Optically active nano-glass-ceramic coatings of Nd3+ doped-80SiO2-20LaF3 prepared by the pre-crystallized nanoparticles sol-gel route

Journal of Non-Crystalline Solids 601 (2023) 122050 Available online 28 November 2022 0022-3093/© 2022 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/). Optically active nano-glass-ceramic coatings of Nd 3+ doped-80SiO 2 -20LaF 3 prepared by the pre-crystallized nanoparticles sol-gel route María Eugenia Cruz a , * , Joaquín Fern´ andez b , Alicia Dur´ an a , Rolindes Balda c , d , Yolanda Castro a a Instituto de Cer´ amica y Vidrio, CSIC, Madrid, Spain b Donostia International Physics Center (DIPC) 20018 San Sebastian, Spain c Dept. Física Aplicada, Escuela Superior de Ingeniería, Universidad del País Vasco (UPV-EHU), 48013 Bilbao, Spain d Centro de Física de Materiales, (CSIC-UPV/EHU), 20018 San Sebastian, Spain ARTICLE INFO Keywords: Glass-ceramics Photonics Sol-gel Neodymium Active coatings ABSTRACT Transparent oxyfluoride glass-ceramic (OxGCs) coatings with composition 1.2Nd 3+ 80SiO 2 - 20LaF 3 were prepared following a sol-gel route labelled “pre-crystallized nanoparticles route”. OxGCs are transparent materials composed by one or more fluoride nanocrystals in a glass matrix. OxGCs have been prepared by sol-gel with encouraging results in powders and bulk glass-ceramics but not in coatings. In the present work, aqueous suspensions of LaF 3 nanoparticles doped with Nd 3+ were prepared by chemical route and then incorporated into a silica sol to obtain 1.2Nd 3+ doped-80SiO 2 -20LaF 3 particulate sols. Nd 3+ -LaF 3 suspensions are characterized by XRD, HRTEM, and XRF revealing the presence of LaF 3 as the unique crystalline phase. Then, 1.2Nd 3+ -80SiO 2 - 20LaF 3 coatings were prepared by dip-coating and characterized by XRD, FTIR, and Ellipsometry. Photoluminescence measurements were performed for LaF 3 nanoparticles and OxGCs coatings, confirming the presence of Nd 3+ in the nanocrystals and showing well-structured crystalline-like emission spectra with lifetimes of 520 and 440 µs respectively. 1. Introduction Transparent glass-ceramics (GC) [1–3] doped with rare earth (RE) ions have received extensive attention during the last decades due to their varied applications as solid-state lasers, fiber amplifiers and tunable lasers among them [4–6]. GCs present properties similar to glass as the excellent mechanical, thermal and electrochemical stability [2,7, 8]. These properties are combined with those of single crystal as an effective optical media for light propagation and luminescence enhancement [1,9–11]. Moreover, GCs are transparent from the visible to the near-infrared region as well as compatible with optical technology. In particular, oxyfluoride glass-ceramics (OxGCs), where the crystalline phase consists of one or more fluoride crystal phases, show interesting optical properties. Fluoride crystals present low maximum phonon energy (300-450 cm −1 ) and low refractive index compared to oxides that can be beneficially used. Furthermore, when RE are used as dopants, the crystal structure of fluorides reduces the probabilities of multiphoton relaxation, resulting in high luminescence efficiencies [12]. The traditional and most used method to prepare OxGCs is the meltquenching (MQ). By MQ, crystals grow during controlled heat-treatment of the parent glass at temperatures close to their transition range [10]. In the pioneering work of Wang and Ohwaki [13], the preparation of Er 3+ -Yb 3+ codopedOxGCs by melting-quenching was reported with highly transparent material with good optical properties. These OxGCs showed around 100 times more efficient luminescence than the parent glass. However, RE ions are located not only in the crystal phase, but they are also usually found in the amorphous phases, only low fractions of fluoride being able to be incorporated. In addition, most studies of OxGCs prepared by MQ are focused in the preparation of bulk materials containing many different fluoride crystal phases with promising optical properties [14–17]. RE doped OxGCs optical fibers showed excellent results, serving as active fibers for optical telecommunication. The research and application of optical fibers have become more and more attractive [18,19]. Although OxGCs can be used to improve the luminescence response of optical devices, there are important applications, as antimicrobial surfaces or environmental monitoring systems based in coatings [4,20, 21]. The possibility to prepare OxGCs as films would allow the creation of new materials for further application in novel fields. However, coatings cannot be obtained by the traditional melt-quenching method, and * Corresponding author. E-mail address: [email protected] (M.E. Cruz). Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol https://doi.org/10.1016/j.jnoncrysol.2022.122050 Received 18 July 2022; Received in revised form 17 November 2022; Accepted 20 November 2022 Journal of Non-Crystalline Solids 601 (2023) 122050 2 alternative routes must be considered. Sol-Gel (SG) is a wet-chemical process with bottom-up approaches to produce high homogeneous materials by using temperatures much lower (<500◦C) than those used in melt-quenching. This process consists on the hydrolysis and polycondensation of metal alkoxide precursors such as tetraethyl orthosilicate (TEOS) in a solvent, typically alcohol, by adding water [22]. After the polycondensation, the formation of an open continuous network, called gelation, takes place, followed by a further densification. SG is a versatile process to prepare bulk, powder, and coatings. In the last three decades, SG was used to produce OxGCs with encouraging results [23–25]. During the last years, OxGCs have been prepared by sol-gel following the Fujihara´s route based on the mixing of the metal alkoxide sol with a solution of rare earth precursors. Fujihara et al. [26] prepared for the first time LaF 3 glass-ceramics coatings by sol-gel. The authors described the LaF 3 crystallization during the heat-treatment, and the preparation of homogenous LaF 3 coatings. Fujihara’s group also reported the preparation of SiO 2 -LaF 3 bulk samples and coatings using tetraethyl orthosilicate (TEOS) as silica precursor and trifluoroacetic acid (TFA) as fluoride source (TFA route), although no optical results are provided. The synthesis process involves the decomposition of TFA around 300◦C, followed by the formation of fluoride nanocrystals (NCs). Many OxGCs were prepared following the TFA route, leading to different fluoride phases [24,27,28]. However, up to 2018, most articles reported the preparation of OxGCs with small nominal crystal fractions (up to 10%). In the same period, Gorni et al. [29] described the crystallization process of OxGCs by sol-gel and concluded that a heat-treatment of 1min is enough to obtain a suitable crystallization in the silica matrix. GlaSS group have also reported the preparation, for the first time, of OxGCs with an active phase amount of 18% molar of LaF 3 calculated by Rietveld [30]. Later, this group prepared OxGCs containing GdF 3 , NaLaF 4 or KLaF 4 NCs with 20% content of active phase [24,31,32]. Even though the luminescent results of this doped-OxGCs are very attractive, were limited to bulk or powders. OxGCs were also prepared as coatings following this route but during the densification of the coating by heat-treatment, the crystal growth is inhibited resulting in crystals smaller than 3 nm, thus avoiding the full incorporation of RE into them, and reducing the efficiency of the optical response [22,33]. Only a few papers described alternative sol-gel routes to achieve OxGCs coatings with suitable optical properties. For example, Rodrigues et al. [34] reported the preparation of LaF 3 -SiO 2 coatings by dispersing Eu 3+ -doped LaF 3 /LaOF NPs into a silica sol by spin coating. However, the presence of LaF 3 was not confirmed and luminescence measurements did not show the emission of Eu 3+ , likely due to the low quantity of NPs incorporated in the silica matrix. GlaSS´s group reported an alternative route, labelled as “Pre-crystallized nanoparticles route” [35] based on the previous synthesis of crystalline fluoride NP aqueous suspensions that are subsequently incorporated to a silica sol [36]. By this way, silica acts as host matrix for NCs of known morphology and size, the densification of the matrix not affecting the NC growth. The synthesis of stable LaF 3 aqueous NPs suspensions was studied in detail using different surfactants and dispersants [37–39]. In a previous work, Cruz et al. [36] reported the preparation of Nd 3+ doped-80SiO 2 -20LaF 3 oxyfluoride glass-ceramic powders with encouraging photonic results. The authors studied the stability of the LaF 3 NPs into the silica matrix before and after the heattreatment at 450◦C. The results revealed a promising method to produce stable fluoride NP suspensions and their incorporation into a silica sol. Thus, the aim of this work is the preparation of Nd 3+ doped 80SiO 2 -20LaF 3 coatings with efficient luminescence following the “pre-crystallized nanoparticles route” preserving the properties of Nd 3+ in LaF 3 crystal. This synthesis route proved to be very