Chia-Erh Liu

Mireille Richard-Plouet

Marie-Paule Besland

David Albertini

Claude Estournès

Luc Brohan

Dip-coated La 2 Ti 2 O 7 as a buffer layer for growth of Bi 3.25 La 0.75 Ti 3 O 12 films with enhanced (0 1 1) orientation

Keywords: A. Films, A. Grain growth, A. Sintering, B. Surfaces, D. Perovskites

Thin-films of La 2 Ti 2 O 7 were obtained by dip-coating process using a precursor salt in nitric acid solution. The effects of solution concentration, withdrawal speed, post-annealing duration and temperature were investigated both on grain size and orientation of the La 2 Ti 2 O 7 thin layers. In addition, a target with the required stoichiometry for PVD deposition of La-substituted Bi 4 Ti 3 O 12 (BLT) was successfully sintered by spark plasma sintering (SPS) at 750 • C. Finally (0 1 1)-oriented BLT ferroelectric films have been grown by RF sputtering on (1 1 0)-oriented La 2 Ti 2 O 7 polycrystalline thin-film. A preferential orientation of BLT thin films has been obtained after annealing at a temperature lower than 650 • C.

Introduction

Among the ferroelectric materials, perovskite type structure and layered derivatives are of great interest due to their particular characteristics: high remnant polarisation, high coercivity, low applied voltage during reading-writing cycles. First, PbZr x Ti 1-x O 3 (PZT) has received a great attention but is known to present a fatigue phenomenon during cycling, especially with Pt electrodes. To overcome this drawback, other perovskites derivatives, exhibiting a layered structure, were then investigated as alternative candidates. Among them, the Aurivillius's phases, SrBi 2 (Nb or Ta) 2 O 9 (SBN or SBT) or Bi 4-x La x Ti 3 O 12 (BLTx), have driven a great interest due to their properties, well adapted to a potential application in non-volatile ferroelectric memories (NVFRAM). These materials are built up of MO 6 octahedra sharing corners as in perovskites but, contrary to the latter only 2 (SBT and SBN) or 3 octahedra are connected in the stacking direction. The so-obtained perovskite sheets are separated by Bi 2 O 2 2+ layers adopting the fluorite Corresponding author. Tel.: +33 240 37 39 96; fax: +33 240 37 39 95. E-mail address: mireille.richard@cnrs-imn.fr (M. Richard-Plouet).

arrangement. Among the Bi 4-x La x Ti 3 O 12 solid solution, the composition x = 0.75, exhibits a high resistance to fatigue during cycling. [START_REF] Park | Lanthanumsubstituted bismuth titanate for use in non-volatile memories[END_REF] To explain such behaviour, different models have been proposed. [START_REF] Chu | X-ray photoemission spectroscopy characterization of the electrode-ferroelectric interfaces in Pt/Bi 4 Ti 3 O 12 /Pt and Pt/Bi 3.25 La 0.75 Ti 3 O 12 /Pt capacitors: possible influence of defect structure on fatigue properties[END_REF][START_REF] Chu | b domains in epitaxial ferroelectric Bi 3.25 La 0.75 Ti 3 O 12 films[END_REF] The great potential of BLT relies on the anisotropic polarisation which lies along [1 0 0]. [START_REF] Cummins | Electrical and optical properties of ferroelectric single crystals[END_REF][START_REF] Takenaka | Grain orientation and electrical properties of hot-forged bismuth titanium oxide Bi 4 Ti 3 O 12 ceramics[END_REF][START_REF] Ramesh | Epitaxial growth of ferroelectric bismuth titanate thin films by pulsed laser deposition[END_REF][START_REF] Esaki | Quantum jump and "Kaizen" toward commercialization of ferroelectric ULSI memories[END_REF] Thus an enhancement of BLT ferroelectric response could be achieved by deposition in thin layers with the [1 0 0] axis aligned perpendicularly to the bottom electrode and to the substrate. In literature such a preferential orientation has been obtained by titanate deposition on a (1 1 0) oriented conductive SrRuO 3 perovskite electrode [START_REF] Lee | Anisotropic ferroelectric properties of epitaxially twinned Bi 3.25 La 0 . 75Ti 3 O 12 thin films grown with three different orientations[END_REF][START_REF] Lee | Growth of uniformly a-axis-oriented ferroelectric lanthanum-substituted bismuth titanate films on silicon substrates[END_REF] or on (1 1 0) Pt faces thanks to the low misfit with the (1 1 0) planes of perovskite. [START_REF] Harp | Epitaxial growth and homoepitaxy of platinum (1 1 0) and copper (1 1 0) on strontium titanate (1 1 0)[END_REF] A second family of layered perovskites also exhibits a great potential: Nanot-Queyroux' phases A n B n X 3n+2 , 2 ≤ n ≤ 6, A = Ln, Sr, Ba, Na, B = Ti, Nb, Ta, X = O, F which are constituted of stacked layers which are cut off along the (1 1 0) plane of the perovskite instead of the (1 0 0) one in SBT or BLT. Indeed, [0 0 1] oriented films exhibit a (1 1 0) plane at the upper surface well suitable as a buffer layer to induce the orientation of BLT along [1 0 0]. In order to minimise the misfit between both titanates, La 2 Ti 2 O 7 [START_REF] Nanot | Formation and stability of A n B n O 3n+2 phases exemplified in lanthanum titanate La 2 Ti 2 O 7 -calcium titanate(IV), neodymium titanate Nd 2 Ti 2 O 7 -calcium titanate(IV) and calcium niobate Ca 2 Nb 2 O 7 -calcium titanate(IV) systems[END_REF][START_REF] Schmalle | The twin structure of lanthanum titanium oxide La 2 Ti 2 O 7 : X-ray and transmission electron microscopy studies[END_REF] (n = 4, denoted LTO) was chosen as a buffer layer. LTO exhibits a high Curie temperature of 1500 • C, piezoelectric and electro-optic effects [START_REF] Nanamatsu | New ferroelectric lanthanum titanate La 2 Ti 2 O 7[END_REF][START_REF] Kimura | Electrooptic and piezoelectric properties of La 2 Ti 2 O 7 single crystal[END_REF] . Since La 2 Ti 2 O 7 melt congruently at 1790 • C, [START_REF] Skapin | Phase stability and equilibria in the La 2 O 3 -TiO 2 system[END_REF] obtaining a pure phase generally require annealing at high temperatures, especially when using a classical solid-state reaction involving lanthanum and titanium oxides. Recently, a flux method was reported to allow the formation of La 2 Ti 2 O 7 at 1100 • C. [START_REF] Arney | New molten-salt synthesis and photocatalytic properties of La 2 Ti 2 O 7 particles[END_REF] Alternative low temperature routes were also developed involving co-precipitation [START_REF] Shcherbakova | Rare earth element titanates[END_REF][START_REF] Takahashi | Vibrational spectroscopic study of structural evolution in the coprecipitated precursors to lanthanum tin oxide La 2 Sn 2 O 7 and lanthanum titanium oxide La 2 Ti 2 O 7[END_REF] or urea precipitation, [START_REF] Suresh | Mixed hydroxide precursors for La 2 Ti 2 O 7 and Nd 2 Ti 2 O 7 by homogeneous precipitation[END_REF] hydrothermal treatments, [START_REF] Chen | Hydrothermal synthesis and characterization of La 2 M 2 O 7 (M = Ti, Zr) powders[END_REF][START_REF] Song | Hydrothermal synthesis of flaky crystallized La 2 Ti 2 O 7 for producing hydrogen from photocatalytic water splitting[END_REF][START_REF] Li | Hydrothermal synthesis and photocatalytic properties of layered La 2 Ti 2 O 7 nanosheets[END_REF] polymerisable complex methods, [START_REF] Milanova | A simple solution route to the synthesis of pure La 2 Ti 2 O 7 and Nd 2 Ti 2 O 7 at 700-800 • C by polymerized complex method[END_REF][START_REF] Milanova | Preparation and characterization of solid-state lanthanum-titanium citrate complexes[END_REF][START_REF] Kim | Photocatalytic water splitting over La 2 Ti 2 O 7 synthesized by the polymerizable complex method[END_REF][START_REF] Todorovsky | On the chemical nature of lanthanum-titanium citric complexes, precursors of La 2 Ti 2 O 7[END_REF][START_REF] Todorovsky | The chemistry of the processes involved in the production of lanthanide titanates by the polymerized-complex method[END_REF][START_REF] Li | Photocatalytic property of La 2 Ti 2 O 7 synthesized by the mineralization polymerizable complex method[END_REF] and more largely sol-gel techniques. [START_REF] Prasadarao | Grain orientation in sol-gel derived Ln 2 Ti 2 O 7 ceramics (Ln = La, Nd)[END_REF][START_REF] Prasadarao | Sol-gel synthesis of Ln 2 (Ln = La, Nd) Ti 2 O 7[END_REF][START_REF] Zhao | Low-temperature synthesis of La 2 Ti 2 O 7 nanocrystal by metalorganic decomposition method[END_REF] The most usual reactant for Titanium oxide is then an alkoxide (Ti(OC 3 H 7 ) 4 or Ti(OC 4 H 9 ) 4 ) and 2methoxyethanol is generally used as solvent, especially for thin film obtained by chemical solution deposition. [START_REF] Prasadarao | Fabrication of lanthanum titanate (La 2 Ti 2 O 7 ) thin films by a sol-gel[END_REF][START_REF] Jeon | Crystallization and dielectric properties of lanthanum titanate thin films by metalorganic decomposition[END_REF][START_REF] Sik | Microstructure and electrical properties of Ln 2 Ti 2 O 7 (Ln = La, Nd)[END_REF][START_REF] Sandstrom | Sol-gel synthesis of textured lanthanum titanate thin films[END_REF][START_REF] Li | Effect of in situ applied electric field on the growth of La 2 Ti 2 O 7 thin films by chemical solution deposition[END_REF] Pulsed laser deposition was also used but requires a severe control of air atmosphere to avoid Ti 4+ reduction. [START_REF] Kushkov | Structure of the lanthanide titanium oxide Ln 2 Ti 2 O 7 thin films prepared by pulsed-laser evaporation[END_REF][START_REF] Dickerson | Spectroscopic ellipsometry characterization of La 2 Ti 2 O 7 thin films[END_REF][START_REF] Ohtomo | Epitaxial growth and electronic structure of LaTiO x films[END_REF][START_REF] Kim | Epitaxial structure and transport in LaTiO 3+x films on (0 0 1) SrTiO 3[END_REF] Recently, researchers suggested that the use of a buffer layer might be an interesting solution to obtain bilayered perovskite films with enhanced dielectric constant. [START_REF] Delage | High-k BST films deposited on MgO by PLD with and without buffer-layer[END_REF][START_REF] Wu | Low temperature deposition of Ba 0.4 Sr 0.6 TiO 3 thin films on LaNiO 3 -buffered electrode by RF magnetron sputtering[END_REF][START_REF] Cukauskas | Morphology and dielectric properties of Ba 0.5 Sr 0.5 TiO 3 thin films on annealed (1 0 0) MgO[END_REF] Therefore, on the basis of previous results on BLT thin films deposition by RF magnetron sputtering, [START_REF] Besland | Comparison of lanthanum substituted bismuth titanate (BLT) thin films deposited by sputtering and pulsed laser deposition[END_REF] we present here first investigations on the effect of a La 2 Ti 2 O 7 buffer layer, previously deposited by dip coating, on both, structure and orientation of the BLT films deposited subsequently by RF magnetron sputtering.

