M Seraiche
email: seraiche28@hotmail.fr

L Guerbous
email: guerbous@yahoo.fr

R Mahiou

A Potdevin

assisted aqueous sol-gel route: Effect of some synthesis parameters

Keywords: GdBO 3 :Eu 3+, Phosphors, Aqueous sol-gel, Photoluminescence 1 Introduction

Synthesis of GdBO3:Eu3+ nanophosphors via acetylacetone assisted aqueous sol-gel route: Effect of some synthesis parameters

M. Seraiche, L. Guerbous, Rachid Mahiou, A.

D 0 → 7 F
2 transition is very sensitive to the symmetry of the crystal field around Eu 3+ ions and its emission intensity is high if the symmetry is low. It is already suggested that the problem of the low chromaticity, can be resolved by lowering of symmetry of the crystal field. Therefore, the position of Eu 3+ ion in REBO 3 :Eu 3+ nanocrystals Orthoborates strongly affects their spectroscopic properties. Several methods have been performed to reduce the Eu 3+ site symmetry: (i) by developing of such material with different crystal structure, in which Eu 3+ ions occupy sites with low-symmetry environment [25], (ii)by synthesizing of nanoscaled REBO 3 :Eu 3+ phosphors. In fact, by decreasing of the crystallite size, the number of Eu 3+ ions occupying the surface with distortion of inversion symmetry can be increased. Therefore, using nanocrystals phosphors could be improved chromaticity [27][28][29]. Furthermore, it is well known that the method of synthesis has an important influence on the properties of prepared nanomaterials [30]. Indeed, GdBO 3 :Eu 3+ micro/nanophosphors have been synthesized by several various methods, such as solid-state reaction, hydrolysis, ultrasonic spray cost alternative approach to synthesizing novel nanoscale inorganic materials activated by the rare earth ions in powders and films forms. This method have becomes very popular due to its versatility, better purity, low processing temperature, and the possibility of controlling the size and morphology of particles. In addition, the doped nanomaterials prepared by sol-gel method are characterized by their ability to provide uniform distribution of the dopant within the host phase. Moreover, within the sol-gel method, different conditions of synthesis influencing the final products are very important parameters such as complexing and chelating agents, dopant concentration, pH value of solution and annealing temperature. The pH value of the solution plays a crucial role and its change can cause some alteration in structural, the morphological and spectroscopic properties [31,32].

In this work, GdBO 3 : Eu 3+ orthoborates vaterite type nanophosphors have been synthesized by a simple aqueous sol-gel method using the acetylacetone (CH 3 COCH 2 COCH 3 ) as chelating agent. A systematic study of the effect of only two synthesis parameters namely the Eu 3+ ion content (0.5, 1, 2, 3, 10, 15 and 20 mol %)

and pH value precursors suspension, extending from acid to basic states (pH = 2, 4, 7, 8 and 10) on the structural, morphology and photoluminescence spectroscopic has been performed. In this study, we begin to fix the pH value (pH = 2) and we optimize the value of Eu 3+ content and after that the pH value is optimized. Thus, structural purity and luminescence properties are the criteria for selecting the best parameter. Furthermore, in this work, we demonstrate that both Eu 3+ ion content and the pH value play a significant role in the determination of the phase purity and the photoluminescence properties of GdBO 3 :Eu 3+ nanoparticles. 3 ) corresponding to 1 mL for 1g of product was added as complexing agent (Fig. 1). The chemical reaction can be described with stoichiometric equation following:

Experimental details

Samples preparation

alt-text: Fig. 1 Fig. 1 To stabilize the solution, stirring was carried out for 30 min. After that, the pH value of precursors suspension was adjusted by adding ammonium hydroxide NH until transforming in to a Xerogel. Finally, the Xerogel were introduced in a furnace and heated in air at 900 °C for 4 h for different Eu 3+ concentration. Secondly, the preparation of GdBO 3 doped with the Eu 3+ obtained optimal concentration at different pH of precursors suspension namely pH = 2, 4, 7, 8 and 10 has been done by the same steps as we saw in the first part of this synthesis (Fig. 1).

Characterization

The phase identification and the related properties of the nanopowders were investigated by X-ray diffraction (XRD) technique a PAN analytical X'Pert (Philips) PRO using CuK α radiation (λ = 1.54059 Å) operated at 45 kV and 40 mA. A symmetric (θ-θ) scans were performed from 10° to 90° 2θ with a step width of 0.02°. All the data were processed by X'Pert High Score plus Software with commercial databases (FWHM deduction and peak identification). Rietveld refinement was done with help of the FullProf software. Thermogravimetric analysis (TG) and differential thermal gravimetry (DTA) were measured by SETARAM SetsysEv 1750 (TGA-DTA 1600) instrumentation. Micrographs were investigated by means of a ZEISS Supra 55VP Scanning Electron Microscope (SEM) operating in high vacuum at 3 kV, using secondary electron detector (Everhart-Thornley detector). Specimens were prepared by sticking powder onto the surface of an adhesive carbon film.

Raman spectra were recorded at room temperature under excitation at 514.5 nm supplied by a Spectra Physics-Stabilite 2017-100 mW argon laser, using a T64000 Jobin-Yvon spectrometer equipped with a CCD detector cooled with liquid nitrogen. For all the samples, the acquisition time was fixed at 30 s and three measurement cycles were performed. The infrared spectra were recorded in the range of 400-4000 cm -1 with a Nicolet-IR 380 Fourier transform infrared (FTIR) spectrometer. The steady and time-resolved photoluminescence spectra were carried out using PerkinElmer (LS-50B) luminescence spectrometer with pulsed Xe lamp excitation at room temperature as described in [33].

3 Experimental results and discussion

Structural analysis

Fig. 2(a)shows the powder XRD patterns of the synthesized Gd 1-x Eu x BO 3 (x = 0.005, 0.01, 0.02, 0.03, 0.1, 0.15 and 0.2) at pH = 2 with 15% excess of H 3 BO

3, annealed at 900 °C for 4h duration. All diffraction peaks in Fig. 2 was indexed according to the hexagonal phase of GdBO 3 as listed in JCPDS files (No. 74-1932). In this system, it is found that crystallographic structure purity is highly dependent on the Eu 3+ concentration and introducing the Eu 3+ dopant drastically changes the host matrix structure. In fact, from the patterns of Fig. 2(a), one can easily observe that it exist three concentration regions, according to the existence of the parasite phases, especially, High temperature phase (HT-GdBO 3 ), indicate by down arrow. The evolution of parasitic phases with Eu 3+ content is as follows: a first is the low Eu 3+ concentration region, from 0.5 at % to that of 3 at %, in which the parasite phase peaks are present. The second region around 10 at %, in which the parasite phase peaks vanished completely and GdBO 3 give a quasi-purest vaterite phase. From 15 to 20 at %, constitute the third region, in which the parasitic phases reappear, which become more intense. Although, all samples are prepared in the same conditions, it did not prevent this highly dependence of GdBO 3 phase on Eu 3+ content. Usually the parasitic phases are not observable for low concentrations of the dopant, but can appear for high concentrations.

In the present work is the opposite was observed, indicating that there are other factors involved and their effects is more important than that of the concentration. This phenomenon has been observed in orthoborates LnBO 3 prepared by solid state reaction [26]. Some authors have already noticed the dependence of the phases produced as a function of the concentration of the dopant. Szczeszak et al. [34] found that in La 1-x Eu x BO 3 and La 1-x Tb x BO 3 nanopowders synthesized by sol-gel Pechini method the aragonite phase purity depend on the Eu 3+ and Tb 3+ concentration, which connected just to the annealing temperature.

The sample with 10 at % Eu 3+ content presents an apparent pure hexagonal vaterite phase compared to the other concentration and it was chosen for the study of the pH value effect.

Fig. 2(b) shows the powder XRD patterns of the GdBO 3 : 10 at % Eu 3+ prepared with 15% excess of H 3 BO 3 and prepared at different pH, ranging from pH = 2 to 10, annealed at 900 °C for 4h. As it was observed that for both pH = 2 and pH = 10, the samples exhibit a pure hexagonal GdBO 3 vaterite structure. For the other pH values (4, 7 and 8), parasitic phases are also observed, such as HT-GdBO 3 , indicated by down arrow. This demonstrated that the pH value has an effect on the purity of GdBO 3 crystallographic structure. Indeed, the explanation of the mechanisms responsible for the formation of the structural phases produced is very complicated. In fact, the B 2 O 3 is much more volatile than the gadolinium and europium oxides leading to stoichiometric imbalance [26]. Furthermore, it is not possible to control the amount of B 2 O 3 which can volatilize during the synthesis in a random manner. This stoichiometric deviation can influences the reaction rate of gadolinium oxide with europium and boron oxides, which can produces additional phase traces such as gadolinium oxides (Gd 2 O 3 ), borates oxide (Gd 3 BO 6 ) [26] and/or other possible parasitic unknown phases. Consequently, from the above, we can say that the formation of GdBO 3 ) during the reaction and depends also to the used synthesis method (pH value, annealing temperature, duration, reagent..).

alt-text: Fig. 2(a) Fig. 2

(a)

. XRD patterns of Gd 1-x Eu x BO 3 (pH = 2) annealed at 900 °C for 4 h synthesized by aqueous sol-gel route.

alt-text: Fig. 2(b) The Rietveld analysis was used to accurately determine the structure and the crystallographic parameters by using Fullproof suite program for all samples as indicated in Fig. 2(c). On this figure we indicate the calculated and observed diffraction profile of GdBO 3 doped at 10 at% Eu 3+ at pH = 10 delivered by the Rietveld refinements as example in hexagonal P6 3 /mmc space group. The line Y obs -Y calc represents the difference between the calculated and observed intensity. The crystallographic data and atomic parameters for GdBO 3 doped at 10 at% Eu 3+ at pH = 10 are summarized in Table 1. The crystal lattice strain generated by the annealing temperature is determined from the Williamson-Hall (W-H) relationship [35]:

Where β is the full width at half maximum, λ is the X-ray wavelength, θ is the diffraction angle, D is the effective crystallite size and η is the effective strain. In Fig. 3(a) and Fig. 3(b), we present the variation of the crystallite size as well as the strain quantities against the Eu 3+ ion content (Fig. 3 , the sample presents the biggest crystallite size. From Fig. 3(a), it is observed that the below 10% mol Eu 3+ , both crystallite size and strain increases from 0.5 to 10% mol, after that they evolve in the opposite direction. The strain follows the same behavior than that of grain size. Both crystallite size and strain behaviors can relate to the phase parasites in addition to the defects, which can exist in the samples. Furthermore, with the variation of pH value of solution the GdBO (

) 1
alt-text: Fig. 2(c)

Fig. 2(c)
The Rietveld profile fits of X-Ray diffraction data of Gd 0.9 Eu 0.1 BO 3 (pH = 10), annealed at 900 °C for 4 h.

alt-text: Table 1 Table 1 Final refined structure parameters of GdBO 3 :Eu 3+ samples using derived from the FullProf refinement of X-ray diffraction data. i The table layout displayed in this section is not how it will appear in the final version. The representation below is solely purposed for providing corrections to the table. To preview the actual presentation of the table, please view the Proof.