efficient for obtaining highly crystalline Nd 3+ -LaF 3 NPs with higher luminescence emission than those reported up to now. 2. Materials and Methods 2.1. Synthesis of Nd 3+ -LaF 3 aqueous suspensions NP suspensions with composition xNd 3+ -LaF 3 (where x=0, 0.9, 1.2 and 1.5 mol%Nd 3+ ) were prepared by mixing lanthanum chloride (LaCl 3 , Alpha Aesar), ammonium fluoride (NH 4 F, Merck) and neodymium acetate (NdAc 3 , Merck) in stoichiometric concentrations with deionized water to reach a La 3+ concentration of 0.04M. The suspensions were maintained at 75◦C for 1.5, 2, 4, 8 and 24 h, respectively. The suspensions were labelled as t-XNd 3+ -LaF 3 where X is the mol% of Nd and t the stirring time. Then, polyvinyl pyrrolidone (PVP, Merck) was incorporated to 0.04M NPs suspensions at 10 wt.% in relation to LaF 3 . The suspensions were concentrated using a rotary evaporator (R-210 with vacuum pump V-700, Buchi) up to a concentration of 0.25M. 2.2. Characterization of Nd 3+ -LaF 3 aqueous suspensions The 0.04M and 0.25M t-XNd 3+ -LaF 3 suspensions were centrifuged at 6000 rpm for 5 min, and the resulted powders rinsed with deionized water; the process was repeated three times. The powders were further dried at 75◦C for 12h and heat treated at 450◦C for 6h in order to remove the PVP [36]. Powders with composition 1.5h-1.2Nd 3+ -LaF 3 , 2h-1.2Nd 3+ -LaF 3 , 4h1.2Nd 3+ -LaF 3 , 8h-1.2Nd 3+ -LaF 3 and, 24h-1.2Nd 3+ -LaF 3 were characterized by X-Ray diffraction using an X-ray powder diffractometer (D8 advance, Bruker) working with CuK α radiation (λ=1.5406 A). The diffraction patterns were acquired in the range 10◦<2Ɵ<70◦with a step of 0.03◦. The crystallite size was calculated by using the Scherrer´s equation, Eq. (1). Dhkl =Kλ  β2−b2−cosθ √(Equation 1) where Dhkl is the calculated crystallite size, K=0.94 for spherical crystals, θ is the Bragg angle and β is the full width of the diffraction peak at half maximum intensity (FWHM). The instrumental broadening was included as b [30,40]. The fits were performed using the origin software and the pseudo-Voigt function. The Williamson-Hall (W-H) plot was also used to estimate the size and strain broadening by considering the peak width as a function of 2θ. The W-H plot was performed calculating the strain (E) for each peak of the XRD pattern using the Eq. (2). The Williamson–Hall plot (β cosθ) vs. (4sinθ) was performed using the D hkl data and E values followed Eq. (3). A linear regression of each plot was generated; the strain component was obtained from the slope and the particle size from the y-intercept [41]. E=β 4cosθ(Equation 2) βcosθ =Kλ 1 Dhkl +4Esinθ (Equation 3) High Resolution Transmission Electron Microscopy (HRTEM) was used to characterize 2h-1.2Nd 3+ -LaF 3 , 8h-1.2Nd 3+ -LaF 3 and 24h1.2Nd 3+ -LaF 3 NPs suspensions dried at 75◦C for 12 h and heat-treated at 450◦C for 6h. The powders were re-dispersed in ethanol followed by dropping on a carbon-coated copper grid (Lacey Carbon, LC-200-Cu 25/ pk). HRTEM images were taken from HRTEM-JEO 2100 microscope and processed using the open-source ImageJ ® software. All the lattice parameters of LaF 3 were determined through the electron diffraction pattern. X-Ray Fluorescence spectroscopy was used to identify the real amount of Nd 3+ present in the 1.5h1.2Nd 3+ -LaF 3 , 2h-1.2Nd 3+ -LaF 3 , 4h-1.2Nd 3+ -LaF 3 , 8h-1.2Nd 3+ -LaF 3 and, 24h-1.2Nd 3+ -LaF 3 powders dried at 75◦C for 12h and heat treated at 450◦C for 6h. The analysis was done using a PANanalytical spectrometer employing Li 2 B 4 O 7 as reference. M.E. Cruz et al. Journal of Non-Crystalline Solids 601 (2023) 122050 3 1.5h-1.2Nd 3+ -LaF 3 , 4h-1.2Nd 3+ -LaF 3 , 8h-1.2Nd 3+ -LaF 3 , and 24h1.2Nd 3+ -LaF 3 and 2hXNd 3+ -LaF 3 (X=0.9, 1.2 and 1.5 mol%) powders obtained from NP suspensions dried and heat-treated for 6h at 450◦C were compacted by uniaxial pressing for 3 min at 1000 MP for luminescence analysis. Then, the samples were excited with a tunable Ti: sapphire ring laser (0.4 cm −1 linewidth) in the 765– 920 nm spectral range. The fluorescence was collected and dispersed by a 0.25 m monochromator, and the signal was detected by an extended IR Hamamatsu H10330A-75 photomultiplier and processed by a lock-in amplifier. The decay times were performed by exciting the samples with a Ti:sapphire laser pumped by a pulsed, frequency-double Nd:YAG laser (9 ns pulse width), and detecting the emission as described above. The photomultiplier output was recorded with a Tektronix oscilloscope. Measurements were performed at room temperature. 