Luminescence Properties and Energy Transfer of Eu³⁺, Bi³⁺ Co-Doped LuVO₄ Films Modified with Pluronic F-127 Obtained by Sol–Gel

Abstract

Nowadays, orthovanadates are studied because of their unique properties for optoelectronic applications. In this work, the LuVO₄:Eu³⁺, Bi³⁺ films were prepared by the sol–gel method, using a new simple route, and deposited by the dip-coating technique. The obtained films are transparent, fracture-free, and homogenous. The sol–gel process was monitored by Fourier-transform infrared spectroscopy (FTIR), and according to X-ray diffraction (XRD) results, the crystal structure was tetragonal, and films that were highly oriented along the (200) low-energy direction were obtained. The morphological studies by scanning electron microscopy (SEM) showed uniformly distributed circular agglomerations of rice-like particles with nanometric sizes. The luminescence properties of the films were analyzed using a fixed concentration of 2.5 at. % Eu³⁺ and different concentrations of Bi³⁺ (0.5, 1.0, and 1.5 at. %); all the samples emit in red, and it has been observed that the light yield of Eu³⁺ is enhanced as the Bi³⁺ content increases when the films are excited at 350 nm, which corresponds to the ¹S₀→³P₁ transition of Bi³⁺. Therefore, a highly efficient energy transfer mechanism between Bi³⁺ and Eu³⁺ has been observed, reaching up to 71%. Finally, it was established that this energy transfer process occurs via a quadrupole–quadrupole interaction.

1. Introduction

Nowadays, phosphor materials with luminescent properties prepared by various techniques are extensively studied because of their great ability to convert UV radiation to the visible spectrum and its multiple applications in different fields, such as integrated photonics [1], optics [2,3], and electronics [4,5]. Most luminescent materials are usually oxides, sulfides, or oxysulfides doped with some transition metal or rare-earth element [6,7,8], but in recent years, some lanthanides known as orthovanadates are currently used as matrices due to their high emission efficiency [9,10,11,12]. Additionally, these materials have unique properties due to the stoichiometric combination of their components and their symmetry with the crystal of the complex oxides; these characteristics increase the luminescent activity in comparison with the simple materials [13,14,15,16,17,18]. Likewise, rare-earth ions that are used as impurities in orthovanadates have shown strong emissions as well as variations in the emission color corresponding to the different activating ions under the excitation of an electron beam or ultraviolet. The luminescent properties presented in these materials depend largely on the location of the 4f energy levels of the dopants and the ratio of the valence and conduction bands (VB and CB, respectively) of the matrix [3,12,19,20,21]. In the energy transfer, the co-doped materials are interesting for optoelectronic applications because of increases in their luminescent effect. Particularly for the Eu³⁺ emission, several ions have been proposed to increase its well-known reddish emission, for example, usually by the use of other rare earths, such as Dy³⁺ [22], Tb³⁺ [23], Tm³⁺ [24], or Sm³⁺ [25]. However, the search of other non-rare earths sensitizers has attracted attention in order not only to increase the light yield, but also to modify the excitation wavelength. Therefore, other ions have been studied, like Sb³⁺ [26] or Li⁺ [27]. Another possibility is the use of Bi³⁺, which increases the luminescent properties and broadens the excitation spectrum of europium ions in LuVO₄ [28] or Lu₂O₃ [29]. In the case of LuVO₄, since it has been shown to have better optical properties than yttrium vanadate [2,8,30], several routes of synthesis have been studied to obtain higher optical properties. However, most of the synthesis routes that have been developed require high temperatures and highly controlled environments to carry out the reaction, for example, in solid state or hydrothermal reaction synthesis. Consequently, new energy-saving simple-synthesis processes are required that can be scalable to industry. In the case of sol–gel synthesis, it allows one to prepare homogeneous, high-purity materials with the possibility of controlling morphologies and impurities at low preparation temperatures without extremely controlled environments. On the other hand, Pluronic F-127 acid was added to the synthesis because it changes the properties of the film, as was shown in Tsotetsi et al. in 2022 [31] and Arconada et al. in 2010 [32], where it was used to increase the thickness because of its viscosity, for providing porosity to the film, and as a morphology controller. Another characteristic of Pluronic F-127 is that it acts as a structure-directing agent in TiO₂ in catalytic and solar cell applications [33,34]. In the present work, transparent films of the LuVO₄:Eu³⁺, Bi³⁺ phosphor were obtained using sol–gel synthesis and the dip-coating method, which can be used in white light emission LEDs, using Bi³⁺ as a sensitizer of Eu³⁺ to improve red emission intensity under UV radiation. The films were characterized in terms of their crystalline structure and excitation/emission intensity spectra, as well as in relation to the effect of the annealing temperature and as a function of the Bi³⁺ content.

