Ines Gharbi

Viviana Palacio-Betancur

Habib Ayeb

Dominique Demaille

Juan J De Pablo

Randall D Kamien

Emmanuelle Lacaze

Liquid Crystal Films as Active Substrates for Nanoparticle Control

Keywords: liquid crystal, gold nanoparticle, nanoparticle control, plasmon extinction, spectrophotometry, AFM, SEM

Liquid crystals (LCs) are the original nanomaterials: subjugated by surface forces, electromagnetic fields, and flow they can be organized, actuated, and switched while maintaining sub-micron order.

 However, forming NP-LC composites through defect engineering is a challenging task since not only does it require the formation of well-defined topological defect arrays, but also it is limited to NP sizes small enough to match the topological defect core sizes and surface chemistries that stabilize the defect morphology of interest. Efforts to use a LC matrix with a low density of defects

ABSTRACT:

Organizing nanoparticles in a controlled way allows us to monitor their optical properties, in particular if it concerns metallic nanoparticles. It is particularly interesting to organize them on top of liquid crystal films to take advantage, in a second step, of the easy actuation of liquid crystals with external parameters like temperature, electric fields, etc…We show that despite their fluidity, nematic and smectic films allow formation of well-ordered hexagonal domains of gold spherical nanoparticles (AuNPs) at their surface but we also show that both nematic films and AuNP domains impact each other. Using Optical Microscopy, Atomic Force Microscopy (AFM), Scanning Electron microscopy (SEM), and Spectrophotometry, we compare nematic, polymer stabilized nematic and smectic films with AuNP domains made of NPs of diameter 6nm. On The liquid crystal films depressions are revealed below the AuNP domains whereas the AuNP domains appear well-organized but with a hexagonal period shortened with respect to AuNP monolayers formed on hard substrates. We interpret these features by the anchoring tilt imposed by the AuNP domains on the molecules. The smectic-A layers characteristic of the nematic surface transform into smectic-C layers which induce formation of depression. The energy penalty associated with the local smectic-A/smectic-C transition induces the shortening of the AuNP domain period in order to decrease the AuNP domain surface. The observed large depth of the polymer stabilized nematic depressions below AuNP domains may be explained either by an increased size of the polymer stabilized smectic layers close to the surface or by an increased number of polymer stabilized smectic liquid crystal smectic layers close to the surface with respect to pure nematic films.

as a substrate for NP self-assembly at the LC surface without the formation of uncontrolled clusters is therefore another interesting challenge. It has been reported that a cholesteric LC film, with a molecular pitch of the order of microns, exhibits a fingerprint texture which guides the NPs into ribbon-like assemblies, revealing the presence of more favourable areas at the surface of these modulated LC matrix. [START_REF] Do | Interactions Between Topological Defects and Nanoparticles[END_REF][START_REF] Mitov | Long-Range Structuring of Nanoparticles by Mimicry of a Cholesteric Liquid Crystal[END_REF][START_REF] Mitov | Fingerprint Patterning of Solid Nanoparticles Embedded in a Cholesteric Liquid Crystal[END_REF][START_REF] Bitar | Cholesteric Liquid Crystal Self-Organization of Gold Nanoparticles[END_REF][START_REF] Tran | Shaping Nanoparticle Fingerprints at the Interface of Cholesteric Droplets[END_REF] However a deep study of the local NP structure that drives the NP optical properties, in particular the light absorption when plasmonic NPs are concerned, and its relationship with the LC matrix specificity is still lacking.

In this work, we demonstrate the stabilization of large collections of gold nanospheres (AuNP) domains on top of defect-free 5CB nematic and polymer stabilized 5CB nematic films. These domains exhibit hexagonal packing, similar to ordered domains of particles on hard substrates.

The presence of NP domains alters the topography of the free-standing 5CB film, while the inter-AuNP distance between AuNPs is shortened and thus red-shifts the plasmonic wavelength associated with AuNP light absorption. We show that similar phenomena are induced at the surface of free-standing 8CB smectic-A films and interpret these mutual modifications as arising from the AuNP-induced variation of anchoring at the interface.

RESULTS

Optical Microscopy

Observations. 5CB Nematic films (see "Materials and Methods" -Figure 7a) and polymer stabilized 5CB nematic films (see "Materials and Methods" -Figure 7b-c) on rubbed polyvinyl alcohol (PVA) substrates were prepared according to "Materials and Methods" section -Figure 7. We will call in the following the polymer stabilized 5CB nematic films by PSNLC for Polymer Stabilized Nematic Liquid Crystal ("Materials and Methods" -Figure 7c) and the 5CB nematic films by NLC for Nematic Liquid Crystal. The nematic texture of PSNLC and of NLC, with and without gold spherical nanoparticles (AuNPs -see "Materials and Methods" -Figure 7d), was first investigated by optical microscopy between crossed polarizers (POM) in transmission. The textures of the NLC and of the PSNLC appear similar (comparison between Figure 1a and 1b and between Figure 1b and SI1). For stability purposes, the results presented here are from PSNLC films unless indicated otherwise (Figure 1).

Profilometer measurements show that the thinnest section of the film is at the border, with a thickness of 6 µm, and the thickest region is at the center, with a thickness of 18 µm. These values correspond to the color of the optical microscopy pictures between crossed polarizers observed in Figure 1a for NLC film, assuming that a hybrid film (planar anchoring on the rubbed PVA substrate and homeotropic anchoring at air) of LC has an effective birefringence of 𝛥𝑛 ̅ 𝐿𝐶 = 𝑛 𝑒𝑓𝑓 -𝑛 0 with no, the ordinary LC index and 𝑛 𝑒𝑓𝑓 , the effective extraordinary index determined by the following equation [START_REF] Miller | Introduction to Optical Methods for Characterizing Liquid Crystals at Interfaces[END_REF] .

