Programmed Self-Assembly of Branched Nanocrystals with an

Sep 5, 2017 - Site-selective surface modification on the shape-controlled nanocrystals is a key approach in the programmed self-assembly of inorganic ...
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Programmed Self-Assembly of Branched Nanocrystals with an Amphiphilic Surface Pattern Yuki Taniguchi,† Muhammad Adli Bin Sazali,‡ Yusei Kobayashi,‡ Noriyoshi Arai,*,‡ Tsuyoshi Kawai,† and Takuya Nakashima*,† †

Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), Ikoma, Nara 630-0192, Japan Department of Mechanical Engineering, Kindai Unversity, Higashiosaka, Osaka 577-8502, Japan

‡

S Supporting Information *

ABSTRACT: Site-selective surface modification on the shape-controlled nanocrystals is a key approach in the programmed self-assembly of inorganic colloidal materials. This study demonstrates a simple methodology to gain selfassemblies of semiconductor nanocrystals with branched shapes through tip-to-tip attachment. Short-chained watersoluble cationic thiols are employed as a surface ligand for CdSe tetrapods and CdSe/CdS core/shell octapods. Because of the less affinity of arm-tip to the surface ligands compared to the arm-side wall, the tip-surface becomes uncapped to give a hydrophobic nature, affording an amphiphilic surface pattern. The amphiphilic tetrapods aggregated into porous agglomerates through tip-to-tip connection in water, while they afforded a hexagonally arranged Kagome-like two-dimensional (2D) assembly by the simple casting of aqueous dispersion with the aid of a convective self-assembly mechanism. A 2D net-like assembly was similarly obtained from amphiphilic octapods. A dissipative particle dynamics simulation using a planar tripod model with an amphiphilic surface pattern reproduced the formation of the Kagome-like assembly in a 2D confined space, demonstrating that the lateral diffusion of nanoparticles and the firm contacts between the hydrophobic tips play crucial roles in the self-assembly. KEYWORDS: self-assembly, nanocrystals, branched structures, hydrophobic effect, dissipative particle dynamics highly ordered liquid crystalline phases31,32 due to the van der Waals interaction in the lateral direction, which is stronger than the dipole−dipole coupling for end-to-end assembly.33 One dimensional (1D) chains and three-dimensional (3D)-ordered superlattices have been obtained through the self-assembly of monodisperse octapod-shaped NCs in an interlocked configuration.34 Surface chemistry plays an important role in self-assembly as well. Heterogeneous surface modifications introduce “patchiness” on the surface property of NCs, bringing about directional interparticle interactions. Particles with a patterned surface modification, in particular, self-assemble into a discrete structure defined by a periodic pattern on the surface of the particle, which serves as a program for self-assembly.35−37 Amphiphilicity is a representative heterogeneity for the surface modification of NCs and can be a versatile characteristic for self-assemblies.38−42 Unmodified inorganic cores can provide

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evelopments in the controlled synthesis of semiconductor nanocrystals (NCs) have afforded NCs with tailored sizes, shapes, compositions, and properties.1−3 Along with such developments, interest is emerging in NCs with uniform shapes and sizes as a building block of selfassembly.4−11 These studies are motivated to mimic selfassemblies in biological systems12−17 and obtain the collective and enhanced properties unique to these self-assembling structures.18−24 Besides template-directed ones,5 self-assemblies of NCs are directed by the interactions25,26 operating between cores and through surface ligands and in most cases result in the formation of closely packed superlattices18,19 with high periodicity and minimum free space. Introduction of anisotropy in the interaction forces and in the shapes of NC successfully makes self-assembling structures anisotropic or more complex. In those cases, the shape of the NCs and their interactions are considered as information for programmed self-assembly.11 For example, the directional dipole−dipole interactions led to the formation of one-dimensional (1D) assemblies, even with the polyhedral-shaped CdTe NCs in water.27,28 The rod-shaped NCs often gave densely packed side-by-side phases29,30 and © 2017 American Chemical Society

Received: July 6, 2017 Accepted: September 5, 2017 Published: September 5, 2017 9312

