Achieving Highly Stable, Reversibly Reconfigurable Plasmonic

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Achieving highly stable, reversibly reconfigurable plasmonic nanocrystal superlattices through the use of semifluorinated surface ligands Maciej Bagi#ski, Ewelina Tomczyk, Andreas Vetter, Radius N. S. Suryadharma, Carsten Rockstuhl, and Wiktor Lewandowski Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03331 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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Chemistry of Materials

Achieving highly stable, reversibly reconfigurable plasmonic nanocrystal superlattices through the use of semifluorinated surface ligands Maciej Bagiński,† Ewelina Tomczyk,† Andreas Vetter,§ Radius N. S. Suryadharma,‡ Carsten Rockstuhl,‡,§ Wiktor Lewandowski†* †

Faculty of Chemistry, University of Warsaw, 1 Pasteura st., 02-093 Warsaw, Poland.

‡

Institute of Theoretical Solid State Physics, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany.

§

Institute of Nanotechnology, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany.

ABSTRACT: Controlling stability, complexity and optical response of reversibly reconfigurable plasmonic nanocrystal superlattices is of critical importance for emergent optoelectronic technologies and can be achieved by engineering the chemical nature of the ligand shell. In this work, we experimentally explore how the design of surface ligands with semifluorinated alkyl chains impacts dynamic self-assembly of nanoparticles. A series of three promesogenic thiols was synthetized and grafted onto plasmonic nanocrystals via ligand exchange reaction. In all cases, after solvent evaporation, we obtained reversibly reconfigurable, thermally responsive assemblies. We examined these nanomaterials using a variety of techniques such as transmission electron microscopy, UV–VIS, and small-angle X-ray scattering. We show that the number of aromatic rings and the length of the fluorinated chain strongly affects symmetry and reconfiguration temperatures of the assemblies. For an optimized material we show that it is possible to achieve relatively quick switching between 3 distinct long-range ordered phases, including non-close packed structures. With modeling we confirm that observed plasmonic response of the material comes from the reconfiguration process. Uniquely, we confirm durability of the material in a 400 cycle switching experiment. Overall, these results guide our understanding of influence chemistry of the ligands on reversible reconfiguration of nanocrystal superlattices.

INTRODUCTION Reversibly reconfigurable assemblies of plasmonic nanoparticles (NPs) have recently sparked great interest1–6 as their stimuli-responsive properties offer transformative potential in the fields of metamaterials,7 active plasmonic materials,8 chiral switches,9 nanoactuators/nanomachines10 and chemical and biological sensors.11 The initial efforts in this field were devoted to developing strategies for achieving reversible modulation of nanoparticle assemblies. One approach that earned substantial interest relies on combining plasmonic nanoparticles with stimuli responsive soft matter in the form of surface ligands,12 matrix material13 or a template.14 The big advantage of this approach is that it allows to use a variety of stimuli to drive the rearrangement – e.g. physical,15 mechanical,16 chemical,17 electronic18 or optical19 stimuli can be utilized allowing for remote control over the assemblies. Of further concern to researchers is how to engineer these materials to precisely control their symmetry20 including non-close-packed structures, maximize functional response, achieve fast reconfiguration and long-term stability. Towards meeting these requirements, a number of advancements in the field of reconfigurable nanoparticle assemblies were proposed. From the structural perspective, DNA-based systems were pushed to the level in which they provide access to micrometer size, long-range ordered

assemblies21 (for DNA ligands) or enable fine tuning angle in nanorods dimers (DNA templates).22 A strong optical response can be realized by using e.g. liquid-crystalline templates that allow orientational control over anisotropic nanoparticles.23 Functional response (switchable character of the surface) can also be achieved by clever terminal functionalization of NPs ligands.24 Improvements of the kinetics of reconfiguration have been achieved by modifying the stimuli addressing polymer wrapped nanoparticles systems (switchability down to µs regime)10 or spatial confinement25 for chemically driven, solvated systems. Among the current advancements, the use of liquid crystalline as nanoparticles surface ligands is an interesting approach to switchable systems (liquid crystalline nanoparticles, LC NPs).23,26–28 In fact, this approach is compatible with different types of nanoparticles,29-31 different stimuli,19 enables anisotropic packing of spherical nanocrystals,32–34 promises fast switching times characteristic to LC materials and was shown to provide access to switchable metamaterials.7 The latter is especially interesting since this is one of a few systems that works without a solvent, thus allows to tune collective interactions of nanoparticles.7 The potentially transformative nature of reconfigurable assemblies of plasmonic nanoparticles is however still hampered by finding nanomaterials that realize all the requirements. This is especially true for long-term stability of the structures -

