Assembly of Branched Colloidal Nanocrystals in Polymer Films Leads

Sep 7, 2016 - Spontaneous self-assembly can be thought as a strategic route to engineer .... Chain flexibility arises from the way octapods are interl...
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Assembly of Branched Colloidal Nanocrystals in Polymer Films Leads to Enhanced Viscous Deformation Resistance Milena P. Arciniegas,*,† Andrea Castelli,†,‡ Luca Ceseracciu,† Paolo Bianchini,† Sergio Marras,† Rosaria Brescia,† and Liberato Manna*,† †

Istituto Italiano di Tecnologia (IIT), via Morego 30, IT-16163 Genova, Italy Dipartimento di Chimica e Chimica Industriale, Università di Genova, via Dodecaneso 31, IT-16146 Genova, Italy

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ABSTRACT: Progress in the integration of nanocrystals with polymers has enabled the creation of materials for applications ranging from photovoltaics to biosensing. However, controlling the nanocrystal segregation and aggregation in the polymer phase remains a challenging task, especially because nanocrystals tend to form amorphous clusters inside the polymer matrix. Here, we present the ability of octapod-shaped particles to overcome their strong entropydriven tendency to aggregate disorderly and form instead centipedelike linear arrays that are randomly oriented and fully embedded in polystyrene films upon controlled solvent evaporation. This behavior cannot be entirely described by short-range van der Waals interactions between the octapods in the polymer solution. An important role here is played by the increment of the viscosity of the medium during the evaporation of the solvent, which prevents disaggregation of the chains once they are formed. We show that increasing the octapod loading in the blends does not impact the length of the linear arrays beyond a critical length, while it favors instead chain demixing to form self-segregated regions of parallel interlocked chains. Our experiments evidence that softening of the polymer matrix by ex situ heating of the films induces a tail-to-tail coupling of the preformed chains and leads to the formation of longer linear structures of octapods, up to 2 μm long. The presence of 1D arrays of octapods in free-standing polystyrene films improves the creep response by a remarkable 37%, owing to an octapod pinning effect of the polymer matrix. KEYWORDS: Octapods, self-assembly, linear arrays, polymer, creep resistance, nanocomposite

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to segregate them orderly at the interface of the polymer films.9,16,17 Control over aggregation becomes particularly difficult when working with multiply branched NCs, and indeed, their behavior in polymers has rarely been investigated.2,18 Ordered structures of NCs with such complex anisotropic shapes can provide additional advantages in the mechanical response of the resulting nanocomposite due to their large surface contact area. In this direction, the advantage of branched NCs over moreinvestigated shapes, such as spheres19 and uniaxial rods20−22 or wires,23−25 has been demonstrated experimentally through the use of, for example, randomly dispersed ZnO tetrapods.26 Correlative simulations has shown that such superiority originates from the NC isotropy that induces an optimal orientation for the stress transfer from matrix to particles and for the covalent bonds at the NC−ligand interface.27 In addition, due to the more intricate morphologies that are

he steady evolution of nanocrystals (NCs) research toward real life applications is encompassing also the development of inorganic−polymer composite films whose properties are enhanced via incorporation of nanostructured architectures.1−4 Engineering these structures in the polymer phase through simple mixing of the two components in a blend would allow any further process to be realized by standard solution-based fabrication methods5−7 so that integration of NCs at low cost and high efficiency could be achieved. An additional step toward the full exploitation of the NCs potentialities is to gain control over their spatial organization in polymer films. Spontaneous self-assembly can be thought as a strategic route to engineer polymers with ordered structures, starting by the simple mixing of the components.8−10 Properly organized particles can boost the properties of the composite: their proximity and orientation can significantly enhance, for instance, energy and charge transport properties.11−14 Still, a challenging goal resides in controlling particles dispersion because NCs tend to segregate in the polymer phase and create tangled domains and polymer-only regions.15 Only in few cases the demixing of particles upon solvent evaporation was found © 2016 American Chemical Society

Received: June 10, 2016 Revised: August 21, 2016 Published: September 7, 2016 6154

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Figure 1. Self-organization of CdSe−CdS octapods in polystyrene thin films: (a) TEM image showing the formation of interlocked chains of octapods randomly dispersed in the polystyrene film prepared with C1 concentration of octapods. (b) A stronger self-segregation of octapods into domains of parallel chains is seen in blends prepared at a higher concentration (C2) of octapods. Scale bars: 0.5 μm. The sketches illustrate top views of the final arrangement of octapods in the C1 and C2 polymer films. The darker regions observed in the films are associated with polymer-rich regions as a consequence of the dewetting of the solutions on the substrates.