The challenge is thus to develop in the LTO layer a grain growth with the c-axis perpendicular to the substrate and to further induce the BLT deposition along a-axis, as illustrated in Fig. 1. The structural arrangement of La 2 Ti 2 O 7 is described in a monoclinic cell with a LTO = 7.812(2), b LTO = 5.5440 (7) and c LTO = 13.010(2), β = 98.66(1) • . [START_REF] Schmalle | The twin structure of lanthanum titanium oxide La 2 Ti 2 O 7 : X-ray and transmission electron microscopy studies[END_REF] The lattice parameters of Bi 3.25 La 0.75 Ti 3 O 12 are a BLT = 5.4181 (6), b BLT = 5.4168(3) and c BLT = 32.8856(4). [START_REF] Wen | of the university of Nantes[END_REF] Along [0 1 0] LTO or BLT (Fig. 1a), the mismatch between c BLT and 4 × a LTO is less than 5%, in the other direction (Fig. 1b) the mismatch is 0.4%. In this ideal configuration, BLT would be in compression along c BLT and in tension along b BLT .

We report here a method allowing the preparation of La 2 Ti 2 O 7 as films, starting from inorganic reactants. Instead of the usual reactants for Ti precursors such as titanium isopropoxide or titanium acetylacetonate, a Ti oxychoride solution is used. Films are heated in air after dip coating in an acid solution containing the stoichiometric reactant salts. Second, among deposition techniques, sputtering or physical vapour deposition technique (PVD) enables to deposit at low temperature wellcrystallised films with a good uniformity over large areas. An optimisation of the deposition conditions allowed us to induce a preferential orientation of the BLT film.

Experimental

Synthesis

The solution containing La and Ti was freshly prepared in decarbonated water (18.2 M , in resistivity). In order to avoid Ti alkoxide or complexing chelate, such as acetylacetonate, we developed a method using only inorganic reactants for Ti, starting from TiOCl 2 solution (Millenium TiOCl 2 •1.4HCl•7H 2 O, 5 mol L -1 in HCl). First, a TiO 2 •nH 2 O gel was precipitated by ammonia (typically 1 mL TiOCl 2 is precipitated with 10 mL NH 4 OH, 6.5 mol L -1 ). Before mixing with the lanthanum nitrate solution, Cl -ions were removed by filtering and carefully washing with de-ionized-water. Second, the solid was dissolved in nitric acid (5 mL HNO 3 , 5 mol L -1 ). In such conditions, a stable solution with a [Ti 4+ ] concentration up to 1 mol L -1 can be prepared. Third, the required amount of lanthanum nitrate was added to this solution in order to adjust the required La 3+ and Ti 4+ concentration in the studied range, i.e. 0.15-0.90 mol L -1 .

Si (1 0 0) wafers, pre-treated with HCl and then boiling acetone or ethanol were used as substrates. Dip coatings were performed with a DC-multi/4 Nima Dip Coater. The substrate was maintained in the solution for 2 min and then pulled out at different rates: 15, 40, 80 and 120 mm min -1 . Then, each layer was allowed to dry for 1 h at room temperature in a 50% RH atmosphere. Six deposition-drying cycles were performed before any annealing at high temperature. Then, the films were annealed at temperature in the 600-1200 • C domain for a duration ranging from 4 to 12 h under air atmosphere.

The target, with required stoichiometry and density, suitable for RF sputtering experiments, was prepared by spark plasma sintering (SPS). The BLT powder was obtained by precipitation with ammonium hydroxide of a solution in nitric acid. The required amount of Bi(NO 3 ) 3 •5H 2 O and La(NO 3 ) 3 •6H 2 O dissolved in concentrated nitric acid were added to a [Ti 4+ ] (0.5 mol L -1 ) solution obtained as described above. After precipitation with ammonium hydroxide, a thermal treatment at 600 • C was performed for hydroxide decomposition before sintering. The powder has been sintered using SPS apparatus (Model Dr Sinter SPS-2080 SPS Syntex INC Japan). The SPS equipment consists of a pulsed (3.3 ms of duration) direct current generator, an hydraulic press system, a vacuum and water cooled chamber, upper and lower punch electrodes graphite spacers, punches and sintering die and finally control systems for the temperature (thermocouple or optical pyrometer), environment pressure inside the chamber and for the positioning of the lower electrode on z-axis which allows us to follow the shrinkage of the sample. The pulse sequence used is 12-2 (pulses-dead time or zero current) as recommended by the SPS equipment constructor. The temperature was monitored and regulated using a thermocouple fixed in a little hole situated at the surface of the die. The temperature was raised from room temperature to 750 • C with a heating rate of 25 • C/min. A 50 MPa uniaxial pressure was applied while increasing temperature from 650 to 750 • C. In these conditions, during the sintering cycle, the voltage reached a maximum value of 2.2 V and the maximum value of current passing through the die reached the applied value of 1480 A. This process led to a pellet, with the following dimensions: 0.3 cm thick and 3 cm diameter, and density very close to the theoretical one, i.e. equal to 97% and thus, suitable as a target for magnetron sputtering deposition.

The BLT films were deposited by RF magnetron sputtering in a customized reactor equipped with a 25 mm diameter planar magnetron cathode as described previously. [START_REF] Besland | Comparison of lanthanum substituted bismuth titanate (BLT) thin films deposited by sputtering and pulsed laser deposition[END_REF] Depositions were performed at 60 W RF power and a total pressure of 47 mTorr, in an argon/oxygen gas mixture containing 10% O 2 . Substrates were located downstream at 7.5 cm from the target. Depositions were performed for 70 min either in a one step or two-step deposition process. [START_REF] Liu | Bi 3.25 La 0.75 Ti 3 O 12 films on La 2 Ti 2 O 7 thin films prepared by chemical solution deposition[END_REF][START_REF] Besland | Improvement of dielectric properties of BLT thin films deposited by magnetron sputtering[END_REF] All BLT depositions were performed without heating the substrate. A 1 h ex situ annealing under flowing oxygen at 650 • C allows obtaining well-crystallised BLT films. For a two-step deposition process, one intermediate and one final ex situ annealing were performed at 650 • C under flowing oxygen during 1 h. The effect of a 3 h ex situ final annealing was also investigated.

Characterisation

Thermal gravimetric analysis (TGA) was performed in argon atmosphere with a TG-92 Setaram equipment at temperature ranging from room temperature to 1200 • C and a heating rate of 10 • C min -1 . A mass spectrometer connected to the TG apparatus allows the identification of the released products during heating.

AFM imaging was performed before thermal treatment using a Nanoscope Ш externder Ex-1, digital instruments Inc in tapping mode.

The X-ray diffraction (XRD) patterns were recorded at a 1 • grazing angle in the 5-55 • , 2θ domain, with a 0.03 • step scan. To enhance the signal to noise ratio, each pattern was accumulated for 3 h. The preferential orientation of the films was estimated from X-ray diffraction intensities using the Lotgering orientation factor f = (pp o )/(1p o ), where p = I (0 0 l) / I (h k l) and p o is the p value for a random powder pattern. [START_REF] Lotgering | Topotactical reactions with ferrimagnetic oxides having hexagonal crystal structures[END_REF] In case of a diffraction peak including several (h k l) reflections, a decomposition of the peak using pseudo-Voigt profiles has been performed to correctly estimate the reflection intensities. As an example, the peak at 27.8 • was decomposed in two components associated to the (0 0 4) and (-211/210) reflections.

Scanning electron microscopy (SEM) images were carried out with a JEOL JSM-6400F microscope (Akishima, Japan) equipped with a field emission gun, operating at 7 kV. Bulk chemical composition of the films was determined by energy dispersive spectrometry (EDS) at 15 kV with a JEOL JSM-5800.

