Hongjun Liu

Ola G Grendal

Susanne Linn Skjaervø

Antoine R M Dalod

Wouter Van Beek

Abderrahime Sekkat

Mari-Ann Einarsrud

David Muñoz-Rojas

Reaction Pathway of the Hydrothermal Synthesis of AgCuO 2 from In Situ Time-Resolved X-ray Diffraction

Keywords: in situ X-ray diffraction, time-resolved, AgCuO 2, hydrothermal, growth mechanism

Introduction

Silver copper mixed oxides are an exciting new family of materials. [START_REF] Muñoz-Rojas | Silver-Copper Mixed Oxides[END_REF] The first member of the family, Ag 2 Cu 2 O 3 was synthesized by Gomez-Romero's group in 1999. [START_REF] Gomez-Romero | Ag 2 Cu 2 O 3 : The First Silver Copper Oxide[END_REF] Ag 2 Cu 2 O 3 , is isostructural with paramelaconite and involves a 3D interpenetrating network of Ag-O and Cu-O chains. After the first member, AgCuO 2 was obtained by electrochemical oxidation of Ag 2 Cu 2 O 3 , 3 ozone oxidation, [START_REF] Muñoz-Rojas | Room Temperature Solid-State Transformation from Ag 2 Cu 2 O 3 to Ag 2 Cu 2 O 4 by Ozone Oxidation[END_REF] co-precipitation [START_REF] Curda | AgCuO 2 -Synthesis, Crystal Structure, and Structural Relationships with CuO and AgIAgIIIO 2[END_REF] and by hydrothermal synthesis. [START_REF] Muñoz-Rojas | High Conductivity in Hydrothermally Grown AgCuO 2 Single Crystals Verified Using Focused-Ion-Beam-Deposited Nanocontacts[END_REF] AgCuO 2 crystallizes in a crednerite-type (CuMnO 2, C2/m) 2D structure, [START_REF] Muñoz-Rojas | Electronic Structure of Ag 2 Cu 2 O 4 . Evidence of Oxidized Silver and Copper and Internal Charge Delocalization[END_REF] with copper atoms being surrounded by 4 oxygen atoms, in an edge-sharing square-planar coordination, forming CuO 2 chains along the b axis (as presented in the supporting information, Figure S1). Contrarily to the 2-fold linear coordination of Cu 1+ in crednerite, Ag ions in AgCuO 2 are coordinated by 6 oxygen ions with a distorted octahedral coordination due to a higher oxidation state than +1, as clearly observed by XPS. [START_REF] Muñoz-Rojas | Electrochemically Induced Reversible Solid State Transformations: Electrosynthesis of Ag 2 Cu 2 O 4 by Room Temperature Oxidation of Ag 2 Cu 2 O 3[END_REF][START_REF] Muñoz-Rojas | High Conductivity in Hydrothermally Grown AgCuO 2 Single Crystals Verified Using Focused-Ion-Beam-Deposited Nanocontacts[END_REF][START_REF] Muñoz-Rojas | Structural Study of Electrochemically-Synthesized Ag 2 Cu 2 O 4 . A Novel Oxide Sensitive to Irradiation[END_REF] In addition, X-ray absorption spectroscopy (XAS) studies confirmed the oxidation state of Ag to be higher than +1, while revealing that Cu atoms are also oxidized above +2 and that the charges are delocalized between all the atoms in the structure. [START_REF] Muñoz-Rojas | Electronic Structure of Ag 2 Cu 2 O 4 . Evidence of Oxidized Silver and Copper and Internal Charge Delocalization[END_REF] These results have also been recently supported by Density Function Theory (DFT) calculations by Carreras et al., in which the high oxidation states of silver and copper are obtained. [START_REF] Carreras | Charge Delocalization, Oxidation States, and Silver Mobility in the Mixed Silver-Copper Oxide AgCuO 2[END_REF] In views of these results, AgCuO 2 is expected to have very interesting transport properties. Sauvage et al. [START_REF] Sauvage | Transport Properties and Lithium Insertion Study in the P-Type Semi-Conductors AgCuO 2 and AgCu 0.5 Mn 0.5 O 2[END_REF] measured a ptype electrical conductivity of AgCuO 2 , three orders of magnitude higher than for Ag 2 CuMnO 4 , another member of the family with oxidation states of +1 and +2 for Ag and Cu, respectively, and no charge delocalization. [START_REF] Muñoz-Rojas | Ag 2 CuMnO 4 : A New Silver Copper Oxide with Delafossite Structure[END_REF] Despite the measurements were made on low density pellets (due to the sintering challenges at high temperature leading to the reduction of Ag cations) a conductivity of 3.2×10 -1 S/cm was obtained for AgCuO 2 . [START_REF] Sauvage | Transport Properties and Lithium Insertion Study in the P-Type Semi-Conductors AgCuO 2 and AgCu 0.5 Mn 0.5 O 2[END_REF] The high conductivity of AgCuO 2 was later confirmed on micron sized single crystals prepared by hydrothermal synthesis. [START_REF] Muñoz-Rojas | High Conductivity in Hydrothermally Grown AgCuO 2 Single Crystals Verified Using Focused-Ion-Beam-Deposited Nanocontacts[END_REF] Along these experimental works, DFT calculations by Feng et al. [START_REF] Feng | Optical Properties of New Photovoltaic Materials: AgCuO 2 and Ag 2 Cu 2 O 3[END_REF] predict a small bandgap of about 1.1 eV. However, the recent work by Carreras et al. [START_REF] Carreras | Charge Delocalization, Oxidation States, and Silver Mobility in the Mixed Silver-Copper Oxide AgCuO 2[END_REF] predicts that AgCuO 2 is a three-dimensional conductive metal. Thus, more detailed studies are needed to understand this difference from the different DFT calculations.

To date it has not been possible to find a suitable deposition method to obtain high quality and pure AgCuO 2 films to carry out a study of the transport and optical properties, and to be implemented in devices. Apart from the synthesis approaches detailed above, which yielded powders, a recent study on the direct electrodeposition of AgCuO 2 was reported by Lu et al. [START_REF] Lu | Electrodeposition of AgCuO 2 Nanoplates[END_REF] However the films were very rough, and no electrical characterization was provided. Reactive sputtering has also been used to deposit Ag-Cu mixed oxide films, but to date no there are no reports on the deposition of phase pure, stoichiometric AgCuO 2 thin films by this approach. [START_REF] Pierson | Effect of the Oxygen Flow Rate on the Structure and the Properties of Ag-Cu-O Sputtered Films Deposited Using a Ag/Cu Target with Eutectic Composition[END_REF][START_REF] Prasad Reddy | Surface Morphological and Optical Properties of Nanocrystalline Cu 2 O Films Prepared by RF Magnetron Sputtering: Substrate Bias Effect[END_REF][START_REF] Reddy | Effect of Substrate Temperature on the Structural, Electrical and Optical Behaviour of Reactively Sputtered Ag -Cu -O Films[END_REF] The hydrothermal method is very appealing for the fabrication of high quality crystalline materials at low temperatures and has been extensively used to fabricate devices based on ZnO nanorod arrays, for example. [START_REF] Parize | ZnO/TiO 2 /Sb 2 S 3 Core-Shell Nanowire Heterostructure for Extremely Thin Absorber Solar Cells[END_REF][START_REF] Yoo | Uniform ZnO Nanorod/Cu 2 O Core-Shell Structured Solar Cells by Bottom-up RF Magnetron Sputtering[END_REF][START_REF] Cossuet | Polarity-Dependent Growth Rates of Selective Area Grown ZnO Nanorods by Chemical Bath Deposition[END_REF] In the case of AgCuO 2 , a hydrothermal approach using AgO suspended in a basic Cu 2+ solution was developed to obtain large single crystals for in situ transport measurements. [START_REF] Muñoz-Rojas | High Conductivity in Hydrothermally Grown AgCuO 2 Single Crystals Verified Using Focused-Ion-Beam-Deposited Nanocontacts[END_REF] AgO contains Ag in the oxidation state of +1 and +3 and thus it is capable of oxidizing the Cu 2+ cations in solution to form AgCuO 2 . No detailed study of the reaction mechanism of the formation of AgCuO 2 has been published, but a better knowledge of the reaction mechanism and the effect of the different parameters is necessary to optimize the synthesis and provide guidance for the deposition of AgCuO 2 thin films.

In this study, we report an in situ time-resolved X-ray diffraction study of the hydrothermal synthesis of AgCuO 2 varying the solution pH and reaction temperature. By comparing the phase transitions during reactions at different pH values, different reaction pathways and final products were observed. From these, a reaction pathway based on the formation of intermediate Ag and Cu complexes is proposed. Also, we applied two different models (Johnson-Mehl-Avrami-Kolmogorov (JMAK) and Austin-Rickett (AR)) to study the reaction kinetics. Our results show that the JMAK model is more suited than the AR to fit our experimental data, but better models need to be developed for this type of reactions. An activation energy of around 70.6±5.1 kJ/mol was estimated.