DOI: 10.1021/acsnano.7b04719 ACS Nano 2017, 11, 9312−9320

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ACS Nano the simplest hydrophobic surfaces.42 Weller, Förster, and coworkers employed hydrophilic polymers as a weakly anchoring capping layer for CdSe/CdS core−shell NCs. 42 The interparticle contact induced the reorganization of the capping layer, making the NCs surface amphiphilic and causing the amphiphilic NCs to self-assemble into vesicles and 1D strings depending on the density of polymer chains.42 Recently, we have achieved an amphiphilic modification on the surface of rod-shaped semiconductor NCs with a wurtzite structure by using water-soluble short-chained thiols with a weakly binding capability as a surface capping ligand.43 Both end surfaces were selectively uncapped by the short-chained thiols because of their weaker affinity to the ligands, while the side wall surface with the better ligand-binding affinity was sufficiently covered. The amphiphilic nanorods self-assembled in an end-to-end manner through the hydrophobic effect operating between both ends. The 1D fibrous assemblies, however, involved a certain degree of freedom in the number and angle of nanorod connections due to the directionless nature of hydrophobic effect (Figure 1), giving a gel-like phase with entangled and

assembly, the amphiphilic tetrapods can be thus considered as a doubly programmed (in the shape and surface pattern) component of self-assembly.11 The CdSe tetrapod NCs ligand exchanged with a cationic thiol were stably dispersed in water first, giving irregular porous tip-to-tip networks in a few days. The aggregate suspension was sonicated, followed by drop casting on the substrate, producing the 2D-ordered Kagomelike network with a hexagonal tetrapod NCs assembly. We also performed molecular simulations using an amphiphilic planar tripod model to confirm the role of the hydrophobic tips in the formation of 2D network structures. While Whitesides and coworkers performed programmable self-assembly in a μm-mm scale,49 we herein demonstrate it using nm-scale components.

RESULTS AND DISCUSSION Synthesis of CdSe Tetrapod NCs and Ligand Exchange. CdSe tetrapod NCs with different arm lengths were synthesized through the continuous precursor injection in the presence of zincblende CdSe NCs.50−52 The four wurtzite arms were grown from four equivalent (1 1 1) facets of the zincblend NCs. The arm length was readily controlled by changing the reaction time with a constant injection rate of arm precursor. We prepared CdSe tetrapod NCs with the arm lengths of 15 and 40 nm; hereafter, CdSe(15) and CdSe(40) tetrapod NCs, respectively (Figure S1). These tetrapod NCs exhibited negligible photoluminescence. The purified CdSe tetrapod NCs were capped with oleate ligands and stably dispersed in organic solvents including chloroform and toluene. The surface ligand was exchanged by short-chained thiolate molecules with an ionic moiety, such as 2-(dimethylamino)ethanethiol hydrochloride (DMAET) and sodium 2-mercaptoethanesulfonate (MES), by a simultaneous phase-transfer method.43,53,54 The DMAET- and MES-capped CdSe tetrapod NCs were clearly dispersed in water just after the ligandexchange and purification procedures. Transmission electron microscopy (TEM) images of DMAET-capped tetrapod NCs clearly show that the ligand-exchange reaction did not change the shape and arm length of the tetrapod NCs (Figures 2 and S1).

Figure 1. Schematic self-assembly models of nanorod and tetrapod NCs with an amphiphilic surface pattern.

cross-linked fibers. The simply patterned surface with hydrophilic-wall and hydrophobic-tips determined the order of assemblies. Given the more complex surface pattern, the degree of freedom in the connectivity of components should be decreased, leading to more ordered self-assemblies. In the present study, we increased the complexity in the shape and surface pattern of self-assembly components by introducing branched structure in NCs. For example, the use of tetrapodshaped NCs with an amphiphilic surface pattern increases the interacting points with defined positions compared to the nanorod system, which should contribute to the unambiguous design of self-assembly (Figure 1). Herein, we report the formation of two-dimensional (2D)ordered self-assembly of CdSe tetrapod and CdSe/CdS core/ shell octapod NCs through tip-to-tip connections. Selfassembly of tetrapod-shaped NCs with homogeneously passivated surface resulted in the formation of 2D and 3D networks,44−46 in which the side-by-side close contacts of arms between NCs dominate the self-assembly. Meanwhile, the tetrapod arm has a crystalline structure equivalent to that of a nanorod.47,48 The use of water-soluble short-chained thiols as a surface-capping ligand is therefore expected to afford a tipselective uncapped hydrophobic surface in a similar manner to nanorods,43 giving an amphiphilic surface with a hydrophilic− hydrophobic pattern. In the sense of programmed self-