TEM observations of cross-sections were performed with a Hitachi HF2000 FEG operating at 200 kV, after samples preparation by mechanical polishing using tripod technique.

Results and discussion

Characterisation of the solid precursor for La 2 Ti 2 O 7

The first step of this study was to characterise the required temperature for La 2 Ti 2 O 7 phase formation in thin films. Therefore, water was evaporated from the reactant solution by drying in a closed container containing P 2 O 5, for 5 days. TGA curve of the obtained solid (Fig. 2) shows four decomposition steps and a weight loss achieved at 600 As a second step, the anhydrous solid is heated at temperatures above 600 • C for 4 h and then the evolution of the X-ray diffraction patterns with temperature has been followed. such low temperature shows that this method leads to a homogenous chemical composition at a very small scale. As reported by Suresh et al., [START_REF] Suresh | Mixed hydroxide precursors for La 2 Ti 2 O 7 and Nd 2 Ti 2 O 7 by homogeneous precipitation[END_REF] a temperature higher than 1000 • C is required to crystallise lanthanum titanate and eliminate any trace of amorphous phase. This preliminary study allowed us to check that our procedure leads undeniably to the crystallisation of La 2 Ti 2 O 7 .

Chemical solution deposition of La 2 Ti 2 O 7

The next objective was to develop in the films a grain growth with the c-axis perpendicular to the substrate. The LTO thin films were obtained at RT by six dip-coating cycles in a [Ti 4+ ] = 0.30 mol L -1 solution, followed by a drying step at RT before any thermal treatment. Fig. 3 shows the AFM image of the surface of the film. A granular morphology is observed exhibiting a homogenous distribution in size in the 30 nm range. The roughness determined on a 5 m × 5 m surface is 1 nm. Finally, as checked by EDX analysis, the La/Ti ratio is equal to 1, as expected.

In Fig. 4, the XRD diffraction patterns of LTO films are presented for different annealing conditions. As already mentioned in the previous section concerning the anhydrous powder-like solid, annealing at 600 • C, for 12 h does not permit the crystallisation of La 2 Ti 2 O 7 (Fig. 4a). Second, the high level of the background in the X-ray diffraction pattern obtained for a film heated for 12 h, at 800 • C, indicates that again only a part of the solid has crystallised (Fig. 4b). After annealing at 1000 • C (Fig. 4c), the quality of the pattern is slightly enhanced with the appearance typical reflections attributable to La 2 Ti 2 O 7 . However, the signal still remains noisy. Increasing the withdrawal speed to 80 mm min -1 , significantly improves the pattern quality, even after a shorter thermal treatment of 4 h at 1000 • C (Fig. 4d). This observation confirms that withdrawal speed is a major parameter in dip-coating technique. Indeed the layer thickness, h, depends on the solution adhesion to the substrate, which is governed through the following Landau-Levich equation: 50 where v is the withdrawal speed, μ the viscosity of the solution, σ the liquid-vapour surface tension and ρ is the density of the solution. This first study indicates that annealing at 1000 • C is required to crystallise La 2 Ti 2 O 7 and that increasing the withdrawal speed by increasing the amount of available precursor favours its crystallisation. Several speeds were thus tested leading to the adhesion of different amount of liquid. Since the SEM images performed with secondary electrons were not contrasted enough for the thinner layers, the thickness was estimated using the backscattered electron mode (Fig. 5). In this mode, the contrast associated with the heavy La atoms is bright then the darker contrast is associated to the SiO 2 layer on top of the Si wafer. SEM images reveal that the thickness of the film is around 50 nm for 15 and 40 mm min -1 (Fig. 5a andc), reaches 75 nm when dipping at 80 mm min -1 (Fig. 5e) and less than 20 nm for 120 mm min -1 (Fig. 5g). The accuracy of the measurement is not higher than 20 nm. For a 200 mm min -1 dipping rate, the film is 400 nm thick as revealed by the SEM cross-view in secondary electron mode (Fig. 5i). SEM images at low magnification allow us to estimate the substrate coverage. As revealed by Fig. 5j, the homogeneity is clearly better for a dipping rate equal to 200 mm min -1 (bottom row of images). The particles size also depends on the withdrawal speed. For all the studied conditions, the grain size distribution is bimodal: a first population of particles exhibits grain size ranging from 50 to 100 nm and a second one with grain size in 100-270 nm as observed in Fig. 5. For a dip performed at 15 mm min -1 , the grain size of both populations is centred on 80 and 130 nm (Fig. 5b). The first population exhibits a grain size in the 100 nm range for the intermediate rates. The grain size of bigger particles increases up to 200-220 nm and even up to 270 nm for 120 mm min -1 (Fig. 5d, f andh). For a 200 mm min -1 dipping rate, the grain size of both populations is centred on 50 and 100 nm (Fig. 5j).

h = 0.94 × μv σ 2/3 σ ρg 1/2 (1)
Fig. 6 shows the evolution of the XRD patterns for thin layers obtained with a [Ti 4+ ] = 0.30 mol L -1 solution at different rates and after a further annealing of 12 h at 1000 • C. Crystallisation of the titanate and its orientation along [0 0 1] could be achieved for dipping rates equal or above 80 mm min -1 (Fig. 6b-d). For lower withdrawal speeds, the diffraction peaks are almost hidden in the background of the pattern, especially for the lowest rate 15 mm min -1 (Fig. 6a). Moreover, for 40 mm min -1 (Fig. 6b) even if the (0 0 l) peaks of La 2 Ti 2 O 7 are detected, the most detectable peaks are attributed to a lanthanum titanium silicate (La 2 Ti 2 SiO 9 , JCPDS file No. 82-1490, marked by black bars in Fig. 6a). We assume that Si atoms may diffuse from the substrate at 1000 • C, leading to the crystallisation of this titano-silicate, usually obtained at 850-950 • C temperature range. [START_REF] Benbertal | Synthesis and crystal structure of a new lanthanum and titanium silicate[END_REF] In addition, as can be seen in Fig. 4d decreasing the temperature and duration of the thermal treatment do not prevent this crystallisation. This silicate seems to be formed to the detriment of La 2 Ti 2 O 7 when there is not enough amount of reactant on the substrate. To verify this hypothesis, depositions were performed with a [Ti 4+ ] = 0.15 mol L -1 solution: they lead to the only formation of the silicate. Unfortunately, this impurity cannot be avoided in the films obtained at rates of 80 and 120 mm min -1 . Nevertheless they mainly exhibit the diffraction peaks of La 2 Ti 2 O 7 and in particular a preferential orientation could be induced along [0 0 1] since the intensity of (0 0 l) reflections is enhanced when compared to the expected intensities (black lines in Fig. 6d). To favour the formation of the titanate to the detriment of the silicate, further attempts with higher concentrated solution were undertaken, namely [Ti 4+ ] = 0.90 mol L -1 . The AFM image of a film surface, obtained before annealing with such a solution at 80 mm min -1 is very similar to Fig. 3,i.e. the one obtained for a film with [Ti 4+ ] = 0.30 mol L -1 . The roughness is again lower than 1.5 nm. In both experimental conditions, the evaporation of the solvent at RT in a controlled humidity induces the formation of colloids which are 30 nm in diameter at the surface of the substrate. Therefore we conclude that the nucleation is most probably homogenous in the liquid phase. Contrary to the lower concentrated solution, the [Ti 4+ ] = 0.90 mol L -1 solution avoids the formation of the silicate, as checked by XRD in Fig. 7. For a higher dipping rate of 80 mm min -1 , a characteristic powder-like La 2 Ti 2 O 7 pattern is recorded (Fig. 7b). Besides, when the deposition is performed at 40 mm min -1 rate, the intensity of the (0 0 l) reflections is enhanced, indicating that a (0 0 1) orientation can be favoured. The measurement of the layer thickness obtained at 40 mm min -1 was too weak to be checked on SEM images. For a dipping rate of 80 mm min -1 , a 260 nm thick porous film was characterised. Furthermore, for a dipping rate of 40 mm min -1 , SEM image allows us to estimate the particle grain size to 110 and 280 nm for the smallest and greatest particles, respectively (Fig. 8a). For a faster rate of 80 mm min -1 , both grain sizes decrease to 70-80 nm and 230 nm, for the smallest and greatest particles, respectively (Fig. 8b).

The f Lotgering factors are reported in Table 1. For a randomly oriented powder the f value is equal to 0 whereas for a textured sample, the f factor reaches 1. However, this simple estimation allows us to determine that the most oriented samples, i.e. 60% oriented are obtained from a [Ti 4+ ] = 0.30 mol L -1 solution with a 80 or 200 mm min -1 withdrawal speed. From the Landau-Levich equation, the corresponding amounts of La 2 Ti 2 O 7 reactant available for the different experimental conditions can be calculated (Table 1, fourth column). The orientation seems to be favoured when the amount of reactant available on the surface lies between 1.6 × 10 -1 and 3.0 × 10 -1 mol cm -2 . For the lower limit and below, the amount of reactant is not high enough to hinder the reaction of SiO 2 with the precursor and the silicate crystallises. In the other case, the grain growth is then favoured and the interfacial energy between the film and the substrate is decreased. Due the layered structure of La 2 Ti 2 O 7 , the grain growth is expected to be faster along the directions belonging to the (a and b) plane and not along the stacking axis. Thus a preferential orientation is obtained along [0 0 1]. It has to be noted, that f factors are only indicative and further texture measurements are required to get more precise information. As already reported, [START_REF] Sandstrom | Sol-gel synthesis of textured lanthanum titanate thin films[END_REF] the degree of texture is closely linked to the amount of reactant available to form the oxide: the most diluted solution led to the highest textured film. In agreement with these results obtained by spin coating, when the concentration of the solution increases, the film is less textured due the enhancement of nucleation to the detriment of growth. Thicker films growth is less governed by the minimisation of interfacial energy and together with the nucleation enhancement, less oriented films are produced.