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usually up to tens of cycles of switching are shown in the literature as a proof of concept,7,10,14,18,21,25,30,35–42 which is at least an order of magnitude less than it is usually reported for example for nanomaterials in battery applications.43 Here, we rely on organic chemistry to design LC-ligand coated nanoparticles that form a reversibly reconfigurable plasmonic nanomaterial with long-term stability. The underlying philosophy for preparing the material was based on the concept of hydro- and oleo-phobic nature of fluorinated chains that provide access to materials with selfcleaning properties,44 recently gained much interest in responsive technologies at the micro-/nano-scale.45–48 By optimizing the LC ligand structure, we identified principles enabling us to achieve a material with unique structural characteristic; namely it exhibits three long-range ordered phases with quick reconfiguration properties surpassing the current state-of-the-art. These structural properties translate to a relatively strong optical response. Notably, we also evidence that even in ambient conditions it is possible to prepare an active nanomaterial with long-term switchability using silver nanoparticles that are usually thought as poorly stable.

RESULTS AND DISCUSSION Hybrid nanoparticle design and synthesis To explore how partial fluorination of ligands affects dynamic self-assembly properties of NPs, we designed a set of nanomaterials based on Au/Ag metallic cores and rod-like, thermo-responsive promesogens. These molecules are promesogenic, which means that they can induce formation of LC phases after grafting onto nanoparticles; for the ease of reading our ligands will be referred to as LC-ligands, hereafter, although by themselves they do not form LC phases. The molecular design of ligands was based on an elongated, central aromatic unit to which two, end-functionalized alkyl chains were attached. One of the alkyl chains served as a flexible spacer unit between nanoparticle surface and aromatic core of the ligand; it was equipped with a mercapto moiety that allowed for grafting the ligand onto the nanoparticles surfaces. The second alkyl chain was semifluorinated, with the fluorinated moiety located at the terminal part of the ligand so that it was exposed at the periphery of the ligand shell. Thus, efficient interactions between fluorinated parts of ligands attached to the same and neighboring nanoparticles were assured. It is worth to note that in our previous research we used semilfuorinated thiols to modify reconfigurable behavior of nanoparticle assemblies,47 however the fluorinated moieties were grafted onto the NPs surface directly; thus, interactions between these moieties from neighbor NPs were hindered and did not allow us to achieve efficient changes in reconfigurability. To study the influence of the size of the central and length of the fluorinated units on dynamic self-assembly of NPs, three final compounds were synthetized (L1, L2, L3, Figure 1a, scheme of the synthetic route and synthetic protocols are given in Figure S1). Structure of obtained ligands was confirmed using 13C and 1H NMR spectroscopies (Figure S2). Polarizing optical microscopy investigations have shown that none of these molecules is mesogenic and they melt directly to the isotropic liquid.

Figure 1. Design and synthesis of hybrid materials. (a) Molecular structures of promesogenic ligands L1, L2, L3 used for grafting nanoparticles. (b) Scheme of ligand exchangereaction. (c) TEM image of Ag_C12 - spherical silver nanoparticles covered with dodecanethiol. Scale bar represents 50 nm. (d) TEM image of Ag_L1 material. Scale bar represents 50 nm. As the nanoparticle platform to build LC NPs, we have decided to use hydrophobic, dodecanethiol coated 5.2+/-0.4 nm diameter Ag (Ag_C12, Figure 1c, Figure S3a) and 3.6+/-0.3 nm diameter Au NPs (Au_C12, Figure S3b). Two important facts regarding size of these NPs are that their small size (sub10 nm diameter) and low size polydispersity (below 10%) favor formation of long-range ordered nanoparticle assemblies. Both proposed types of NPs exhibit plasmonic properties which enabled us to investigate not only structural, but also functional switchability of hybrid nanoparticle assemblies. On one hand, the use of Ag NPs was preferred since they exhibit stronger plasmonic properties than Au NPs of the corresponding size, while on the other hand the use of gold should potentially yield materials with greater stability than in the case of silver. To prepare the final hybrid materials, a solution phase ligand-exchange procedure was employed as schematically shown in Figure 1b, allowing to introduce organic molecules L1-L3 to the NPs surface. Time of the reaction and ligand/nanoparticle ratio were chosen based on our previous


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experience7,49,50 to assure that 40-60% of the native ligands covering nanoparticles were exchanged. The exchange rate was confirmed using thermogravimetric analysis (TGA, Figures S4a,b). Importantly, this reaction did not affect the mean diameter of NPs but only slightly increased the metallic core size distribution as evidenced by the TEM (Figure 1d, Figures S3c-d). TEM imaging also evidenced enlarged internanoparticle spacing in self-assembled monolayers (L1-L3 ligands are larger than dodecanethiol) further confirming successful derivatization of nanoparticles (Figure 1d).