Chemical formula

Also, from Fig. 3 can think that these points can be related to the purity of the GdBO 3 vaterite phase, especially for the Eu 3+ ion concentration parameter.

Thermal analysis

Thermogravimetric curve of GdBO 3 :10 at % Eu 3+ for pH = 10 precursor pointed in blue in Fig. 4 consists in three mass losses regions. The first mass loss was observed between 80 °C and 210 °C and is attributed to the elimination of absorbed species, such as alcohol molecules or water. The second important mass loss observed between 210 °C and 300 °C, corresponds to GdBO3 condensation step, and is in parallel with a large endothermic peak (around 282 °C) in the DTA curve. This endothermic effect is attributed to the removal of H2O molecules. Otherwise, in the range of 210 °C to 300 °C, there is formation of inorganic compounds. A third observed mass loss is between 300° and to 490 °C depending on the formation of GdBO3 and corresponds to the first GdBO3 crystallization temperature range. This final mass loss is characteristic of ultimate formation of the crystallites growth of the GdBO3 network and final disposal of residual OH groups. Above 600 °C, no weight loss was observed. It may be noted that the total mass loss is large (almost -91%) occurred during the heat treatment to 600 °C. From the recorded curves, we can deduce that the best crystallization temperatures, which were chosen, should be in excess of 1000 °C. The formation of hexagonal structure GdBO3 is observed above 600 °C, which is also seen in the curve of the DTA as a nearly linear and stable curve between 600 °C and 1100 °C.

Morphology analysis

It is well known that the morphology and particle size of the phosphors are important for their in lighting devices applications. In Fig. 5 we show that the morphology and particle size of the GdBO3: 10 at % Eu 3+ for pH = 10 powder for different pH value. From this figure, one can see that the morphology and size of the particles were dependent on the pH values. In fact, one can observe that the particles are slackly bound and connected to each other as networks with the particle size depend on the pH value. Indeed, for low pH = 2 the sample presents a small particle size, and slightly increases for pH = 4 and 8. The sample prepared with pH = 10 exhibit a large particle size. Although, the crystallite size at pH = 10 is small, it has the largest particle size. The sizes of these primary particles were estimated about 150 and 250 nm, which can be considered as the agglomeration of three alt-text: Fig. 4 Fig. 4 Thermogravimetric analysis of Gd 0.9 Eu 0.1 BO 3 (pH = 10) synthesized by aqueous sol-gel route.

to five crystallites estimated by W-H method [36]. EDAX analysis, clarifies the qualitative analysis of the elements of the respective phosphor composition i.e. Gd 3+, Eu 3+ and O 2- . Furthermore, EDAX spectra elucidate the absence of any impurity elements, during sol gel reaction (Fig. 5).

Vibrational analysis

It is well known that the trivalent rare-earth orthoborates presents three-isostructural forms, which are calcite, aragonite, or vaterite. Depending on the size of lanthanide cations, their orthoborates usually present the aragonite-type (La-Nd), vaterite-type (Sm-Yb), or calcite-type (Lu) structure Furthermore, both two structures calcite-type and the aragonite-type take part triangular planar (BO 3 ) 3- groups, but the vaterite-type borates participate (B 3 O 9 ) 9- groups. Thereby, the vibrational spectra of vaterite orthoborates type are found to be significantly different from those of calcite and aragonite types. Fig. 6(a) shows the FT-IR spectra of Gd reduce the amount of the water in the final product [37]. Furthermore, one can observe that the FTIR vibrational spectra reflect the XRD analysis results. For all Eu 3+ content, the samples exhibit the absorption bands in the region. 666-12250 cm -1 , ascribed to the characteristic vibration modes of (B 3 O 9 ) 9- alt-text: Fig. 5 Fig. 5 SEM morphology of Gd 0.9 Eu 0.1 BO 3 (pH = 2, 4, 8 and10) powders, annealed at 900 °C/4h synthesized by aqueous sol-gel route.

groups in the GdBO 3 vaterite orthoborates [12,38,and39]]. The absorption intensity depends on Eu 3+ concentration, which is related to the structure purity. In fact, for low Eu 3+ content the samples present intense vibrational absorption, however, for high concentration (15 and 20 at % of Eu 3+ ) weak absorption. This difference can be related to the presence of phase impurity in GdBO 3 vaterite structure, generated by the high Eu 3+ concentration. Furthermore, an intense absorption bands in 1225-1500 cm -1 region is observed for all sample except 10 at % Eu 3+ . This band can be related to the asymmetric stretching of B-O in triangular (BO 3 ) 3- groups in the calcite-type, with the analogy of FTIR spectra of calcite LuBO 3 orthoborates [38]. The FTIR spectra analysis shows that it exist some boron atoms are in 3-fold coordination and the GdBO 3 : Eu 3+ structure made of the (BO 3 ) 3- groups. To confirm the presence of GdBO 3 orthoborates in other structure together with vaterite type, two Raman spectra was registered (Fig. 6(b)) for the two Eu 3+ content namely 3 and 10 at %. In fact, for 3 at % Eu 3+ the sample holds the parasite phase and for 10 at % Eu 3+ is pure. As shown in Fig. 6 presents some additional peaks mainly an intense one at 951.07 cm -1 and some wreaks in 280-380 cm -1 region related to (BO 3 ) 3- groups [39].

alt-text: Fig. 6(a) Earlier and in his work, Ren et al. [START_REF] Ren | [END_REF] conclude that the high-temperature (HT) phase GdBO 3 crystallizes in a calcite, which consists of planar triangular borate group (BO 3 ) 3- . However, Szczeszak et al.

[39] prepared new monoclinic GdBO 3 orthoborates by a modified Pechini method crystallized in the C 2 /c space group. The same author Szczeszak et al. [25] has assigned the presence of borate group (BO 3 ) 3- to new triclinic structure of GdBO 3 :Eu 3+ with P1̅ space group prepared by hydrothermal method. Furthermore, in our previous work [36],

on the basis of the VUV excited emission, it is supposed that GdBO 3 :Eu 3+ calcite type is presents as parasitic phase.

Fig. 6(c) shows the FTIR spectra for the as synthesized of GdBO 3 :10 at % Eu 3+

, annealed at 900 °C for 4 h samples prepared at different pH values. Same absorption bands characteristic of (B 3 O 9 ) 9- groups in GdBO 3 vaterite structure and (BO 3 ) 3- group phase impurity discussed above. It is important to note that for pH = 2, absorption bands in 1225-1500 cm -1 region is not observed and which considered as a pure vaterite structure.

One can note that the intensity of FTIR absorption bands at around 3500 cm -1 increases with increasing of pH values of solution. In fact, for high pH value the reaction is governed by (OH) molecules [37].

The Raman spectra of the Gd 1-x E x BO 3 : a) 3%mol Eu 3+ , b) 10%mol Eu 3+

, annealed at 900 °C for 4 h, synthesized by aqueous sol-gel route.

alt-text: Fig. 6(c)

Fig. 6(c)

Photoluminescence spectroscopy

At the beginning, we have measured the room temperature emission spectra of Gd 1-x Eu x BO 3 (x = 0.005, 0.01, 0.02, 0.03, 0.1, 0.15, 0.2, pH = 2 and 15% excess of H 3 BO

