Self-Organization of Nanorods into Ultra-Long Range Two

Dec 15, 2014 - We present a two-dimensional (2-D), millimeter-scale network of colloidal CdSe nanorods (NRs) in monolayer thickness through end-to-end...
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Self-Organization of Nanorods into Ultra-Long Range TwoDimensional Monolayer End-to-End Network Dahin Kim,† Whi Dong Kim,† Moon Sung Kang,‡ Shin-Hyun Kim,*,† and Doh C. Lee*,† †

Department of Chemical and Biomolecular Engineering (BK21+ Program), KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea ‡ Department of Chemical Engineering, Soongsil University, Seoul, Korea S Supporting Information *

ABSTRACT: Highly uniform large-scale assembly of nanoscale building blocks can enable unique collective properties for practical electronic and photonic devices. We present a two-dimensional (2-D), millimeter-scale network of colloidal CdSe nanorods (NRs) in monolayer thickness through end-to-end linking. The colloidal CdSe NRs are sterically stabilized with tetradecylphosphonic acid (TDPA), and their tips are partially etched in the presence of gold chloride (AuCl3) and didecyldimethylammonium bromide (DDAB), which make them unwetted in toluene. This change in surface wetting property leads to spontaneous adsorption at the 2-D air/toluene interface. Anisotropy in both the geometry and the surface property of the CdSe NRs causes deformation of the NR/toluene/air interface, which derives capillary attraction between tips of neighboring NRs inward. As a result, the NRs confined at the interface spontaneously form a 2-D network composed of end-to-end linkages. We employ a vertical-deposition approach to maintain a consistent rate of NR supply to the interface during the assembly. The rate control turns out to be pivotal in the preparation of a highly uniform large scale 2-D network without aggregation. In addition, unprecedented control of the NR density in the network was possible by adjusting either the lift-up speed of the immersed substrate or the relative concentration of AuCl3 to DDAB. Our findings provide important design criteria for 2-D assembly of anisotropic nanobuilding blocks. KEYWORDS: Nanorod, self-assembly, end-to-end, interfacial adsorption, capillary interaction

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applied to a transparent layer for display due to the low solid fraction compared to the close packing.17 Additionally, end-toend assembly, with a controllable degree of network opening, can be used in efficient opto-electric devices by engineering the energy transfer among neighboring NRs.18 Despite intensive demand and their potential, end-to-end linking of colloidal NRs has been relatively underexplored because strong van der Waals (vdW) or dipole−dipole attraction keeps the NRs aligned sideby-side.19,20 One way to overcome the strong attraction toward side-byside alignment is to exploit the high reactivity of NR tips. For example, growing gold nanoparticles (NPs) on NR tips enables selective welding between the gold tips.21,22 The short alkyl chains on the tips of the NRs and the long chains on their sidewalls increase tip-to-tip attraction by relatively weakening steric hindrance in that direction.23,24 However, this approach produces big 3-D aggregates of uncontrolled size and shape, which severely restricts practical applications as most electronic devices require two-dimensional (2-D) panels.23,25−28 Alter-

olloidal nanomaterial assemblies can have ensemble properties uniquely suited for various target applications, which are otherwise difficult to achieve with individual colloids.1−4 In particular, aggregated nanorods (NRs) often exhibit unconventionally polarizable optical properties or high conductivity, making them appealing for photonics, electronics, or energy-harvesting applications.5−8 NRs can form either sideby-side or end-to-end alignments, and the resulting ensembles demonstrate different characteristics depending on the population ratio between the two alignments. Side-by-side packing has been observed in assemblies prepared by solvent evaporation, occasionally assisted by templates or other external forces, with NRs forming either parallel or perpendicular alignment to a substrate.9−13 The orientational and positional ordering of NRs resembles that of liquid crystal molecules, in a sense that the ordering is typically thermodynamically favorable.14 These NR assemblies can provide linearly polarized emission or enhanced optical absorption.15,16 On the other hand, end-to-end networks of NRs would open up new interesting opportunities. For example, NRs in an end-to-end linked cluster serves as a continuous pathway for carrier transport with intrinsic properties of individual NRs, such as quantum confinement, unimpaired. Also, the networks can be © XXXX American Chemical Society