𝑛 𝑒𝑓𝑓 = 1 𝑒 ∫ 𝑛 𝑜 𝑛 𝑒 √𝑛 𝑒 2 𝑐𝑜𝑠 2 𝜃(𝑧)+𝑛 𝑜 2 𝑠𝑖𝑛 2 𝜃(𝑧) 𝑑𝑧 𝑒 0
(1) with e the overall thickness, (z) the molecular orientation throughout the LC film (the director profile), ne and no the extraordinary and ordinary 5CB indexes.

For homeotropic anchoring at air, using no = 1.537 and ne = 1.735, it can be calculated that 𝛥𝑛 ̅ 𝐿𝐶 is of the order of 0.1. [START_REF] Lelidis | A New Optical Method to Measure Angular Tilts for Planar Anchored Nematic Liquid Crystals[END_REF] It decreased by 0.09 with respect to the nominal 5CB birefringence (ne -no) = 0.19. The birefringence of monomer RM257 (with no = 1.508 and ne = 1.687) [START_REF] Mohammadimasoudi | Thin Film Polarizer and Color Filter Based on Photo-Polymerizable Nematic Liquid Crystal[END_REF] and that of 5CB are of the same order and we don't expect the polymer birefringence to significantly modify the PSNLC one.

On top of NLC, droplets of gold spherical nanoparticles (AuNPs of diameter 6 nm and covered by dodecanethiol ligands -see "Materials and Methods" -Figure 7d) in toluene have been deposited. For PSNLC with AuNPs the polymerization occurred after AuNP deposition. Large filaments or cluster structures are observed as shown in Figure 1b (for AuNP structures on NLC film, see Figure S1). The difference between figures 1a and 1b suggests that these domains are composed of AuNPs and the change in color indicates that their presence modifies the structure of the LC film underneath.

Figures 1c-d show a magnified area of the PSNLC-AuNP polymerized films, where the areas with NPs appear blue-shifted. For example, the free nematic in red changes to yellow, and the free nematic in yellow changes to blue. This blue-shift is systematic but can be more or less visible depending on the LC thickness and most probably also on the size and density of the NP domains.

We might attribute this blue-shift to two main phenomena: the domains of AuNPs form depressed regions on the PSNLC film altering its thickness, or the AuNPs impose a change of birefringence in the film. The observation of a large and well-visible blue shift of POM colors in the presence of AuNPs would imply non-negligible thickness changes with respect to the overall film thickness such that the second phenomenon is more likely to occur. In order to study the localization and organization of AuNPs in the domains identified by POM and understand the origin of the local birefringence changes, we have studied PSNLC films with AuNPs using Atomic Force Microscopy (AFM). The birefringence change can be estimated according to the following formula:

𝛥𝑛 ̅ = 𝑘 ∆𝜆 𝑒 (2)
where 𝛥𝑛 ̅ is the variation of birefringence with respect to 𝛥𝑛 ̅ 𝐿𝐶 , k is the optical order, Δλ is the difference in wavelength between the zones with and without AuNPs identified in the POM pictures and e is the overall thickness. For the region between yellow and blue (Figure 1d), Δλ of the order of 100 nm and k = 2. Solving formula (2) for 𝛥𝑛 ̅, the change in birefringence should be about 0.02. We calculate 𝑛 𝑒𝑓𝑓 as a function of the molecular orientation throughout the LC film.

Firstly, we assume that the director profile (the average LC molecule orientation) follows the analytical solution for a hybrid film described in eq. ( 1). In such a case a numerical solution for eq. ( 1) yields an angle θ=17° if Δn is increased by 0.02 with respect to homeotropic anchoring at the surface. Secondly, if we assume that the texture in the areas with nanoparticles matches the adjacent texture in the areas without nanoparticles from the PVA (θ=90°) up until some tilt angle θ2 and then remains at that constant angle all the way to the AuNP interfaces, we find that θ2=39°

for an increase 𝛥𝑛 of 0.02. This appears to be the maximum LC tilt below the AuNP domains consistent with the observed birefringence. Such tilted anchoring appears consistent with previous results of the literature, since the dodecanethiol ligands grafted on an Au surface can induce tilted anchoring for short enough alkyl chains. We can now ask the reverse question: if and how the AuNPs themselves are affected by the presence of the nematic film below. We have thus studied PSNLC films with AuNPs by SEM (Figure 3) on a slightly more concentrated area than the one studied by AFM. The AuNP domains (in white on Figure 3) are indeed more interconnected than on Figure 2. In contrast with AFM measurements, a zoom on the islands (from Figure 3a to Figure 3c) easily displays the AuNP structure associated with formation of hexagonal networks. Surprisingly, we observed a very well-organized monolayer on the nematic film, characterized by a hexagonal organization similar to the one that can be obtained on a hard substrate (Supporting Figure S2 for comparison). The strong contrast associated with the AuNPs is consistent with the absence of nematic LC above the AuNPs, indeed already suggested by AFM results.

