Photoluminescence Investigations of Dy³⁺-Doped Silicate Xerogels and SiO₂-LaF₃ Nano-Glass-Ceramic Materials

Abstract

In this work, the series of Dy³⁺-doped silicate xerogels were synthesized by sol-gel technique and further processed at 350 °C into SiO₂-LaF₃:Dy³⁺ nano-glass-ceramic materials. The X-ray diffraction (XRD) measurements, along with the thermal analysis, indicated that heat-treatment triggered the decomposition of La(TFA)₃ inside amorphous sol-gel hosts, resulting in the formation of hexagonal LaF₃ phase with average crystal size at about ~10 nm. Based on the photoluminescence results, it was proven that the intensities of blue (⁴F_9/2 → ⁶H_15/2), yellow (⁴F_9/2 → ⁶H_13/2), and red (⁴F_9/2 → ⁶H_11/2) emissions, as well as the calculated yellow-to-blue (Y/B) ratios, are dependent on the nature of fabricated materials, and from fixed La³⁺:Dy³⁺ molar ratios. For xerogels, the emission was gradually increased, and the τ(⁴F_9/2) lifetimes were elongated to 42.7 ± 0.3 μs (La³⁺:Dy³⁺ = 0.82:0.18), however, for the sample with the lowest La³⁺:Dy³⁺ molar ratio (0.70:0.30), the concentration quenching was observed. For SiO₂-LaF₃:Dy³⁺ nano-glass-ceramics, the concentration quenching effect was more visible than for xerogels and started from the sample with the highest La³⁺:Dy³⁺ molar ratio (0.988:0.012), thus the τ(⁴F_9/2) lifetimes became shorter from 1731.5 ± 5.7 up to 119.8 ± 0.4 μs. The optical results suggest, along with an interpretation of XRD data, that Dy³⁺ ions were partially entered inside LaF₃ phase, resulting in the shortening of Dy³⁺-Dy³⁺ inter-ionic distances.

1. Introduction

For the past few decades, optical materials doped with luminescent rare earths (RE³⁺) have attracted immense attention because of their plenteous application in photonic devices, like displays, lasers, light-emitting diodes (LEDs), and sensors [1,2,3,4]. Among RE³⁺, the visible luminescence of Dy³⁺ ions inside blue (488 nm, ⁴F_9/2 → ⁶H_15/2 transition), and yellow (570 nm, ⁴F_9/2 → ⁶H_13/2 hypersensitive transition) regions makes Dy³⁺-doped optical materials promising candidates for utilization as white light emitters. For instance, barium silicate glasses doped with Dy³⁺ are suitable for a white light generation defined by chromaticity coordinates equal to (0.31|0.34), which are lying near the standard point for the while illuminant (0.33|0.33) [5]. Similarly, the chromaticity coordinates for Dy³⁺-doped lithium zinc borosilicate glasses (e.g., (0.318|0.357) or (0.321|0.347)) were also found to be located inside the white light region, and the calculated correlated color temperatures (CCT) are above 5700 K, which indicates that the glasses emit cool white light [6]. Further, it was reported that the lithium aluminum borate glasses co-doped with Gd³⁺/Dy³⁺ ions are able to produce neutral white light (0.363|0.402) with CCT equal to 4556 K [7], but the warm white light was obtained for selected Dy³⁺-doped glass-ceramics containing Na₃Gd(PO₄)₂ phase [8]. Moreover, since the populations of the ⁴I_15/2 and the ⁴F_9/2 excited levels of Dy³⁺ ions are governed by the Boltzmann statistics, they are thermally coupled, which makes it possible to apply them in optical thermometry. Factually, Bu et al. [9] found for Dy³⁺-doped glass-ceramics containing LaF₃ crystal phase that the intensities of emissions located at 480 nm (⁴F_9/2 → ⁶H_15/2) and 572 nm (⁴F_9/2 → ⁶H_13/2) are decreased, but conversely, the luminescence within the blue light scope at 454 nm (⁴I_15/2 → ⁶H_15/2) gradually increased as temperature rose from 275 to 550 K. Similarly, Komar et al. [10] stated that the fluorescence intensity ratio between the ⁴I_15/2 → ⁶H_15/2 and the ⁴F_9/2 → ⁶H_15/2 emission lines for La₃Ga_5.5Ta_0.5O₁₄ crystal could be treated as a temperature-dependent parameter up to 773 K. Further, the materials co-doped with Dy³⁺ and transition metals could also be used in the field of luminescence thermometry. Indeed, Lin et al. [11] identified that the ²E_g → ⁴A_2g transition of Mn⁴⁺ is very sensitive to temperature fluctuations because of the thermal quenching of the luminescence as a result of strong electron–phonon coupling. Contrary, the authors found that the shielded 4f⁹-4f⁹ emissions of Dy³⁺ ions are relatively negligibly affected by the lattice environment and thus, the ratio between emission intensities of Mn⁴⁺ and Dy³⁺ ions is dependent on temperature.

The spectroscopy of trivalent Dy³⁺ ions is widely described for various types of glassy hosts fabricated by the conventional melt-quenching technique and the derivative glass-ceramics [12,13,14,15,16], as well as phosphors like Li⁺-doped CaWO₄:Dy³⁺ [17], BaSrY₄O₈:Dy³⁺ [18], or SrLaAlO₄:Dy³⁺ [19], and complexes [20,21,22]. However, the literature concerning optical properties of Dy³⁺-doped sol-gel materials is far less exhausting. In this field, the studies published by the research group of B. Grobelna focused on Dy³⁺-doped silicate xerogels containing selected types of tungstates, e.g., CaWO₄ [23], and Ln₂(WO₄)₃ (Ln = La or Gd) [24,25]. The characteristic blue (⁴F_9/2 → ⁶H_15/2) and yellow (⁴F_9/2 → ⁶H_13/2) luminescence of Dy³⁺ ions were generated via the energy transfer from WO₄^2- using an excitation wavelength from the mid-UV area (λ_ex = 240 nm). Thus, the authors stated that fabricated optical materials could be potentially used in solar cells to enhance conversion efficiency. Additionally, the spectroscopy of Dy³⁺ ions in sol-gel materials was described for Dy³⁺/Tb³⁺ co-doped 90SiO₂-10YF₃ (mol%) [26] and 95SiO₂-5LaF₃ (mol%) [27] nano-glass-ceramics, and according to the energy transfer from Dy³⁺ to Tb³⁺ those systems are considered as promising candidates for solar cells applications. The synthesis of zirconate xerogels and aerogels containing Dy³⁺ ions were presented in work [28], and their characterization provided the thermal and structural analysis; however, the luminescence measurements of Dy³⁺ luminescence were not the aim of the presented studies. Therefore, such a relatively small number of papers devoted to Dy³⁺ ions spectroscopy in sol-gel materials makes investigating those types of optical materials highly meaningful and necessary.

