Understanding the structural and optical evolution of Eu3+ and Dy3+ co-doped YVO4 phosphors across concentration series for lighting applications

Review Understanding the structural and optical evolution of Eu 3+ and Dy 3+ co-doped YVO 4 phosphors across concentration series for lighting applications Houssem Eddine Sekrafi a,b,* , Eya Hergli a,b , Stanislav Ferdov a,b , Paulo J.G. Coutinho a,b , Luis Rebouta a,b a Physics Centre of Minho and Porto Universities (CF-UM-UP), University of Minho, Braga 4710-057, Portugal b Laboratory of Physics for Materials and Emergent Technologies, LaPMET, University of Minho, Braga 4710-057, Portugal ARTICLE INFO Keywords: Nanophosphors White LED Photoluminescence YVO 4 :x(Eu 3+ , Dy 3+ ) Band gap Energy transfer ABSTRACT YVO 4 nanoparticles co-doped with Eu 3+ and Dy 3+ ions were successfully synthesized using a conventional coprecipitation method. The resulting phosphors exhibited a single-phase trigonal YVO 4 structure, with nanoparticles averaging approximately 60 nm in size, obtained by measurements carried out with scanning electron microscope images. UV–visible diffuse reflectance spectroscopy analysis reveals that the direct band gaps of our samples fall within the range of 3.6–3.75 eV. After a thorough analysis, the optimal doping concentration was identified as 2 at% Eu 3 ⁺ and 2 at% Dy 3+ ions co-doping, exhibiting the strongest up-conversion emission intensity under 310 nm excitation. YVO 4 :x(Eu 3+ , Dy 3+ ) phosphors exhibit distinct bands corresponding to transitions of Dy 3+ and Eu 3+ ions from their 4 F 9/2 and 5 D 0 excited states, respectively. The energy transfer from Dy 3+ to Eu 3+ is validated through electric dipole–dipole interaction, with a critical distance of 15.63 Å. YVO 4 nanoparticles codoped with Eu 3+ and Dy 3+ ions exhibit wide-range control over their photoluminescence color by regulating the concentration of both dopants. These findings suggest great potential for applications in current industrial settings. 1. Introduction Reducing global energy consumption is a pressing challenge for researchers worldwide [1]. Developing and improving white light-emitting diodes (WLEDs) has gained significant attention as a potential replacement for conventional fluorescent and incandescent lamps [2]. This is due to WLEDs’ numerous advantages, including low energy consumption, long lifespan, high luminous efficiency, and environmental friendliness [3]. WLEDs are a crucial component of modern lighting technology, with a wide range of applications in general lighting, backlighting, and automotive lighting. A common approach for achieving white light utilizes a blue LED chip coated with a yellow phosphor, known as the "blue-pumping" method [4]. However, this method suffers from limitations like low color rendering index (CRI) and high correlated color temperature (CCT) [5]. An alternative method employs a red, green, and blue (RGB) LED chip combination, enabling a wider color gamut and higher CRI [5]. However, this approach necessitates precise control over the intensity and wavelength of each LED chip, increasing complexity and cost. A promising approach utilizes a single, dual-emitting phosphor, like YVO 4 :Bi 3+ , Eu 3+ [6,7], which simplifies manufacturing and reduces costs while maintaining high CRI and low CCT. This phosphor exhibits high thermal stability and excellent luminescence properties, making it ideal for WLED applications. Furthermore, Li + co-doping in the YPO 4 :Dy 3+ system presents exciting possibilities for display devices [8]. The co-doping enhances Dy 3+ emission intensity and improves thermal stability, making the phosphor suitable for high-power WLEDs [9]. Different trivalent rare-earth (RE) ions, such as Eu 3+ , Dy 3+ , Tm 3+ , and Sm 3+ , are commonly used as activators in the vanadate host lattice [10,11]. These RE ions can also be co-doped to enhance emission efficiency through energy transfer mechanisms. However, the co-doping of divalent ions can also significantly impact the emission spectra of the material. Furthermore, yttrium vanadate (YVO 4 ) doped with various RE ions exhibits remarkable versatility, covering nearly the entire visible spectrum with its diverse emission colors. YVO 4 is a critical host material due to its exceptional combination of properties. These include a * Corresponding author at: Physics Centre of Minho and Porto Universities (CF-UM-UP), University of Minho, Braga 4710-057, Portugal. E-mail address: [email protected] (H.E. Sekrafi). