Photoluminescence characteristics and energy transfer phenomena in Ce³⁺-doped YVO₄ single crystal

1 Introduction

Yttrium orthovanadate (YVO₄) is a promising optical material because it has a broad transparency spectra ranging from visible to far-infrared regions (400–5,000 nm), strong birefringence that ensures the polarization state of emitted light ( Δ n = 0.2225 for 633 nm), high damage threshold, high conductivity, good mechanical properties, and chemical stability [1,2,3]. The pure YVO₄ single crystal has been widely used as polarizers, optical isolators, and beam displacers [4,5,6,7,8,9]. As a good host for a variety of optical applications, rare-earth (RE)-doped YVO₄ materials have attracted significant attention. Owing to potential technological applications in the fields of display phosphors, laser materials, and fluorescent lamps, many researchers pay more attention to investigating the structural, magnetic, optical, and other properties of the YVO₄ doped with different rare-earth elements. For example, the Eu³⁺-activated YVO₄ powder synthesized by the solid-state reaction, solution combustion, sol-gel, and the sonochemical method suggests high-efficiency fluorescence signal and higher thermal stability on electron-beam excitation so that it has been considered as an important commercial red phosphor used in cathode ray tube, fluorescent lamps, plasma display panels, and scintillator in image detectors [10,11,12,13]. The YVO₄ doped with Nd³⁺ ions single crystal is an excellent laser material. Due to its low pumping threshold, large emission section, large absorption coefficient, strongly polarized laser output, and low susceptibility to electron irradiation, the Nd³⁺-doped YVO₄ single crystal has been widely used in the diode-pumped solid lasers and self-frequency-doubling solid lasers [14,15,16,17]. In addition, Er³⁺-doped YVO₄ and Yb³⁺-doped YVO₄ crystals are very attractive potential directly diode-pumped laser materials with emission around 1.0 and 1.53 µm [18]. The Tm³⁺/Ho³⁺/Er³⁺/Yb³⁺ co-doped YVO₄ phosphors exhibit bright white up-conversion emission upon 980 nm near-infrared excitation, which is a promising material for white light-emitting diodes [19].

The structural properties, elastic properties, hardness, and electronic structure of YVO₄ have been investigated extensively [3,20,21,22]. The YVO₄ belongs to the tetragonal crystallographic system (zircon-type phase) with space group D 4 h 19 − I 4 1 / a m d and lattice parameters a = b = 0.71192 nm, c = 0.62898 nm , and α = β = γ = 90 ° [23]. The experimental and calculated theoretical band gaps E g are 3.7 and 3.0 eV, respectively [3,24,25]. The central metal ion V⁵⁺ in isolated VO 4 3 − group is coordinated by four oxygen ions in a tetrahedral symmetry [26]. The maximum phonon energy attributed to the totally symmetrical vibration (A_1g) in YVO₄ is about 890 cm⁻¹ [13]. The primitive unit cell of the YVO₄ single crystal contains four molecules of YVO₄ and RE³⁺ ions occupy the Y³⁺ lattice sites.

Up to now, the growth and spectral properties of RE³⁺-doped YVO₄ (RE = Nd, Eu, Er, Tm, and Yb) phosphors and crystals have been reported extensively in the literature. In this paper, we focus on the photoluminescence properties and energy transfer phenomena in the Ce³⁺-doped YVO₄ single crystal via a combination of absorption, excitation, and emission spectroscopy. More attentions were paid to ascertaining the corresponding transition states, analyzing luminescent mechanism, and revealing the energy transfer from VO 4 3 − to Ce³⁺ ions. Meanwhile, the concentration effect on the spectral properties is also investigated (the Ce³⁺ doping concentration varies from x = 0.005 to x = 0.06 in YVO₄).

2 Experimental procedures

Pure YVO₄ and Ce³⁺-doped YVO₄ crystals were grown by the Czochralski method in a medium frequency induction furnace. High-purity synthesized powders (>99.99%) of commercially available Y₂O₃, V₂O₅, and Ce₂(CO₃)₃ were applied to crystal growth. These raw materials were weighed and mixed in stoichiometric amounts. It is to be noticed that the Ce₂O₃ is unstable at room temperature. Generally, the CeO₂ is used as a raw material of Ce³⁺-doped optical crystals in reducing atmosphere. Nevertheless, it cannot rule out the possibility of some Ce⁴⁺ ions existing in crystals. In this work, the Ce₂(CO₃)₃ is used to grow the Ce³⁺-doped YVO₄ crystals in nitrogen atmosphere, which is considered as the donor of Ce³⁺ ions. We believe that the reaction obeys the following equations:

The concentrations of Ce³⁺ ions in YVO₄ single crystal vary from x = 0.005 to x = 0.06. The process of crystal growth has been reported in our previous work [27].

