Programmed Self-Assembly of Branched Nanocrystals with an

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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 ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b04719 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 7, 2017

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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

*e-mail: [email protected], [email protected]

1

ACS Paragon Plus Environment

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ABSTRACT Site-selective surface modifications 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 self-assemblies of semiconductor nanocrystals with branched shapes through tip-to-tip attachment. Short-chained water-soluble 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 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

2


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Developments 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 self-assembly.4-11 These studies are motivated to mimic self-assemblies 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 either between cores and through surface ligands and 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 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 chains and three-dimensional 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 inter-particle interactions. Particles with a patterned surface modification, in particular, 3


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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 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 inter-particle 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 one-dimensional 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 crosslinked 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 4


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components by introducing branched structure in NCs. For example, the use of tetrapod-shaped 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).

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

Herein, we report the formation of 2D-ordered self-assembly of CdSe tetrapod and CdSe/CdS core/shell octapod NCs through tip-to-tip connections. Self-assembly 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 tip-selective 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-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 5


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stably dispersed in water firstly, 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 Kagome-like 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 co-workers 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 DMAET6


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and MES-capped CdSe tetrapod NCs were clearly dispersed in water just after the ligand-exchange 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 (Figure 2 and Figure S1).

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 two 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-to-tip connections between the tetrapod NCs as observed by scanning electron microscopy (SEM), and TEM zooming in a less dense area (Figure 3, also see Figure S2 for CdSe(15) tetrapod NCs capped with DMAET and MES). The assembling 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, 7


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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 tip-selective removal of capping ligands.57

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 tip-to-tip connections between tetrapods. 8


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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) tetrapod NCs before and just after the ligand-exchange and after the self-assembly. The CdSe tetrapod samples prepared via re-precipitation 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 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 9


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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 one 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).

Se 3d 5/2 , 3d 3/2

Cd 3d 5/2 , 3d 3/2

CdSe

Cd Se (Cd 3d 5/2 ) Oxid ized Se (Se, SeO2,3 )

(c)

(b )

In ten sity/ a.u .

Cd Se (Cd 3d 3/2 )

In ten sity/ a.u .

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(a) 50

(c)

(b )

(a)

55 60 65 400 405 410 415 420 Bin d in g en erg y/ eV Bin d in g en erg y/ eV

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.

Two-Dimensional 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 10


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grid. The energetically-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 three-dimensional 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 were 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 stereo-images observed with the specimen tilt angle of 10º (Figure 5c) and 3D tomography TEM measurement (Figure 5d, Figure S7 and Supplementary Movie 1) 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 11


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(Figure 5c and Figure S6a) correspond to upright pillars like unbound dangling bonds. The electron beam irradiation for 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).

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. (f) Representative TEM image of the self-assembly of CdSe(40) tetrapod NCs.

The hexagonal assemblies of CdSe tetrapod NCs are most likely formed during the evaporation 12


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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 (Figure 5a, Figure 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, 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 ºC) in the aqueous solution and deposition temperature (35 ºC). We also adopted a slow evaporation method,46 which did not lead to obvious improvement in regularity in the self-assemblies. 13


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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 cupper grid to observe the self-assembly. As shown in Figure 6, the octapod NCs also gave tip-to-tip self-assemblies on the carbon-coated TEM grid (also see Figure S12). They form, one-dimensional 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

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

Dissipative Particle Dynamic Simulations. We performed dissipative particle dynamics (DPD) 14


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simulation to further study the 2D self-assembling behavior of amphiphilic nanoparticles.37,665,66 To simplify the simulation condition, 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 unit) 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 2,3). The DPD simulation successfully reproduced the self-assembly of branched NCs via hydrophobic tip-to-tip connection including a hexagonally arranged Kagome-like 2D assembly for tetrapod NCs (Figures 5b, 7a) and a net-like 2D assembly for octapod NCs (Figures 6b, 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 a 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 15


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hydrophilic thiolate ligand (Supplementary Movie 4). 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 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.

