger than that of Zn²⁺ and Ge⁴⁺. For the Zn²⁺ and Ge⁴⁺ ions, the ionic radii are 0.60 Å (Zn²⁺, CN = 4), and 0.58 Å (Ge⁴⁺, CN = 4), respectively. Therefore, the cell parameters and volumes of Zn₂Ge_0.99O₄:0.01RE are larger than that of Zn_1.99GeO₄:0.01RE because the radius of Ge⁴⁺ is smaller than that of Zn²⁺. SEM images of Zn_1.99GeO₄:0.01RE and Zn₂Ge_0.99O₄:0.01RE are measured and the similar shapes are observed for each sample. As a representative, Fig. 3 shows the SEM images of Zn_1.99GeO₄:0.01Ce³⁺ and Zn₂Ge_0.99O₄:0.01Ce³⁺. It is clearly seen that these samples have ball shapes with micrometer size, and the EDX spectra confirm the presence of Zn, Ge, Ce and O in Zn_1.99GeO₄:0.01Ce³⁺ and Zn₂Ge_0.99O₄:0.01Ce³⁺.

Table 1 Refined main structure of Zn_1.99GeO₄:0.01RE and Zn₂Ge_0.99O₄:0.01RE (RE = Ce³⁺, Eu³⁺, Tb³⁺, Dy³⁺) samples derived from the GSAS refinement of XRD data

Sample	RE	Cell parameters		Volume
		a/b (Å)	c (Å)	V (Å^3)
Zn1.99GeO4:0.01RE	Ce^3+	14.238	9.528	1672.83
	Eu^3+	14.238	9.524	1671.06
	Tb^3+	14.238	9.526	1672.40
	Dy^3+	14.238	9.525	1672.38
Zn2Ge0.99O4:0.01RE	Ce^3+	14.275	9.550	1685.50
	Eu^3+	14.259	9.541	1680.10
	Tb^3+	14.260	9.545	1680.83
	Dy^3+	14.247	9.535	1676.05

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Fig. 3 SEM images and EDX spectra of Zn_1.99GeO₄:0.01Ce³⁺ (a) and Zn₂Ge_0.99O₄:0.01Ce³⁺ (b).

3.2 Luminescent property

In order to further verify the influence of host compositions on luminescence and crystal field, we select Ce³⁺ as the doping ion, due to that the positions of the 4fⁿ⁻¹5d levels are greatly influenced by the crystal field interaction. According to our previous reports,²⁸ Zn₂GeO₄ exhibits a broad emission band ranges from 350 to 650 nm due to the existence of O vacancy , single ionized Zn interstitial , Ge vacancy (V_Ge) and ionized Zn , and the excitation band locates at 270 nm. The diffuse reflection (DR) spectra of Zn_1.92GeO₄:0.08Ce³⁺ and Zn₂Ge_0.92O₄:0.08Ce³⁺ are shown in Fig. 4a and b, respectively, it can be seen that the absorption bands extending from 200 to 350 nm are attributed to the native defects of host and 4f–5d transitions of Ce³⁺ ion. The excitation spectra (λ_em = 510/515 nm) show a absorption band extending from 200 to 300 nm with the maxima at 270 nm, which are attributed to the electronic transition from the 4f ground state to the 5d excited state of the Ce³⁺ ion.^29,30 Besides that, the excitation spectrum of Zn_1.92GeO₄:0.08Ce³⁺ exhibits another band, peaking at 310 nm monitored at 510 nm, which is due to that Ce³⁺ ions occupy two independent 4-fold coordinated Zn²⁺ sites, as shown in the inset of Fig. 4a. Meanwhile, DR spectra ranging from 300 to 350 nm show different trends. Under 270 nm excitation, an emission band of Zn_1.92GeO₄:0.08Ce³⁺ was fitted by a sum of three Gaussian peaks in which two are attributed to the Ce³⁺ band centered at 459 (21 786 cm⁻¹) nm and 510 (19 608 cm⁻¹) nm with an energy difference of 2178 cm⁻¹, and another Gaussian peak is assigned to the Zn₂GeO₄ host emission band centered at 419 nm. The emission spectra of Zn₂Ge_0.92O₄:0.08Ce³⁺ was deconvoluted using Gaussian profiles into two Gaussian peaks centered at 463 (21 598 cm⁻¹) nm and 515 (19 418 cm⁻¹) nm with an energy difference of 2180 cm⁻¹ and another Gaussian peak at 420 nm, which is attributed to the Zn₂GeO₄ host. The energy differences of Zn_1.92GeO₄:0.08Ce³⁺ and Zn₂Ge_0.92O₄:0.08Ce³⁺ are found to be in good agreement with the theoretical energy value (2000 cm⁻¹), due to the ground state splitting between the ²F_7/2 and ²F_5/2 levels of Ce³⁺ ions.³¹

