Tailoring Anisotropic Interactions between Soft Nanospheres Using

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Tailoring Anisotropic Interactions Between Soft Nanospheres Using Dense Arrays of Smectic Liquid Crystal Edge Dislocations. Delphine Coursault, Jean-François Blach, Johan Grand, Alessandro Coati, Alina Vlad, Bruno Zappone, David Babonneau, Georges Levi, Nordin Felidj, Bertrand Donnio, Jean-Louis Gallani, Michel Alba, Yves Garreau, Yves Borensztein, Michel Goldmann, and Emmanuelle Lacaze ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b02538 • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 9, 2015

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Tailoring Anisotropic Interactions between Soft Nanospheres Using Dense Arrays of Smectic Liquid Crystal Edge Dislocations. Delphine Coursault,†,‡ Jean-Francois Blach,¶ Johan Grand,§ Alessandro Coati,k Alina Vlad,k Bruno Zappone,⊥ David Babonneau,# Georges Lévi,§ Nordin Félidj,§ Bertrand Donnio,@,4 Jean-Louis Gallani,∇ Michel Alba,†† Yves Garreau,‡‡,¶¶ Yves Borensztein,†,‡ Michel Goldmann,†,‡ and Emmanuelle Lacaze∗,†,‡ CNRS UMR 7588, Institut des NanoSciences de Paris (INSP), 4 place Jussieu, 75005 Paris, France, Sorbonne Universités, UPMC Univ Paris 06, UMR7588, Institut des NanoSciences de Paris (INSP), 4 place Jussieu, F-75005, Paris, France, UMR 8181, Unité de Catalyse et de Chimie du Solide - UCCS, Univ. Artois, Faculté des Sciences Jean Perrin, SP18, F-62300 Lens, France, Interfaces, Traitements, Organisation et Dynamique des Systèmes (ITODYS), CNRS : UMR7086 Université Paris VII - Paris Diderot, Synchrotron SOLEIL - SixS beamline LOrme des Merisiers Saint Aubin, BP 48 91192 Gif sur Yvette CEDEX, France, CNR-IPCF, Liquid Crystal Laboratory, Università della Calabria, cubo 33/B, Rende, 87036, Italy, Institut Pprime, Département Physique et Mécanique des Matériaux, UPR 3346 CNRS, Université de Poitiers, SP2MI, 11 Boulevard Marie et Pierre Curie, BP 30179, 86962 Futuroscope Chasseneuil Cedex, France, Institut de Physique et de Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504, CNRS-Université de Strasbourg, BP 43, 23 rue du Loess, F-67034 Strasbourg Cedex 2, France, Complex Assemblies of Soft Matter Laboratory (COMPASS), UMI 3254 (CNRS-RHODIA/SOLVAY-University of Pennsylvania), CRTB, 350 George Patterson Boulevard, Bristol, PA 19007, USA , Institut de Physique et de Chimie des Matériaux de 2 Strasbourg (IPCMS), UMR 7504, CNRS-Universit´ e de Strasbourg, BP 43, 23 rue de Loess, ACS Paragon Plus Environment

F-67034 Strasbourg Cedex 2, France, Laboratoire Léon Brillouin UMR12 CNRS-CEA,

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Abstract We investigated composite films of gold nanoparticles (NPs)/liquid crystal (LC) defects as a model system to understand the key parameters, which allow for an accurate control of NPs anisotropic self-assemblies using soft templates. We combined spectrophotometry, Raman spectroscopy and grazing incidence small-angle X-ray scattering (GISAXS) with calculations of dipole coupling models and soft sphere interactions. We demonstrate that dense arrays of elementary edge dislocations can strongly localize small NPs along the defect cores, resulting in formation of parallel chains of NPs. Furthermore, we show that within the dislocation cores, the inter-NPs distances can be tuned. This phenomenon appears to be driven by the competition between ”soft (nano-)sphere” attraction and LC-induced repulsion. We evidence two extreme regimes controlled by the solvent evaporation: i) when the solvent evaporates abruptly, the spacing between neighboring NPs in the chains is dominated by Van der Waals interactions between interdigitated capping ligands, leading to chains of close-packed NPs. ii) when the solvent evaporates slowly, strong interdigitation between the ligands is avoided, leading to a dominating LC-induced repulsion between NPs associated with the replacement of disordered cores by NPs. The templating of NPs by topological defects, beyond the technological inquiries may enable creation, investigation and manipulation of unique collective features for a wide range of nanomaterials.