The X-ray diffraction (XRD) was measured at room temperature and performed by a Rigaku D/max 2,500 v/PC diffractometer in the 2θ range from 10° to 90° in a step scan mode, with steps of 0.02°, using CuKα radiation of wavelength 0.154056 nm (40 kW/200 mA).

Absorption spectra were measured by a Shimadu UV-3101 PC Spectrometer in the ultraviolet-visible and near infrared ranges (200–3,000 nm). The emission and excitation spectra were recorded with the Jobin-Yvon FL3221 TCSPC spectrophotometer and SPEX F-212 fluorescence spectrometer. Photoluminescence photographs were taken with Kimmom Koha IK Series Helium Cadmium 325 nm (He–Cd) laser systems in the darkroom. All tested samples are slices cutting from Ce³⁺-doped YVO₄ single crystals with different Ce³⁺ doping concentrations. The thickness of these slices is 2.00 mm after optically polishing. All the measurements were carried out at room temperature.

We did not carry out the X-ray fluorescence (XRF) measurement so that the actual Ce³⁺ doping concentrations in the Ce³⁺-doped YVO₄ crystals are not obtained through the XRF. The final heat treatment and annealing process have an effect on the photoluminescence, but don’t affect the luminescent mechanism. In addition, the segregation phenomenon also leads to the difference between actual cerium ion concentration and theoretical concentration in different parts of the as-grown crystal. Therefore, the theoretical calculations of optical parameters related to Ce³⁺ doping concentration are not carried out in this paper, such as absorption cross, integrated absorption cross, integrated emission cross-section, energy transfer probability, critical distance, and so on.

3 Results and discussion

Figure 1(a) and (b) show the photographs of as-grown undoped YVO₄ single crystals and Ce³⁺-doped YVO₄ single crystals with various cerium doping concentrations, respectively. The test specimens are Ce³⁺-doped YVO₄ single-crystal slices, as shown in Figure 1(c). The XRD patterns of the pure YVO₄ and Ce³⁺-doped YVO₄ crystals are presented in Figure 2. The strong XRD peaks are collected in a wide angle range and some evident peaks are assigned. The diffraction peaks and relative intensity agree well with the standard data of YVO₄ (JCPDS card No. 72-0341), which indicates that the obtained phase is YVO₄ without any parasitic phase, such as the Y₂O₃ or V₂O₅ phases. The Ce³⁺ ions enter into the YVO₄ crystal and occupy the Y³⁺ lattice sites. The ionic radii of Ce³⁺ and Y³⁺ ions are 0.102 and 0.1015 nm [28,29,30], respectively. It is obvious that the radius of Ce³⁺ ions is very similar to that of the Y³⁺ ions so that the lattice distortion caused by Ce³⁺ ions is very small, which contributes to the crystal growth. However, the higher Ce³⁺ ions are doped in the crystal, the more severe lattice distortion is. The samples doped with different Ce³⁺ concentrations exhibit the pure tetragonal YVO₄ crystalline phase. It is indicated that partial substitution of Ce³⁺ for Y³⁺ does not affect the structure of YVO₄ phase and the Ce³⁺ ions doped into the host lattice effectively [31].

Figure 1

Photographs of as-grown YVO₄ and Ce³⁺-doped YVO₄ single crystal. (a) Pure YVO₄ single crystals, (b) Ce³⁺-doped YVO₄ single crystals with different doping concentrations, (c) Ce³⁺-doped YVO₄ single-crystal slices (the Ce³⁺ doping concentrations vary from x = 0.005 to x = 0.06. The thickness of these slices is 2.00 mm after optical polishing).

Figure 2

XRD patterns of the pure YVO₄ and Ce³⁺-doped YVO₄ crystals.

For a free Ce³⁺ ion, its electronic configuration is 4f¹. The free-ion Hamiltonian (including electron-electron and spin-orbit interaction terms) produces the ^2S+1L_J multiplets. We only consider the spin-orbit coupling of trivalent rare earth and do not take account of Stark splitting under the influence of the crystal field. Based on the Hund’s rules, the ground state is ²F_5/2 and the excited state is ²F_7/2. The energy gap between ²F_7/2 and ²F_5/2 states is about 2,200 cm⁻¹ [27,32,33]. The 5d energy levels of the excited 4f⁰5d¹ configuration form a 2D term splitting into ²D_3/2 and ²D_5/2 states, which are influenced strongly by the crystal field of the host. The energy gap between ²D_3/2 and ²D_5/2 states is about 1,890 cm⁻¹ [34,35]. The energy of the 5d levels is so low that the spectra of 5d–4f transition can be observed in the near-ultraviolet and visible ranges.