RF magnetron sputtering of BLT on oriented LTO

In the synthesis of bismuth titanates, the major difficulty is the volatility of Bi 2 O 3 which melts at 830 • C. [START_REF] Drache | Polymorphism of Bi 1-x Ln x O 1 . 5 phases (0 < x < 0.40). Characterization of a new compound Bi 4 Ln 2 O 9 (x = 0.33; Ln = La, Pr, Nd)[END_REF] To avoid this drawback, many syntheses are performed with an excess of Bi to counterbalance any Bi 3+ deficiency. A second alternative was involved, based on a promising technique: SPS, performed at lower sintering temperature for short duration. [START_REF] Omori | Principles and Mechanisms of the SPS Process. What's, SPS[END_REF] In addition, this technique enables to reach high densities suitable for RF magnetron sputtering. Prior being sintered, a low-temperature process involving precipitation with ammonium hydroxide was developed to obtain powder with the required stoichiometry. Fig. 9a shows the XRD pattern of the powder annealed at 600 • C exhibits a pattern which is characteristic of an Aurivillius's phase. The theoretical intensities are calculated with PowderCell software 54 using the atomic parameters obtained by a Rietveld refinement of the XRD pattern of Bi 3.25 La 0.75 Ti 3 O 12 . [START_REF] Wen | of the university of Nantes[END_REF] Such a low temperature for crystallisation is attributed to the high homogeneity of the precipitate and its high reactivity. The XRD peaks are rather broad and according to Sherrer's law, the crystal size can be evaluated in the 25 nm range. After SPS performed at 750 • C, the pellet was annealed at 800 • C for 3 h under oxygen in order to remove the carbon layer and to re-oxidise Bi metal formed at the interface with the graphite matrix (Fig. 9b).

The XRD pattern is again typical of a Bi(La) titanate (Fig. 9c). The EDX analysis reveals an average chemical composition Bi 3.24 La 0.33 for 3Ti atoms, quite far from the required one with an important loss La 2 O 3 . After polishing, the average chemical composition is 3.2Bi-0.8La for 3Ti close enough to the expected one and thus suitable for RF magnetron sputtering.

It is worth noting that the X-ray diffraction pattern exhibits some modifications, namely a decrease of (h 0 0) peak intensity and a broadening of the peaks before and after polishing.

The full width at half maximum (FWHM) increases from 0.25 • to 0.40 • , before and after polishing, respectively. This grain growth at the interface with the electrodes can be attributed to the higher temperature reached in their vicinity during the SPS process. The target was then used to sputter BLT films on previously obtained La 2 Ti 2 O 7 buffer layer oriented along c-axis (withdrawal speed = 120 mm min -1 , less oriented but exhibiting a more homogenous surface). Using the experimental conditions previously reported, a single PVD deposition leads, after 1 h ex situ annealing at 650 • C, to a 230 nm thick BLT film, as revealed by the X-ray diffraction pattern (Fig. 10a) and the cross-view SEM image (Fig. 10b). The SEM image (Fig. 10c) shows a film surface with a grain size ranging from 20 to 80 nm. A further annealing for 3 h of this film leads to a grain growth up to 150 nm (Fig. 10f). In an attempt to increase the thickness of the ferroelectric layer, a second deposition was performed in the same experimental conditions. As expected, the thickness is increased up to 500 nm (Fig. 10e) and a good adhesion occurs between the two BLT layers: no evidence for a separation between both depositions is observed. A second 3 h annealing did not promote further grain growth but led to a surface with melted aspect which seems to extend under the surface. Nevertheless, after annealing, the X-ray diffraction patterns are characteristic of an Aurivillius's phase. By comparison with the theoretical intensities, all BLT films deposited on LTO buffer layers present a significant enhancement of the (2 0 0) reflection (Fig. 10d).

Thus, this preliminary study gives additional evidence that a one-step deposition process is not suitable, in our experimental conditions, to obtain an oriented BLT film, even after ex situ annealing at 650 • C. A two-step process, with an intermediate and a final thermal treatment, as recently published, leads to a better orientation of the BLT films. In addition, a longer annealing period induces a further crystallisation process which is prejudicial to the expected (0 1 1) orientation of the BLT layer on the preferential [0 0 1] LTO. These results need to be further investigated to understand more deeply the parameters which favour the preferential orientations both of LTO and BLT layers. To get more insight on the effect of the two-step process, HRTEM investigations are under progress.

Conclusions

An alternative route to the preparation of titanates without involving organo-metallic precursors has been developed. The dipping solution was prepared by dissolution in nitric acid of a freshly TiO 2 •nH 2 O precipitate with the required amount of La nitrate is added. A uniform film was obtained by dip coating in the nitric acid solution. For a given concentration, the withdrawal speed can be optimised to favour a preferential orientation. The amount of available reactants on the substrate was optimised though the concentration of the solution and the withdrawal speed. LTO films with c-axis orientation were obtained and used as first buffer layer to favour the deposition of [1 0 0] oriented BLT films. Such a buffer layer exhibiting crystallographic features equivalent to a (1 1 0) perovskite plane was used to favour the preferential growth of Bi 3.25 La 0.75 Ti 3 O 12 layers along aaxis. The target of such a complex chemical composition-like Bi 3.25 La 0.75 Ti 3 O 12 was successfully sintered by SPS at 750 • C. Last, involving a two-step RF magnetron sputtering process at low O 2 content, high pressure and low RF power, including an intermediate and a final thermal treatment at low temperature (650 • C, for 1 h), we succeed to deposit well crystallised BLT layers exhibiting a preferential orientation along [1 0 0]. These preliminary results obtained on Si substrates are very promising for further applications. Same deposition process is currently investigated on multi-layer structures including bottom and top Pt electrodes. Electrical measurements will be finally performed to control the effect on BLT layer polarisation and its ferroelectric response.

Fig. 1 .

 1 Fig. 1. Representations of the epitaxy relationship between LTO and BLT along: (a) b LTO and b BLT and (b) a LTO and c BLT .

  • C. According to Gobichon et al. works, 49 the thermal decomposition of lanthanum nitrate hexahydrate can be usually divided in three steps: first, dehydration below 200 • C, second formation of LaONO 3 between 240 and 380 • C with elimination of two nitrate groups and finally formation of La 2 O 3 with nitrate loss between 400 and 520 • C. Thus, in our case, the first step, between room temperature and 240 • C, is associated to a water departure, corresponding to the partial dehydration of TiO 2 •nH 2 O and La(NO 3 )•6H 2 O. The second one, occurring between 240 and 360 • C, corresponds to a further release of water and nitrates (as NO, O 2 and N 2 O and N 2 ), and is mainly related to the beginning of LaONO 3 formation and the further degradation of TiO 2 •nH 2 O. The two last weight losses, occurring, respectively in the 360-460 • C and 460-600 • C ranges can be attributed to the decomposition of La(NO 3 ) 3 .

  Fig. 2. TGA and mass spectrum performed on the solid obtained after evaporation of water from the HNO 3 solution containing the reactant, [Ti 4+ ] = [La 3+ ] = 0.30 mol L -1 .

Fig. 3 .

 3 Fig. 3. AFM image of a film obtained by dip coating in a [Ti 4+ ] = 0.30 mol L -1 solution, withdrawal speed of 40 mm min -1 , dried at RT after each 6 dips.

Fig. 4 .

 4 Fig. 4. X-ray diffraction of films dried at RT after 6 dips in a [Ti 4+ ] = 0.30 mol L -1 solution, withdrawal speed of 40 mm min -1 , annealed for 12 h at (a) 600 • C, (b) 800 • C, (c) 1000 • C, withdrawal speed of 80 mm min -1 , annealed at 1000 • C for (d) 4 h bars: diffraction peaks of La 2 Ti 2 O 7 , JCPDS file no. 81-1066.

Fig. 5 .

 5 Fig. 5. SEM images of La 2 Ti 2 O 7 films obtained by dip coating in a [Ti 4+ ] = 0.30 mol L -1 solution then annealed at 1000 • C for 12 h in air, at various withdrawal speeds 15 mm min -1 (a) backscattered imaging of a cross-view, (b) high magnification, 40 mm min -1 , (c) backscattered imaging of a cross-view, inset: TEM cross-view, (d) high magnification, 80 mm min -1 , (e) backscattered imaging of a cross-view, low and (f) high magnification, 120 mm min -1 , (g) backscattered imaging of a cross-view, (h) high magnification secondary images and 200 mm min -1 , (i) backscattered imaging of a cross-view, and (j) high magnification secondary images.

Fig. 6 .

 6 Fig. 6. X-ray diffraction patterns of various films, prepared at different withdrawal speeds: (a) 15 mm min -1 (lower line) and 40 mm min -1 (upper line), bars: diffraction peaks of La 2 Ti 2 SiO 9 , JCPDS file no. 82-1490, (b) 80 mm min -1 , (c) 120 mm min -1 , and (d) 200 mm min -1 , annealed at 1000 • C for 12 h, bars: diffraction peaks of La 2 Ti 2 O 7 , JCPDS file no. 81-1066.

Fig. 7 .

 7 Fig. 7. X-ray diffraction patterns of films, dried at RT after 6 dips in a [Ti 4+ ] = 0.90 mol L -1 solution prepared at different withdrawal speeds: (a) 40 mm min -1 , (b) 80 mm min -1 , annealed at 1000 • C for 12 h (black bars: diffraction peaks of La 2 Ti 2 O 7 , JCPDS file no. 81-1066).