2. Materials and Methods

Synthesis

LuVO₄:Eu³⁺, Bi³⁺ films were prepared using the sol–gel method and dip-coating technique. First, lutetium acetate (CH₃CO₂)₃Lu•xH₂O (99.9%, Sigma-Aldrich, Saint Louis, MO, USA) was dissolved in ethanol (99.5, C₂H₆O, Fermont, Monterrey, México) under vigorous magnetic stirring at 60 °C, obtaining a sol with a 0.15 M lutetium concentration. Later, a second sol was prepared from ammonium metavanadate NH₄VO₃ (99.0%, Fermont, Monterrey, México) dissolved in an ammonium hydroxide NH₄OH (30%, Fermont, Monterrey, México) solution in distilled water (36 M), obtaining a sol with a 0.09 M vanadium concentration after vigorous magnetic stirring at 80 °C for 1 h. Later, the two sols were combined to obtain the LuVO₄ sol-precursor, and nitric acid was added (0.5 M) to obtain a clear, homogeneous, and bluish solution after 2 h of stirring at 80 °C. On the other hand, to add the Eu³⁺ and Bi³⁺ ions to the lutetium vanadate sol, europium nitrate Eu(NO₃)₃·5H₂O (99.5%, Alfa Aesar, Tewksbury, MA, USA) was incorporated in order to obtain 2.5 mol % Eu³⁺ samples, and bismuth was incorporated from a solution prepared by the dissolution of bismuth nitrate Bi(NO₃)₃·5H₂O (98%, Sigma-Aldrich, Saint Louis, MO, USA) at a 1:1 molar ratio with ethanol:diethyleneglycol (C₄H₁₀O₃, 98%, Sigma-Aldrich, Saint Louis, MO, USA), obtaining a Bi³⁺ 0.05 M concentration. Later, a fixed volume was incorporated into the lutetium vanadate sol to obtain a Bi³⁺ 0.5, 1, and 1.5 at. % doping-level. Finally, to modify the viscosity of the previously obtained sol and therefore obtain homogeneous films, diethylene glycol (HOCH₂CH₂)₂O (99% Sigma-Aldrich, Saint Louis, MO, USA) was first added (5.7 M), followed by acetylacetone (6.8 M) CH₃COCH₂COCH₃ (99% Sigma-Aldrich, Saint Louis, MO, USA), 0.8 M of HNO₃, and 0.006 M Pluronic F-127 (C₃H₆O·C₂H₄O, 99% Sigma-Aldrich, Saint Louis, MO, USA). The final sol can be used to obtain powders or films; for powders, the sol was dried at 100 °C for 24 h and then annealed at several temperatures from 200 to 1000 °C for 3 h for each temperature.

