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

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.


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 Eu 3+ 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 Dy 3+ [22], Tb 3+ [23], Tm 3+ [24], or Sm 3+ [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 Sb 3+ [26] or Li + [27]. Another possibility is the use of Bi 3+ , which increases the luminescent properties and broadens the excitation spectrum of europium ions in LuVO 4 [28] or Lu 2 O 3 [29]. In the case of LuVO 4 , 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 TiO 2 in catalytic and solar cell applications [33,34]. In the present work, transparent films of the LuVO 4 :Eu 3+ , Bi 3+ phosphor were obtained using sol-gel synthesis and the dip-coating method, which can be used in white light emission LEDs, using Bi 3+ as a sensitizer of Eu 3+ 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 Bi 3+ content.

Materials and Methods Synthesis
LuVO 4 :Eu 3+ , Bi 3+ films were prepared using the sol-gel method and dip-coating technique. First, lutetium acetate (CH 3 CO 2 ) 3 Lu•xH 2 O (99.9%, Sigma-Aldrich, Saint Louis, MO, USA) was dissolved in ethanol (99.5, C 2 H 6 O, 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 NH 4 VO 3 (99.0%, Fermont, Monterrey, México) dissolved in an ammonium hydroxide NH 4 OH (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 LuVO 4 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 Eu 3+ and Bi 3+ ions to the lutetium vanadate sol, europium nitrate Eu(NO 3 ) 3 ·5H 2 O (99.5%, Alfa Aesar, Tewksbury, MA, USA) was incorporated in order to obtain 2.5 mol % Eu 3+ samples, and bismuth was incorporated from a solution prepared by the dissolution of bismuth nitrate Bi(NO 3 ) 3 ·5H 2 O (98%, Sigma-Aldrich, Saint Louis, MO, USA) at a 1:1 molar ratio with ethanol:diethyleneglycol (C 4 H 10 O 3 , 98%, Sigma-Aldrich, Saint Louis, MO, USA), obtaining a Bi 3+ 0.05 M concentration. Later, a fixed volume was incorporated into the lutetium vanadate sol to obtain a Bi 3+ 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 (HOCH 2 CH 2 ) 2 O (99% Sigma-Aldrich, Saint Louis, MO, USA) was first added (5.7 M), followed by acetylacetone (6.8 M) CH 3  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 LuVO 4 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.

Chemical Evolution and Structural Properties
To observe the xerogel chemical evolution during the annealing process in the sol-gel process for the synthesis of LuVO 4 : 2.5% at. Eu 3+ and 1.5% at. Bi 3+ 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 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 LuVO 4 can be observed at~810-830 cm −1 and~446-455 cm −1 , which correspond to the V-O bonds of the vanadate group (VO 2 3− ) and Lu-O, respectively [2]. It is important to notice that, from 700 • C, all the organic groups have been completely removed from the LuVO 4 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 Bi 3+ (not shown). All the previous observations can be verified by the diffraction patterns obtained by XRD.
First, the XRD patterns of LuVO 4 : 2.5 at. % Eu 3+ , X at. % Bi 3+ (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 LuVO 4 , and the only appreciable impurity, observed at ≈ 26.05 • , could be related to the formation of a secondary vanadate phase, LuVO 3 (98-018-5830 ICSD). It is important to notice that no evidence of the possible formation of impurities from Bi 3+ or Eu 3+ ions has been detected. This result is expected for Eu 3+ 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 BiVO 4 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 LuVO 4 peak presents a gradual shift to the lower degree with the increase in Bi 3+ content, implying the lattice expansion of LuVO 4 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 YVO 4 system [37], who reported the appearance of the BiVO 4 phase until the 4.0 at. % of Bi 3+ . Lu M. et al. [38]. also reported the absence of impurities for YVO 4 : 8% Eu, 10% Bi nanoparticles Finally, Zeng L. et al. [39] also reported the substitution of Lu 3+ by Bi 3+ ions in nanoparticles of LuVO 4 obtained by a hydrothermal method. Bi 3+ . 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 Lu 3+ by Bi 3+ 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. Eu 3+ , 1.5% at. Bi 3+ 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  On the other hand, Figure 3 shows the structural evolution of the LuVO 4 : 2.5% at. Eu 3+ , 1.5% at. Bi 3+ 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 LuVO 4 (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 αhkl was calculated, which is defined as [42]: where Ihkl is the relative intensity of the corresponding diffraction. Table 1 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 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]: where I hkl is the relative intensity of the corresponding diffraction. Table 1 shows the (200) preferential orientation-index α 200 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: 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.