Figure 3 shows the absorption spectra of pure YVO₄ and Ce³⁺-doped YVO₄ crystals in the ultraviolet-visible and near infrared ranges at room temperature. The pure YVO₄ crystal exhibits a very wide transparency spectrum ranging from 500 to 3,000 nm, while an intense broad absorption bands below 350 nm can be recorded. The same intense absorption bands below 350 nm are also observed in the samples with different dopant concentrations. Up to now, this phenomenon has been well-understood. The central metal ion V⁵⁺ in isolated VO 4 3 − group is coordinated by four oxygen ions in a tetrahedral symmetry [31,36]. The theoretical studies have proven that the molecular orbital of V⁵⁺ ion in isolated VO 4 3 − group gives rise to a ¹A₁ ground state with configuration t 1 6 e 0 t 2 0 and four excited states with configuration t 1 5 e 1 t 2 0 , which are ³T₁, ³T₂, ¹T₁, and ¹T₂, respectively [37,38,39]. Therefore, the intense absorption band below 350 nm is ascribed to the intrinsic absorption of VO 4 3 − , which is the charge transfer absorption transitions from the ¹A₁ ground state to the ³T₁, ³T₂, ¹T₁, and ¹T₂ excited states. Compared with pure YVO₄ single crystal, the YVO₄ crystals with Ce³⁺ doping concentration of x = 0.005, 0.01, 0.02, 0.05 exhibit the broad absorption band (centered at 460 nm) ranging from 380 to 880 nm. Moreover, the absorption becomes stronger and stronger with increasing the Ce³⁺ doping concentration. The Ce³⁺ doping concentration of x = 0.06 leads to the whole absorption below 640 nm. It is obvious that the absorption band is associated with typical transition of the Ce³⁺ ions, which are assigned to the superposition of the ²F_5/2 → ²D_3/2 and ²F_5/2 → ²D_5/2 absorption transitions. Broader absorption band and larger absorption cross-section serve to absorb the pump light and improve the conversion efficiency of pump light [40].

Figure 3

Ultraviolet-visible and near infrared absorption spectra of undoped YVO₄ and Ce³⁺-doped YVO₄ single crystals at room temperature. Noticed that the vertical axis represents the optical density.

Figure 4 shows the visible emission spectra of Ce³⁺-doped YVO₄ crystals with different cerium doping concentrations (from x = 0.005 to x = 0.06). On a basis of the absorption spectrum (Figure 3), the excitation wavelength at 460 nm is applied to pumping all the samples. As can be seen from the Figure 4, a very broad emission band centered at about 620 nm is observed in the region from 480 to 800 nm. Except for the emission intensity, the obtained spectra are identical in the range of 480–800 nm. The 460 nm excitation wavelength corresponds to the ²F_5/2 → ²D_3/2 absorption transition of Ce³⁺ ions. However, it is very difficult to determine the energy levels of emission transition. Wang et al. [41] studied the same broad emission band and concluded that the broad emission band centered at 620 nm was attributed to the charge transfer transition of the Ce⁴⁺–O²⁻ ions pairs (CTS₂ → CTS₁). Generally speaking, the charge transfer transition of rare-earth ions is usually located in the ultraviolet range, such as Lu₂Si₂O₇:Ce³⁺ [42,43], YVO₄:Eu³⁺ [13,44,45], Na₂YMg₂(VO₄)₃:Yb³⁺ [31,46]. The red emission band at 620 nm corresponding to the ³T₁ → ¹A₁ transition has been also observed in the pure Zn₃(VO₄)₂, Ca₅Mg₄(VO₄)₆, Ca₅Zn₄(VO₄)₆, and NaYMg₂(VO₄)₃ single crystals with isolated VO₄ tetrahedra [26,31,47]. Moreover, the influences of phonons and Stark split induced by crystal field cannot be neglected. In the view of above analysis, we assume that the observed broad band centered at about 620 nm can be assigned to the ³T₁ → ¹A₁ transition (the charge transfer from oxygen ligands to the central vanadium atom inside the VO 4 3 − ). On the other hand, it can be also assigned to the ²D_3/2 → ²F_7/2 transition under the joint action of the phonons and crystal filed. Thus, it can be concluded that the broad is attributed to the overlap of the ³T₁ → ¹A₁ transition and phonon-assisted ²D_3/2 → ²F_7/2 transition under the influence of ligand field.