Among various types of optical materials based on fluorides (e.g., CaF₂, SrF₂, BaF₂, YF₃), LaF₃ is one of the most frequently and willingly studied host, as evidenced by plentiful works reported in the current literature [29,30,31,32,33,34,35]. LaF₃ is characterized by exceptionally low phonon energy (~350 cm⁻¹) arising from the strong ionicity of La³⁺-F⁻ bond, compared with other RE³⁺-F⁻ or Na⁺-F⁻ [36,37]. LaF₃ crystallizes as the trigonal/hexagonal phase, but the cubic polymorphic form is also known and reported [38,39]. Moreover, LaF₃ is also characterized by good transmission within the range between 0.13 and 11 μm [40,41]. The similarity in ionic radii of La³⁺ cation and other RE³⁺ dopants allows for relatively easily substituting them in the parent fluoride crystal lattice to improve the optical properties by suppressing the non-radiative losses of photon energy [42]. Thus, these peculiarities clearly point out the great utility potential of glass-ceramic materials containing LaF₃ crystals doped with optically active RE³⁺ ions. Moreover, in this regard, it should be noted that the fabrication of oxyfluoride glass-ceramic materials using the sol-gel method allows for overcoming the fundamental drawback of the melt-quenching technique, which is often correlated with the high risk of the evaporation of fluorides (even about 30–40 mol%) [43]. The appropriate chemical reactions (i.e., hydrolysis of the metal/semi-metal alkoxide, further condensation, and polycondensation) during sol-gel synthesis are usually performed at room temperature (or slightly elevated); it is used as an alternative route for the fabrication of the oxyfluoride glass-ceramic materials. As a result, this low temperature approach allows for the fabrication of nano-glass–ceramics with a greater fluoride crystal fraction.

In this study, the series of silicate xerogels doped with Dy³⁺ ions were fabricated using the sol-gel technique, and further processed into SiO₂-LaF₃:Dy³⁺ nano-glass-ceramics. The molar ratio of La(CH₃COO)₃:Dy(CH₃COO)₃ acetates used during the sol-gel synthesis was changed as follows (1 − x):x, where x = 0.012, 0.03, 0.06, 0.12, 0.18, and 0.3. The thermal analysis and XRD technique were used to verify the structural transformation during performed controlled heat-treatment of precursor silicate xerogels. The impact of La³⁺:Dy³⁺ molar ratio, as well as the influence of xerogels’ evolution into nano-glass-ceramics on photoluminescence properties was discussed based on excitation and emission spectra, along with the decay analysis from the ⁴F_9/2 excited state of Dy³⁺ ions.

2. Materials and Methods

The sol-gel preparation method used to synthesize the series of xerogels doped with Dy³⁺ was described with details elsewhere [44]. All reagents were taken from Sigma Aldrich Chemical Co. (St. Louis, MO, USA). The subsequent chemical reactions, which undergo during sol-gel evolution, e.g., hydrolysis, condensation, and polycondensation of precursor (tetraethoxysilane, TEOS), were carried out in a solution of ethyl alcohol (EtOH), deionized water, and acetic acid (AcOH), with molar ratio equals to 1:4:10:0.5. In parallel, the appropriate amounts of La(AcO)₃ and Dy(AcO)₃ acetates were dissolved in water and trifluoroacetic acid (TFA), and the resultant mixtures were added dropwise to TEOS-based solutions. The molar ratio of TFA:Ln³⁺ (La³⁺ and Dy³⁺) was set at 5:1, which finally varied as follows: TFA:La³⁺:Dy³⁺ = 5:(1 − x):x, where x = 0.012, 0.03, 0.06, 0.12, 0.18, and 0.3. The as-prepared sols were poured into beakers that were kept sealed until rigid xerogels were formed. The sol-gel evolution from the silicate sols, through wet-gels, up to solid xerogels was performed at 35 °C for the next several weeks, and the following samples were denoted as XG1-XG6. The transformation of xerogels into oxyfluoride nano-glass-ceramics was conducted at 350 °C for 10 h. The fabricated SiO₂-LaF₃:Dy³⁺ nano-glass-ceramics were marked in the text as GC1-GC6.

The thermogravimetry and differential scanning calorimetry (TG/DSC) were carried out using a Labsys Evo system with a heating rate of 10 °C/min in argon atmosphere (SETARAM Instrumentation, Caluire, France). The prepared sol-gel materials were characterized by X-ray diffraction (XRD) analysis using an X’Pert Pro diffractometer supplied by PANalytical with CuKα radiation with λ = 1.54056 Å wavelength (Almelo, the Netherlands). The luminescence measurements were performed on a Photon Technology International (PTI) Quanta-Master 40 (QM40) UV/VIS Steady State Spectrofluorometer (Photon Technology International, Birmingham, NJ, USA), supplied with a tunable pulsed optical parametric oscillator (OPO) pumped by the third harmonic of a Nd:YAG laser (Opotek Opolette 355 LD, OPOTEK, Carlsband, CA, USA). The laser system was coupled with a xenon lamp, a double 200 mm monochromator, and a multimode UV/VIS PMT detector. The excitation and emission spectra were recorded with a resolution of 0.5 nm. The luminescence decay curves were recorded by a PTI ASOC-10 (USB-2500) oscilloscope with ±0.1 μs accuracy. All structural and optical measurements were carried out at room temperature.

3. Results and Discussion

3.1. Analysis of TG/DTG and DSC Results for Dy³⁺-Doped Xerogels

Figure 1 presents the TG/DSC curves recorded for fabricated xerogels in an inert gas atmosphere in a temperature range from 30 to 430 °C. The TG technique involves the measurement of weight losses as a function of temperature; therefore, the TG curves demonstrate the thermal stability of the studies samples. According to the analysis of TG curves (solid lines), there are two distinguishable degradation steps for all xerogels: first, identified within a temperature range from ~55 to ~220 °C, and second, between ~220 and ~380 °C. The indicated degradation steps are marked in Figure 1, and the appropriate temperature ranges for individual xerogels doped with Dy³⁺ ions are collected in

Table 1. The parameters from TG, DTG, and DSC analysis for studied XG1-XG6 silicate xerogels doped with Dy³⁺ ions.