Nanoparticle aggregates based on L1 molecules In order to test the hypothesis that semifluorinated chains can enhance the bundling of mesogenic ligands (leading to anisotropic arrangement of nanoparticles), we first prepared hybrid NPs using L1 ligand and Ag_C12 NPs. On the one hand, L1 was chosen since it should assure the highest segregation tendency out of the planned ligands (larger central unit and long fluorinated moiety). On the other hand, silver was chosen since it exhibits the strongest plasmonic properties. Thus, the planned material (Ag_L1) seems a plausible choice for achieving a switchable plasmonic superlattice.

Figure 2. Structural and optical characterization of Ag_L1 material. (a) Temperature evolution of SAXRD pattern during heating. (b) Comparison of XRD peaks location at temperatures of 30°C and 170°C as obtained by integration of suitable patterns. (c) Extinction spectra of the material taken at 30°C and 170°C revealing a shift of plasmon band maxima. (d) Observed shifts of plasmon band maxima during the subsequent measurement cycles. To investigate self-assembly properties of Ag_L1 small angle X-ray diffraction (SAXRD) measurements were conducted. For this purpose a small amount of Ag_L1 concentrated suspension in dichloromethane was dropcasted onto kapton foil, annealed at 120°C and cooled slowly (5°C/min) to room temperature. Both thermal annealing and slow cooling support formation of LC NPs aggregates with long-range order (narrow SAXRD peaks) thus making the SAXRD diffractograms analysis easier in comparison to the as

dropcasted material. In the case of Ag_L1, SAXRD studies of an annealed sample at 30 ° C revealed two commensurate Bragg rings with the main peaks periodicity is 11.2 nm. We further tested whether the material exhibits thermal switchability using temperature-dependent SAXRD measurements. Evolution of the SAXRD pattern was observed while raising the temperature in consecutive steps by 5 ° C. While raising the temperature, only a slight shift of the position of Bragg peaks was observed up to ca. 160°C. At this point a clear phase transition occurred; above this temperature two, relatively broad signals centered at 9.4 nm and 5.9 nm were observed (Figures 2 a,b). Heating of the material above 180 ° C results in decomposition of the nanoparticles. Due to broadness of diffraction signals we did not pursue unequivocal determination of symmetry of the assembled sample, however we appreciate that it is thermally switchable. Since the obtained material is structurally switchable we decided to investigate the influence of the phase transition on the plasmonic resonance of Ag_L1 material. UV-VIS spectra were collected at 30 ° C and 170 ° C, that is at temperatures which correspond to different ordering of nanoparticles. The first heating resulted in a large, 46 nm blue shift of the plasmonic peak maxima position (Figures 2 c,d) indicating larger distance among neighboring particles in the uppertemperature phase, which causes reduced dipole-dipole interactions and tunes with that the entire observable band. Unfortunately, with each subsequent heating/cooling cycle magnitude of the plasmonic peak maximum shift lowered: 32, 28, 19 and 13 nm shifts were observed in cycles 2-5 (Figure 2d). The lack of reproducibility can be explained by low stability of Ag NPs at temperatures above 160°C.51 Thus, it is clear that a high phase transition temperature does not allow us to achieve truly reversible reconfiguration of nanoparticle superlattice. To resolve this problem we decided to prepare an analogous material based on gold nanoparticles which should exhibit higher thermal stability. Structural properties of Au_L1 material were investigated in a manner analogous to the silver-based hybrid. At low temperatures SAXRD measurements of the annealed sample revealed a diffractogram (Figure 3a) which can be interpreted as a lamellar (Lm) system. The Lm phase is a lamellar structure with well-defined distance between layers of nanocrystals and liquid-like (short range) hexagonal order of nanoparticles within a single plane. The diffraction pattern of such a phase (in case of an aligned sample, e.g. by shearing) should consist of a series of narrow (evidencing long-range order), commensurate signals along the common direction (layer normal) and a diffused signal (evidencing short range order) in an orthogonal direction. Indeed, such a pattern (inset in Figure 3a) has been recorded for the sample that we dropcasted onto a kapton foil and mechanically sheared at ca. 120 ° C (which allowed us to prepare a quasi-monodomain structure). Positions of the recorded signals can be recalculated to inter-layer spacing of 11.0 nm, and intra-layer periodicity of 6.6 nm (Figures 3 a,b and Figure S5e). Further confirmation of lamellar ordering of NPs was obtained using TEM measurements. A small amount of diluted Au_L1 solution was dropcasted onto the TEM grid and thermally annealed, as in the case of SAXRD measurements. The obtained TEM micrograph (Figure 3b) show rows of NPs



with ca. 11.2 nm inter-row distance, which corresponds well to the spacing between layers determined with SAXRD measurements. These results confirm a non-uniform distribution of ligands around the metallic core and evidencing bundling of the ligands prompted us to investigate temperature-dependent behavior of the material.

