Intrinsic [VO₄]³⁻ emission of cesium vanadate Cs₅V₃O₁₀

Abstract

Polycrystalline Cs₅V₃O₁₀ micro-particles were synthesized by a solid-state reaction. The vanadate shows intrinsic self-activated luminescence of a single broad band with a peak at 520 nm, extending from about 400 nm to 720 nm. This asymmetric band is composed of two bands due to the electronic transitions from the ³T₁ and ³T₂ excited states to the ¹A₁ ground state in [VO₄]³⁻ centers. The same emission band was obtained for micro- and nanoparticles. It is suggested that the broadening of the emission band arises from single [VO₄]³⁻ molecules. The emission and decay curve profiles indicate that the emission is not due to different kinds of [VO₄]³⁻ centers but only one kind of [VO₄]³⁻ center. When the temperature is increased from 10 K to 450 K, the emission intensity increases below 150 K and decreases above 150 K, and an unusual blue shift is observed. The observed temperature dependence can be understood by the relaxation processes of the emitting ³T₁ and ³T₂ states including the thermal feeding by the lower-energy ³T₁ state to the higher-energy ³T₂ state.

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1. Introduction

Luminescent materials have been widely employed in optoelectronic technologies, e.g., for photonic/electronic integration, solid-state lighting, and as labels in biological research etc.^1–4 Of these various luminescent materials, vanadates have also been used for solid state lighting, displays, and pigments.^5,6 Vanadates are compounds which contain the vanadium V ion surrounded by oxygen O ions. Many kinds of vanadate materials have been synthesized. The samples are YVO₄, AVO₃ (A: Li, Na, K, Rb, Cs), M₂V₂O₇ (M: Mg, Ca, Sr, Ba, Zn), M₃V₂O₈ (M: Mg, Ca, Sr, Ba, Zn), Zn₃(VO₄)₂, CsK₂Gd[VO₄]₂, and Ca₂NaMg₂V₃O₁₂. Recently a lot of studies have investigated vanadates doped with lanthanide ions. Lanthanide-doped vanadate nanoparticles exhibit efficient emission from the lanthanides by energy transfer from the vanadate host, and wide colour tuning is easily done by selecting different lanthanide elements; these can be applied in many fields, such as cathode ray tubes, lamps, X-ray detectors, biosensors, and solid-state lasers.^7–13 In these nanoparticles, emission from the vanadate host is not observed or is considerably weak because of the highly efficient energy transfer from the host to the activator. It has been observed that non-doped vanadates show a broad emission band in the visible spectral region, as mentioned later. We are interested in the luminescence of vanadate itself, i.e., the intrinsic and self-activated luminescence from non-doped vanadates.

The origin of the intrinsic luminescence from vanadates has been assigned to the charge transfer (CT) transitions from the HOMO (highest occupied molecular orbital) level, which is composed of O 2p nonbonding orbitals, to the LUMO (lowest unoccupied molecular orbital) level, which is composed of antibonding V 3d orbitals and O 2p orbitals, in tetrahedral [VO₄]³⁻.^14–16 These molecular orbitals form the ground ¹A₁ state and the excited ¹T₁, ¹T₂, ³T₁, and ³T₂ states.^17,18 The electronic transitions of ¹A₁ → (¹T₁, ¹T₂) give rise to a doublet-structured broad and intense CT absorption band in the UV region in all the vanadates, while a broad and intense CT emission band is observed in the 400–720 nm region, which is due to the transitions (³T₁, ³T₂) → ¹A₁.^19–22 This luminescence mechanism is based on the electronic transitions in [VO₄]³⁻, which is called the [VO₄]³⁻ model hereafter.