Sample	Thermal Degradation
	1st Step		2nd Step
	Temperature Range (°C)	Maximum of DTG Peak (°C)	Temperature Range (°C)	Maximum of DTG Peak (°C)	DSC Peak Position (°C)
XG1	55–223	129	223–383	307	307
XG2	56–222	134	222–382	303	302
XG3	50–224	135	224–382	307	307
XG4	57–226	157	226–378	291	298
XG5	51–217	132	217–386	307	307
XG6	45–201	111	201–375	307	309

.

The first degradation step, which occurred in lower temperatures (in a range from ~55 to ~220 °C), is correlated with the evaporation of residual organic solvents (ethyl alcohol, acetic acid, unreacted TFA) and water desorption from the porous silicate sol-gel network, and is observed as a gentle degradation. Actually, xerogels are porous solid materials, with pores that are usually filled by liquids. Despite the vibrations characteristic for the silicate sol-gel network (~1200 cm⁻¹ and below), there are additional bands identified as the vibrations of OH groups (>3000 cm⁻¹), C=O moieties (1650 cm⁻¹), and C–H bonds (~1390 cm⁻¹, ~1460 cm⁻¹), which clearly indicate the presence of water and organic compounds residues in xerogels before any heat-treatment [44]. Indeed, the boiling points of indicated chemical compounds under atmospheric pressure (i.e., 72 °C for TFA, 78 °C for C₂H₅OH, 100 °C for H₂O, and 118 °C for CH₃COOH). Hence, according to the 1st step of degradation, the evaporation of the compounds mentioned above is expected in a given temperature range. Supplementarily, the derivative thermogravimetry (DTG) expresses the results of TG by providing the first derivative curve as a function of temperature. DTG is a type of thermal analysis in which the rate of xerogels’ mass changes upon heating is plotted against temperature. Therefore, the temperature at which the maximum of the first DTG peak (dashed lines) occurs indicates the temperature at which the evaporation of water and organic compounds undergo the maximum rate. The temperatures from DTG curves for an individual sol-gel sample are identified in a range from 111 °C (XG6) to 157 °C (XG4) and are summarized in Table 1.

According to the processing of nano-glass-ceramic materials, the 2nd step of thermal degradation is essential because it is directly related to the crystallization of the LaF₃ fluoride phase, preceded by La(TFA)₃ decomposition. Indeed, this process involved a chemical reaction in which the compounds LaF₃, (CF₃CO)₂O, CO₂, and CO are obtained. The investigations of the mechanism of this reaction allowed us to conclude that such thermolysis led to cleavage of C–F bonds inside −CF₃ groups from TFA ligand, and the fluorine anions (F⁻) tend to react with La–O bonds, forming LaF₃ phase resultantly [45]. From TG analysis (solid lines), the indicated transformation within the structure of prepared sol-gel materials is observable as a significant decrease in the mass. Accordingly, the temperatures at which the maximum of the DTG peaks for an individual sample are identified as approximately 300 °C. The indicated weight losses associated with La(TFA)₃ thermal decomposition are estimated at 29.76 (XG1), 27.99 (XG2), 27.97 (XG3), 29.54 (XG4), 26.90 (XG5), and 28.02% (XG6). Moreover, for each fabricated xerogel, a strong exothermic DSC peak (dotted line) in this temperature range is recorded with a maximum near 300 °C. Therefore, such a degradation step is according to the release of energy and mass. The location of DSC peaks is consistent with the maxima of DTG peaks, which correspond to temperatures at which the transformation occurs the most rapidly.

The obtained results are consistent with the literature data, which clearly indicate that the thermal decomposition of metal trifluoroacetates and crystallization of appropriate fluoride phases occur at about 300 °C [44,46,47]. Additionally, based on the TG analysis, it should be noted that the prepared sol-gel samples are characterized by good thermal resistance at temperatures close to 350 °C. According to data collected in Table 1, it could be assumed that co-doping with Dy³⁺ ions was not influenced the thermal parameters of the prepared silicate xerogels. Based on collected data from TG (DTG) and DSC measurements, the temperature of a heat-treatment process to fabricate nano-glass-ceramics was assessed at 350 °C.

3.2. Structural Characterization of Fabricated Dy³⁺-Doped Sol-Gel Materials

Figure 2 shows the XRD diffractograms of Dy³⁺-doped precursor xerogels and samples obtained during controlled heat-treatment at 350 °C. For xerogels, the XRD patterns showed no sharp diffraction lines but only a broad hump with a maximum located near ~22°, which confirmed their amorphous nature devoid of long-range order [48]. Conversely, the sharp XRD lines are well-visible for heat-treated samples, and the lines are attributed to the hexagonal LaF₃ phase crystallized in P6₃cm space group (ICDD card no. 00-008-0461). According to the literature, in the nearest framework around La³⁺ cations, there are nine F⁻ anions with four non-equivalent sites, including 3F1, 3F2, 2F3, and 1F4 [49]. The broadening of the diffraction lines was used to calculate the average diameter (D) of the crystallized LaF₃ phase using the Scherrer equation [50]:

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

(1)

where K is a shape factor (in our calculations it was taken K = 1), λ is a wavelength of X-ray (0.154056 nm, Kα line of Cu), β_hkl is a broadening of the (hkl) diffraction peak at half of the maximum intensity, and θ is a Bragg’s angle. The average crystallite size was estimated from 11.9 ± 0.1 (GC6) to 21.3 ± 0.5 nm (GC1). Additionally, the Williamson–Hall theorem was also used to determine the average size of LaF₃ phase [51]:

$${\mathsf{\beta }}_{\mathrm{hkl}}\cos \mathsf{\theta }=\frac{\mathrm{K}\mathsf{\lambda }}{\mathrm{D}}+4\mathrm{Z}\sin \mathsf{\theta }$$