Fig. 4 Diffuse reflection (DR), emission and excitation spectra of Zn_1.92GeO₄:0.08Ce³⁺ (a) and Zn₂Ge_0.92O₄:0.08Ce³⁺ (b), the inset shows the diagram of substitution.

The emission intensities of the three Gaussian peaks as a function of doping concentration (x) in the Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺ are illustrated in Fig. 5a–f, respectively. The emission intensities first increase and then decrease with the doping level, and the optimal values of 0.01 and 0.03 are obtained for Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺, respectively. The emission intensities decrease due to the concentration quenching effect. The concentration quenching characteristics of Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺ can be further understood by evaluating the critical distance (R_c) of Ce³⁺ ions. The R_c value can be estimated by following formula:³²

where V denotes the volume of the unit cell, x_c is the critical doping concentration, and N represents the number of total doping sites in the unit cell. Thus, the R_c was calculated to be 26.16 Å and 18.14 Å for the Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺, respectively. For the exchange interaction, the R_c is around 5 Å (forbidden transition), and the emission and excitation spectra do not overlap well. Based on the Dexter theory, we deduce that electric multipolar interaction leads to the concentration quenching between the two nearest activator centers.³³

Fig. 5 Gaussian peak fittings of Zn_2−xGeO₄:xCe³⁺ at Peak 1 = 419 nm (a), Peak 2 = 459 (b) nm, Peak 3 = 510 nm (c) and Zn₂Ge_1−xO₄:xCe³⁺ at Peak 4 = 420 nm (d), Peak 5 = 463 (e) nm, Peak 3 = 515 nm (f).

Fig. 6a–f show the normalized spectra of Zn_2−xGeO₄:xCe³⁺ at Peak 1 = 419 nm, Peak 2 = 459 nm, Peak 3 = 510 nm and Zn₂Ge_1−xO₄:xCe³⁺ at Peak 4 = 420 nm, Peak 5 = 463 nm, Peak 6 = 515 nm, respectively. For the host emission bands (Peak 1 and Peak 4), they show blueshift at first and then appear redshift while the concentration of Ce³⁺ increases, which illustrates that the native defects have influence on the doping ions due to mismatched valence. For Zn_2−xGeO₄:xCe³⁺, the emission bands of Peak 2 and Peak 3 shift to the lower wavelength for x ≤ 0.02. The blueshift could be explained by a higher 4f⁶d¹ band position due to the nephelauxetic effect with doping ions increasing.^34,35 The electronegativity of Zn, Ge and Ce are 1.65, 2.01 and 1.12, respectively. The difference of electronegativity between cation and anion increases due to that the electronegativity of Ce is lower than that of Zn and Ge. Hence, the covalency of Zn_2−xGeO₄:xCe³⁺ decreases with increase Ce³⁺ concentration, which causes a blueshift of Ce³⁺ emission due to the nephelauxetic effect. However, the Ce³⁺ emission bands appear a redshift for x > 0.02 due to the crystal field spitting effect. Generally, the crystal field splitting (D_q) trends with bond length can be determined by the following equation:^36–39