For the dip-coating procedure, the sols were filtered using a 0.2 µm filter, and carefully cleaned silica glass substrates (QSI quartz, Quartz Scientific Inc., Lake County, OH, USA, refractive index = 1.417) were dipped into the prepared sol and pulled up at a constant rate of 4 cm·s⁻¹. After each dipping, the films were dried at 100 °C for 10 min to remove the water and the most volatile organics components. Subsequently, the film was annealed at 300 °C and 500 °C for 10 min each to remove the organic remnants. The dipping cycle was repeated 3 times. Finally, to crystallize the LuVO₄ phase, the films were annealed from 600 to 1000°C for 3 h.

The Fourier-transform infrared spectroscopy (FTIR) spectra of the samples were recorded in the range of 4000–400 cm⁻¹, using FTIR (FTIR 2000, Perkin Elmer, Waltham, MA, USA) and the KBr pelleting technique. The phase composition of the powders was identified by X-ray diffraction (XRD) at room temperature on a powder diffractometer (Bruker D8Advance, Billerica, MA, USA), using Cu Kα radiation (1.5418 Å). The morphology studies were carried out with a Zeiss Supra 55VP scanning electron microscope. The luminescence study was carried out at room temperature with a fluorescence spectrophotometer Hitachi F-7000, equipped with a 150 W xenon lamp and an R955 photomultiplier tube.

3. Results and Discussion

Chemical Evolution and Structural Properties

To observe the xerogel chemical evolution during the annealing process in the sol–gel process for the synthesis of LuVO₄: 2.5% at. Eu³⁺ and 1.5% at. Bi³⁺ sample, the FTIR study at different temperatures (from 200 to 1000 °C) was carried out (Figure 1). From 200 to 400 °C, strong bands at 3300 cm⁻¹ (ν), 1650 cm⁻¹ (δ), and 750 cm⁻¹ (δ) are related to the O-H groups due to the presence of water and alcohols (ethanol and diethilenglycol). All these bands disappear at 600 °C, which indicates that this temperature is adequate to eliminate the O-H groups present in the samples.

In addition, bands associated with the carbonyl groups (-COOH) ~1725–1700 cm⁻¹ are observed and attributed to the stretching bond of the lutetium acetate precursor employed, and are completely removed at 600 °C, which is an indicator of the moment of the beginning of crystallization process [8]. Bands around ~1430–1100 cm⁻¹ are observed, which correspond to the symmetric and asymmetric vibrations of the C-H bonds, probably from the use of Pluronic F-127; however, as can be observed, they disappear at 700 °C [35,36]. Next, at 500 °C, bands related to the crystallization of LuVO₄ can be observed at ~810–830 cm⁻¹ and ~446–455 cm⁻¹, which correspond to the V-O bonds of the vanadate group (VO₂³⁻) and Lu-O, respectively [2]. It is important to notice that, from 700 °C, all the organic groups have been completely removed from the LuVO₄ system, and, therefore, all the luminescent properties correspond only to the ceramic film. The same results have been obtained for the 0, 0.5, and 1.0 at. % of Bi³⁺ (not shown). All the previous observations can be verified by the diffraction patterns obtained by XRD.

First, the XRD patterns of LuVO₄: 2.5 at. % Eu³⁺, X at. % Bi³⁺ (X = 0.5, 1, 1.5) powders derived from the same sol used to prepare the films, annealed at 1000 °C, are presented in Figure 2. It can be observed that all samples are exactly in agreement with the corresponding 98-041-9281 ICSD from LuVO₄, and the only appreciable impurity, observed at ≈ 26.05°, could be related to the formation of a secondary vanadate phase, LuVO₃ (98-018-5830 ICSD). It is important to notice that no evidence of the possible formation of impurities from Bi³⁺ or Eu³⁺ ions has been detected. This result is expected for Eu³⁺ because, being a rare earth, it can occupy the position of Lu in the vanadate structure. On the other hand, regarding the case of bismuth, since it would also present a tetragonal structure forming BiVO₄ and the same valence as Lu, it can be possible to have at least a partial dissolution of Bi in the structure. Furthermore, the inset in Figure 2 shows that the principal LuVO₄ peak presents a gradual shift to the lower degree with the increase in Bi³⁺ content, implying the lattice expansion of LuVO₄ when the Lu atoms in the lattices are gradually substituted by Bi, which has a bigger radius. Similar results have been reported previously by for the YVO₄ system [37], who reported the appearance of the BiVO₄ phase until the 4.0 at. % of Bi³⁺. Lu M. et al. [38]. also reported the absence of impurities for YVO₄: 8% Eu, 10% Bi nanoparticles Finally, Zeng L. et al. [39] also reported the substitution of Lu³⁺ by Bi³⁺ ions in nanoparticles of LuVO₄ obtained by a hydrothermal method.