3 ), annealed at 900 °C for 4 h duration, under 235 nm excitation wavelength (Charge transfer Band CTB) shown in Fig. 7(a). The spectra consist of sharp peaks ranging from 570 to 660 nm assigned the transitions from the excited 5 D 0 level to the ground states of 7 F J (J = 1, 2 and 3) of the Eu 3+ activator [40]. The major emission transitions are centered at 591 nm ( 5 D 0 → 7 F 1 ), orange-red magnetic dipole (MD) and at 614, 624 nm ( 5 D 0 → 7 F 2 ), which is the hypersensitive red electric dipole (ED). The weak peaks observed at 648 and 671 nm are ascribed to 5 D 0 → 7 F 3 . The less intense emission peak at 578 nm is attributed to 5 D 0 → 7 F 0 forbidden transition. No intraconfigurationnel emission transitions from the highest excited states 5 D J (J=1-4) of Eu 3+ or Gd 3+ are observed. Furthermore, it is observed from Fig. 7 (a), the orange emission corresponding to the 5 D 0 → 7 F 1 magnetic dipole transition (591 nm) is greater than the red electric dipole transitions in the region ( 5 D 0 → 7 F 2 situated at 614 nm and 624 nm) which is transitions from Eu 3+ ions located in a non-inversion symmetry site. As shown in Fig. 7(a)inset, the concentration quenching occurs at 10 mol% of Eu 3+ ions concentration, which is associated with the energy transfer, occurs from one Eu 3+ ions to other ions and finally quenches the emission intensity [41]. Two effects can tuned the orange-red Eu 3+ emission: (i) the existence of parasitic phases and the type of symmetry in which Eu 3+ is located (centrometric or non-centrometric), (ii) the change in crystallite size. In fact, the reduction of size into nanoscale which increase the number of Eu 3+ ions located in the surface area, leading to lowering the symmetry of crystal field and thereby increases the red emission ( 5 D 0 → 7 F 2 ) relative to the orange one ( 5 D 0 → 7 F 1 ). Therefore, the asymmetry ratio parameter R, which is the ratio of the integrated intensities of the luminescence transitions of 5 D 0 → 7 F 2 on 5 D 0 → 7 F 1 transitions, is introduced (Fig. 7(b)) [42,43]. In fact, this parameter can give FTIR spectra of Gd 0.9 Eu 0.1 BO 3 with different pH value of precursors suspension, annealed at 900 °C for 4 h, synthesized by aqueous sol-gel route.

information about the Eu 3+ ions local symmetry distortion in the host material. In addition, as a consequence of nanoscale materials, the value of R parameter is usually higher for the nanoscale material relative to its bulk form. From Fig. 7(b), one can observe that as the Eu 3+ concentration increases from 0.5 to 3 at % Eu 3+ , the R parameter increases and roughly stabilized between 3 and 10 at % Eu 3+ to come back increases after that. The increasing in R parameter is related to the Eu 3+ site symmetry lowering which can be generated by existence of parasite phases or defects. For the constant quasi-constant value of R is connected to the GdBO 3 hexagonal phase purity. Although, this low symmetry can gives better chromaticity but with lower emission intensity.

alt-text: Fig. 7 The room temperature excitation spectrum (Fig. 7(c)) of the GdBO 3 :10 at % Eu 3+

, annealed at 900 °C for 4 h was recorded by monitoring the emission peak at 614 nm. Fig. 7 (c) reveals a broad and strong excitation band in the 200-265 nm range, which indicates the charge-transfer (CT) transition between O 2- and Eu 3+ . The weak excitation peak observed at 275 nm is assigned to the 8 S 7/2 → 6 I J transitions of Gd 3+ ion. The presence of this peak indicates that it exist an energy transfer from Gd 3+ to Eu 3+ [44]. Several peaks from 300 to 540 nm (Fig. 7 (c) inset) are likely to have originated from the intra-4f forbidden transitions of Eu 3+ ions. the excited Gd 3+ states than the 5 D 0 state of Eu 3+ ; consequently, Gd 3+ ions can be play an intermediate for the energy transfer and sensitize Eu 3+ luminescence.

In fact, the charge-transfer (CT) transition process corresponds to the transfer of an electron from the 2p orbitals of the oxygen ligands to the empty 4f orbitals of the Eu 3+ ion, giving rise usually to the charge transfer band (CTB). In most cases, CTB exhibits a stronger intensity than that of the 4f-4f interconfigurational transitions that appearing as narrow peaks with low intensities in excitation spectra. It is known that this interconfigurational transition of type (np q , 4f 6 → np q-1 4f 7

) has a high absorption cross-section compared to that of 4f 6 -4f 6 intraconfigurationnel transitions, which explain difference in intensity between them on the excitation spectra.

Also, this process can induce a significant luminescence of the dopant ion Eu 3+ doped oxides material if the population of radiative levels 5 D J by a process of energy transfer is itself important [45].

The room temperature steady photoluminescence excitation and emission, of GdBO alt-text: Fig. 7(c) The variation of the asymmetry ratio parameter R in function of pH value of solution is displayed in Fig. 8(b).

The values of R do not follow a well-defined trend and presents like zigzag shape between pH values. We can think that the value of R is very sensitive to several parameters, which can be changed by switching from one pH to another following, and thus influencing the symmetry of the site occupied by the Eu 3+ ion in GdBO 3 host material. Also, from this figure, we can note that for some pH values we can promotes the 5 D 0 → 7 F 2 red emission and can achieve better chromaticity, despite their lower luminescence compared to that pH = 10. Then, there is a tradeoff between the chromaticity and intensity of luminescence.

The excitation spectra for different pH value, annealed at 900 °C for 4 h were measured for the red emission (614 nm), and presented in Fig. 8(c). From this figure, the charge-transfer (CT) transition between O 2- and Eu 3+ were observed between 200 and 265 nm in addition to Gd 3+ excitation transition and those of intra-4f forbidden transitions (Fig. 8(c) inset) of Eu 3+ ions.

Fig. 9(a) displays the room temperature time-resolved phosphorescence emission spectra of GdBO3:10 at% Eu 3+

, for pH = 10 annealed at 900 °C for 4 h duration as an example measured in phosphoresce mode. The extracted integrated photoluminescence decay curves of the 5 D 0 → 7 F 1 (591 nm) transition under UV excitation (CTB-235 nm) for different Eu 3+ content and different pH value of solution is presented in Fig. 9(b) and Fig. 9 (c) respectively. Referring to the decay curves, the effective decay times (τ AV ), which is considered as the experimental lifetime (t exp ) for 5 D 0 level of Eu 3+ the can be determined using the following expression [33]:

Where I(t) is the luminescence intensity at the considered time. The 5 D 0 level lifetime as a function of Eu 3+ concentration was presented in Fig. 9(b) inset. From Fig. 9(b) inset, one can observe that there are three regions with respect to the Eu 3+ content. The first region situated between 0.5 and 2 at % Eu 3+ , in which the 5 D 0 level lifetime decreases. The second one from 2 to 10 at % Eu 3+ , the 5 D 0 level lifetime increases. The third last one, from 10 to 20 at % Eu 3+ the lifetime decreases again. Several mechanisms could be used to explain the deceasing of 5 D 0 luminescence lifetimes in these three regions. For the first and the third region, the reducing of the lifetime can be connected with cross-relaxation between the Eu 3+ ions [46] and the existence of the parasitic phases, playing a quencher role. Indeed, the Eu 3+ ions can interact in their excited states, and also, this interaction can happen between Eu 3+ its excited state and Eu 3+ neighboring ions in a ground state, which result in energy transfer and depopulation of Eu 3+ ions in the 5 D 0 state. For the third region, the increasing of the lifetime can be related to fact that GdBO 3 becomes the more quasi-purest hexagonal vaterite structure and presents the biggest crystallite size with respect to Eu 3+ concentration.

(

) 2
alt-text: Fig. 9(a) F 1 transition of Gd 1-x Eu x BO 3 (pH = 2) powders, annealed at 900 °C/4h.

From Fig. 9(c) inset, on can see variation that 5 D 0 level lifetime depend strongly on the pH value of solution. In fact, the value of 5 D 0 level lifetime connected to several related parameters such as the synthesized method, the phase purity, structure defects, atomic or molecular impurity in the host material and the crystallite size. Although GdBO 3 :Eu 3+ samples prepared at pH = 2 and 10 present the purest hexagonal vaterite structure their 5 D 0 level lifetime values are smallest compared to the intermediate pH values. One can think that the crystallite size is responsible this smallest lifetime. Indeed, for pH = 10 the sample presents the highest orange-red emission and a small crystallite size. Another possible cause is the presence the H 2 O absorbed band with high quantity at pH = 10, which quench the emission leading a shortness lifetime. Furthermore, in nano-size optical materials, several parameters such as the structure, the size have an important impact on the radiative and nonradioactive lifetimes, energy transfer, and thermalization phenomena. In fact, it was found that rare earth doped nano-sized inorganic materials, can exhibits a prolonged lifetimes [47], which further explain the increases of the lifetime in the third region in Fig. 9(b) inset and the variation with respect of pH values.

In addition, for evaluating the performance of such GdBO 3 :Eu 3+ nanophosphor, the chromaticity coordinate is one of the important parameters. Fig. 10 The table layout displayed in this section is not how it will appear in the final version. The representation below is solely purposed for providing corrections to the table. To preview the actual presentation of the table, please view the Proof.

  (a)) and pH value (Fig. 3(b)) in GdBO 3 host material. At 10 at % Eu 3+

  3 : 10%mol Eu 3+ crystallite size change (Fig. 3(b)). The crystallite size increases from pH = 2 to pH = 4 and decreases after from pH = 4 to pH = 10. Although that the sample with pH = 4 presents some parasite phase (Fig. 3(b)) it exhibits the biggest crystallite size and with the highest strain value.

Fig. 2

 2 Fig. 2(b)

  Fig.3(a)

Fig. 6

 6 Fig.6(a)

Fig

  Fig. 7(a)

Fig. 7

 7 Fig. 7(c)

Fig. 9 Fig

 9 Fig. 9(a)

  Fig. 9(c)

Fig. 10 Chromaticity

 10 Fig. 10

GdBO 3

	Crystalsystem	Hexagonal
	Space group	P6 3 / mmc
	a (A°) = b (A°)	3.8326 (6)
	c (A°)		8.909 (2)
	V (Å 3	)	113.33
	Crystallite size (nm)	74
	R exp %	26.91
	R wp %	36.6
	Rp %		39.4
	χ2		1.85

  displays the color coordinates for GdBO 3 :10 at % Eu3+ , for different pH value of solution. The CIE coordinates of the synthesized samples given in Table2. It can be clearly seen from the figure and the table that most of the chromaticity coordinates are located in the orange-red region. In

addition, One can that CIE coordinates are closer to the NTSC (National Television Systems Committee) value of red phosphors (0.670, 0.330) [48]. These results indicate that this kind of phosphor can be useful in the development of LEDs applications.