2 O 3 Fig. 8 .

 238 Fig. 8. SEM images of La 2 Ti 2 O 7 films obtained by dip coating in a [Ti 4+ ] = 0.90 mol L -1 solution at various withdrawal speeds then annealed at 1000 • C for 12 h in air: (a) 40 mm min -1 and (b) 80 mm min -1 .

- 1 a

 1 μ = 1.0553 Pa s, ρ = 1055.8 kg m -3 (viscosity and volumic mass of a 1.7 mol L -1 HNO 3 solution), σ = 0.072 J m 2 (water). b μ = 1.4385 Pa s, ρ = 1166.2 kg m -3 (viscosity and volumic mass of a 5.1 mol L -1 HNO 3 solution), σ = 0.072 J m 2 (water).

Fig. 9 .

 9 Fig. 9. XRD patterns of BLT pellet: (a) after annealing at 600 • C for 3 h under air compared with theoretical intensities for BLT45 (bars), (b) after SPS compared with Bi metal (stars, according to JCPDS file no. 44-1246), C for carbon peak, (c) target annealed at 800 • C for 3 h under O 2 and (d) after oxidation and polishing, inset: target with the following dimensions, 3 cm in diameter and 0.3 cm thickness.

Fig. 10 .

 10 Fig. 10. X-ray diffraction patterns of a BLT film sputtered on an oriented LTO buffer layer, on Si substrate, comparison with the calculated intensity of BLT45: (a) one deposition annealed for 1 h at 650 • C, and corresponding SEM images, (b) cross-and (c) plane views, (d) two depositions annealed for 1 h and then 3 h at 650 • C, and corresponding SEM images, (e) cross-and (f) plane views.

Table 1

 1 Parameters describing the preferential orientation along c-axis of La 2 Ti 2 O 7 , for different films.

	[Ti 4+ ] (mol L -1 )	Pulling speed (mm min -1 )	f = (p -p o )/(1 -p o ) (%)	Adherent liquid thickness (m)	Amount of La 2 Ti 2 O 7 reactant (mol cm -2 )
	0.30 a	15	-	3.5	0.5 × 10 -1
		40	-	6.8	1.0 × 10 -1
		80	61	10.8	1.6 × 10 -1
		120	46	14.1	2.1 × 10 -1
		200	67	19.9	3.0 × 10 -1
	0.90 b	40	28	7.9	3.6 × 10 -1
		80	9	12.6	5.7 × 10

Acknowledgments

The authors are deeply indebted to Alain Barrault at the service commun de microscopie électronique à balayage for his assistance in recording SEM images and to Eric Gautron for TEM cross-section preparation. In addition, Pierre Barroy, Nicolas Leclair and Marc Souilhat are acknowledged for target preparation and analysis.


Emilie Moulin

Joseph J Armao

Nicolas Giuseppone

Triarylamine-based Supramolecular Polymers: Structures, Dynamics, Functions

CONSPECTUS

Triarylamine molecules and triarylamine-based covalent polymers have been extensively investigated for more than sixty years in academics and industry because of their intriguing electronic and optical characteristics. However, despite the profusion of studies made on these derivatives, only very recently have the first examples of supramolecular polymers based on the triarylamine motif been described in the literature. Specifically, our research group has shown that, by adding supplementary hydrogen bonding moieties such as amide functions in their periphery, it becomes possible to tightly pack triarylamine molecules in columnar supramolecular stacks presenting a collinear arrangement of their central nitrogen atoms. These supramolecular polymers can self-assemble into various soft hierarchical structures such as helical fibers, nanorods, nanospheres, and nanoribbons in the sol and in the gel states, into liquid-crystalline mesophases, as well as into highly organized supramolecular frameworks and their single crystals thereof.

Interestingly, the associated supramolecular polymerization mechanism involves a nucleation step

of high activation energy which requires the flattening of the triarylamine core. Because of this singularity, and although dependent on the precise chemical nature of the building blocks, it has been demonstrated that their supramolecular polymerization can be triggered by original tools such as light irradiation or electrochemistry, and that it can display autocatalytic growth behaviors, remarkably strong amplifications of chirality, as well as complex and competing thermodynamic and kinetic self-assembly pathways. Further, from a functional point of view, it has been highlighted that a partial oxidation of the triarylamine molecules results in an enhanced throughspace delocalization of the charge carriers along the π-π stacked supramolecular polymers, a feature that confers to these nanowires exceptional transport properties. Upon increasing the charge carrier concentration, the electronic nature of these soft materials can be switched from a semiconducting to a metallic behavior, and the presence of highly delocalized unpaired electrons in supramolecular polaronic band structures has been further exploited to implement plasmonic properties within subwavelength organic interconnects and microscopic optical waveguides.

Finally, by making use of the unusual dynamics and functions of triarylamine-based nanostructures, it becomes possible to precisely address their self-construction within confined environments or within nano-and micrometer scale devices. This has been demonstrated for instance in between nanoparticles and in between electrodes, inside inorganic nanopores and inside phospholipid bilayers, as well as at the liquid-liquid interface. Such a meeting point between bottom-up and top-down technologies is of high interest to envision further developments and applications for this entirely new class of supramolecular polymers which combine a unique relationship between their structures, their dynamics, and their subsequent emerging functional properties.

MAIN TEXT

Introduction

Triarylamine (TAA) derivatives represent a well-known class of organic molecules constructed around the triphenylamine (TPA) core. They have been intensively used for decades because of their properties as electroactive and photoactive materials. [START_REF] Shirota | Photo-and Electroactive Amorphous Molecular Materials -Molecular Design, Syntheses, Reactions, Properties, and Applications[END_REF][START_REF] Wang | Triarylamine: Versatile Platform for Organic, Dye-Sensitized, and Perovskite Solar Cells[END_REF] In 2010, our research group discovered that some TAA derivativesi.e. TAA substituted with at least one lateral amide group can undergo supramolecular polymerization. [START_REF] Moulin | The Hierarchical Self-Assembly of Charge Nanocarriers: A Highly Cooperative Process Promoted by Visible Light[END_REF] In this original report, we provided the first structural description of TAA-based supramolecular polymers which was later on revealed as a general feature for a large family of analogues. The main characteristic of these supramolecular polymers is a columnar packing of TAA units with a collinear arrangement of their central nitrogen atoms. This supramolecular structure is mostly stabilized by a combination of i) π-π stacking interactions between the phenyl groups of the TAA, and of ii) hydrogen bonds provided by their lateral amide groups. In the present account, we will first describe in detail the various columnar TAA structures that have been published in the literature so far. In particular, we will highlight how the precise molecular nature of the TAA can influence fine local variations of their primary packing, as well as their various hierarchical organizations at higher length scales. We will then describe the supramolecular polymerization mechanisms that have been proposed depending on the TAA molecular structures and on the environmental conditions imposed to the systems. We will finally show that TAA supramolecular polymers provide an original platform for a number of intriguing functional features including ionic, electronic, and photonic transport properties.

Structural aspects of triarylamine-based supramolecular polymers

The primary structure of TAA-based supramolecular polymers is governed by a limited number of parameters, but their combination can lead to important variations which influence the polymer chirality, its self-assembly mechanism, its mesoscale organization, as well as its functional properties. It is thus of crucial importance to understand the basic relationships that link the molecular configuration and conformation of TAA molecules with their supramolecular structures.

Three main molecular parameters have been shown to modify the primary packing of TAA units: i) their chiral conformation, ii) the number of amides substituting the TPA core, and iii) the nature of the solubilizing chains at their periphery.