possible to achieve through their self-assembly,28,29 branched NCs might improve polymer chain bridging between two neighboring particles, particularly in the case of one-dimensional (1D) interlocked structures. However, the formation of linear arrays in the polymer phase, albeit demonstrated in few systems by using particles of simpler shapes,30−32 has so far not been attempted with branched structures. Here, we report the case of branched NCs made of a CdSe core and eight CdS pods, to which we refer as octapods, and their ability to form self-organized 1D arrays fully embedded in polystyrene (PS) films upon controlled solvent evaporation. We show that the solution-phase self-assembly of octapods in the polymer is different from the mechanism of nucleation and growth that yields superstructures in bulk solvent; they cluster to form stable interlocked chains, resulting in randomly dispersed centipede-like structures of octapods in the polymer films. This behavior is strongly influenced by local variations of octapods density during the drying of the solvent; that is, the availability of NCs in a region impacts the number of particles that can add up to form a chain, hence the chain length (which could be up to 1 μm). However, the average chain length in the polymer cannot be increased by simply loading a higher number of particles in the blend. In this case, at a higher concentration of NCs, octapods cluster in domains formed by multiple (from two to seven) parallel interlocked chains rather than assembling into longer structures. Therefore, the assembly process of octapods in the PS matrix can be explained by (i) strong van der Waals forces between octapods in the octapod− PS blend and (ii) viscosity fluctuations of the blend during solvent evaporation. These significant changes in viscosity over time limit the octapod motion and thus restrict the formation of longer chains. We also show that softening the polymer matrix by heating does not disassemble the chains into

individually dispersed octapods; on the contrary, it allows mobility of the centipedes structures, thus promoting the formation of longer linear structures by a tail-to-tail coupling of preformed octapod chains. The process reported here for the fabrication of polymer−NC blends is scalable, as we could prepare free-standing composite films in which the selfassembled chains of octapods increase the mechanical performance of the polymer (specifically, the resistance to deformation upon a constant stress (creep)). We hypothesize that the octapod-interlocked 1D structures exploit both the effect of larger surface area and optimal orientation as well as the interchain bridging, so that the concave shape of octapod chains acts as pinning points with the surrounding polymer and restricts polystyrene chains slippage and reorientation. To perform the experiments, we prepared suspensions of octapods in toluene following a synthesis protocol previously reported by our group.33 The CdSe−CdS octapods have an average tip-to-tip pod length (L) of 75 ± 2 nm and a pod diameter (D) of 12 ± 2 nm for an aspect ratio (L/D) of ∼6.0 (see Figure S1a,b). The polystyrene pellets were dissolved at 1% volume in toluene, a highly compatible solvent for both components. We then added an aliquot of the polystyrene solution to different dilutions of the octapod suspension (C1 ≈ 5 × 10−8 M and C2 ≈ 1.6 × 10−7 M) for a final polymer concentration of 0.2% volume (see the Methods section for details). After sonication of the blends, we simply dip-coated the substrates to get a thin liquid layer of the solution covering their surface. Immediately after, we placed the substrates horizontally in a homemade chamber with a toluene saturated atmosphere for 4 h. The chamber was then opened to allow complete evaporation of the toluene from the octapod−PS blend. To avoid disordered aggregation of octapods in the polymer phase induced by an excess of organic surfactants 6155

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Figure 2. Details of octapod chains in the polystyrene films. (a) Transmission electron microscopy (TEM) images evidencing that short segments (top left) of octapods in the C1 polymer films preferentially exhibit a tighter interlocking than longer chains (bottom left) that present instead a rather loose interlocking; at a higher concentration of octapods in the blends, the particles form a loose 2D packing (top right) of chains in less-dense regions of octapods, and a tighter interlocked (bottom right) packing is observed in denser regions in the polymer films. Scale bars: 100 nm. The sketches depict the top projections of both tight (I and III) and loose (II) interlockings configurations. d1 and d2 denote the distance between two closer octapods with a crosslike projection. (b) High-angle annular dark-field scanning TEM images extracted from a single tilt series of a group of chains (framed in white at −70°) induced in the C1 polymer films. This series evidences that the chains of octapods are fully embedded in the polymer film. The tilt axis direction and corresponding tilt angle are indicated in each panel.