(2)

in which β_hkl is a broadening of the (hkl) diffraction line, θ is a diffraction angle, λ is an X-ray wavelength, D is an average crystal size, and Z is an effective strain. The lattice strain and the crystallite size were deduced from the intercept of βcos θ/λ versus sin θ/λ. The average crystal sizes of LaF₃ from the Williamson–Hall method are similar for all fabricated SiO₂-LaF₃:Dy³⁺ nano-glass-ceramics and were estimated from 8.2 ± 0.1 (GC2) to 10.6 ± 0.1 nm (GC1). As can be seen from the obtained results, there is a noticeable difference in the size of the crystallites obtained by the Scherrer and Williamson–Hall methods. The difference is because the Scherrer method does not consider the share of internal stresses in the half-width of the XRD diffraction line. Contrary, the Williamson–Hall method separates the half-width into parts associated with the average crystallite size and parts related to internal stresses. If there would be no internal stresses in the material, the results of methods are convergent. If there are no internal stresses in the material, the results of the methods are convergent. However, the dysprosium ions caused some internal stress, so the estimated crystallite sizes obtained by these methods are slightly different. In the case of fabricated samples, Dy³⁺ ions, the inset of Figure 2 displays the high-resolution transmission electron microscope (HR-TEM) image of the prepared GC1 sample. Based on it, it was stated that the size of LaF₃ nanocrystals is consistent with the average crystal size estimated from XRD analysis.

Figure 2 also shows an evident shift of (002), (110), and (111) diffraction lines toward higher angles as the content of Dy³⁺ ions increases in the subsequent samples in the prepared series. The shift in the position of (110) diffraction line (∆θ), compared with pure LaF₃ phase, is about from 0.01 to 0.40° for GC2 and GC6 nano-glass-ceramics, respectively. These results indicate that the lattice parameters for the cation-exchanged LaF₃:Dy³⁺ phase are smaller than for the pure fluoride phase without any admixtures of Dy³⁺ ions. So, because Dy³⁺ ions have a slightly smaller ionic radius (r = 1.083 Å) compared with La³⁺ cation (r = 1.216 Å) [52], some lattice distortions and intra-stress occur, as was presented in

Table 2. Crystal lattice parameters of LaF₃ phase in prepared SiO₂-LaF₃:Dy³⁺ nano-glass-ceramics. The asterisk (*) refers to parameters of undoped LaF₃ phase according to ICDD card no. 00-008-0461.

Sample	Lattice Parameter [Å]		Crystallite Size [nm]		Lattice Strain [%]
	LaF3 (*)	Sol-Gel Sample	Scherrer	Williamson–Hall
GC1	a0 = 7.184 c0 = 7.351	a0 = 7.181(8) c0 = 7.359(4)	21.3 ± 0.5	10.6 ± 0.1	0.24 ± 0.01
GC2		a0 = 7.172(4) c0 = 7.351(9)	15.3 ± 0.3	8.2 ± 0.1	0.27 ± 0.01
GC3		a0 = 7.161(0) c0 = 7.343(3)	12.7 ± 0.1	9.0 ± 0.1	0.16 ± 0.01
GC4		a0 = 7.147(7) c0 = 7.321(9)	12.3 ± 0.1	9.0 ± 0.1	0.14 ± 0.01
GC5		a0 = 7.139(6) c0 = 7.311(5)	12.6 ± 0.1	9.8 ± 0.1	0.11 ± 0.01
GC6		a0 = 7.077(2) c0 = 7.242(9)	11.9 ± 0.1	9.0 ± 0.1	0.13 ± 0.01

. Indeed, a general tendency to a progressive decrease in the cell parameters of fluoride nanocrystals was denoted (from a₀ = 7.181(8) Å, c₀ = 7.359(4) Å for GC1 up to a₀ = 7.077(2) Å, c₀ = 7.242(9) Å for GC6) in comparison with that of pure and undoped LaF₃ phase (a₀ = 7.184 Å, c₀ = 7.351 Å). So, since the ionic radius of dopant (Dy³⁺) and cation from parent fluoride crystal lattice (La³⁺) are slightly different, the substitution of La³⁺ by Dy³⁺ modifies the inter-ionic distances and induces the perturbation in the lattice parameters. It generates stress inside the nanocrystal lattice, and for LaF₃:Dy³⁺ system the compressive strain could be observed [53,54]. The lattice strain derived from the Williamson–Hall formula for fabricated sol-gel samples was estimated from 0.11 ± 0.01% to 0.27 ± 0.01%, indicating some lattice distortion. Interestingly, conversely to the above tendency, a very slight increase in c₀ parameter for GC1 sample (c₀ = 7.359(4) Å) in comparison with those of the pure LaF₃ phase (c₀ = 7.351 Å) was observed. It may be correlated with a peculiar property of crystals in the nanoscale, as was also denoted e.g., for CeO₂ [55,56], BaF₂ [57], or Pt nanoparticles [58].

3.3. Optical Properties of Dy³⁺-Doped Xerogels

Figure 3 illustrates the excitation spectra for the series of Dy³⁺-doped xerogels, registered on collecting the yellow emission at λ_em = 570 nm. Within the near-UV and VIS ranges, the 4f⁹-4f⁹ intra-configurational transitions originating from the ⁶H_15/2 ground state of Dy³⁺ ions to the various excited levels were noted, appropriately labeled as the ⁶P_3/2 (326 nm), ⁴I_9/2 (340 nm), ⁶P_7/2 (352 nm), ⁴I_11/2 (366 nm), ⁴F_7/2 (388 nm), ⁶G_11/2 (427 nm), ⁴I_15/2 (452 nm), as well as ⁴F_9/2 (474 nm). It could be observed that the intensities of individual excitation bands have grown with decreasing La³⁺:Dy³⁺ molar ratio as the content of Dy³⁺ ions increased. On the other hand, since the intensities of excitation bands for XG5 and XG6 samples are comparable, it could be stated that the energy transfer processes between Dy³⁺ ions started to occur, suggesting the concentration quenching. The emission spectra of Dy³⁺-doped xerogels are presented in Figure 4. The spectra were recorded upon excitation at λ_ex = 352 nm and show three luminescence bands at 477, 570, and 655 nm, according to the following transitions: ⁴F_9/2 → ⁶H_15/2 (blue), ⁴F_9/2 → ⁶H_13/2 (yellow), and ⁴F_9/2 → ⁶H_11/2 (red), as was also presented in the energy level scheme in Figure 5. For fabricated xerogels, the intensities of recorded bands increased with decreasing in La³⁺:Dy³⁺ molar ratio from XG1 to XG5 sample, but for XG6 (with the highest content of Dy³⁺) the luminescence started to quench, suggesting the occurrence of the energy transfer (ET) process between neighboring Dy³⁺ ions in the host.