The broadband emission and UV absorption properties make vanadates suitable for solar cells, because the visible photoluminescence (PL) which is generated under excitation with UV light from the sun is absorbed by current silicon solar cell materials. Of the many kinds of vanadates, some vanadates show a high PL quantum efficiency (PQE). For example, RbVO₃ and CsVO₃ show PQEs of 79% and 87%, respectively, compared to a PQE of 4% for KVO₃.¹⁶ Similarly, Zn₃V₂O₈ shows a high PQE value, 52%, compared to 6% of Mg₃V₂O₈, although the two M₃V₂O₈ (M: Mg and Zn) materials show the same broad emission band extending from 410 nm to 900 nm.¹⁶

Recently, we reported that lanthanide-free Cs₅V₃O₁₀ shows [VO₄]³⁻ emission with a high PQE of 85.2% under UV light. This vanadate also gives intense scintillation emission under X-ray excitation.²³ Therefore Cs₅V₃O₁₀ is expected to be useful for lighting, displays, and scintillation. However, its photo-physical nature has not been clarified. For example, (1) the reason why a broad emission band is observed for Cs₅V₃O₁₀ is unknown, (2) it is unknown whether the width and peak wavelength of the emission band change with particle size, (3) it is unknown whether only one or different kinds of luminescent [VO₄]³⁻ centers are present in Cs₅V₃O₁₀, and (4) the relaxation process of the excited states of [VO₄]³⁻ is still unknown. The present work was undertaken to try to clarify these points.

It is suggested that an unusual broadband emission observed from CdSe is caused by ultra-small CdSe nanocrystals.²⁴ In our previous paper, we synthesized Cs₅V₃O₁₀ nanoparticles with diameters of 47–135 nm by the Pechini method.²³ In the present work, we make Cs₅V₃O₁₀ micro-crystals by a solid state reaction, to clarify whether the size of Cs₅V₃O₁₀ particles is responsible for the broadening of the emission band or not. Besides the above purposes of the present work, there is another purpose. Recently Wang et al. observed a broad emission band with a peak at 546.4 nm, which extends from 420 to 700 nm, from Zn₃(VO₄)₂ vanadate microspheres.²⁵ Although the [VO₄]³⁻ model has been used in many papers,^14–22 they have suggested that the 546.4 nm vanadate emission might be ascribed to zinc vacancies existing in Zn₃(VO₄)₂. Here we investigate whether the vacancy model is applicable to the emission of Cs₅V₃O₁₀.

2. Experimental

The preparations of Cs₅V₃O₁₀ powders were carried out by solid state reactions. The starting chemicals were high-purity Cs₂CO₃ and V₂O₅. Stoichiometric amounts of these materials were thoroughly mixed together. Firstly, the mixtures were heated at 350 °C for 6 h. Then the powders were thoroughly ground to a homogeneous mixture; secondly, the mixture was sintered at 680 °C for 6 h.

The photoluminescence (PL) and PL excitation (PLE) spectra at room temperature were recorded using a Perkin-Elmer LS-50B luminescence spectrometer and a Hitachi F-4500 fluorescence spectrophotometer. The PL and PLE spectra at various temperatures between 10 K and 450 K were measured with a Spex Fluorolog-3 spectrophotometer under a 500 W Xe-lamp excitation. PL lifetime measurements were made using the third harmonics (355 nm) of a Spectron Laser Sys. SL802G pulsed Nd:YAG (yttrium aluminum garnet) laser with a pulse energy of 5 mJ, repetition rate of 10 Hz, and duration of 5 ns at 10–300 K (gas helium flow). The luminescence was dispersed by a 75 cm monochromator (Acton Research Corp. Pro-750) and observed with a photomultiplier tube (Hamamatsu R928). The decays were recorded using a 500 MHz digital oscilloscope (Tektronix DPO3054). A filter was used to avoid the intense singles from the laser scattering.

3. Results and discussion

The X-ray diffraction (XRD) pattern of the synthesized Cs₅V₃O₁₀ was examined to characterize the phase purity and crystallinity. Fig. 1 shows the XRD pattern. It is found that all the diffraction peaks are in good agreement with the standard card PDF2# 50-0027 in the International Center for Diffraction Data (ICDD) database. No impurity lines are observed. The results indicate that the sample has a pure crystal formation of Cs₅V₃O₁₀. From the XRD, it is confirmed that the tetrahedral [VO₄]³⁻ ions are formed.

Fig. 1 The typical XRD patterns of the Cs₅V₃O₁₀ phosphor and the standard PDF2 card no. 50-0027.