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom https://doi.org/10.1016/j.jallcom.2024.177389 Received 4 September 2024; Received in revised form 28 October 2024; Accepted 3 November 2024 Journal of Alloys and Compounds 1010 (2025) 177389 Available online 6 November 2024 0925-8388/© 2024 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). high melting point of approximately 1810 ◦C, isotropic light emission with a refractive index of around 2.02, low phonon energy (<900 cm −1 ), and a direct bandgap of approximately 3.6 eV [12]. Dy 3+ ions, with their characteristic 4 f [13] electronic configuration, exhibit emission spectra primarily originating from the 4 F 9/2 energy level. This results in two distinct narrow emission lines: a blue light emission (470–500 nm) corresponding to the 4 F 9/2 → 6 H 15/2 transition, and a yellow light emission (570–600 nm) corresponding to the 4 F 9/2 → 6 H 13/2 transition [13,14]. Notably, the intensity ratio of these yellow and blue emissions is highly sensitive to the surrounding crystal environment of the Dy 3+ ions. By carefully selecting a suitable host material and doping it solely with Dy 3+ , white-emitting phosphors can be achieved through manipulation of this intensity ratio [15]. In contrast, Eu 3+ ions are widely used as efficient activators for red-emitting phosphors. Their emission lines arise from electronic transitions originating from the excited 5 D 0 level to the various 7 F J (J =0, 1, 2, 3, 4, 5, 6) levels within the 4 f [16,17] electronic configuration [14,18]. Importantly, the emission spectra of Eu 3+ are strongly influenced by the symmetry of the sites they occupy within the host material. This sensitivity arises from the combined effects of electronic-dipole and magnetic-dipole interactions of the internal 4 f electrons. Therefore, the selection of an appropriate host material plays a critical role in determining the luminescent properties of inorganic phosphors by providing a suitable crystal field environment for the activator ions. To further enhance the luminescent characteristics of YVO 4 , doping with specific RE ions, such as Eu 3+ and Dy 3+ , presents a viable strategy. These ions introduce novel energy levels within YVO 4 ’s band gap, resulting in heightened luminescence efficiency and customizable emission colors. Building on this concept, researchers have investigated co-doping orthovanadates with Eu 3+ and Dy 3+ ions. They typically optimize the photoluminescence intensity by first doping the YVO 4 matrix with a single dopant (either Eu 3+ or Dy 3+ ) at varying concentrations. Subsequently, they introduce a series of different concentrations of Eu or Dy into the pre-doped YVO 4 :Eu 3+ or YVO 4 :Dy 3+ matrix. Our work presents a novel approach by directly co-doping the YVO 4 matrix with equal concentrations of both Eu 3+ and Dy 3+ ions. The nanophosphors of YVO 4 : x(Eu 3+ , Dy 3+ ) (where x=1 at%, 2 at%, 3 at%, and 5 at%, the same x atomic concentration for both) were synthesized using a coprecipitation technique. Subsequently, a comprehensive analysis of their structural, compositional, and optical characteristics was meticulously conducted. 2. Experimental 2.1. Materials The raw materials employed in the current study were sourced from Sigma-Aldrich (St. Louis, MO, USA), including sodium orthovanadate (Na 3 VO 4 ) with a purity of 99.98 % (trace metals basis), yttrium nitrate hexahydrate (Y(NO 3 ) 3 •6 H 2 O) with a purity of 99.9 %, europium (III) nitrate pentahydrate (Eu (NO 3 ) 3 •5 H 2 O) with a purity of 99.9 %, and dysprosium (III) nitrate hydrate (Dy (NO 3 ) 3 •xH 2 O) with a purity of 99.9 % in the desired proportions. 2.2. Synthesis of YVO 4 :x(Eu 3+ , Dy 3+ ) (x =1–5 at%) nanoparticles The nanophosphors were prepared through co-precipitation technique [19]. One mmol of Na 3 VO 4 was first dissolved in 3.5 mL of distilled water, and then, Y(NO 3 ) 3 •6 H 2 O (1 mmol) was dissolved in 0.5 mL of distilled water. For each sample, the desired amounts of Eu (NO 3 ) 3 •5 H 2 O and Dy(NO 3 ) 3 •xH 2 O were subsequently dissolved in separate aliquots of 0.5 mL of distilled water. Different samples were prepared with Eu and Dy dopant concentrations of 1, 2, 3, and 5 at. percent (at%). The solution was stirred using a magnetic stirrer for 10 minutes at room temperature. The resulting precipitate was then centrifuged, washed with distilled water, and dried at 50 ◦C. Subsequently, the final product was subjected to heating in an electric muffle furnace at 800◦C for a duration of 2 h. The yield of the product containing both Eu and Dy is 65 %. 