2.3. Synthesis of 1.2Nd 3+ doped-80SiO 2 -20LaF 3 sols Sols of composition 1.2Nd 3+ doped-80SiO 2 -20LaF 3 were prepared using tetraethyl orthosilicate (TEOS, Sigma Aldrich) and methyltriethoxysilane (MTES, ABCR) as silica precursors in a molar ratio of 50TEOS/50MTES. The as-prepared and concentrated 2h-1.2Nd 3+ -LaF 3 aqueous suspension was mixed with the TEOS and MTES precursors under continuous stirring at room temperature. Hydrochloric acid was added under vigorous stirring and after 5 min; the sol was immersed in an ice bath for 15 minutes and then, ethanol absolute (EtOH) was added. 2.4. Deposition and characterization of 1.2Nd 3+ doped-80SiO 2 -20LaF 3 coatings Glass-ceramic coatings were prepared by dip-coating on glass-slides substrates (SLIU-010-050, Labox), at 25cm.min −1 and using the 1.2Nd 3+ doped-80SiO 2 -20LaF 3 sol and the aqueous NPs suspension synthesized for 2h. In this part, the appropriate heat treatment procedure was selected, similar to the process described in a previous paper [36]. Coatings were dried at 180◦C for 3h and then sintered at 450◦C for 1h. Multilayer coatings were prepared by repeating the dip-coating process for 2 and 3 times. The coatings were characterized by spectroscopic ellipsometry in the range of wavelength (λ) 350-950nm using a M2000UTM ellipsometer (J.A. Woollam Co., Lincoln, NE, USA). The incidence angle varied between 50◦to 60◦and the acquisition time was 10s. Data modeling were performed using the Complete EASE software (version 5.20, J.A. Woolam Co.) and the Cauchy’s equation model, shown in Eq. (4). N(λ) = A+B λ2+C λ4+D λ6…(Equation 4) Where A, B, C, D, etc. are Cauchy’s coefficients, and N the refractive index. The 1.2Nd 3+ doped-80SiO 2 -20LaF 3 coatings heat treated at 180◦C from 0 to 180 min were scraped and analyzed by Fourier Transform Infrared Spectroscopy (FTIR). The results were recorded with a PerkinElmer Spectrum 100 FT-IR instrument in the range 4000-450 cm −1 , with a resolution of 4 cm −1 . 1.2Nd 3+ -80SiO 2 -20LaF 3 coatings heat treated at 180◦C for 3 h and sintered at 450◦C for 1h were analyzed by Grazing Incidence X-Ray diffraction (GI-XRD). GI-XRD spectra were obtained using a X-pert PRO Theta/2Theta diffractometer (Panalytical). The 2θ scanning angle varied from 20 to 60◦, and the step used was 0.04◦with a grazing angle of 0.5◦. High Resolution Transmission Electron Microscopy (HRTEM) was used to characterize the scraped 1.2Nd 3+ -80SiO 2 -20LaF 3 heat treated at 180◦C for 3h and sintered at 450◦C for 1h. The sample preparation and image processing followed the experimental setup described in Section 2.2. The steady-state emission and excitation spectra and luminescence decay curves of the 1.2Nd 3+ doped80SiO 2 -20LaF 3 coatings were performed by using the experimental setup described in Section 2.2. The coatings were excited with a tunable Ti-sapphire ring laser, and the incident beam formed a 25◦angle with the sample normal. The emitted light was collected along the direction perpendicular to the coatings. 3. Results and discussion 3.1. Characterization of t-XNd 3+ -LaF 3 NPs Stable t-1.2 Nd 3+ -LaF 3 NPs suspensions were prepared at different reaction times from 1.5 to 24h. A small amount of each suspension was dried at 75 ◦C and then heat treated at 450◦C for 6h to study the evolution of the NPs with the reaction time. Fig. 1a shows the X-ray diffraction (XRD) patterns of t-1.2Nd 3+ -LaF 3 -NPs powders, taken in the range 20◦<2Ɵ<70◦. As observed, the XRD patterns show clear and intense peaks associated with the crystallization of LaF 3 as hexagonal phase as unique phase, corresponding to the pattern JCPDS 00-0320483. In the XRD, it is observed that the intensity of the peaks increases with the reaction time. The crystal size calculated using the Scherrer´s equation and W-H equation are shown in Table 1. For a reaction time of 1.5h the crystallite size, calculated by Scherrer equation is around 10nm, increasing slightly to 11, 12 and 14 nm for longer reaction times 2h, 4h and 8h, respectively; the maximum crystal size, 28nm, is reached for 24h. Moreover, Fig. 1a shows the amplification of the double peak corresponding to 23◦<2Ɵ<26◦. This doublet becomes more defined with the reaction time, reaching a good resolution at 24 h, confirming the advance of crystallinity for longer reaction times [42]. The average particle size and micro-strain have also been calculated using W–H equation. Fig. 1b shows the plotting of W-H equation for t-1.2 Nd 3+ -LaF 3 NPs with t from 1.5 to 24h. The slope of the lines provides the value of the intrinsic strain, and the intercept gives the average particle size (Table 1). The authors