between octapods are the same hard-core repulsive and attractive van der Waals interactions described there. It also indicates that the growth of chains, just like in the absence of polymer, starts with the addition of individual octapods and continues following consecutive interlocking steps, fed by the arrival of free octapods, until most of the available individual octapods are either consumed in the process or blocked in the film before reaching a neighboring particle. However, the observation of a side-to-side attachment of octapods chains when the number of octapods is increased in the starting solution (concentration C2) can be associated with a second stage of the assembly in the polymer film that precedes the formation of ordered 3D assemblies. However, we did not observe this last stage of the octapod assembly in the polymer films, which led us to conclude that constraints to the initially high mobility of octapods in the mixtures are imposed by (i) the diminished volume available for both single octapods and chains during solvent evaporation and (ii) the increased viscosity of the solution over time. In line with the formation of linear arrays from NCs with simpler geometries via more complex protocols35,36 or induced by external forces,23,37,38 we found that 1D octapod structures exhibit a relatively strong bending flexibility in the polymer, as previously observed in bulk solution.29 Chains subjected to such deformations do not break into shorter chains but rather curve within the films (see Figure S4). A key role is presumably played by the viscous drag of the polymer matrix, induced by local fluctuations of the solvent evaporation rate. Chain flexibility arises from the way octapods are interlocked to each other. This type of interlocking, especially in longer chains, leaves some residual degree of rotational freedom to each individual octapod. Figure S5 shows details of octapod chains formed in a supported film after a treatment with oxygen plasma to remove the polymer. Consistently, we found a significant larger core-to-core distance between two octapods with a cross-projection in loose interlocked structures (71 ± 5 nm) compared to the particle interdistance in tightly interlocked chains (54 ± 3 nm). The loose structures are

coming from the synthesis, the octapod suspensions were repeatedly washed before the addition of the polymer aliquots. Analysis via Fourier transform infrared spectroscopy (FTIR) confirmed the presence of a mixture of phosphorus-based ligands in the octapod surfaces after removing the excess ligands from the solution (Figure S1c). Results and Discussion. Through the procedure described above, we have obtained an interlocked organization of octapods in linear arrays that extend over the micrometer scale regions inside the films, as observed from the top view of Figures 1a and S2. With the C1 particle concentration, octapod chain lengths vary from 5 to 20 particles. In projection, these chains appear as centipedes-like structures (see the sketch in Figure 1a). The aggregation of octapods was strongly affected by the content of particles in the blend, as evidenced in Figure 1b: at C2 concentration, we observed octapod-rich regions formed by parallel interlocked chains of different lengths, as depicted in the sketch of the figure. This denotes the NC-densitydependent behavior of the octapod assemblies, from randomly dispersed chains at low octapod concentration, to side-to-side chain attachment at a higher concentration of particles. Such behavior is in agreement with our previous work (which is also supported by computational analysis) that revealed that in a denser system, the side-by-side chain configuration is more energetically favored than the growth of chain by single addition of particles to its ends.34 It should be noted that the transition point between the two assembly geometries appears to be solely related to density of particles in the system because in both cases other parameters, such as solvent and polymer concentration, were kept constant. However, a relatively low PS molecular weight was required to obtain the assemblies of octapods in the PS films, as a strong three-dimensional (3D) disordered clustering was seen when working with a highermolecular-weight polymer (see Figure S3). The fact that we observe the same type of octapod configuration in the C1 films as in bulk solution,29,34 where no polymer was used, is an indication that the primary forces 6156

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Figure 3. Effect of heating on the chains of octapods formed in the polymer films: (a) (top) short segments of chains formed in a C1 film at room temperature (TR) and (bottom) a longer chain of octapods formed by a higher number of short chains, observed after heating of the film at 80 °C. Scale bar: 500 nm. (b) Length distribution profiles of the octapod chains in the polystyrene film before and after heating, evidencing a chain length increment with the heating. (c) Cartoons explaining the interlocking sequence of octapods inside of the polymer at TR and interchains interaction to form an end-to-end coupling when the film is heated at a temperature close to Tg. Such changes can be related to the mobility gain induced by changes of the mixture viscosity.

octapod−octapod interactions, repulsive ligand−polymer interactions contribute to the ordered clustering of octapods in the polymer. Worthy of note is that a fast evaporation of the solvent from the blends did not induce the formation of self-assembled structures. We instead observed strong dewetting of the mixture on the substrates (Figure S6a), and the particles followed the classical dewetting pattern of polystyrene;39,40 that is, they formed patterns of dots, nanorings, or both (see Figure S6c). Also, we did not observe self-assemblies of octapods when we applied the protocol described above to a PS-free suspension of octapods (Figure S1d). This indicates that the presence of polystyrene in the blends and the slow evaporation of the solvent both play a crucial role in the formation of the interlocked structures. These factors simultaneously bring changes on the viscosity of the solution during film formation, which impacts the free (Brownian) motion of octapods in the blend and, thus, their interactions with neighboring particles. The viscosity of the blend indeed reduces the octapod diffusion coefficient (D) in the solution from a value estimated for octapods (with a hydrodynamic radius of ∼37 nm) dispersed in pure toluene of Doct−tol ≈ 9.6 × 10−14 m2/s29 to Doct−PS ≈ 7.6 × 10−14 m2/s,41 i.e., they are 20% less diffusive in the PS solution than in pure toluene. This change in viscosity over the time might explain the presence of chains consisting of octapods with a noninterlocked configuration, as can be seen in some segments in Figure S4 (i.e., octapods were frozen in a noninterlocked configuration as the viscosity of the medium reached a critical threshold). To investigate the effect of viscosity variations on preformed chains, we conducted ex situ heating experiments of C1 films to a temperature (80 °C), close to the polystyrene glass transition regime, Tg (107 °C; see the Methods section). By surveying the chain length of over 50 assembled structures of octapods before and after heating the films, we found that the thermal treatment forces the octapod chains to neither disaggregate nor to coalesce into 2D aggregates. Instead, we observed notably longer structures in the films after heating (as the examples