The chiral conformations of TAA molecules are enforced by the fact that the phenyl rings of the TPA core have to rotate out of the molecular main plane in order to get rid of steric repulsions between them. As a result, TAA molecules are chiral propellers with two limit conformational enantiomers Λ and Δ (Figure 1A). As suggested early by our experimental observations and theoretical approaches together with Semenov, [START_REF] Moulin | The Hierarchical Self-Assembly of Charge Nanocarriers: A Highly Cooperative Process Promoted by Visible Light[END_REF][START_REF] Nyrkova | Supramolecular Self-Assembly and Radical Kinetics in Conducting Self-Replicating Nanowires[END_REF] the mutual attraction between aromatic rings of TPA propellers can result in two distinct conformations of very similar energies and which share a comparable collinear arrangement of their central nitrogen atoms. In the "snowflake" situation, TPA cores alternate their Λ and Δ chiralitythus leading to a global achiral supramolecular columnar packingand display a rotation angle of 60° together with a N-N distance of 0.48 nm between two stacks (Figure 1B(i)). Such a snowflake conformation was for the first time observed experimentally in 2010 on double columns of TAA mono-amides. [START_REF] Moulin | The Hierarchical Self-Assembly of Charge Nanocarriers: A Highly Cooperative Process Promoted by Visible Light[END_REF] In the "Mercedes-Benz" situation, TPA cores of similar chirality are stacked with a N-N distance of 0.5 nm between them and with a minimal rotation angle of 1.5°, leading to either M or P helicities for a given supramolecular column (Figure 1B(ii)). Such a helical conformation was for the first time observed experimentally in 2014 on single columns of triarylamine tris-amides (TATA). [START_REF] Armao | Healable Supramolecular Polymers as Organic Metals[END_REF] Importantly, in both the achiral and chiral supramolecular columns, because of the out-of-plane tilt of the propeller phenyl rings, and despite an average distance between the nitrogen centers of ~ 0.48-0.50 nm, additional short distances of ~ 0.31-0.36 nm were measured by small angle X-ray scattering (SAXS). They correspond to edge-to-face π-π stacking of the TPA cores in the snowflake conformation, and to face-to-face π-π stacking of the TPA cores in the Mercedes-Benz one. [START_REF] Nyrkova | Supramolecular Self-Assembly and Radical Kinetics in Conducting Self-Replicating Nanowires[END_REF][START_REF] Armao | Healable Supramolecular Polymers as Organic Metals[END_REF] The number of amide substituents on the TPA core is also an important parameter to understand the stacking behavior as well as the thermodynamic and kinetic stabilities of their corresponding supramolecular polymers. The amide functions can either interact with one another along a single supramolecular column, or between distinct columns by lateral interactions. For instance, C3symmetric TATAs with bulky lateral alkyl chains have been shown to usually produce single columnar arrangements made of Mercedes-Benz stacks connected by three infinite strings of intracolumnar hydrogen bonds. To adapt the characteristic hydrogen bond distance with the N-N distance, the rotational angle between two consecutive stacks was experimentally shown to increase up to 20° (as compared with the 1.5° of a simple TPA model), therefore decreasing the pitch of the corresponding helical polymer (Figure 1C). [START_REF] Armao | Healable Supramolecular Polymers as Organic Metals[END_REF][START_REF] Kim | Dynamic Propeller Conformation for the Unprecedentedly High Degree of Chiral Amplification of Supramolecular Helices[END_REF][START_REF] Armao | Anisotropic Self-Assembly of Supramolecular Polymers and Plasmonic Nanoparticles at the Liquid-Liquid Interface[END_REF][START_REF] Adelizzi | Unravelling the Pathway Complexity in Conformationally Flexible N -Centered Triarylamine Trisamides[END_REF] It should be noted that carbonyl-bridged triarylamine tris-amides, presenting an achiral flat TPA core substituted with large chiral alkyl chains, also lead to similar helical supramolecular polymers. [START_REF] Haedler | Long-Range Energy Transport in Single Supramolecular Nanofibres at Room Temperature[END_REF][START_REF] Haedler | Pathway Complexity in the Enantioselective Self-Assembly of Functional Carbonyl-Bridged Triarylamine Trisamides[END_REF] In striking contrast, the use of similar C3-symmetric TATAs with a minimal steric hindrancei.e. with three amides substituted by methyl groupsleads to a snowflake conformation and to a highly ordered 3D supramolecular framework organized by inter-columnar hydrogen bonds, as recently characterized by X-ray diffraction (XRD) of single crystals (Figure 1D). [START_REF] Armao | Long-Range Energy Transport via Plasmonic Propagation in a Supramolecular Organic Waveguide[END_REF] In the case of mono-amide TAA derivatives, only snowflake conformations were characterized by SAXS and XRD to date, revealing that the amide groups are here instrumental to self-assemble two columnar stacks along a zipper-like string of hydrogen bonds (Figures 1E). [START_REF] Nyrkova | Supramolecular Self-Assembly and Radical Kinetics in Conducting Self-Replicating Nanowires[END_REF][START_REF] Schneider | Columnar Self-Assemblies of Triarylamines as Scaffolds for Artificial Biomimetic Channels for Ion and for Water Transport[END_REF] Going further, the primary structures of these monodimensional supramolecular polymers can often lead to higher order aggregation processes which produce gels, mesophases, and colloidal mesostructures of various aspects. The mesoscale organization was shown to be dependent on several factors including the steric bulkiness of the TAA monomers, the mutual affinity of their substituents ruling lateral aggregations, the contour and persistence length of the primary fibrils, the number of TAA moieties contained in the monomer, etc. Obviously concentration and temperature also play an important role as it is the case for any supramolecular self-assembly.

Interestingly, TAA derivatives were shown to produce columnar stacks even when bearing very large substituents in solution, [START_REF] Moulin | Light-Triggered Self-Assembly of Triarylamine-Based Nanospheres[END_REF][START_REF] Kumar | Enhanced Photovoltaic Efficiency via Light-Triggered Self-Assembly[END_REF][START_REF] Wolf | PH and Light-Controlled Self-Assembly of Bistable [c2] Daisy Chain Rotaxanes[END_REF][START_REF] Moulin | Self-Assembly of Benzene-Tris(Bis(p-Benzyloxy)Triphenylamine)Carboxamide[END_REF] in thin films, [START_REF] Kumar | Enhanced Photovoltaic Efficiency via Light-Triggered Self-Assembly[END_REF][START_REF] Aytun | Self-Assembling Tripodal Small-Molecule Donors for Bulk Heterojunction Solar Cells[END_REF] or in the bulk with the formation of various liquid-crystalline mesophases (Figure 1F). [START_REF] Domoto | Control over Nanostructures and Associated Mesomorphic Properties of Doped Self-Assembled Triarylamine Liquid Crystals[END_REF][START_REF] Gorbunov | Switchable Charge Injection Barrier in an Organic Supramolecular Semiconductor[END_REF] Strikingly, even when TAA are conjugated to substituents as large as fullerene moieties, 1D or 3D supramolecular polymers can be obtained (Figure 1G). [START_REF] Busseron | Light-Controlled Morphologies of Self-Assembled Triarylamine-Fullerene Conjugates[END_REF][START_REF] Gavat | 3D Supramolecular Self-Assembly of [60]Fullerene Hexaadducts Decorated with Triarylamine Molecules[END_REF] Overall, after a few years of investigations, more and more evidence highlights that the propensity of TAA derivatives to stack with one another embodies a quite general rule. As soon as the TPA core is substituted with at least one amide group, it appears very tolerant to a number of supplementary chemical modifications and it tends to form supramolecular polymers. To go further with the basic knowledge and potential implementations of these self-assemblies, it appears of first importance to better understand the thermodynamic and kinetic parameters that govern their supramolecular polymerization.

Dynamic aspects of triarylamine-based supramolecular polymers

Supramolecular polymerization of triarylamine mono-amides

The first mechanistic investigations on the supramolecular polymerization of triarylamines were performed with TAA mono-amides. [START_REF] Nyrkova | Supramolecular Self-Assembly and Radical Kinetics in Conducting Self-Replicating Nanowires[END_REF] This was motivated by the intriguing behavior of molecule 1 in chloroform (c ≈ 1 -10 mM) (Figure 2A). Although highly stable when kept in the dark, the yellow solution turns slightly green when exposed to visible light for a few minutes. This color change is caused by the production of a small quantity (typically between 0.1% and 1%) of triarylammonium radicals 1 •+ (Figure 2A), which subsequently triggers a supramolecular polymerization process leading to the nanorods shown in Figure 1E(iii).

This polymerization was shown fully reversible by using thermal heating or dissociating solvents.

The involved mechanism was studied in depth by a large combination of experimental and theoretical tools, and it was shown to proceed as follows (Figure 2B). The first activation step is related to the metastable character of unimer 1 because of its conformation. Indeed, neutral 1 adopts a slightly pyramidal conformation of its sp 3 nitrogen center with a strong out-of-plane rotation of the aromatic rings which avoid polymerization on its own. However, 1 •+ becomes flat on its sp 2 nitrogen center and the ring rotation angles decrease significantly (Figure 2B(i)). We further detailed how the flat radicals form strings of dipoles with Cl -and then facilitate dipoledipole and stacking interactions, and how TAA combine in a snowflake conformation to produce hydrogen-bonded double columnar nuclei involving six molecules of 1 •+ from an extremely low critical concentration of 10 -8 M (Figure 2B(ii)). Once double columnar nuclei are formed, they can attract neutral TAA 1 because the energy gain of attachment is now sufficient to flatten the free propellers from the solution (Figure 2B(iii)). Therefore, the nucleation step provides a platform for the end-growth of the double-columnar supramolecular polymer that can in turn bundle in larger fibers and nanorods by making use of van der Waals interactions provided by lateral alkyl chains (Figure 2B(iv)). Interestingly, the same supramolecular polymerization process can be triggered in the absence of light but in the presence of a catalytic quantity of chemical oxidant (tetrabromoquinone), or by seeding the solution of TAA unimers with a catalytic quantity of preformed fibers in suspension. In these three modes of activation (light, chemical oxidant, and seeding) it was shown that sigmoidal aggregation profiles are obtained with lag times that can reach more than one hour for very low quantities of activator (Figure 2C(i)). This autocatalyticself-replicatingbehavior was modeled by a universal equation taking into account the nucleation rate and the scission rate of the fibers which contribute to the acceleration of the process by increasing over time the concentration of polymer ends on which free neutral unimers can associate. This general model of nucleation/growth was further confirmed using advanced light scattering experiments (Figure 2C(ii)), [START_REF] Jouault | Light Scattering Strategy for the Investigation of Time-Evolving Heterogeneous Supramolecular Self-Assemblies[END_REF] and several other light-triggered supramolecular aggregations of mono-, bis-, and tris-amide TAA derivatives were additionally reported independently in the literature. [START_REF] Kumar | Enhanced Photovoltaic Efficiency via Light-Triggered Self-Assembly[END_REF][START_REF] Kim | Induction and Control of Supramolecular Chirality by Light in Self-Assembled Helical Nanostructures[END_REF][START_REF] Ma | Supramolecular Self-Assembly Carbazolyl Radicals Nanospheres Triggered by Ultraviolet Light for Explosives Sensing[END_REF] Interestingly, the analytical equations supporting our model also indicate that the length of the polymer chains decreases for higher oxidation levels (and thus for higher light intensity), whereas the number of nuclei (and therefore the number of fibrils) increases. [START_REF] Nyrkova | Supramolecular Self-Assembly and Radical Kinetics in Conducting Self-Replicating Nanowires[END_REF] This was for instance illustrated by irradiating TAA-fullerene conjugates with different light sources. [START_REF] Busseron | Light-Controlled Morphologies of Self-Assembled Triarylamine-Fullerene Conjugates[END_REF] It was shown that with a halogen lamp, the quantity of generated radicals (and the number of nuclei) was small, thus leading to long supramolecular columns coagulating in larger ribbons (Figure 2D(i)). However, when using UV light at the same concentration, the high number of nuclei leads to short rods of about 10 stacks, which in turn hierarchically aggregate in micrometric spheres (Figure 2D(ii)). This example illustrates the interest to deeply understand such nucleation/growth mechanisms in order to tune the morphologies of supramolecular polymers at various length scales.