report that the crystallite size and lattice strain increase with the peak width [43]. This difference is attributed to the fact that Scherrer´s equation only considers that all the peak width for determining the particle size, whereas by W-H model the peak width has two contributions (strain +crystallite size) [44]. On the other hand, the strain values, shown in Table 1, slightly change from 1.5h-1.2Nd 3+ -LaF 3 NPs to 2h-1.2Nd 3+ -LaF 3 NPs (from 0.0997 to 0.0927) indicating a similar atomic arrangement and morphology. The strain is decreased until 0.0034 for NPs synthesized during 4h, associated with the completed formation of elongated NPs. Moreover for 8h-1.2Nd 3+ -LaF 3 NPs, where rounded NPs started to appear (shown in the HR TEM images in Fig. 2) the strain increased until 0.0305, related to the lattice strain corresponding to the morphology change in the NPs. Finally, the lattice strain up to 0.0165 decreases with the complete formation of rounded NPs and the increase of crystallization. Fig. 2 shows the HRTEM images of t-1.2Nd 3+ -LaF 3 powders and the respective NPs size distribution graphs for reaction times of t=2, 8 and 24h and heat-treated at 450◦C. Fig. 2a and b show the 2h-1.2Nd 3+ -LaF 3 NPs with elongated form, mainly nano-rods, along with some spherical ones, obtained by agglomeration of nano-rods primary particles. The average size of the spherical particles is 11 nm and, 13 nm for elongated ones (Fig. 2c and d, respectively). These values are discordant with those calculated by W-H plot (Fig. 1), probably associated with the formation of polycrystalline aggregates, resulting in NP sizes higher than the crystallite sizes [43]. In the case of 8h-Nd 3+ -LaF 3 powder (Fig. 2e and f), aggregated NPs grow quickly reaching a more spherical shape compared to 2h-1.2Nd 3+ -LaF 3 NPs, although elongated NPs and remain in more amount. The dimensionless shape factor between rounded (R) and elongated (E) NPs was calculated as R/E, Table 1. Furthermore, for reaction times from 2h to 8 h, an increment in the particles size is noticed for both shapes of NPs, from 14 nm to 25 nm for the elongated NPs and from 11 to 16 nm for spherical ones. For reaction time of 24h, a complete transformation from elongated to rounded shapes is observed (Fig. 2i and j). In this case, the medium size of the spherical NPs is around 25 nm. The amplification images and filtering using the fast Fourier M.E. Cruz et al. Journal of Non-Crystalline Solids 601 (2023) 122050 4 transform (FFT) of 2h-1.2Nd 3+ -LaF 3 , 8h-1.2Nd 3+ -LaF 3 and 24h-1.2Nd 3+ - LaF 3 NPs are shown in Fig. 2b, f and j, revealing the lattice planes with a constant spacing of 2.0Å, associated with (113) atomic plane of LaF 3 hexagonal phase. Similar behavior was reported by Liu and Chen [45], where LaF 3 NPs were prepared by salvo-thermal reaction, observing the transformation from elongated to spherical shape with the increment of synthesis temperature. Other authors such as Vanetsev et al. [42] reported the preparation of LaF 3 by different routes; in particular, by co-precipitation route less crystalline materials with mainly elongated shape were obtained while more crystalline NPs with rounded/ hexagonal shapes were obtained by hydrothermal micro-wave treatment. In general, the kinetic parameters such as temperature or reaction time affect the morphology and crystal size of the LaF 3 NPs. 1.2Nd 3+ -LaF 3 powders were analyzed by X-ray fluorescence spectroscopy to quantify the real amount of Nd 3+ incorporated into the LaF 3 NPs (FF). XRF measurements were performed for 1.5h1.2Nd 3+ -LaF 3 , 2h-1.2Nd 3+ -LaF 3 , 4h-1.2Nd 3+ -LaF 3 , 8h-1.2Nd 3+ -LaF 3 and 24h-1.2Nd 3+ - LaF 3 samples (Table 1). The analysis reveals that the concentration of Nd 3+ incorporated in the NPs depends on the reaction time. For a reaction time of 2h, only 1.02 mol% of Nd 3+ has been incorporated in the NPs instead of 1.2 mol%. Thus, the incorporation of Nd 3+ increases with the reaction time, reaching a 100% (1.2 mol%) for 24h-1.2Nd 3+ - LaF 3 . 