typical of long chains, while short chains or densely packed 2D arrays of chains adopt a tighter configuration (Figure 2a). The loose interlocking configuration observed in longer chains is an indication of the crucial role of the polymer viscosity that might act as a barrier to the inflow of additional octapods coming from neighboring regions and hamper any further contact between closer particles. Similar tightly interlocked chains observed at higher concentration of octapods in the blends indicate that changes on the number of closer octapods also affect their interlocking: in regions with a low particle density, octapods tend to pack loosely (top-right panel in Figure 2a), while in denser regions of particles, they form a tighter 2D packing (bottom-right panel in Figure 2a). The sketches in the figure highlight the tight (I and III) and loose (II) interlocking of octapods observed in the TEM images, where d1 and d2 represent the core-to-core distances between two closer octapods with a crosslike projection. To gain insight into the assembly process, we acquired single of high-angle annular dark-field scanning TEM (HAADF-STEM) images for various groups of chains at the C1 concentration. We found that the octapods chains are not segregated to the boundaries of the bulk film, nor do they form isolated polymerfree regions in the C1 films. The tilted images demonstrate that linear arrays of octapods span the whole thickness of the polystyrene films and are fully embedded in the films. An example of a group of chains is displayed in Figure 2b, where the tilt to high angles (up to 70°) evidences that part of the chains are closer to the top boundary, while others remain deeply immersed in the film. This observation denotes a sufficient chemical compatibility at the interface between the octapods and the polystyrene to avoid strong isolation and clumping of the particles in the film, as it usually occurs when NCs passivated with phosphorus-based ligands are mixed with polymers.2,9,17 Still, such affinity is weak enough to allow the aggregation of single octapods into (ordered) arrays; this weak interaction at the component interface is supported by the absence of individual octapods randomly dispersed in the film. Hence, although the system is dominated by strong attractive 6157

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Figure 4. Nonlinear excitation luminescence response of octapods in free-standing polystyrene films: (a) correlative TEM with nonlinear excitation luminescence imaging of a group of octapods without polymer. A closer view of the red-framed region is shown in the bottom panel; scale bar: 200 nm. (b) Nonlinear excitation luminescence response of the free-standing PS films evidencing the aggregation of octapods with a resolution of about four octapod lengths; scale bar: 5 μm. A magnified view of the white-framed region is shown in the left panel; scale bar: 1 μm. (c) TEM image of octapod chains embedded in a supported PS film and the equivalent simulated luminescence image obtained by the convolution of 2PE point spread function; scale bars: 1 μm. The resemblance of the features shown in the right panels in (b) and (c) suggests that the acquired luminescence is generated by clusters of octapod chains that span over the thickness of the free-standing films.

microscopy techniques are not feasible to observe particles distribution within thick films without damaging them,42 we exploited near-infrared (NIR) ultrashort pulse laser light to two-photon excite (2PE) the luminescence on both polymerfree octapods (to evidence their footprint under 1PE and 2PE at 458 and 860 nm, respectively), and thick composite films to attain a large view of their organization and distribution. The use of NIR light allows a better penetration in scattering samples, and the nonlinear excitation enhances signal and contrast of the imaging.43,44 Figure 4a displays a correlative TEM and nonlinear luminescence images of a region containing polymer-free octapods; a closer TEM view of the red-framed region can be seen in the bottom panel of the figure. The octapods exhibit two clear features (bottom-right spectrum in Figure S8): one related to their second harmonic generation at 430 nm when excited at two-photons, which is attributed to their 4-fold symmetry, and one to the emission from their CdS pods at ∼500 nm, which is also observed in the spectrum collected at one-photon excitation (bottom-left spectrum in Figure S8). The same analysis conducted over regions of 50 × 50 μm2 on free-standing octapod−PS films allowed us to recognize the presence of octapods by their features (top panels in Figure S8) in aggregates larger than 400 nm, as can be appreciated in Figure 4b (left panel). The changes in the luminescence intensity of the images are associated with variations of octapod density and distribution thorough the thickness of the films. We noticed that octapod aggregates in the free-standing PS films appear to have an elongated shape, as observed in the magnified view (right panel) of the white-frame region in Figure 4b, while groups of octapods without polymer give a spherical shape projection (see Figure S9). As indirect evidence that such aggregates are formed by chains, we found that their morphology follows that observed in similar images obtained by simulation of the point