Figure 2. TEM images of DMAET-capped (a) CdSe(15) and (b) CdSe(40) tetrapod NCs deposited from aqueous solutions just after the dispersion.

Both CdSe tetrapod NCs exhibited less dispersion stability in the aqueous solution and formed precipitates after aging for 2 days at room temperature, which could be attributed to the strong interparticle interactions. The precipitates were composed of 3D porous network structures with the tip-totip connections between the tetrapod NCs as observed by scanning electron microscopy (SEM) and TEM zooming in a 9313

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tetrapod NCs before and just after the ligand exchange and after the self-assembly. The CdSe tetrapod samples prepared via reprecipitation from chloroform and aqueous solutions before and just after the ligand exchange, respectively, provided almost identical profiles in these core level spectra (Figure 4a,b). The peak positions for Cd 3d and Se 3d well accorded

less dense area (Figure 3, also see Figure S2 for CdSe(15) tetrapod NCs capped with DMAET and MES). The assembling

Figure 4. Core level spectra of Se 3d and Cd 3d for CdSe(15) tetrapod NCs (a) before (black) and (b) just after the ligand exchange and (c) after the self-assembly.

with those for bulk CdSe and CdSe NCs in the literature.58,59 The CdSe tetrapod NCs before and just after the phase transfer are homogeneously capped with oleate and DMAET ligands, respectively. The peak pattern of Cd 3d remained unchanged after the self-assembling process, while a new broad peak appeared with a high binding energy shift (∼57.5 eV) relative to the main Se 3d peak corresponding to CdSe (Figure 4c). This new peak with a higher binding energy could be assigned to the oxidized selenium such as elemental Se and SeOx (x = 2, 3).60 The peak integral ratio of the new broad peak relative to the main one was estimated to be 3%, which is reasonable considering the tip-selective oxidation of CdSe tetrapod with sharpened arms (see Figure S3 for the estimation). The arms of tetrapod NCs have a crystalline structure equivalent to that of wurtzite nanorods.47,48 The use of short-chained charged ligands facilitates the ligand desorption both on the side and tip surfaces due to the electrostatic repulsion between neighboring like-charged groups. Upon desorption of the ligands, the topmost Cd atoms are etched selectively at the tip surface by the co-existing anionic species (Cl − for DMAET),43,61 giving bare Se-rich tip surfaces. The oxidation of the Se-rich surface under the ambient condition could provide a tip surface covered by elemental Se and/or selenium oxide monolayer with a hydrophobic nature in a similar manner to the end surfaces of CdSe nanorods.43 The tip-selective hydrophobic effect drives the self-assembly of CdSe tetrapod NCs into a porous network structure. Although the side wall is expected to be stabilized by the balanced adsorption− desorption of ligands, the chemical etching and subsequent oxidation advanced to the side surfaces by further aging in the aqueous solution for another 1 week, disturbing the tetrapod shape of NCs (Figures S4 and S5). The shortened arms of tetrapods also indicate that the chemical etching started at the tips and then penetrated to the side wall (Figure S5). 2D Self-Assembly of CdSe Tetrapod NCs. We then sonicated the precipitate suspension in a bath sonicator (100 W, 38 kHz) for 5 min to disassemble the tetrapods from the irregular 3D porous network, and the resulting dispersion was drop-cast on a carbon-coated TEM grid. The energetically

Figure 3. (a) SEM and (b) TEM images of porous network structures formed by DMAET-capped CdSe(40) tetrapod NCs. Inset in (b): enlarged portion of (b) showing the representative tipto-tip connections between tetrapods.