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of the peak indicates a decreasing length of correlation of nanoparticles positions (Figure S5g). This effect most probably results from growing size polydispersity of the material, that is, its deterioration. How the observed structural behavior translates to plasmonic properties of the Au_L1 material was investigated using ultraviolet-visible (UV-VIS) spectroscopy; spectra were collected at 120°C and 200°C to enable direct comparisons between structural and plasmonic properties of the aggregate. A clear, 26 nm shift of the plasmonic peak maximum (from 598 to 572 nm) was observed in the first cycle (Figure 3c); lowering of the shift was also evidenced after few cycles (Figure 3d), however rate of the degradation was much slower than in the case of Ag_L1 material. The lower plasmon resonance can be well explained by the larger interparticle distance in the two arrangements. The performed measurements for both L1-based hybrid materials confirmed that ligands equipped with a semifluorinated terminal chain support achieving reversibly reconfigurable nanoparticles assemblies with dynamically controlled optical properties. However, high temperatures of the phase transition translate to deterioration of the materials with each consecutive switching cycle, even if more stable Au NPs cores were used in the place of Ag NPs. Thus, further work was needed to lower the switching temperatures of hybrid NPs. Nanoparticle aggregates based on L2 molecules

Figure 3. Structural and optical characterization of Au_L1 material. (a) Temperature evolution of SAXRD pattern during heating. Inset shows SAXRD diffractogram of a mechanically sheared sample; arrow indicates the direction of rubbing. (b) TEM image of an annealed sample. Scale bar represents 20 nm. (c) Extinction spectra of the material taken at 120°C and 200° C revealing a shift of plasmon band maxima. (d) Reversible shifts of plasmon band maxima position in consecutive heating/cooling cycles. SAXRD studies performed on heating (Figure 3a, Figures S5a,b) evidenced structural stability of the Au_L1 lamellar phase up to 195°C. On further heating a phase transition to a long-range ordered structure was observed as revealed by the change of the diffractogram and appearance of three narrow Bragg peaks. The XRD profile of this phase can be well fitted assuming a face centered cubic (FCC) arrangement of NPs with lattice parameter a = 14.3 nm (at 210°C, Figure S5f). The FCC phase is stable up to 215 ° C at which temperature fast decomposition of the sample occurs. SAXRD measurements were also performed on cooling (Figure S5c). In such a case, phase transition from FCC to lamellar structure was observed at 150 ° C, that is 40 ° C lower than for Au_L1 during heating measurements. Such hysteresis is not surprising and is often observed for liquid-crystalline materials.52 Further, we decided to test whether Au_L1 nanoparticles can be switched multiple times between Lm and FCC phases. For this purpose we performed SAXRD measurements at 120 and 200°C in consecutive heating/cooling cycles (Figure S5d). Diffractograms characteristic to Lm and FCC phases were obtained, however, the broadness of the Bragg peaks in consecutive measurements grew – already within 7 cycles the peak of the main, (111), FCC signal, was doubled. Broadening

To lower the phase transition temperature of our hybrid nanomaterials we turned to reports on LCs and LC NPs that show strong influence of the number of aromatic rings of LC molecules on soft matter melting temperature.34 Therefore, to lower switching temperatures of the investigated NPs, we decided to obtain a two-aromatic ring analogue of L1 ligand. The obtained L2 molecule was then used to prepare a hybrid material with silver cores, since it should exhibit a stronger plasmonic response than Au NPs. Thus Ag_L2 hybrid material was obtained. An annealed sample of Ag_L2 material was subject to temperature dependent XRD measurements (Figure 4a, Figures S6 a,b). At low temperatures (30-90°C), three narrow Bragg reflections were observed confirming formation of a long-ordered structure. Positions of the peaks varied with temperature; phase assignment was performed using patterns registered at 80°C when all three signals were well separated. Given the broadness of the peaks and similar structure of the ligand in comparison to L1, we assume that the analyzed pattern can be indexed assuming a 2D lamellar (Lm) symmetry of the aggregate with inter-layer distance 10.9 nm and intra-layer periodicity 6.9 nm (Figure S6d). Above 90°C, a clear change of SAXRD pattern was observed evidencing a relatively broad phase transition. Narrow Bragg peaks coming from the high-temperature phase were observed above 110°C, suggesting the formation of a 3D long-range ordered structure. We identified the symmetry of the high temperature phase as body centered cubic (BCC) with lattice parameter a = 12.0 nm at 120 ° C (Figure S6d). Above 150 ° C, broadening of the peaks was observed suggesting degradation of the material. Both Lm and BCC structures were investigated when the sample was cooled down from 150°C, with the only difference that reorganization of NPs positions occurred at 85° C (Figure S6a).


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TEM images of an annealed Ag_L2 sample (Figure 4b) revealed short rows of NPs with inter-row distance of ca. 9.510 nm, which roughly correlates to the inter-layer distance of the Lm phase. Next, using SAXRD we confirmed that reconfiguration of NPs positions between the two phases (Lm at 30°C and BCC

lower (by almost 100°C) than in the case of L1-based materials. Consequently, Ag_L2 exhibits greater stability in heating/cooling cycles than Ag_L1 since the switching is performed further away from the decomposition temperature of NPs. However, to fully realize the set goals further lowering of phase transition temperature was required.