Localized Surface Plasmon Extinction. In order to understand if the AuNP organization is similar to that on a hard substrate, we have compared the localized surface plasmon (LSP) extinction of AuNPs directly deposited on top of the rubbed PVA substrate with that of the AuNPs organized in domains at the PSNLC surface (Figure 4). Figure 4 shows that for both systems a large red-shift of the LSP resonance wavelength is observed with respect to isolated AuNPs in solution (Supporting Figure S3). A similar red-shift is measured with NLC films with AuNPs (Supporting Figure S4). This shift reflects the electromagnetic coupling between NPs that occurs when they are packed within a hexagonal lattice. We have measured the LSP extinction for incident light parallel and perpendicular to the rubbing of PVA. Consistent with the symmetry of the hexagonal packing, we find no difference in the measurements and thus we only show nonpolarized data in Figure 4.

A striking difference between the two data sets in Figure 4 is that the red-shift is significantly larger on top of the nematic film (69 nm) as compared to the case on hard substrate without the LC (41 nm). It is known that for these NPs covered by ligands the optical index responsible for the LSP resonance wavelength is mostly imposed by the ligands. [START_REF] Do | Interactions Between Topological Defects and Nanoparticles[END_REF][START_REF] Mulvaney | Surface Plasmon Spectroscopy of Nanosized Metal Particles[END_REF] Thus the fact that ne = 1.735 is significantly larger than the optical index of PVA (n=1.5) compared to no=1.537 is not at the origin of the observed larger red-shift for AuNPs at NLC surface. This is indeed confirmed by the

DISCUSSION

When the AuNP in toluene solution droplet is deposited on the nematic there is an almost complete dissolution of the nematic film. As the toluene evaporates, the nematic film reforms, leading to the AuNP localization at the air interface. This suggests that in absence of topological defects in the LC film, localization of AuNPs at the LC film surface may be the equilibrium state of the composite nematic film-AuNPs. Similar observations have been obtained for cholesteric films formed in the presence of platinum or gold nanoparticles, [START_REF] Mitov | Long-Range Structuring of Nanoparticles by Mimicry of a Cholesteric Liquid Crystal[END_REF][START_REF] Bitar | Cholesteric Liquid Crystal Self-Organization of Gold Nanoparticles[END_REF] but, in this latter case, competing with formation of aggregates in the bulk. [START_REF] Mitov | Fingerprint Patterning of Solid Nanoparticles Embedded in a Cholesteric Liquid Crystal[END_REF] Neither, in the NLC nor in the PSNLC film, aggregate is formed. This could be related to the preparation of the composites via the deposition of a droplet at the surface that somehow may favor localization at the air interface. In comparison with the cholesteric film, there is no modulation on the nematic surface. As a result no stripes of AuNPs are observed. [START_REF] Mitov | Long-Range Structuring of Nanoparticles by Mimicry of a Cholesteric Liquid Crystal[END_REF][START_REF] Bitar | Cholesteric Liquid Crystal Self-Organization of Gold Nanoparticles[END_REF] Instead the AuNPs organize in a process that looks similar to the one leading to the well-known organization on top of a solid substrate after solvent evaporation (Supporting Figure S2). The filaments made of islands of ordered hexagonal AuNP networks revealed by combined POM, AFM and SEM may be connected to the end of solvent evaporation at the LC film surface. The islands are the signature of the last solvent droplets that bring AuNPs close to each other before full solvent evaporation. The observed NP organization is the usual result of inter-AuNP Van der Waals interactions that favor hexagonal organization of nanospheres.

However spectrophotometric data demonstrate that on PSNLC (Figure 4) and on NLC (Supporting Figure S4) the period of the hexagonal network is shortened with respect to the one on a hard, rubbed PVA surface (without a LC). This suggests that an additional interaction takes place together with the inter-NP Van der Waals interactions. The inter-NP spacing in the hexagonal network has been measured on rubbed PVA surface (without a LC) by combined spectrophotometric measurements and synchrotron Grazing Incidence X-ray Scattering (GISAXS). It is equal to 1.9 nm [START_REF] Do | Interactions Between Topological Defects and Nanoparticles[END_REF][START_REF] Do | From Chains to Monolayers: Nanoparticle Assembly Driven by Smectic Topological Defects[END_REF][START_REF] Goubet | Which Forces Control Supracrystal Nucleation in Organic Media?[END_REF][START_REF] Gauvin | Mechanical Properties of Au Supracrystals Tuned by Flexible Ligand Interactions[END_REF] . If we analyze the red-shift of the plasmonic absorption of Figure 4 using the dipolar approximation (see "Materials and methods"), we find that the inter-NP gap decreases from 1.9 nm without LC [START_REF] Do | Interactions Between Topological Defects and Nanoparticles[END_REF][START_REF] Do | From Chains to Monolayers: Nanoparticle Assembly Driven by Smectic Topological Defects[END_REF][START_REF] Goubet | Which Forces Control Supracrystal Nucleation in Organic Media?[END_REF][START_REF] Gauvin | Mechanical Properties of Au Supracrystals Tuned by Flexible Ligand Interactions[END_REF] to 1.1nm on top of PSNLC, leading to a shortening of 0.8 nm. The additional interaction thus leads to quite a large shortening of the inter-AuNP spacing that requires high distortion and interdigitation of the dodecanethiol ligands that have an extended length of 1.8 nm. The nematic material allows for the formation of a well-organized AuNP network at the surface, however with a significantly enhanced electromagnetic coupling between AuNPs, able to significantly modify their optical properties. It is important to understand the origin of such shortening of lattice spacing. It may be related to the formation of the depression in the LC film below the AuNP domains. Since the observed shortening of the AuNP lattice spacing occurs independently of the polymerization in the nematic material (we find a similar redshift for plasmonic resonance with and without polymerization), this would imply that polymerization is not directly responsible for the establishment of the depressions. In order to check it and to understand the origin of the observed depressions, we have studied AuNP aggregation on smectic films without polymer.