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 a 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 dominate the self-assembling process. Further improvements in the conditions including the 16


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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 non-closely 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 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), oleylamine (0.5 mL) using 1-octadecene (1-ODE).47 The CdSe seeds (3 × 10-7 mol) solution in the mixture of 1-ODE (21.3 mL), OA (2.25 mL), n-trioctylphosphine (TOP, 1.5 mL) and cetyltrimethylammonium bromide (0.21 mmol) was heated up to 260 ºC under N2 flow. After the temperature reached 260 ºC, 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 17


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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. 1 mmol of Cu(acac)2 was dispersed in the mixture of oleylamine (9.5 mL) and 1-dodecanethiol (DT, 3 mL), which was heated to 60 ºC in vacuo for 1 h. The mixture was then heated to 220 ºC 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 in a glove box filled with Ar. In the glove box, the seed NCs were purified by repetitive precipitation and re-dispersion 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 three necked flak, CdO (60 mg), CdCl2 (6 mg), ODPA (0.29 g), hexylphosphonic acid (80 mg) and tri-octylphosphine oxide (3 g) were charged and heated to 130 ºC in vacuo for 1 h followed by heating to 350 ºC 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), TOPS solution (500 µL, 32 mg/mL) was injected to the reaction mixture. The temperature was recovered to 350 ºC 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, 18


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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 ºC 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 JEM-3100FE operating at 300 kV. A 3D image was reconstructed from 141 images accumulated by tilting the sample stage in 1º increments from –70º to +70º. 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 ULVAC-PHI PHI 5000 Versa Probe II with an MgKα source (1253.6 eV). The 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 self-assembling behavior of a confined amphiphilic NCs solution. The 19


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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

= = ∑ + +

(1)

where m is the mass, v is the velocity vector, FC 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

− 1 − , ≤ ! = , 0, > !

(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

= $

−%& ( ∙ ) , ≤ ! 0, > !

(3)

= $

*& + ,- .//1 , ≤ ! 0, > !

(4)

and

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, 20


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* 1 = 2%34 5 ,

(5)

1

1 − , ≤ ! & = 6& 7 = . 0, > ! 1

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

89:: (; ) = < =>89:: 89:: (−; ? + 2; @ − 2; + 1)A/;,

(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 9 000 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 21


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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 Supporting Information The Supporting Information is available free of charge on the ACS Publications website: Supplementary data including tomographic images and XPS profiles (PDF) Supplementary Movie 1, reconstructed 3D tomography image demonstrating a hexagonal arrangement of CdSe(15) tetrapods (AVI) Supplementary Movie 2, DPD simulation result for amphiphilic planar tripods (AVI) Supplementary Movie 3, DPD simulation result for amphiphilic planar tetrapods (AVI) Supplementary Movie 4, DPD simulation result for tripods with homogeneous surface (AVI) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected]

The authors declare no competing financial interests. ACKNOWLEDGMENTS 22


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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. 2. Costi, R.; Saunders, A.E.; Banin, U. Colloidal Hybrid Nanostructures: A New Type of

Functional Materials. Angew. Chem. Int. Ed. 2010, 49, 4878-4897. 3. Chen, O.; Zhao, J.; Chauhan, V. P.; Cui, J.; Wong, C.; Harris, D. K.; Wei, H.; Han, H. S.;

Fukumura, D.; Jain, R. K.; Bawendi, M. G. Compact High-Quality CdSe-CdS Core-Shell Nanocrystals with Narrow Emission Linewidths and Suppressed Blinking. Nat. Mater. 2013, 12, 445-451. 4. Glotzer, S. C.; Solomon, M. J. Anisotropy of Building Blocks and Their Assembly into

Complex Structures. Nat. Mater. 2007, 6, 557-562. 5. Grzelczak, M.; Vermant, J.; Furst, E. M.; Liz-Marzan, L. M. Directed Self-Assembly of

Nanoparticles. ACS Nano 2010, 4, 3591-3605. 6. Nie, Z.; Petukhova, A.; Kumacheva, E. Properties and Emerging Applications of

Self-Assembled Structures Made from Inorganic Nanoparticles. Nat. Nanotechnol. 2010, 5, 15-25. 7. Gao, Y.; Tang, Z. Design and Application of Inorganic Nanoparticle Superstructures: Current

Status and Future Challenges. Small 2011, 7, 2133-2146. 23


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

8. Xu, L.; Ma, W.; Wang, L.; Xu, C.; Kuang, H.; Kotov, N. A. Nanoparticle Assemblies:

Dimensional Transformation of Nanomaterials and Scalability. Chem. Soc. Rev. 2013, 42, 3114-3126. 9. Kotov, N. A.; Weiss, P. S. Self-Assembly of Nanoparticles: A Snapshot. ACS Nano 2014, 8,