Nanoparticle aggregates based on L3 molecules Besides the number of aromatic rings, also the presence of functional groups strongly influences phase transition temperatures of liquid crystals by introducing specific interactions (e.g. hydrogen bonding, dipole-dipole interactions). In the case of the L2 molecule, one of the factors that might strongly influence the L2 melting behavior is the segregation tendency of the fluorinated alkyl chains. By lowering the segregation tendency, it should be possible to achieve low temperature switchability. Thus, we have decided to obtain L3 ligand, which is based on L2 molecules. However, it is equipped with a shorter fluorinated part (4-carbons long, twice shorter than in the case of L1 and L2). We have then used the L3 molecule to prepare LC NPs with silver core to maximize the optical response of the final nanomaterial, achieving Ag_L3 NPs.

Figure 4. Structural and optical characterization of Ag_L2 material. (a) Temperature evolution of SAXRD pattern during heating. (b) TEM image of an annealed sample. Scale bar represents 20 nm. (c) Extinction spectra of the material taken at 30°C and 120°C revealing a shift of plasmon band maxima. (d) Reversible shifts of plasmon band maxima position in consecutive subsequent fast heating/cooling cycles. at 120°C) is possible in a few cycle heating/cooling experiment (Figure S6c). Although from temperature dependent SAXRD measurements it seems that the material is well stable at 120° C, broadness of the Bragg peaks varied in consecutive cycles, as shown for the main BCC signal (Figure S6e). Within the first cycles a narrowing of the Bragg peak indicates an increasing correlation length of nanoparticles positions (larger monodomains are formed), that is the cycles act as thermal annealing. However, from the forth cycle the broadness increases rapidly, which as in the case of Au_L1 material can be ascribed to increasing polydispersity of NPs. Within 7 cycles doubling of the lowest value is registered (Figure S6e). Reversibility of plasmonic properties of the material were also investigate using UV-VIS absorption measurements. When cycled between the 30°C and 120°C spectra revealed a clear, 19 nm shift of the plasmonic peak maximum from 470 to 451 nm (Figure 4c); notably it was well-reversible in further cycles (Figure 4d) with only small degree of deterioration that can be explained by lowering the correlation length of nanoparticles positions derived from XRD measurements. Based on the above results for Ag_L2 sample, it can be concluded that the proposed change in the structure of the ligand has brought us closer to achieving the assumed goal. We obtained a nanomaterial with reconfigurable structure. Importantly, the phase transition temperature was much

Figure 5. Structural characterization of Ag_L3 material. (a) Temperature evolution of SAXRD pattern during heating. (b) Temperature evolution of SAXRD pattern during cooling. (c) SAXRD profiles taken at 30, 80 and 130 ° C. (d) FWHM evolution of the (111) XRD peak of the BCC phase in 400 consecutive heating/cooling cycles; the inset shows data for the first 15 cycles. As with the above-described materials, we have first focused on assessing structural reconfigurability of the Ag_L3 material. An annealed sample of Ag_L3 was subject to temperature dependent SAXRD measurements on heating (Figure 5a). Based on the changes of the collected diffractograms, we have identified 3 distinct long-range ordered (as judged from low FWHM of the peaks) structures



formed by Ag_L3 NPs at 30 - 60, 60 - 110 and 110 - 160°C ranges. To identify the symmetry of these aggregates we have fitted diffractograms taken at 30, 70 and 130 °C using lamellar (Lm, inter-layer spacing 11.0 nm; intra-layer periodicity 6.5 nm), body centered cubic (BCC, a = 11.4 nm) and face centered cubic (FCC, a = 15.0 nm) symmetries, respectively (Figure 5c, Figure S7c). Above 160°C decomposition of NPs was observed.