Whereas it is very difficult to obtain reliable SEM or AFM pictures using nematic films without polymer, it is nonetheless possible to perform AFM measurements on smectic films. [START_REF] Michel | Optical Gratings Formed in Thin Smectic Films Frustrated on a Single Crystalline Substrate[END_REF] We prepared 8CB smectic films within TEM copper grids inducing homeotropic texture (see "Materials and Methods"), thus avoiding distortion and the creation of defects. [START_REF] Brake | An Experimental System for Imaging the Reversible Adsorption of Amphiphiles at Aqueous-Liquid Crystal Interfaces[END_REF] After deposition of the same AuNP droplets on top of the smectic films, a similar anchoring modification of 8CB induced by the AuNPs is observed as revealed by POM (Figure 5a-b). Moreover, we observe the same localization of AuNPs at the 8CB surface, leading to the formation of similar depressions, as revealed by AFM measurements (Figure 5c-e). These results obtained with smectic films without polymerization confirm that polymerization in PSNLC, even if it impacts the depth of the depression, is not directly responsible for the formation of these domains at the LC surface nor for the formation of depression below AuNP domains at the surface of nematic or smectic films. They also suggest that the domains could correspond to the real equilibrium state of the composite nematic/smectic film-AuNPs, in agreement with the observations that the smectic films with AuNP domains at the surface remain unchanged several months after their preparation. Finally the fact that similar depression features below the AuNPs are observed in smectic and nematic films suggests that the origin of the depressions below AuNPs is similar in both phases. In the smectic phase the depressions can be viewed as the result of the tilted anchoring imposed below the AuNP domains: to allow for tilted anchoring and flat smectic layers, parallel to a flat interface, a (local) transition from a smectic-A phase to a smectic-C phase is necessary. However, along with the tilt comes layer compression, as in the smectic-A to smectic-C transition. The accommodation of compressed smectic-C layers and neighboring non-compressed smectic-A ones thus creates a depression below the AuNP domains (see scheme of Figure 6). For an anchoring tilt of θ below AuNP domains and a depression depth of about h = 15 nm corresponding to the one observed on How does this bear on our nematic data for 5CB? It has been observed in both experiment and modelling that nematics can display surface "freezing" and form several layers of smectic phase near a homeotropic surface. [START_REF] Fukuto | Wetting of Liquid-Crystal Surfaces and Induced Smectic Layering at a Nematic-Liquid Interface: An x-Ray Reflectivity Study[END_REF][START_REF] Sadati | Molecular Structure of Canonical Liquid Crystal Interfaces[END_REF] Since merely changing the anchoring condition from homeotropic to a tilt whose value ranges between 17° and 39° could not modify the nematic density, we postulate that a surface smectic-A phase forms and that, upon contact with the AuNPs, becomes a smectic-C. By analogy with the true equilibrium smectic of 8CB, this should lead to the formation of a depression below AuNP domains. If we postulate that the polymerization has no influence on the depression feature, then the observed depression depth of 82 nm (the average value associated with more than 100 measurements) requires that the number of smectic-A/smectic-C layers to be very large, ranging from around 130 to 670, as shown by the N values obtained for an anchoring tilt of 39° and 17°, respectively, taking the 5CB smectic-A layer size close to the surface to be 2.6 nm. [START_REF] Fukuto | Wetting of Liquid-Crystal Surfaces and Induced Smectic Layering at a Nematic-Liquid Interface: An x-Ray Reflectivity Study[END_REF] This number is definitely too large when compared to previous results suggesting that 5CB has short-range smectic order with at most 3 to 4 layers near the air interface [START_REF] Fukuto | Wetting of Liquid-Crystal Surfaces and Induced Smectic Layering at a Nematic-Liquid Interface: An x-Ray Reflectivity Study[END_REF][START_REF] Sadati | Molecular Structure of Canonical Liquid Crystal Interfaces[END_REF] . This leads us to conclude that the polymerization must contribute to the observed large depression depth in PSNLC with AuNP monolayers. For instance, cybotactic clusters of polymers may co-organize with 5CB into layers close to the interface, with the polymer orientation being imposed by the 5CB thus inducing an increased layer size. [START_REF] Noh | Taming Liquid Crystal Self-Assembly: The Multifaceted Response of Nematic and Smectic Shells to Polymerization[END_REF] The increased size of the polymer stabilized smectic layers would induce a large depth of the depressions below the AuNP domains. The polymerization may also favor the surface "freezing" of the nematic structure into smectic one as previously shown in ref [START_REF] Noh | Taming Liquid Crystal Self-Assembly: The Multifaceted Response of Nematic and Smectic Shells to Polymerization[END_REF] leading to a large number of polymer stabilized smectic layers close to the interface. Such a phenomenon may be enhanced by the directional UV-irradiation leading to the well-known favored polymerization close to the air-AuNP interface. [START_REF] Kamal | A Liquid Crystal Polymer Based Single Layer Chemo-Responsive Actuator[END_REF] The smectic-C structure induced by the tilted anchoring below AuNP domains incurs an energy penalty. To decrease this energy as much as possible the area of the AuNP domains will shrink, leading to the observed shortening of the lattice spacing. We can compare this shortening to prior work on AuNPs trapped in smectic topological defects. For small AuNPs, of diameter D = 4 nm with the same dodecanthiol ligands (with a size smaller than the core of the defects) no shortening is observed [START_REF] Coursault | Tailoring Anisotropic Interactions between Soft Nanospheres Using Dense Arrays of Smectic Liquid Crystal Edge Dislocations[END_REF] . The AuNPs used here, of diameter D = 6 nm, have also been embedded in 2D smectic topological defects [START_REF] Do | Interactions Between Topological Defects and Nanoparticles[END_REF][START_REF] Do | From Chains to Monolayers: Nanoparticle Assembly Driven by Smectic Topological Defects[END_REF] . There, the 2D confinement induces formation of hexagonal networks similar to the one observed at the 5CB surface but instead embedded within the 2D defects. Shortening is also observed, suggesting that these AuNPs are larger than the defect core size in contrast with AuNPs of diameter D = 4 nm. The shortening is only 0.6 nm, of the same order yet smaller than that observed here on 5CB (with both PSNLC and NLC film). In the former case the advantage of being embedded within a topological defect core may be responsible of the smaller shortening. This however shows that the disorder induced by the AuNPs outside of the topological defect core has an energy per unit area of the same order of magnitude as the one associated with the putative smectic-C/smectic-A induced by the AuNPs.