On the other hand, Figure 3 shows the structural evolution of the LuVO₄: 2.5% at. Eu³⁺, 1.5% at. Bi³⁺ film as the annealing temperature increases. As observed, the crystallization process begins at 650 °C, which confirms the FTIR observation. The diffraction peaks can be ascribed to the diffraction pattern of tetragonal lutetium vanadate LuVO₄ (98-041-9281, ICSD), space group I41/amd, and with the lattice systems a (7.01 Å), b (7.01 Å), and c (6.19 Å).

It is evident that the increment in the annealing temperature increases the crystallization of the films. At 600–650 °C, the broad peak centered at ~25–25.5° is typical for the amorphous structure obtained by the xerogel formation by the sol–gel process [40]. This amorphous structure disappears at 700 °C and with the increment in temperature, other peaks characteristic of the lutetium vanadate structure do not appear, which indicates that the films are highly oriented towards the (200) direction. In general, this result can probably be attributed to the films which, when deposited, tend to be structured first in the direction of lower surface energy, in this case, the (200) direction. Additionally, the presence of F127 probably tends to decrease the surface energy of the deposited xerogel; thus, the films maintain this preferential orientation. Furthermore, it has been stated [41] that the slow evaporation of high-molecular-weight alcohol, upon heating, like the diethylene glycol used in the present work, allows the structural orientation of the film before crystallization, preferable in the kinetically favored orientation along the c-axis (200). In order to quantify this observation, the preferential orientation parameter α_hkl was calculated, which is defined as [42]:

$${\propto }_{hkl}=\frac{{\mathrm{I}}_{hkl}}{\sum {\mathrm{I}}_{hkl}}$$

(1)

where I_hkl is the relative intensity of the corresponding diffraction.

Table 1. Crystallite size and preferential orientation index of LuVO₄: 2.5% at. Eu³⁺, 1.5% at. Bi³⁺ films as a function of annealing temperature.

System	Annealing Temperature (°C)	Crystallite Size (nm)	Preferential Orientation Index α200
LuVO4: Eu3+ 2.5 at. %, Bi3+ 1.5 at. %	700	68.2 ± 6.2	0.92
	800	71.6 ± 7.8	0.91
	900	66.9 ± 4.2	0.91
	1000	80.8 ± 6.1	0.90

shows the (200) preferential orientation-index α₂₀₀ of the prepared films, and as observed, it practically does not present changes due to the effect of temperature increase, remaining at approximately 0.90, which shows that the orientation process was carried out from low temperatures and, once the material crystallized, the system tends to maintain that orientation. It is worth mentioning that when Pluronic F-127 acid is used, in some studies, it has been shown that it changes the size of the particles and their orientations [36,37,38,39,40,41,42,43], which is a very desirable property for luminescent films since oriented films tend to minimize the scattering of light during emission process, therefore incrementing its quantum efficiency [44,45]. On the other hand, the crystallite size does not present a significant difference with the increment in the annealing temperature, as can be observed in Table 1. In this regard, the crystallite size was computed by the Scherrer equation:

$$\mathrm{D}=\frac{K\mathsf{\lambda }}{\mathsf{\beta }\cos \mathsf{\theta }}$$

(2)

where D is the crystallite size, K is a constant (0.9), λ is the X-ray wavelength (Cu-Kα = 0.15418 nm), β is the full width at half maximum (FWHM), and θ is the half diffraction angle of the peak centroid.