where D_q is the magnitude of the 5d energy level separation, Z represents the anion charge or valence, e is the electron charge, r is the radius of the d wavefunction, and R is the bond length. The band length of Ce–O will decrease due to that the radius of Ce³⁺ is larger than that of Zn²⁺, and the crystal field splitting will become stronger, which causes a redshift. Fig. 7 shows the schematic diagram for the effects of crystal field spitting and nephelauxetic effect on the energy level of a Ce³⁺ ion. In this system, the shift of emission band is a combination of crystal field spitting and nephelauxetic effect. The emission band appears blueshift when nephelauxetic effect plays a dominant role, on the contrary, it shows redshift.

Fig. 6 Normalized spectra of Zn_2−xGeO₄:xCe³⁺ at Peak 1 = 419 nm (a), Peak 2 = 459 (b) nm, Peak 3 = 510 nm (c) and Zn₂Ge_1−xO₄:xCe³⁺ at Peak 4 = 420 nm (d), Peak 5 = 463 (e) nm, Peak 6 = 515 nm (f).

Fig. 7 Schematic mechanism accounting for the blueshift and redshift of Ce³⁺ emission.

For Zn₂Ge_1−xO₄:xCe³⁺, the emission bands of Peak 5 and 6 show a blueshift for x ≤ 0.01, and then appear a redshift with the increment of doping concentration. However, the emission band of Peak 6 again shows a blueshift for x ≥ 0.05. The reason is that there are two effects influence the shift, that is, blueshift and redshift are due to that nephelauxetic effect and crystal field spitting effect are the dominant type, respectively. As compared to Zn_2−xGeO₄:xCe³⁺, the different shift is caused by different crystal field splitting. Ce³⁺ ions doped in Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺ by substituting Zn²⁺ and Ge⁴⁺ ions, because the radius of Ge⁴⁺ (0.58 Å) is smaller than that of Zn²⁺ (0.60 Å), which results in a smaller crystal field splitting in Zn₂Ge_1−xO₄:xCe³⁺.

Fig. 8a and b show the decay curves of Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺ monitored at 419 and 420 nm, respectively, and the lifetime (τ) of host luminescence can be expressed as⁴⁰

where I(t) is the lifetime dependent luminescence intensity at 419 and 420 nm. The corresponding lifetimes are calculated to be 1.95 ms, 1.98 ms, 2.06 ms, 1.89 ms and 1.85 ms for Zn_2−xGeO₄:xCe³⁺ phosphors with x = 0.001, 0.01, 0.03, 0.08 and 0.10, respectively. For Zn₂Ge_1−xO₄:xCe³⁺, the lifetimes are 1.95 ms, 1.96 ms, 1.58 ms, 1.53 ms and 1.51 ms, corresponding to x = 0.001, 0.01, 0.03, 0.08 and 0.10, respectively. According to our previous study, the lifetime of Zn₂GeO₄ host emission is 1.94 ms.²⁸ Hence, the emission bands centered at 419 and 420 nm of Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺ are attributed to Zn₂GeO₄ host. The lifetimes are first increase along with the doping concentration, and then decrease when the doping levels exceed their respective quenching concentrations. Fig. 9a and b show the decay curves of Ce³⁺ for Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺ monitored at 510 and 515 nm, respectively. All decay curves were fitted employing a double exponential decay function:⁴¹

I(t) = I₀ + A₁e^−t/τ₁ + A₂e^−t/τ₂ (4)

where I(t) is luminescence intensity; A₁ and A₂ are constants; t is time; and τ₁ and τ₂ are the lifetimes for the exponential components. The decay processes of these samples can be further evaluated by the average lifetime values (τ_av).

τ_av = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (5)

Fig. 8 Decay curves of Zn_2−xGeO₄:xCe³⁺ monitored at 419 nm (a) and Zn₂Ge_1−xO₄:xCe³⁺ monitored at 420 nm (b).