A typical scanning electron microscope (SEM) micrograph of Cs₅V₃O₁₀ prepared by the solid-state reaction is shown in Fig. 2. According to the image, the sizes of the synthesized particles are mainly 2–5 μm. Particles with a size of less than 1 μm are rare, indicating the success of the micro-particle synthesis. Unlike the round-shaped nanoparticles synthesized by the Pechini method,²³ the particles have rectangle-like shapes with smooth surfaces.

Fig. 2 A typical SEM micrograph of the Cs₅V₃O₁₀ particles.

The elemental composition was checked using energy-dispersive X-ray spectroscopy (EDS). Fig. 3 is the EDS spectrum of the synthesized Cs₅V₃O₁₀. It confirmed that the synthesized sample has the elements Cs, O, and V. The average Cs/V ratio was measured to be about 1.76. This value is close to the theoretical value (5/3 = 1.67) in the stoichiometric chemical formula of Cs₅V₃O₁₀. These EDS and XRD results indicate our successful synthesis of Cs₅V₃O₁₀, which does not contain any impurity.

Fig. 3 EDS spectrum of Cs₅V₃O₁₀, with an indication of the elements.

We compare the PL spectrum of the micro-particles synthesized in the present work with the PL spectrum of the nanoparticles which were presented in our previous paper,²³ to investigate whether the size of the Cs₅V₃O₁₀ particles is responsible for the broadening of the emission band. Fig. 4 shows the PL spectra of the micro- and nanoparticles. The same broad emission band with a peak at 520 nm which extends from 400 nm to 750 nm is obtained for the micro- and nanoparticles. The size effect was expected to strengthen the nanoparticle emission compared to the micro-particle emission. However, such an effect was not found. This indicates that the particle size is not responsible for the broadening of the emission band due to [VO₄]³⁻. Observation of the emission due to [VO₄]³⁻ confirms the absence of unintended impurities in our samples, because it has been reported that impurities randomly located in the crystal lattice induce quenching of the luminescence of the YVO₄ vanadate at room temperature.¹³

Fig. 4 PL spectra of the micro-particles (red line) excited at 330 nm and the nano-particles excited at 300 nm (blue) and 360 nm (black).

The emission from single molecules which are dispersed in solution is broad due to the charge transfer and electron–phonon interactions within the single molecules.²⁶ Therefore it is suggested that the broad emission of vanadates arises from single [VO₄]³⁻ molecules which are distributed uniformly in the micro- and nanoparticles without any electronic interaction with neighboring VO₄³⁻ molecules and with defects. As a result, the Cs₅V₃O₁₀ nano- and micro-particles show the same emission band.

The 520 nm emission band is asymmetric. This band is composed of two sub-bands (called Em₁ and Em₂ at the high- and low-energy sides, respectively) as shown in Fig. 5. Similar compositions have been observed in various vanadates.^16,20,22 The Em₁ and Em₂ sub-bands with peaks at about 507 and 588 nm are attributed to the transitions from the ³T₂ and ³T₁ states to the ground state ¹A₁, respectively, as indicated in the inset of Fig. 5. Fig. 5 also presents the PLE spectrum of the 450 nm Em₁ emission and the PLE spectrum of the 600 nm Em₂ emission of the Cs₅V₃O₁₀ micro-particles. Each of the two PLE spectra consists of two bands with the same peaks at 280 and 365 nm. The 280 and 365 nm PLE bands are called Ex₁ and Ex₂, respectively. These two absorption bands in the PLE spectra correspond to the transition from the ground state ¹A₁ to the ¹T₂ and ¹T₁ states, respectively, as indicated in the inset of Fig. 5. The Ex₁ and Ex₂ absorption bands are observed for the Cs₅V₃O₁₀ nanoparticles too.²³ The double peak of Ex₁ and Ex₂ in the PLE spectrum has also been observed in various vanadates such as M₂V₂O₇ (M: Mg, Ca, Sr, Ba, and Zn),²⁰ AVO₃ (A: K, Rb, Cs) and M₃V₂O₈ (M: Mg, Zn),¹⁶ and GdVO₄.²⁷ This confirms that the Ex₁ and Ex₂ bands arise from the [VO₄]³⁻ ion which is a common component of vanadate materials.