Received: November 6, 2014 Revised: December 11, 2014

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absence of additives, the NRs are packed side-by-side (Figure S2b). To elucidate this unique assembly behavior and expand on the strategy to grow a uniform network structure over a wide area, we employed a vertical dip-coating technique. Figure 1a shows a schematic illustration of the dip-coating procedure. During the vertical coating, toluene evaporates predominantly at the meniscus and causes a convective influx. This NR-laden flow helps supply the NRs to the interface and therefore

natively, end-to-end alignment has been achieved by placing NRs in exclusive confinements. NRs are assembled within nanostructures developed by microphase separation of block copolymers or guided by lithographically prepared nanoscale trenches.29−31 However, matrix-free networks with high density of NRs transferable onto another substrate have remained elusive. For practical device applications, 2-D end-to-end NR networks that are highly uniform over a large area is an important challenge. In this work, we establish a simple, one-step method for assembling 2-D end-to-end networks of CdSe NRs in monolayer thickness over a large length scale, using anisotropic capillary interaction between NRs confined at the air/liquid interface. To the best of our knowledge, 2-D end-to-end linking of colloidal NRs has not been systematically investigated or achieved. We changed the surface of the NRs by selectively peeling the surfactant layer from the tips using gold chloride (AuCl3), facilitating spontaneous anchoring of the NRs at the air/liquid interface. The etched tips of the NRs then deform the local interface, yielding strong capillary attraction between the tips of neighboring NRs and thus leading to preferential end-toend linkages. With a vertical dip-coating technique, we can create and transfer the network structure with high uniformity onto a 2-D solid substrate. During the dip-coating, the supply of the NRs to the interface, end-to-end assembly, and transfer of the assembled structure onto a solid substrate all occur simultaneously, keeping the density of the network constant and thus enabling a homogeneous network over a large area. Furthermore, the density of the NR networks can be controlled by adjusting the rate of NR supply and the etching activity of AuCl3. The resulting networks possess high connectivity between NRs and low solid fraction at the same time and could potentially serve as efficient percolating electrical channel with high transparency. More importantly, this approach provides new insight into forming complex 2-D clusters of diverse anisotropic colloids using interfacial self-assembly. Isotropic microparticles anchored at a free interface do not perturb the interface and cause no capillary interaction.32 By contrast, anisotropic microparticles, such as ellipsoids and cylinders, deform the interface and induce directional capillary attraction to minimize the deformation of the interface.33,34 Inspired by directional assembly of micro-objects, we sought to create 2-D network structures of CdSe NRs by inducing anisotropic capillary attraction on nanobuilding blocks stranded at the interface. Although the capillary force caused by shape anisotropy is much weaker at nanoscale, anisotropy in surface property can magnify the interfacial deformation. In addition, the high diffusivity of nanobuilding blocks facilitates the assembly as long as the capillary force is sufficiently large to hold the blocks. In our case, anisotropic surface properties of CdSe NRs were augmented by the use of AuCl3, which selectively etches their tips. Although AuCl3 is typically used as a precursor to grow gold particles on NR tips, it can also etch the tip in the absence of a reducing agent.35 Once tetradecylphosphonic acid (TDPA), the steric stabilizer on the surface of the NRs, is selectively peeled from the tips, the NRs are no longer wettable by toluene and hence adsorb spontaneously at the air/toluene interface. The NR solution containing AuCl3 and DDAB form a film at the interface (Figure S1).36−38 When the solution is deposited on a transmission electron microscopy (TEM) grid, the aggregated NRs are linked in an end-to-end fashion (Figure S2a). In the

Figure 1. Interfacial and bulk assembly of CdSe nanorods (NRs) using vertical dip-coating. (a) Schematic illustration of the assembly process of colloidal CdSe NRs into two distinct morphologies: 2-D network structure composed of end-to-end linkages is prepared through interfacial assembly with gold chloride (AuCl3) and didecyldimethylammonium bromide (DDAB), and a series of bands composed of side-by-side stacks is prepared through concentration of NRs under the meniscus without additives. (b, c) Transmission electron microscopy (TEM) images of a uniform CdSe NR network prepared with AuCl3 and DDAB. Inset of (b) shows high uniformity of the network over a wide area. (d, e) TEM images showing a band composed of side-by-side packing of CdSe NRs formed in the absence of AuCl3 and DDAB. Inset of (d) shows a series of bands. Scale bars in inset (b) and (d) represent 750 μm. B