Figure 4

Visible emission spectra of Ce³⁺-doped YVO₄ crystals with various doping concentrations. The excitation wavelength is 460 nm corresponding to the ²F_5/2 → ²D_3/2 absorption transition of Ce³⁺ ions.

Since the radial wave function of the excited 5d electron extends spatially well beyond the closed 5s²5p⁶ shells, the ²D_3/2 and ²D_5/2 states are strongly perturbed by the ligand field of the host. The absorption and emission wavelength of Ce³⁺-doped different oxides have been published in ref. [35]. These data are listed in Table 1. The different hosts lead to a great change in the f–d transition, which is different from the f–f transition of 4f ^N electronic configuration.

In order to investigate the spectral properties and analyze the energy transfer mechanism from VO 4 3 − (the host) to Ce³⁺ ions, the emission spectra of Ce³⁺-doped YVO₄ crystals with various dopant concentrations from x = 0.005 to 0.6 were measured at room temperature, as shown in Figure 5. The excitation wavelength was chosen to be 325 nm corresponding to the ¹A₁ → ¹T₂ absorption transition of VO 4 3 − . Besides the broad emission bands centered at 440 nm from 350 to 600 nm for all samples showing up obviously, a weak emission band centered at 620 nm was also recorded. Except for the emission intensity, all obtained spectra are identical in the range of 350–750 nm. It is well-known that the energy transfer from VO 4 3 − to the doped rare-earth ions is so efficient that the intrinsic luminescence of the host cannot be observed at room temperature. Although some literatures have reported the intrinsic emission of VO 4 3 − in the rare-earth-doped YVO₄ materials at room temperature, the doping concentration of rare-earth ions is very low (less than x = 0.01) [2,48,49,50,51]. When the doping concentration reaches more than x = 0.01, the intrinsic luminescence tends to disappear in the emission spectra. Therefore, we believe that the broad emission band centered at 440 nm from 350 to 600 nm must be attributed to the luminescence of the Ce³⁺ ions, which is the overlap of the ²D_3/2 → ²F_7/2 (P₂ centered at 470 nm) and ²D_3/2 → ²F_5/2 (P₁ centered at 428 nm) emission transitions. Figure 6 shows the fitting curves of the emission spectra. The luminescent photographs of the Ce³⁺-doped YVO₄ single crystals excited at 325 nm are shown in Figure 7, which are taken in darkroom. Intense blue emissions are observed at room temperature. According to the fitting data, the energy gap between the ²F_7/2 and ²F_5/2 states is about 2,088 cm⁻¹, which is in good agreement with theoretical values 2,200 cm⁻¹ [27,32,33]. As can be seen from Figure 5, a narrow emission band centered 620 nm appears in the emission spectra excited at 325 nm, which is also observed in other pure undoped single crystals with isolated VO 4 3 − (act as a luminescent center) tetrahedra [26,31,46,47]. According to the energy level of VO 4 3 − group in other materials, we can assume that the narrow emission band is assigned to the ³T₂, ³T₁ → ¹A₁ (620 nm) transitions. However, the intrinsic luminescence of the host in the highly Ce³⁺-doped YVO₄ crystal (more than x = 0.01) is not acceptable. Zhou et al. [2] have investigated the unusual intrinsic luminescence of YVO₄:Er³⁺ phosphors (dopant concentration x = 0.005, 0.02, 0.04, and 0.06). They believe that this phenomenon means very little amount of rare-earth ions successfully entered the host or the energy transfer between VO 4 3 − and rare-earth ions was not efficient. The analysis of the XRD patterns has proven that the obtained phase is YVO₄ without any parasitic phase, such as the Y₂O₃ or V₂O₅ phases. Moreover, the radius of Ce³⁺ ions is very similar to that of the Y³⁺ ions so that the lattice distortion caused by Ce³⁺ ions is very small and the Y³⁺ lattice sites are occupied by the Ce³⁺ ions easily. In addition, the ref. [2,31] all reported that the intrinsic luminescence of the host could decrease with the doping concentration. The Figure 5 displays the opposite behavior, which suggests this narrow emission band may correspond to luminescence properties of the Ce³⁺ ions. Moreover, if the 620 nm emission would arise from VO 4 3 − , it should be a very broad band [52]. This emission band is very sharp and weak so that it also looks like the transition from the 5D(0) state to the 7F(J) (J = 0, 1, 2, 3, 4) states of unintentionally contaminated Eu³⁺ ions. But all the experimental specimens with various Ce³⁺ doping concentrations show the same characterization, which basically rules out the possibility of the unintentionally contaminated Eu³⁺ ions. This phenomenon always puzzles us and the reason why the intrinsic emission appears in the emission spectra needs further study.