3101-3103. 10. Kovalenko, M. V.; Manna, L.; Cabot, A.; Hens, Z.; Talapin, D. V.; Kagan, C. R.; Klimov, V. I.;

Rogach, A. L.; Reiss, P.; Milliron, D. J.; Guyot-Sionnnest, P.; Konstantatos, G.; Parak, W. J.; Hyeon, T.; Korgel, B. A.; Murray, C. B.; Heiss, W. Prospects of Nanoscience with Nanocrystals. ACS Nano 2015, 9, 1012-1057. 11. Cademartiri, L.; Bishop, K. J. Programmable Self-Assembly. Nat. Mater. 2015, 14, 2-9. 12. Kotov, N. A. Inorganic Nanoparticles as Protein Mimics. Science 2010, 330, 188-189. 13. Xia, Y.; Nguyen, T. D.; Yang, M.; Lee, B.; Santos, A.; Podsiadlo, P.; Tang, Z.; Glotzer, S. C.;

Kotov, N. A. Self-Assembly of Self-Limiting Monodisperse Supraparticles from Polydisperse Nanoparticles. Nat. Nanotechnol. 2011, 6, 580–587. 14. Zhou, Y.; Marson, R. L.; van Anders, G.; Zhu, J.; Ma, G.; Ercius, P.; Sun, K.; Yeom, B.;

Glotzer, S. C.; Kotov, N. A. Biomimetic Hierarchical Assembly of Helical Supraparticles from Chiral Nanoparticles. ACS Nano 2016, 10, 3248-3256. 15. Yang, M.; Chan, H.; Zhao, G.; Bahng, J. H.; Zhang, P.; Kral, P.; Kotov, N. A. Self-Assembly of

Nanoparticles into Biomimetic Capsid-Like Nanoshells. Nat. Chem. 2017, 9, 287-294. 16. Yue, M.; Li, Y.; Hou, Y.; Cao, W.; Zhu, J.; Han, J.; Lu, Z.; Yang, M. Hydrogen Bonding

Stabilized Self-Assembly of Inorganic Nanoparticles: Mechanism and Collective Properties. ACS Nano 2015, 9, 5807-5817. 17. Miao, L.; Han, J.; Zhang, H.; Zhao, L.; Si, C.; Zhang, X.; Hou, C.; Luo, Q.; Xu, J.; Liu, J.

Quantum-Dot-Induced Self-Assembly of Cricoid Protein for Light Harvesting. ACS Nano 2014, 8, 3743-3751. 18. Talapin, D. V.; Lee, J. S.; Kovalenko, M. V.; Shevchenko, E. V. Prospects of Colloidal

Nanocrystals for Electronic and Optoelectronic Applications. Chem. Rev. 2010, 110, 389-458. 24


Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

19. Boles, M. A.; Engel, M.; Talapin, D. V. Self-Assembly of Colloidal Nanocrystals: From

Intricate Structures to Functional Materials. Chem. Rev. 2016, 116, 11220-11289. 20. Kuzyk, A.; Schreiber, R.; Fan, Z.; Pardatscher, G.; Roller, E. M.; Hogele, A.; Simmel, F. C.;

Govorov, A. O.; Liedl, T. DNA-Based Self-Assembly of Chiral Plasmonic Nanostructures with Tailored Optical Response. Nature 2012, 483, 311-314. 21. Lavieville, R.; Zhang, Y.; Casu, A.; Genovese, A.; Manna, L.; Di Fabrizio, E.; Krahne, R.