Fig. 5 Photoluminescence (PL) and PL excitation (PLE) spectra of Cs₅V₃O₁₀ at 300 K. Inset shows a diagram showing the processes of excitation and emission.

It has been observed in the Cs₅V₃O₁₀ nanoparticles that the excitations into the Ex₁ and Ex₂ bands give the same emission band,²³ as shown in Fig. 4 by the blue and black curves, respectively. The same was also observed for the Cs₅V₃O₁₀ micro-particles. It is noted that the PLE spectrum for the Em₁ emission gives a different intensity ratio of the Ex₁ and Ex₂ bands compared to the PLE spectrum for the Em₂ emission, although the same Ex₁ and Ex₂ peak wavelengths are observed for the two PLE spectra (Fig. 5). From this result, it might be suggested that different luminescent [VO₄]³⁻ centers are present in the Cs₅V₃O₁₀ micro-particles. If so, it is difficult to explain the reasons (1) why the same Ex₁ and Ex₂ peak wavelengths are observed for the two PLE spectra and (2) why the same emission band is observed for the Ex₁ and Ex₂ excitations. The reason for the different intensity ratios of the Ex₁ and Ex₂ bands is suggested to be as follows, based on only one kind of luminescent center. The probability of a non-radiative transition from the excited ¹T₂ and ¹T₁ states to the ³T₂ state, which leads to the Em₁ emission, is different from the probability of a non-radiative transition from the excited ¹T₂ and ¹T₁ states to the ³T₁ state, which leads to the Em₂ emission. As the result, a different intensity ratio of the Ex₁ and Ex₂ bands was observed.

The idea of the presence of only one kind of luminescent center is supported by PL lifetime measurements. Fig. 6 shows the PL decay curves of the 450 and 600 nm emissions. 450 and 600 nm emissions are selected as these are the wavelengths of emission for Em₁ and Em₂, respectively. A fast drop is observed at about 0–0.008 μs (i.e., 0–8 ns) in each of the two emission decay curves. This drop is due to the pulsed laser which was used for excitation, because the laser has a duration of 5 ns. The two decay curves are quite similar to each other. This indicates that (1) the Em₁ and Em₂ emissions occur after a fast thermal equilibrium between the ³T₂ and ³T₁ excited states, and (2) the emission arises not from different kinds of [VO₄]³⁻ luminescent center, but from only one kind of [VO₄]³⁻ luminescent center. If different kinds of luminescent centers are present, it is expected that the Em₁ and Em₂ emissions would give quite different PL lifetimes.

Fig. 6 PL decay curves of the 600 and 450 nm emissions at 10 K.

As mentioned above, we have suggested that the broad emission of vanadates arises from [VO₄]³⁻ molecules which do not have electronic interaction with defects. To check their non-interaction with defects, we investigated the dependence of the emission intensity on the excitation intensity. It is known that the PL intensity shows saturation at high excitation intensities if the emission arises from permanent defects and traps.^28,29 Fig. 7 shows the [VO₄]³⁻ emission intensity plotted against the excitation intensity. It is observed that the emission intensity increases linearly upon increasing the excitation power from 0.2 to 0.96 W. No PL saturation is observed. These results indicate that defects are not responsible for the emission.

Fig. 7 The dependence of the emission intensity of Cs₅V₃O₁₀ on the excitation intensity; inset is the comparison of the emission spectra between the powers of 0.2 W and 0.96 W.

If defects such as photo-generated traps are generated at high excitation intensities, the emission band shape changes with increasing excitation intensity.^30,31 No change was observed in the band-shape of the emission from Cs₅V₃O₁₀ upon increasing the excitation intensity. For example, the same emission line shape is obtained at low and high excitation powers of 0.2 W and 0.96 W, respectively (inset of Fig. 7). This indicates that emissive defects such as photo-generated traps are not generated in Cs₅V₃O₁₀ at high excitation intensities.

Fig. 8 shows the temperature dependence of the emission spectra of Cs₅V₃O₁₀ and the peak wavelength of the emission band. The emission intensity is plotted against temperature in Fig. 9. Two unusual results are found regarding the emission intensity and emission peak shift.