4. Morphological Studies

The obtained films were transparent and fracture-free, as can be observed in Figure 4a. Additionally, the luminescent macroscopic behavior under short and large UV- Vis excitation are presented in Figure 4b,c, respectively.

The scanning micrographs of the films, shown in Figure 5, correspond to LuVO₄: 2.5 at. % Eu³⁺, 1.5 at. % Bi³⁺ film, deposited and annealed at 700 °C (Figure 5a) and 1000 °C (Figure 5b) at 10,000×.

Micrographs of films containing LuVO₄: 2.5 at. % Eu³⁺, 1.5 at. % Bi³⁺ show a distribution of circular agglomerations of rice-like particles (according to NIST), morphology that is typical of vanadate [46,47,48], and as observed, these particles are distributed over the whole surface. The particle size was measured directly from the micrographs of 180 individual particles for both samples in such a way that it was possible to determine the average size is 579.3 ± 96.7 nm and 716.4 ± 123.8 nm, for the 700 °C and 1000 °C, respectively. The increment in the particle size can be explained since the energy excess with the higher temperature promotes the growth of particles. From Figure 4c, which corresponds to the film annealed at 1000 °C at 500×, it is possible to observe that these particle growth zones are distributed over the surface. Additionally, from the results shown in XRD, it was determined that the films grow preferentially in the (200) direction; therefore, the morphology of the particles corresponds to the growth in this direction. Similar morphologies have been observed in other yttrium vanadate films [49]. Finally, it is important to note that this preferential growth of the films is probably the product of the presence of Pluronic F-127 and diethylene glycol, which, as evaporate, would promote the formation of some pores [50,51].

5. Photoluminescence Study

Figure 6 shows the excitation and emission spectra corresponding to the LuVO₄: 2.5% at. Eu³⁺, 1.5% at. Bi³⁺ films annealed at 1000 °C. The excitation spectrum shows a wide band at 250–380 nm, where the absorption of vanadate groups at λ_em = 615 nm can be assigned (VO₄³⁻) according to Liang et al. [22]. On the other hand, the emission spectrum of the films of the LuVO₄:Eu³⁺, Bi³⁺ system is shown under excitation at 350 nm, and peaks can be observed at 594, 615, 650, and 700 nm that correspond to the transitions of Eu³⁺ ions: $5D_0^{}\to 7F_J^{}$ (J = 1, 2, 3, 4), respectively, of which the one located at 615 nm is the most acute, therefore it is the one with the highest emission. The band of bismuth emission, located at 550 nm and generated with 350 nm excitation, decreases, as shown by the curve in the blue color, which has been scaled by ten. The best luminescent properties were reported in the case of the higher bismuth content, which could be explained by the charge transfer of the metal-metal bond (Bi³⁺, V⁵⁺) and the subsequent energy transfer to Eu³⁺ [20], and a better index of refraction, density, possibly a reduction of the pores, and a structural alignment of the films, which reduces the dispersion of the emissions and for that reason, better patterns are obtained [52].

On the other hand, the luminescence properties of bismuth ions were analyzed. Figure 7a presents the excitation and emission spectra of Bi³⁺ for a mono-doped LuVO₄: 1.0 at. % Bi³⁺ film. As can be observed, congruent results are obtained when comparing with the observations of the co-doped sample, being the wavelength excitation at 332 nm and the greenish emission at 550 nm, products of the energy transition ¹S₀→³P₁ for the excitation process and vice versa for the emission one. However, as also observed in Figure 6, for the co-doped samples, the greenish emission almost disappears, which clearly indicates an energy transfer process from Bi to Eu. Figure 7b presents the reduction of the 550 nm emission from Bi³⁺ in the co-doped samples, compared with the mono-doped one. As the Bi³⁺ content increases, the 550 nm emission almost disappears because of the energy transfer.