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.

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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: where D is the crystallite size, K is a constant (0.9), 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.

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. % Eu 3+ , 1.5 at. % Bi 3+ 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. % Eu 3+ , 1.5 at. % Bi 3+ 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 1 cm 1 cm  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].  Figure 6 shows the excitation and emission spectra corresponding to the LuVO4: 2.5% at. Eu 3+ , 1.5% at. Bi 3+ films annealed at 1000 °C. The excitation spectrum shows a wide band at 250-380 nm, where the absorption of vanadate groups at λem = 615 nm can be assigned (VO4 3− ) according to Liang et al. [22]. On the other hand, the emission spectrum of the films of the LuVO4:Eu 3+ , Bi 3+ 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 Eu 3+ ions: 5 → 7 (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 (Bi 3+ , V 5+ ) and the subsequent energy transfer to Eu 3+ [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]. Micrographs of films containing LuVO 4 : 2.5 at. % Eu 3+ , 1.5 at. % Bi 3+ 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]. Figure 6 shows the excitation and emission spectra corresponding to the LuVO 4 : 2.5% at. Eu 3+ , 1.5% at. Bi 3+ 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 (VO 4 3− ) according to Liang et al. [22]. On the other hand, the emission spectrum of the films of the LuVO 4 :Eu 3+ , Bi 3+ 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 Eu 3+ ions: 5D 0 → 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 (Bi 3+ , V 5+ ) and the subsequent energy transfer to Eu 3+ [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 Bi 3+ for a mono-doped LuVO4: 1.0 at. % Bi 3+ 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 1 S0→ 3 P1 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 Bi 3+ in the co-doped samples, compared with the mono-doped one. As the Bi 3+ content increases, the 550 nm emission almost disappears because of the energy transfer.