Experimental Section a. Materials and preparation

To study the reaction mechanism and activation energy, the hydrothermal synthesis of AgCuO 2 from AgO and CuSO 4 was reproduced following a previously described protocol. [START_REF] Muñoz-Rojas | High Conductivity in Hydrothermally Grown AgCuO 2 Single Crystals Verified Using Focused-Ion-Beam-Deposited Nanocontacts[END_REF] To maximize the diffraction peak intensity, a dense slurry (suspension) was prepared by mixing 0.262 g (2.11 mmol) of AgO (99.5 %, Sigma Aldrich) and 0.522 g (2.09 mmol) of CuSO 4 •5H 2 O (99 %, Sigma Aldrich) in 6 mL of deionized water, which was mechanically stirred for 5 min. Based on the pH chosen for the experiment, a given amount of KOH (99 %, Sigma Aldrich) (giving 0, 0.3 and 1 M) was added to the suspension, which was mixed for a further 5 min. To facilitate the injection into the reaction cell, the slurry was first mildly ground using an agate mortar. Due to the difficulty of recovering the reaction product from the capillary after the in situ experiments, parallel syntheses were conducted in a conventional approach to be able to characterize the reaction products ex situ and to perform pH measurement before and after the hydrothermal treatment. These experiments were performed by mixing 0.524 g (4.22 mmol) AgO and 1.044 g (4.18 mmol) of CuSO 4 •5H 2 O in 12 mL deionized water followed by the addition of 0.68 g (12 mmol), 0.20 g (3.6 mmol) or 0 g of KOH to have the different pH tested in situ (1, 0.3 and 0 M KOH, respectively). The reactions were performed in Pyrex (25 ml) bottles (that were maintained in the dark) at different temperatures from 50 to 120 °C for 10 h. Table S1 in the SI summarizes the pH values and the color of the solution and the resulting powders for the different reactions.

b. Characterization

The in situ X-ray diffraction (XRD) experiments were conducted at the Swiss-Norwegian Beamlines (BM01) at the European Synchrotron Radiation Facilities (ESRF), Grenoble, France (Table 1). The experiments were performed in transmission mode using the PILATUS@SNBL platform with a wavelength λ = 0.6999 Å. [START_REF] Dyadkin | A New Multipurpose Diffractometer PILATUS@SNBL[END_REF] The reaction cell has been described in detail in previous works, [START_REF] Skjaervø | Formation Mechanism and Growth of MNbO 3 , M=K, Na by in Situ X-Ray Diffraction[END_REF][START_REF] Jensen | Revealing the Mechanisms behind SnO 2 Nanoparticle Formation and Growth during Hydrothermal Synthesis: An in Situ Total Scattering Study[END_REF][START_REF] Becker | Experimental Setup for in Situ X-Ray SAXS/WAXS/PDF Studies of the Formation and Growth of Nanoparticles in near-and Supercritical Fluids[END_REF] and is shown in Figure 1. A single crystal sapphire capillary serves as a miniature reaction cell, with 1.15 ± 0.1 mm outer and 0.8 ± 0.08 mm inner diameters. The slurries were injected into the reaction cell with a plastic syringe. Once the capillary was filled liquid chromatography pump, Shimadzu LC-10ADVP, Shimadzu Corporation). A pressure of 250 bar was applied to avoid boiling during the reaction. In order to evaluate a possible effect of pressure, experiment S1 was also performed without applying pressure and no significant effect of pressure on the final composition or refined parameters was observed. A high temperature heat blower (Leister LE Mini 800) with N 2 gas flow was used for heating.

Acquisition times were optimized for the different reaction rates, 5 s for the fastest reactions and 10 s for the slower reactions. In order to eliminate the possibility of having light-induced reactions (Ag compounds being photosensitive), the whole reaction and characterization process was carried out in the dark. Parasitic regions (spots from sapphire capillary and shadow from beam stopper) were masked from the raw data using the Bubble software, as described in previous works. [START_REF] Dyadkin | A New Multipurpose Diffractometer PILATUS@SNBL[END_REF] The masked 2D data were integrated into 1D also using the Bubble software. Temperature calibrations were performed with boron nitride powder [START_REF] Pease | An X-Ray Study of Boron Nitride[END_REF] inside a sapphire capillary. The instrumental broadening was calibrated with a NIST 660a LaB 6 standard. [START_REF] Skjaervø | Formation Mechanism and Growth of MNbO 3 , M=K, Na by in Situ X-Ray Diffraction[END_REF][START_REF] Dalod | Controlling Oriented Attachment and in Situ Functionalization of TiO 2 Nanoparticles during Hydrothermal Synthesis with APTES[END_REF] All suspensions were prepared right before the diffraction experiments (no more than 10 minutes). The morphology characterization of the product from the parallel syntheses conducted in Pyrex bottles were carried out using a SEM-FEG Environmental FEI QUANTA 250 Scanning Electron Microscope (SEM).

c. Parameter refinement Rietveld refinements were performed using Topas v. 5 to fit a model to the XRD patterns for experiments conducted at 50, 80 and 120 °C. Strain and unit cell parameters of Ag 2 O were refined from the last frame and these values were kept fixed. Then, the same was conducted for CuO and AgO from the first frame (this was performed individually for all experiments). All these parameters were kept fixed during the batch Rietveld refinements, which was necessary for maintaining the stability of the refinements, since several of the reflections were overlapping (see supplementary information for details). For the batch Rietveld refinement, scale factor, unit cell parameters (a, b, c and β), strain and thermal parameters for Ag and Cu in AgCuO 2 phase were refined for all frames.

Results and discussion

For reaction S1, in which 1 M KOH was used, AgCuO 2 was obtained, as expected. In Figure 2a evolution of the diffraction patterns, the corresponding 3D plot is shown in Figure S2b). In this case, AgCuO 2 was not obtained. At low diffraction angles, two emerging reflections were observed corresponding to the final Cu 4 (SO) 4 (OH) 6 phase. Similar to sample S1, a continuous evolution with no intermediate phases was observed. From the diffraction pattern, an increase of the background signal was presented between 10° and 20°, which can be correlated to the higher concentration of dissolved silver ions, [START_REF] Burgess | Ions in Solution: Basic Principles of Chemical Interactions[END_REF] since the Ag-containing phases were barely seen in the final product (unreacted AgO and metallic Ag, both in small amounts). The precipitation of the basic copper sulfate results in an acidification of the solution (Table S1 presents the initial pH values of the solutions for the different reactions), with a concomitant dissolution of the AgO, as evidenced by the decrease of the intensity of the reflection at 14.5°. would imply that the reaction proceeds via the formation of soluble Cu and Ag basic species that then would coprecipitate. For Cu, it is known that it can form soluble hydroxide complexes in basic solutions. For Ag, the formation of such complexes has also been described in the past, [START_REF] Dirkse | The Stability and Solubility of AgO in Alkaline Solutions[END_REF] and the sulfate ions could also play a role in the formation of soluble complexes. 29 Combining the results obtained from the different reactions discussed so far, we propose a reaction pathway for the hydrothermal synthesis of AgCuO 2 , as illustrated in Figure 3. At the beginning of the reaction, the dissolution of CuSO 4 in the 1 M KOH solution induces the precipitation of CuO, [START_REF] Yagi | Potential-PH Diagrams for Oxidation-State Control of Nanoparticles Synthesized via Chemical Reduction[END_REF] as confirmed by the reflections at the beginning of the reaction (Figure 2b). Therefore, the actual reaction precursors in 1 M KOH solution are CuO and AgO. In this starting slurry, there is always an equilibrium between the metal oxide and the metal hydroxide complex, as shown in Equations 1 and 2. [START_REF] Dirkse | The Stability and Solubility of AgO in Alkaline Solutions[END_REF] In the case of AgO, the solubility of such complexes is rather low (in the range of 10 -14 mol/L). [START_REF] Dirkse | The Stability and Solubility of AgO in Alkaline Solutions[END_REF][START_REF] Amlie | Solubility and Stability of Silver Oxides in Alkaline Electrolytes[END_REF] (

Then, AgCuO 2 forms as a result of a co-precipitation between [Ag(OH) x ] 2-x and [Cu(OH) x ] 2-x , as shown in Equation 3. It should be noted that we are dealing with diffraction data only, so the possibility of having mixed metal complexes or an amorphous precursor of AgCuO 2 rapidly crystallizing to the final crystalline sample, cannot be excluded. The fact that AgCuO 2 forms at high pH conditions implies that it is much less soluble than AgO or CuO in basic solutions. [START_REF] Lu | Electrodeposition of AgCuO 2 Nanoplates[END_REF][START_REF] Wang | Determination of AgCuO 2 Discharge Mechanism in Alkaline Electrolyte[END_REF] Therefore, the silver and copper hydroxide complexes in solution are consumed by AgCuO 2 precipitation, displacing the equilibrium between the metal oxides and metal hydroxide complex towards the latter.

In addition to studying the effects of pH on the reaction outcome and pathway, the effects of reaction temperature were also investigated. The KOH concentration was maintained at 1 M with 250 bar pressure and the reaction temperature was varied from 50 to 120 °C, as shown in Table 1. Since the initial suspension used had the same concentration in all cases, the quantity of injected reactants in the capillary is assumed to be the same. For all the reaction temperatures, AgCuO 2 was obtained as the final product and similar reaction evolution was observed, with the intensity of XRD reflections corresponding to AgO and CuO decreasing while the reflections for AgCuO 2 increased in intensity (The evolution with time of the weight fraction of the different phases for different temperatures, as extracted from the refinements, is plotted in Figure S3).