Table 2

 2 (x,y)Chromaticity coordinates at different pH values of precursor suspension of Gd 0.9 Eu 0.1 BO 3 annealed at 900 °C/4h. The standard NTSC red color coordinates are presented in ref[48].

		CIE Coordinate
	pH value
		x y
	2	0.6318	0.3678
	4	0.6334	0.3662
	7	0.6336	0.3660
	8	0.6361	0.3634
	10	0.6355	0.3641
	NTSC	0.670	0.330

Intense orange-red emitting GdBO 3 orthoborate based nanophosphors doped with Eu 3+ ion have been successfully synthesized via simple and low-cost, using acetylacetone assisted aqueous sol-gel process.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Highlights

• GdBO 3 :Eu 3+ nanophosphors were prepared using acetylacetone assisted aqueous sol-gel method; with two essential parameters: Eu 3+ concentration and pH value.

• Effects of Eu 3+ content and the pH value on the morphology, structural and luminescence spectroscoy properties have been investigated and highlighted.

• GdBO 3 :qEu 3+ hexagonal pure phase shows a correlation on the europium content and pH value.

• High orange-red color emission from 5 D 0 levels to the 7 F J levels (J = 0-3) of Eu 3+ ion was observed.

• It was found that 10% concentration of Eu 3+ presents the quenching of emission and sample pH = 10 exhibit the highest emission intensity in GdBO 3 .

Query: Please confirm that given names and surnames have been identified correctly and are presented in the desired order and please carefully verify the spelling of all authors' names.

Answer: M. Seraiche a *, L. content and the pH value on the morphology, structural and luminescence properties were investigated.• GdBO 3 :Eu 3+ hexagonal pure phase shows a correlation on the europium content and pH value.• High orange-red color emission from 5 D 0 levels to the 7 F J levels (J = 0-3) of Eu 3+ ion was observed. • For 10 % Eu 3+ content the quenching was found for pH = 10 exhibiting the highest emission intensity.


Jeff L Nyalosaso

Audrey Potdevin

François Réveret

Pierre Disseix

Rachid Mahiou

Geneviève

Geneviève Chadeyron

Enhancement of light extraction in Y3Al5O12:Tb3+ thin films through nanopatterning

Keywords:

come

Introduction

Developing high efficiency White Light-Emitting Diodes (WLEDs) for different optical applications such as displays and lighting devices has been a topic of interest for several years. WLEDs generally combine a semiconductor chip emitting in the ultraviolet or blue wavelength region to one or more phosphor(s) deposited in the form of coatings usually using either on-chip or remote configurations. [START_REF] Khanna | Fundamentals of Solid-State Lighting: LEDs, OLEDs, and Their Applications in Illumination and Displays[END_REF][START_REF] Mckittrick | Review: down conversion materials for solid-state lighting[END_REF][START_REF] Piquette | On choosing phosphors for near-UV and blue LEDs for white light[END_REF] Even if actually commercialized LEDs-based display and lighting devices are characterized by good photometric parameters, their optical performances can be further improved notably their luminous efficacy. [START_REF] Geng | Enhancement of light output power from leds based on monolayer colloidal crystal[END_REF][START_REF] Khan | Non-destructive analysis on nano-textured surface of the vertical LED for light enhancement[END_REF] One of the factors we can act on is the Light Extraction Efficiency (LEE), which represents the ratio between the number of photons extracted to the air and the number of photons generated in the active layer. [START_REF] Geng | Enhancement of light output power from leds based on monolayer colloidal crystal[END_REF]Lenef et al., 2018;[START_REF] Narendran | Extracting phosphor-scattered photons to improve white LED efficiency[END_REF][START_REF] Tian | Highly enhanced luminescence performance of LEDs via controllable layer-structured 3D photonic crystals and photonic crystal beads[END_REF] Actually, the emitted light is hard to extract into the air since there is a gap between the high refractive index of the phosphor (n ~1.8 for the most common used Y 3 Al 5 O 12 :Ce 3+ phosphor) and the air (n ~1). Total internal reflection (TIR) thus occurs and a large fraction of the light produced in LED devices is backscattered to the chip or trapped in dielectric structures inherent to the substrate (luminescent coatings). (Lenef et al., E-mail address: audrey.potdevin@sigma-clermont.fr (A. Potdevin) 2018; [START_REF] Zhmakin | Enhancement of light extraction from light emitting diodes[END_REF] The trapped light is then, either emitted from the substrate edge or re-absorbed then converted to heat, which entails in both cases the decrease in forward light emission, limiting the LEE value. The key to increase the escape probability of photons and so on LEE value, is to give them more chances to find the escape cone defined by

, n air and n AL being refractive indexes of the air and of the Active Layer (AL) (for example the phosphor) respectively. [START_REF] Kim | Enhanced luminescence characteristics of remote yellow silicate phosphors printed on nanoscale surface-roughened glass substrates for white light-emitting diodes[END_REF][START_REF] Sun | Enhancement of yellow light extraction efficiency of Y 3 Al 5 O 12 : Ce 3+ ceramic converters using a 2-D TiO 2 hexagonal-lattice nanocylinder photonic crystal layer[END_REF] It can occur either by angular randomization of photons by scattering them from a structured surface [START_REF] Kim | Enhanced luminescence characteristics of remote yellow silicate phosphors printed on nanoscale surface-roughened glass substrates for white light-emitting diodes[END_REF][START_REF] Kumar | Analytical treatment of light extraction from textured surfaces using classical ray optics[END_REF][START_REF] Schnitzer | 30% external quantum efficiency from surface textured, thin-film light-emitting diodes[END_REF][START_REF] Sun | Enhancement of yellow light extraction efficiency of Y 3 Al 5 O 12 : Ce 3+ ceramic converters using a 2-D TiO 2 hexagonal-lattice nanocylinder photonic crystal layer[END_REF] or by using an intermediate layer with an appropriate refractive index between the AL and the air. [START_REF] Fang | Nanoparticle stacks with graded refractive indices enhance the omnidirectional light harvesting of solar cells and the light extraction of light-emitting diodes[END_REF][START_REF] Khan | Non-destructive analysis on nano-textured surface of the vertical LED for light enhancement[END_REF] In both cases, TIR will be attenuated and LEE value will increase which has been demonstrated on LED chips. [START_REF] Geng | Enhancement of light output power from leds based on monolayer colloidal crystal[END_REF] Among the existing light extraction management strategies based on these principles, one can cite surface roughening (roughened or corrugated substrates) [START_REF] Kim | Enhanced luminescence characteristics of remote yellow silicate phosphors printed on nanoscale surface-roughened glass substrates for white light-emitting diodes[END_REF][START_REF] Sohn | Effect of corrugated substrates on light extraction efficiency and the mechanism of growth in pulsed laser deposited Y 2 O 3 :Eu 3+ thin-film phosphors[END_REF][START_REF] Youn | Corrugated sapphire substrates for organic light-emitting diode light extraction[END_REF], photonic crystals [START_REF] Devys | Extraction length determination in patterned luminescent sol-gel films[END_REF], nanorods or nanopillars [START_REF] Ding | Enhanced light extraction with silicon nanoantenna arrays for white light LED applications[END_REF][START_REF] Yang | Light extraction efficiency enhancement of colloidal quantum dot light-emitting diodes using large-scale nanopillar arrays[END_REF]) and many others. In order to reduce the waveguide effect and consequently to optimize the forward light extraction, we have chosen to directly structure the surface of a thin phosphor layer into a hexagonal network at the nanometric scale.

The surface nanostructuring of luminescent phosphor coatings has been widely investigated by several research groups in recent years. By replacing the luminescent coatings currently used in LED devices with https://doi.org/10.1016/j.ces.2019.115416 0009-2509/© 2019. nanostructured coatings, it was thus expected to improve the performances of the LEDs lighting devices while maintaining or even reducing their cost. The reported works concerned not only the field of LED lighting but also that of displays for which surface structuration leads to better resolution of screens for example. In this context, different types of phosphors have been studied: organic (europium chelates [START_REF] Devys | Extraction length determination in patterned luminescent sol-gel films[END_REF][START_REF] Revaux | Photonic crystal patterning of luminescent sol-gel films for light extraction[END_REF] or inorganic (mainly sesquioxides of Y 2 O 3 : RE 3+ -RE = Er, Eu [START_REF] Sohn | Effect of corrugated substrates on light extraction efficiency and the mechanism of growth in pulsed laser deposited Y 2 O 3 :Eu 3+ thin-film phosphors[END_REF] or Gd 2 O 3 :Eu 3+ types [START_REF] Do | Enhanced light extraction efficiency in pulse laser deposited Gd 2 O 3 : Eu 3+ thin-film phosphors on 2-D PCLs[END_REF], but also sulfides [START_REF] Ko | Enhanced light extraction from SrGa 2 S 4 : Eu 2+ film phosphors coated with various sizes of polystyrene nanosphere monolayers[END_REF] and more recently the YAG:Ce matrix [START_REF] Cho | Nanopatterned yttrium aluminum garnet phosphor incorporated film for high-brightness GaN-based white light emitting diodes[END_REF][START_REF] Ge | Enhanced forward emission of YAG:Ce 3+ phosphor with polystyrene nanosphere coating[END_REF][START_REF] Gorsky | Directional light emission enhancement from LED-phosphor converters using dielectric Vogel spiral arrays[END_REF][START_REF] Park | Selecting morphology of Y 3 Al 5 O 12 : Ce 3+ phosphors for minimizing scattering loss in the pc-LED package[END_REF][START_REF] Sun | Enhancement of yellow light extraction efficiency of Y 3 Al 5 O 12 : Ce 3+ ceramic converters using a 2-D TiO 2 hexagonal-lattice nanocylinder photonic crystal layer[END_REF]Wang et al., 2016a)). As an example, [START_REF] Gorsky | Directional light emission enhancement from LED-phosphor converters using dielectric Vogel spiral arrays[END_REF] recently measured a peak extraction enhancement by a factor of 2 from YAG:Ce converters patterned with TiO 2 nanodisks compared to a flat reference. Among existing literature, only Wang et al. 2016a reported a direct micropatterning of the YAG matrix, using a short-pulse laser (microwells array with spacing between adjacent microwells around 7 µm). Indeed, due to its high mechanical strength and excellent chemical stability, YAG matrix is very harsh to pattern directly regardless of the method. Consequently, main works intended for enhancing forward YAG:Ce 3+ emission involve the use of foreign materials such as SiO 2 , TiO 2 or SiN x . [START_REF] Cho | Nanopatterned yttrium aluminum garnet phosphor incorporated film for high-brightness GaN-based white light emitting diodes[END_REF][START_REF] Ge | Enhanced forward emission of YAG:Ce 3+ phosphor with polystyrene nanosphere coating[END_REF] The durability of these structures is not well known because they could be deteriorated or delaminated from the phosphor surface over time notably under operating conditions.