It is well known that assembling nanoparticles (NPs) causes changes in their individual properties due to the electromagnetic coupling with the neighboring NPs. 1–3 These changes ∗

To whom correspondence should be addressed INSP-CNRS ‡ UPMC ¶ UCCS § ITODYS-Paris VII k SOLEIL ⊥ Licryl # Pprime @ IPCMS 4 COMPASS ∇ IPCMS †† LLB-CEA ‡‡ SOLEIL ¶¶ MPQ-ParisVII †


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increase with the coupling strength and obviously rely on the distance between the NPs in the assembly. For metallic NPs, localized surface plasmon resonance (LSPR) become shifted. 4 Exciton coupling for fluorescent NPs can be observed 5 or plasmon-exciton coupling for hybrid systems. 6 If, in addition, single oriented chains of NPs can be formed, anisotropic properties are expected, 7 like creation of oriented giant spins for magnetic NPs; 8 or plasmon coupling, 9 and exciton 10 coupling activated by light polarization for metallic NPs and quantum dots. In this context, directed self-assembly of nano-colloids provides ways to build original nanostructured materials on a large scale. 11 One major advantage of such approaches is that ultra-small distances between NPs can be achieved, in contrast with other microfabrication techniques such as e-beam lithography. However, despite the numerous works devoted to the achievement of this goal over the past decade, combining large scale NP assembly of controlled geometry together with a nanoscale control of the NP distance remains challenging. The success of these nanoscale self assemblies depend crucially on our ability to understand and ”engineer” the interactions between nanoscopic particles. 12 In this work, we demonstrate that liquid crystal topological defects can lead to the formation of oriented single chains of NPs and we address the relation between this directed assembly and the interaction between NPs that monitor the side-to-side spacing between NPs in the chains. A main research thematic focuses on the use of LCs to tune NPs properties. 13–18 It is wellknown that various LC matrices allow for an anisotropic directed assembly of micrometersize colloids, the first examples corresponding to nematic cells. 19,20 These assemblies are mediated by the interaction between liquid crystal topological defects associated with each colloid. 21,22 The phenomenon occurs for sizes of the colloids as small as 20 nm, 23,24 however, for really small nanoparticles, with a diameter of some nanometers, topological defects can no more be defined. As a result, an easy and uncontrolled aggregation of the nanoparticles is observed in most of the liquid crystals. 17 A number of techniques must be developed to overcome this problem, 25 including advanced functionalization of the nanoparticles. 26,27 An


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interesting alternative strategy consists in confining nanoparticles in specific zones in the manner of nanoparticles which can be trapped at liquid-liquid interface for simple liquids. 28 In this frame, LCs present the advantage to easily present topological defects, without the prior presence of nanoparticles, either in droplets, 29,30 or in oriented films. 17 Hence, an emerging class of LC systems for directed assembly of NPs, relies on topological defect networks. NPs can be trapped and accumulated in the defect cores, reducing the molecular disorder and the free energy of the LC phase. 9,31–34 This leads to LC phases stabilized in temperature, in presence of NPs. 31,32 In addition, LC topological defects of well-defined size, shape and orientation may lead to well-defined size shape and orientation of NPs assemblies. 9 However does the LC nature of topological defects significantly influence the NPs spacing? For conventional 2D and 3D assemblies, the ligands grafted onto the NPs to avoid their flocculation play an essential role in controlling the spacing. 35–37 The control of the solvent evaporation allows the modification of swelling of these ligands by the surrounding medium 38 and thus modification of the inter-particle spacing. Is this process modified for NPs trapped in topological defects? We have succeeded in creating NPs chains using oriented patterns of linear smectic defects, known as oily streaks , displaying anisotropic plasmonic properties. 9 We now aim at probing the structure of the smectic LC/NPs composites down to the molecular scale. We use grazing incidence small-angle X-Ray scattering (GISAXS), together with UV-vis and Raman spectroscopies, leading in addition to the demonstration that such a combination of techniques is particularly suitable for the nanometric characterization of plasmonic/organic hybrid materials. First the nature of the topological defects, acting as traps for the NPs and leading to the formation of chains has been evidenced together with their location. Second, the dual origin of the spacing between NPs is evidenced, being at the same time controlled by the ligands interdigitation and by the topological defect core disorder. We establish the possibility to tune the spacing between NPs in LC topological defects if strong interdigitation between the ligands can be avoided. Considering that smectic dislocations constitute


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a model system, we believe that our findings can be generalized to composites based on topological defects of lamellar compounds.