Generally, the relative intensities of the ⁴F_9/2 → ⁶H_15/2 (ΔJ = 3, forbidden transition) and the ⁴F_9/2 → ⁶H_13/2 emissions (ΔJ = 2, hypersensitive electric–dipole transition) are influenced by the symmetry in the nearest framework around Dy³⁺ ions [59]. Based on recorded spectra, yellow-to-blue (Y/B) ratios were calculated, and the obtained values were equaled to 2.83, 2.37, 2.27, 2.34, and 2.31 for XG2-6, respectively. For XG1 xerogel sample, the Y/B-ratio was not calculated due to the presence of a broad band in a blue light region with a maximum at λ = 434 nm (not shown in the figure), which coincides with the ⁴F_9/2 → ⁶H_15/2 emission of Dy³⁺ ions. The indicated background is associated with defects inside the amorphous sol-gel host, as was stated in the literature [60]. Indeed, it is attributed to photon recombinations from plentiful defects associated with dangling bonds inside the sol-gel skeleton, and its appearance is independent of the introduced rare-earth dopant, as was proven in our earlier works concentrated on Tb³⁺ and Eu³⁺ spectroscopies [46,47]. For the same reason (correlated with overlapping of this broad band with blue emission of Dy³⁺ ions), the Y/B-ratio for XG2 sample is higher than the values calculated for other XG3-XG6 samples characterized by greater intensities of emission lines from Dy³⁺ ions. Our experimental results for XG3-XG6 samples indicate that Y/B-ratio values are set at a nearly constant level, despite La³⁺:Dy³⁺ molar ratio and Dy³⁺ content, which suggests no significant changes in the local environment around optically active ions in samples before heat-treatment. In general, such high Y/B-ratio values obtained for precursor xerogels specify a relatively high covalent nature of bonds between Dy³⁺ and the host [13], and they are comparable with the values declared in the literature for selected amorphous systems depicted in

Table 3. Y/B-ratios for different types of amorphous optical materials doped with Dy³⁺ ions.

Amorphous Material	Y/B-Ratio	Reference
35.7SiO2-25.5B2O3-17BaO-3.4K2O-3.4Al2O3-15BaCl2 (mol%):0.1–1wt% Dy2O3 2	2.88–2.98	[59]
XG2-XG6 1	2.83	[this work]
	2.37	[this work]
	2.27	[this work]
	2.34	[this work]
	2.31	[this work]
50B2O3-(25 − x)CaO-15Al2O3-10CaF2-xDy2O3 (x = 0.5–5) wt% 2	1.94–2.18	[61]
20SiO2-(40 − x)B2O3-10Al2O3-20NaF-10ZnO-xDy2O3 (x = 0.1–2.5) mol% 2	1.66–1.77	[62]
35B2O3-20SiO2-(15 − x)Al2O3-15ZnO-15Na2CO3-xDy2O3 (x = 0.1–2.5) mol% 2	1.61–1.75	[63]
45SiO2-20Al2O3-10CaO-24.9CaF2-0.1Dy2O3 mol% 2	1.52	[64]
73TeO2-4BaO-3Bi2O3-18SrF2-2Dy2O3 mol% 2	1.50	[65]
Ba2O3-PbO-Al2O3-WO3-Dy2O3 wt.% 2 (B2O3:PbO molar ratio changed from 2:1 to 1:8)	1.04–1.22	[66]
(20 − x)Na2O-5BaF2-5CaF2-60B2O3-10TeO2-xDy2O3 (x = 0.5–2.5) mol% 2	0.86–1.11	[12]
15ZnO-5PbO-(20 − x)Al2O3-60B2O3-xDy2O3 (x = 0.1–2.0) mol% 2	0.68–0.78	[67]
TEOS-based xerogels 1	0.51–0.76	[68]

¹ materials prepared by the sol-gel method. ² materials prepared by the conventional melt-quenching technique.

[12,59,61,62,63,64,65,66,67,68]. Indeed, similar Y/B-ratio values (above 2) have been reported for 35.7SiO₂-25.5B₂O₃-17BaO-3.4K₂O-3.4Al₂O₃-15BaCl₂:0.1–1Dy₂O₃ [59], and 50B₂O₃-(25−x)CaO-15Al₂O₃-10CaF₂-xDy₂O₃ (x = 0.5–5) [61] glassy systems. The data collected in Table 3 clearly indicate the strong correlation between Y/B-ratios and modifications in chemical compositions of glasses and amorphous sol-gel materials.

The further characterization of Dy³⁺-doped xerogels involved the luminescence decay analysis from the ⁴F_9/2 excited state, and the resultant curves are presented in Figure 6 (λ_ex = 352 nm, λ_em = 570 nm). The registered luminescence decay curves followed the second-order exponential nature, and the average lifetimes were calculated using the following formula:

$${\mathsf{\tau }}_{\mathrm{avg}}=\frac{{\mathrm{A}}_1{\mathsf{\tau }}_1^2+{\mathrm{A}}_2{\mathsf{\tau }}_2^2}{{\mathrm{A}}_1{\mathsf{\tau }}_1+{\mathrm{A}}_2{\mathsf{\tau }}_2}$$

(3)

where A₁ and A₂ are residual weighting factors and τ₁ and τ₂ are decay components. The resultant τ_n(⁴F_9/2):Dy³⁺ lifetimes with A₁ and A₂ parameters are depicted in

Table 4. Decay components (τ_n), residual weighting factors (A_n), and average decay times (τ_avg) of the ⁴F_9/2 state of Dy³⁺ in fabricated silicate xerogels.

Sample	Decay Components (μs)		Residual Weighting Factors (%)		Average Decay Time, τavg (μs)
	τ1	τ2	A1	A2
XG1	8.0 ± 0.1	35.8 ± 0.8	68.76	31.24	26.6 ± 0.7
XG2	9.4 ± 0.2	34.3 ± 0.6	50.76	49.76	28.9 ± 0.5
XG3	10.8 ± 0.3	39.8 ± 0.9	56.79	43.21	32.2 ± 0.8
XG4	12.7 ± 0.1	47.8 ± 0.3	49.16	50.84	40.6 ± 0.3
XG5	15.0 ± 0.2	50.4 ± 0.3	48.28	51.72	42.7 ± 0.3
XG6	13.0 ± 0.1	40.3 ± 0.1	46.52	53.48	34.3 ± 0.1

.