Photoluminescence Study
(a) (b) On the other hand, the luminescence properties of bismuth ions were analyzed. Figure 7a presents the excitation and emission spectra of Bi 3+ for a mono-doped LuVO 4 : 1.0 at. % Bi 3+ 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 1 S 0 → 3 P 1 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 Bi 3+ in the co-doped samples, compared with the mono-doped one. As the Bi 3+ content increases, the 550 nm emission almost disappears because of the energy transfer.
Therefore, the previous results establish an energy transfer process from Bi 3+ to Eu 3+ . Similar behavior has been observed by Zhu et al. (2013) [53], and it is possible to propose the mechanism for LuVO 4 :Eu 3+ , Bi 3+ presented in Figure 8. First, when LuVO 4 is excited by the UV light, the energy could be absorbed by Bi 3+ via the 1 S 0 → 3 P 1 transition. Next, there are two different types of excited states of Bi 3+ , 3 P 1 , which relaxes to the 1 S 0 ground state, producing a broad luminescence band that could transfer energy to the Eu 3+ . Thereafter, the absorbed energy can be transferred to the 5 D 2 level of Eu 3+ , and the emission of Bi 3+ partly quenches. At the same time, Eu 3+ ions can also excite to 5 D 2 from the ground state, 7 F 0 , and all the energy falls to the ground state, 5 D 0 . Immediately, red light emission peaks at 615 nm occur because of the 5 D 0 →F 2 characteristic transitions of Eu 3+ ions.
the greenish emission at 550 nm, products of the energy transition S0→ P1 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 Bi 3+ in the co-doped samples, compared with the mono-doped one. As the Bi 3+ content increases, the 550 nm emission almost disappears because of the energy transfer.  Therefore, the previous results establish an energy transfer process from Bi 3+ to Eu 3+ . Similar behavior has been observed by Zhu et al. (2013) [53], and it is possible to propose the mechanism for LuVO4:Eu 3+ , Bi 3+ presented in Figure 8. First, when LuVO4 is excited by the UV light, the energy could be absorbed by Bi 3+ via the 1 S0→ 3 P1 transition. Next, there are two different types of excited states of Bi 3+ , 3 P1, which relaxes to the 1 S0 ground state, producing a broad luminescence band that could transfer energy to the Eu 3+ . Thereafter, the absorbed energy can be transferred to the 5 D2 level of Eu 3+ , and the emission of Bi 3+ partly quenches. At the same time, Eu 3+ ions can also excite to 5 D2 from the ground state, 7 F0, and all the energy falls to the ground state, 5 D0. Immediately, red light emission peaks at 615 nm occur because of the 5 D0→F2 characteristic transitions of Eu 3+ ions. The influence of the Bi 3+ content on the luminescence of Eu 3+ is observed in Figure 9a at λexc = 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 λexc = 308 nm, to observe that there is indeed a process of increment in the emission of Eu 3+ due to the presence of bismuth. Furthermore, the emission of Eu 3+ is highly dependent on the surroundings of the cation. It is possible to set the ratio R = I ( 5 D0 → 7 F2)/I ( 5 D0 → 7 F1) as a reference to the coordination state and symmetry since the intensity of the 5 D0 → 7 F1 magnetic dipole transition (centered at 594 nm) is independent of the surroundings of the Eu 3+ , whereas the intensity of the 5 D0 → 7 F2 electric dipole transition (centered at 618 nm) is very sensitive to site symmetry. If the R value is low, the Eu 3+ cation tends to localize at a centrosymmetric site or a high-symmetry site. In contrast, when the R value is high, the Eu 3+ cation tends to localize at a non-centrosymmetric site or a low-symmetry site [54]. For the LuVO4: 2.5% at. Eu 3+ , X% at. Bi 3+ , 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 Bi 3+ content promotes the emission of the The influence of the Bi 3+ content on the luminescence of Eu 3+ 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 Eu 3+ due to the presence of bismuth. Furthermore, the emission of Eu 3+ is highly dependent on the surroundings of the cation. It is possible to set the ratio R = I ( 5 D 0 → 7 F 2 )/I ( 5 D 0 → 7 F 1 ) as a reference to the coordination state and symmetry since the intensity of the 5 D 0 → 7 F 1 magnetic dipole transition (centered at 594 nm) is independent of the surroundings of the Eu 3+ , whereas the intensity of the 5 D 0 → 7 F 2 electric dipole transition (centered at 618 nm) is very sensitive to site symmetry. If the R value is low, the Eu 3+ cation tends to localize at a centrosymmetric site or a high-symmetry site. In contrast, when the R value is high, the Eu 3+ cation tends to localize at a non-centrosymmetric site or a low-symmetry site [54]. For the LuVO 4 : 2.5% at. Eu 3+ , X% at. Bi 3+ , 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 Bi 3+ content promotes the emission of the low-symmetry sites. as observed in Figure 9b, shows that as the Bi 3+ content increases, the energy transfer efficiency from Bi 3+ →Eu 3+ 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 Eu 3+ , an ET of 68% was obtained for Tm 3+ →Eu 3+ in Li-LaSiO4:Tm 3+ , Eu 3+ [56]; 78% for Tb 3+ →Eu 3+ in Ba2SiO4 [57]; or 17% for Sm 3+ →Eu 3+ on TeO2−GeO2-ZnO glasses [24].
(a) (b) 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]: where η0 and η represent the quantum efficiencies of Bi 3+ ions in the absence and presence of Eu 3+ ions, respectively, the ratio 0/ can be calculated by the ratio of luminous intensities ( 0/ S), and C is the total molar concentration of Bi 3+ and Eu 3+ , 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 Bi 3+ to Eu 3+ in the LuVO4 system occurs via quadrupole-quadrupole interactions. This electronic interaction is in good agreement with other Bi 3+ →Eu 3+ energy transfer processes on other systems; for example, the same process has been observed in a Bi 3+ /Eu 3+ -doped SiO2-Al2O3-CaO-B2O3-La2O3-K2O glass, with an ET efficiency of 86% [58], and also on the Ca10(PO4)6F2:Bi 3+ , Eu 3+ , with an ET efficiency of 50% [53]. On the other hand, it is possible to calculate the energy transfer efficiency (ET) between Bi 3+ →Eu 3+ [55]: where I S is the luminous intensity of Bi 3+ ions in the presence of Eu 3+ ions, and I S0 is the luminous intensity of Bi 3+ ions in the absence of Eu 3+ ions. The energy transfer efficiency, as observed in Figure 9b, shows that as the Bi 3+ content increases, the energy transfer efficiency from Bi 3+ →Eu 3+ 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 Eu 3+ , an ET of 68% was obtained for Tm 3+ →Eu 3+ in LiLaSiO 4 :Tm 3+ , Eu 3+ [56]; 78% for Tb 3+ →Eu 3+ in Ba 2 SiO 4 [57]; or 17% for Sm 3+ →Eu 3+ on TeO 2− GeO 2− 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]: where 0 and η represent the quantum efficiencies of Bi 3+ ions in the absence and presence of Eu 3+ ions, respectively, the ratio η 0 /η can be calculated by the ratio of luminous intensities (I S0 /I S ), and C is the total molar concentration of Bi 3+ and Eu 3+ , 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 Bi 3+ to Eu 3+ in the LuVO 4 system occurs via quadrupole-quadrupole interactions. This electronic interaction is in good agreement with other Bi 3+ →Eu 3+ energy transfer processes on other systems; for example, the same process has been observed in a Bi 3+ /Eu 3+ -doped SiO 2 - with an ET efficiency of 86% [58], and also on the Ca 10 (PO 4 ) 6 F 2 :Bi 3+ , Eu 3+ , with an ET efficiency of 50% [53].
To corroborate the energy-transfer process, the Bi 3+ fluorescence decay behavior has been studied. Figure 11 shows the fluorescence decay curves of LuVO4:Eu 3+ , Bi 3+ 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]: = + + (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 Bi 3+ sample, which corroborates the energy mechanism process between Bi 3+ ions to Eu 3+ ions. Furthermore, as the relation of Eu 3+ /Bi 3+ increases (from 1.5 to 0.5 Bi 3+ samples), the fluorescence lifetime decreases. This effect can be explained by the fact that, as this Eu 3+ /Bi 3+ ratio increases, the Bi 3+ →Eu 3+ ion distance must shorten, and the interaction gradually increases, so the fluorescence lifetime of Bi 3+ 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]. Figure 10. Fitting relationship of I S0 /I S of Bi 3+ on C(Bi 3+ +Eu 3+ ) 6/3 for a dipole-dipole interaction; C (Bi 3+ +Eu 3+ ) 8/3 for a dipole-quadrupole interaction, and C(Bi 3+ +Eu 3+ ) 10/3 for a quadrupolequadrupole interaction.
To corroborate the energy-transfer process, the Bi 3+ fluorescence decay behavior has been studied. Figure 11 shows the fluorescence decay curves of LuVO 4 : Eu 3+ , Bi 3+ 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]: where I(t) is the luminescence intensity at time t, τ 1 and τ 2 are rapid and slow decay times for the exponential components, respectively, and A 1 and A 2 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 τ 1 and τ 2 , it can be observed that the values of the co-doped samples are smaller compared with the mono-doped Bi 3+ sample, which corroborates the energy mechanism process between Bi 3+ ions to Eu 3+ ions. Furthermore, as the relation of Eu 3+ /Bi 3+ increases (from 1.5 to 0.5 Bi 3+ samples), the fluorescence lifetime decreases. This effect can be explained by the fact that, as this Eu 3+ /Bi 3+ ratio increases, the Bi 3+ →Eu 3+ ion distance must shorten, and the interaction gradually increases, so the fluorescence lifetime of Bi 3+ ions decreases. The fast decay component (τ 1 ) is derived from the fluorescence decay, whereas the slow decay component (τ 2 ) 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 colo matching function, the correlated color temperature (CCT), and the color purity (CP), to understand the fluoresce center [66][67][68][69]. Table 2 presents the CIE color coordinates fo 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 sil icate 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 mov ing to the red emission region is due to the energy transfer, as explained earlier, which i congruent with similar reports [70,71]. Table 2 also shows the RGB coordinates, calculated from the x and y coordinate val ues, and the hex values, from the RGB coordinates [72,73]. The hex code represents a use ful 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. 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 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].  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].