morphology was apparently different from those obtained by CdSe tetrapod NCs homogeneously capped with long-chained ligands giving arm-by-arm close contacts, as exemplified by Figure S1c,d.44−46 The arm-by-arm close contacts were sufficiently suppressed in the assemblies of DMAET-capped CdSe tetrapod NCs because of the electrostatic repulsion between the side wall of arms capped with the positively charged thiolate, resulting in the highly porous structure. The attractive van der Waals force-based arm-by-arm interaction was thus replaced with the electrostatic repulsion between the cationic surfaces after the ligand exchange. In combination with this repulsive interaction between the side wall, the attractive hydrophobic effect operating between the tips facilitates the formation of the porous tip-to-tip assemblies in a similar manner to the formation of the gel-like phase composed of fibrous nanorod assemblies.43 An acid treatment of CdSe(core)/CdTe(arms) tetrapods with alkylphosphonic acid ligands led to a similar tip-to-tip network structure, in which the higher reactivity of tips55,56 was also exploited for the tipselective removal of capping ligands.57 To obtain a greater insight into the self-assembly mechanism, we performed X-ray photoelectron spectroscopy (XPS) measurements, elucidating the change in the chemical states of components after the self-assembly. The X-ray photoelectron signals from Cd 3d and Se 3d were compared for CdSe(15) 9314

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Figure 5. (a) TEM, (b) STEM, (c) SEM, (d) reconstructed 3D tomography images, and (e) schematic model of Kagome-like hexagonal assembly by DMAET-capped CdSe(15) tetrapod NCs. One hexagonal unit is colored with purple in the tomography image for clarity (d). (f) Representative TEM image of the self-assembly of CdSe(40) tetrapod NCs.

hours in the tomography measurement damaged the NCs, resulting in the bending of those unbound arms (Figure 5d). The tetrapods’ hexagonal arrangement was also observed for the CdSe(40) tetrapod NCs with the longer arms, while the regularity of ordering was not as prominent as that of CdSe(15) tetrapod NCs with the shorter arms due to the longer distance between the interaction points and their shape imperfection (Figures 4f and S8). The hexagonal assemblies of CdSe tetrapod NCs are most likely formed during the evaporation process with the aid of convective self-assembly mechanism.5,63 The solvent evaporation caused the convection flow toward the meniscus to compensate the loss of water molecules, which also carries the NCs, bringing them close to each other. For the amphiphilic CdSe tetrapod NCs with hydrophobic tips, it is important to consider interactions between NCs as well as between NCs and the substrate, the carbon film on the TEM grid. The effect of the hydrophobic carbon surface may constrain the tetrapods to stand on the substrate using three pods, limiting the 3D rotation of NCs. The minimum unit in an ordered assembly is a dimer formed by double-point contacts between the hydrophobic tips (Figure S9). The hexagonal assembly can be grown through step-by-step addition of monomeric units to the oligomer and terminated by the attachment of NCs on the hydrophobic surface, denying the mobility of the NCs. The growing domains hardly merge into a single domain because the edge shape of each domain must fit like a jigsaw puzzle, resulting in the multidomain assembly (Figures 5a and S6). The lateral diffusion of NCs was maintained in a liquid film, which affords the assembly−disassembly equilibrium. After the solvent was evaporated, the adsorption of NCs on the substrate prohibited the diffusion of NCs to give imperfect assemblies (Figure S10). The interplay among adsorption equilibrium,