3.2. Luminescence properties of Nd 3+ -LaF 3 NPs To analyze the influence of the reaction time on the luminescence, the emission of 1.2 mol% Nd 3+ - doped LaF 3 NPs was obtained for t=1.5, 2, 4, 8, and 24 h. The 4 F 3/2 → 4 I 11/2 steady-state fluorescence spectra were performed at room temperature by exciting with a Ti:sapphire laser at 786 nm in resonance with the 4 I 9/2 → 4 F 5/2 , 2 H 9/2 absorption band. As observed in Fig. 3a, the emission increases from 1.5 to 2 h and then significantly decreases as reaction time increases. Therefore, the most efficient emission corresponds to a reaction time of 2 h. As it is revealed in the section before, in Table 1, the content of Nd 3+ in the NPs is increased from 0.95% to 1.2% when reaction time increases from 1.5h to 24h. Particularly, the sample corresponding to a reaction time of 24h shows a real content equal to the nominal one (1.2%). The observed luminescence behavior for reaction times longer than 2h can be attributed to concentration quenching due to the high content of Nd 3+ in the NPs at long reaction times, as well as changes of Nd 3+ distribution with NPs morphology. Thus, Nd 3+ -LaF 3 NPs suspension prepared at a reaction time of 2h was identified as the most efficient and selected to the rest of studies [46–48]. On the other hand, the luminescence properties of LaF 3 NPs prepared with a reaction time of 2 h and different amount of dopant (0.9, 1.2, and 1.5 mol% Nd 3+ ) heat-treated at 450◦C-6h were measured to determine the optimum concentration of Nd 3+ . As observed in Fig. 3b the spectra are similar for the three samples. The peaks position and linewidth do not change when increasing Nd 3+ concentration; however, the emission intensity is reduced for the sample doped with 1.5% indicating the presence of non-radiative processes. These processes present for concentrations higher than 1.2 mol% Nd 3+ are also reflected in the lifetime values of the 4 F 3/2 state, reduced from 520 µs for the sample doped with 1.2 mol% Nd 3+ to 439 µs for 1.5 mol%. The spectral features of the emission spectra together with the lifetime value of 520 µs correspond to Nd 3+ ions in LaF 3 crystal confirming the incorporation of the rare-earth ion in LaF 3 NCs. Fig. 1. (a) XRD patterns and (b) W-H plot from the XRD of t-1.2Nd 3+ -LaF 3 powders prepared at different reaction times; t =1.5, 2, 4, 8 and 24h, and heat-treated at 450◦C for 6h. Table 1 Crystallite size calculated by Scherrer´s equation and by W-H plot, strain calculated from W-H plot, elongated and rounded particle size measured from HR-TEM images and, rounded/elongated shape ratio (R/E) with the increment of reaction time effective incorporation of Nd 3+ in the LaF 3 NPs (FF). Cristallite Size (nm) Nanoparticle size (nm) Reaction time (h) Scherrer Equation (þ/- 0.1nm) W-H Plot (þ/- 0.1nm) Strain Elongated Particle size obtained from HR-TEM (þ/- 0.5nm) Rounded Particle size obtained from HR-TEM (þ/- 0.5nm) Roundedl/ Elongated NPs molNd 3þ : molLaF3 (R/E) (FF) 1.5 10 2.9 0.0997 – – 20 0.95 2 11 3 0.0927 16 11 — 1.02 4 12 3.12 0.0034 – – 55 1.035 8 14 4 0.0305 25 16 — 1.05 24 28 8 0.0165 – 25 100 1.2 M.E. Cruz et al. Journal of Non-Crystalline Solids 601 (2023) 122050 5 3.3. Characterization of 1.2Nd 3+ doped-80SiO 2 -20LaF 3 coatings Homogeneous and transparent 1.2Nd 3+ doped-80SiO 2 -20LaF 3 coatings, were prepared after the incorporation of the Nd 3+ -LaF 3 NPs suspension synthesized for 2h (2h-1.2Nd 3+ -LaF 3 ). The coatings were prepared by dip-coating in air atmosphere and room temperature and subjected to a thermal treatment at 180◦C followed by heating 450◦C/ 1h. It is crucial to select the best sintering conditions to densify the coatings and to completely remove the PVP. The combustion and decomposition of the organic matter must occur before the densification of the coatings, otherwise the porous entrap the PVP during the densification of the network avoiding the elimination of surfactant and affecting the optical properties. In this part, the appropriate heat treatment procedure was selected, similar to the process described in a previous paper [36]. Fig. 4 shows the FTIR analysis of the scraped 1.2Nd 3+ doped-80SiO 2 - Fig. 2. (a) and (b) HR-TEM images of 2h-1.2Nd 3+ -LaF 3 NPs, (c) and (d) particle size distribution of rounded and elongated 2h-1.2Nd 3+ -LaF 3 NPs, respectively, (e) and (f) HR-TEM images of 8h-1.2Nd 3+ -LaF 3 2h NPs, (g) and (h) particle size distribution of rounded and elongated 8h-1.2Nd 3+ -LaF 3 NPs, respectively, (i) and (j) HRTEM images of 24h-1.2Nd 3+ -LaF 3 8h NPs and (k) particle size distribution of rounded 24h-1.2Nd 3+ -LaF 3 NPs. Selected areas of electron diffraction (SAED) patterns and crystalline structures with interplanar distances associated to hexagonal phase are shown in each pair of pictures. Fig. 3. Room temperature emission spectra of the 4 F 3/2 → 4 I 11/2 transition of (a) LaF 3 NPs doped with 1.2% Nd 3+ obtained for t=1.5, 2, 4, 8, and 24 h and (b) LaF 3 NPs doped with three different Nd 3+ molar concentrations for t=2h. M.E. Cruz et al. Journal of Non-Crystalline Solids 601 (2023) 122050 6 20LaF 3 coatings heat treated at 180◦C at different