Meanwhile, weak X-ray reflections of Ag 2 O were also observed, which could be due to the thermal decomposition of AgO, as stated above. Conversely, metallic Ag was never detected in the XRD patterns. The evolution of the lattice parameters, unit cell volume and micro strain for the AgCuO 2 crystals varied differently with regard to the reaction time and temperature, as presented in Figure 4 (the refinement of crystallite size was also performed but no reliable values could be obtained). In Figure 4a, 4b and 4c, the unit cell parameters a, b, c generally exhibited a rapid increase once the reaction started, before decreasing to smaller final values. For cell parameter a, at 120 °C, the value reached a maximum almost instantly after the reaction was initiated, and stabilized at about 5.878 Å. Meanwhile at the lower reaction temperatures of 80 and 50 °C, the increase was less drastic and a reached a maximum followed by a slight decrease. The value of a stabilized around 5.873 Å at 50 °C. This variation of the parameter a (value increase and then decrease) could be related to the re-organization of the lattice. It has to be taken into account that the AgCuO 2 formation takes place even at room temperature [START_REF] Muñoz-Rojas | High Conductivity in Hydrothermally Grown AgCuO 2 Single Crystals Verified Using Focused-Ion-Beam-Deposited Nanocontacts[END_REF] and thus we expect small amounts of AgCuO 2 to be present from the first XRD pattern acquired. A similar evolution was observed for parameter b (Figure 4b), and similar values are obtained at 120 and 50 °C. Conversely, a clear difference of about 0.015 Å was observed for the c parameter: 6.098 Å at 120 °C and 6.083 Å at 50 °C (Figure 4c). The c parameter showed more variation as the reaction proceeded, compared to parameters a and b. The unit cell volume presented in Figure 4e increased after the initiation of the reaction followed by a decrease to a lower final value (i.e. following the evolution observed for the cell parameters). Larger unit cells are observed for reactions at higher temperature, 100.8 Å 3 at 120 °C and 95.8 Å 3 at 50 °C, as expected from the thermal expansion of the lattice. As in previous studies, the values are nevertheless higher than what would be expected for a solid sample, but the presence of hydroxyl ions or water molecules within the structure has been proposed to account for it. [START_REF] Dalod | Controlling Oriented Attachment and in Situ Functionalization of TiO 2 Nanoparticles during Hydrothermal Synthesis with APTES[END_REF][START_REF] Grendal | Facile Low Temperature Hydrothermal Synthesis of BaTiO 3 Nanoparticles Studied by In Situ X-Ray Diffraction[END_REF][START_REF] Philippot | Coupling in Situ Synchrotron Radiation with Ex Situ Spectroscopy Characterizations to Study the Formation of Ba 1-xSr x TiO 3 Nanoparticles in Supercritical[END_REF] Lastly, the strain was also monitored for the different temperatures (Figure 4f). For crystals formed at higher temperatures, the strains were generally lower than those formed at lower temperatures, again as expected since higher temperatures are favoring a better crystallization and strain relaxation. At 120 °C, about 0.00128±0.00003 of strain was observed and about 0.00143±0.00002 for 50 °C, in agreement with the diffraction reflection width observed in Figure 4a, 4b and4c.

In addition to the unit cell parameters, the atomic thermal displacement factor (B iso ) of Ag and Cu atoms in the AgCuO 2 crystals were refined for the different temperatures and results are shown in Figure 5. For Ag, the B iso value decreased rapidly once the reaction was initiated (Figure 5a). Conversely, for Cu there was a rapid initial decrease followed by an increase of the B iso values (Figure 5b). The B iso values obtained are rather large, which is similar to what we have observed in previous in situ studies. [START_REF] Grendal | Facile Low Temperature Hydrothermal Synthesis of BaTiO 3 Nanoparticles Studied by In Situ X-Ray Diffraction[END_REF] Again, the presence of hydroxyl groups could be the reason for the higher B iso values obtained in these type of studies. 34 For AgCuO 2 crystals formed at all the reaction temperatures, the Ag atom B iso values are generally larger than those for Cu atoms, indicating a higher thermal motion for Ag ions in the AgCuO 2 crystals. In any case, the high B iso values obtained for AgCuO 2 are in agreement with previous XAS data, and are the result of the peculiar electronic structure of this oxide, with intermediate oxidation states and charge delocalization. [START_REF] Muñoz-Rojas | Electronic Structure of Ag 2 Cu 2 O 4 . Evidence of Oxidized Silver and Copper and Internal Charge Delocalization[END_REF] In particular, Ag B iso values are always larger than for Cu, which could be related to the high mobility of silver anions, as for example in AgI, where Ag ions are mobile inside the fixed lattice of I -anions. [START_REF] O'sullivan | Silver-Ion Disorder in -AgI: A Computer Simulation Study[END_REF] This is in agreement with the recent DFT calculations by Carreras et al., which predict an easy slide of Ag atoms along the chains of linearly coordinated Ag cations. [START_REF] Carreras | Charge Delocalization, Oxidation States, and Silver Mobility in the Mixed Silver-Copper Oxide AgCuO 2[END_REF] To estimate the activation energy for the hydrothermal formation of AgCuO 2 from AgO and CuO in a basic solution, two models were compared, namely, the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model and the Austin-Rickett (AR) model. In these models the extent of reaction, f, is described, respectively, by Equations 9 and 10.

(1) (10) k is the reaction rate constant, t is the time after t 0 of appearance of the phase, and n is linked to the growth mechanism. Both models have been used to interpret precipitation and phase-change reactions. [START_REF] Starink | Kinetic Equations for Diffusion-Controlled Precipitation Reactions[END_REF][START_REF] Peterson | A Kinetic Analysis of the Transformation from Akaganeite to Hematite : An in Situ Time-Resolved X-Ray Diffraction Study[END_REF] The normalized fitting results are shown in Figure S4 and the fitted results of n, k values are presented in Table 2. To evaluate the reaction durations at a range of temperatures, we set 90 % as the indication of an accomplished reaction. As observed from the fittings, the obtained k(T) (fitted line slope) using both models are rather similar, showing an exponential increase with temperature. However, the n values deduced from the different fitting models exhibit a clear difference, as shown in Table 2 and Figure 6a. From the AR model, the n values obtained increased from 2.7 to almost 3.8 with increasing reaction temperature. Meanwhile, from the JMAK model the fittings are rather reasonable for all reactions, n values around 2 being obtained. A comparison of both models (Figure S4) shows that the JMAK model fits better than the AR model in our case. The obtained n values around 2 would indicate that the growth is restricted to 1 or 2 dimensions, according to the litterature. [START_REF] Weinberg | Crystallization Kinetics and the JMAK Equation[END_REF][START_REF]The Johnson-Mehl-Avrami-Kolmogorov Model: A Brief Review[END_REF][START_REF] Starink | An Analysis Method for Nucleation and Growth Controlled Reactions at Constant Heating Rate[END_REF] This would imply that in all cases the particles obtained would have similar shapes, but SEM images of the different products obtained from hydrothermal reactions performed sealed Pyrex bottles (see experimental details) show two different type of particles at low and high temperatures (Figure 7). The AgCuO 2 particles tend to be bi-dimensional with a platelet-type shape for reaction temperatures from 50 to 80 °C. Conversely, for 90 and 120 °C, the particles are more spherical (see figure S5 in the SI for lower magnification images). Thus, it seems clear that none of the two models can fully explain the reaction taking place in this case, especially as reaction temperature decreases, as evidenced by the worse fittings obtained (figure S4). More studies and new models are therefore needed. In particular, it has been shown that when dealing with nanomaterials, particle growth via the oriented attachment mechanism (as opposed to the classical Ostwald ripening model) often plays a key role at hydrothermal conditions. [START_REF] Lu | The Use of Mixed-Metal Single Source Precursors for the Synthesis of Complex Metal Oxides[END_REF][START_REF] Zhang | Recent Advances in Oriented Attachment Growth and Synthesis of Functional Materials: Concept, Evidence, Mechanism, and Future[END_REF] The development of new kinetic models taking oriented attachment into account have been tackled, requires an exhaustive HRTEM study of the samples. [START_REF] Zhang | Progress of Nanocrystalline Growth Kinetics Based on Oriented Attachment[END_REF] The different kinetics and particle shape of the samples obtained at different temperatures could in fact be due to oriented attachment taking place at lower temperatures, but a deeper study should be performed to evaluate this hypothesis. From our fittings (Figure 6b), the activation energies for the whole reaction obtained from the two models are rather similar, at 70.6 kJ/mol. Given the limited applicability of these models for our cases, as discussed above, this value needs to be taken as approximated. In our case, AgCuO 2 forms at room temperature and is detectable at the beginning of the experiments even before heating, thus meaning that both the dissolution of initial oxides and the precipitation of AgCuO 2 can take place at room temperature. With this in mind, and given that the activation energy obtained for the reaction is comparable to other similar precipitation reactions, [START_REF] Inskeep | An Evaluation of Rate Equations for Calcite Precipitation Kinetics at PCO Less than 0 . 01 Atm and PH Greater than 8[END_REF][START_REF] Koutsoukos | Precipitation of Calcium Carbonate in Aqueous Solutions[END_REF] we propose that it corresponds to the nucleation of AgCuO 2 from the complexes.

Conclusions

In this work, we present an in situ time-resolved XRD study of the reaction mechanism of the hydrothermal synthesis of AgCuO 2 evaluating the effect of both solution pH and reaction temperature. By varying the solution pH, different reaction intermediates and final products were formed, and AgCuO 2 could only be obtained when using 1 M KOH solutions in this work. Based on our analyses, we have proposed a reaction pathway that is based on the co-precipitation of silver and copper hydroxide complexes in alkaline solution. The cell parameters increased when increasing the reaction temperature and the c and b parameters were the most sensitive to the temperature. Relatively high B iso values comparable to similar studies were observed. In all cases, the B iso values for Ag where higher than for Cu. This is believed to be due to the intrinsic high mobility of Ag atoms, which could be favored in AgCuO 2 given the peculiar electronic structure and 6-fold coordination of the Ag atoms. Finally, we compared the JMAK and AR models for the calculation of the n values and the activation energy. Although in our case the JMAK model provides a better fitting of the data, the n values obtained do not match the morphology of the particles obtained and thus better models need to be developed. An approximative activation energy of 70.6±5.1 kJ/mol was obtained, which we propose corresponds mainly to the nucleation of the AgCuO 2 phase. The insight gained on the formation pathway of AgCuO 2 should allow an optimization of the conditions to obtain high quality, dense AgCuO 2 films. For example, AgCuO 2 nuclei obtained from reactions carried out at low temperature could be deposited on a substrate to then grow a film by performing the electrodeposition or hydrothermal reaction at higher temperatures.