These results finally show that the LC anchoring modifications induced by the AuNP domains allow to manipulate the AuNP domain lattice spacing and to enhance the coupling between AuNPs.

For metallic NPs this induces a significant red-shift of the plasmonic resonance known to be associated with an enhanced exaltation of the electromagnetic field between neighboring NPs under light irradiation. This will be particularly appropriate to use these composite surfaces as Surface Enhanced Raman Scattering (SERS) surfaces, for sensing applications. This effect could even be monitored using different ligands around AuNPs to further modify the LC anchoring below AuNP domains and thus still enhancing the coupling between NPs. It will be also interesting (Figure 7c), the samples were irradiated in a nitrogen atmosphere with UV light of 0.15 mW.cm -2 intensity at wavelength 365 nm for 20 min at room temperature. [START_REF] Dierking | Polymer Network-Stabilized Liquid Crystals[END_REF][START_REF] Gharbi | Persistence of Smectic-A Oily Streaks into the Nematic Phase by UV Irradiation of Reactive Mesogens[END_REF] Smectic LC Film Preparation with AuNPs. Homeotropic substrates were prepared by spincoating 50 μl of a 0.1 wt% DMOAP solution on clean glass and were baked at 100 °C for 1 hour.

Hexagonal Cu grids were placed on the substrate to ensure uniform thickness and that a uniform homeotropic smectic film is formed. [START_REF] Brake | An Experimental System for Imaging the Reversible Adsorption of Amphiphiles at Aqueous-Liquid Crystal Interfaces[END_REF] The LC, in this case 8CB, purchased from Sigma-Aldrich, was heated above its clearing temperature and deposited on the grids. The film cools down and at room temperature forms a smectic phase. Optical microscopy images were taken to confirm a homeotropic film was formed. To deposit AuNPs on the smectic film, a 10 μl solution of AuNPs and toluene is deposited 1 μl at a time, and enough time is given to allow the toluene to evaporate.

Samples Characterization:

POM: The manufactured films are visualized by a LEICA DMRX microscope, surmounted by a SONY CCD camera. A Xenon lamp illuminates the sample in white light, placed so that the rubbing direction is oriented at 45° from the polarizer and the analyzer. This position allows us to have the maximum intensity.

AFM: AFM images were obtained on PSNLC films and on smectic films with and without AuNPs using a Veeco model Nanoscope Dimension 3100 Atomic Force Microscope and a 500 kHz cantilever/stylus in tapping mode.

SEM: SEM images were obtained on PSNLC films with AuNPs using a ZEISS scanning electron microscope.

Spectrophotometry:

The extinction spectra for spherical isolated AuNPs in toluene was measured using a Cary 5000 UV-Vis-NIR. The experimental data were fitted with a Gaussian curve. Spectra for the AuNPs of 6 nm in diameter display a plasmon resonance of 516 nm.

A Maya 2000 Pro micro spectrophotometer coupled to the Leica optical microscope through optical fiber was used to measure the light absorption of PSNLC-AuNP and NLC samples. The sample was illuminated by a white light source, and areas of 150 µm x 150 µm are probed with a 50x objective lens. All spectra were taken at room temperature in the 190-1100 nm spectral range.

A reference made of PSNLC (or NLC) of similar thickness but without AuNP was used to extract the optical absorption of AuNPs only.

The distance between AuNPs was extracted from the extinction spectra using a dipole coupling model in the quasistatic approximation (NPs diameter D ≪ wavelength λ). [START_REF] Kreibig | On the Mutual Potential Energy of a Plane Network of Doublets[END_REF] In this approximation the multipolar interactions between AuNPs as well as the retardance effect are not considered, this latter assumption being obviously correct due the small size of AuNPs. In this case the resonance condition can be calculated using the expression of the effective polarizability of an infinite AuNP hexagonal monolayer: nm). Σ is the lattice sum associated with the AuNPs assembly, AuNPs being considered as periodic dipoles. For monolayers considered as an infinite hexagonal network, the sum Σ is Σ ≈ 5.5. 44,[START_REF] Van Der Hoff | A Method for the Evaluation of Some Lattice Sums Occurring in Calculations of Physical Properties of Crystals[END_REF] The AuNP dielectric function was recalculated based on Johnson and Christy data [START_REF] Johnson | Optical Constants of the Noble Metals[END_REF] to take into account the small size effect and the influence of the chemical interface, leading to broadening and blue-shifted localized surface plasmon resonance of the isolated NPs in toluene. [START_REF] Garcia | Surface Plasmon Resonance of Capped Au Nanoparticles[END_REF] εm is taken as equal to 2.28, previously shown to be mostly induced by the dodecanethiol ligands for these same

eff Σ / 4D + s           (3)

Figure 1 .