stable assembling structure for the tetrapods with four sticky tips should be a 3D diamond-like network, in which all the tips with high surface energy are equally used for the connections between NCs (Figure 1). To achieve such a highly ordered 3D assembly, the component tetrapods must possess absolute uniformity in the size, angle between arms, and the arm lengths. Unlike the superlattices formed by spherical NCs, even the small distributions in those parameters and the imperfection of shape cannot be afforded in the highly ordered tip-to-tip 3D assembly of tetrapods, resulting in the irregular porous network (Figures 3 and S2). We herein anticipated the reorganization of amphiphilic CdSe tetrapod NCs from the irregular tip-to-tip 3D network to more ordered assemblies by limiting the 3D free rotational motion of tetrapods in a confined 2D space formed during the solvent evaporation.62 Both DMAET-capped CdSe tetrapod NCs with the different arm lengths gave a thin layer of assemblies (Figure S6). Interestingly, a Kagome-like network structure with regularly arranged hexagonal assemblies of CdSe tetrapod NCs was observed along with irregular networks including the 3D porous assemblies. The monolayer of CdSe(15) tetrapod NCs was composed of domains with a number of hexagonally arranged tetrapods (Figure 5a). The largest regular domain was formed by ca. 60 tetrapod NCs (Figure 5b). The SEM stereoimages observed with the specimen tilt angle of 10° (Figure 5c) and 3D tomography TEM measurement (Figures 5d and S7 and Supplementary Movie S1) clearly demonstrated that each tetrapod uses three arms for the interparticle connections and the remaining arm sticks out in a vertical direction as depicted in the model image in Figure 5e. The periodically arranged bright spots observed in the SEM images (Figure 5c and Figure S6a) correspond to upright pillars like unbound dangling bonds. The electron beam irradiation for 9315

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ACS Nano lateral diffusion, and reversible hydrophobic contacts of NCs determines the regularity in the 2D-ordered assemblies.64 The images shown in Figure 5 were obtained in the optimized condition by controlling the aging time (48 h) and temperature (35 °C) in the aqueous solution and deposition temperature (35 °C). We also adopted a slow evaporation method,46 which did not lead to obvious improvement in regularity in the selfassemblies. Tip-to-Tip Self-Assembly of Octapod NCs. The CdSe(core)/CdS(arms) octapod NCs (Figure S11) were also subjected to a ligand-exchange procedure using a short-chained thiolate molecule, sodium 2-mercaptoethenesulfonate (MES). The anionic ligand MES worked better than DMAET for the ligand exchange of octapod NCs with CdS arms covered by octadecylphosphonic acid (ODPA). In a similar manner to the tetrapod NCs, the MES-capped octapod NCs were well dispersed in water. The aqueous solution of MES-capped CdSe/CdS octapod NCs aged for a week was deposited on a TEM copper grid to observe the self-assembly. As shown in Figure 6, the octapod NCs also gave tip-to-tip self-assemblies

Figure 7. Snapshots of self-assembled structure of planar tripod NCs (a) and planar tetrapod NCs (b) under equilibrium. Color code: solvent beads (water molecules) in cyan; hydrophobic beads in red; hydrophilic beads in blue.

tip-to-tip connection including a hexagonally arranged Kagomelike 2D assembly for tetrapod NCs (Figures 5b and 7a) and a net-like 2D assembly for octapod NCs (Figures 6b and 7b). The DPD simulations also suggested the certain degree of reversibility of the tip-to-tip attachments, and components are exchanged between the assembly domains in monomeric or oligomeric units, contributing to the growth of each regular domain. However, the reversibility does not seem enough to achieve the long-range ordering of NCs as observed for the homogeneously passivated branched NC assemblies driven by van der Waals force-based NC−NC interactions.34,45,46 Meanwhile, the DPD simulation using a tripod composed only of solvophilic beads resulted in a randomly dispersed state without assembly, which may represent the dispersion behavior of tetrapod NCs just after the ligand exchange with a hydrophilic thiolate ligand (Supplementary Movie S4). The DPD simulations thus clearly revealed that the hydrophobically modified tips are indispensable and the lateral diffusion of NCs and the hydrophobic contacts between the tips play a crucial role in the formation of 2D network assemblies of branched NCs.

Figure 6. Tip-to-tip assemblies of CdSe/CdS octapod NCs with possible schematic models: (a) chain-like and (b) 2D-hexamer assemblies.