Supporting Information

Crystal structure of AgCuO 2 (Figure S1), 3D phase evolution of reaction S1(Figure S2), pH and solution color before and after the hydrothermal reactions (Table S1), evolution with time of the weight fraction of the different phases (Figure S3), comparison of fitting by JMAK and AR models (Figure S4), and SEM images of AgCuO 2 powders (Figure S5).

Figure 1 .

 1 Figure 1. a). 3D Scheme of the capillary in situ cell used (drawn with Autodesk(R) Inventor LTTM by Anders Bank Blichfeld). 22 b). In situ XRD experimental setup at line BM01A, ESRF,

  Figure2b(the corresponding 3D phase evolution plot is presented in FigureS2ain the SI). From

Figure 2 .

 2 Figure 2. a) representative XRD patterns during reaction S1 and b) phase evolution, with 1 M

Figure 3 .

 3 Figure 3. Illustration of the proposed AgCuO 2 phase formation pathway at high pH facilitating

Figure 4 .

 4 Figure 4. Time resolved refined values for AgCuO 2 . a), b), c) and d) unit cell parameters (lattice

  Figure 4d shows the variation in β and the difference at the highest and lowest reaction temperature was about 0.25°. As stated above the different evolution and final values of the cell parameters at different temperatures could be due to lattice rearrangements. Parameter b lies parallel to the CuO 2 chains in which Cu atoms are coordinated by 4 oxygen atoms (with edge-sharing square-planar coordination), and to the chains of Ag octahedra. The similar final values of parameter b for the different reaction temperatures are thus rather logical. Contrarily, twisting of the CuO 2 chains or any other lattice rearrangement is more likely to take place in the ac plane, which again agrees with the evolution and different final values observed for parameters a and c.

Figure 5 .

 5 Figure 5. B iso values for Ag a) and Cu b) in AgCuO 2 as a function of time.

Figure 6 .

 6 Figure 6. a) Fitted n values from both JMAK and AR models. b) Activation energy estimated

Figure 7 .

 7 Figure 7. SEM images of AgCuO 2 powders synthesized in 1 M KOH solution in sealed Pyrex

Table 1 .

 1 Overview of in situ XRD measurements performed including the

different parameters used during the hydrothermal synthesis and the phases obtained. S1-S3 were prepared varying the KOH concentration while the T1-T5 samples were synthesized at different temperatures. Sample name Temperature (°C) KOH solution concentration (M) Main Obtained phase (weight fraction in parenthesis, secondary phases in italics) Estimated completion time (s)

	T2	70		AgCuO 2 (83) AgO (4) CuO (8), Ag 2 O (5)	864
	T3	80		AgCuO 2 (92) CuO (7), Ag 2 O (1)	297
	T4 (=S1)	90		AgCuO 2 (83) CuO (8), Ag 2 O (9)	286
	T5	120		AgCuO 2 (92) CuO (5), Ag 2 O (2)	52
	S1	90	1	AgCuO 2 (83) CuO (8), Ag 2 O (9)	N/A
	S2		0.3	Cu 4 (SO 4 )(OH) 6 (AgO, Ag)
	S3		0	0 Cu 4 (SO 4 )(OH) 6 (with intermediate phases)
	T1	50	1	AgCuO 2 (83) CuO (11), Ag 2 O (6)	3980

  3 •2H 2 O to Cu 3 (SO 4 )(OH)4 and finally stabilized as the Cu 4 (SO 4 )(OH) 6 phase. Meanwhile, the reflections corresponding to AgO totally disappeared after about 10 min of reaction. This is again caused by the acidification of the suspension as the basic copper sulfate phases precipitate and transform (see table S1 in SI).Indeed, the formation of different types of copper sulfate hydroxide phases was reported in a work by Tanaka et al.[START_REF] Tanaka | Thermogravimetry of Basic Copper(II) Sulphates Obtained by Titrating NaOH Solution with CuSO4 Solution[END_REF] In their work, a CuSO 4 solution (1 M) was titrated against a NaOH solution (0.01 M, pH 12). As different quantities of CuSO 4 solution were added, different copper sulfate hydroxide phases were formed. Initially, Cu(OH) 2 was obtained as expected, and then other basic copper sulfates formed. For instance, Cu 4 (SO 4 )(OH) 6 formed when the solution had neutral pH and even at pH as low as 4 after further additions of CuSO 4 solution. Reactions carried out with only AgO as starting precursor, in both 0 and 1 M KOH solutions did, on the Hence, the pH of the solution plays a critical role in AgCuO 2 formation. Different pH values during the reaction led to different reaction schemes between CuSO 4 and AgO, resulting in different final products. The fact that AgCuO 2 is only obtained when high pH values are used,

other hand, not result in any observable reaction or dissolution, AgO being the only phase detectable by XRD after the hydrothermal reaction, as indicated in Table

S1

.

Table 2 .

 2 Fitted n and k values from JMAK and AR models.

	Temperature	k j	(JMAK fitted	n j (JMAK fitted) k a	(AR	fitted	n a (AR fitted)
	(K)	reaction	rate		reaction	rate
		constant)			constant)
	323	0.0359±0.0002		1.94±0.03	0.0451±0.0006	2.84±0.11
	343	0.1329±0.001		1.78±0.02	0.1697±0.0020	2.62±0.09
	353	0.4813±0.0023		1.95±0.03	0.6015±0.008	2.95±0.11
	363	0.6568±0.0036		1.95±0.03	0.8192±0.008	2.92±0.08
	393	3.6605±0.1629		1.90±0.03	4.5830±0.1368	3.87±0.21

Acknowledgements HL acknowledges the Ministère de l'Education nationale, de l'Enseignement supérieur et de la Recherche in France for a Ph.D. grant. We acknowledge SNBL at ESRF, Grenoble, France for the granted beam time. DMR acknowledges funding through the Marie Curie Actions (FP7/2007-2013, Grant Agreement No. 631111). Financial support from NTNU Norwegian University of Science and Technology and The Research Council of Norway under the Toppforsk program to the project (No 250403) "From Aqueous Solutions to oxide Thin films and hierarchical Structures" is gratefully acknowledged.

For Table of Contents


Viet Huong Nguyen

César Abderrahime Sekkat

Arturo Masse

La Huerta

Fadi Zoubian

Chiara Crivello

Juan Rubio-Zuazo

Moustapha Jaffal

Marceline Bonvalot

Christophe Vallée

Olivier Aubry

Arturo César

Masse De La Huerta

Chi- Ara Crivello

Oliv- Ier Aubry

Hervé Rabat

Dunpin Hong

David Muñoz-Rojas

Atmospheric Plasma-Enhanced Spatial Chemical Vapor Deposition of SiO 2 Using Trivinylmethoxysilane and Oxygen Plasma

ABSTRACT: SiO2 constitutes one of the most widely used dielectric materials in microelectronics, packaging and optical industries. Therefore, the development of new processes to deposit SiO2 at low temperature and in an affordable and scalable way are desirable. In this work, we present a low-temperature, open-air process based on spatial atomic layer deposition (SALD) that yields high purity SiO2 films at temperatures down to room temperature. The films were obtained by operating our SALD system in CVD mode (i.e. allowing precursor crosstalk), using an oxygen plasma in combination with Trivinylmethoxysilane (TVMS). TVMS is an appealing precursor since it is highly volatile, affordable and does not contain halogen elements, thus being very suitable for application in atmospheric-pressure spatial deposition systems. Conversely, water, oxygen, hydrogen peroxide or ozone did not show any reactivity with TVMS at temperatures up to 260 °C. Thus, when operating our system in ALD mode, no film could be obtained due to the lack of reactivity of the precursor with OH * surface groups. 3D printing was employed to fabricate custom heads integrating both the precursor injector and the atmospheric plasma generator. Our results show that compact, conformal, pinhole-free SiO2 thin films can be deposited by our atmospheric plasma-enhanced spatial chemical vapor deposition (APE-SCVD) approach at low temperatures (RT -180 °C) on different substrates, including silicon wafers, micro-glass slides, or even on polymeric substrates with a high growth rate up to 2 -5 nm/min. The deposition rate increased when increasing the power applied to the plasma reactor but decreased when increasing the deposition temperature due to de faster decay of the metastable oxygen radical species. FTIR results showed no differences for films deposited with different plasma powers. Conversely, temperature had an effect on the ratio between the AS1 and AS2 bands. Even though the deposition of SiO2 was carried out at low temperatures in the open air using a metalorganic precursor, no contamination from SiNx or SiCx, was observed by FTIR and XPS measurements. Our results open the door to the low-temperature, fast printing of Si-based devices.