Therefore, the previous results establish an energy transfer process from Bi³⁺ to Eu³⁺. Similar behavior has been observed by Zhu et al. (2013) [53], and it is possible to propose the mechanism for LuVO₄:Eu³⁺, Bi³⁺ presented in Figure 8. First, when LuVO₄ is excited by the UV light, the energy could be absorbed by Bi³⁺ via the ¹S₀→³P₁ transition. Next, there are two different types of excited states of Bi³⁺, ³P₁, which relaxes to the ¹S₀ ground state, producing a broad luminescence band that could transfer energy to the Eu³⁺. Thereafter, the absorbed energy can be transferred to the ⁵D₂ level of Eu³⁺, and the emission of Bi³⁺ partly quenches. At the same time, Eu³⁺ ions can also excite to ⁵D₂ from the ground state, ⁷F₀, and all the energy falls to the ground state, ⁵D₀. Immediately, red light emission peaks at 615 nm occur because of the ⁵D₀→F₂ characteristic transitions of Eu³⁺ ions.

The influence of the Bi³⁺ content on the luminescence of Eu³⁺ is observed in Figure 9a at λ_exc = 350 nm. In this case, the intensity at 615 nm increases when the atomic content of bismuth is bigger, while the emission observed at 538 nm (corresponding with the bismuth emission) reduces in ratio with the increase in europium emission because an energy transfer occurs according to the energy diagram (Figure 8). For comparison purposes, the emission spectrum of an Eu-mono-doped sample has been added, with λ_exc = 308 nm, to observe that there is indeed a process of increment in the emission of Eu³⁺ due to the presence of bismuth. Furthermore, the emission of Eu³⁺ is highly dependent on the surroundings of the cation. It is possible to set the ratio R = I (⁵D₀ →⁷F₂)/I (⁵D₀ →⁷F₁) as a reference to the coordination state and symmetry since the intensity of the ⁵D₀ →⁷F₁ magnetic dipole transition (centered at 594 nm) is independent of the surroundings of the Eu³⁺, whereas the intensity of the ⁵D₀ →⁷F₂ electric dipole transition (centered at 618 nm) is very sensitive to site symmetry. If the R value is low, the Eu³⁺ cation tends to localize at a centrosymmetric site or a high-symmetry site. In contrast, when the R value is high, the Eu³⁺ cation tends to localize at a non-centrosymmetric site or a low-symmetry site [54]. For the LuVO₄: 2.5% at. Eu³⁺, X% at. Bi³⁺, the R factor increases from 3.91→4.73→4.81 for X = 0, 0.5, 1, and 1.5, respectively, showing that the increment in the Bi³⁺ content promotes the emission of the low-symmetry sites.

On the other hand, it is possible to calculate the energy transfer efficiency (ET) between Bi³⁺→Eu³⁺ [55]:

$${ŋ}_{ET}=1-\frac{I_S}{I_{S0}}$$

(3)

where I_𝑆 is the luminous intensity of Bi³⁺ ions in the presence of Eu³⁺ ions, and I_𝑆0 is the luminous intensity of Bi³⁺ ions in the absence of Eu³⁺ ions. The energy transfer efficiency, as observed in Figure 9b, shows that as the Bi³⁺ content increases, the energy transfer efficiency from Bi³⁺→Eu³⁺ also increases, reaching high values up to 71% in the 1.5 at. % sample. The ET values vary depending on the matrix and the co-doped ion. For comparison, for other proposed sensitizers for Eu³⁺, an ET of 68% was obtained for Tm³⁺→Eu³⁺ in LiLaSiO₄:Tm³⁺, Eu³⁺ [56]; 78% for Tb³⁺→Eu³⁺ in Ba₂SiO₄ [57]; or 17% for Sm³⁺→Eu³⁺ on TeO₂₋GeO₂₋ZnO glasses [24].