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. Eu 3+ , X% at. Bi 3+ (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-  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].

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. Eu 3+ , X% at. Bi 3+ (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-  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].

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. Eu 3+ , X% at. Bi 3+ (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-  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].

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. Eu 3+ , X% at. Bi 3+ (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-  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].

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. Eu 3+ , X% at. Bi 3+ (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- 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].  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].

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. Eu 3+ , X% at. Bi 3+ (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- Figure 12. CIE chromaticity coordinates for the LuVO 4 : 2.5 at. % Eu 3+ , X at. % Bi 3+ (X = 0.5, 1, 1.5) films and a Bi 3+ mono-doped sample.

Conclusions
A new sol-gel synthesis was carried out to easily obtain films of the LuVO 4 system at different concentrations of bismuth, LuVO 4 : 2.5% at. Eu 3+ , X% at. Bi 3+ (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 Bi 3+ to Eu 3+ , which effectively increases the light yield of the 5 D 0 → 7 F 2 transition of Eu 3+ . Furthermore, it was shown that the energy transfer increases as the Bi 3+ concentration increases, reaching an energy transfer efficiency for Bi 3+ →Eu 3+ of 71% with 1.5 at. % Bi 3+ , 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 Eu 3+ . The produced films could have different possible applications in white emission LEDs, mobile devices screens, lasers, and optoelectronic devices.