Silicon dioxide (SiO2) is used in various fields such as microelectronics, packaging, or optical communication due to its excellent physical properties including corrosion resistance, dielectric nature, optical transparency, hardness, gas and humidity barrier, etc. [START_REF] Kim | Thermal Atomic Layer Deposition of Device-Quality SiO 2 Thin Films under 100 °C Using an Aminodisilane Precursor[END_REF][START_REF] Kischkat | Mid-Infrared Optical Properties of Thin Films of Aluminum Oxide, Titanium Dioxide, Silicon Dioxide, Aluminum Nitride, and Silicon Nitride[END_REF][START_REF] Chang | Intrinsic SiOx-Based Unipolar Resistive Switching Memory. II. Thermal Effects on Charge Transport and Characterization of Multilevel Programing[END_REF] SiO2 thin films can be obtained using various techniques, including chemical vapor deposition (CVD), [START_REF] O'neal | Chemical Vapor Deposition (CVD) of Silicon Dioxide Films Using Oxygen-Silicon Source Reactants and a Free Radical Promoter[END_REF][START_REF] Fujino | Silicon Dioxide Deposition by Atmospheric Pressure and Low-Temperature CVD Using TEOS and Ozone[END_REF] plasma-enhanced chemical vapor deposition (PECVD), [START_REF] Patrick | Plasma-Enhanced Chemical Vapor Deposition of Silicon Dioxide Films Using Tetraethoxysilane and Oxygen: Characterization and Properties of Films[END_REF][START_REF] Mallick | High Quality Silicon Oxide Films by Remote Plasma CVD from Disilane Precursors[END_REF][START_REF] Ho | Thick PECVD Silicon Dioxide Films for MEMS Devices[END_REF] atomic layer deposition (ALD) [START_REF] Bachmann | Self-Catalytic, Atomic Layer Deposition of Silicon Dioxide[END_REF][START_REF] Karg | Atomic Layer Deposition of Silica on Carbon Nanotubes[END_REF][START_REF] Ma | Systematic Study of the SiOx Film with Different Stoichiometry by Plasma-Enhanced Atomic Layer Deposition and Its Application in SiOx/SiO2 Super-Lattice[END_REF] and physical techniques such as thermal oxidation, [START_REF] Deal | General Relationship for the Thermal Oxidation of Silicon[END_REF] magnetron sputtering, [START_REF] Bhatt | Silicon Dioxide Films by RF Sputtering for Microelectronic and MEMS Applications[END_REF] among others. For most of the mentioned techniques, SiO2 deposition is usually performed at low pressure in a vacuum-chamber, which has a substantial repercussion on the cost of scaling up to industrial applications. Developing alternative approaches for the open-air deposition of Si-based thin films at low temperatures and with high throughput is therefore desirable.

Recently, Atmospheric pressure spatial ALD (AP-SALD) has become an appealing alternative to conventional ALD, in that it allows deposition of highly conformal and uniform films even in the open atmosphere and with ultrafast deposition rates (up to two orders of magnitude faster than standard ALD). [START_REF] Poodt | Spatial Atomic Layer Deposition: A Route towards Further Industrialization of Atomic Layer Deposition[END_REF][START_REF] Musselman | Nanomanufacturing: High-Throughput, Cost-Effective Deposition of Atomic Scale Thin Films via Atmospheric Pressure Spatial Atomic Layer Deposition[END_REF][START_REF] Muñoz-Rojas | Spatial Atomic Layer Deposition[END_REF] In addition, AP-SALD can be easily scaled up to large surfaces, since no vacuum chamber is needed. As a result, AP-SALD has been widely applied to the deposition of functional materials for new generation photovoltaics, [START_REF] Muñoz-Rojas | Spatial Atmospheric Atomic Layer Deposition: A New Laboratory and Industrial Tool for Low-Cost Photovoltaics[END_REF][START_REF] Muñoz-Rojas | Spatial Atomic Layer Deposition (SALD), an Emerging Tool for Energy Materials. Application to New-Generation Photovoltaic Devices and Transparent Conductive Materials[END_REF][START_REF] Illiberi | Spatial-ALD of Transparent and Conductive Oxides[END_REF][START_REF] Illiberi | Atmospheric Spatial Atomic Layer Deposition of In-Doped ZnO[END_REF] or for the encapsulation of flexible electronics. [START_REF] Maydannik | Spatial Atomic Layer Deposition : Performance of Low Temperature H2O and O3 Oxidant Chemistry for Flexible Electronics Encapsulation Spatial Atomic Layer Deposition : Performance of Low Temperature H 2 O and O 3 Oxidant Chemistry for Flexible Electronics[END_REF][START_REF] Hoffmann | Gas Diffusion Barriers Prepared by Spatial Atmospheric Pressure Plasma Enhanced ALD[END_REF][START_REF] Franke | Alumina Films as Gas Barrier Layers Grown by Spatial Atomic Layer Deposition with Trimethylaluminum and Different Oxygen Sources[END_REF][START_REF] Nguyen | Low-Cost Fabrication of Flexible Transparent Electrodes Based on Al Doped ZnO and Silver Nanowire Nanocomposites: Impact of the Network Density[END_REF] Similar to conventional ALD and CVD processes, plasma activation of SALD has been already demonstrated by using atmospheric pressure plasmas. [START_REF] Van Den Bruele | Atmospheric Pressure Plasma Enhanced Spatial ALD of Silver[END_REF][START_REF] Hoffmann | Atmospheric Pressure Plasma Enhanced Spatial Atomic Layer Deposition of SnO x as Conductive Gas Diffusion Barrier[END_REF] In open-air SALD approaches, a gas manifold injector is used and placed very close to the substrate to avoid crosstalk of precursors across the inert gas barriers. [START_REF] Levy | Stable ZnO Thin Film Transistors by Fast Open Air Atomic Layer Deposition[END_REF][START_REF] Hoye | Synthesis and Modeling of Uniform Complex Metal Oxides by Close-Proximity Atmospheric Pressure Chemical Vapor Deposition[END_REF] The deposition area is thus only related to the size of the gas injector and its scanning amplitude over the substrate. While such approach was designed to perform ALD, the distance between the substrate and the injector (deposition gap) can be increased to facilitate the crosstalk of precursors in the gas phase, resulting in film deposition via CVD mode. [START_REF] Hoye | Synthesis and Modeling of Uniform Complex Metal Oxides by Close-Proximity Atmospheric Pressure Chemical Vapor Deposition[END_REF][START_REF] Musselman | Rapid Open-Air Deposition of Uniform, Nanoscale, Functional Coatings on Nanorod Arrays[END_REF][START_REF] Masse De La Huerta | Influence of the Geometric Parameters on the Deposition Mode in Spatial Atomic Layer Deposition: A Novel Approach to Area-Selective Deposition[END_REF] This so-called spatial chemical vapor deposition (SCVD) mode allows depositing films with higher growth rates as compared to SALD while still retaining the main advantages of the latter, i.e. operating at low temperatures and in open air, as well as film quality and conformality inherent to ALD. [START_REF] Hoye | Synthesis and Modeling of Uniform Complex Metal Oxides by Close-Proximity Atmospheric Pressure Chemical Vapor Deposition[END_REF] Furthermore, similar atmospheric plasma generators used in spatial reactors could also be used in SCVD mode, thus allowing more flexibility in processing conditions, such as a low deposition temperatures (< 100 °C) or a wider choice of precursors. The combination of atmospheric plasma generator and the SCVD technique would therefore be particularly beneficial to industrial-scale applications, such as for instance deposition of thin films for solar cells, barrier coatings, or layers for electronic device encapsulation. [START_REF] Muñoz-Rojas | High-Speed Atmospheric Atomic Layer Deposition of Ultra Thin Amorphous TiO 2 Blocking Layers at 100 °C for Inverted Bulk Heterojunction Solar Cells: AALD for Inverted Bulk Heterojunction Solar Cells[END_REF][START_REF] Ali | High Rate Rollto-Roll Atmospheric Atomic Layer Deposition of Al 2 O 3 Thin Films towards Gas Diffusion Barriers on Polymers[END_REF] So far, the most used precursors for the chemical vapor deposition of SiO2 thin films are silane (SiH4) and halogencontaining precursors, such as SiCl4 [START_REF] Sneh | Atomic Layer Growth of SiO2 on Si(100) Using SiCl4 and H2O in a Binary Reaction Sequence[END_REF][START_REF] Ezhovskii | Growth and Properties of Al2O3 and SiO2 Nanolayers on III-V Semiconductors. Inorg[END_REF] , Si2Cl6 [START_REF] Lee | Atomic Layer Deposition of Silicon Oxide Thin Films by Alternating Exposures to Si2Cl6 and O3. Electrochem[END_REF] or SiH2Cl2 36 because of their ability to provide high-purity films. The fabrication of SiO2 films using these conventional precursors is often carried out at relatively high temperatures (> 400 °C). A manner to reduce the deposition temperature is to employ catalysts such as NH3 or pyridine. [START_REF] Klaus | Atomic Layer Deposition of SiO2 at Room Temperature Using NH3-Catalyzed Sequential Surface Reactions[END_REF][START_REF] Du | Mechanism of Pyridine-Catalyzed SiO2 Atomic Layer Deposition Studied by Fourier Transform Infrared Spectroscopy[END_REF] Although the halogen-containing precursors are highly volatile and reactive, they are also pyrophoric and their use in SiO2 fabrication produces corrosive gases. When considering the safety, toxicity, and compatibility with open-air low-cost chemical vapor deposition approaches, halide-free metalorganic precursors such as tetraethoxysilane (TEOS), [START_REF] Fujino | Silicon Dioxide Deposition by Atmospheric Pressure and Low-Temperature CVD Using TEOS and Ozone[END_REF][START_REF] Patrick | Plasma-Enhanced Chemical Vapor Deposition of Silicon Dioxide Films Using Tetraethoxysilane and Oxygen: Characterization and Properties of Films[END_REF][START_REF] Shareef | Subatmospheric Chemical Vapor Deposition Ozone/TEOS Process for SiO2 Trench Filling[END_REF][START_REF] Ferguson | ALD of SiO2 at Room Temperature Using TEOS and H 2 O with NH 3 as the Catalyst[END_REF][START_REF] Bange | Growth and Characterization of SiO2 Films Deposited by Flame Hydrolysis Deposition System for Photonic Device Application[END_REF][START_REF] Ponton | Development of a Kinetic Model for the Moderate Temperature Chemical Vapor Deposition of SiO2 Films from Tetraethyl Orthosilicate and Oxygen[END_REF] tetramethylsilane (TMS) [START_REF] Grill | Low Dielectric Constant Films Prepared by Plasma-Enhanced Chemical Vapor Deposition from Tetramethylsilane[END_REF][START_REF] Fonash | Low Temperature, High Quality Silicon Dioxide Thin Films Deposited Using Tetramethylsilane (TMS) for Stress Control and Coverage Applications[END_REF] , di-isopropylaminosilane (DIPAS) [START_REF] Shin | Plasma-Enhanced Atomic Layer Deposition of Low Temperature Silicon Dioxide Films Using Di-Isopropylaminosilane as a Precursor[END_REF][START_REF] Lee | Low Temperature Atomic Layer Deposition of SiO2 Thin Films Using Di-Isopropylaminosilane and Ozone[END_REF] or tris(dimethylamino)silane (TDMAS) [START_REF] Burton | SiO2 Atomic Layer Deposition Using Tris(Dimethylamino)Silane and Hydrogen Peroxide Studied by in Situ Transmission FTIR Spectroscopy[END_REF] are indeed more attractive choices. Another potential candidate is trivinylmethoxysilane (TVMS) because of its high vapor pressure, non-toxicity and appealing cost. These qualities make TVMS very appealing in particular for use in atmospheric-pressure chemical vapor deposition systems. However, Burton et al. have demonstrated that this precursor is indeed not suitable for ALD due to its low reactivity towards SiOH* surfaces, as well as H2O in the range of temperatures up to 400 °C. [START_REF] Burton | SiO2 Atomic Layer Deposition Using Tris(Dimethylamino)Silane and Hydrogen Peroxide Studied by in Situ Transmission FTIR Spectroscopy[END_REF] Moreover, although a recent theoretical study by Vajeeston et al. indicates that TVMS should be a suitable candidate for the deposition of Si by ALD, to the best of our knowledge, this precursor has never been successfully used for both SiO2 and Si thin film deposition via chemical vapor deposition techniques.