2.3. Characterization X-ray diffraction (XRD) patterns were collected using a Bruker D8 Discover diffractometer with CuK α radiation (λ =1.5406 Å). The instrument operated in the θ/2θ scan mode over a range of 10◦-70◦with a step size of 0.04◦and an integration time of 1 second. Energy-dispersive X-ray spectroscopy (EDS) using an EDAX Pegasus X4M analyzer, with an acceleration voltage of 15 keV, determined the chemical composition of the particles. Particle size and morphology were investigated using an FEI NOVA NanoSEM 200 scanning electron microscope (SEM). ImageJ software was employed to analyze the particle size distributions. We manually selected 100 particles from each SEM image, excluding holes, for size measurement. ImageJ can characterize both spherical and nonspherical particles using various parameters, including longest and shortest diameters, perimeter, projected area, or equivalent spherical diameter. The reflectance of the compound was measured using a Shimadzu UV-3600i Plus UV-Vis-NIR spectrophotometer over a wavelength range of 250–850 nm with a step of 1 nm. The total reflectance was obtained with an incidence angle of 8◦using a 150 mm diameter integrating sphere (model ISR-1503, Shimadzu). A homemade portable setup equipped with a 310 nm light-emittingdiode (LED) excitation source and a CCS200 spectrometer from Thorlabs was used to measure the photoluminescence of Eu 3+ and Dy 3+ co-doped YVO 4 nanoparticles. All characterizations were performed at room temperature. 3. Results and discussion 3.1. XRD patterns, Phase structure and Rietveld refinement Fig. 1.a displays X-ray diffraction (XRD) patterns of YVO 4 phosphorous with varying atomic percentages of Eu 3+ and Dy 3+ ions, synthesized using the co-precipitation method and subsequently thermally annealed at 800 ◦C. All diffraction peaks observed in the patterns can be assigned to the reference YVO 4 phase (tetragonal, space group I41/amd, JCPDS 00–016–0250). Broad and intense peaks in the X-ray diffraction pattern indicate that the nanophosphors synthesized are wellcrystallized and have dimensions on the nanoscale [20]. No evidence of additional crystalline phases was found. The Rietveld refinement analysis of YVO 4 :3 % (Eu 3+ , Dy 3+ ), chosen as the subject of analysis, was performed using the FULLPROF suite program. The Pseudo-Voigt function was utilized to fit multiple parameters of the data points [21]. Fig. 1.b illustrates the Rietveld refinement of the XRD pattern for selected nanophosphor samples with x=3 mol%. The tetragonal structure of YVO 4 was initially used as a reference to refine the data towards the actual crystal structure. In the refinement plot, the solid line represents the calculated XRD pattern, while dots depict the observed XRD patterns. Vertical green bars indicate the Bragg positions of the tetragonal phase, and the lower profile shows the discrepancy between observed and calculated XRD patterns. The quality of the refined data was assessed by lower values of goodness of fit (GOF) = χ 2 ; for optimal refinement results, GOF should approach unity. Table 1 summarizes the calculated lattice parameters (a, b, c), as well as the unit cell volume (V). In the same table, we have also compiled the reliability factor (R F ), the Bragg R-factor, and the GOF. Based on the calculated values, it is evident that the refined parameters show minor deviations from the standard data for YVO 4 . This observation indicates the effective and consistent integration of Europium and Dysprosium ions into the YVO 4 host matrix. This integration is made possible by the similar ionic radius and chemical reactivity shared among Eu 3+ , Dy 3+ , H.E. Sekrafi et al. Journal of Alloys and Compounds 1010 (2025) 177389 2 and Y 3+ ions [22,23]. Moreover, this substitution can potentially impact the photoluminescent properties of the material, particularly the emission spectrum. The crystallite size of the nanophosphor was determined by utilizing the most prominent diffraction peak (200) and applying Debye-Scherrer’s formula as presented in Equation [24,25]: DSC =Kλ βcosθ(1) D SC represents the crystallite size, where K is a constant (K ≈0.89), λ denotes the wavelength of the x-rays used (0.15406 nm), θ corresponds to the angle of diffraction, and β represents the full width at half maximum (FWHM in radian). The calculated values of crystallite size revealed a range of 20–36 nm, as listed in the Table 1. The FWHM of XRD diffraction peaks