shown in the bottom panel of Figure 3a and in Figure S7) compared to preformed shorter arrays in the films; see the top panel in Figure 3a. The length distribution analysis displayed in Figure 3b indicates a significant shift toward longer chains, up to micrometric length. Although this behavior is not associated with an increased octapod−octapod distance in the chains (a close view is shown in Figure S7), our observations reveal that the longer structures observed in the post-treated films are formed by an end-to-end coupling of short segments of chains with different orientations, clearly surrounded by the polymer. This can be likely the result of the polymer softening, due to heating, which enables the octapod chains to break their kinetic arrest in this out-of-equilibrium system, gain mobility, and eventually merge; the polymer shrinkage during cooling might also affect chain mobility.32 The observed coupling of preformed chains is here favored because their motion in the polymer phase toward a side-by-side attachment would need higher energy. The sketches in Figure 3c depict the hypothesized mechanism for the thermal induced chain growth of octapods as a function of their mobility gain induced by changes of polymer viscosity: (i) starting from a solvent-rich mixture (high fluidity) at room temperature, its increment over time during solvent evaporation favors the formation of octapod chains; and (ii) in a dry film under heating, the lack of solvent does not allow the ripening of small chains, but the lower viscosity of the system promotes centipedes motion and end-to-end coupling of a significant number of interlocked segments. To further explore the potential reinforcement offered by octapod linear arrays in the polymer composites, we first checked if similar types of assemblies would form in much thicker free-standing films. We processed the blends via cast molding, starting from a C1 solution (see the Methods section). Using this approach, we obtained films with a thickness ranging from 30 to 50 μm. Because standard optical or electronic 6158

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Figure 5. Mechanical response of free-standing octapod−PS films. Nonlinear luminescence images from an octapod−PS film before (a) and after (b) plastic stretching at 120 °C, evidencing the orientation of octapods chains that follow the direction of the applied stress; scale bars: 5 μm. (c) Typical curves of creep modulus as a function of test time for pure PS and octapod−PS free-standing films. The three stages of creep are also highlighted. The top panel shows the creep moduli ratio. The blue arrows indicate the steady region in which a significant enhanced creep response of around 37% is observed from the composites. The black arrow indicates an apparent peak in the ratio, caused by the failure of some PS samples before the end of the test. (d) Cartoon of the proposed reinforcing mechanism showing the PS chains, both surrounding the octapod structures (in blue) and trapped between octapods in the chain (in cyan) that are aligned (red and yellow, respectively) by applying a stress, σt, at a temperature close to the glass transition one. The pinning effect restricts chains and entanglements mobility thanks to the interlocking of polymer within NC chains.

spread function (PSF; see the Methods section) using TEM images of groups of chains in supported PS films (Figure 4c). Finally, we investigated the stretchability of octapod chains in the polymer by performing two-photon excitation microscopy on free-standing films subjected to viscous deformation at high temperature (120 °C), reaching up to twice the original length (10 mm). To conduct the experiments, we applied a constant load of 100 mN to the films, corresponding to a tensile stress of about 5 MPa. We then increased the temperature at a constant rate of 5 °C/min. Once 100% of deformation was reached, we froze the samples in this deformed state by quickly cooling them to room temperature. Figure 5a displays the selected images acquired from a region of 65 × 65 μm2 located in the center of free-standing octapod−PS films before and after stretching (necking zone). By comparing the morphology of the features observed in both films, we can clearly appreciate the strong strain-induced orientation of the octapod domains in the film as a consequence of the alignment of the PS chains under deformation, as it occurs in nanocomposite materials loaded with 1D nano- and microstructures.22,45 It is important to note that, despite the large strain applied to the films, most octapods domains appear to be only slightly longer compared to their size before stretching (see Figure S10); that is, they are not disrupted during polymer chain alignment, suggesting a relatively high intrachain strength. To experimentally evidence the potential effect of octapod chains on time-varying mechanical resistance of the free-standing polymer films, we tested the creep response (that is, the viscous

deformation under constant load) of octapod−PS and pure PS free-standing films, the latter prepared using the same process as for the composite films and with the same polymer concentration. We chose a relatively high temperature (ca. 70 °C) to detect differences in all stages of creep deformation within a short testing time after abruptly applying a low tensile stress of 2 MPa. Figure 5b shows the typical creep behavior observed from the films in terms of creep modulus (σ/ε, stress over deformation) as a function of time. The octapod−PS freestanding films exhibit a higher average creep modulus than the pure PS one, as evidenced in the creep moduli ratio in the top panel of Figure 5b, throughout the classical three stages of creep: an early phase (stage I) that is characterized by a rapid decay of the creep modulus over time, which is defined by a logarithmic deformation with values in these conditions of ca. 310 MPa for the octapod−PS and ca. 178 MPa for the PS freestanding films. This stage is followed by a steady linear deformation (stage II), in which the composite films showed average creep moduli values around 37% higher than the PS films, with values of 56 ± 6 versus 41 ± 7 MPa at the end of this stage. At the beginning of the last phase of the test (stage III), where nonrecoverable deformation mechanisms are activated and the deformation rate is increased, differences are less appreciable. It should be pointed out that, in half of the cases, the PS films failed before completion of the test, whereas the octapod−PS films always reached the end. Contrary to the typical limited or detrimental contribution of surfactant-capped NCs to the mechanical response of polymer composites,2,46,47 6159