 1 Figure 1. POM images in transmission between crossed polarizers where the molecules are

Figure 2 .

 2 Figure 2. (a) AFM topographical and (b) phase data corresponding to the surface of a

28

 28

Figure 3 .

 3 Figure 3. SEM images of the PSNLC-AuNPs sample prepared in the same conditions as the

Figure 4 .

 4 Figure 4. LSP extinction of AuNPs on a hard

Figure 5 .

 5 Figure 5. Optical microscopy images: (a) in transmission with parallel polarizer revealing

Figure 5 ,

 5 Figure 5, the number of smectic layers involved in the smectic-C/smectic-A transition is larger

Figure 6 .

 6 Figure 6. Schematic presentation of the depression occurring below AuNP domains due to a

  to study the possible activation of plasmonic resonance as a function of temperature, the temperature being increased above the nematic/isotropic transition (35.2°C for 5CB) where the LC anchoring phenomenon is lost. Increase of temperature above the transition may eliminate the lattice spacing shortening. Temperature may thus become an external activator for control of the coupling between NPs in ordered NP networks when they are formed on top of nematic or smectic surfaces.CONCLUSIONIn conclusion, we have shown that nematic films can serve as substrates for the formation of wellordered hexagonal monolayers of gold nanoparticles (AuNPs) of diameter D = 6 nm. This has been shown by combining Optical Microscopy on Polymerized Stabilized 5CB (PSNLC) and pure 5CB (NLC) films, Atomic Force Microscopy (AFM), Scanning Electron microscopy (SEM) on PSNLC films, and Spectrophotometry on PSNLC films and NLC films. The same features have been shown on smectic surfaces by combining Optical Microscopy and AFM. However we also show how both AuNP networks and nematic films are mutually impacted by each other. The AuNP network period is shortened by the surrounding nematic film and depressions are created at the nematic surface, where the AuNP domains are formed. We show that it is related to a tilt of the nematic anchoring induced below the AuNP monolayers, ranging from 17° to 39°. In the smectic film the anchoring tilt transforms smectic-A layers into smectic-C layers. Depressions through layer-shrinkage may then become necessary to accommodate the neighboring smectic-A (without AuNPs) and smectic-C (with AuNP monolayers) areas. The same phenomenon occurs with nematic films in relation with the known surface-induced smectic-A structure of the nematic film at the interface transformed into a smectic-C one. The shortening of the AuNP network period may be induced by the necessary minimization of the surface of the smectic-C areas in relation to the energy cost of the local smectic-A/smectic-C transition. We find that when the nematic film is polymer stabilized the depression depth is increased either in relation with an increased smectic layer size or with an increased number of polymer stabilized smectic layers close to the surface with respect to NLC films. These results not only open the route for a control of the coupling between NPs at LC surface through the use of various ligands around NPs that would modify the LC anchoring underneath the AuNP domains, but also for activated control through temperature changes reaching the nematic/smectic transition temperature.MATERIALS AND METHODSSolid substrate preparation. PVA was spin-coated onto clean glass slides, baked for at least one hour at 120°C to polymerize, and rubbed unidirectionally with a commercial rubbing cloth to promote planar anchoring at the surface. This treatment is used on the silicon (SI) substrate to visualize the AuNPs without the LC, on glass slides for NLC and PSNLC samples, and on those coated with a thin transparent layer of Indium Thin Oxide (ITO) to visualize PSNLC samples by SEM.

Figure 7 .

 7 Figure 7. Chemical structures of (a) 5CB and (b) RM257. (c) Schematic presentation of the

  with (D/2) 3 (ε -εm) / (ε + 2εm) is the polarizability of a single AuNP, leading for the plasmonic resonance to the following equation: ε = εm (Σ + 8(s/D + 1)[START_REF] Choudhary | Advances in Gold Nanoparticle-Liquid Crystal Composites[END_REF] )/ (Σ -4(s/D + 1) 3 )[START_REF] Kreibig | On the Mutual Potential Energy of a Plane Network of Doublets[END_REF] (4) that relates ε the AuNP dielectric function, εm the dielectric function of the homogeneous surrounding, s the inter-AuNP gap in the hexagonal network and D the AuNP diameter (D = 6

  The Supporting Information is available free of charge on the ACS Publications website at DOI: Optical microscopy image of nematic liquid crystal film in the presence of AuNPs, SEM image of a self-organized AuNPs on unidirectionally rubbed PVA on a silicon substrate, LSP extinction of isolated AuNPs suspended in toluene and of AuNPs on nematic film (PDF)

We would like to thank Abdelhafidh Gharbi for precious comments and fruitful discussions, Christophe Blanc for useful help with the polymerization test, Denis Limagne for technical supports. Ines Gharbi was supported by the Tunisian Ministry for Higher Education, Research, and Technology and by the CNRS-UMR7588. Ines Gharbi's travel between Tunis and Paris was supported by a grant from Université de Tunis El Manar and more specifically from LR99ES16. Viviana Palacio-Betancur is thankful to the Fulbright commission in Colombia and Colciencias, as well as the Chateaubriand Fellowship program and the Eiffel Excellence Scholarship program. Randall D. Kamien was supported by NSF MRSEC Grant DMR-1720530 and by a Simons Investigator Grant from the Simons Foundation. We are grateful to Brigitte Pansu for providing Au nanoparticles and thank Soleil synchrotron and Doru Constantin for the measurement of the diameter and polydispersity in size of AuNPs using SAXS on the beamline Swing.