To confirm reversibility of the reconfiguration we have performed XRD measurements on cooling (Figure 5b). As a starting point we have chosen 140 ° C since this is a temperature at which FCC structure should be already well developed but no sign of decomposition of the material was observed at the time scale of XRD measurements (as confirmed by XRD measurements on heating). FCC and BCC symmetries of the Ag_L3 NPs assembly were reproduced with analogous unit cell sizes and only Lm structure was less developed in this temperature range. Notably, reduction of phase transition temperatures to about 85°C (FCC/BCC) and 40°C (BCC/Lm) was observed. The latter is one of the lowest phase transition temperature for LC NPs observed so far. We have also confirmed the ability of the material to relatively quickly (with the heating/cooling rate of 40 ° C/min) reconfigure between chosen symmetries (Figures S7 a,b) when reversibly changing the temperature. It is worth to note that FCC/BCC reconfiguration mechanism can be explained by Bian deformation,53 however the time scale of our SAXS measurements did not allow us to follow this process in more detail. TEM analysis of the thermally annealed sample confirmed the tendency of the material to form ordered structures (Figure S7e). Since we achieved significant reduction in phase transition temperature (Lm/BCC reconfiguration) in comparison to the previous samples we decided to test long-term switchability of the Ag_L3 material. Nanoparticles were subject to 400 heating/cooling cycles in which temperature was varied between 30 and 70 ° C (Figure 5d, Figure S7d). Notably, this reconfigurability test was performed in air, for a sample that was 3 months old and for each cycle XRD measurements were performed with our in-house equipment, which means that in total the sample was kept at the elevated temperature for over 12h. Thus, these conditions were far from optimal and further optimization could be envisaged e.g. by assuring an inert atmosphere.54,55 To analyze the collected XRD diffractograms we have first analyzed the FWHM of the main peak, (110), registered for BCC structure (Figure 5d). Within the first 15 cycles are analyzed the change is practically unnoticeable, in clear contrast to Au_L1 and Ag_L2 materials. After 400 cycles, ca. 35% increase was observed, which is still much below what was observed for above L1- and L2-based structures. Notably, these results highlight the need for long-term measurements of reconfigurability since even tens of repetitions might not reveal long-term reconfigurability of the nanoparticle assemblies. Additionally, we have chosen to follow the position of the main XRD peaks characteristic to each phase (Figure S7d). This way we were able to assess changes of the anisotropy of the organic coating of Ag_L3 nanoparticles, or in other words, the ability of LC ligands to bundle over long term. For the Lm structure, the position of the main peak varied within a 3Å range with slight tendency to degrade as judged from the slope of a linear regression to the data.

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The observed variation can be ascribed to a relatively high noise in these measurements, which is a consequence of our intention to keep the measurement times as short as possible. In the case of a BCC structure, the narrower character of the main peak lowered the influence of the noise on the estimated position of the peak. In this case a clear shift of the position of the maxima was observed – by 3Å after 400 cycles. Ligands migration over nanoparticle surface, surface oxidation resulting in ligands detachment and Ostwald/digestive ripening processes are all phenomena that can contribute to the observed tendency;56,57 still after 400 cycles the sample exhibited a clear, reversible reconfiguration. Ag_L3 material exhibited promising structural properties, thus we have further turned to investigating its switchable plasmonic behavior. For this purpose UV-VIS extinction spectra were collected at 30, 70 and 130°C, corresponding to Lm, BCC and FCC symmetries of the aggregate, respectively. Heating the sample was accompanied by a clear shift of the plasmonic band maxima: from 479 nm at 30°C, through 442 nm at 70°C, to 438 nm at 130°C (Figure 6a). To confirm that the shift results from reconfiguration of nanoparticles positions we have modeled the extinction of Lm, BCC and FCC phases made of silver nanoparticles with unit cell sizes corresponding to the ones derived from XRD measurements. For that purpose, a multiple scattering code with periodic boundary conditions in the transverse directions has been used.58 With that, an infinite numbers of particles in transverse directions were considered. In the longitudinal direction, three periods have been considered. The silver nanoparticles were taken as perfectly spherical with a radius of 2.65 nm. The lattice vectors were taken as extracted from the prior measurements. In the simulations, for the material dispersion of silver a Drude model with a size correction of the imaginary part is used.59,60 The ambient material was assumed to have a nondispersive refractive index of 1.7.

Figure 6. Optical characterization of Ag_L3 material. (a) Extinction spectra of the material taken at 30, 70 and 130°C revealing a shift of plasmon band maxima. Inset shows a magnified region of the plasmonic bands maxima. (b) Extinction spectra simulated for structures corresponding to


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those shown in panel a. (c) Observed shifts of the plasmonic peak maximum during the subsequent measurement cycles. Very good agreement was obtained between modeled and experimental results (Figure 6b). The spectral positions of the resonances are clearly predicted and indeed, they are largely caused by the interaction with the nearest neighbor in the lattice. The line width is underestimated in the simulation. This can be easily explained, as we assume in the simulation a perfect periodic arrangement of identical spheres, something that is not met in real-world samples. Disorder in the samples, by any means, causes an inhomogeneous line broadening of the plasmonic band; something we observe in the experiments. Besides the ability to understand the plasmonic spectra, we have also confirmed that the measured response is fully reversible (Figure 6c). We may add that temperaturedependent material properties particularly of silver could have an impact on the observed plasmon shifts. However, it has been found by numerical simulations and using temperature dependent material properties as documented in literature 61 that these shifts are at most a few nanometers, i.e. in the range between 4 nm and 8 nm depending on the structural geometry considered. The details of these simulations are documented in the Supporting Information (Figure S8). Therefore, temperature-dependent material properties may have an impact but they cannot explain the overall observed spectral shifts. In conclusion, the match between modeled and experimental results confirms correct interpretation of SAXRD data and attest that structural reconfiguration is the major cause for the change of the plasmonic properties of Ag_L3 material.