Charge Transport in Nanoscale "All-Inorganic" Networks of Semiconductor Nanorods Linked by Metal Domains. ACS Nano 2012, 6, 2940-2947. 22. Zhao, H.; Sen, S.; Udayabhaskararao, T.; Sawczyk, M.; Kucanda, K.; Manna, D.; Kundu, P. K.;

Lee, J. W.; Kral, P.; Klajn, R. Reversible Trapping and Reaction Acceleration within Dynamically Self-Assembling Nanoflasks. Nat. Nanotechnol. 2016, 11, 82-88. 23. Rao, A.; Roy, S.; Unnikrishnan, M.; Bhosale, S. S.; Devatha, G.; Pillai, P. P. Regulation of

Interparticle Forces Reveals Controlled Aggregation in Charged Nanoparticles. Chem. Mater. 2016, 28, 2348-2355. 24. Pillai, P. P.; Kowalczyk, B.; Grzybowski, B. A. Self-Assembly of Like-Charged Nanoparticles

into Microscopic Crystals. Nanoscale 2016, 8, 157-161. 25. Bishop, K. J.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A. Nanoscale Forces and Their Uses in

Self-Assembly. Small 2009, 5, 1600-1630. 26. Batista, C. A.; Larson, R. G.; Kotov, N. A. Nonadditivity of Nanoparticle Interactions. Science

2015, 350, 1242477. 27. Tang, Z. Y.; Kotov, N. A.; Giersig, M. Spontaneous Organization of Single Cdte Nanoparticles

into Luminescent Nanowires. Science 2002, 297, 237-240. 28. Nair, P. V.; Thomas, K. G. Hydrazine-Induced Room-Temperature Transformation of CdTe

Nanoparticles to Nanowires. J. Phys. Chem. Lett. 2010, 1, 2094-2098. 29. Baker, J. L.; Widmer-Cooper, A.; Toney, M. F.; Geissler, P. L.; Alivisatos, A. P. Device-Scale

Perpendicular Alignment of Colloidal Nanorods. Nano Lett. 2010, 10, 195-201. 30. Wang, T.; Zhuang, J.; Lynch, J.; Chen, O.; Wang, Z.; Wang, X.; LaMontagne, D.; Wu, H.; 25


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

Wang, Z.; Cao, Y. C. Self-Assembled Colloidal Superparticles from Nanorods. Science 2012, 338, 358-363. 31. Li, L.-s.; Walda, J.; Manna, L.; Alivisatos, A. P. Semiconductor Nanorod Liquid Crystals. Nano

Lett. 2002, 2, 557-560. 32. Diroll, B. T.; Greybush, N. J.; Kagan, C. R.; Murray, C. B. Smectic Nanorod Superlattices

Assembled on Liquid Subphases: Structure, Orientation, Defects, and Optical Polarization. Chem. Mater. 2015, 27, 2998-3008. 33. Ghezelbash, A.; Koo, B.; Korgel, B. A. Self-Assembled Stripe Patterns of Cds Nanorods. Nano

Lett. 2006, 6, 1832-1836. 34. Miszta, K.; de Graaf, J.; Bertoni, G.; Dorfs, D.; Brescia, R.; Marras, S.; Ceseracciu, L.;

Cingolani, R.; van Roij, R.; Dijkstra, M.; Manna, L. Hierarchical Self-Assembly of Suspended Branched Colloidal Nanocrystals into Superlattice Structures. Nat. Mater. 2011, 10, 872-876. 35. Chen, Q.; Bae, S. C.; Granick, S. Directed Self-Assembly of a Colloidal Kagome Lattice.

Nature 2011, 469, 381−384. 36. Zhang, Z.; Glotzer, S. C. Self-Assembly of Patchy Particles. Nano Lett. 2004, 4, 1407-1413. 37. Arai, N.; Yausoka, K.; Zeng, X. C. Self-Assembly of Triblock Janus Nanoparticle in Nanotube.

J. Chem. Theory Comput. 2013, 9, 179-187. 38. Nie, Z.; Fava, D.; Kumacheva, E.; Zou, S.; Walker, G. C.; Rubinstein, M. Self-Assembly of

Metal-Polymer Analogues of Amphiphilic Triblock Copolymers. Nat. Mater. 2007, 6, 609-614. 39. Song, J.; Cheng, L.; Liu, A.; Yin, J.; Kuang, M.; Duan, H. Plasmonic Vesicles of Amphiphilic

Gold Nanocrystals: Self-Assembly and External-Stimuli-Triggered Destruction. J. Am. Chem. Soc. 2011, 133, 10760-10763. 40. Niikura, K.; Iyo, N.; Higuchi, T.; Nishio, T.; Jinnai, H.; Fujitani, N.; Ijiro, K. Gold

Nanoparticles Coated with Semi-Fluorinated Oligo(Ethylene Glycol) Produce Sub-100 Nm Nanoparticle Vesicles without Templates. J. Am. Chem. Soc. 2012, 134, 7632-7635. 41. Lee, H. Y.; Shin, S. H.; Drews, A. M.; Chirsan, A. M.; Lewis, S. A.; Bishop, K. J.