reflects both lattice strain and crystallite size [26]. The Williamson-Hall equation quantifies this relationship, as shown below [25,27]: βcosθ=4 ε sinθ+Kλ DW−H (2) By utilizing a linear regression model to analyze the curve, focusing on the relationship between 4sin (θ) (x-axis) and βcos (θ) (y-axis) as shown in Fig. 2, we successfully calculated the strain ε by identifying the slope of this graph. Simultaneously, the crystallite size D W-H was determined by locating the point where the curve intersects the vertical axis at sin (θ) =0. The obtained values are detailed in Table 2. The crystallite size estimates obtained from the Scherrer’s and WilliamsonHall (W-H) plots exhibit minimal variation. This is because the strain component in Scherrer’s equation is typically assumed to be negligible. However, the W-H plot reveals that the 2 % and 5 % co-doped samples of Eu 3+ and Dy 3+ ions exhibit relatively lower strain values and higher crystallite sizes, which is certainly correlated. In this context, it is suggested that elevated strain levels could potentially result in a reduction of fluorescence intensity [28]. The Table 2 also includes dislocation density (δ), calculated as δ =1/D 2 (D =crystallite size from Fig. 1. (a) X-ray diffraction pattern of YVO 4 :x(Eu 3+ , Dy 3+ ) with x=1, 2, 3 and 5 mol% (b) Rietveld refinement was performed on the XRD pattern of the selected compound at a concentration of 3 mol%. Table 1 Refinement results and crystal structure data for the YVO 4 :x(Eu 3+ , Dy 3+ ) series (x =1 %, 2 %, 3 %, and 5 %) were compared to the reference standard of YVO 4 (space group I41/amd, JCPDS 1–0250). Parameters YVO 4 PDF#16–0250 YVO 4 :x(Eu 3þ , Dy 3þ ) 0 % 1 % 2 % 3 % 5 % a ¼b (Å) 7.1230 7.116 (3) 7.1164 (12) 7.114 (3) 7.1230 (17) c (Å) 6.2910 6.290 (3) 6.2888 (10) 6.289 (2) 6.2919 (14) c/b 0.8832 0.8839 0.8837 0.8840 0.8833 V (Å 3 )319.19 318.51 318.48 318.31 319.24 Space group I41/amd (141) Y/Eu/Dy X Y Z - 0.0000 0.7500 0.1250 0.0000 0.7500 0.1250 0.0000 0.7500 0.1250 0.0000 0.7500 0.1250 V     X Y Z - 0.0000 0.2500 0.3750 0.0000 0.2500 0.3750 0.0000 0.2500 0.3750 0.0000 0.2500 0.3750 O     X Y Z - 0.0000 0.4360 0.2038 0.0000 0.4376 0.2045 0.0000 0.4350 0.1990 0.0000 0.4370 0.2030 R Bragg - 7.88 7.02 8.39 9.03 R F - 5.83 7.35 7.28 8.18 GoF - 1.28 1.62 1.31 1.41 Fig. 2. W-H plot of YVO 4 :x(Eu 3+ , Dy 3+ ) nanophosphors from x=1, 2, 3 and 5 mol%. H.E. Sekrafi et al. Journal of Alloys and Compounds 1010 (2025) 177389 3 Debye-Scherrer) [29]. The data reveals an inverse relationship between crystallite size, dislocation density, and strain value. This is due to the surface area effect: smaller crystallites have a higher surface area to volume ratio, leading to increased strain and dislocation density in nanomaterials [30]. 3.2. Morphological analysis The surface morphology of a luminescent material is a critical factor that significantly influences its optical properties [31]. Surface features, irregularities, and nanostructures can alter light absorption, emission efficiency, and overall optical performance. Controlling and understanding the surface morphology is essential for optimizing the optical characteristics and performance of luminescent materials across various applications. Fig. 3.a displays the SEM micrographs of as-synthetized YVO 4 :x (Eu 3+ , Dy 3+ ) (for x=1 %, 2 %, 3 %, and 5 %) nanoparticles. The SEM results reveal a porous and agglomerated morphology, consisting of polycrystalline nanoparticles. The porous structure provides a high surface area-to-volume ratio, which can be beneficial for various applications, such as catalysis, adsorption, or energy storage. The agglomeration of the nanoparticles is a common phenomenon due to the high surface energy of the nanoscale materials, leading to the formation of larger, polycrystalline structures. By comparing the SEM images, it was found that the material co-doped with 2 % Eu 3+ and Dy 3+ exhibits uniform and well-dispersed quasi-spherical nanoparticles. Conversely, the sample co-doped with 5 % dopants displays significantly larger grain sizes, suggesting their formation during precipitation through the fusion of smaller precursor particles. This observation aligns with previous studies, indicating that dopant concentration influences the degree of particle aggregation and size [19]. The average particle sizes observed in the SEM images (Fig. 3.b) range from 35 to 65 nm. This is larger than the corresponding crystallite sizes obtained from the XRD results, which confirms the agglomeration of the nanophosphor particles. The EDS analysis was employed to assess the chemical purity and elemental composition of the YVO 4 :x(Eu 3+ , Dy 3+ ) materials (Fig. 4). The EDX spectra confirmed the presence of Y, V, Eu, Dy, and O through the observation of their characteristic peaks. The absence of any additional peaks suggests that the prepared samples are free from significant elemental impurities. 