On the other hand, the energy transfer from the sensitizers to the activators could be achieved through electric multipole interactions. According to the Dexter energy level transfer, the electric multipole interactions can be related to [3]:

$$\frac{I_S}{I_{S0}}\approx \frac{{ŋ}_0}{ŋ} \propto C^{n/3}$$

(4)

where ŋ₀ and ŋ represent the quantum efficiencies of Bi³⁺ ions in the absence and presence of Eu³⁺ ions, respectively, the ratio 𝜂₀/𝜂 can be calculated by the ratio of luminous intensities (I_𝑆0/I_S), and C is the total molar concentration of Bi³⁺ and Eu³⁺, whereas n = 6, 8, or 10 correspond to electric dipole–electric dipole, electric dipole–electric quadrupole, or electric quadrupole–electric quadrupole interactions, respectively. The corresponding plots are presented in Figure 10, and as observed, the best linear fit is exhibited when n = 10/3, implying that the most probable interaction for the energy transfer from Bi³⁺ to Eu³⁺ in the LuVO₄ system occurs via quadrupole–quadrupole interactions. This electronic interaction is in good agreement with other Bi³⁺→Eu³⁺ energy transfer processes on other systems; for example, the same process has been observed in a Bi³⁺/Eu³⁺-doped SiO₂–Al₂O₃–CaO–B₂O₃–La₂O₃–K₂O glass, with an ET efficiency of 86% [58], and also on the Ca₁₀(PO₄)₆F₂:Bi³⁺, Eu³⁺, with an ET efficiency of 50% [53].

To corroborate the energy-transfer process, the Bi³⁺ fluorescence decay behavior has been studied. Figure 11 shows the fluorescence decay curves of LuVO₄: Eu³⁺, Bi³⁺ films at 550 nm when the systems are excited at 350 nm. The best fit of the experimental data was obtained using the double exponential equation [59]:

$$I\left(t\right)=I_0+A_1{e}^{-\raisebox{1ex}{$t$}\!\left/ \!\raisebox{-1ex}{${\tau }_1$}\right.}+A_2{e}^{-\raisebox{1ex}{$t$}\!\left/ \!\raisebox{-1ex}{${\tau }_2$}\right.}$$

(5)

where I(t) is the luminescence intensity at time t, τ₁ and τ₂ are rapid and slow decay times for the exponential components, respectively, and A₁ and A₂ are constants.

That the double exponential equation presented the best fit can be explained since, from classical kinetics, there are two emitting species [60], which is the case in co-doped systems. S Dutta et al. [61] established that this can occur for systems with low concentrations of lanthanides and that it can be due to three factors: (1) the difference in the nonradiative probability of decays for lanthanide ions at or near the surface and lanthanide ions in the core; (2) an inhomogeneous distribution of the doping ions in the host material, leading to the variation in the local concentration; and (3) the transfer of excitation energy from the donor to lanthanide activators [62,63,64].

From the calculation of the decay values of τ₁ and τ₂, it can be observed that the values of the co-doped samples are smaller compared with the mono-doped Bi³⁺ sample, which corroborates the energy mechanism process between Bi³⁺ ions to Eu³⁺ ions. Furthermore, as the relation of Eu³⁺/Bi³⁺ increases (from 1.5 to 0.5 Bi³⁺ samples), the fluorescence lifetime decreases. This effect can be explained by the fact that, as this Eu³⁺/Bi³⁺ ratio increases, the Bi³⁺→Eu³⁺ ion distance must shorten, and the interaction gradually increases, so the fluorescence lifetime of Bi³⁺ ions decreases. The fast decay component (τ₁) is derived from the fluorescence decay, whereas the slow decay component (τ₂) possibly results from the luminescence caused by defects of phosphors on the surface and in the bulk structure [65].