Further, we recently proposed that the initial light-irradiation step can also be replaced by a direct electrochemical oxidation, and we reported what can be considered as the first example of a "supramolecular electropolymerization" process. [START_REF] Ellis | Supramolecular Electropolymerization[END_REF] The mechanism involved is very similar to the one proposed in Figure 2B, but with the nuclei being here produced by electro-oxidation at the anode. Interestingly, solvents with a low dielectric constant were used to trigger the nucleation at the anode surface only and, subsequently to directly interconnect the anode with the cathode by precisely orienting the fibers growth in the electric field, as demonstrated in microelectronic devices of various configurations such as ITO cells or interdigitated electrodes (Figure 2E(i-iii)).

This unprecedented spatial precision of the supramolecular polymerization was demonstrated by bridging microarray electrodes of less than 5 µm in diameter and over a distance of 50 µm in length (Figure 2E(iv)).

Supramolecular polymerization of triarylamine tris-amides (TATA)

TATA molecules tend to stack more easily than their monoamide counterparts as they are less metastable in ambient conditions. We demonstrated for the first time in 2014 that solutions of TATA can stack in a racemic mixture of well-defined M and P helical supramolecular polymers, even at low concentration and room temperature, and that such supramolecular polymers can produce thermoreversible physical gels at higher concentrations (Figure 3A). [START_REF] Armao | Healable Supramolecular Polymers as Organic Metals[END_REF] The remarkable stability of these structures, and their propensity to polymerize without external trigger, are permitted by the supplementary set of hydrogen bonds compared to their mono-amide counterparts. Although being crossed spontaneously, the activation barrier related to the flattening of the propeller core in TATA is still present, and is partly responsible for large thermal hysteresis in the polymerization processes. While organogels based on TATA were previously reported in one paper of the literature, no suggestion of a supramolecular structure was made at that time. [START_REF] Yasuda | 4,4',4''-Tris(Stearoylamino)Triphenylamine as a Novel Material for Functional Molecular Gels[END_REF] Recently, based on the demonstrated helical nature of the TATA stacks, [START_REF] Armao | Healable Supramolecular Polymers as Organic Metals[END_REF] a number of mechanistic studies related to their supramolecular polymerization were published by several research groups. [START_REF] Kim | Dynamic Propeller Conformation for the Unprecedentedly High Degree of Chiral Amplification of Supramolecular Helices[END_REF][START_REF] Adelizzi | Unravelling the Pathway Complexity in Conformationally Flexible N -Centered Triarylamine Trisamides[END_REF][START_REF] Kim | Induction and Control of Supramolecular Chirality by Light in Self-Assembled Helical Nanostructures[END_REF][START_REF] Kim | Helicity Control of Triphenylamine-Based Supramolecular Polymers: Correlation between Solvent Properties and Helicity in Supramolecular Gels[END_REF][START_REF] Adelizzi | Supramolecular Block Copolymers under Thermodynamic Control[END_REF][START_REF] Adelizzi | Painting Supramolecular Polymers in Organic Solvents by Super-Resolution Microscopy[END_REF][START_REF] Osypenko | Temperature Controlled Hierarchical Supramolecular Polymerization of Chiral Triarylamines[END_REF][START_REF] Seo | Peculiar Triarylamine-Based Co-Assembled Supramolecular Polymers That Exhibit Two Transition Temperatures in the Formation of a Coiled Helical Bundle[END_REF] For instance, by adding chiral side chains to the TATA core, Miyajima and Aida demonstrated the possibility to control the M or P helicity of the corresponding supramolecular polymers, and further to fully bias the handedness of a racemate of supramolecular helices made of TATA having no chiral side chains by using only 0.2 mol% of enantiopure TATA (Figure 3B). [START_REF] Kim | Dynamic Propeller Conformation for the Unprecedentedly High Degree of Chiral Amplification of Supramolecular Helices[END_REF] The control of 500 soldiers by only 1 sergeant represents an unusually strong amplification of chirality attributed to the peculiar dynamics of the TATA propellers. The sensitivity of chiral TATA derivatives to environmental parameters was also nicely demonstrated by Jung and collaborators, who have revealed correlations between solvent properties and the selective expression of either left-handed or right-handed helicities in organogels. [START_REF] Kim | Helicity Control of Triphenylamine-Based Supramolecular Polymers: Correlation between Solvent Properties and Helicity in Supramolecular Gels[END_REF] Interestingly, Kim and collaborators published a study in which they observed a full control on the supramolecular chirality of TATA helices by simply using visible circularly polarized light (CPL). [START_REF] Kim | Induction and Control of Supramolecular Chirality by Light in Self-Assembled Helical Nanostructures[END_REF] Indeed, irradiation of TATA in chlorinated solvents with r-or l-CPL induces their self-assembly with opposite chirality, and moreover the resulting handedness could be totally switched by irradiating with a CPL of the opposite rotational direction (Figure 3C).

In parallel, the group of Meijer highlighted a number of very interesting mechanistic aspects. For instance, they proved that the helicity of single supramolecular columns in solution can be switched to the opposite handedness as a function of temperature (Figure 3D). [START_REF] Adelizzi | Unravelling the Pathway Complexity in Conformationally Flexible N -Centered Triarylamine Trisamides[END_REF] The related cooperative nucleation/growth process was shown to produce a given helicity at high temperature, whereas the opposite helicity was obtained when cooling down the system below room temperature through a transition from the first assembled state. The helical transformation was attributed to a molecular conformational inversion of the TATA coupled to an internal helical inversion of the entire column, i.e. taking place without a depolymerization step as suggested by the unusually sharp transition observed when cooling the system. Meijer and collaborators also demonstrated that, in the case of monocolumnar systems made of flat carbonyl bridged TATAs, isodesmic and nucleation/growth pathways can coexist, and that each enantioselective pathway can be selectively accessed by processing the temperature profile. [START_REF] Haedler | Pathway Complexity in the Enantioselective Self-Assembly of Functional Carbonyl-Bridged Triarylamine Trisamides[END_REF] In addition, our group demonstrated that the cooling rate used to induce the gelation of some TATA derivatives can modify the hierarchical chirality expressed at the supramolecular level (Figure 3E). [START_REF] Osypenko | Temperature Controlled Hierarchical Supramolecular Polymerization of Chiral Triarylamines[END_REF] Indeed, we observed that, whereas P-helical fibers were formed from a toluene solution by fast cooling, Msuperhelical fibers were produced upon slow cooling. Temperature-dependent CD measurements revealed a large thermal hysteresis (due to the activation energy necessary to flatten the chiral propellers), as well as strong memory effects illustrating an unusually complex stepwise combination of primary and secondary nucleation events that are at the origin of this structural bifurcation. These last results illustrate that pathway complexity in supramolecular polymerization can be encountered in systems which produce similar primary nucleation steps and primary growths, but which diverge when reaching their secondary or higher organizational order. Finally, Meijer and Palmans recently made use of the dynamics of TATA derivatives to construct supramolecular block copolymers under thermodynamic equilibrium conditions. [START_REF] Adelizzi | Supramolecular Block Copolymers under Thermodynamic Control[END_REF][START_REF] Adelizzi | Painting Supramolecular Polymers in Organic Solvents by Super-Resolution Microscopy[END_REF] By using original super-resolution microscopic techniques, they revealed that block copolymers can be formed either by mixing preassembled homopolymers, or by copolymerization of monomers under slow cooling (Figure 3F). The stability of the multiblock structure was shown to be governed by the fine balance between favorable hydrogen-bonding interactions and small mismatch penalties between two different monomers that avoid complete mixing.

To summarize, a number of recent studies have been performed to understand the mechanisms at work in the supramolecular polymerization of TAA derivatives. The propeller shape of the TAA core plays here again a central role as it requires the crossing of a high energy barrier to allow stacking, and as it drives the chirality of the self-assembly. It also provides a number of possibilities to play with thermodynamically stable and metastable states in order to access different polymer morphologies, or in order to trigger the polymerization by various external effectors such as temperature, concentration, light, chemical oxidants, seeding, or even electric field. These tools afford new possibilities to gain spatial and temporal precision in the supramolecular polymerization processes and, as illustrated in the next section, they also offer new opportunities to provide TAA-based materials with advanced functionalities.