3.3. UV-Visible reflectance spectroscopy UV–Visible total reflectance spectra of YVO 4 :x (Eu 3+ , Dy 3+ ) doped with different concentrations of Eu 3+ and Dy 3+ (1, 2, 3, 5 mol%) has been depicted in Fig. 5. These measurements were carried out at ambient temperature over a wavelength range spanning from 250 nm to 850 nm. The uniform doping variation in reflectance occurs between 1 % and 5 % for the Eu 3+ and Dy 3+ dopants. The inset of Fig. 5 highlights the presence of a sharp absorption band at approximately 305 nm, attributed to the photoexcitation of electrons from the valence band of O 2- (2p electrons) to the conduction band of Y 3+ (4d level) within the host lattice [32]. This phenomenon can be further elucidated as follows: the granular shape morphology has revealed the presence of voids in Eu 3+ and Dy 3+ doped YVO 4 samples. Additionally, surface defects such as Frenkel defects existing in the lattice structure contribute to absorption at various wavelengths [33]. There are also small peaks at 756 nm and 810 nm due to the f-f transition of Dy 3+ and Eu 3+ ions [34]. It is noteworthy that while indirect transitions may occur concurrently with direct transitions, they often go undetected in the absorption spectrum due to their high energy and low probability. The most significant transition governs the semiconductor’s behavior; nevertheless, establishing the corresponding transition experimentally poses considerable challenges. The band gap energy of the co-doped YVO 4 :Eu 3+ , Dy 3+ nanophosphors was determined by analyzing the reflectance spectra using the Kubelka-Munk (K-M or F(R)) method. The K-M function is expressed as follows [35]: F(R) = (1−R)2 2R(3) In this context, R represents the reflectance of an infinitely thick sample, while the function F(R) is directly proportional to the extinction coefficient ( α ). The relation given by [36] allows us to visualize the spectra based on the calculated reflectance values: [F(R).hv]n=A(hv −Eg)(4) In above equation, h υ denotes the energy of the incident photon, E g stands for the energy of the optical bandgap, and A denotes a constant referred to as the band tailoring parameter. The value of ’n’ varies depending on the type of transition: 2 for direct allowed transitions, 3 for direct forbidden transitions, 1/2 for indirect allowed transitions, and 3/2 for indirect forbidden transitions. The direct band gap energy (E gd ) of co-doped YVO 4 : Eu 3+ , Dy 3+ (1, 2, 3, and 5 %) was calculated by plotting the square of the K–M function [F (R).h ν ] 2 versus the energy in electron volts and shown in Fig. 6.a. The linear part of the curve was extended to F(R) 2 =0 in order to determine the direct band gap energy. As shown, the band gap energy exhibits a slight increase with the concurrent co-doping of Eu and Dy, resulting in E gd values of 3.6 eV, 3.65 eV, 3.69 eV, and 3.72 eV for doping levels of 1 %, 2 %, 3 %, and 5 %, respectively. The determined optical bandgap value indicates that the analyzed compound qualifies as a semiconductor material, given that E g is lower strictly than 4 eV [37]. The observed rise in the optical band gap resulting from doping could be associated with changes in the local atomic structure or lattice configuration [38]. Another interpretation is associated with the Burstein-Moss effect (BM) [39], which clarifies the relationship between the band gap (E g ) and doping concentration. In heavily doped semiconductors, the Fermi level can shift into the conduction band (CB) due to the abundance of free charge carriers. This transition results in a notable change in the semiconductor’s optical properties, leading to an apparent widening of the band gap [40]. To validate the bandgap value and its nature, Fig. 6.b shows the first derivative of reflectance (dR/dλ) for our compounds. This derivative reveals peaks at approximately 3.67 eV, 3.65 eV, 3.72 eV, and 3.76 eV, corresponding to each doping level, respectively. These peaks confirm a direct bandgap, consistent with Marotti et al. [41]. 