sintering time from 0 to 3 h. In the spectrum of the scraped coating dried at room temperature (Fig. 4; t =0 h), a broad peak around 3000-3500cm −1 appears associated to –OH (3000-3500cm -1 ) of molecular water together with small peaks assigned to CH 2 groups (2978cm −1 ) from the remainders of PVP. This broad band is also observed for coatings treated at 180◦C during 1 and 2h [49]. However, after 3h of heat treatment, the peaks disappear indicating the total elimination of PVP. In conclusion, the heat treatment conditions selected was 180◦/3h followed by 450◦C/1h. The effect of stacking coatings on the optical properties was also investigated. Homogeneous and crack-free multilayer coatings were prepared by stacking a maximum of 3 layers of 1.2Nd 3+ doped80SiO 2 - 20LaF 3 (Fig. 5a) by dipping. The multilayer system was characterized by ellipsometry to obtain the optical constant and the thickness of each stacking coating. Fig. 5b shows the ellipsometer measures together with the fitted curves using the Cauchy model. The measure of the ellipsometric angles Ψ and Δ is in good agreement with the calculated values, confirming the quality of the coatings. Fig. 5c shows the linear increment of coating thickness with the number of coatings, from 1.2 µm for one layer to 2.9 µm for three coatings. The refractive index was measured using the Cauchy model and considering one, two or three stack layers depending on the number of layers deposited. A value of 1.44 at λ=700 nm were obtained in all the case. Fig. 6a and b shows HR TEM images of the scrapped coating with composition 1.2Nd 3+ doped-80SiO 2 -20LaF 3 . Fig. 6a and b shows elongated and rounded NPs, respectively, both with similar morphology and size (13nm and 14nm, rounded and elongated NPs respectively) to those NPs shown in HR TEM images of the 2h-1.2Nd 3+ -LaF 3 NPs. The HRTEM interplanar distance shown in Fig. 6a was determined 0.32nm, corresponding to the lattice plane (111) of the hexagonal LaF 3 (JPCD 00-320483). These results confirm the presence of the 2h-1.2Nd 3+ -LaF 3 NPs into the silica matrix of the 1.2Nd 3+ doped-80SiO 2 -20LaF 3 OxGCs coating as well as the stability of the NPs through the whole process. Fig. 6c shows the XRD pattern of the tree-layers 1.2Nd 3+ doped80SiO 2 -20LaF 3 coating heat treated as described below. The lack of definition of the peaks is related to the fact that coatings have been prepared on soda-lime glasses and, to the thickness of the coating that is much lower than that of the substrate. In the XRD pattern the peaks Fig. 4. FTIR analysis by scraping the 1.2Nd 3+ doped-80SiO 2 -20LaF 3 coatings heat-treated at 180◦C at different treatment times: 0h (black line), 1h (red line), 2h (blue line) and 3h (green line). Fig. 5. (a) Ellipsometry characterization of 1.2Nd 3+ doped-80SiO 2 -20LaF 3 coatings with 1, 2 and 3 layer-by-layer assembly. Fitting performed with Cauchy layer model (dashed lines) for the ellipsometric angle Ψ in the range of 450nm<λ<900nm. (b) Variation of the coating thickness as a function of the number of layers deposited. (c) Transparent triple-layer 1.2Nd 3+ doped-80SiO 2 -20LaF 3 coating after heat treatment at 180◦C for 3h and 450◦C for 1h. M.E. Cruz et al. Journal of Non-Crystalline Solids 601 (2023) 122050 7 associated to the hexagonal phase of LaF3 JCPDS 00-032-0483 are shown. The XRD patterns reveals the presence of LaF 3 NPs incorporated into the OxGCs coating and it is possible to observe that the most intense peak, corresponding to 2θ=27.45◦is associated to the plane (111), the same observed in HR TEM images. 3.4. Luminescence properties of 1.2Nd 3+ doped-80SiO 2 -20LaF 3 coatings Once the optimum Nd 3+ concentration and reaction time were selected, the luminescence properties of transparent coatings of 1.2Nd 3+ doped-80SiO 2 -20LaF 3 heat-treated 3h at 180◦C and 1h at 450◦C were measured. Fig. 7a shows, as an example, the emission and excitation spectra of a transparent coating prepared with two layers. The emission spectrum, like that obtained for the NPs, shows two main peaks at around 1047 and 1063 nm. This spectrum confirms that Nd 3+ ions are in the LaF 3 NCs in the coatings [46,50]. The emission (Fig. 7a) was obtained by exciting at 786 nm, the most intense peak in the excitation spectrum (Fig. 7b). The excitation spectrum performed in the 775-890 nm range by collecting the luminescence at the maximum of the emission shows sharp peaks characteristics of the excitation spectrum of Nd 3+ in LaF 3 NCs [46]. Similar results are obtained for the three layers coatings (not shown). Further evidence of the presence of Nd 