Fig. 9 Decay curves of Zn_2−xGeO₄:xCe³⁺ monitored at 510 nm (a) and Zn₂Ge_1−xO₄:xCe³⁺ monitored at 515 nm (b).

It can be seen that the lifetimes increase first, and then decrease with the increasing Ce³⁺ doping concentration due to the concentration quenching, which is matched well with the emission spectra characteristics. The lifetimes of Zn₂Ge_1−xO₄:xCe³⁺ are larger than that of Zn_2−xGeO₄:xCe³⁺, because Zn₂Ge_1−xO₄:xCe³⁺ are in a relatively weak crystal field. The reason is that Ce³⁺ ions substitute for Zn²⁺ and Ge⁴⁺ ions in Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺, and the radius of Ge⁴⁺ is smaller than that of Zn²⁺.

As shown in Fig. 10, the CIE chromaticity coordinates and corresponding photographs of the Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺ (x = 0.01, 0.03 and 0.05) under 254 nm UV lamps. The luminescence colors of Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺ are blue and blue green, respectively. The photographs matched with their color tones, which are consistent with the emission spectra.

Fig. 10 CIE chromaticity diagram of Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺ (x = 0.01, 0.03 and 0.05) and photographs under 254 nm UV lamps.

The luminescence of different doping level for R (R = Eu³⁺, Tb³⁺, Dy³⁺) doped Zn_2−aGeO₄ and Zn₂Ge_1−aO₄ are measured, and similar luminescence characteristics are observed for each sample. As a representative, we only select the results of a = 0.08. Fig. 11 exhibits the diffuse reflection (DR), excitation and emission spectra of Zn_1.92GeO₄:0.08Eu³⁺, Zn₂Ge_0.92O₄:0.08Eu³⁺, Zn_1.92GeO₄:0.08Tb³⁺, Zn₂Ge_0.92O₄:0.08Tb³⁺, Zn_1.92GeO₄:0.08Dy³⁺ and Zn₂Ge_0.92O₄:0.08Dy³⁺. It can be seen from Fig. 11a and b that all of the excitation spectra are consistent with the DR spectra. The excitation spectra of Zn_1.92GeO₄:0.08Eu³⁺, Zn₂Ge_0.92O₄:0.08Eu³⁺ (λ_em = 620 nm) show a broad absorption band peaking at 270 nm, which is attributed to charge-transfer band (CTB) between Eu³⁺ and the surrounding oxygen ions,^42–44 and other sharp bands are due to the ⁷F₀ → ⁵L₆, ⁵D₂, ⁵D₁ transition of Eu³⁺. Comparing to the excitation spectra of Zn_1.92GeO₄:0.08Eu³⁺ and Zn₂Ge_0.92O₄:0.08Eu³⁺, it can be seen that the ratio of excitation bands intensities are changed, which is caused by the different crystal field due to different host compositions. In addition, Eu³⁺ doped Zn_1.92GeO₄ and Zn₂Ge_0.92O₄ show the similar emission peaks, including a broad Zn₂GeO₄ host emission band peaking at 430 nm and several bands with peaks at 584, 594, 615 and 625 nm due to the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3) transition of Eu³⁺. For Zn_1.92GeO₄:0.08Tb³⁺ and Zn₂Ge_0.92O₄:0.08Tb³⁺, the excitation bands peaking at 257 nm are attributed to the spin-allowed transition from the 4f to the 5d state of Tb³⁺, and 270 nm band is ascribed to Zn₂GeO₄ host due to the energy transfer from Zn₂GeO₄ to Tb³⁺,²⁸ as shown in Fig. 11c and d. Under excitation of 270 nm, phosphors show a host emission band peaking at 430 nm, and four emission bands centered at 494, 554, 590 and 629 nm due to the ⁵D₄ → ⁷F₆, ⁷F₅, ⁷F₄, ⁷F₃ transitions of Tb³⁺, respectively. Zn_1.92GeO₄:0.08Tb³⁺ and Zn₂Ge_0.92O₄:0.08Tb³⁺ exhibit similar emission and excitation bands due to the energy transfer. Fig. 11e and f show the excitation spectra of Zn_1.92GeO₄:0.08Dy³⁺ and Zn₂Ge_0.92O₄:0.08Dy³⁺, and the excitation spectra consist of a CTB of Dy³⁺ centered at 270 nm and two bands centered at 315 and 385 nm, corresponding to the transition from ⁶H_15/2 to ⁶P_3/2 and ⁴F_7/2, respectively. The emission band centered at 430 nm is attributed to the Zn₂GeO₄ host, and other bands peaking at 530 and 586 nm are due to ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺. The ⁴F_9/2 → ⁶H_13/2 transition is corresponding to the forced electric dipole transition (J = 2), therefore it is strongly influenced by the chemical environment surrounding of Dy³⁺.⁴⁵ Hence, the 530 nm emission band is caused by the Stark energy splitting. There is an obvious difference observed by comparing Dy³⁺ doped Zn_1.92GeO₄ and Zn₂Ge_0.92O₄ that the relative intensity of emission band peaking at 530 m in Zn_1.92GeO₄:0.08Dy³⁺ is greater than that of in Zn₂Ge_0.92O₄:0.08Dy³⁺, which is caused by the different crystal field environment due to different host compositions. To summarize, there is no influence on the position of emission and excitation bands, due to that the RE ions based on 4f–4f transitions (Tb³⁺, Eu³⁺, Dy³⁺) are less affected by the crystal field environment.