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facilitates the interfacial assembly which occurs near the contact line.39 At the same time, continuous drying of toluene moves the contact line downward, transferring the NR network near the contact line onto the substrate. The simultaneous assembly and transfer process facilitates the growth of a fractal network of uniform monolayer thickness on a large scale (Figure 1b and Figure S3). Over the entire area of the substrate, a 3 × 3 mm carbon-coated Cu grid, few aggregates or voids are observed. Because the NRs adsorbed at the interface are allowed to migrate only laterally, the convective flow does not concentrate the NRs under the meniscus, effectively suppressing any piling up of the NRs and enabling the formation of a uniform film as shown in the inset of Figure 1b. Also, the network comprises purely end-to-end linkages of the NRs with negligible side-byside contact, where most of the NR chains participate in the formation of a single network (see Figure 1c). This network has powerful potential in creating efficient percolating electrical channels for applications including quantum rod-based transistors and bulk heterojunction solar cells. When toluene does not contain AuCl3 and DDAB, the NR suspension is highly stable against aggregation or sedimentation, and dip-coating the suspension yields a completely different morphology. Figure 1d shows a low-resolution TEM image of CdSe NRs forming coffee rings. This coffee ring formation results from contact line pinning due to impurity or defects on the substrate.40 When the contact line is pinned, NRs are concentrated at the line by convective flow and stacked, forming a band. As toluene evaporates, the pinning force no longer staves off the gravitational force, and the contact line slips and pins to a lower level, forming the next band of stacked NRs. The magnified TEM image in Figure 1e reveals that each coffee ring consists of a pile of NRs packed side-by-side. This stick−slip deposition results in a nonuniform distribution of NRs. It is noteworthy that NR assembly in the presence of AuCl3 and DDAB effectively suppresses coffee ring formation and produces a monolayer fractal film over a large area through interfacial adsorption of the NRs.41,42 Formation of a continuous network spanning the solid substrate and the air/toluene interface leads to gentle movement of the contact line instead of stick−slip motion. During the dip-coating process, the rate of NR supply to the air−toluene interface is determined by the drawing speed of the substrate and the evaporation rate of toluene. The primary focus of our initial investigation was the effect of the drawing speed as the area of the gas/liquid interface turned out to make little impact to the network density. To probe the quantitative effect of the drawing rate, we varied the rates between 0, 0.189, 0.567, and 0.944 μm/min, while maintaining the concentrations of NRs, AuCl3, and DDAB. Figure 2a−d exhibits a dramatic change in the density of NRs in the resultant network structures. Increasing the lift-up speed from 0 to 0.189 μm/ min reduces the areal number density of NRs, ρAN, in the network from 1.33 × 10−3 NRs/nm2 to 9.04 × 10−4 NRs/nm2. The network prepared at 0.189 μm/min still retains overall continuity. Increasing the speed further to 0.567 μm/min reduces the rate of NR supply and results in only local linkages due to insufficient value of ρAN, 5.35 × 10−4 NRs/nm2, losing the overall continuity. As one may predict, a higher speed of 0.944 μm/min lowers the value of ρAN to 1.99 × 10−4 NRs/ nm2, resulting in individual NRs or NR pairs. Figure 2e demonstrates that the average number of NRs forming a single linkage, NNRs, has a similar tendency to ρAN (see Figure S4 for detail). NNRs is as high as 2.6 in a dense network prepared

Figure 2. Influence of the vertical drawing rate of a solid substrate on the density of NRs in the network. (a−d) TEM images of network structures prepared by vertical dip-coating at four different upward drawing rates: (a) 0 μm/min, (b) 0.189 μm/min, (c) 0.567 μm/min, and (d) 0.944 μm/min. (e) Influence of drawing rate on average number of NR arms at a linkage, NNRs (●, left axis) and areal number density of NRs, ρAN (◊, right axis).