On the other hand, the quality of the light emission is evaluated by the CIE 1931 color matching function, the correlated color temperature (CCT), and the color purity (CP), to understand the fluoresce center [66,67,68,69].

Table 2. Chromaticity coordinates (x, y), correlated color temperature (CCT), color purity (CP), RGB coordinates, and Hex color.

Sample	CIEE Color Coordinates (x, y)	CTTK	CP	RGB			Hex	Hex Color
				R	G	B
Eu3+ mono-doped	(0.6423, 0.3416)	1026	0.92	255	34	0	#FF2200
Bi3+ mono-doped	(0.3611, 0.5124)	4939	0.59	189	255	80	#BDFF50
Eu3+ 2.5 at. %, Bi3+ 0.5 at. %	(0.5713, 0.3734)	1620	0.78	255	97	33	#FF6121
Eu3+ 2.5 at. %, Bi3+ 1.0 at. %	(0.5721, 0.3724)	1620	0.79	255	96	33	#FF0021
Eu3+ 2.5 at. %, Bi3+ 1.5 at. %	(0.5999, 0.3684)	1680	0.86	255	81	0	#FF5100

presents the CIE color coordinates for different concentrations of bismuth; the mono-doped Bi film presents coordinates (0.3611, 0.5124) that are congruent with the results of Krishnan (2018), which studied bismuth silicate glasses for white light generation and bismuth silicate oxyfluoride glasses for LED applications. The calculated CCT values are below 3000 K in the co-doped films and, therefore, could be useful for warm light sources. When Bi is incorporated with 2.5 at. % Eu, the CP improved to 0.86. This highest CP and low CCT with the CIE coordinates moving to the red emission region is due to the energy transfer, as explained earlier, which is congruent with similar reports [70,71].

Table 2 also shows the RGB coordinates, calculated from the x and y coordinate values, and the hex values, from the RGB coordinates [72,73]. The hex code represents a useful tool to present the actual color display used in LED displays. As observed, all the co-doped samples present reddish color, whereas the Bi mono-doped sample has a greenish color.

Finally, the CIE chromaticity coordinates (Figure 12) proved that when the bismuth concentration is 1.5 at. %, the color of the emission is reddish, according to the diagram. A displacement from emitting a yellow-green color to a red color appears when the pure bismuth (100%) is combined with a constant europium concentration (2.5 at, %); however, it is evident that the best energy transfer occurs with a greater amount of bismuth because in the case of 0.5 and 1% europium the displacement is minimal between them, but emission changes to the color red. This result is congruent with that obtained by Zhang (2018) [28].

6. Conclusions

A new sol–gel synthesis was carried out to easily obtain films of the LuVO₄ system at different concentrations of bismuth, LuVO₄: 2.5% at. Eu³⁺, X% at. Bi³⁺ (X = 0, 0.5, 1, and 1.5), using the dip-coating method at low temperatures. The results show that the first bond of Lu-O is obtained from 500 °C, and it is possible to obtain completely structured films from 700 °C. Furthermore, it has been stated that, due to the presence of Pluronic F-123 and diethylene glycol during the sol–gel process, it has been possible to obtain highly oriented films along the (200) direction. Luminescence studies show that there is an energy transfer mechanism from Bi³⁺ to Eu³⁺, which effectively increases the light yield of the ⁵D₀→⁷F₂ transition of Eu³⁺. Furthermore, it was shown that the energy transfer increases as the Bi³⁺ concentration increases, reaching an energy transfer efficiency for Bi³⁺→Eu³⁺ of 71% with 1.5 at. % Bi³⁺_, and it was observed that the energy transfer takes place via a quadrupole–quadrupole interaction. Finally, the actual real color from the co-doped samples is reddish, which is typical for the presence of Eu³⁺. The produced films could have different possible applications in white emission LEDs, mobile devices screens, lasers, and optoelectronic devices.