AUTHOR INFORMATION

Corresponding Author

*E-mail: emmanuelle.lacaze@insp.jussieu.fr

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT


Raphaëlle Barbier

Skander Ben Yahia

Pascal Le Masson

Benoit Weil

EXPANDING USAGES OF EARTH OBSERVATION DATA: A CO-DESIGN APPROACH TO GROW AN ECOSYSTEM OF EFFICIENT SERVICE DESIGNERS

Keywords: design, Earth Observation, value creation from data, open-data, data-based ecosystems

Earth Observation data has the potential to provide significant benefits to a large variety of socio-economic stakeholders. However, creating new usages of these data is particularly challenging as it requires connecting distant data and usages ecosystems. 'Co-designing' services based on Earth Observation data appears to be a promising path to overcome insufficiencies of 'open-data' strategies. However, in this challenging context, 'co-design' cannot be limited to the mere adjustment between user demands and data supply. Based on design theory, we propose a comprehensive framework for such a 'co-design' approach, aiming at growing an ecosystem of efficient service designers. It is experimented in the e-shape project. First results show that: (1) such co-design involves the implementation of a dynamic process of specific types of co-design actions, to unlock the different blocking points occurring in the growth of the ecosystem over time, (2) each co-design action aims at creating a 'resilient fit' between stakeholders.

INTRODUCTION

Earth Observation (EO) refers to the production of information about the planet and its environment, based on different types of instruments (satellites, in-situ sensors etc). Initially produced mainly for scientific goals, EO data are now made available to every economic actor, through 'open-data' policies. Socio-economic applications of this data seem to be diverse and promising, however, in practice, developing usages from EO data seems to be particularly challenging. Indeed, this effort could be schematically described as connecting very distant socio-economic ecosystems: the ecosystem of data and the various ecosystems of potential usages. These ecosystems are called "distant" as they do not share the same dynamics, time horizons (e.g. very long cycles to develop new instruments compared to short timeline of actions in the data usage context), performance logics and competencies (e.g. data processing might require very specific technical expertise while data usages might also require specific domain expertise).

In order to connect distant data and usages ecosystems, several approaches have been promoted and implemented by the EO community in the last decades. The first one consists in having each ecosystem bridging independently half the distance, through an 'open-data' strategy [START_REF] Borzacchiello | The impact on innovation of open access to spatial environmental information: a research strategy[END_REF]: on the one side, data are made available to everyone, on the other side, the different stakeholders take advantage of these resources by integrating them in their own usage context. Despite being necessary to broaden the usages of data, this approach has proved to be insufficient, as the stakeholders tend to have difficulty making use of EO data spontaneously.

This accounts for the current efforts of the EO community to operate a second approach that consists in connecting the distant ecosystems of data and usages by encouraging the development of operational services based on Earth Observation data, through specific 'co-design' activities. An important stream of literature documents the implementation of such an approach in the case of climate services (based on climate-related data, being a certain type of EO data). Co-design (also referred as 'co-production' or 'co-development' depending on the authors) mostly relates to the involvement of data users in order to adjust user demands and the supply of useful information [START_REF] Mcnie | Delivering Climate Services: Organizational Strategies and Approaches for Producing Useful Climate-Science Information[END_REF]. Without appropriate processes, this might lead to ad hoc small-scale and short-lived data-based services. However, despite being implemented in several projects through dedicated processes, recent research also underlines that what is understood by 'co-design' is not systematically discussed and formalized [START_REF] Goodess | Advancing climate services for the European renewable energy sector through capacity building and user engagement[END_REF].

Based on the research work carried out in e-shape, a project funded within the EU Horizon 2020 program [START_REF] Ranchin | and all e-shape partners, 'e-shape -EuroGEO Showcases: application powered by Europe contribution to EuroGEO and to the development of the EO industry[END_REF], this paper aims at proposing an analytical framework for codesign in the Earth Observation context, clarifying codesign ambitions and the operational tools that could effectively contribute to the expansion of EO data usages.

CO-DESIGN SEEN FROM THE DESIGN AND MANAGEMENT FIELD

Literature in design and in management gives interesting insights on different approaches of co-design and Paper submitted at IGARSS conference, Brussels, Belgium, Jul 2021 its evolution over time [START_REF] Dubois | From organizing for innovation to innovating for organization: how co-design fosters change in organizations[END_REF]. Co-design reported in the Earth observation field seems to mainly corresponds to a first approach consisting in building specific interactions between users and service designers in order to fit the developed service to user needs. This approach of co-design, as supply and demand adjustment, has largely developed since the years 2000s [START_REF] Steen | Co-Design as a Process of Joint Inquiry and Imagination[END_REF]. However, it is interesting to notice that co-design were previously used in completely different situations, aiming at addressing other blocking points of the development of products or services concerning actors other than the user:

• First, in the 70s, for the development of embedded systems [START_REF] Wolf | Hardware-software co-design of embedded systems[END_REF]: co-design referred to hardware and software integration, as the issue was to make different fields of expertise cooperate, a list of requirement being already defined • Later in the 90s, co-design referred to reshaping collaborations between buyers and suppliers, beyond usual price negotiation, to design new required components (e.g. in the automotive industry, new modules to increase comfort and reduce pollutant emission of cars) [START_REF] Spina | Factors influencing co-design adoption: drivers and internal consistency[END_REF].