CONCLUSIONS We have demonstrated that it is possible to chemically engineer the surface ligands of nanoparticles to achieve reconfigurable plasmonic assemblies with long-lasting switchability. The philosophy behind this research was based on introducing terminally semifluorinated chains into the structure of thermoresponsive, promesgenic ligands. By exploring a series of ligand structures we were able to fine tune the ligand-ligand interactions towards lowering phase transition temperature of hybrid nanoparticles, while still keeping the ability of ligands to form anisotropic organic shell around spherical nanocrystals. With the presented approach we were able to achieve thermally responsive reconfigurable assemblies of plasmonic nanoparticles that exhibit properties interesting from the applicative point of view: it is based on Ag which has the strongest plasmonic properties, exhibits reversible switchability between three distinct long range-ordered phases (not reported so far for Ag NPs) and can be relatively quickly switched between different symmetries. Notably, the first transition temperature is low enough to assure high durability of the nanomaterial, which we confirmed in a 400 heating/cooling cycles experiment, an order of magnitude more than it is usually explored. Thus, we can say that with our approach we combine the best features of state-of-the-art reconfigurable assemblies: durability that can be expected for polymer coated nanoparticles (which also have lowtemperature switching, but form close-packed arrangements) and the ability to form anisotropic assemblies similar to the DNA-coated nanoparticles (which are usually less stable). We also confirmed that the reconfiguration is accompanied by

plasmonic response, which is relatively strong for a coupled, non-solvated systems. In future it would be of great value to follow the reconfiguration process in situ e.g. using TEM. We believe that our results will contribute to unlocking the applicative potential of reconfigurable plasmonic nanoparticles assemblies by further underlying the importance of chemical engineering of nanoparticle surface ligands towards functional nanomaterials,62–64 especially in the context of terminally functionalized ligands.24

EXPREIMENTAL SECTION Material and methods for organic synthesis. All reagents and solvents were obtained from Sigma-Aldrich. Before use solvents were dried over activated molecular sieves for 48 h. Substrates were used without purification. All reactions were carried out under a nitrogen conditions in dried glassware and ensuring efficient magnetic stirring. Purification of reaction products was carried out by column chromatography using RushanTaiyang silica gel 60 (230-400 mesh) at atmospheric pressure or by crystallization. Analytical thin-layer chromatography (TLC) was performed using Silica Gel 60 Å F254 (Merck) pre-coated glass plater (0.25 mm thickness) and visualized using iodine vapor and/or UV lamp (254 nm). Yields refer to chromatographically and spectroscopically (1H NMR) pure materials. The 1H NMR and 13C NMR spectra were recorded using 500 MHz NMR Varian Unity Plus. Proton chemical shifts are reported in ppm (δ) relative to the internal standard – tetramethylsilane (TMS δ =0.00 ppm). Carbon chemical shifts are reported in ppm (δ) relative to the residual solvent signal (CDCl3, δ=77.0 ppm). Details about used procedures for synthesizing ligands and analysis of their structure are available in Supporting Information (Figure S1 and Figure S2). Nanoparticle synthesis We prepared silver (Ag_C12) and gold (Au_C12) spherical nanoparticles (NPs) coated with dodecanethiol according to modified literature method.65 Dodecylamine (3g) was dissolved in cyclohexane (100 ml), then 12 ml of aqueous formaldehyde solution (37%) was added. After vigorously stirring for 10 min organic phase was separated out and washed twice with water (2x30 ml). Next, an aqueous silver nitrate (0.4 g AgNO3 in 20 ml of water) or tetrachloroauric acid (0.08 g HAuCl4 in 20 ml of water) was added under vigorously stirring. After further stirring for 40 min cyclohexane phase was separated out by centrifuged. Next, excess of dodecanethiol (2 ml) was added and the reaction mixture was stirred overnight. The formed precipitate was centrifuged (10 min, 6000 rpm), and then 150 ml of acetone was added. The precipitate was centrifuged (10 min, 6000 rpm), collected and dissolved in a small amount of cyclohexane (10 ml). The precipitation procedure was repeated additional two times. After that obtained solution was again centrifuged (10 min, 8000 rpm), in order to get rid of insoluble aggregates. To get fractions of NPs with slight size distribution, the fractionation process was performed. A small amount of ethanol was added to the cyclohexane NPs solution until turbidity appeared. The precipitate was centrifuged (10 min, 6000 rpm), dissolved in a small amount of cyclohexane, and to remaining supernatant further portion of ethanol was added.