3+ ions in the NCs in the coatings is provided by the fluorescence decay curves of the 4 F 3/2 level. Fig. 8 shows the experimental decay of the 1.2Nd 3+ doped-80SiO 2 - 20LaF 3 coating obtained under excitation at 786 nm and collecting the luminescence at 1063 nm. The decay can be described to a good approximation by a single exponential function with a lifetime of 440 μ s, shorter than that obtained for the NPs. The experimental decay corresponding to Nd 3+ in LaF 3 NPs is also represented in Fig. 8 (red line). This reduction of the lifetime cannot be attributed to the presence of PVP, fully eliminated after 3h at 180◦C (Fig. 4), a possible origin could be related to small aggregations of LaF 3 NPs. These results confirm that 1.2Nd 3+ doped-80SiO 2 -20LaF 3 coatings Fig. 6. (a) and (b) HR TEM images of the scraped 1.2Nd 3+ doped80SiO 2 -20LaF 3 heat treated at 180◦C for 3 h and 450◦C for 1h and (c) XRD pattern of the 1.2Nd 3+ doped80SiO 2 -20LaF 3 coating heat treated at 180◦C for 3 h and 450◦C for 1h. Fig. 7. (a) Room temperature emission spectrum of the 4 F 3/2 → 4 I 11/2 transition of 1.2Nd 3+ doped80SiO 2 -20LaF 3 coating obtained by exciting at 786 nm. (b) Room temperature excitation spectrum obtained by collecting the luminescence at 1063 nm. Fig. 8. Semi-logarithmic plot of the experimental decay of the 1.2Nd 3+ doped80SiO 2 -20LaF 3 coating heat treated 3h at 180◦C and 1h at 450◦C obtained under 786 nm excitation by collecting the luminescence at 1063 nm (black line) and 1.2Nd 3+ doped LaF 3 NPs (red line). M.E. Cruz et al. Journal of Non-Crystalline Solids 601 (2023) 122050 8 preserve the luminescence properties of Nd 3+ in LaF 3 NCs being a promising route to prepare glass-ceramic coatings with relevant photonic applications. 4. Conclusions The synthesis of the LaF 3 NPs aqueous suspensions was optimized, selecting a reaction time of 2h as the most appropriated to achieve the best properties of size, shape, and crystallized fraction. An exhaustive physicochemical characterization allows us to conclude that the crystal size and the shape of the NPs, as well as the amount of dopant, are key parameters to take into account for achieving good mechanical and spectroscopic performances. Furthermore, it was found that 1.2 mol.% Nd 3+ is the highest possible amount of Nd 3+ dopant in LaF 3 NPs before concentration quenching occurs. Transparent OxGCs coatings with composition 1.2Nd 3+ doped80SiO 2 -20LaF 3 were obtained for the first time by the “Pre-crystallized NPs route” from the incorporation of the NPs suspension into silica sol precursors. Nd 3+ -LaF 3 NPs are stable in the silica sol as well as in the coatings, maintaining their composition, morphology, and optical properties. The luminescence emission of the 1.2 Nd 3+ -80SiO 2 -20LaF 3 coatings confirms the efficiency of the incorporation of the Nd 3+ into the crystals and the stability of the LaF 3 along the present sol-gel route. The excited-state lifetime value was found to be 440 µs. Although, there is enough room to further improve the synthesis process by adjusting the rare earth concentration, the present results confirm that this processing route appears as promising and suitable for preparing Ln: Oxyfluoride sol-gel coatings with high luminescence efficiency. Disclosures All authors declare that they have no conflicts of interest. Confirmation of authorship We, the undersigned, confirm that we are the joint authors of the above paper. We confirm that all the authors have had material input into the submission. We confirm that, to our knowledge, all the claims, statements and conclusions are true and are our jointly held opinions. We confirm that we all accept the terms of publication of the publisher. Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Maria Eugenia Cruz reports financial support was from MICINN under projects PID2020-115419GB-C21C22/AEI / 10.13039/ 501100011033. Data availability No data was used for the research described in the article. Acknowledgements The authors acknowledge financial support from MICINN under projects PID2020-115419GB-C21C22/AEI / 10.13039/501100011033 and from the Basque Country University under project GIU21/006. This article is a part of dissemination activities of the project FunGlass, which has received funding from the European Union´s Horizon 2020 research and innovation program under grant agreement No 739566. References [1] R.K. Sharma, P. Ghosh, Lanthanide-doped luminescent nanophosphors via ionic liquids, Front. Chem. 9 (2021) 1–26. [2] H. Guo, F. Li, J. Li, H. 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