Fig. 11 Diffuse reflection (DR), emission and excitation spectra of Zn_1.92GeO₄:0.08Eu³⁺ (a), Zn₂Ge_0.92O₄:0.08Eu³⁺ (b), Zn_1.92GeO₄:0.08Tb³⁺ (c), Zn₂Ge_0.92O₄:0.08Tb³⁺ (d), Zn_1.92GeO₄:0.08Dy³⁺ (e) and Zn₂Ge_0.92O₄:0.08Dy³⁺ (f).

Fig. 12a–f show the decay curves of Zn_1.92GeO₄:0.08Eu³⁺ (λ_em = 620 nm), Zn₂Ge_0.92O₄:0.08Eu³⁺ (λ_em = 620 nm), Zn_1.92GeO₄:0.08Tb³⁺ (λ_em = 554 nm), Zn₂Ge_0.92O₄:0.08Tb³⁺ (λ_em = 554 nm), Zn_1.92GeO₄:0.08Dy³⁺ (λ_em = 586 nm) and Zn₂Ge_0.92O₄:0.08Dy³⁺ (λ_em = 586 nm) when specimens were excited at 270 nm, respectively, the lifetimes can be calculated using formula (3). The corresponding lifetimes are calculated to be 3.51 ms, 3.37 ms, 163.82 μs, 157.27 μs, 4.23 ms and 3.99 ms for Zn_1.92GeO₄:0.08Eu³⁺, Zn₂Ge_0.92O₄:0.08Eu³⁺, Zn_1.92GeO₄:0.08Tb³⁺, Zn₂Ge_0.92O₄:0.08Tb³⁺, Zn_1.92GeO₄:0.08Dy³⁺ and Zn₂Ge_0.92O₄:0.08Dy³⁺, respectively. By comparison, we found that Eu³⁺, Tb³⁺, Dy³⁺ doped Zn_1.92GeO₄ and Zn₂Ge_0.92O₄ have similar lifetimes, which are not influenced by the crystal field environments.

Fig. 12 Decay curves of Zn_1.92GeO₄:0.08Eu³⁺ (a), Zn₂Ge_0.92O₄:0.08Eu³⁺ (b), Zn_1.92GeO₄:0.08Tb³⁺ (c), Zn₂Ge_0.92O₄:0.08Tb³⁺ (d), Zn_1.92GeO₄:0.08Dy³⁺ (e) and Zn₂Ge_0.92O₄:0.08Dy³⁺ (f).