In this work, we explore a simple surface patterning strategy that we have carried out specifically on YAG matrix-based coatings for the purpose of enhancing their luminescence performances. This strategy represents an innovation insofar it allows directly structuring any kind of oxides or fluorides available through the sol-gel process. In this way, it involves both colloidal lithography and Langmuir-Blodgett technique (LB). This approach relies on the self-assembly of a monolayer of beads of a given diameter on a substrate. These beads are generally silica nanospheres, polystyrene or PMMA [START_REF] Vogel | Advances in colloidal assembly: the design of structure and hierarchy in two and three dimensions[END_REF] and the substrate may be bare or already coated with the selected material of interest. The monolayer of beads then serves as a physical mask. [START_REF] Ai | Advanced colloidal lithography beyond surface patterning[END_REF][START_REF] Vogel | From soft to hard: the generation of functional and complex colloidal monolayers for nanolithography[END_REF] The material to be structured is deposited on this mask by physical techniques [Physical vapor deposition (PVD)] or chemical way (impregnation) so as to partially or completely fill the interstices that are formed between the beads. The monolayer of beads is then removed (by heating, ultrasound or washing) leaving a periodic structure of the selected material. This technique has already been used successfully to improve the efficiency of blue LEDs by structuring the silicon wafer or the GaN chip. [START_REF] Geng | Enhancement of light output power from leds based on monolayer colloidal crystal[END_REF] Obtaining a good quality network obviously involves checking the homogeneity of the (mono)layer(s) of beads. Different deposition techniques have been used and reported in the literature. [START_REF] Liang | Self-assembly of colloidal spheres toward fabrication of hierarchical and periodic nanostructures for technological applications[END_REF][START_REF] Vogel | Advances in colloidal assembly: the design of structure and hierarchy in two and three dimensions[END_REF] Among them, "floating-transfer" and spin-coating techniques seem to be the most convenient to implement. [START_REF] Colson | Experimental design applied to spin coating of 2D colloidal crystal masks: a relevant method?[END_REF][START_REF] Lee | Enhanced extraction efficiency of Y 2 O 3 : Eu 3+ thin-film phosphors coated with hexagonally close-packed polystyrene nanosphere monolayers[END_REF][START_REF] Vogel | From soft to hard: the generation of functional and complex colloidal monolayers for nanolithography[END_REF] However, in our work, the technique of Langmuir-Blodgett was favored to achieve monolayers of polystyrene nanospheres (NPS). The LB process, which was first used as an efficient technique for the assembly of molecular monolayers, relies on the controlled compression of monolayers consisting of amphiphilic compounds located at the water / air interface. [START_REF] Bardosova | The langmuir-blodgett approach to making colloidal photonic crystals from silica spheres[END_REF][START_REF] Ulman | An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly[END_REF] A substrate, generally treated to be hydrophilic and initially immersed in water, is gently removed from this water subphase while the amphiphilic molecules adhere to its surface under the effect of pressure exerted by the barriers of the LB system. Today, this technique has the great advantage of controlling the number of deposited layers by varying the number of "soak-withdrawal" cycles of the substrate [START_REF] Tian | Highly enhanced luminescence performance of LEDs via controllable layer-structured 3D photonic crystals and photonic crystal beads[END_REF]. It is also very robust (very good reproducibility) [START_REF] Reculusa | Synthesis of colloidal crystals of controllable thickness through the Langmuir-Blodgett technique[END_REF] and can be used to cover large surfaces with compact assemblies of par ticles. [START_REF] Colson | Nanosphere lithography: a powerful method for the controlled manufacturing of nanomaterials[END_REF][START_REF] Vogel | Advances in colloidal assembly: the design of structure and hierarchy in two and three dimensions[END_REF] Despite these benefits, there is little research on the use of the LB technique to perform "NanoSphere Lithography" (NSL) [START_REF] Bardosova | The langmuir-blodgett approach to making colloidal photonic crystals from silica spheres[END_REF] and most of them are dated after 2010. [START_REF] Szabó | Regular ZnO nanopillar arrays by nanosphere photolithography[END_REF]Wang et al., 2016b) In our work, a mask based on a monolayer of polystyrene nanoparticles was used and deposited by the technique of LB on sol-gel derived coatings, after having optimized all the parameters of LB deposition.

Sol-gel chemistry has proved to be a very efficient method for the production of thin layers of oxides on different types of substrates. [START_REF] Brinker | Sol-Gel Science: The Physics and Chemistry of Sol-Gel[END_REF] In the field of optics, this method offers the possibility of controlling the density of the deposited films and consequently their refractive index. [START_REF] Akaltun | The relationship between refractive index-energy gap and the film thickness effect on the characteristic parameters of CdSe thin films[END_REF][START_REF] Mergel | Density and refractive index of TiO 2 films prepared by reactive evaporation[END_REF] It also promotes in particular a better dispersion of the optically active centers (rare-earth ions for example) in inorganic or composite dielectric matrices. [START_REF] Brinker | Sol-Gel Science: The Physics and Chemistry of Sol-Gel[END_REF][START_REF] Shionoya | Phosphor Handbook[END_REF] In our study, we used a sol-gel process developed in-house (Potdevin et al., 2006a;[START_REF] Potdevin | Waveguiding terbium-doped yttrium aluminum garnet coatings based on the sol-gel process[END_REF] to elaborate luminescent amorphous and crystallized Y 3 Al 5 O 12 :Tb 3+ (YAG:Tb) coatings. This phosphor and derived Tb 3+ doped garnets have been shown to possess interesting features for solid-state lighting devices. (Potdevin et al., 2010b) Furthermore, this study is used as a proof of concept to investigate the influence of nanostructuring of sol-gel derived thin films on their light extraction efficiency and on their angle-dependent light emission. The morphological and structural properties of the films were studied using standard methods (AFM, SEM, XRD) at each step of the nanostructuring process, when necessary. Then, the photoluminescence measurements were carried out in order to determine the influence of the nanostructuring on the light extraction efficiency.

Material and methods

Preparation processes

The nanopatterning process is schematized in Fig. 1.

Preparation of the YAG:Tb sol (step 1)

YAG:Tb (20%) precursor sol was prepared by using the sol-gel method, following a process already described in the literature. (Potdevin et al., 2006a, b) In this preparation, anhydrous yttrium and terbium chlorides (YCl 3 and TbCl 3 , 99.9% purity, Sigma-Aldrich) and aluminum isopropoxide (Al(OiPr) 3 , ≥98% purity, Sigma-Aldrich) were used as precursors for sol-gel reaction. First, YCl 3 and TbCl 3 were dissolved in isopropanol under argon atmosphere. Atomic ratio [Tb 3+ /(Y 3+ + Tb 3+ )] in the YAG:Tb sol was set to 20 mol% since this doping rate has been determined to lead to the best optical properties upon blue or UV radiation. (Potdevin et al., 2006b) After complete dissolution of YCl 3 and TbCl 3 , potassium chunks (K, 98% purity in mineral oil, Sigma-Aldrich) were added to the reaction mixture at 85 °C so that to initiate sol-gel reaction. A KCl white precipitate was directly observed. Al(OiPr) 3 was added directly after 1 h reflux. After two more hours of reflux, the sol was stabilized by acetylacetone (acacH, 99% purity, Merck) used as a chelating agent. Indeed, it forms stable complexes when mixed with Tb 3+ , Al 3+ and Y 3+ ions. [START_REF] Potdevin | A thorough spectroscopic study of luminescent precursor solution of Y 3 Al 5 O 12 :Tb 3+ : influence of acetylacetone[END_REF] The final molar composition of the reaction mixture was 1 Tb : 4 Y : 5 Al : 9 K : 5 acacH. The KCl precipitate was separated from the sol by centrifugation.