Figure 1: Oily streaks structure. a) Top view: optical microscopy image in transmission between crossed polarizers. b) Simplified 3D schematic of the oily streaks where smectic layers are arranged in flattened hemicylinders, with the molecular orientation within the smectic layers shown in black. The flattened hemicylinders are separated by curvature walls (W) and are associated with a rotating grain boundary around C, the axis of curvature of the smectic layers. 39–41 Dislocations, parallel to the hemicylinder axis, are expected to be concentrated in these two areas. 40,42

Composite films Composite NP/ smectic LC films, with an average thickness around 200 nm, were prepared using two series of gold NPs coated with dodecanethiol molecules, respectively NP1s (spheres with diameter 3.9 nm, dispersity 13 % , spectral position of the LSPR λLSPR =513 nm in toluene) and NP2s (spheres with diameter 4.2 nm, dispersity 18 %, λLSPR =516 nm in toluene). The mixture NP/LC were prepared in toluene. This solvent allows for an homogeneous dispersion of the NPs, preventing the NPs from aggregating prior to the formation of the composite NP/LC film. The strong affinity of the toluene molecules with the ”dodecane”(thiol) molecules leads to a swelling of the ligands around the NPs, stabilizing the dispersion. For NP1s, two cases were investigated to allow the formation of strongly, 9 or respectively poorly interacting NPs. In a first case, referred to as ”abrupt evaporation”, the 6 ACS Paragon Plus Environment

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solution was deposited on the substrate and left evaporating (for 2 min. at 60◦ C) before cooling down to room temperature. In a second ”controlled evaporation” case, the solvent evaporation was controlled by confining the solvent droplets between the substrate at 60◦ C and a pipette for 1 minute. Then the solvent was evaporated (2 minutes at 60◦ C), and finally the sample was cooled at room temperature. The second protocol allowed for a slower evaporation of toluene, also leading to a more homogenous film in contrast to the first protocol. For NP2s, only the ”abrupt evaporation” was applied. At the nematic-smectic transition, 8CB stripes (fig.1a), known as smectic oily streaks, appear in optical microscopy, parallel to X direction. Oily streaks have been evidenced on crystalline substrates, MoS2 39,40 and mica, 43,44 on rubbed PVA polymer, 9,41 but also in closed cells, where their structure can be controlled by the electric field. 45 In open cells, they have been studied by Optical Microscopy, AFM, X-ray diffraction 39,40,43,44 and ellipsometry. 41 Their presence is independent of thesolvent evaporation process and occurs with or without NPs. They are created by the competition between the unidirectional planar anchoring of the LC director (mesogens (LC molecules) parallel to the substrate) along the rubbing of the PVA substrate (Y direction - fig 1b), opposed to the homeotropic (normal) anchoring induced at the air interface (Z direction). These antagonistic boundary conditions lead to curved smectic layers in flattened hemicylinders parallel to the substrates (X direction - fig.1b), 40,41 the 8CB molecules rotating with the smectic layers in the (Y, Z) plane perpendicular to the hemicylinders which appear as straight stripes in optical microscopy (X direction - fig. 1a). 39,43 The oily streaks contain regions where the lamellar structure is strongly distorted. This is in particular the case of the curvature walls, W, between neighboring hemicylinders (fig. 1 b) 39,42 and the region around the rotation axis C of the smectic layers (fig 1b). 40,41 These regions are expected to be composed of edge dislocations all parallel to the hemicylinder axis (X direction - fig.1). 40–42 Smectic dislocations are topological defects, made of elastically deformed smectic layers around a linear core defect with a diameter of the order of the smectic layer width, i.e. of the order of 3 nm for 8CB. 46 Oily


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streaks may form a network of topological defects, composed with dense arrays of straight edge dislocations, all oriented parallel to the stripes (X direction). These dislocations are potential traps for the NPs, in agreement on one hand with the formation of straight chains of gold NPs along the stripes direction, 9 on the other hand with the orientation of nanorods parallel to the stripes. 47

Results NP Chains:LSPR Measurements. In order to understand how the LC can modulate the electromagnetic coupling between NPs, we focus on the spacing between NPs in chains formed in 8CB oily streaks using LSPR (fig. 2) and GISAXS (fig. 3) measurements. The spacing, s, is defined such that the distance between two NP centers, is d, d = D + s, with D the diameter of the gold core. In the smectic oily streaks, for NP1s ”abrupt evaporation”, the formation of chains is detected through the LSPR red-shift with respect to isolated NPs in toluene, occurring for polarization parallel to the 8CB stripes (detected by optical microscopy) only. 9 The extinction spectra for polarization parallel and perpendicular to the stripes are presented in fig. 2 for a volume density of NPs, φ = 0.13%. Since the molecules rotate only in the plane (Y, Z) perpendicular to the substrate and to the stripes, for a normal incidence, there is no modification of the incident light polarization induced by the birefringent LC matrix neither parallel nor perpendicular to the stripes. 41 Systematically, when the light polarization is perpendicular to the 8CB stripes, very little difference is observed with respect to isolated NPs in toluene. On the other hand, for parallel polarization, a red-shift is evidenced (fig 2b), of value varying between ∆λ = 30 nm and ∆λ = 40 nm for 5 different locations on a given sample. 9 The role of the liquid crystal birefringence can be ruled out for the interpretation of the observed anisotropic red-shift. 9 The 8CB molecules rotating in the (Y, Z) plane perpendicular