Fig. 13 shows the CIE chromaticity coordinates and corresponding photographs of Zn_1.92GeO₄:0.08Eu³⁺, Zn₂Ge_0.92O₄:0.08Eu³⁺, Zn_1.92GeO₄:0.08Dy³⁺, Zn₂Ge_0.92O₄:0.08Dy³⁺, Zn_1.92GeO₄:0.08Tb³⁺ and Zn₂Ge_0.92O₄:0.08Tb³⁺ (point 1–6) under 254 nm UV lamps. It can be seen that the luminescence colors of Zn_1.92GeO₄:0.08Eu³⁺, Zn₂Ge_0.92O₄:0.08Eu³⁺ are blue, which is due to that the intensity of host emission is larger than that of Eu³⁺ ions. The luminescence colors of Zn_1.92GeO₄:0.08Dy³⁺, Zn₂Ge_0.92O₄:0.08Dy³⁺ are green and blue green, respectively. The reason of different colors is that the 530 nm intensity of Dy³⁺ for Zn_1.92GeO₄:0.08Dy³⁺ is stronger than that for Zn₂Ge_0.92O₄:0.08Dy³⁺. Zn_1.92GeO₄:0.08Tb³⁺ and Zn₂Ge_0.92O₄:0.08Tb³⁺ show similar green colors, due to the energy transfer from Zn₂GeO₄ host to Tb³⁺. In addition, their corresponding photographs are consistent with the color tones.

Fig. 13 CIE chromaticity diagram of Zn_1.92GeO₄:0.08Eu³⁺, Zn₂Ge_0.92O₄:0.08Eu³⁺, Zn_1.92GeO₄:0.08Dy³⁺, Zn₂Ge_0.92O₄:0.08Dy³⁺, Zn_1.92GeO₄:0.08Tb³⁺ and Zn₂Ge_0.92O₄:0.08Tb³⁺ (point 1–6) and corresponding photographs under 254 nm UV lamps.

4 Conclusions

Zn_2−aGeO₄:aRE³⁺ and Zn₂Ge_1−aO₄:aRE³⁺ (RE = Ce³⁺, Eu³⁺, Tb³⁺, Dy³⁺) with different compositions were synthesized by a high-temperature solid-state method. The increase in cell parameters and volumes with the substitution of RE for Zn²⁺ and Ge⁴⁺ confirms the formation of phosphors. For the Ce³⁺ ions based on 5d–4f transition, Zn_1.92GeO₄:0.08Ce³⁺ and Zn₂Ge_0.92O₄:0.08Ce³⁺ show two emission bands centered at 459, 510 nm and 463, 515 nm, respectively. The CIE chromaticity coordinates and the luminescence photographs of Zn_2−xGeO₄:xCe³⁺ and Zn₂Ge_1−xO₄:xCe³⁺ show different characteristics. The emission bands show blueshift and redshift with the increment of Ce³⁺ ions concentration, and the blueshift and redshift are caused by the nephelauxetic effect and crystal field splitting, respectively. For Zn_2−aGeO₄:aR and Zn₂Ge_1−aO₄:aR (R = Tb³⁺, Eu³⁺, Dy³⁺), the positions of emission and excitation bands are not influenced by the crystal field, however the relative intensities of each emission and excitation bands are changed. The reason is that the rare earth ions based on 4f–4f transitions are less influenced by the crystal field environment. The proposed luminescence tuning can be realized by controlling host composition, and it is potential to extend to other phosphors.

Acknowledgements

The work is supported by the National Natural Science Foundation of China (No. 50902042), the Funds for Distinguished Young Scientists of Hebei Province, China (No. A2015201129), the Natural Science Foundation of Hebei Province, China (No. A2014201035, E2014201037), the Education Office Research Foundation of Hebei Province, China (No. ZD2014036, QN2014085), China Postdoctoral Science Foundation funded project (No. 2015M581311), and the Midwest Universities Comprehensive Strength Promotion Project.

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