on the carbon-coated TEM grid (also see Figure S12). They form 1D chain-like oligomers (Figure 6a) as well as 2D assemblies (Figure 6b). The hexamer was the largest number of octapod components in an ordered 2D assembly. Since the number of interacting points per single NCs increased by more than double from tetrapod (three) to octapod (eight) for 2D assembly, it was difficult to achieve well-ordered crystalline assembly in a wide area, unlike the self-assembly with an interlock mechanism.34 Dissipative Particle Dynamic Simulations. We performed dissipative particle dynamics (DPD) simulation to further study the 2D self-assembling behavior of amphiphilic nanoparticles.37,65,66 To simplify the simulation conditions, we replaced the tetrapod and octapod structures with a planar tripod and a planar tetrapod, respectively, and confined the simulation space into a thin layer to limit the motion of NCs to a lateral diffusion and rotation. These tripod and tetrapod arms were modified in an amphiphilic manner, wherein the tip parts are composed of solvophobic beads and the rest of them are formed by solvophilic beads. We thus assigned interaction parameters of 75 (in kBT units) between the solvophobic and solvent beads, 25 between the solvophilic and solvent ones and 75 between solvophobic and solvophilic ones. As such, larger interaction parameters represent the higher repulsive interaction. The representative 2D assembly snapshots under equilibrium are shown in Figure 7 (also see Supplementary Movies S2 and S3). The DPD simulation successfully reproduced the self-assembly of branched NCs via hydrophobic

CONCLUSIONS This study has demonstrated the self-assembly of amphiphilic branched NCs through tip-to-tip connections. The tip surface of tetrapod NCs with high reactivity was selectively uncapped by using weakly bound short-chained thiolate molecules, giving the partly oxidized bare inorganic surface with hydrophobic nature. The convective self-assembly induced by drop-casting a suspension of amphiphilically modified tetrapod NCs led to the formation of 2D assemblies with a Kagome-like hexagonal arrangement of NCs. DPD simulations clearly revealed the formation mechanism of the well-ordered assemblies, in which the lateral diffusion of NCs and hydrophobic contacts between the tips dominates the self-assembling process. Further improvements in the conditions including the aspect ratio of NCs, aging period, temperature, substrate, humidity, and solvent evaporation rate might expand the size of the regular hexagonal domains. The present methodology can be applied, in principle, to a variety of anisotropic NCs with a partial structure of wurtzite arms, including luminescent NCs. The 3D and 2D nonclosely packed self-assemblies with an appreciable internal volume should find potential applications as sensors, photocatalysts, and photovoltaics. We also successfully modified the surface of branched NCs with cationic and anionic ligands. These charged NCs with branched shapes 9316

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precipitated powder samples were placed on a piece of carbon tape for the measurements. The binding energy shift was calibrated with the peak position of C 1s from the substrate. Simulation Method. We employed the dissipative particle dynamics (DPD) simulation method68,69 to reproduce the selfassembling behavior of a confined amphiphilic NCs solution. The DPD method has been proven to be an effective mesoscopic simulation tool to study fluid events occurring in millisecond time scales and micrometer length scales. The fundamental equation in the DPD method is Newton’s equation of motion. For a particle i, each DPD bead is subjected to three types of forces: conservative, dissipative, and random. Newton’s equation of motion for the particle i is given by