Functional aspects of triarylamine-based supramolecular polymers

Our group early suggested that the red-ox properties of TAA derivatives, if combined with their ability to stack along continuous supramolecular pathways of overlapping π-orbitals, should potentially lead to hole-transporting materials. [START_REF] Moulin | The Hierarchical Self-Assembly of Charge Nanocarriers: A Highly Cooperative Process Promoted by Visible Light[END_REF] The initial experimental evidence of such properties was published in 2012 by performing the direct self-assembly of TAA in between gold nanoelectrodes separated by a distance of 80 nm. [START_REF] Faramarzi | Light-Triggered Self-Construction of Supramolecular Organic Nanowires as Metallic Interconnects[END_REF] The fabrication process took advantage of the in situ supramolecular polymerization triggered by light of a chloroform solution in the presence of an electric field (Figure 4A(i)). The first interesting aspect is that charged TAA nuclei are attracted to the electrode, and that subsequent supramolecular growth follows the electric field lines until bridging the entire gap. This self-construction process produces a highly ordered array of nanowires as probed by AFM (Figure 4A(ii)). The second interesting aspect is the outstanding conducting properties measured for such organic interconnects, with a striking increase of the conductance by six orders of magnitude during the supramolecular closure of the gap (Figure 4B(i)). In addition, the nanowires display ohmic conductivity of 5x10 3 S.m -1 and sustain a current density of 2 x 10 6 A.cm -2 , which are exceptional performances taking into account the interface contact (Figure 4B(ii)). It was also highlighted, by lowering the temperature down to 1.5 K, that the sum of ohmic contact resistance and intrinsic resistance of the organic supramolecular polymer display a metallic character (i.e. with a large density of freely moving charges within the nanowire). Such characteristics are exceptional for purely organic nanostructures and are only surpassed by ballistic carbon nanotubes with highly transparent resistance contacts. Based on these results, theoretical approaches on identical TAA stacks were performed independently by the group of Sanvito using density functional theory (DFT). [START_REF] Akande | First Principles Study of the Structural, Electronic, and Transport Properties of Triarylamine-Based Nanowires[END_REF] Their study confirmed that TAA nanowires are remarkable hole conductors, with a theoretical upper mobility as high as 12 cm 2 .V - 1 .s -1 , a rather large value for any one-dimensional object (except for carbon nanotubes).

Experimentally, very high conductivities were also reported for TATA in the gel state. [START_REF] Armao | Healable Supramolecular Polymers as Organic Metals[END_REF] We showed that such materials can be switched from a semiconducting to a conducting behavior when doped by light irradiation. Various signatures of enhanced charge transport were evidenced upon photodoping, revealing for instance a half-filled polaronic band structure of the nanowires, the presence of temperature-independent delocalized Pauli spins, and a metallic reflectance (Figure 4C). The observation of such supramolecular (i.e. through space) polarons is of high novelty and interest, especially within a soft healable organic material. Interestingly, the groups of Kumar and Stupp reported independently the improvement of solar cells by implementing TAA supramolecular polymers within the active layer. [START_REF] Kumar | Enhanced Photovoltaic Efficiency via Light-Triggered Self-Assembly[END_REF][START_REF] Aytun | Self-Assembling Tripodal Small-Molecule Donors for Bulk Heterojunction Solar Cells[END_REF] In thin films, supramolecular columnar liquid crystals made of TATA can act as a switchable charge injection barrier by combining semiconducting and dipolar functionalities. [START_REF] Gorbunov | Switchable Charge Injection Barrier in an Organic Supramolecular Semiconductor[END_REF] Besides, TAA nanowires were built within insulating mesoporous silica layers in order to access an hybrid conducting material functioning as a biocathode. [START_REF] Licsandru | Self-Assembly of Supramolecular Triarylamine Nanowires in Mesoporous Silica and Biocompatible Electrodes Thereof[END_REF] In a complementary direction, the group of Sanvito investigated by DFT and Monte Carlo simulations the ability of TAA nanowires to behave as organic semiconductors for spintronic applications. [START_REF] Bhattacharya | Spin Transport Properties of Triarylamine-Based Nanowires[END_REF] They reported on the finding of a spin diffusion length of 100 nm at room temperature, a rather large value compared to other organic materials. Independently, Meijer and Naaman first implemented TATA derivatives to control electrons' spin in a water splitting photoelectrochemical cell in order to suppress the undesired production of H2O2 coming from the recombination of OH • radical. [START_REF] Mtangi | Control of Electrons' Spin Eliminates Hydrogen Peroxide Formation During Water Splitting[END_REF] For that, they coated anodes with chiral TAA supramolecular polymers in order to control the spin state of the electronic potential on which the reaction occurs.

In this configuration, the spins of the hydroxyl radicals are aligned in a parallel fashion by the helical structures, so that their recombination into H2O2 cannot take place and thus water splitting is enhanced (Figure 4D).

Optical properties of TAA-based supramolecular polymers have also emerged recently as an intriguing domain of investigations. For instance, a straightforward consequence of the stacked configuration is the appearance of an aggregation induced emission behavior in the visible region upon polymerization (λmax ≈ 500 nm), 10 related to the locking of the free rotations in the propeller. This was implemented with the light-triggered self-assembly of TAA-based nanospheres for sensing explosives with detection limits as low as 120 nM. [START_REF] Adelizzi | Supramolecular Block Copolymers under Thermodynamic Control[END_REF] A more unusual optical property of TATA columns is their capacity to transport light energy over extremely long distances. For instance, Hildner and Schmidt proved possible to transport excitons over 4 µm at room temperature within a single columnar fiber made of carbonyl-bridged TATA H-type aggregates (Figure 4E). [START_REF] Kumar | Enhanced Photovoltaic Efficiency via Light-Triggered Self-Assembly[END_REF] The experimental observations suggest a coherent transport mechanism and make these supramolecular polymers promising candidates for light-harvesting and quantum information technologies. In addition, our research group observed waveguiding properties in single crystals of TATA over distances larger than 10 µm. [START_REF] Moulin | Self-Assembly of Benzene-Tris(Bis(p-Benzyloxy)Triphenylamine)Carboxamide[END_REF] Here, the mechanism proposed to explain such a long range transport was correlated with the presence of metallic electrons in the doped crystals as revealed by their optical, electric, and magnetic properties (Figure 4F,G). Because the waveguiding observed does not amount to a photoluminescence mechanism, we proposed the partial involvement of a plasmonic mechanism. Such an active mechanism couples the incident light and the delocalized metallic electrons within the structures, akin to what is commonly observed in metallic nanowires. This explanation is in agreement with the fact that the energy transport occurs i) only when the crystals are excited by the laser source at their tips, and ii) with different laser excitation wavelengths, even in regions where the crystal does not absorb.

Plasmonic transport was also proposed to explain the optical behavior of hybrid nanostructures connecting arrays of gold nanoparticles by doped TATA nanowires (Figure 4H). [START_REF] Armao | Supramolecular Organic Nanowires as Plasmonic Interconnects[END_REF] In such devices, the optical conductivity of the plasmonic layer increases by about 20 times when compared to the undoped layer, suggesting an efficient exchange of metallic electrons between the gold nanoparticles through the subwavelength organic interconnects. As a whole, the optical properties of TAA-based materials appear extremely exciting for a number of new fundamental endeavors and potential applications.

Finally, one should also mention that TAA derivatives can take advantage of their intrinsic dynamic organizational properties to drive the formation of complex self-assemblies within soft confined spaces. For instance, we have shown that photo-doped fibers of TATA can orientate upon centrifugation in large nematic domains at the chloroform/water interface, and subsequently drive the perfect alignment of strings of oppositely charged gold nanoparticules on their top (Figure 4I). [START_REF] Schneider | Columnar Self-Assemblies of Triarylamines as Scaffolds for Artificial Biomimetic Channels for Ion and for Water Transport[END_REF] In a totally different approach, we have also demonstrated in collaboration with Barboiu and Lehn that TAA substituted with crown ethers can be stacked in confined phospholipid bilayers to form artificial potassium channels (Figure 4J). [START_REF] Aytun | Self-Assembling Tripodal Small-Molecule Donors for Bulk Heterojunction Solar Cells[END_REF] XRD on single crystals revealed that a wire made of alternating K + cation and water molecules is formed during the transport process, as observed in biology for the KcsA potassium channel.

Conclusions

This account describes the discovery of a new family of supramolecular polymers based on the stacking of triarylamine units. It highlights that they can tolerate a large variety of molecular variations and that their formation can be understood by a set of common general rules. The rich thermodynamic and kinetic characteristics of the triarylamine monomers lead to a large collection of self-assembly pathways including nested hierarchical dynamic structures, and permit original modes of activation and control. Further, their emergent electronic, magnetic, and optical properties reveal unprecedented behaviors of great fundamental value and of high potential interest for their implementation as new functional materials. Thus, triarylamine-based supramolecular polymers appear not only as a new class of supramolecular objects, but they also stimulate our imagination by pushing further the boundaries of the current scientific knowledge.

FIGURESFigure 1 .

 1 FIGURES

Figure 2 .

 2 Figure 2. Supramolecular dynamics of triarylamine mono-amides. (A) Oxidation of TAA to

27 Figure 3 .

 273 Figure 3. Supramolecular dynamics of triarylamine tris-amides. (A) Typical gel made of

Figure 4 .

 4 Figure 4. Functional triarylamine-based supramolecular polymers. (A) (i) Schematic device

ACKNOWLEDGMENTS

The authors are deeply grateful to all their collaborators, who have vastly contributed to build the knowledge presented in this paper. They also thank the European Research Council (ERC StG agreement n°257099), the LabEx CSC, the CNRS, the University of Strasbourg and the Institut Universitaire de France (IUF). † This Accounts is dedicated to the memory of our friend and co-author Joe Armao, one of the most brilliant young scientists we have met, and who left us on November 20 th 2018. Joseph J. Armao IV was born and educated in San Diego. He did his PhD under the supervision of Prof. Nicolas Giuseppone and pursued his scientific training by a post-doctoral stay with Prof.

Jean-Marie Lehn. His interests range from science in general to philosophy.

Nicolas Giuseppone is distinguished Professor of chemistry at the University of Strasbourg and member of the Institut Universitaire de France. His research interests are focused on supramolecular chemistry, molecular machines, and functional materials.

AUTHOR INFORMATION Corresponding Author

* giuseppone@unistra.fr

Notes

The authors declare no competing financial interests.

BIOGRAPHICAL INFORMATION

Emilie Moulin completed her