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stand the aggregation of branched particles in the polymer phase and thereby facilitate their use in many applications. Methods. Chemicals. Copper chloride (CuCl, 99.999%), tri-n-octylphosphine oxide (TOPO, 99%), and tri-n-octylphosphine (TOP, 97%) were purchased from Strem Chemicals. nOctadecylphosphonic acid (ODPA) and n-hexylphosphonic acid (HPA) were purchased from Polycarbon Industries. Cadmium oxide (CdO, 99.99%), cadmium chloride (CdCl2, 99.99%), and sulfur (S, 99.98%) were purchased from SigmaAldrich. Anhydrous methanol and toluene were purchased from Carlo Erba reagents. All chemicals were used as received. Synthesis of the Nanocrystals. The CdS quasispheres were synthesized according to the procedure described by S. Christodoulou and co-workers, with slight modifications.52 The suspensions of CdSe−CdS rods were prepared using a previously reported protocol by our group.53 The details for the synthesis of octapods are as follows: Octapod-shaped NCs were synthesized following previous protocols reported by our group.33,54 Briefly, 0.060 g of CdO, 0.006 g of CdCl2, 0.290 g of ODPA, 0.080 g of HPA, and 3.000 g of TOPO were mixed and degassed under vacuum at 120 °C for 1 h in a 25 mL three-neck flask. The mixture was then heated to 380 °C, and 2.6 mL of TOP solution was injected into the flask. In the glovebox, 100 μL of a suspension of Cu2−xSe NCs in TOP (prepared according to our previously reported procedure54 and with a concentration of NCs equal to 3 × 10−6 M) were mixed with 0.5 g of a TOP−S solution (the latter prepared by dissolving 96 mg of S in 1 mL of TOP). The resulting mixture was quickly injected into the reaction flask at 380 °C, and the reaction was cooled to room temperature after 10 min. The resulting product was purified several times to remove the excess of surfactant deriving from the synthesis and to ensure a more direct contact with the polymer. This process consisted of repeatedly washing the octapods with toluene and methanol, followed by heating of the solution at 70 °C for 5 min and centrifugation at 3000 rpm for 3 min. Aliquots were taken from the fresh solution of octapods in toluene and dispersed in pure solvent to reach low and high concentrations of particles (in this case, 5 × 10−8 M (C1) and 1.6 × 10−7 M (C2), as determined by inductively coupled plasma optical emission spectroscopy on suspensions digested in a mixture of HCl−HNO3 (3:1, v/v). The samples from the repeatedly washed octapod solution, with no polymer added, were prepared by drop-casting and drying in air on the substrates. Films Fabrication and Morphology Studies. The polystyrene solutions were prepared by dissolving the polymer pellets from Sigma-Aldrich (molecular weight, Mw = 192.000 g/ mol; Tg = 107 °C) at a 1% volume in toluene and kept under strong shaking for 5 h prior to use. A polystyrene solution was also prepared using a higher Mw polymer (Mw = 350.000 g/ mol) with the same solvent and polymer concentration. The octapod−PS blends were prepared by the addition of 50 μL of the polystyrene solution to 200 μL of octapod suspension at different concentrations of octapods. The blends were then sonicated for 5 min, and the films were prepared by dip-coating followed by a controlled solvent evaporation of 4 h. The latter step was conducted by placing the wet substrates inside of a closed chamber saturated with a toluene atmosphere. For the preparation of the C1 free-standing octapod−PS films, a polystyrene solution was prepared at 5% volume, and the blends were prepared by adding 200 μL of this sonicated polystyrene solution to 300 μL of the fresh C1 octapod solution for a final polymer concentration of 2% vol. The blend was