The average decay times continuously elongate as the content of Dy³⁺ ions increased in the following order: 26.6 ± 0.7 (XG1), 28.9 ± 0.5 (XG2), 32.2 ± 0.8 (XG3), 40.6 ± 0.3 (XG4), and 42.7 ± 0.3 μs (XG5). However, for the sample with the highest content of Dy³⁺ ions (XG6), an evident shortening in the τ_avg(⁴F_9/2):Dy³⁺ value to 34.3 ± 0.1 μs was denoted, and it clearly corroborates with Dy³⁺-Dy³⁺ ET process. Generally, according to the numerous works in the literature, another factor that indicates the occurrence of the ET process among neighboring Dy³⁺ ions is the non-exponential behavior of the decays [12,14,64,67]. Based on this conception, we should assume that ET began to appear in the sample with the lowest content of Dy³⁺ (XG1), although its influence on the overall luminescence is negligible (indeed, we could observe the continuous elongation of the decays up to XG5 sample, simultaneously with growing intensities of the emission bands, as was presented in Figure 4). Therefore, for XG1-XG5 luminescence is proportional to the number of centers in an excited state. Further, for the XG6, the Dy³⁺-Dy³⁺ inter-ionic distances are the shortest in the series of fabricated xerogels, which makes the participation of ET enough to observe the shortening in the τ_avg(⁴F_9/2) lifetime value and quenching the emission. The τ(⁴F_9/2):Dy³⁺ lifetimes reported in the current literature for other amorphous systems, i.e., calcium boroaluminate glasses (510–800 μs) [61] or zinc-alumino-borosilicate glasses (296.5–673.7 μs) [62] are significantly longer compared with the decay times obtained for xerogels in this work. However, we could assume that the observed tendency should be related to the limited content of OH groups in glassy hosts prepared by the melt-quenching technique (in comparison with xerogels), which play a crucial role in quenching of the luminescence originating from Dy³⁺ ions. Indeed, the τ_avg(⁴F_9/2):Dy³⁺ lifetimes for studied silicate xerogels are in the order of microseconds, and such relatively short luminescence lifetimes are strictly correlated with the presence of plentiful OH groups originated from silanol Si-OH moieties as well as residual organic solvents and water, inside a highly porous silicate network [44]. Since the ⁴F_9/2 → ⁴F_1/2 energy gap of Dy³⁺ ions equals only ΔE = ~7000 cm⁻¹ [12], merely two high-energy phonons of OH groups (~3500 cm⁻¹) are required to promote a non-radiative relaxation from the ⁴F_9/2 excited state. As was also presented earlier by us for Eu³⁺ and Tb³⁺-doped samples [46], the non-radiative deactivation of the ⁴F_9/2 level could also be partially caused by TFA ligands from RE³⁺ coordination sphere, containing carbonyl groups (~1665 cm⁻¹; four groups to cover the energy gap) and C–F bonds (~1200 cm⁻¹; six groups to cover the energy gap).

3.4. Luminescence Behavior of Dy³⁺-Doped Nano-Glass-Ceramics

Figure 7 shows the excitation spectra of SiO₂-LaF₃:Dy³⁺ nano-glass-ceramics, recorded by monitoring the characteristic yellow emission at λ_em = 570 nm. The spectra revealed the eight bands corresponding to the following electronic transitions: ⁶H_15/2 → ⁶P_3/2 (326 nm), ⁶H_15/2 → ⁴I_9/2 (339 nm), ⁶H_15/2 → ⁶P_7/2 (351 nm), ⁶H_15/2 → ⁴I_11/2 (364 nm), ⁶H_15/2 → ⁴F_7/2 (389 nm), ⁶H_15/2 → ⁶G_11/2 (427 nm), ⁶H_15/2 → ⁴I_15/2 (453 nm), and ⁶H_15/2 → ⁴F_9/2 (472 nm). Conversely to precursor xerogels, the excitation bands’ intensities gradually decrease with decreasing La³⁺:Dy³⁺ molar ratios in the subsequent GC1-GC6 samples. Hence, the concentration quenching phenomenon is observed from the nano-glass-ceramic with the lowest content of Dy³⁺ ions.

The emission spectra of Dy³⁺-doped nano-glass-ceramics, recorded upon excitation at λ_ex = 351 nm are presented in Figure 8. Similarly, as for xerogels, the characteristic emission bands of Dy³⁺ were identified at 478 nm (⁴F_9/2 → ⁶H_15/2), 570 nm (⁴F_9/2 → ⁶H_13/2), and 657 nm (⁴F_9/2 → ⁶H_11/2). A progressive decrease in the relative intensities of recorded bands for subsequent nano-glass-ceramics was observed. It could be stated that the concentration quenching has occurred even from the sample with the lowest content of Dy³⁺ ions (GC1), as was also observed in excitation spectra (Figure 7). Generally, the concentration quenching could be realized through the resonant energy transfer (RET) or possible non-radiative cross-relaxation channels CR1-CR3, as was presented in the energy level scheme in Figure 5. According to these channels, an excited Dy³⁺ ion (donor, D) makes a downward transition, whereas a coupled unexcited neighbor Dy³⁺ (acceptor, A) simultaneously makes an appropriate upward transition. The electronic transitions involved in each of the individual channel could be denoted as follows:

RET: ⁴F_9/2 (D) + ⁶H_15/2 (A) → ⁶H_15/2 (D) + ⁴F_9/2 (A),

CR1: ⁴F_9/2 (D) + ⁶H_15/2 (A) → (⁶H_9/2 + ⁶F_11/2) (D) + ⁴F_3/2 (A),

CR2: ⁴F_9/2 (D) + ⁶H_15/2 (A) → ⁶F_5/2 (D) + (⁶H_7/2 + ⁶F_9/2) (A),

CR3: ⁴F_9/2 (D) + ⁶H_15/2 (A) → ⁶F_3/2 (D) + (⁶H_9/2 + ⁶F_11/2) (A).