Fig. 8 Temperature dependence of the emission spectra of Cs₅V₃O₁₀ (a and b) and the peak wavelength (inset of (a)).

Fig. 9 The temperature dependences of the integrated emission intensity (right scale) and the average lifetime (left scale). The intensities were normalized at the maximum of the emission intensity at 150 K.

Firstly, the emission intensity increases from 10 K to 150 K, and decreases quickly above 150 K (Fig. 9). This thermal quenching can be described using eqn (1), using the model by Struck and Fonger.^32–35 This equation was obtained for a luminescent center where the electrons in the emitting state are relaxed to the ground state by (1) a radiative transition process and (2) a non-radiative transition process through thermal activation with activation energy ΔE.

I_T = I₀[1 + c exp(−ΔE/kT)] (1)

Here I_T is the intensity at temperature T, I₀ is the intensity near 0 K, and c is the rate constant for thermally activated escape. In the case of the emission from Cs₅V₃O₁₀, ΔE is attributed to the energy from the upper emitting state ³T₂ to the crossing point of the ³T₂ state and the ¹A₁ ground state in the configuration coordinate diagram. The electrons which are thermally excited to the crossing point are relaxed to the ¹A₁ ground state non-radiatively.

Eqn (1) indicates that the emission intensity is nearly constant upon increasing the temperature from 0 K at low temperatures until a certain temperature (called the quenching temperature, T_q) and suddenly decreases nearly exponentially at high temperatures above T_q. The luminescence of Cs₅V₃O₁₀, however, does not obey eqn (1) especially at low temperatures (10–150 K). The intensity increases at temperatures lower than the quenching temperature of 150 K as shown in Fig. 9. By taking into account thermal feeding by the ³T₁ state to the upper ³T₂ state,¹⁸ we suggest that this unusual increase in the emission intensity at low temperatures can be understood as follows.

At low temperatures below 150 K where thermal activation to the crossing point is not effective, the ³T₂ state is thermally fed by the lower-energy ³T₁ state with increasing temperature from 10 K. The transition probability between the ¹T₂ state and the ¹A₁ state is higher than the probability of transition between the ¹T₁ state and ¹A₁ state.¹⁸ It is suggested that the same is true for the case of the transition probabilities from the ³T₂ and ³T₁ states, i.e., the transition probability between the ³T₂ state and the ¹A₁ state is higher than the probability of transition between the ³T₁ state and the ¹A₁ state. In fact, it is observed that the Em₁ emission from the ³T₂ state is more intense than the Em₂ emission from the lower-energy ³T₁ state (Fig. 5). Therefore, the increase in the total luminescence below 150 K is due to the increase in population of the ³T₂ state, which is enhanced with increasing temperature.

Secondly, unlike most luminescent centers, the emission of Cs₅V₃O₁₀ shifts to high-energy with increasing temperature (inset of Fig. 8a). It is suggested that the observed blue-shift can be understood as follows. Of the two emitting states ³T₁ and ³T₂, the population of the upper ³T₂ state increases with increasing temperature due to thermal feeding from the lower ³T₁ state, leading to an enhancement in the intensity of the emission from the higher-energy ³T₂ state relative to the intensity of the emission from the lower-energy ³T₁ state. In this way we can understand the observed blue shift. This is consistent with the suggestion³⁶ that thermally active phonons assist the jumping of electrons from a lower-energy excited state to a higher-energy excited state.

Fig. 10 shows the PL decay profiles of Cs₅V₃O₁₀ at various temperatures. All the decay curves are non-exponential. The decay curve at 10 K extends over 0.7 μs to 1.4 μs (see also Fig. 6), and the decay curve at 300 K extends to 0.2 μs. Such long times of these decay profiles indicate that the emission is not fluorescence (which usually occurs on a nano-second time scale, such as the emission from a LED), but phosphorescence. This supports the idea that the emission of Cs₅V₃O₁₀ is caused by the electronic transitions from the triplet ³T₁ and ³T₂ states to the ground state ¹A₁. These transitions are spin-forbidden. However, intense emission has been observed in various vanadates including Cs₅V₃O₁₀. The intense emission is caused by the heavy atom effect due to the vanadium ion in [VO₄]³⁻, which leads to the mixing of the singlet ¹T₁ and ¹T₂ states and the triplet ³T₁ and ³T₂ states by strong spin–orbit coupling.