3.4. Photoluminescence Rare earth phosphors can be broadly categorized into two types based on their emission characteristics: broad-band emission phosphors and narrow-band emission phosphors. Broad-band emission phosphors, such as those doped with Eu 2+ or Ce 3+ , demonstrate emission due to 5d4f electronic transitions [42]. In contrast, narrow-band emission phosphors, such as those doped with Eu 3+ , Gd 3+ , Sm 3+ , Dy 3+ , Nd 3+ , Tm 3+ or Er 3+ exhibit emission arising from 4f-4f electronic transitions [43]. Considering the 310 nm excitation wavelength used in previous work with YVO 4 :Eu 3+ 2 at% [19] and findings reported in the literature for YVO 4 :Dy 3+ 2 at% [44] phosphors, we adopted the same excitation Table 2 The average crystallite size and lattice strain in the synthesized samples were estimated using Debye-Scherrer’s formula and the Williamson-Hall equation. Phosphor YVO 4 :x (Eu 3+ , Dy 3+ ) (at. mole%) Crystallite size D (nm) Strain ε (×10 −3 ) Dislocation density (nm −2 ) from Scherrer formula δ (×10 −4 ) D SC D WH 1 20 25 1.12±0.02 25 2 35 43 0.59±0.01 8 3 25 30 1.35±0.02 16 5 34 44 0.65±0.03 8 H.E. Sekrafi et al. Journal of Alloys and Compounds 1010 (2025) 177389 4 wavelength for our current investigation. Fig. 7 displays the emission spectra of the YVO 4 :Ln 3+ (Ln=Eu and/or Dy) nanocrystals, each doped with 2 at% Ln, under UV excitation. For Dy 3+ ions, the emission peaks are centered at 486 nm ( 4 F 9/2 → 6 H 15/2 ) and 577 nm ( 4 F 9/2 → 6 H 13/2 ), which can produce a yellow-green light from the YVO 4 :Dy 3+ phosphors [45,46]. The intensity of the yellow emission is stronger than the green emission due to the D 2d point symmetry of Dy 3+ ions without an inversion center in the YVO 4 host. The 4 F 9/2 → 6 H 13/2 transition is a hypersensitive forced electric dipole transition [47]. Its intensity can vary significantly by orders of magnitude depending on the local site symmetry of the emitting ion. In contrast, the intensity of the 4 F 9/2 → 6 H 15/2 transition is relatively insensitive to the surrounding environment [48]. The emission spectrum of YVO 4 :Eu 3+ arises from the 5 D 1 → 7 F 1 , 5 D 1 → 7 F 2 and 5 D 0 → 7 F J (J =1, 2, 3, 4) transitions of Eu 3+ at 597 nm, 619 nm, 622 nm, 652 nm and 702 nm, which is characteristic of red phosphors [19,49]. The presence of a red peak at 619 nm and 622 nm signifies the 5 D 0 → 7 F 2 transition of europium ions within the YVO 4 lattice. This transition is hypersensitive, further confirming the europium ions occupy sites lacking inversion symmetry [50]. PL spectra (Fig. 7) reveal that YVO 4 :2 %Dy 3+ and YVO 4 :2 %Eu 3+ phosphors emit yellow and red light, respectively. Notably, both possess intrinsic excitation peaks in the UV–visible region. Therefore, Fig. 3. (a) SEM images (b) particle distributions histogram of YVO 4 :x(Eu 3+ , Dy 3+ ) (with x=1 %, 2 %, 3 % and 5 %). H.E. Sekrafi et al. Journal of Alloys and Compounds 1010 (2025) 177389 5 incorporating Dy 3+ and Eu 3+ ions simultaneously into the YVO 4 host lattice is expected to achieve tunable color in a single-phase phosphor by adjusting the activator concentration. Remarkably, the intensity of YVO 4 :(Eu 3+ , Dy 3+ ) 2 at% is higher than that of both Eu-YVO 4 and DyYVO 4 . This observation suggests that the presence of an energy transfer mechanism between Eu 3+ and Dy 3+ within the YVO 4 host material, as already reported [51,52]. To investigate the effect of co-doping concentration on photoluminescence and determine the maximum allowable co-doping concentration, a series of samples were prepared with varying co-doping amounts of Eu 3+ and Dy 3+ (1 %, 2 %, 3 %, and 5 %), as shown in Fig. 8. The PL spectra consist of peaks at 486, 577, 597, 612, 619, 622, 655, 702, and 707 nm. There are no shifts in the peak positions of Eu 3+ and Dy 3+ ; however, the intensity of the dopant ions increases with varying co-dopant concentrations. Inset of Fig. 8 reveals a significant enhancement in the emission intensity for YVO 4 :(Eu 3+ , Dy 3+ ) 2 at% nanophosphor compared to other co-doping concentrations. This increase in luminescence intensity might be attributed to a decrease in the strain within the crystal lattice, when compared with sample with 1 at%. In contrast, the intensity significantly decreased when the doping concentration exceeded 3 mol%. This decrease in intensity, observed with increasing concentrations of Eu 3+ and Dy 3+ , can be attributed to a wellknown phenomenon in phosphors called concentration quenching [53, 54]. The concentration quenching tends to decrease the intensity. This is due to the cross-relaxation