Preparation of the YAG:Tb films (step 2)

Spray-coating, spin-coating and dip-coating are among various methods that exist for depositing phosphors thin films. [START_REF] Brinker | Sol-Gel Science: The Physics and Chemistry of Sol-Gel[END_REF][START_REF] Potdevin | Waveguiding terbium-doped yttrium aluminum garnet coatings based on the sol-gel process[END_REF][START_REF] Shionoya | Phosphor Handbook[END_REF] Under the experimental conditions developed during our work, the two first methods of deposition did not provide a smooth and even film surface. Spin-coating method led to irregular dense films with significant edge effects whereas spray-coating method generally leads to films characterized by a very rough surface. (Potdevin et al., 2006a;[START_REF] Potdevin | Structural, morphological and scintillation properties of Ce 3+ -doped Y 3 Al 5 O 12 powders and films elaborated by the sol-gel process[END_REF] The dip-coating method has been chosen for its simplicity and efficiency. This method offered optical quality coatings with easily controllable thicknesses and good surface homogeneity. The YAG:Tb amorphous films obtained from this method have all a smooth and homogeneous microcracks-free surface. [START_REF] Potdevin | Waveguiding terbium-doped yttrium aluminum garnet coatings based on the sol-gel process[END_REF] In our process, previously prepared YAG:Tb sol (actually, YAG:Tb precursor) was deposited by dip-coating on quartz slides in order to form a first amorphous coating (sublayer) of YAG:Tb, further annealed at 400 °C. It has been demonstrated that the intensity of photoluminescence (PL) is dependent in particular on the thickness of the coating. [START_REF] Cho | Optical properties of sol-gel derived Y 2 O 3 :Eu 3+ thin-film phosphors for display applications[END_REF] Based on our previous work, [START_REF] Potdevin | Waveguiding terbium-doped yttrium aluminum garnet coatings based on the sol-gel process[END_REF] we chose to prepare films enough thick to have significant PL efficiency; hence we worked with a coating of 1 μm which represents a good compromise between the PL efficiency and a reasonable number of layers (see Fig. S1 which evidences the film thickness). Indeed, in the used deposition conditions, such a thickness required 20 cycles of dip-coating with a concentrated sol of YAG:Tb(20 mol%). This concentration results in a weakly viscous sol but allows avoiding delamination phenomenon which generally occurs if the thickness of each coating is too important. The thermal annealing process we used has been optimized to lead to homogeneous thin films free of cracks (necessary condition to have a subsequent PS beads deposition of best quality). Actually, this step has to be mastered to avoid microcracks which are the consequence of the difference in thermal expansion coefficients between the quartz substrate and the YAG: Tb film. [START_REF] Cho | Optical properties of sol-gel derived Y 2 O 3 :Eu 3+ thin-film phosphors for display applications[END_REF] It is on this first amorphous YAG:Tb coating (sublayer) that the colloidal lithography involving the polystyrene (PS) NPs of 460 nm (NPS-460, 10%, Sigma-Aldrich) was carried out. This method consisted in depositing by the LB technique (with a Kibron MicroTroughXS® system) a compact monolayer of NPS on the amorphous layer of YAG:Tb. Consecutively, a very small quantity of YAG:Tb sol (several hundred of µL) was again deposited on this monolayer so as to penetrate into the interstitial voids left between the NPS and thus to form a continuity with the YAG:Tb sublayer. The calcination of NPS at 400 °C conse quently led to the removal of NPS and to a nanostructuring resulting from the negative imprint left by the NPS monolayer. The subsequent crystallization of YAG:Tb structure was induced by annealing at 1100 °C for 1 h. In this study, we compared the structural and optical properties of YAG :Tb films in their different forms : amorphous and crystallized, unstructured and nanostructured.

U N C O R R E C T E D P R O O F

Characterization techniques

The X-ray diffraction (XRD) patterns of the coatings were obtained with a Philips Xpert Pro diffractometer operating with the Cu-Kα 1 radiation (λ = 1.5406 Å). Recording of the XRD patterns was carried out at 10°<2θ <70°with a scanning speed (2θ) of 1°/min. The scanning electron microscope (SEM) images of the coatings were collected on a ZEISS Supra 55 FEG-VP instrument at 2MAtech (Clermont-Ferrand). The observations were carried-out under high vacuum at 3 kV and using a secondary electron detector (Everhart-Thornley detector). Prior to observation, the samples (coated-quartz slides) were attached on one face to an adhesive carbon and then metallized using an Au coating. The surface topography of YAG:Tb coated quartz slides was measured by atomic force microscope (Innova AFM from Bruker).

Excitation spectra in the VUV range were performed at the Deutsches Elektronen Synchotron (DESY, Hamburg) using the facility of SUPER-LUMI station [START_REF] Zimmerer | SUPERLUMI: A unique setup for luminescence spectroscopy with synchrotron radiation[END_REF] at HASYLAB. The measurements were made at room temperature (RT) and the luminescence was detected by a HAMAMATSU R6358P PMT photomultiplier (200-800 nm range). Excitation spectra were scanned with the primary 2 m monochromator in 15°McPherson mounting (resolution of 3.2 Å) and they have been corrected from the spectral response of the incident photon flux.

In order to characterize qualitatively and quantitatively the influence of nanostructuring on guided light extraction, angle-resolved photoluminescence measurements were carried out by exciting the sample (coated-quartz slides) under normal incidence with the fourth harmonic of a continuous wave YAG:Nd laser (266 nm). The angular-resolved emission spectra of the sample were measured by using microphotolu minescence (µPL) far-field imaging where the Fourier plane is imaged on the monochromator entrance slit. The μPL signal was detected through a microscope objective with a numerical aperture of 0.5, giving access to a large range of emission angles (-30°to + 30°). The angular resolution was 0.2°. The normal direction lies at 0°. The resulting spectra were gathered on a 2-dimensional map representing emission intensity (color scale) versus angle and wavelength. The signal was focused on the slit of a 1 m focal monochromator and detected by a 1024 × 256 CCD camera. Room temperature measurements were made on both patterned and unpatterned samples between 400 and 800 nm. The integration of the photoluminescence spectra was used to determine and compare the light extraction efficiency.

Results and discussion

Crystalline structure of YAG:Tb

Structural properties of both unpatterned and patterned coatings have been studied by means of X-Ray Diffraction (XRD). It has to be noticed that the same sol has been used to elaborate both sublayer and overlayer in patterned samples as well as the layer of the unpatterned one. Fig. 2 shows the XRD patterns of the nanostructured and unstruc tured YAG:Tb (20%) films annealed at 1100 °C for 1 h. It can be observed for both samples a large signal around 20°(2θ), related to the quartz substrate. Diffraction lines of the samples mainly correspond to the diffraction reflections of the Y 3 Al 5 O 12 (YAG -JCPDS-file 88-2047). However, we can notice the presence of two reflections of weak intensity, which could be ascribed to Y 4 Al 2 O 9 (YAM) on both luminescent films (marked by an asterisk). The presence of this possible parasitic phase can be related to the sol and is generally avoided as we have already shown in previous papers. [START_REF] Potdevin | Waveguiding terbium-doped yttrium aluminum garnet coatings based on the sol-gel process[END_REF] Since both unstructured and nanostructured samples contain this parasitic phase in similar contents, the results concerning the optical properties could be compared and the influence of the nanostructuring on these properties can be nevertheless assessed.

The coatings annealed only at 400 °C remain amorphous but they will be designed elsewhere in the text as either "amorphous YAG:Tb" or "YAG:Tb films/coatings annealed at 400 °C" (by abuse of language and for clarity).

Morphological properties

The quality of the nanoparticles (NPs) monolayer is decisive for the achievement of a regular nanostructuring. This was obtained by the LB technique, which is very sensitive to the experimental conditions. The formation of a monolayer of close packed particles was optimized thanks to a thorough study of the influence of several factors such as temperature, compression rate, surface pressure and the amount of nanospheres (see Fig.s S2 and S3 in the SI showing the influence of the concentration of NPs suspensions and that of the withdrawal speed of the substrate respectively). In addition, as aforementioned, the surface of the continuous sublayer of YAG:Tb must be smooth and homogeneous in order to obtain a 2D compact and regular packing of nanospheres. Fig. 3 gathers the SEM images of the layers obtained at each step of the patterning.

The YAG:Tb film annealed at 400 °C consists of amorphous film with a smooth and homogeneous surface (Fig. 3a) whereas YAG:Tb film crystallized at 1100 °C exhibits cracked, rough and porous surface (Fig. 3b). For these reasons, the deposition of NPs was carried out on the amorphous coating precursor of YAG:Tb.

Polystyrene (PS) NPs which surface has been modified with functional groups of sulphate type are partially of hydrophobic nature. During LB deposition, the PS NPs are initially deposited on the surface of

U N C O R R E C T E D P R O O F

the water subphase over which they float. The capillary attraction forces guided by the concave meniscus, which in turn is induced by gravity between the neighboring colloidal particles, favor compact packing. (Vogel et al., 2012) However, the PS NPs, which float on the surface of the water, are so light that they do not sufficiently deform the planar surface of the water so as to cause the phenomenon of compact packing through capillary attraction forces. The compression of the NPs favors the packing of the particles in the hexagonal close packing mode (self-assembly) but maintains a kind of mobility because of the moderate polarity of their surface generated by the functional sulphate groups. The resulting NPs monolayer obtained after evaporation of the water on the substrate and through capillary forces arises therefore as a regular hexagonal packing with some voids between them (Fig. 3c).

The next step of nanostructuring involved the deposition of YAG:Tb precursor sol (overcoating) that uniformly covers the PS NPs as shown in Fig. 3d, while filling the interstices between PS NPs. Removal of PS NPs obtained by calcination at 400 °C results in a relatively homogeneous structure of the luminescent amorphous YAG:Tb coating. The obtained structure is similar to a honeycomb with irregular alveoli (Fig. 3e). This honeycomb structure is consistent with the fact that the PS NPs monolayer was not fully compact. Otherwise, triangular structures not connected to one another would have been obtained. [START_REF] Zhang | Colloidal lithography-the art of nanochemical patterning[END_REF] Sintering at 1100 °C of the honeycomb structure led to a crystallized YAG:Tb film exhibiting a hexagonal 2D pattern (Fig. 3f). Due to crystallization, the alveoli observed in the amorphous film give way to a more homogeneous hexagonal structure that recalls the hexagonal shape of the PS NPs array. Furthermore, some cracks associated with the densification of the YAG phase appear on the surface of the crystallized film. The same cracks are found on the unpatterned coating sintered at 1100 °C (Fig. 3b). The sintering is also responsible for the fact that the alveoli (hexagonal cell) are less pronounced (thinner and shallower edges between alveoli) than those of the amorphous coating. This step has required the optimization of several parameters, in particular the heating rate. For example, a too high heating rate (4 °C/min) caused a too fast subsequent removal of the organic residues, leading to the partial destruction of the structured network. It should be noted that a sufficiently thick sublayer (of the order of 400 nm minimum before sintering at 1100 °C) is necessary to guarantee the appearance of a well-organized pattern. Here, as explained in the experimental section, we worked with a sublayer characterized by a thickness around 1 µm. Indeed, during our first attempts of patterning (thickness less than 100 nm), the coating retracted preventing structuration to occur as shown in Fig. 4a. On the other hand, the amount of YAG:Tb sol used for the overcoating should not be too high, otherwise the PS NPs will be completely covered. In this case, a nanostructuring is obtained but largely covered by an unpatterned hollow overlayer, as illustrated in Fig. 4b (hollow spheres let by the removal of PS spheres may be guessed by darker round areas visible on the overlayer at low magnification).