As was discussed according to the gradual shift of XRD diffraction lines (Figure 2), we could expect that part of Dy³⁺ ions were entered into LaF₃ nanocrystal lattice, which significantly promotes the shortening of the Dy³⁺-Dy³⁺ inter-ionic distances. The incorporation of Dy³⁺ ions inside fluoride nanophase could also be stated based on a decrease in Y/B-ratio values compared to those for precursor xerogels: 2.71 (GC1), 2.53 (GC2), 2.34 (GC3), 2.20 (GC4), 2.06 (GC5), and 1.94 (GC6). Indeed, the denoted alterations in Y/B-ratios indicate that the bonding between Dy³⁺ ions and the local environment is less covalent in prepared nano-glass-ceramics than in xerogels, but it should be noted that the verified decrease in calculated ratios is slight. Furthermore, compared with Y/B-ratios declared in the literature (

Table 5. Y/B-ratios for glass-ceramics doped with Dy³⁺ ions.

Type of Crystal Phase	Y/B-Ratio	Reference
LaF3 1 (350 °C)	2.74	[this work]
	2.53
	2.34
	2.20
	2.06
	1.94
CaF2 2 (650 °C, 700 °C)	1.58 1.68	[64]
β-NaGdF4 2 (700 °C)	1.51	[69]
PbF2 2 (380 °C/2 h)	1.18	[70]
PbF2 2 (380 °C/5 h)	1.05
PbF2 2 (380 °C/10 h)	1.22
NaGd(WO4)2 2 (450 °C)	1.0–1.1	[71]
SrWO4 2	0.787–0.881	[72]
Gd2(WO4)3 1,*	0.23–1.37	[25]
La2(WO4)3 1,*	0.26–1.21	[25]

¹ materials prepared by sol-gel method. ² materials prepared by conventional melt-quenching technique. * the crystal phase was not formed by in situ nucleation during controlled heat-treatment.

[25,64,69,70,71,72]), the values calculated for prepared SiO₂-LaF₃:Dy³⁺ materials remained relatively high. Thus, we could suppose that calculated Y/B-ratio values are correlated with the presence of a broad band attributed to the photon recombinations from structural defects inside the silicate sol-gel host (still visible even after controlled heat-treatment of xerogels; the maximum of this band was shifted from ~434 nm (before heat-treatment) to ~465 nm (after heat-treatment)), which directly overlaps with the characteristic emission lines of Dy³⁺ ions within the blue (⁴F_9/2 → ⁶H_15/2) and the green (⁴F_9/2 → ⁶H_13/2) light spectral scopes. On the other hand, a clear trend could be noticed according to gradually decreasing Y/B-ratios for the subsequent SiO₂-LaF₃:Dy³⁺ nano-glass-ceramics as the content of optically active Dy³⁺ ions grow. It suggests an increasing tendency to accumulate Dy³⁺ ions in the LaF₃ phase, which is also confirmed by the continuous shift of the XRD diffraction lines.

Additionally, the comparison of emission spectra recorded for xerogels and nano-glass-ceramics for individual La³⁺:Dy³⁺ molar ratios in samples’ compositions was presented in Figure 9. Based on this comparison, it can be stated that for La³⁺:Dy³⁺ molar ratios equal to 0.988:0.012, 0.97:0.03, and 0.94:0.06, the heat-treatment process enhances the intensity of the emission bands originated from Dy³⁺ ions. The most remarkable difference in the bands’ intensity can be observed when the content of Dy³⁺ is the lowest in the series of obtained samples (La³⁺:Dy³⁺ = 0.988:0.012). The correlation between luminescence intensities and La³⁺:Dy³⁺ molar ratio starts to change as the content of Dy³⁺ ions increases. For La³⁺:Dy³⁺ molar ratios equal to 0.88:0.12, 0.82:0.18, and 0.70:0.30, the emission intensities of luminescent bands of Dy³⁺ ions are greater for xerogels than for glass-ceramic materials. It is caused by the progressing concentration quenching, particularly for the highest content of Dy³⁺ ions, due to a significant shortening in the inter-ionic Dy³⁺-Dy³⁺ distances correlated with the incorporation of Dy³⁺ ions into the LaF₃ fluoride phase.

The luminescence decay curves of the ⁴F_9/2 state of Dy³⁺ for the series of prepared SiO₂-LaF₃ nano-glass-ceramics are illustrated in Figure 10. For all GCs, the decay curves follow the second-order exponential nature, which could, according to the distribution of Dy³⁺ ions, be either between a silicate xerogel host and fluoride nanocrystals with different decay rates, but could also indicate the ET process between neighboring Dy³⁺ ions in the host. The resultant τ_m(⁴F_9/2):Dy³⁺ lifetimes with A₁ and A₂ parameters are depicted in

Table 6. Decay components (τ_n), residual weighting factors (A_n), and average decay times (τ_avg) of the ⁴F_9/2 level of Dy³⁺ in prepared nano-glass-ceramics containing LaF₃ phase.

Sample	Decay Components (μs)		Residual Weighting Factors (%)		Average Decay Time, τavg (μs)
	τ1	τ2	A1	A2
GC1	302.7 ± 2.1	1920.6 ± 6.8	45.64	54.36	1731.5 ± 5.7
GC2	223.3 ± 0.8	1317.8 ± 3.0	55.92	44.07	1124.1 ± 2.5
GC3	143.1 ± 0.6	782.5 ± 3.4	66.51	33.49	612.2 ± 3.0
GC4	64.2 ± 0.5	305.1 ± 2.6	67.44	32.56	232.0 ± 2.3
GC5	51.4 ± 0.4	197.6 ± 1.8	69.12	30.88	143.8 ± 1.5
GC6	42.9 ± 0.1	180.2 ± 0.5	76.76	23.24	119.8 ± 0.4