without drawing (Figure 2a) and 2.2 in a loose network prepared at a drawing rate of 0.189 μm/min (Figure 2b). At faster drawing rates, NNRs is lower than 2 (Figure 2c and d), indicating unlinked dead ends and no overall continuity. To ensure continuity, the average distance between the centers of trapped NRs should be shorter than ca. 33 nm, approximated from ρAN−1/2 at the drawing rate of 0.189 μm/min (Figure 2b). Figure 3a and b shows HR-TEM images of a NR network formed in the presence of AuCl3. NR tips are completely fused, forming robust connections without any noticeable separation (see also Figure S5). No TDPA likely remains at the tip. There are few side-by-side alignments, which show a narrow gap between the NRs, as shown in the inset of Figure 3b. This corroborates the existence of TDPA on the sidewalls. We attribute this selective local etching to the low density of capping molecules at the rounded tip. Chloride ions, included in AuCl3, can access the surface of CdSe at the tip, while being distanced from the sidewalls by the densely packed capping molecules.43 The chloride anions can attack the electrondeficient Cd atoms and detach them, making the (001) plane of C

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Figure 3. Interparticle fusion in end-to-end networks and interparticle separation in side-by-side assembly. (a, b) High-resolution TEM (HR-TEM) images of a NR network with complete fusion of tips. Inset of (b) reveals that there is separation between side walls. (c) Scheme illustrating selective peeling of TDPA from NR tips and directional tip-to-tip attraction at the interface. (d, e) TEM images of side-by-side packing of NRs with inter-NR separation. (f) Scheme illustrating that NRs capped entirely with TDPA are attracted by van der Waals and dipole−dipole coupling in a bulk solution.

Figure 4. Surface composition of NRs. (a, b) TEM images of an end-to-end network taken at the same position (a) before and (b) after e-beam exposure for 30 s. Gold NPs grow on the tips of CdSe NRs during e-beam irradiation. Inset of (b) shows HR-TEM image with characteristic lattice spacings of Au indexed. (c, d) X-ray photoelectron spectroscopy (XPS) spectra of (c) as-prepared CdSe NRs and (d) NRs treated with AuCl3 and DDAB, with deconvoluted spectra. The curve with two peaks at binding energy 406.8 and 414 eV in the first panel of (d) represents oxidized Cd48 and the three curves with a peak each at binding energy 55.2, 56.7, and 52.6 eV in the second panel represent Se0, oxidized Se, and Au−Se bond, respectively.49,50 In the right panel, Au species are detected in the form of Au3+, Au+, and Au0,51 which means that the NR surface is partially oxidized and Au species are deposited during the etching.

the [001] direction.44 Through selective etching, the NRs are rendered to have poor wettability in toluene, which causes

NRs bare and unstable. A similar mechanism has been employed to grow CdSe nanowires from spherical NPs along D

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of AuCl3 with DDAB leads to low activity due to stable encapsulation. Because the etching rate influences the rate of interfacial adsorption and therefore assembly, the relative concentration of AuCl3 (CAuCl3) to DDAB (CDDAB) provides another means to control the density of NRs in the resultant network. When CAuCl3 increases from 0.65 × 10−5 M to 2.82 × 10−5 M, while maintaining CDDAB at 8.53 × 10−5 M, the density of NRs in the resulting network also increases (Figure S10). In a similar manner, when we increase CDDAB from 4.27 × 10−5 M to 17.1 × 10−5 M with constant CAuCl3 of 1.41 × 10−5 M, the network dramatically loosens as shown in Figure 5a−c and Figure S11. At CDDAB of 4.27 × 10−5 M ([DDAB]/[AuCl3] = 3.03), AuCl3 etches not only the tips but also the sidewalls, leading to formation of dense networks. A small portion of side-