In this work, we have used an atmospheric plasma-enhanced spatial chemical vapor deposition (APE-SCVD) approach to deposit SiO2 thin films at low temperatures (RT -180 °C) in open air with high deposition rates (2 -5 nm/min), on different substrates including glass slides, silicon wafers or flexible polymeric substrates. We have used a custom-designed and 3D printed one-block gas injection head incorporating the atmospheric plasma generator. In such a configuration, a confined reaction zone is obtained, limited to the volume where the different precursors can meet over the substrate surface. This mixing zone is considered as a virtual CVD chamber whose volume can be easily controlled by modifying the injector-substrate gap. This enables more flexibility in controlling the growth rate, the roughness as well as the density of the deposited films. Trivinylmethoxysilane (TVMS) has been successfully used for the first time as a silicon precursor to deposit SiO2 thin films with oxygen plasma as a co-reactant. The effect of different parameters on the SiO2 film quality, including substrate temperature, oxygen plasma power, the gap between the substrate and the injection head, as well as the influence of the head design on the film quality is presented and discussed. Our work demonstrates that the spatial separation concept can also be applied to CVD reactors even when using plasma activation.

Different depositions were performed in a home-build APE-SALD system equipped with a close-proximity manifold head. As described previously, by adjusting different parameters, the system can operate in a CVD mode, [START_REF] Masse De La Huerta | Influence of the Geometric Parameters on the Deposition Mode in Spatial Atomic Layer Deposition: A Novel Approach to Area-Selective Deposition[END_REF] while still showing a constant growth per cycle (GPC) and a high conformality. [START_REF] Musselman | Rapid Open-Air Deposition of Uniform, Nanoscale, Functional Coatings on Nanorod Arrays[END_REF] Trivinylmethoxysilane (Si(C2H3)3(OCH3), TVMS, used as received from Fluorochem) was used as the silicon precursor, while oxygen plasma was employed as the oxidizing agent. The latter was produced using a homebuilt dielectric barrier discharge (DBD) plasma source, which contains two interdigitated copper grids and a fused quartz layer of 2 mm-thick in between as dielectric barrier. [START_REF] Zoubian | Development and Characterization of an Atmospheric Pressure Plasma Reactor Compatible with Spatial ALD[END_REF] The DBD source was integrated in a custom-designed deposition head that was fabricated by 3D printing using a Formlabs II 3D printer. The precursor bubbler was kept at room temperature. TVMS was delivered to the substrate by bubbling a 50 sccm nitrogen gas flow through the liquid precursor, which was then diluted with an extra nitrogen flow (100 sccm) before being sent to the injection head. The oxygen plasma was generated by delivering 200 sccm of pure oxygen to the DBD plasma source. The maximum voltage and frequency that can be applied to the DBD are 12 kV and 10 kHz, respectively. The silicon substrates and micro-glass slides used in this work were cleaned using ethanol, deionized water, and then dried under pure nitrogen. On samples used for TVMS surface reactivity tests, an extra 2 min long oxygen plasma cleaning for 2 min was employed to functionalize the substrate surface before exposing it to TVMS.

The oxygen plasma was characterized by using a high-resolution optical emission spectroscopy (OES) system (Spec-traPro HRS 750 from Princeton Instruments). The spectrum was obtained (in the dark) by pointing an optical fiber toward the DBD plasma source while oxygen was being injected.

The chemical composition of the deposited films was analysed by using Fourier Transform Infrared spectroscopy (FTIR, Bruker VERTEX 70v) and X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed at BM25-SpLine laboratory using a standard ultra-high vacuum chamber with a pressure of 1 × 10 -10 mbar. A hemispherical analyzer equipped with a channel-plate and a non-monochromatic Mg X-ray source ( h =1254.6 eV) were used for the measurements. The energy scale was calibrated with the C1s line from contamination carbon at 284.8 eV. Analyses were carried out at a constant angle of 90° between the sample surface and the analyzer. The thickness, surface morphology, and mass density of deposited films were analyzed by ellipsometry (Film Sense FS-1), Xray reflectometry (D500 Siemens, using Cu Kα radiation (λ = 0.15406 nm), 0.01 °/step, 2 s/step) and field emission gun-scanning electron microscopy (FEG-SEM Environmental FEI Quanta 250).

A combination of three modules from Comsol Multiphysics was used for 2D simulation: heat transfer in fluids, laminar flow and transport of diluted species. The gas injector/substrate distance and the gas flow per outlet were kept at 150 or 500 µm and 100 sccm, respectively.

As a first step, the reactivity of TVMS precursor with respect to different oxidizing agents and various substrates was investigated. As stated above, Vajeeston et al. [START_REF] Vajeeston | Search for Potential Precursors for Si-Atomic Layer Deposition -A Quantum Chemical Study[END_REF] have used state-of-the-art density-functional calculations to predict that TVMS should be a suitable precursor for the ALD growth of Si and SiO2 thin films because of a favorable surface adsorption on an -OH * terminated surface, and reaction of the precursor with itself in the case of Si deposition. [START_REF] Vajeeston | Search for Potential Precursors for Si-Atomic Layer Deposition -A Quantum Chemical Study[END_REF] However, to our best knowledge, there are no reports in which TVMS has been successfully used to deposit SiO2 or Si thin films by CVD or ALD techniques. In addition, Burton et al. [START_REF] Burton | SiO2 Atomic Layer Deposition Using Tris(Dimethylamino)Silane and Hydrogen Peroxide Studied by in Situ Transmission FTIR Spectroscopy[END_REF] have used a conventional ALD reactor and FTIR to experimentally demonstrate that Si-OCH3 bonds are relatively inert towards Si-OH * surface groups, while breaking Si-C bonds to form Si-O bonds may have an activation barrier that is kinetically limiting.

Thus, the ALD reactivity of TVMS with micro-glass slides, silicon substrates or oxygen plasma-treated silicon substrates has been tested, as well as its reactivity The substrate/injector gap was maintained at 500 µm in this simulation, [START_REF] Masse De La Huerta | Influence of the Geometric Parameters on the Deposition Mode in Spatial Atomic Layer Deposition: A Novel Approach to Area-Selective Deposition[END_REF] and the color bar corresponds to the zone where both TVMS and atomic oxygen are present; c) Variation of SiO2 film thickness as a function of high voltage applied to the DBD plasma source; d) Intensity variation of the oxygen atomic desexcitation peaks at 777 nm for different applied high voltages ranging from 7.8 kV to 9.5 kV; e) FTIR spectra of a 45 nm thick SiO2 film deposited on a silicon substrate by APE-SCVD at 75 °C; f) Si 2p core level XPS spectrum of 45 nm thick APE-SCVD SiO2 film deposited at 75 °C.

with various oxidizing agents. Firstly, the surface adsorption activity of TVMS was tested by exposing it to microglass slides and two different silicon substrates (with and without extra plasma treatment, see experimental details) for 15 min at 100 °C, using our SALD system. More details related to the reactivity tests using a standard SALD injection head can be found in the SI, Figure S2. If TVMS adsorption on these surfaces takes place, one would expect the surface to be terminated by hydrophobic groups, i.e. -C2H3 since the dissociation enthalpy of the SiO-CH3 bond at 298 K (358 kJ/mol) [START_REF] Allendorf | Theoretical Study of the Thermochemistry of Molecules in the Si-O-H System[END_REF] is much lower as compared to that of the Si-C2H3 bond (527 kJ/mol). [START_REF] Allendorf | Theoretical Study of the Thermochemistry of Molecules in the Silicon-Carbon-Hydrogen System[END_REF] However, no difference in wettability of the substrates could be observed before and after exposure to TVMS. In addition, FTIR measurements (in the case of Si substrates) didn't show any trace of TVMS on the substrate surface, even for plasma treated substrates which contain a high density of -OH * groups (see more details in the SI, Figure S1). This indicates that no TVMS adsorption happened under the test conditions, thus implying that TMVS is not a suitable ALD precursor, at least when working at atmospheric pressure.