would be unique components of electrostatically self-assembled supercrystals.25

EXPERIMENTAL SECTION Synthesis of CdSe Tetrapod NCs. CdSe tetrapod NCs were synthesized by the continuous precursor injection method.50 The zincblende CdSe seeds with a diameter of 5.4 nm were prepared according to the reported procedure from CdO (39 mg), myristic acid (137 mg), Se (12 mg), oleic acid (OA, 0.5 mL), and oleylamine (0.5 mL) using 1-octadecene (1-ODE).47 The CdSe seed (3 × 10−7 mol) solution in the mixture of 1-ODE (21.3 mL), OA (2.25 mL), ntrioctylphosphine (TOP, 1.5 mL), and cetyltrimethylammonium bromide (0.21 mmol) was heated up to 260 °C under N2 flow. After the temperature reached 260 °C, the arm precursor solution47 containing Cd(OA)2 and TOPSe was injected at the constant rate of 0.2 mL/min to the seeds solution by using a syringe pump. CdSe tetrapods with arm lengths of 15 and 40 nm were obtained by controlling the injection periods for 15 and 40 min, respectively. The synthesized NCs were purified by successive dispersion−precipitation cycles using chloroform and acetone, respectively. Synthesis of CdSe(core)/CdS(arms) Octapod NCs. CdSe/CdS octapod NCs were prepared according to the literature.67 The Cu2−xSe seeds with a diameter of 15 nm were first prepared. One mmol of Cu(acac)2 was dispersed in the mixture of oleylamine (9.5 mL) and 1dodecanethiol (DT, 3 mL), which was heated to 60 °C in vacuo for 1 h. The mixture was then heated to 220 °C under N2 flow. At that temperature, the Se-precursor solution containing Se (158 mg), oleylamine (1 mL), and DT (1 mL) was injected, and the reaction mixture was stirred for another 30 min. The reaction flask was cooled to room temperature and quickly transferred to a glovebox filled with Ar. In the glovebox, the seed NCs were purified by repetitive precipitation and redispersion cycles using methanol and toluene, respectively. The obtained Cu2−xSe seeds were dispersed in 10 mL of toluene. The CdS arms were grown on the seed NCs. In a threenecked flask, CdO (60 mg), CdCl2 (6 mg), ODPA (0.29 g), hexylphosphonic acid (80 mg), and trioctylphosphine oxide (3 g) were charged and heated to 130 °C in vacuo for 1 h, followed by heating to 350 °C under N2 flow. 2.5 mL of TOP was further added, and then the seed solution containing the Cu2−xSe seed NCs in toluene (200 μL), TOP (1 mL), and TOPS solution (500 μL, 32 mg/mL) was injected to the reaction mixture. The temperature was recovered to 350 °C and kept at that temperature for 10 min. After that, the reaction flask was cooled to room temperature, and the octapod NCs were purified by washing with toluene and methanol. Ligand Exchange. Ligand-exchange reaction of NCs from ligands with long-alkyl chains to water-soluble thiolates was carried out by the simultaneous phase-transfer procedure.43,53,54 Briefly, 1 mL of a chloroform solution of NCs was put in a vessel, and 1 mL of an aqueous solution of thiolates (0.5 M) was then added on top, followed by the addition of 0.5 mL of acetone. 2-Dimethylaminoethanethiol hydrochloride (DMAET) and sodium 2-mercaptoethanesulfonate (MES) were used as water-soluble thiolate ligands. The vessel was quickly placed in a water-bath set at 80 °C and vigorously stirred. The transfer of NCs to the aqueous phase was easily detectable by its color change. After the phase transfer, NCs were removed from the vessel as the aqueous solution and precipitated by the addition of ethanol. The NCs were collected by centrifugation. The repetitive dispersion− precipitation cycles with water and ethanol, respectively, successfully removed an excess amount of thiolate ligands. Characterizations. TEM measurements were performed by JEOL JEM 2200FS electron microscopy operating at 200 kV. SEM and STEM images were taken with a Hitachi SU9000. Electron 3D tomography measurement was carried out by using JEOL JEM3100FE operating at 300 kV. A 3D image was reconstructed from 141 images accumulated by tilting the sample stage in 1° increments from −70° to +70°. Specimens for these electron microscopic studies were prepared by the drop-casting of NC solutions on carbon-film-coated copper mesh grids. XPS measurements were performed by ULVACPHI PHI 5000 Versa Probe II with an MgKα source (1253.6 eV). The

mi

d vi = fi = dt

∑ (FCij + FijD + FijR ) (1)

j≠i C

where m is the mass, v is the velocity vector, F is the conservative force, FD is the dissipative force, and FR is the pairwise random force. The force acting on the NP is summed over all the interbead forces between particles i and j. The conservative force is soft repulsive and is given by

FCij

⎧ ⎛ rij ⎞ ⎪ ⎪− aij⎜1 − ⎟nij, rij ≤ rc rc ⎠ ⎝ =⎨ ⎪ ⎪ 0, rij > rc ⎩

(2)

where r is the position vector, rij = rj − ri, r = |rij|, and nij = rij/|rij|. Here, aij is a parameter to determine the magnitude of the repulsive force between beads i and j, and rc is the cutoff distance. Dissipative and random forces are given by D ⎧ ⎪− γω (rij)(nij · vij)nij , rij ≤ rc FijD = ⎨ ⎪ rij > rc 0, ⎩