Fig. 10 The decay profiles of Cs₅V₃O₁₀ at 10, 50, 200, 250, 273 and 300 K under excitation with a 355 nm laser.

If a vacancy gives rise to the vanadate emission as suggested by Wang et al. who observed the broad band emission from Zn₃(VO₄)₂,²⁵ it is difficult to explain why phosphorescence is observed. It is conceivable that defects in the lattice such as vacancies give rise to quenching of emission.¹³ If a vacancy gives rise to emission, the peak wavelength and band width of the observed emission band are expected to depend largely on the metal elements in vanadates, as in the cases of color centers in alkali halide crystals.³⁷ However, the emission band profiles observed in various vanadates are almost the same. Therefore it is suggested that the [VO₄]³⁻ model is more reliable than the vacancy model to understand the various optical properties observed such as phosphorescence, strong emission intensity, and the doublet structure of the PL and PLE bands.

The non-exponential decay curves can be fitted to the average lifetime expressed by eqn (2):³⁸

where I(t) is the emission intensity at time t after the excitation. From 200 K to 300 K, the average lifetime decreases from 0.089 to 0.0245 μs.

The emission lifetime is plotted against temperature in Fig. 9. It is observed that the PL lifetime decreases gradually in the range of 10–200 K, while it decreases quickly from about 200 K with increasing temperature. The shortening of the lifetime at high temperatures can be understood as follows.

The parabolic potential of the emitting states ³T₁ and ³T₂ has crossing points with the potential of the ¹A₁ ground state in the configuration coordinate diagram. Electrons which are thermally excited to the crossing points from the ³T₁ and ³T₂ states are relaxed to the ¹A₁ ground state non-radiatively at high temperatures. That is, the ³T₁ and ³T₂ states have two relaxation processes to the ¹A₁ state: one is a radiative direct transition and the other is a non-radiative transition through the crossing points. The coexistence of these two processes leads to the shortening of the lifetime with increasing temperature because the thermal activation rate is enhanced with increasing temperature. This explanation is based on the [VO₄]³⁻ model, not on the defect model. Therefore, it is confirmed from the temperature dependence of the PL lifetime that the [VO₄]³⁻ model is more reliable than the defect model.

4. Conclusions

Cs₅V₃O₁₀ shows intrinsic self-activated luminescence of a single broad band with a peak at 520 nm, extending from about 400 nm to 720 nm. The same emission band is obtained for micro- and nanoparticles. It is suggested that the broadening of the emission band arises from single [VO₄]³⁻ molecules. This emission band is asymmetric, and is composed of two emission bands, Em₁ and Em₂, due to the electronic transitions from the ³T₂ and ³T₁ excited states to the ¹A₁ ground state in [VO₄]³⁻ centers, respectively. The same emission band is obtained for micro- and nanoparticles. Each of the PLE spectra for the Em₁ and Em₂ emissions of the Cs₅V₃O₁₀ micro-particles consists of the same peaks at 280 and 365 nm which are called Ex₁ and Ex₂, respectively. These Ex₁ and Ex₂ bands are attributed to the transitions from the ground state ¹A₁ to the ¹T₂ and ¹T₁ states, respectively. The excitations into the Ex₁ and Ex₂ bands give the same emission band. In addition to the decay curve profiles of the Em₁ and Em₂ emission, the emission profiles indicate that the emission is not due to different kinds of [VO₄]³⁻ centers but only one kind of [VO₄]³⁻ center. When the temperature is increased from 10 K to 450 K, the emission intensity increases below 150 K and decreases above 150 K, and an unusual blue shift is observed. The emission lifetime decreases with increasing temperature, largely above 150 K. The observed temperature dependences can be understood by the thermal feeding by the lower-energy ³T₁ state to the higher-energy ³T₂ state, which is effective above about 50 K, and by the thermal activation process from the ³T₂ state to the ¹A₁ state which is effective above 150 K.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013R1A1A2009154) and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China.

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