Figure 5

Emission spectra of Ce³⁺-doped YVO₄ crystals with different cerium doping concentrations. The excitation wavelength is 325 nm corresponding to the ¹A₁ → ¹T₂ absorption transition of VO 4 3 − .

Figure 6

Gaussian fitting curve of the emission spectra excited at 325 nm.

Figure 7

Luminescent photographs of the YVO₄ (a) and Ce³⁺-doped YVO₄ single crystals excited at 325 nm (Doping concentration is x = 0.005 (b), 0.01 (c), 0.02 (d), 0.05 (e), and 0.06 (f), respectively).

Figure 8 shows the excitation spectra of Ce³⁺-doped YVO₄ single crystal for 620 nm (a), 440 nm (b), and 470 nm (c) emissions at room temperature. Monitoring the emissions at 620, 440, and 470 nm, the specimen exhibits a broad excitation band extending from 260 to 360 nm, which is attributed to the ¹A₁ → ¹T₂, ¹T₁ charge transfer band (CT) of V⁵⁺–O²⁻ (from oxygen ligands to the central vanadium stom inside VO 4 3 − groups in the YVO₄) [53,54,55]. Under excitation of UV lights (260–360 nm), the emission intensities of 440 and 470 nm are stronger than those of 620 nm. This result is consistent with analysis of photoluminescence emission spectra (Figure 5). Differing from Figure 8(b) and (c), a strong excitation band located at about 565 nm and two weak excitation bands at 395 and 460 nm are recorded in Figure 8(a). The two weak excitation bands are associated with typical transitions of the Ce³⁺ ions, which are assigned to the ²F_5/2 → ²D_5/2 (395 nm) and ²F_5/2 → ²D_3/2 (460 nm) transitions, respectively. The observed strong excitation band at 565 nm could be assigned to the charge transfer transition between V⁵⁺ and O²⁻, described as ¹A₁ → ³T₂, ³T₁ transition. Compared to the typical excitation bands of the Ce³⁺ ions, the charge transfer band is more suitable for 620 nm emission. The excitation spectra displaying a strong charge transfer band means that the host ( VO 4 3 − group) can absorb the pump energy (UV-photon) intensively and then transfer the energy to the nearby Ce³⁺ ions efficiently. This process can improve the conversion efficiency of the pump light and promote the luminescent properties of rare-earth ions.

Figure 8

Excitation spectra of Ce³⁺-doped YVO₄ single crystal for 620 nm (a), 440 nm (b), and 470 nm (c) emissions at room temperature.

The CIE coordinates and correlated color temperatures (CCTs) are two important characteristics for luminescent materials [56]. The color coordinates can be obtained on a basis of emission spectra (Figures 4 and 5). According to the CIE chromaticity coordinate, the correlated color temperature also can be calculated through the following approximate formula proposed by McCamy [19,57,58,59].

where n = (x − x _e )/(y − y _e ) is the inverse slope line, and x _e = 0.3320, y _e = 0.1858.

The CIE chromaticity coordinates and correlated color temperatures of Ce³⁺-doped YVO₄ crystals have been calculated under different excitation wavelengths, as listed in Table 2. Figure 9 presents the CIE chromaticity coordinates (x, y) positions of Ce³⁺-doped YVO₄ single crystals in the Commission Internationale de I’Eclairage (CIE) 1931 chromaticity diagram. It is clear that the samples excited at 325 nm are located in blue region, which is in good agreement with Figure 7. When the excitation wavelength is 460 nm, the samples are located in yellow region. In addition, the Ce³⁺ doping concentration has little effect on the emitting color.

Figure 9

CIE chromaticity coordinates (x, y) positions of Ce³⁺-doped YVO₄ single crystals excited at 325 nm (a) and 460 nm (b) in the Commission Internationale de I’Eclairage (CIE) 1931 chromaticity diagram.

4 Conclusion

In summary, the high-quality YVO₄ and Ce³⁺-doped YVO₄ single crystals were grown by the Czochralski method. The X-ray diffraction (XRD) patterns indicated that the obtained phase was YVO₄ without any parasitic phase. Optical properties of Ce³⁺-doped YVO₄ single crystal with different doping concentrations were investigated via a combination of absorption, excitation, and emission spectra. Particularly, more attentions were paid to ascertaining the corresponding transition states, analyzing the luminescence mechanism and revealing the charge transfer (CT) transition in Ce³⁺-doped YVO₄ single crystals. Based on the experimental data, CIE chromaticity coordinates and correlated color temperatures of Ce³⁺-doped YVO₄ crystals were calculated and depicted in CIE chromaticity diagram.