When deposition parameters are well-defined, the hexagonal network obtained in the crystallized coating is preserved over a long range with great fidelity. From SEM images, the periodicity "a" was measured at 490 nm corresponding to the negative imprint left by the PS nanospheres of 460 nm.

Atomic force Microscopy (AFM) was also used to study nanostructured coatings. Fig. 5 gathers some results obtained from the sample coated with PS NPs whereas Fig. 6 presents AFM images and depth profile of the amorphous nanopatterned coating. Other depth profiles are available in the SI. The AFM images of the PS NPs layer (Fig. 5(a) and (b)) confirm that the PS spheres are homogeneously hexagonally closely packed and the depth profile (Fig. 5 (c)) shows a period of approximately 400 nm between two spheres which is relatively concordant with the SEM images (Fig. 3(c)) and the expected diameter of these commercial PS NPs which is of 460 nm. The comparison of different depth profiles (see Fig. S4(a) in the SI) settles the homogeneity of the NPs deposition, which ensured an optimal first step for the following nanostructuring.

On the AFM images of the nanopatterned amorphous coating (Fig. 6(a) and (b)), one can observe the presence of hexagonally-organized microwells, as evidenced by SEM images (Fig. 3(e)). Sidewalls seem to be relatively smooth. The depth profile (Fig. 6(c)) shows a period between two edges around 500 nm, which is concordant with the SEM study. The comparison of different depth profiles (see Fig. S4(b)) well establishes the high homogeneity of the nanostructuring.

Furthermore, the mean roughness of the nanostructured amorphous coating was evaluated around 50 nm (RMS) thanks to Gwyddion software whereas the unpatterned one was characterized by a RMS around 0.5 nm. [START_REF] Potdevin | Waveguiding terbium-doped yttrium aluminum garnet coatings based on the sol-gel process[END_REF] This should have consequences on the optical properties of these samples since an increased RMS generally entails a decrease in the waveguiding properties of the coating. [START_REF] Kim | Enhanced luminescence characteristics of remote yellow silicate phosphors printed on nanoscale surface-roughened glass substrates for white light-emitting diodes[END_REF] The RMS slightly decreases for the crystallized nanostructured sample (RMS around 40 nm) which is probably due to the phenomenon of densification accompanying the crystallization. Sidewalls and microwells are then less sharp and deep respectively as schematized in the Fig. S5. In this case, the differences between the rugosity of the unpatterned crystallized coating and that of the patterned one are less pronounced than between the two amorphous samples.

Luminescence properties and characterization of the light extraction by the surface patterning

Fig. 7 shows the room-temperature excitation spectra of unpatterned amorphous (Fig. 7(a)) and crystallized (Fig. 7b) YAG:Tb dip-coated layers in the VUV and in a part of the UV wavelength ranges. It is assumed that patterned samples are characterized by similar excitation profiles.

The crystallized layer (Fig. 7b) presents an excitation spectrum close to those obtained for YAG:Tb powders [START_REF] Potdevin | Optical properties upon vacuum ultraviolet excitation of sol-gel based Y3Al5O12:Tb3+, Ce3+ powders[END_REF] with several broad bands. The main one is located around 172 nm and corresponds to the band absorption of the host lattice [START_REF] Mayolet | Investigation in the VUV range of the excitation efficiency of the Tb 3+ ion luminescence in Y 3 (Al x , Ga y ) 5 O 12 host lattices[END_REF][START_REF] Park | VUV excitation of Y3Al5O12: Tb phosphor prepared by a sol-gel process[END_REF] since the band gap energy of YAG is approximately 7.0 eV. This band originates from a partial energy transfer between the host and the luminescent centers. Besides, three other absorption bands located at 203, 227 nm and 271 nm are observed. Tb 3+ ions exhibit a 4f 8 configuration for which the ground state is 7 F 6 . The 4f 7 5d 1 first excited configuration presents two spin states: the high spin 9 D J (or 9 E) and the low-spin 7 D J (or 7 E) (J = 0…5). The bands at 227 and 271 nm can be attributed to the spin-allowed 7 F 6 → 7 D 4 and 7 F 6 → 7 D 5 transitions respectively. The weak signal located at 203 nm is also related to 4f → 5d Tb 3+ transitions and more particularly to the spin-forbidden 7 F 6 → 9 D 3 transition [START_REF] Dorenbos | Exchange and crystal field effects on the 4f(n-1)5d levels of Tb 3+[END_REF], which explains why this signal is so weak. One can also observe the onset of a signal around 320 nm, already evidenced in sol-gel derived YAG:Tb powders (Potdevin et al., 2010b) and which can be ascribed to another spin-forbidden transition, 7 F 6 → 9 D 2 [START_REF] Dorenbos | Exchange and crystal field effects on the 4f(n-1)5d levels of Tb 3+[END_REF]. This excitation profile, characteristic of YAG:Tb 3+ phase, confirms the negligible effect of the presence of a weak amount of YAM phase evidenced by XRD (Fig. 2).

Concerning the amorphous coating (Fig. 7a), we can notice that the main absorption band corresponding to the host excitation has vanished, which is consistent with the amorphous character of the sample. Two very broad excitation bands can be observed around 191 and 233 nm. Based on previous studies led on amorphous powders, (Potdevin et al., 2010a) we can assume they are related to inter-shell 4f → 5d Tb 3+ transitions. The spectral distribution of these transitions is strongly influenced by the strength and symmetry of crystal-field undergone by the active ions. [START_REF] Blasse | Luminescent Materials[END_REF] In the aspect of crystal-field theory, amorphous phases exhibit lower symmetry and weaker crystal-field strength induced by the neighboring ions if compared with crystallized ones. These differences in the Tb 3+ local environment logically entail a blue-shift of excitation bands of the amorphous coating (Fig. 7a) compared to those obtained for the crystallized sample (Fig. 7b).

Even if it is not the best excitation wavelength for the amorphous samples, we recorded emission spectra under a 266 nm excitation (imposed by the experimental set-up). Fig. 8 gathers emission spectra of the nanostructured and unstructured films for both forms of YAG:Tb, amorphous and crystallized. These emission spectra exhibit profiles characteristic of both amorphous and crystallized YAG:Tb powders with a main emission band peaking in the green region. (Potdevin et al., 2010a). It is important to emphasize that crystallization significantly enhanced emission intensity, as already reported in [START_REF] Potdevin | Waveguiding terbium-doped yttrium aluminum garnet coatings based on the sol-gel process[END_REF] and as shown in Fig. S6 gathering emission spectra of amorphous and crystallized unpatterned samples. In Fig. 8, scales have been chosen to be as adapted as possible to the data and are not comparable between Fig. 8a andb.

Photographs corresponding to the samples are also presented in Fig. 8. These pictures have been taken using a camera that automatically adjusted recording conditions to have the best sharpness. As a result, photographs are not comparable. However, they evidence the bright green luminescence of all samples and the diffusive effect brought by the patterning.

The spectra of the amorphous coatings show broad emission bands linked to the Tb 3+ ions multisite character (Fig. 8a) while the Stark components are well distinguished on the spectra of the crystallized coatings (Fig. 8b).

The observed bands can be ascribed to the electronic transitions from the 5 D 4 first excited level of the Tb 3+ ions to the 7 F J manifolds (J = 6…0) of the ground state. The most intense band (λ em = 544 nm) corresponds to the transition between the 5 D 4 and 7 F 5 levels of the Tb 3+ ions. The insets of the Fig. 8 represent this main transition, highlighting the influence of the nanopatterning on the emission intensity, deduced from the integrated area of this band in all cases. The emission intensity corresponding to the 5 D 4 → 7 F 5 transition is more important for the patterned films. This enhancement reaches 26% in the case of crystallized films and 131% in the case of amorphous films, while the amount of phosphor remains similar when comparing unpatterned and patterned samples. This gain can also be defined by the so-called "enhanced extraction factor" (EEF) which corresponds to the ratio between the intensities of nanostructured films and unstructured ones, phosphors amounts being equal in both samples. The EEF is 1.1 in the case of crystallized films and 2.3 in the case of amorphous samples.