. Indeed, for the subsequent Dy³⁺-doped samples, the progressive shortening of the lifetimes was observed, and the average decay times equaled: 1731.5 ± 5.7 (GC1), 1124.1 ± 2.5 (GC2), 612.2 ± 3.0 (GC3), 232.0 ± 2.3 (GC4), 143.8 ± 1.5 (GC5), and 119.8 ± 0.4 μs (GC6). That denoted tendency to shortening of the decay times clearly indicates the continuous concentration quenching. It should also be noted that the ET is much more noticeable for nano-glass-ceramics than for xerogels (the shortening of the τ_avg(⁴F_9/2) was reported only for XG6 sample with La³⁺:Dy³⁺ molar ratio equals to 0.70:0.30), which is strictly associated with substantial decreasing in Dy³⁺-Dy³⁺ distances due to their partial entering into LaF₃ nanocrystal lattice. Another noteworthy issue is related to the substantial elongation of the τ(⁴F_9/2):Dy³⁺ lifetimes for nano-glass-ceramic materials in accordance with precursor xerogels, especially for samples with lower content of Dy³⁺, and it is associated with the low-phonon energy of LaF₃ nanocrystal lattice (350 cm⁻¹ [42]), which provides the low probability of depopulation of the excited states. Indeed, about 20 phonons of such fluoride phase would be needed to cover the energy gap between the ⁴F_9/2 level and the ⁶F_1/2 state of Dy³⁺ to quench the luminescence. Additionally, the remaining part of Dy³⁺ ions (which did not accumulate inside the fluoride lattice but are still located inside the amorphous sol-gel host) are surrounded by Q³ [SiO₄] groups (1045 cm⁻¹) with lower oscillation energy than OH moieties, which also reduces the probability of the ⁴F_9/2 state depopulation. According to our previous research concentrated on the impact of structure on photoluminescence of RE³⁺ [46], it has been proven that the proposed thermal treatment conditions (350 °C/10 h) cannot trigger the complete elimination of OH groups from the sol-gel network; nevertheless, their amounts are significantly reduced compared to the xerogels. As a result, OH groups do not have a crucial impact on Dy³⁺ luminescence quenching. According to the literature data the τ(⁴F_9/2):Dy³⁺ lifetimes for glass-ceramic materials with PbF₂ [70], NaGd(WO₄)₂ [71], KNbO₃ [73], or Ca₂Ti₂O₆ [74] crystal phases do not exceed the value of 1 ms, while for studied SiO₂-LaF₃:Dy³⁺ nano-glass-ceramics (with lower contents of Dy³⁺ ions), longer lifetimes of about ~1.8 ms (GC1, La³⁺:Dy³⁺ = 0.988:0.012) and ~1.1 ms (GC2, La³⁺:Dy³⁺ = 0.97:0.03) were obtained. For higher contents of Dy³⁺ ions (when La³⁺:Dy³⁺ molar ratio equals 0.94:0.06, 0.88:0.12, and 0.70:0.30), the luminescence lifetimes are comparable with the values declared in the literature for those glass-ceramic systems [70,71,72,73,74].

Finally, it should be also pointed out that photoluminescence quantum yield (PLQY) is one of the essential spectroscopic parameters for RE³⁺-doped materials to judge their suitability for device fabrication, e.g., as visible light or infrared irradiation emitters. In the paper published by N. Maruyama et al. [75], the quantum yields for Dy³⁺-doped glass with 40BaO-20TiO₂-40SiO₂-0.5Dy₂O₃ and derivative nano-glass-ceramic were evaluated directly from measurements using an integrating sphere. As a result of the crystallization of precursor glasses, the intensities of emission bands according to the ⁴F_9/2 → ⁶H_J (J = 15/2, 13/2, 11/2) transitions of Dy³⁺ ions significantly increased. As a result, the estimated quantum yield for Dy³⁺-doped nano-glass-ceramic is close to 15.2%, while for precursor glass it equaled 4.1%. Therefore, the quantum yield for nano-glass-ceramic is nearly 4-fold higher than for glass. Indeed, for Dy³⁺-doped sol-gel materials described in this work, the sum of the integrated intensities of individual blue (⁴F_9/2 → ⁶H_15/2), yellow (⁴F_9/2 → ⁶H_13/2), and red (⁴F_9/2 → ⁶H_11/2) emissions is at least 4.5-fold higher for SiO₂-LaF₃:Dy³⁺ nano-glass-ceramics compared with silicate xerogels before controlled heat-treatment. Nevertheless, it should be noted that this correlation is observed only for samples with low concentrations of Dy³⁺ ions in sol-gel hosts (with La³⁺:Dy³⁺ molar ratios equaled 0.988:0.012 and 0.97:0.03) when concentration quenching for glass-ceramics is inhibited. Thus, we believe that for those of fabricated nano-glass-ceramics, the quantum yield will be higher than for xerogels due to the preferable location of Dy³⁺ ions inside LaF₃ fluoride nanocrystals and effective shortening of Dy³⁺-Dy³⁺ inter-ionic distances. These important aspects, according to the evaluation of luminescence quantum yields, will be examined in the future.

4. Conclusions

This paper presents the optical characterization of Dy³⁺-doped silicate xerogels and nano-glass-ceramics containing LaF₃ phase, according to the structural modifications and variable La³⁺:Dy³⁺ molar ratios in the samples’ composition. The thermal degradation of La(TFA)₃ and its transformation into the fluoride phase was verified by TG/DSC analysis, and XRD measurements confirmed the crystallization of LaF₃ in the nanoscale. The luminescence characterization of prepared sol-gel samples involved the registration of excitation and emission spectra, along with the decay analysis from the ⁴F_9/2 excited level of Dy³⁺. For amorphous xerogels, the concentration quenching occurs from the sample with the lowest proposed La³⁺:Dy³⁺ molar ratio (0.70:0.30, XG6), when the Dy³⁺-Dy³⁺ distances are the shortest in the series. The considerable differences in τ(⁴F_9/2):Dy³⁺ lifetimes, the decrease in calculated Y/B-ratio, as well as the results from XRD analysis suggest the partial migration of Dy³⁺ from amorphous xerogel host into crystallized LaF₃ nanophase during heat-treatment. Indeed, it was found that luminescence lifetimes are strongly dependent on Dy³⁺-Dy³⁺ inter-ionic distances determined by the content of optically active Dy³⁺ ions and the nature of prepared sol-gel materials (correlated with the vibrational energies in the immediate vicinity of optically active dopant). The embedding of Dy³⁺ inside LaF₃ phase of prepared glass-ceramics resulted in continuous shortening of the inter-ionic distances, thus, the progressive quenching of the luminescence is observable even from the lowest content of Dy³⁺ (La³⁺:Dy³⁺ = 0.988:0.012, GC1). Simultaneously, the incorporation of Dy³⁺ into fluoride nanocrystals with low phonon energy resulted in substantial elongation of the τ(⁴F_9/2) lifetimes compared with xerogels. The obtained results suggest that the fabricated Dy³⁺-doped materials could be predisposed for application as visible light emitters, like color screens or three-dimensional displays.