Described above process was repeated three additional times, yielding fractions containing smaller and smaller NPs. Details about obtained gold and silver nanoparticles- TEM images and details about size distribution are available in Supporting Information (Figure S3). Hybrid materials synthesis We introduced promesogenic ligands into NPs surface by ligand-exchange reaction. To 20 mg of NPs coated with dodecanethiol dissolved in 5 ml of cyclohexane, 20 mg of given promesogenic ligand (L1-L3) dissolved in 5 ml of dichloromethane was added. The reaction mixture was slowly stirred overnight. Then, solvents were concentrated to about 3 ml, NPs were precipitated with 20 ml of ethanol and centrifuged (10 min, 6000 rpm). The supernatant containing unbound thiol ligands was rejected and precipitate was dissolved in 2 ml of toluene. NPs were again precipitated by addition of 10 ml of cyclohexane and centrifuged. This washing procedure was repeated additional two times- after that no unbound ligand was presented as was determined by thin-layer chromatography. After the last centrifugation, NPs were dissolved in 5 ml of dichloromethane. SAXRD measurements The small angle X-ray diffraction (SAXRD) measurements were performed with the Bruker Nanostar system (CuK α radiation, parallel beam formed by cross-coupled Goebel mirrors and 3-pinhole collimation system, area detector VANTEC 2000). The temperature of the sample was controlled with precision of 0.1 K. Samples were prepared as a thin films on kapton tape substrate. For all samples temperature dependent measurements were performed in the same manner – data was collected every 5 K for 300 s with 30 K/min heating or cooling rate between consecutive data collection points. Quasimonodomain samples were prepared by mechanical shearing at elevated temperatures (below the phase transition point) on heating table. Fitting of the experimental diffractograms and simulation of the patterns were done using Topas 3 software (Bruker). Each procedure started with choosing the most probable symmetry of the lattice. Then, the unit cell parameters, intensities of the (Pseudo-Voigt) signals and (1/x) background intensity were independently adjustable parameters. Transmission electron microscopy Transmission electron microscopy (TEM) measurements were performed using Zeiss Libra 120 microscope, with LaB6 cathode, equipped with OMEGA internal columnar filters and CCD camera. For TEM imaging materials were dropcasted into TEM grids and thermally annealed two times on heating table. UV-VIS spectroscopy The UV-Vis spectrum was measured using a Cary 5000 spectrometer (Agilent). The solutions of functionalized particles in dichloromethane were held in standard PMMA cuvettes (VWR) with a 10 mm optical path, while the aggregates were recorded on a quartz substrate in a transmission mode. Thermogravimetric analysis Thermogravimetric (TGA) analysis were performed with a TA Q50 V20.13 (TA Instruments) analyzer. The measurements were carried out in 100-900°C range with 10°C/min heating rate in nitrogen atmosphere. The weight loss between 100 and 260 ° C for nanoparticles covered with dodecanethiol was attributed to the removal of the organic shell and recalculated to the number of surface

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alkyl thiols as described below. In case of hybrid nanomaterials the weight loss was substantially larger and we observed two distinct steps of this weight loss (Figure S4a), as can be judged from derivative of the wight loss (Figure S4b). The first one took place below 260°C and corresponds to the removal of the alkyl coverage of nanoparticles- (dodecanethiol molecules). A wider loss at temperature above 260°C is due to the removal of promesogenic ligands molecules. To recalculate the obtained data we first calculated the mass of a single nanoparticle, using average diameter derived from SAXRD and TEM and the bulk density of metals. The mass of organic compounds (Morg) removed from a single nanoparticle was calculated using % of mass left after the analysis (%Mleft) and % of mass loss (%Mloss): Morg = MAg(orAu) /%Mleft*%Mloss. To determine the number of ligands per nanoparticle Morg has to be divided by the mass of the ligand responsible for the given mass drop. For nanoparticles after the ligand exchange reactions mass losses below and above 260 ° C were treated separately. In the exemplary case of Ag_L3 material (Figure S4a,b) the number of alkyl ligands was calculated to be 364, while there was 250 promesogenic ligands on a single NP surface. The total number of ligands is in good agreement with our previous reports for smaller Ag NPs27 and theoretically predicted number of ligands for nanoparticles of this size.66

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental details, synthesis and characterization organic ligands by 1H and 13C NMR spectrometry, UV/vis spectra, SAXRD data, TGA data.

AUTHOR INFORMATION Corresponding Author * E-mail:[email protected] (W.L.)

Funding Sources This work was supported by the REINFORCE project (agreement No. First TEAM2016-2/15) carried out within the First Team programme of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund and by PRELUDIUM project 2016/21/N/ST5/03356 of the National Science Centre, Poland. This work was supported by the German Science Foundation (project RO 3640/4-1). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sk ł odowska-Curie grant agreement No 675745. A. V. and R. N. S. S. also acknowledge support by the Karlsruhe School of Optics and Photonics (KSOP).

Notes

Any additional relevant notes should be placed here.

ACKNOWLEDGMENT REFERENCES


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Achieving Highly Stable, Reversibly Reconfigurable Plasmonic

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