This means that nanostructuring could be used to increase the extraction of the light produced by luminescence previously entrapped in the substrate. Furthermore, the deeper the nanostructuring is (the higher the RMS is), the more efficient the light extraction will be. [START_REF] Devys | Extraction length determination in patterned luminescent sol-gel films[END_REF] Indeed, the nanopatterning of the amorphous coatings is more marked than that of the crystallized coatings (see Fig. 3e andf and RMS measurements by AFM), as illustrated by the schemes gathered in Fig. S5. This phenomenon has already been observed by [START_REF] Kim | Enhanced luminescence characteristics of remote yellow silicate phosphors printed on nanoscale surface-roughened glass substrates for white light-emitting diodes[END_REF] on roughened surfaces used for the deposition of a silicate phosphor for LEDs applications. It can be ascribed to the growth in opportunities for photons to find broader escape angles with increasing surface roughness. [START_REF] Khan | Non-destructive analysis on nano-textured surface of the vertical LED for light enhancement[END_REF][START_REF] Kim | Enhanced luminescence characteristics of remote yellow silicate phosphors printed on nanoscale surface-roughened glass substrates for white light-emitting diodes[END_REF] Unstructured amorphous samples are those in which waveguide phenomena are the most present, RMS the weakest and therefore those for which the extraction efficiency is worst. They are transparent under daylight and very smooth, which promoted the waveguiding phenomena. When they are patterned, they become more diffusing, and their RMS significantly increase, which surely entails a decrease in the number of guided modes within the layer. [START_REF] Gorsky | Directional light emission enhancement from LED-phosphor converters using dielectric Vogel spiral arrays[END_REF][START_REF] Kim | Enhanced luminescence characteristics of remote yellow silicate phosphors printed on nanoscale surface-roughened glass substrates for white light-emitting diodes[END_REF] As mentioned before, the light diffusing effect on the pat terned coatings is clearly demonstrated in the photos of the coated substrates in Fig. 8.

Qualitative and quantitative angle-resolved luminescence measurements

In order to study the effect of nanostructuring on the directionality of the PL, angular emission measurements were carried out on both unstructured and nanostructured crystallized YAG:Tb films. The luminescence diagrams I (color scale) given in Fig. 9a-1 and b-1 represent the emission spectra for the main emission band located between 540 and 560 nm as a function of the detection angle θ. The diagrams were recorded from θ = -30°to θ = +30°and between 540 nm and 560 nm (0°corresponding to a light perpendicular to the layer). The highest the color temperature is, the strongest the emission intensity is. As aforementioned, emission profiles are characteristic of Tb 3+ ions in crystalline matrices, with an intense emission band peaking at 544 nm corresponding to the 5 D 4 → 7 F 5 transition. The luminescence diagrams are different depending on whether the film is patterned or not.

For the nanostructured surface, the false color intensity diagram shows that the intensity of the spectrum is maximum at θ = 0°. This result demonstrates that the emission intensity of Tb 3+ depends on the detection angle θ when the surface is nanostructured in a hexagonal lattice. The light extraction is therefore directional in this case.

The EEF was calculated by integrating the emission bands in the region of 535-550 nm over θ =+30°, θ = 0°and θ = -30°, and by making the ratio of the calculated areas. Fig. 9a-2 and b-2 represent the superposition of these emission bands for both the nanostructured and unstructured films. In the case of the unstructured film, no difference was observed between the emission bands on the three detection angles. The emission intensity was found isotropic indicating that no direction was favored. On the other hand, in the case of the structured film, the emission distribution was preferably centered on θ = 0°.

U N C O R R E C T E D P R O O F

Moreover, we found an EEF of 1.5 between the emission at θ = 0°and θ = ± 30°. Due to nanostructuring, 1.5 times more light intensity was extracted at θ = 0°(forward emission). By analogy with the study of the influence of the depth of photonic crystals patterns on the light extraction reported by [START_REF] Devys | Extraction length determination in patterned luminescent sol-gel films[END_REF], this phenomenon can be associated with a coupling between the waveguide and the hexagonal compact (shape factor) network in a given direction, in this case at θ = 0°.

In conclusion, with similar amounts of phosphor, the structuration allows a significant increase in the emission intensity and can act on the angular dependence of this intensity, by increasing the extraction of light perpendicularly to the surface. It should be pointed out that, through structuration, it will therefore be possible to control the light output angle in order to have a stable light quality over angular ranges from (-30°/ +30°) to (-60°/ +60°) depending on the targeted application, instead of the 4π steradians obtained without structuration.

Finally, these results are a proof of concept that can be applied to all matrices synthesizable by sol-gel, in particular the YAG:Ce used in the more conventional solid state lighting devices. As indicated in the introduction, the YAG:Tb matrix was used in this study because it is characterized by a bright green luminescence readily detectable with the naked eyes upon standard UV lamps and during angular resolved PL. Based on several reports and on first simulations led on YAG:Ce, light extraction enhancement can be further improved by optimizing the geometric parameters of the patterned network as well as by working with textured substrate (resulting in texture on both sides of the AL). [START_REF] Gorsky | Directional light emission enhancement from LED-phosphor converters using dielectric Vogel spiral arrays[END_REF][START_REF] Kim | Enhanced luminescence characteristics of remote yellow silicate phosphors printed on nanoscale surface-roughened glass substrates for white light-emitting diodes[END_REF][START_REF] Mao | Surface patterning of nonscattering phosphors for light extraction[END_REF]

Conclusion

Patterned films of YAG:Tb (20%) were prepared for the first time on quartz slides using an innovative and versatile approach combining sol-gel chemistry with colloidal lithography carried out by the LB technique. These films were compared with their unpatterned equivalents in their amorphous and crystallized states for the purpose of studying forward light extraction. It was demonstrated in this study that nanopatterning YAG:Tb films in a hexagonal close-packing network with a 460 nm grating has an influence on the light path by reducing the losses mainly linked to internal reflections. Photoluminescence spectroscopy was used to determine the gain in forward photoluminescence, due to nanostructuring, which was found to be 26% in the case of crystallized films and 131% in amorphous films. This study was first devoted to YAG:Tb matrix with the purpose of establishing a proof of concept before extending the structuring process to other matrices such as the YAG:Ce, used in current LED lighting systems. Because this structuration process can be applied to many different sol-gel-derived phosphors, it represents a very promising means to create patterned luminescent coatings, for which patterning parameters could be tuned (by changing the size of PS beads for example) to match the targeted application. Indeed, as shown by these first results, by engineering the shape of the structure (especially the height and the sharpness of the walls of the microwells) we can access to a controlled roughness that should entail a significant enhancement of luminescence of these coatings, which is very interesting in the aim of combination with LEDs. Further works should be dedicated to the patterning of YAG:Ce matrix and the study of the influence of the patterning network parameters on the optical properties of structured coatings, based on both simulations and experiences. Preliminary experiments involving the YAG:Ce matrix are currently under investigation and first simulations concerning these patterned coatings have foreseen significant changes on emission spectral distribution.

Funding sources

This work was supported by the Région Auvergne-Rhône-Alpes (Optilum project); European Community for its financial support during the access to VUV spectroscopy facilities DESY (Contract RII3- CT-2004-506008).

Fig. 1 .

 1 Fig. 1. Sol-gel and deposition processes of YAG:Tb (20%) nanostructured thin films.

Fig. 2 .

 2 Fig. 2. XRD patterns of quartz substrate (a), unpatterned (b) and patterned (c) YAG:Tb (20%) thin films.

Fig. 3 .

 3 Fig. 3. SEM images of : (a) unpatterned amorphous (annealed at 400 °C) YAG:Tb film; (b) unpatterned crystallized (annealed at 1100 °C) YAG:Tb film; (c) 2D hexagonal close packing NPs LB film; (d) 2D hexagonal close packing NPs LB film with an overcoating YAG:Tb; (e) 2D patterned surface of YAG:Tb amorphous film and (f) 2D patterned surface of YAG:Tb crystallized film.

Fig. 4 .

 4 Fig. 4. SEM images illustrating the need to control both (a) the quality of the sub-layer and (b) the quantity of the sol used over the PS beads.

Fig. 5 .

 5 Fig. 5. (a) 3D AFM image, (b) 2D AFM image and (c) depth profile of 2D hexagonal close packing PS NPs obtained by the LB method on the sub-layer of amorphous YAG:Tb precursor.

Fig. 6 .

 6 Fig. 6. (a) 3D AFM image, (b) 2D AFM image and (c) depth profile of the amorphous nanopatterned coating after removal of PS NPs.

Fig. 7 .

 7 Fig. 7. Excitation spectra of (a) amorphous and (b) crystallized (heated at 1100 °C) unpatterned coatings.

Fig. 8 .

 8 Fig. 8. Room-temperature emission spectra (λ exc = 266 nm) of Tb 3+ for patterned (black) and unpatterned (grey) films annealed at a) 400 °C and b) 1100 °C. Insets are the superposition of the patterned (black) and unpatterned (grey) films spectra (λ em : 535-565 nm) in order to highlight the difference in intensity and integrated area.

Fig. 9 .

 9 Fig. 9. Luminescence maps corresponding to the emission spectra detected at different angle positions for : (a-1) unpatterned and (a-2) patterned films of crystallized YAG:Tb. The colour code represents the emission intensity. (a-2) and (b-2) represent the emission spectra recorded at angles -30°, 0°and + 30°for the unpatterned and patterned films of crystallized YAG:Tb, respectively.

Acknowledgements

The authors thank the Région Auvergne-Rhône-Alpes for its financial support (Optilum project), Christelle Blavignac, Claire Szczepaniak, Lorraine Novais Gameiro (Centre Imagerie Cellulaire Santé -Université Clermont Auvergne, France) and Anne-Marie Gélinaud (2Matech, Aubière, France) for their technical support and expertise for SEM observations as well as David Bourgogne (ICCF, Clermont-Ferrand) for his advices during AFM study. The authors also acknowledge DESY and the European Community respectively for the access of VUV spectroscopy facilities and the financial support (Contract RII3- CT-2004-506008).

CRediT authorship contribution statement

Jeff Nyalosaso: Investigation, Writing -original draft. Audrey Potdevin: Supervision, Writing -original draft, Writing -review & editing, Visualization, Funding acquisition. François Réveret: Investigation. Pierre Disseix: Supervision. Rachid Mahiou: Investigation. Geneviève Chadeyron: Conceptualization, Funding acquisition.

Declaration of Competing Interest

The authors declared that there is no conflict of interest.

Appendix A. Supplementary material

Supplementary data to this article can be found online at https://doi. org/10.1016/j.ces.2019.115416.