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Figure 2: a) Extinction spectra for NP1s/LC composites with average concentration φ = 0.13%. Measurements with incident light polarized parallel and perpendicular to the 8CB stripes are plotted in red and black, respectively. The blue dotted line is indicative of the position of the LSPR maximum of NP1s in toluene. b) Calculated extinction spectrum for an infinite chain of NPs in dipole coupling model for a side-to-side spacing between NPs along the chain of s=0.68nm.


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to the stripes (figs 1 b-c), we expect a smaller optical index, no for a polarization parallel to the stripes, than for a polarization perpendicular to the stripes, the optical index being then an average between no and ne . 9 As a consequence, if an influence of the LC birefringence had occurred, we would expect a redshift perpendicular to the stripes with respect to parallel to the stripes. On the contrary, the large observed red-shift parallel to the stripes can only be due to an anisotropic coupling between NPs, occurring along the stripes, in agreement with NPs chains parallel to the stripes. When the NPs concentration is further increased, the redshift remains roughly constant along the stripes, at ∆λ around 40 nm with an increasing of intensity only. Perpendicular to the stripes, the signal varies due to an electromagnetic coupling between the chains (see suppl. S1). It is known that, for a fixed spacing between NPs, the red-shift increases with the number of interacting NPs until a saturation is reached. 48–50 The largest observed redshift for single chains (∆λ = 40 nm) therefore corresponds to long NP1s chains that will be considered as infinite single chains in the following. This value of ∆λ = 40 nm has been measured on 3 different samples, all prepared similarly. The same result has been obtained for NP2s with a maximum red-shift of 35 nm with respect to isolated NP2s in toluene, occurring systematically and being measured on more than 10 various locations on the same sample and this for 4 samples (see suppl. S1). For very small particles, the extinction is dominated by the absorption process, the scattering phenomenon can be neglected. 51,52 We have thus evaluated the spacing between spherical NPs, using a dipole coupling model in the quasistatic approximation (NPs radius R = 2.09 nm ± 0.04 for φ = 0.026 %. We have measured three samples of three different concentrations, each time over 11 different areas. The spacing decreased from < s1 > = 2.09 nm ± 0.04, for φ = 0.026 %, down to a limit value < s1 > = 1.92 nm± 0.02 when φ increased up to φ = 0.13 %. The NPs spacings within the chains obtained by LSPR analysis and GISAXS experiments are summarized in table 1. The comparison between LSPR and GISAXS results was performed on the NP2 sample prepared by ”abrupt evaporation” and gave close result s2 =0.97 nm from GISAXS and s2 =0.92 nm from LSPR. This shows the consistency between the two


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techniques in order to establish the spacing values within the chains. In the case of ”abrupt evaporation”, whereas s1 was smaller than s2 , the two values, for NP1s and NP2s, remain smaller than 1nm. This is associated with a strong deformation and interdigitation of the ligands. In contrast, for a ”controlled evaporation”, we observe for NP1s a large increase of the spacing, from s1 = 0.68 nm (”abrupt evaporation”) to s1 = 1.92 nm (”controlled evaporation”).

Raman measurements To better understand the trapping phenomenon in oily streaks, the nature of the NPs trapping sites allowing for NPs chain formation was investigated using polarized micro-Raman spectroscopy. Smectic composites were prepared at low NPs volume fraction to favor single chains, either with NP1s -”abrupt evaporation” (φ = 0.13 %), NP1s-”controlled evaporation” (φ = 0.065 %) or with NP2s-”abrupt evaporation” (φ = 0.13 %). Pure 8CB films of thickness 200 nm were also studied as reference. Using polarized micro-Raman spectroscopy we probe the average orientation of the LC molecules. 67 The rigid carbon-carbon (double) bonds of the phenyl rings display a symmetric stretching mode at 1600 cm−1 , along the 8CB molecular axis, thus indicative of the 8CB molecule orientation. 68 This Raman band is highly polarized: In a set-up of parallel polarizers, each molecule illuminated by the beam participates to the Raman intensity. The Raman intensity is defined by the diagonal elements of Raman tensor, a and b, related by the angle θ between the molecule and the polarizers: 67

I = (a cos θ + b sin θ)2

(2)

The signal is thus maximum (resp. minimum) when the molecule is parallel (resp. perpendicular) to the polarizers. Fig. 4a shows the reference spectra (pure 8CB) . The larger signal corresponds to the polarized Raman intensity for polarization perpendicular to the stripes (black spectrum) but Raman scattering is also detected for a parallel polarization (red spec-


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trum). In a model of smectic flattened hemicylinders, the molecules remain oriented in the plane perpendicular to the stripes (Y, Z). The scattered intensity parallel to the stripes (X direction) could be due to the contribution of b (in eq.2) and may be also associated with the molecular disorder within the dislocations.