Self-Assembly of Nanoparticle Amphiphiles with Adaptive Surface Chemistry. ACS Nano 2014, 26


Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

8, 9979-9987. 42. Nikolic, M. S.; Olsson, C.; Salcher, A.; Kornowski, A.; Rank, A.; Schubert, R.; Fromsdorf, A.;

Weller, H.; Forster, S. Micelle and Vesicle Formation of Amphiphilic Nanoparticles. Angew. Chem., Int. Ed. 2009, 48, 2752-2754. 43. Taniguchi, Y.; Takishita, T.; Kawai, T.; Nakashima, T. End-to-End Self-Assembly of

Semiconductor Nanorods in Water by Using an Amphiphilic Surface Design. Angew. Chem., Int. Ed. 2016, 55, 2083-2086. 44. Goodman, M. D.; Zhao, L.; Derocher, K. A.; Wang, J.; Mallapragada, S. K.; Lin, Z.

Self-Assembly of CdTe Tetrapods into Network Monolayers at the Air/Water Interface. ACS Nano 2010, 4, 2043-2050. 45. Zanella, M.; Bertoni, G.; Franchini, I. R.; Brescia, R.; Baranov, D.; Manna, L. Assembly of

Shape-Controlled Nanocrystals by Depletion Attraction. Chem. Commun. 2011, 47, 203-205. 46. Mishra, N.; Wu, W.-Y.; Srinivasan, B. M.; Hariharaputran, R.; Zhang, Y.-W.; Chan, Y.

Continuous Shape Tuning of Nanotetrapods: Toward Shape-Mediated Self-Assembly. Chem. Mater. 2016, 28, 1187-1195. 47. Manna, L.; Scher, E. C.; Alivisatos, A. P. Synthesis of Soluble and Processable Rod-, Arrow-,

Teardrop-, and Tetrapod-Shaped CdSe Nanocrystals. J. Am. Chem. Soc. 2000, 122, 12700-12706. 48. Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Controlled Growth of

Tetrapod-Branched Inorganic Nanocrystals. Nat. Mater. 2003, 2, 382-385. 49. Bowden, N. B.; Weck, M.; Choi, I. S.; Whitesides, G. M. Molecule-Mimetic Chemistry and

Mesoscale Self-Assembly. Acc. Chem. Res. 2001, 34, 231-238. 50. Lim, J.; Bae, W. K.; Park, K. U.; zur Borg, L.; Zentel, R.; Lee, S.; Char, K. Controlled

Synthesis of Cdse Tetrapods with High Morphological Uniformity by the Persistent Kinetic Growth and the Halide-Mediated Phase Transformation. Chem. Mater. 2013, 25, 1443-1449. 51. Pang, Q.; Zhao; Cai, Y.; Nguyen, D. P.; Regnault, N.; Wang, N.; Yang; Ge; Ferreira, R.;

Bastard, G.; Wang CdSe Nano-Tetrapods: Controllable Synthesis, Structure Analysis, and 27


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

Electronic and Optical Properties. Chem. Mater. 2005, 17, 5263-5267. 52. Asokan, S.; Krueger, K. M.; Colvin, V. L.; Wong, M. S. Shape-Controlled Synthesis of CdSe

Tetrapods Using Cationic Surfactant Ligands. Small 2007, 3, 1164-1169. 53. Nakashima, T.; Kobayashi, Y.; Kawai, T. Optical Activity and Chiral Memory of Thiol-Capped

CdTe Nanocrystals. J. Am. Chem. Soc. 2009, 131, 10342-10343. 54. Devatha, G.; Roy, S.; Rao, A.; Mallick, A.; Basu, S.; Pillai, P. P. Electrostatically Driven

Resonance Energy Transfer in "Cationic" Biocompatible Indium Phosphide Quantum Dots. Chem. Sci. 2017, 8, 3879-3884. 55. Kim, D.; Kim, W. D.; Kang, M. S.; Kim, S. H.; Lee, D. C. Self-Organization of Nanorods into