These elements lead us to make the following proposition regarding co-design in the Earth Observation context: co-design objective could be described as growing an ecosystem of efficient service designers by unlocking the different blocking points in the development of EO-based services, going beyond adjusting supply and demand between data users and service designers. This paper proposes to test this proposition based on e-shape experience, and to address the two resulting questions:

1. How to describe the blocking points occurring in the development of EO-based services? 2. What types of tools would be needed to unlock these blocking points?

METHODOLOGY

Our research relies on the work carried out within the EU-funded e-shape project, bringing together a team of 54 experienced partners from academia, industry, institutional entities and user communities to develop 27 pilot applications, spanning 7 thematic areas (food security, health, renewable energy, biodiversity, water resources, disaster resilience and climate). Given the large number of partners, and the variety of application sectors, e-shape appears to be a particularly favorable context to have a comprehensive understanding of the issues faced in the EO field. Within a dedicated work package led by the authors of the paper, a co-design approach is being progressively designed and experimented with e-shape partners, based on recent advances in design theory [START_REF] Le Masson | Design Theory: Methods and Organization for Innovation[END_REF]. The following process has been set up to assess co-design needs for each pilot:

• Questionnaire sent to each pilot

• Answers used to classify the different blocking points faced by the pilots (also called "co-design needs") and make a first diagnosis for each pilot • Interview of one hour and a half with each pilot to validate the diagnosis of co-design needs

RESULTS

The outcomes of this analysis process are summarized in Table 1 for 22 pilots (anonymized), the analysis of the 5 remaining pilots being still to be validated.

Co-design going beyond supply-demand adjustment

A first noticeable result is the validation of the proposition according to which co-design needs are not restricted to adjusting supply and demand between data users and service designers. Indeed, this perspective only corresponds to a first type of co-design needs (referred as Type 1 in the Table 1), whereas three other types of codesign needs have also been identified. These four types of co-design needs correspond to four types of actions, each type corresponding to a certain blocking point requiring the design of the relationship with a specific actor (cf Table 2). For each type, the initial state, blocking point to be addressed and expected outcomes always include two dimensions:

1. A dimension related to the design of the service, described with two terms usefulness and usability, as commonly used in literature on climate services [START_REF] Lemos | Narrowing the climate information usability gap[END_REF]. Usefulness refers to the general potential seen by users, whereas usability refers to the effective integration in users' operations. Literature on climate services highlights that both aspects need to be addressed to successfully develop services, and that specific efforts are Table 1: Analysis outcomes of e-shape pilots especially needed to move from useful to usable information, i.e. narrowing the so-called "usability gap". 2. A dimension related to the design of a specific relationship. This second dimension is crucial as the development of a service cannot be done only through collective work phases but also requires separate work phases. Agreeing on cooperation modalities is therefore crucial to guarantee the continuation of alternate collective and separate work phases over time. In a way, 'co-design' has to put a strong emphasis on designing the 'co', and not only the service itself.

Co-design as a dynamic interplay of four types of actions to connect distant data and usages ecosystems

A second major result is the consideration of codesign as a dynamic interplay of these four types of actions. It is first an interplay of actions because at every moment, each service designer might be confronted with several codesign needs. For example, the service designer might at the same time need a co-design type 1 to strengthen the relationship with a certain user, but also consider a type 2 to explore a new type of user community, and prepare for the operationalization of the service through a type 3. This interplay is also dynamic because each service designer goes through different co-design types at different moments in time, depending on its evolution and the issues faced all along. This appears in Table 1 through the integration of both short-term and long-term time horizons.

Implementation of specific tools for each type of action, in a "resilient-fit" perspective

A third important result concerns the type of tools needed to support these four types of actions. It was first noticed that e-shape partners were already experienced in making some parts of these actions on their own, however they were often faced with the issue of making these efforts of connecting data and usages sustainable over time. Consequently, it appears that specific tools are required, aiming at establishing a "resilient fit" between stakeholders, rather than a "quick fit".

"Quick-fit" actions only focus on finding one type of interaction between data and usages ecosystems (single list of requirements with one user, in a punctual relationship). Whereas, "resilient-fit" actions aim at generating a range of alternatives (regarding the lists of requirements, the stakeholders involved, the types of partnerships), allowing a better adaptation to future surprises or unexpected constraints arising later in the process. These specific tools for "resilient-fit" actions are currently under experimentation within e-shape, but illustrations of "quickfit" and "resilient-fit" actions are already given for each type of co-design in Table 2.

CONCLUSION

This paper introduces an analytical framework for co-design adapted to the Earth Observation context, that has been designed and experimented within e-shape. The overall ambition of such a co-design approach is to progressively connect and expand the distant ecosystems of data and usages. In this perspective, we highlighted that co-design should not be restricted to the adjustment of supply and demand between users and service designers, but should be considered as a way of growing an ecosystem of efficient EO-based service designers. It is based on a continuous process involving four types of actions aiming at unlocking blocking points occurring in the development of the services. Each type corresponds to the design of a certain committed form of relationship and should target a "resilient fit" between relevant stakeholders, in order to make sure that these efforts are sustainable over time. This approach of co-design and the related tools will be further experimented in the coming years within e-shape.

Figure 1 :

 1 Figure 1: Schema of the four types of co-design actions aiming at connecting the distant data & usages ecosystems

ACKNOWLEDGMENT

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 820852.