our results show that there is an enhanced creep response in the octapod−PS films. A possible reason for this improvement is that octapod chains act as pinning points so that, thanks to the concave shape of octapods, a large surface area interacts with the surrounding polymer, thus restricting polystyrene chains slippage and reorientation, as it occurs instead in composite films with rodlike NCs.48 However, such a pinning mechanism is especially effective when the deformation is relatively large, corresponding to a permanent displacement of the polymeric chains.49 Under such conditions, the interlocked configuration of octapod chains appears to play a key role by partially trapping polymer in their hollow spaces between NCs during solvent evaporation and chain assembly (see the cartoon in Figure 5c). Although such an event has no demonstrated impact on the elastic deformation,27 it appears to be effective on preventing large deformation at high temperatures, thus reinforcing the system during viscous deformation in comparison to other 1D structures.21,48,50,51 The proposed pinning mechanism is strongly dependent on the concave shape of octapods and on their ability to form chains. To confirm that, we performed tests on composite films made by embedding in the polymer NCs with shapes different from octapods. To this aim, we tested CdS particles with roughly spherical shapes (16 nm in diameter)52 and CdSe−CdS rods53 (56 nm long and 4 nm thick), both synthesized with phosphorus-based ligands, as for the synthesis of the octapods (Figure S11). The films were prepared following the same procedure as the octapod−PS films and using a concentration of either spheres or rods chosen to match the volume occupied by the octapods (see the Methods section). With the rods, the resistance to creep of the films was in most cases lower than that of octapod−PS composites at large deformation, with failure of samples before the end of the test, whereas octapods resisted throughout the test (Figure S12). Films prepared with spheres presented a brittle behavior and therefore could not be tested. The positive comparison with rod−PS films is a further confirmation that the improvement given by octapods, particularly efficient to hinder large viscous deformation, can be attributed to the mechanism of polymer trapping within the intrachain hollow spaces. In conclusion, we have demonstrated the ability of octapodshaped NCs to form linear arrays by interlocking single particles in a polystyrene solution upon controlled solvent evaporation. We have studied how these structures in the polymer films change both with increments in octapod concentration and with thermal treatment. We found that increasing the content of octapods in the starting mixture creates locally segregated 2D packing of chains within the polymer rather than inducing disordered aggregation or longer 1D assemblies. The latter, instead, are the resulting structures when films are heated close to the polystyrene glass transition temperature and are formed by tail-coupling of preformed chains. We relate this behavior to chain motion enabled by the softening of the polymer matrix with temperature as well as to a contribution of the polymer shrinkage during cooling. We also showed through correlative TEM with nonlinear luminescence imaging analysis that 1D arrays are formed in thicker polymer films as well. Such assemblies could then enhance the timedependent mechanical properties of the material thanks to the polymer chains trapped between interlocked octapods. The growth of complex interconnected structures in polymers is a highly desired feature for the exploitation of nanoscale materials; thus, our results contribute significantly to under6160

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Nano Letters sonicated for 5 min and cleared of air bubbles. A total of 450 μL of the mixture were injected into an open aluminum mold of 30 × 10 × 3 mm3 that was previously polished, cleaned, and coated with Marbocote 227 CEE release agent from Marbo Italia SPA. The samples were allowed to dry at room temperature under inert atmosphere overnight and then released from the mold for a final dry at open air of 72 h before testing. Each films was cut in prismatic shapes of ca. 10 × 8 mm2 for mechanical stretching and photoluminescence imaging. For comparison purposes, octapod-free PS films were prepared from a PS solution at 2% following the same steps described above for free-standing films. In the case of the sphere−PS and rod−PS films, the blends were prepared using the concentration needed to reach the volume occupied by octapods in the octapod−PS films with the C1 concentration. This corresponded to concentrations of 6.2 × 10−7 and 2 × 10−6 M for spheres and octapods, respectively. The ex situ heating experiments were conducted by preparing C1 films on Carbon coated TEM grids and placing them in an oven (Genlab Limited MINO/6) at 80 °C for 1 h at a heating rate of 3 °C/min. TEM images were acquired from three different samples before heating, and the average length chain was estimated based on 20 locations of the samples before the experiments; seven locations were considered to measure the chain length after heating. The images acquired from the same location in the films, before and after the experiment, did not show any significant change in the arrangement of the octapods, which can be attributed to a permanent damage of the region by the electron beam; thus, we examined regions far from the FOV in the post-treated films. Transmission Electron Microscopy Analyses. Bright field transmission electron microscopy (TEM) analyses were conducted on a 100 kV JEOL JEM 1011 microscope. HAADF-STEM images were acquired using a JEOL JEM2200FS TEM, equipped with an in-column (Omega) energy filter and operated at 200 kV. The tilted HAADF-STEM images were acquired by means of a Fischione holder (Model 2030). High-resolution scanning electron microscopy (HRSEM) analysis was carried out using a JEOL JSM-7500FA microscope equipped with a cold field emission gun (single crystal tungsten emitter, ultimate resolution of 1 nm). Images were acquired at 10 kV using an in-lens secondary electron (SE) detector. Samples were prepared by drop-casting the solution on ultraflat 5 × 5 mm2 (type P) silicon substrates. The substrates were cleaned before use by using 10 min of ultrasonic bathing in acetone, distilled water, and, finally, isopropanol; the substrates were then dried with pressurized air. A plasma reactor (Gambetti Tucano Multipurpose Plasma System) with an oxygen flow was used for the removal of the polymer from the Si substrate. The plasma exposure time was 5 min at room temperature with a bias power level of 200 W. The thickness of the thinner films supported by the substrates was measured by an AMBiOS XP-2 Technology optical profilometer. A micrometer gauge was used to measure the thickness of the free-standing films. Nonlinear Photoluminescence Imaging. All of the images were acquired by a multiphoton, confocal, and super-resolution Leica TCS SP5 STED-CW gated microscope (Leica Microsystems, Mannheim, Germany). The microscope is equipped with a white light and an argon and two HeNe lasers covering the visible spectrum from 458 to 690 nm. An ultrafast Ti− sapphire excitation Mai Tai laser (Spectra Physics, Santa Clara, CA) with a repetition rate of 80 MHz repetition rate and a