Ultra-Long Range Two-Dimensional Monolayer End-to-End Network. Nano Lett. 2015, 15, 714-720. 56. Chakrabortty, S.; Guchhait, A.; Ong, X.; Mishra, N.; Wu, W. Y.; Jhon, M. H.; Chan, Y. Facet to

Facet Linking of Shape Anisotropic Inorganic Nanocrystals with Site Specific and Stoichiometric Control. Nano Lett 2016, 16, 6431-6436. 57. Franchini, I. R.; Cola, A.; Rizzo, A.; Mastria, R.; Persano, A.; Krahne, R.; Genovese, A.; Falqui,

A.; Baranov, D.; Gigli, G.; Manna, L. Phototransport in Networks of Tetrapod-Shaped Colloidal Semiconductor Nanocrystals. Nanoscale 2010, 2, 2171-2179. 58. Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron

Spectroscopy ; Perkin-Elmer: Eden Prairie, MN, 1992. 59. J. E. B. Katari, J. E. B.; Colvin, V. L.; Alivisatos, A. P. X-Ray Photoelectron Spectroscopy of

CdSe Nanocrystals with Applications to Studies of the Nanocrystal Surface. J. Phys. Chem. 1994, 98, 4109-4117. 60. de la Cueva, L.; Lauwaet, K.; Otero, R.; Gallego, J. M.; Alonso, C.; Juarez, B. H. Effect of

Chloride Ligands on CdSe Nanocrystals by Cyclic Voltammetry and X-Ray Photoelectron Spectroscopy. J. Phys. Chem. C 2014, 118, 4998-5004. 61. Lim, S. J.; Kim, W.; Jung, S.; Seo, J.; Shin, S. K. Anisotropic Etching of Semiconductor

Nanocrystals. Chem. Mater. 2011, 23, 5029-5036. 28


Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

62. Qi, W.; de Graaf, J.; Qiao, F.; Marras, S.; Manna, L.; Dijkstra, M. Ordered Two-Dimensional

Superstructures of Colloidal Octapod-Shaped Nanocrystals on Flat Substrates. Nano Lett. 2012, 12, 5299-5303. 63. Han, W.; Lin, Z. Learning from "Coffee Rings": Ordered Structures Enabled by Controlled

Evaporative Self-Assembly. Angew. Chem. Int. Ed. 2012, 51, 1534-1546. 64. Tanoue, R.; Higuchi, R.; Enoki, N.; Miyasato, Y.; Uemura, S.; Kimizuka, N.; Stieg, A. Z.;

Gimzewski, J. K.; Kunitake, M. Thermodynamically Controlled Self-Assembly of Covalent Nanoarchitectures in Aqueous Solution. ACS Nano 2011, 5, 3923-3929. 65. Arai, N.; Yasuoka, K.; Zeng, X. C. Self-Assembly of Janus Oligomers into Onion-Like Vesicles

with Layer-by-Layer Water Discharging Capability: A Minimalist Model. ACS Nano 2016, 10, 8026-8037. 66. Kobayashi, Y.; Arai, N. Self-Assembly and Viscosity Behavior of Janus Nanoparticles in

Nanotube Flow. Langmuir 2017, 33, 736-743. 67. Castelli, A.; de Graaf, J.; Prato, M.; Manna, L.; Arciniegas, M. P. Tic-Tac-Toe Binary Lattices

from the Interfacial Self-Assembly of Branched and Spherical Nanocrystals. ACS Nano 2016, 10, 4345-4353. 68. Hoogerbrugge, P. J.; Koelman, J. M. V. A. Simulating Microscopic Hydrodynamics Phenomena

with Dissipative Particle Dynamics. Europhys. Lett. 1992, 19, 155−160. 69. Espanõl, P.; Warren, P. B. Statical-Mechanics of Dissipative Particle Dynamics. Europhys. Lett.

1995, 30, 191−196. 70. Groot, R. D.; Rabone, K. L. Mesoscopic Simulation of Cell Membrane Damage, Morphology

Change and Rupture by Nonionic Surfactants. Biophys. J. 2001, 81, 725-736. 71. Arai, N.; Yasuoka, K.; Zeng, X. C. Self-Assembly of Surfactants and Polymorphic Transition in

Nanotubes. J. Am. Chem. Soc. 2008, 130, 7916-7920.

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Programmed Self-Assembly of Branched Nanocrystals with an

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