mechanisms of Eu +3 and Dy +3 , which exhibit strong quenching behavior [55]. Energy transfer from sensitizer (Dy 3+ ) to activator (Eu 3+ ) ions in the YVO 4 matrix is a multistage interaction that typically occurs via two primary mechanisms: exchange interaction and multipolar interaction [56,57]. The exchange interactions dominate when the critical distance (R c )is less than 5 Å, while other interactions are prominent when R c exceeds 5 Å. According to Blasse’s formula [58], the critical distance is roughly equivalent to twice the radius of a sphere that has the same volume: RC≈2(3V 4 π XcN)1/3 (5) In Equation (5), V, X C and N denote the unit cell volume, the critical concentration, and the number of lattice sites that can occupied by the activator ions, respectively. For YVO 4 : Eu 3+ , Dy 3+ (1, 2, 3, and 5 %) phosphors, the values of these variables are known and equal to V = 319.19 Å 3 , N =4, and X C =0.04. Notably, X C represents the total concentration of dopants required to achieve maximum energy transfer efficiency. Based on the aforementioned parameters, R c was determined to be 15.63 Å, exceeding the effective distance of 5 Å for energy exchange interactions. Consequently, in this photoluminescent system, energy transfer occurs through the multipolar interaction mechanism, which is also the primary cause of concentration quenching. According to Dexter’s energy resonance theory, the interaction type, denoted by the symbol (θ), can be expressed by the following formula [59]: I x=k(1+β(x)θ/3)−1(6) Fig. 4. The analysis of chemical elements in YVO 4 nanoparticles co-doped with Eu 3+ and Dy 3+ were analyzed using EDS (with x=2 % selected). Fig. 5. Experimental UV–visible reflectance spectrum of YVO 4 :x(Eu 3+ , Dy 3+ ) (with x=1 %, 2 %, 3 % and 5 %). The inset present the absorption spectrum. H.E. Sekrafi et al. Journal of Alloys and Compounds 1010 (2025) 177389 6 The Eq. (6) describes the relationship between integral intensity (I), activator concentration (x), and a constant term (k). The parameter β is another constant specific to the excitation conditions of the host crystal. According to this equation, the value of θ reflects the type of energy transfer occurring. A value of θ =3 indicates energy transfer between nearest neighbor ions. Conversely, values of θ =6, 8, and 10 correspond to dipole-dipole (d-d), dipole-quadrupole (d-q), and quadrupolequadrupole (q-q) interactions, respectively [60]. The multipolar character (θ) can be determined by plotting a graph of Log (I/x) versus Log (x), as shown in Fig. 9. The slope of the line, θ/3, was approximated to be −2.23 for our samples, which leads to a calculated value of θ that is approximately 6. Therefore, the concentration quenching observed in the Eu and Dy co-doped YVO 4 nanophosphor can be attributed to d-d interactions. 3.5. CIE chromaticity The Chromaticity Coordinates (CIE, Commission International de l’Eclairage [61]) of YVO 4 :x(Eu 3+ , Dy 3+ ) phosphors were determined from the emission spectra at different x values, which are depicted in Fig. 10. The emitted light exhibits a wide color gamut, transitioning smoothly from yellow to orange and ultimately to red. This tunability suggests that the composition of YVO 4 :x(Eu 3+ , Dy 3+ ) can be precisely controlled by adjusting the x value, allowing for the targeted manipulation of the luminescent material’s output color. The quality of a light source can be often characterized by its correlated color temperature (CCT). This metric can also be determined for YVO 4 :x(Eu 3+ , Dy 3+ ) phosphors using McCamy’s well-established empirical formula [62], as shown below: CCT = − 449n3+3525n2−6823n+5520.33 (7) Fig. 6. Kubelka–Munk plots for of YVO 4 :x(Eu 3+ , Dy 3+ ) (with x=1 %, 2 %, 3 % and 5 %): (a) dependence of F(R) 2 and (b) dependence of F(R) 1/2 with gap energy. (c) Portrays the evolution of dR/dλ with wavelength. H.E. Sekrafi et al. Journal of Alloys and Compounds 1010 (2025) 177389 7 Here, n= (x−xe)/(x−ye)represents the inverse slope of the line, with x e =0.332 and y e =0.186 denoting the coordinates of the epicenter. Table 3 presents the calculated CCT values for YVO 4 :x(Eu 3+ , Dy 3+ ) phosphors. Under 310 nm excitation, YVO 4 : (Eu 3+ , Dy 3+ ) nanophosphors co-doped with 2 % total rare earth cations display the strongest emission in the warmer orange-red spectral region, evident from their 1735 K (CCT). YVO 4 : (Eu 3+ , Dy 3+ ) nanophosphors doped with 2 % total rare earth cations exhibit the most intense emission in the warmer orange-red region of the spectrum, as evidenced by their (CCT) of 1735 K under 310 nm excitation. 