pulse width of 80 fs is coupled to the microscope. A pair of laser lines were used at 458 and 860 nm for one-photon and twophoton excitation, respectively. The images were acquired with a HCX PL APO CS 63.0× 1.20 NA water immersion objective, and the signals collected in the spectral window of 480−570 (exciting at 458 nm) and 420−570 nm (exciting at 860 nm). The simulations were performed by thresholding the TEM images so that only octapod structures were selected; the resulting images were convolved with the simulated point spread function (PSF) of the microscope. Finally, the number of pixels were reduced by binning, and Poissonian noise was added to match the pixel size and the signal-to-noise ratio of the acquired nonlinear excitation images. Mechanical Stretching of the Octapods−PS Free-Standing Films. Stretching of the polymer−NCs samples was performed on a Dynamic Mechanical Analyzer (Q800, TA Instruments) equipped with uniaxial tension clamp. A constant load of 100 mN was applied to the sample, corresponding to a tensile stress of about 5 MPa, and then temperature was increased with the constant rate of 5 °C/min until 100 °C was reached. After that, the temperature was increased more slowly, at 2 °C/min, so as to reach the temperature for fast creep deformation without such a large softening that would induce failure of the film. A trigger was added to the stretching control to interrupt the deformation and lock the moving shaft as soon as the deformation would reach 100%. Indeed, such deformation was reached rapidly after temperature had overcome 115 °C, a value slightly larger than the typical Tg temperature of polystyrene (107 °C). After the target deformation was reached, the temperature was then quickly lowered to freeze the sample in the deformed state. Creep Characterization. Samples of pure PS and of NCs− PS composites were cut in prismatic films (approximately 10 × 2 × 0.05 mm3) and mounted on a DMA testing machine (TA Instruments, New Castle, DE) equipped with a uniaxial tensile clamp. Temperature was raised to 70 °C and kept constant 2 min, and then an abrupt stress (2 MPa) was applied to the material and kept constant for 20 min. Creep modulus, defined as stress over creep strain, was recorded throughout the test. A total of five repetitions were performed for each material.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b02371. Additional details on TEM details on polymer-free octapods and impact of different processing conditions on the octapod-arrangement in the PS films. Figures showing schematic representation of an octapod, an EM image of a group of octapods standing on four pods after solvent drying at open air, an FTIR absorption spectrum collected from dried octapods, an SEM image evidencing the lack of interlocked chains from a suspension of octapods, a secondary electron imaging scanning electron microscopy large-view image revealing the self-assembly of octapods into interlocked chains, low- and highmagnification TEM images evidencing the octapod strong clustering behavior in films, a collection of TEM images showing the bending flexibility of the linear arrays of octapods in the PS films, SEM images of a short and a longer chain formed in a PS film supported by a Si 6161

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substrate, effect of the different fabrication processes on the octapod arrangement in the PS films, TEM images showing larger chains of octapods observed in the C1 samples after heating, representative luminescence emission spectra, linear luminescence response of a group of polymer-free octapods standing on a formvar film-coated Ni TEM grid and observed at the same magnification, spectra collected from two regions on the left image evidencing the emission of octapods, selected and processed images acquired from 25 μm thick freestanding octapod−PS films before and after the application of a strain and their histograms, TEM images of CdS quasi-spheres and CdSe−CdS rods on carboncoated Cu TEM grids after drop casting at open air, and curves of creep modulus as a function of test time for rod−PS free-standing films. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*M.A. e-mail: [email protected]. *L.M. e-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.M. and M.P.A. acknowledge financial support from European Union through the FP7 starting ERC grant NANO-ARCH (contract no. 240111). M.P.A. thanks F. De Donato for the synthesis of the quasi-spheres and rods nanocrystals and T. Catelani for the technical support on TEM image acquisition.

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