As detailed above, our spatial system provides the possibility of intermixing precursor and co-reactant in the gas phase above the substrate surface by adjusting the gap between the head and the substrate (a scheme of a standard SALD head used in our system is shown in Figure S2 in the SI). Thus, we explored the use of conventional oxidizers, namely H2O, H2O2 and ozone, to test whether a SCVD growth mode leading to silicon dioxide deposition could be obtained using TVMS. In this case, no film growth was observed neither on glass nor on both standard and plasmatreated silicon substrates, thus confirming again the limited reactivity of TVMS against these co-reactants at atmospheric pressure and low temperatures (< 260 °C). In view of these results, the use of plasma assistance was explored, since a variety of highly reactive radicals are formed, among which atomic oxygen (O*) can be used as an efficient oxidizing agent to react with TVMS. Figure 1a shows a 3D schematic drawing of the APE-SCVD reactor used, which was fabricated by 3D printing (see inset). Taking advantage of the possibilities offered by 3D printing, we could design both the oxygen plasma generator and the TVMS gas delivering outlet in a single one-block head. Figure 1b illustrates the result of a 2D simulation performed using Comsol Multiphysics (see more details in part 4 of the SI). It shows the mixture of TVMS and oxygen plasma in the gas phase above the substrate surface as a result of an increased deposition gap of 500 µm (an oscillation speed of approximately 3 cm/s was considered). The colored zone shown in Figure 1b represents the concentration of the TVMS/oxygen plasma mixture in the gas phase at room temperature. The so-called CVD reaction zone corresponding to the volume of mixture can actually be simply monitored by adjusting the injection head-substrate gap. Similar to SALD, the thin film deposition is then obtained by exposing this CVD reaction zone to the substrate that moves underneath the precursor injector.

Figure 1c shows a tremendous increase in SiO2 film thickness from 22 nm to 152 nm (for the same deposition duration) when the high voltage applied to the DBD varies from 7.75 kV to 10 kV, while all other experimental parameters such as carrier gas flow of TVMS (100 sccm), substrate temperature (180 °C) or the substrate/gas injector gap (150 µm) are maintained constant. This can be taken as an indication that the film growth rate is proportional to the concentration of atomic oxygen generated. This assertion is supported by OES measurements shown in Figure 1d, in which an increase in the intensity of the atomic oxygen characteristic triplet (777.19 nm, 777.42 nm and 777.54 nm) [START_REF] Milosavljević | Influence of Plasma Chemistry on Oxygen Triplets[END_REF] with the applied high voltage is clearly observed. It should be also noted that, even though the oxygen plasma is generated in open-air conditions, no emission from nitrogen species were observed in the plasma when pure oxygen was flown through the DBD (200 sccm). This proves that the DBD integrated 3D printing gas injector used in this work is very efficient to prevent nitrogen contamination in the plasma (further details can be found in Figure S3a in the SI). Consistently, FTIR and XPS spectra of a 45 nm thick SiO2 sample (grown by APE-SCVD at 75 °C using 10 kV and 8 kHz for plasma generation) do not show any trace of silicon nitride or carbide, as shown in Figure 1e-f. Si-O-Si peaks only can be detected, which are located at 1062 and 1130 cm -1 in the FTIR spectrum, and at 103.4 eV in the XPS one (SiNx peaks are located in the 900-950 cm -1 FTIR spectral region, and at 101.8 eV in XPS spectra, [START_REF] Diéguez | Optical Gratings Coated with Thin Si3N4 Layer for Efficient Immunosensing by Optical Waveguide Lightmode Spectroscopy[END_REF] while SiCx peaks are at about 760 cm -1 and 100.5 eV, respectively, in FTIR and XPS spectra). [START_REF] Sun | High Temperature Gas Sensing Performances of Silicon Carbide Nanosheets with an n-p Conductivity Transition[END_REF] This remarkable observation demonstrates the possibility of using APE-SCVD and TVMS as silicon precursor to produce SiO2 at low temperature in ambient condition, leading to thin films with low carbon and nitrogen contaminations. In view of this result, it is expected that the deposition of nitride films such as SiNx could be achieved using open-air spatial approaches with a nitrogen DBD plasma. In the plasma activated spatial CVD mode used here, a separation effect between the precursor and plasma outlets should be expected, given that plasma species have a short lifetime. Figure 2 shows three schematic designs of 3 different deposition heads with varying distance values between the oxygen plasma and the TVMS (2 mm, 6 mm and 12 mm) output channels. In the third one, an exhaust channel was also present between the TVMS and plasma outlets, as is the case in typical SALD heads. Figure 3 shows the XRR patterns of SiO2 films deposited under identical conditions of gas flows and temperature (75 °C) using these different heads. Using the first two heads results in thin films with rather similar thickness, 47.4 ± 0.5 nm and 38.5 ± 0.5 nm, respectively. Despite yielding the thickest films, the first head design leads to rough films (0.40 ± 0.25 nm of roughness), as indicated by the absence of oscillation in the pattern. This can be explained by the fact that the concentration of oxygen atoms right at the two outlets is high, with the TVMS outlet located right next to them, making the CVD reaction very efficient. The second head design yielded films with a smoother surface (0.17 ± 0.1 nm of roughness), as indicated by the clear oscillations in the XRR pattern. Finally, the third design with a large distance between the TMVS and the oxygen plasma outlets, along with the addition of an exhaust channel, resulted in very thin SiO2 film (5 ± 0.2 nm). This can actually be explained by the fact that the concentration of atomic oxygen decreases when travelling from the oxygen plasma outlets to the TMVS one. For this work, only the samples shown in the part concerning the effect of DBD voltage were deposited using the first injector design, while all other samples were deposited using the second one. In conventional CVD, a higher deposition temperature usually leads to a higher growth rate due to a more efficient decomposition of CVD precursors. However, in our case the film thickness significantly decreases when slightly increasing the deposition temperature (see Figure 4). Indeed, the decay processes of atomic the recombination coefficient for oxygen atoms and substrate temperature has been experimentally addressed in several works, for instance Greaves & Linnette, [START_REF] Greaves | Recombination of Atoms at Surfaces. Part 6.-Recombination of Oxygen Atoms on Silica from 20°C to 600°C[END_REF] and Brovikova. [START_REF] Brovikova | The Kinetics of Production and Loss of O(3P) Oxygen Atoms in Air Plasma[END_REF] In these studies, an increase of the recombination coefficient for oxygen atoms was observed when the surface temperature varies from 20 °C to 150 °C. Therefore, a higher substrate temperature would lead to a decrease in the oxygen atom concentration, which agrees with the temperature-dependent growth rate of SiO2 films observed in our case.

In addition to the growth rate, the substrate temperature also has an impact on the chemical composition of the deposited films. Figure 5a shows FTIR spectra of SiO2 films deposited with our APE-SCVD system at different substrate temperatures, varying from RT to 150 °C. Figure 5b shows a normalized close-up of these spectra from 720 cm -1 to 1600 cm -1 . Carbon contamination is usually found in SiO2 films prepared by CVD techniques, mainly originating from the organic nature of CVD precursors and uncompleted chemical reactions leading to SiO2 film formation. [START_REF] Kim | Characterization of Low-Dielectric-Constant SiOC Thin Films Deposited by PECVD for Interlayer Dielectrics of Multilevel Interconnection[END_REF][START_REF] Kim | Characteristics of Low-k SiOC(-H) Films Deposited at Various Substrate Temperature by PECVD Using DMDMS/O2 Precursor[END_REF][START_REF] Mori | Effects of Dilution Gas on Characteristics of SiOC(H) Films Synthesized by Atmospheric Pressure Plasma CVD[END_REF] But in our case, it is remarkable that neither Si-CHx nor Si-N groups are observed, even at deposition temperatures as low as RT and in open-air conditions. This means that all carbon species are efficiently removed during the APE-SCVD process thanks to the high reactivity of the close-proximity oxygen plasma. For the film deposited at RT, small C-H and C=O peaks are observed, which can be attributed to residues of TVMS or by-products from the reaction between atomic oxygen and TVMS. Moreover, when deposition is performed at low temperatures, Si-OH group formation and water absorption from the atmosphere may also increase the intensity of Si-OH and H-OH peaks located at wavenumbers around 928 cm -1 and 3400 cm -1 . While temperature clearly affects the quality of the deposited films, the plasma power had no sensible effect on FTIR spectra (see more details in Figure S3c-d of the SI). A closer analysis in the [720 cm -1 , 1600 cm -1 ] region containing the main Si-O-Si peaks is shown in Figure 5b and Figure 5c. The Si-O-Si transverse optical asymmetric stretch with adjacent O atoms in phase and with adjacent O atoms out of phase generates the two main peaks AS1 and AS2, respectively. The ratio of these peaks can be used as an index of the density and stoichiometry of the SiO2 films. Indeed, a lower AS2/AS1 ratio corresponds to a higher quality of SiO2 films (denser and stoichiometry closer to 1:2). Figure 5d shows a comparison of the ratio of our APE-SCVD samples deposited at various substrate temperatures with the ratios calculated for samples prepared by other techniques. [START_REF] Park | Microstructure and Interfacial States of Silicon Dioxide Film Grown by Low Temperature Remote Plasma Enhanced Chemical Vapor Deposition[END_REF][START_REF] Bange | Growth and Characterization of SiO2 Films Deposited by Flame Hydrolysis Deposition System for Photonic Device Application[END_REF] Firstly, the AS2/AS1 ratio decreases from 0.56 to 0.31 when the deposition temperature increases from RT to 150 °C.