4. Summary In this study, a series of YVO 4 :Eu 3+ , Dy 3+ samples were successfully synthesized using the co-precipitation method and then underwent thermal annealing at 800◦C. XRD analysis confirmed the successful incorporation of Eu 3+ and Dy 3+ ions into the YVO 4 lattice without altering its tetragonal crystal structure. SEM revealed that the YVO 4 : Eu 3+ , Dy 3+ phosphors possess a well-defined, spherically-deformed morphology with an average particle size of approximately 60 nm. This particle size range suggests promising potential for practical applications. EDX confirmed the presence of signals from yttrium, vanadium, oxygen, europium, and dysprosium, indicating also the successful doping of these ions into the YVO 4 lattice. UV–vis diffuse reflectance spectra reveal absorption peaks corresponding to an optical direct band gap in the range of 3.6–3.75 eV. In the YVO 4 :x(Eu 3+ , Dy 3+ ) phosphors, the optimal concentration of Eu 3+ and Dy 3+ ions for achieving the maximum up-conversion emission intensity under 310 nm excitation is 2 mol%. This concentration ratio results in the most efficient energy transfer and highest luminescence intensity. In YVO 4 :x(Eu 3+ , Dy 3+ ) phosphors, the energy transfer critical distance between Dy 3+ and Eu 3+ ions was calculated to be 15.63 Å. Concentration quenching in this system is attributed to dipole-dipole interactions, which dominate the energy transfer mechanism. Based on these findings, the YVO 4 :(Eu 3+ , Dy 3+ ) 2 %at. nanophosphor exhibits significant potential for use in solid-state lighting technology, particularly as a promising WLED candidate. Fig. 7. Photoluminescence emission spectra of YVO 4 matrices doped with 2 % Eu 3+ and Dy 3+ , as well as matrices co-doped with the same concentration of both rare-earth ions. Fig. 8. Photoluminescence emission spectra of YVO 4 :x(Eu 3+ , Dy 3+ ) with x=1, 2, 3 and 5 mol% and inset show the enlarged view of highest emission intensity with a series of concentrations. Fig. 9. Dual logarithmic graph illustrating the relationship between the photoluminescence (PL) emission intensity per co-dopant (Log(I/x)) and the concentration of Eu 3+ and Dy 3+ ions (Log(x)) in YVO 4 :x(Eu 3+ , Dy 3+ ) (with x=1 mol%, 2 mol%, 3 mol%, and 5 mol%) nanophosphors. Fig. 10. CIE 1931 diagram for Eu 3+ and Dy 3+ co-doped YVO 4 phosphors (YVO 4 :x(Eu 3+ , Dy 3+ ) where x=1 %, 2 %, 3 %, and 5 %). Table 3 Color chromaticity coordinates (x, y) and correlated color temperature (CCT, K) of the YVO 4 :x(Eu 3+ , Dy 3+ ) phosphors. Compound code Chromaticity coordinates CCT(K) x y YVO 4 :(Eu 3+ , Dy 3+ ) 1 %at. 0.569 0.371 1620 YVO 4 :(Eu 3+ , Dy 3+ ) 2 %at. 0.608 0.365 1735 YVO 4 :(Eu 3+ , Dy 3+ ) 3 %at. 0.546 0.367 1639 YVO 4 :(Eu 3+ , Dy 3+ ) 5 %at. 0.594 0.362 1694 H.E. Sekrafi et al. Journal of Alloys and Compounds 1010 (2025) 177389 8 CRediT authorship contribution statement Houssem Eddine Sekrafi: Writing – original draft, Software, Methodology, Investigation, Formal analysis, Data curation. Eya Hergli: Investigation. Stanislav Ferdov: Writing – review & editing, Visualization, Validation, Data curation. Paulo J. G. Coutinho: Writing – review & editing, Data curation. Luis Rebouta: Writing – review & editing, Supervision, Resources. Declaration of Competing Interest All authors confirm that there are no known conflicts of interest associated with this publication and that there has been no significant support for this work that could have influenced the presented results. Acknowledgements “This work was supported by the European Structural and Investment Funds in the FEDER Component through the Operational Competitiveness and Internationalization Programme (COMPETE 2020) under Advanced Decision Making in productive systems through Intelligent Networks (ADM.IN) Project 055087 (POCI-01–0247-FEDER055087), and partially supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2020.” Data availability Data will be made available on request. References [1] N. Bardsley, S. Bland, M. Hansen, L. Pattison, M. Pattison, K. Stober, M. Yamada, Solid-State Light. Ramp 2015 (2015), https://doi.org/10.2172/1220487 (D Plan -). [2] H. Zhang, H. Zhang, A. Pan, B. Yang, L. He, Y. Wu, Rare earth-free luminescent materials for WLEDs: recent progress and perspectives, Adv. Mater. Technol. 6 (2021), https://doi.org/10.1002/admt.202000648. [3] R. Mueller-Mach, G.O. 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