Figure 4: Raman spectra for polarization parallel (red) and perpendicular to the oily streaks (black) a) for pure 8CB thin film; b) for 8CB doped with 0.13% NP2s.

In fig. 4b, we present the Raman signal of 8CB film in presence of NP2s chains (φ = 0.13 %). The intensity is strongly increased perpendicular to the stripes (≈ 4 times) with respect to pure 8CB films (Y direction). Such an increase may be associated with a surface enhanced Raman scattering (SERS) effect, due to the strong amplification of electric field around the NPs displaying LSPR. 69,70 As a consequence, the major part of the Raman intensity shown in fig. 4b may be associated with the molecules in the vicinity of the NPs. In contrast with the perpendicular polarisation, we observe no signal increase with respect to the reference for the polarization parallel to the stripes (X direction). This suggests first that 16 ACS Paragon Plus Environment

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the contribution from b is small (and thus also for pure 8CB film); Second that, in the vicinity of the NPs, the scattered intensity from the mesogens (LC molecules) is almost zero for a projection parallel to the stripes (X direction, perpendicular to the Y anchoring direction). There is consequently almost no disordered molecule around the NPs. Otherwise, a SERS effect would also be present for polarization parallel to the stripes, similarly to the data obtained for pure 8CB films. This observation is in perfect agreement with the assumption that the trapping sites for the NPs are the dislocation cores as also recently suggested by trapping of oriented nanorods. 47 Dislocation cores are expected to be constituted by disordered mesogens with a number of molecules partially oriented along the stripes (X direction). For φ = 0.13 %, the formation of long chains of closed-packed NPs would expel most of the disordered 8CB molecules from the dislocations and thus from the vicinity of NPs (fig.4b). The assumption of trapped chains within dislocation cores is further supported by the observation for low NP concentration (φ = 0.065 %, NP1-”controlled evaporation”) of SERS effect parallel to the stripes as well (see suppl. S3). On the areas with the smallest local NP concentration, isolated NPs may be trapped in disordered dislocation cores, together with chains, small enough to present a non negligible influence of their extremities. Raman signal of the remaining disordered molecules in contact with the isolated NPs and the NPs chains extremities is thus measured, but strongly decreases for locally larger concentrations (suppl. S3-a). For long chains of closed-packed NPs, the absence of disordered 8CB molecules in the surrounding of NPs implies also that the NPs are larger or of the same order of magnitude than the dislocation cores. In oily streaks, for NPs concentration small enough to lead to the formation of single chains in majority, the trapping sites are consequently the elementary dislocation cores, whose width is known to be of the order of the smectic period, 46 thus of the same order of magnitude as the NPs diameter. Moreover a large Raman signal for polarization perpendicular to the stripes suggests that the surrounding molecules in the vicinity of the NPs is in majority composed of planar 8CB molecules. Therefore NPs may


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be localized in elementary dislocations close to the substrate. The formation of single chains with elementary dislocations appears possible due to the high density of dislocations. When the density of dislocations decreases, like for example in wedge cells, NP ribbons are obtained with an accumulation of the NPs, not only in the dislocation cores, but also in the dislocation region, where layers are dilated. 34,71

Comparison with NP monolayers In order to understand the role of the smectic LC for the establishment of the spacing between NPs, we have compared the distances between the NPs organized in chains within the smectics to the ones obtained for NPs deposited on rubbed PVA without LC. We have used the same two protocols of deposition, ”abrupt” and ”controlled” evaporation. Without LC, a LSPR red-shift, insensitive to light polarization, is observed. It is independent of the NPs concentration and is consistent with the expected formation of close-packed domains with hexagonal symmetry. 9,12 Again the large observed red-shift suggests that the 2D assemblies are equivalent to infinite networks, allowing to employ the dipole coupling model to extract the spacing values from the measured LSPR wavelength. The s values summarized in Table 1 are obtained. The calculations for 2D monolayers and for chains were performed under the same conditions. They allowed for a fair comparison of the evaluated distances between NPs in chains in smectics and NPs monolayers without LC, in particular considering that NPs are surrounded by dodecanethiols. However they may lead to a slight overestimation of the spacing for monolayers, because, in contrast with NPs embedded in LC, NPs are surrounded by ligands and by air. Table 1 evidences a different LC role on the spacing, depending on the protocol used. For ”abrupt evaporation”, NPs spacings within monolayers without LC are similar to the ones in chains embedded in LC, for NP1s and NP2s. In contrast, for ”controlled evaporation” and NP1s, the spacing in LC is clearly larger than the one without LC (1.92 nm versus 0.94 nm). In addition, table 1 reveals that, without LC, spacing values are larger for ”controlled 18 ACS Paragon Plus Environment