The value of the AS2/AS1 ratio of thermal SiO2 is 0.16, while it is estimated to be about 0.35, 0.33 for samples prepared by PECVD, [START_REF] Park | Microstructure and Interfacial States of Silicon Dioxide Film Grown by Low Temperature Remote Plasma Enhanced Chemical Vapor Deposition[END_REF] and flame hydrolysis deposition [START_REF] Bange | Growth and Characterization of SiO2 Films Deposited by Flame Hydrolysis Deposition System for Photonic Device Application[END_REF] indeed a small area localized within the gap between the gas injector and the heated substrate. Thanks to an appropriate design of the gas injector, and by a careful adjustment of the distance between the injector and the substrate, as well as the other deposition parameters, the size of this 'virtual CVD chamber' can be easily monitored. Figure 6a and Figure 6b show simulation images of the CVD reaction zone obtained using 2D Comsol Multiphysics for two values of injectorsubstrate gaps of 150 µm and 500 µm, respectively. As can be observed, the mixture volume of TVMS and atomic oxygen is clearly larger and more efficient, as evidenced by a deeper color intensity, at larger substrate-injector gap (500 µm), which means that for a constant precursor concentration at the inlet, a higher TVMS-to-SiO2 conversion efficiency should be found. Interestingly, SEM micrographs of SiO2 deposited on glass substrates using these two gap values show a clear difference in the deposited film thickness, i.e. 145 nm versus 290 nm, as shown in Figure 6c and Figure 6d. In contrast, although the higher gap value roughly yields twice as large as growth per cycle, the obtained films are rather rough with visible co-deposited particles. A similar situation was also observed when high-aspect-ratio features, such as patterned silicon substrates, were coated with SiO2 thin layers, as shown in Figure 6e and Figure 6f. Accordingly, using the higher substrate-injector gap, i.e. 500 µm, yields a 87 nm-thick coating layer, along with the presence of particles, while using the closer gap allows depositing a highly conformal, 45 nm-thick layer without any visible co-deposited nanoparticles. In both cases, the deposition is highly conformal. This result can be attributed to the large vertical distribution of the TVMS/atomic oxygen mixture in the case of larger which leads nanoparticle formation in the gas phase that may then be deposited over the For this reason, a thinner CVD mixture zone, or in other words, a smaller gap between the gas injector and the substrate, should generate a better film quality. Nevertheless, one can adjust the deposition parameters in the APE-SCVD system as a function of the targeted application, to produce highquality, smooth films or higher coating rates when roughness is not an issue.

In summary, we have demonstrated in this work the possibility of growing SiO2 thin films at low temperatures (RT -180 °C), in open-air conditions using the atmospheric plasma-enhanced spatial chemical vapor deposition (APE-SCVD) technique. Taking advantage of the 3D printing technology, both precursor injector and the plasma generator were integrated into a single head, which is then placed at very close distance (150 -500 µm) to the substrate. This provides a very efficient supply of highly reactive chemical species (atomic oxygen in this case) to activate the CVD reaction on the substrate surface. We have shown that SiO2 deposition is not possible at temperatures up to 260 °C using TVMS as silicon precursor and H2O, H2O2 or O3 as co-reactant. However, SiO2 deposition was successful when combining TVMS with an oxygen plasma containing highly reactive atomic oxygen. Our experimental results show that compact, conformal, pinhole-free SiO2 thin films can be deposited with a high growth rate of 2 up to 5 nm/min on different substrates including silicon, glass and polymeric substrates. The APE-SCVD technique leads to SiO2 thin films with non-detectable SiNx or SiCx contamination levels, as confirmed by XPS or FTIR measurements, despite the use of a metalorganic precursor and low processing temperatures.

Our findings open up a new approach for the deposition of functional thin films with a larger choice of available precursors and lower input thermal energy budget.

Supporting Information.

* Corresponding authors: huong.nguyenviet@phenikaa-uni.edu.vn david.munuz-rojas@grenoble-inp.fr

Figure 1 .

 1 Figure 1. a) 3D schematic view of the APE-SCVD injection head (inset: a photograph of the 3D printed APE-SCVD head used in this work); b) a 2D simulation obtained by Comsol Multiphysics,showing the mixture of TVMS and Oxygen plasma in the gas phase just above the substrate. The substrate/injector gap was maintained at 500 µm in this simulation,[START_REF] Masse De La Huerta | Influence of the Geometric Parameters on the Deposition Mode in Spatial Atomic Layer Deposition: A Novel Approach to Area-Selective Deposition[END_REF] and the color bar corresponds to the zone where both TVMS and atomic oxygen are present; c) Variation of SiO2 film thickness as a function of high voltage applied to the DBD plasma source; d) Intensity variation of the oxygen atomic desexcitation peaks at 777 nm for different applied high voltages ranging from 7.8 kV to 9.5 kV; e) FTIR spectra of a 45 nm thick SiO2 film deposited on a silicon substrate by APE-SCVD at 75 °C; f) Si 2p core level XPS spectrum of 45 nm thick APE-SCVD SiO2 film deposited at 75 °C.

Figure 2 .

 2 Figure 2. Three designs of APE-SCVD head with various distances between the oxygen plasma and TVMS output channel: 2 mm (left), 6 mm (middle) and 12 mm (right). An exhaust channel was added to the 3 rd case to further reduce the CVD mixture volume.

Figure 3 .

 3 Figure 3. XRR patterns of SiO2 samples prepared by APE-SCVD at 75 °C using different designs. The distance between the head and the substrate was maintained at 150 µm.

Figure 4 .

 4 Figure3shows the XRR patterns of SiO2 films deposited under identical conditions of gas flows and temperature (75 °C) using these different heads. Using the first two heads results in thin films with rather similar thickness, 47.4 ± 0.5 nm and 38.5 ± 0.5 nm, respectively. Despite yielding the thickest films, the first head design leads to rough films (0.40 ± 0.25 nm of roughness), as indicated by the absence of oscillation in the pattern. This can be explained by the fact that the concentration of oxygen atoms right at the two outlets is high, with the TVMS outlet located right next to them, making the CVD reaction very efficient. The second head design yielded films with a smoother surface (0.17 ± 0.1 nm of roughness), as indicated by the clear oscillations in the XRR pattern. Finally, the third design with a large distance between the TMVS and the oxygen plasma outlets, along with the addition of an exhaust channel, resulted in very thin SiO2 film (5 ± 0.2 nm). This can actually be explained by the fact that the concentration of atomic oxygen decreases when travelling from the oxygen plasma outlets to the TMVS one. For this work, only the samples shown in the part concerning the effect of DBD voltage were deposited using the first injector design, while all other samples were deposited using the second one. In conventional CVD, a higher deposition temperature usually leads to a higher growth rate due to a more efficient decomposition of CVD precursors. However, in our case the film thickness significantly decreases when slightly increasing the deposition temperature (see Figure4). Indeed, the decay processes of atomic oxygen such as O + O + N2 → O2 +N2 or O + O + surface → O2 are temperature-dependent. The relationship between

Figure 5 .

 5 Figure 5. a) FTIR spectra of SiO2 samples prepared at various substrate temperatures, b-c) a zoom into the main Si-O peaks of the normalized spectra shown in Figure 5a, and an example of fitting the main peaks with different components: Si-O-Si transverse optical asymmetric stretch with adjacent O atoms in phase (AS1), and with adjacent O atoms out of phase (AS2), d) comparison of the AS2/AS1 ratio of APE-SCVD samples with SiO2 samples prepared by other techniques. 60,41 SiCl4 was used for SiO2 deposition by PECVD and Flame Hydrolysis Deposition.

Figure 6 .

 6 Figure 6. a) and b): simulation images of the CVD reaction zone obtained from 2D Comsol Multiphysics for two values of the injector/substrate gap (150 µm and 500 µm), the color intensity indicates the concentration of TVMS and atomic oxygen co-existing in the gap; c) and d): SEM micrographs of SiO2 thin film deposited on glass substrates using two values of the gap, 150 µm and 500 µm, respectively. Samples were prepared at 50 °C; e) and f) SEM micrographs of SiO2 thin coating deposited on patterned silicon substrates using two values of the gap, 150 µm and 500 µm, respectively. Samples were prepared at 75 °C.

The authors declare no competing financial interest. This work was funded by the Agence Nationale de Recherche (ANR, France) via the programs ANR-16-CE05-0021 (DESPATCH), ANR-17-CE05-0034 (OXYGENE), and the "Investissements d'avenir" program (ANR-15-IDEX-02) through the project Eco-SESA. D.M.-R. also acknowledges support from the European Union's Horizon 2020 FETOPEN-1-2016-2017 research and innovation program under grant agreement 801464. Funding from the University Grenoble Alpes is acknowledged through the AGIR POLE 2016 program under the grant SURPASS. This work was benefited from funding from the Consejo Nacional de Ciencia y Tecnología (CONACYT) from Mexico (No. 456558). The authors acknowledge the Consortium des Moyens Technologiques Communs (CMTC)-Grenoble INP platform-for the characterization equipment.