(3)

and −1/2 R ⎧ nij, rij ≤ rc ⎪ σω (rij)ζijΔt FijR = ⎨ ⎪ rij > rc 0, ⎩

(4)

respectively, where vij = vj − vi, γ is the friction parameter, σ is the noise parameter, and ζ is the random fluctuating variable with Gaussian statics. The dissipative and the random forces are linked by the fluctuation−dissipation theorem,69 thereby one of the two weight functions appearing in eqs 3 and 4 can be chosen arbitrarily with a relation between the amplitude and kBT:

σ 2 = 2γkBT

(5)

2 ⎧⎛ r ⎞ ⎪ ⎜1 − ij ⎟ , r ≤ r ⎪ ij c rc ⎠ ωD(rij) = [ωR (rij)]2 = ⎨ ⎝ ⎪ ⎪ 0, rij > rc ⎩

(6)

For eqs 5 and 6, kB is the Boltzmann constant and T is temperature. Usually, reduced units are adopted for reporting the DPD results. The DPD unit of length is the cutoff radius, rc, the unit of mass is the bead mass, m, and the unit of energy is kBT. In this simulation, we have adopted the method by Groot and Rabone for scaling of length and time.70 For the DPD simulations of the nanoslit system, the periodic boundary condition is applied in the x−y directions. The planar tripod NC is composed of both hydrophobic and hydrophilic DPD beads on a diamond lattice with a lattice constant α = 0.47 nm. Each nanoparticle consists of 90 DPD beads: 21 are solvophobic and another 69 are solvophilic. The length of the arm is 2.83 nm. The solvent bead represents single DPD bead. The inner surface of a nanoslit is treated as a smooth wall.37,71 The force field of the smooth 9317

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ACS Nano

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wall is derived based on a structured wall by summing the DPD forces between every pair of DPD beads and wall beads and is given by

Fwall (R ) =

1 πρ a wall(− R4 + 2R3 − 2R + 1)R/R 6 wall

(7)

where ρwall is the number density of the structured wall, awall is the interaction parameter between the wall and the DPD bead, R is the normal vector from the bead to the wall surface, R = |R| is the distance between the wall and the bead. In this simulation, ρwall and awall are 10.0 and 50, respectively. The supercells for the planar tripod and tetrapod systems contain 76,966 and 79,966 beads among which 9000 and 12,000 beads are used to construct NCs, respectively, and remaining 67,966 beads are used as solvent for both systems. The total numbers of NCs are therefore set to 100 and 120 in the planar tripod and tetrapod systems, respectively. The volume of the simulation box is 97 × 97 × 0.71 nm3 for both systems. The noise parameter σ is set to 3.0, and the friction parameter γ is set to 4.5. All simulations are performed in the constant-volume and constant-temperature ensemble.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04719. Supplementary data including tomographic images and XPS profiles (PDF) Movie S1: Reconstructed 3D tomography image demonstrating a hexagonal arrangement of CdSe(15) tetrapods (AVI) Movie S2: DPD simulation result for amphiphilic planar tripods (AVI) Movie S3: DPD simulation result for amphiphilic planar tetrapods (AVI) Movie S4: DPD simulation result for tripods with homogeneous surface (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yusei Kobayashi: 0000-0002-5690-2033 Noriyoshi Arai: 0000-0002-5254-7329 Takuya Nakashima: 0000-0002-5311-4146 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was funded by JSPS KAKEN-HI grand number JP16H06522 in Scientific Research on Innovative Areas “Coordination Asymmetry”. We thank Ms. S. Fujita for her assistance with TEM tomography measurements, Mr. Y. Okajima for his help in XPS measurements, and Prof. L McDowell for proofreading the entire text in its original form. We are also grateful to Prof. M. Kunitake at Kumamoto University for his valuable comments on the present work. REFERENCES (1) Li, H.; Kanaras, A. G.; Manna, L. Colloidal Branched Semiconductor Nanocrystals: State of the Art and Perspectives. Acc. Chem. Res. 2013, 46, 1387−1396. 9318

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DOI: 10.1021/acsnano.7b04719 ACS Nano 2017, 11, 9312−9320