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Table 1: Calculated side-to-side spacing between NPs, determined using LSPR measurements with parallel incident polarization. Values obtained from GISAXS experiments are in bold. NP1s, diameter 3.9 nm NP2s, diameter 4.2 nm Abrupt evap. Controlled evap. Abrupt evap. λLSP R (nm) s(nm) λLSP R (nm) s(nm) λLSP R (nm) s(nm) NPs chains in LC 553 0.68 Ø 1.92 551 0.92 (0.97) NPs monolayer 567 0.6 549 0.94 553 0.98 evaporation” with respect to ”abrupt evaporation”. This allows for a more precise qualification of these two regimes. It has been recently shown in 3D crystals formed with NPs of diameter around 5 nm, also coated by dodecanethiols, that the distance between NPs, obtained after solvent evaporation, could be tuned by the solvent rate of evaporation. 36 These results evidence that, when solvent evaporation is slow enough, the swelling of the ligand by the solvent can occur leading to an increased distance between NPs, with respect to fast evaporation. 36,72,73 There is a strong affinity between the dodecanethiol and toluene molecules that allow for the solvent molecules to penetrate close to the gold core., 74 and also induce a more stretched geometry of the ligands that may enhance the repulsive interaction between NPs with ligands. This phenomenon may explain as well the larger distance observed in the present 2D monolayers with ”controlled” evaporation regime compared with the ”abrupt” one (0.94 nm versus 0.6nm). As a result, we expect less entangled ligands for ”controlled” evaporation. The small spacing values obtained for NP1s and NP2s with the ”abrupt” evaporation regime suggest in contrast a 2D monolayer structure with highly entangled ligands. For ”abrupt evaporation”, it can be postulated that the swelling of the ligands by the solvent is negligible due the fast evaporation. This leads to a smaller distance between NPs, characterized by highly entangled ligands and close to the structure simulated for gold NPs dimers and trimers, again covered by dodecanethiols. 75,76 We can thus use the phenomenological rule predicted in these simulations to establish the spacing values, scalc = 0.25×2R, 75 the rule being valid either for 1D (dimers) or 2D assembly (trimers). 76 For NP1s, the experimen19 ACS Paragon Plus Environment

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tal spacing is even smaller than the phenomenologically expected one (s1exp =0.6 nm versus s1calc = 0.97nm). This rule has been established for nanocrystals fully covered by ligands. This suggests that NP1s, purchased from a commercial producer, may have lost grafted dodecanethiols, since repulsion between ligands is diminished for a smaller ligand density, leading to a decrease of spacing. 75 A spacing in agreement with the phenomenological rule has been obtained for NP2s (synthesized in our laboratory) s2exp =0.98 nm versus s2calc = 1.05nm.

Discussion In smectic liquid crystals, the core of an elementary dislocation is a region with high energy density. In 8CB, the corresponding line tension has been measured in free standing smectic films to be equal to 0.5 kT/˚ A. 77 Replacing a portion of the core with a NP of diameter 4 nm reduces the core energy by an amount 20 kT. Regardless of any attraction due to elastic LC deformation, 33 this is sufficient to generate an effective attractive potential that stably traps the NP in the dislocation core during solvent evaporation. The orientation of the induced NP chains is imposed by the one of the dislocations. We here demonstrate that the spacing between NPs in the chains is also specific to the LC structure with topological defects: it is equal or larger than the one obtained in monolayers without liquid crystals, in contrast with NPs aggregates formed in the LC cholesteric bulk without topological defects. 58 In cholesteric LCs, the LC matrix induces a compression between NPs, leading for NP2s, to s2 = 0.6 nm, 58 to be compared with s2 = 0.98 nm for monolayers without LC and with s2 = 0.92 nm for chains in smectics (Table 1). This small s in cholesterics was related to the disorder induced around the NP2s in the LC matrix, which can be diminished by decreasing the spacing between NPs. 58 This behaviour is in contrast with the one observed here in oily streaks which shows similar spacings than in the monolayers without LC for ”abrupt evaporation” and larger ones for ”controlled evaporation” (Table 1). This further confirms


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the presence of topological defects in oily streaks, no additional disorder being induced since the NPs are trapped in already disordered dislocations cores. On the contrary, the disorder may be decreased thanks to the expelling of 8CB disordered molecules in presence of NPs from the dislocation cores. We do not yet fully understand the origins of the intercolloidal forces that control the spacing of the nanoparticles, but infer that it may arise from local ordering of the mesogens in the defects. The presence of the LC dislocations may induce an additional repulsion between NPs along the chains. If NPs are stabilized into dislocation cores, we expect them to be organized in order to expel the maximum of disordered 8CB molecules. As a result the region between NP core, extended over the so-called spacing s, should be filled by the (alkylthiol) ligands only. Accordingly, as shown by the Raman measurements, there are no disordered 8CB molecules in between NPs. For a given number of NPs, N , involved in the chain, the length L over which disordered 8CB molecules of the dislocation core are expelled is L = N (D + s), D, being the diameter of the NP gold core. The energy advantage of localizing the chain in the dislocation core is L × ELT with ELT the 8CB dislocation line tension. To reduce the dislocation core energy, for a fixed NP number, N, the length has to be increased, so s has to be increased, through modification of alkylthiol configuration and interdigitation, in relation with an LC-induced repulsion. Together with the steric repulsion between NPs, this LC-induced repulsion should compete with Van der Waals attraction between NPs. We use ELT = 0.5kT /˚ A to interpret our data, together with the identification of two regimes: the first regime is characterized by an ”abrupt evaporation” and no real influence of the LC on the spacing value; The second regime is characterized by a ”controlled evaporation” and a larger spacing in LC with respect to 2D monolayers without LC. In order to understand how we can pass from one regime to another, it appears appropriate to compare in the two regimes the expected LC-induced repulsion between NPs to the expected Van der Waals attraction. For NP1s-”abrupt evaporation” (first regime), if the Van der Waals attractions are associated with the ones simulated for NPs dimers and trimers in air, they are expected to


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be dominated by the interactions between interdigitated ligands. 75,76 For NPs coated with dodecanethiols in air, the corresponding force has been calculated to be 5.8 kT/˚ A for a NP diameter of 1.9 nm. 75 It increases with the NPs size, up to 8.6 kT/˚ A for NPs of diameter 2.7 nm. The likely reason why the spacing between NPs measured in the chains is the same as in the 2D monolayers without LC is that LC-induced repulsion, 0.5 kT/˚ A, is much smaller than Van der Waals attractions. The similarity of the distances between NPs in 2D NPs monolayers without LC and NPs in chains in smectics also suggests that, despite the modification of environment, from air to LC, the Van der Waals attraction does not significantly vary. This may be due to the dominating influence of the surrounding thiol shell, at least for closed distances between NPs. For NP1s-”controlled evaporation” (second regime), due to the former swelling of the solvent, the ligands may be less interdigitated after solvent evaporation. This is confirmed by a larger spacing for NP1s-”controlled evaporation” without LC with respect to NP1s”abrupt evaporation” without LC (Table 1). The Van der Waals attraction between ligands thus decreases and may become of the same order of magnitude of the line tension in LC. A low limit value of Van der Waals attractions is the one considering the attraction between the Au NP cores in an organic medium. For a spacing between NPs, s=0.94nm, the equilibrium spacing measured in 2D monolayers without LC, we calculated the corresponding attractive force to be 0.37kT /˚ A (using the known Au Hamaker constant in toluene 73 ). Therefore, the LC-induced repulsion is no longer negligible compared to Van der Waals attraction and we accordingly observe an increase from s = 0.94 nm in 2D monolayers without LC to s = 1.92 nm for chains in smectic oily streaks. This interpretation may be confirmed in the future by the demonstration that, under ”controlled evaporation”, the distance between NPs can be controlled by the nature of the topological defects through the value of their line tension. The interaction driving the particularly large increase of spacing values for ”controlled evaporation” in oily streaks may be connected with a larger line tension of the dislocations in oily streaks compared to the one


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in free standing films, possibly due to the large disorientation between the smectic layers from each side of the dislocations in oily streaks. The fact that large spacings are favored between NPs in smectics with topological defects shows that in presence of 8CB molecules, attractive depletion forces 12 remain negligible, probably due to the small size of the NPs with a diameter of the order of the dislocation core. The particularly large observed increase from NP spacings in 2D monolayers to NP spacings in chains in oily streaks could also be due to a swelling by 8CB molecules between the NPs in the chains. This hypothesis appears contradictory first with the fact that the 8CB molecule size is about 2.1nm, larger than spacing between the NPs ; second with the Raman results (see Suppl. S3), which evidence quasi-no 8CB molecules parallel to the stripes in the vicinity of the NPs, for large enough NP chains. The swelling by toluene may be also more efficient in LC with respect to 2D monolayers without LC. It is thus worth noting that the increase of NP spacing in dislocations is expected to depend on the phenomenon of disentanglement of the ligands which is monitored by the solvent evaporation. It consequently may depend on the swelling efficiency by the solvent, this latter one being related to the density of ligands around the NPs. It would be now interesting to test this hypothesis through a systematic measurement of the relationship between ligand density around the NPs and induced spacing in NP chains trapped in oily streaks.

Conclusion We have shown that in presence of a high density of elementary edge dislocations, gold NPs can be strongly confined along the defect cores leading to chain formation and anisotropic plasmonic properties, the LSPR being controlled by the NPs spacing in the chains. It is known that, in absence of defects, for appropriate ligands, NPs can be well dispersed in LCs, 78 or, if the ligands induce disorder, they form aggregates with LC-induced compression between NPs. 58,79 Systems with topological defects like smectic LC therefore bridge the gap


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between these two extreme cases, maintaining the NPs confined within the disordered core, with a spacing controlled by the ligands and by the LC. We have shown that the control of the solvent evaporation appears as a key parameter to tune NPs spacings. When NPs are confined in smectic edge dislocations, an induced LC repulsion between the NPs within the defect cores may take place, associated with the high energy of the line tension of the smectic edge dislocation. It is not able to compete with the strong Van der Waals attraction between NPs ligands 35 for an abrupt solvent evaporation. However, for a controlled solvent evaporation able to disentangle ligands during the linear NPs self-assembly, the LC-induced repulsion appears to control, the spacing between NPs. Together with the induced linear assembly of NPs along the topological defect cores, the control of the spacing between NPs is a unique feature of the LC film. Directed assembly of NPs using topological defects as template may thus enable manipulation of NPs interactions and investigation of their unique collective properties, with the benefit of revealing the intimate and obscure structure of the organic matrix.

Methods Materials Samples were created by depositing a droplet of a mixture of 8CB (4-n-octyl-4’-cyanobiphenyl, smectic LC at room temperature, c=0.01M, ) and gold NPs in toluene onto a polyvinyl alcohol (PVA) polymer film, initially spin-coated and rubbed on a glass substrate (0.5cm2 ). NP1 are purchased from Aldrich, NP2 synthesis are described elsewhere. 58

Micro-spectroscopy techniques We measure extinction properties of the samples using a LOT Oriel MS260i spectrometer coupled to an upright optical microscope (Olympus BX 51) to probe a 40 × 40 µm

2

areas.

The signal was collected through an air objective (×50, N.A.= 0.52). The composite films 24 ACS Paragon Plus Environment

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were excited with linearly polarized light either along or perpendicular to the oily streaks. Raman spectra were obtained with a XY-800 DILOR spectrometer equipped with an Olympus microscope (objective ×100, N.A.= 0.9) through which the pump beam (Ar/Kr ion laser at 514.5 nm) is focused onto the sample. This allows to probe 1 µm2 areas.

GISAXS Experiments were performed with a square incident X-ray beam of width 300 µm, at an incident angle 0.25◦ , covering several millimeters of the sample along the beam direction (SIXS beamline at French synchrotron SOLEIL, photons energy 18keV). For an incident Xray beam perpendicular to the 8CB stripes (fig.3a), the assembly along the stripes is probed: the horizontal component of the wave vector (qx in the referential of the sample) is parallel to the chain direction and the NPs spacing in the chains can be measured. For an incident X-ray beam parallel to the 8CB stripes, the NPs organization perpendicular to the stripes is probed (fig.3b). The experimental data were analysed using FitGISAXS package 66 to determine the average interparticle distance within the chains.

Acknowledgement We thank Denis Limagne and Christophe Rafaillac respectively for sample holder design and elaboration. We acknowledge Laurent Pelliser and Anja Seidel for their active performing during GISAXS experiment and for helping to extract the experimental data with Frederic Picca. We also thanks Romain Greget for the NP2s synthesis, Dominique Demaille for SEM and TEM experiments, Mohamed Selmane, Stephanie Hajiw and Marianne Imperor for SAXS measurement of the colloids and finally Rolfe Petschek for useful discussions. This work was partly supported by the PNANO program of the French national research agency (ANR) through grant ANR-09-Nano-003, Nanodiellipso and by the Partner University Fund, administered by the Embassy of France in the United States.


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Supporting Information Available Extinction properties and GISAXS pattern for NP2s ”abrupt evaporation”. Raman spectra for NP1s ”controlled evaporation”. This material is available free of charge via the Internet at http://pubs.acs.org/.

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