spontaneous anchoring at the air/toluene interface. The etched tips are also likely to deform the interface and form multipoles,45,46 which increase end-to-end attraction (Figure 3c). Once the ends are brought into contact, the unstable tips fuse to reduce interfacial energy.47 The formation of robust end-to-end connections enables the stable transfer of the NR network onto a substrate without structural deterioration. By contrast, NRs decorated with TDPA in the absence of AuCl3 form side-by-side packing. High magnification images in Figure 3d and e show that the NRs are separated by approximately 2 nm due to the TDPA layer on the surface. While the steric layer keeps the NRs from aggregation or interfacial adsorption, NRs can attract their neighbors by vdW and dipole−dipole coupling (Figure 3f). The attractive energy between neighboring NRs packed side-by-side is estimated to be one-order-of-magnitude higher than that between NRs aligned end-to-end, considering vdW and dipole−dipole forces are major impetuses for the respective interactions (see Table S1 for details).19,20 Therefore, the parallel arrangement is predominant in bulk solution (Figure 1e). Figure 4a−b and Figure S6 show TEM images of CdSe NR networks before and after the networks are exposed to incident electron beam of TEM. Interestingly, gold NPs grow at the linkages of the network under e-beam irradiation within a minute. Au species is likely to cover the tips of CdSe NRs during the selective etching, even though the NRs were washed prior to the TEM analysis.52 X-ray photoelectron spectroscopy (XPS) analysis corroborates the existence of amorphous Au species on the surface of CdSe NRs (Figure 4c and d). While the CdSe NRs show characteristic peaks around Cd 3d and Se 3d, the NRs exhibit peaks of oxidized Cd, oxidized Se, and Au species, after the AuCl3 treatment. In addition, there is no peak of Cl in the XPS spectrum of Au-coated CdSe NRs (Figure S7). The absence of a Cl signal implies that it is Au, not AuCl3, that covers the NR surface. Despite the absence of a reducing agent, Au ions can be reduced into Au0 by oxidizing a fraction of selenium on the NR surface.53,54 No Au crystal peaks were detected in an X-ray diffraction (XRD) pattern of CdSe NRs treated with AuCl3 (Figure S8), which again means Au stays as an amorphous Au shell, possibly covering the etched tips of the NRs. This encapsulation has been observed on the surface of CdSe NPs at lower ligand densities and incomplete Au reduction, which is similar to our experimental conditions.52 To decouple the effects of etching from gold deposition, we used HCl as the tip-etching agent, instead of AuCl3.43 Similar, end-to-end assembly is observed although side-by-side packing coexists (see Figure S9a and S9b in Supporting Information). To etch the tips further and increase the fraction of end-to-end linkages, we used a higher concentration of HCl. However, increased HCl results in shortening of NR length in the end-toend linkages (Figure S9c and d), which is attributed to nonuniform etching of tips by HCl. Nevertheless, we can still conclude that tip-etching leads to spontaneous interfacial adsorption and end-to-end networking. Notably, a higher concentration of AuCl3 does not yield shorter NRs, but significantly influences the density of the network (Figure S10). This clear contrast between AuCl3 and HCl implies that the amorphous Au layer, deposited during surface etching, protects against additional etching and enables the NRs to retain their length. The etchant, AuCl3, is insoluble in toluene and thus forms inverse micelles with DDAB. Therefore, the relative amount of AuCl3 to DDAB strongly affects etching activity: high coverage

Figure 5. Influence of concentration of DDAB, CDDAB, on the density of NRs in the network. (a−d) TEM images of NR-assembled structures with four different CDDAB: (a) 4.27 × 10−5 M, (b) 8.53 × 10−5 M, (c) 17.1 × 10−5 M, and (d) 25.6 × 10−5 M. The concentrations of AuCl3 and NRs, CAuCl3 and CNR, are kept constant at 1.41 × 10−5 M and 3.18 × 10−9 M, respectively. As CDDAB increases, the network loosens, and eventually side-by-side packing forms. Insets of (a) and (d) are high magnification images of corresponding structures. Scale bars represent 50 nm. (e) Influence of CDDAB/CAuCl3 on the average number of NR arms at a linkage (●, left axis) and absorbance of suspension after incubation for 9 h at 640 nm wavelength normalized with the value before incubation (⧫, right axis). E

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by-side packing exists (Figure 5a). Interestingly, the sidewalls are brought into contact without a gap. In bulk suspension, the NRs become unstable due to lack of steric repulsion at this relative concentration and form random segregates without the dip-coating process. Once segregation occurs, NR clusters sediment in 9 h, leaving behind a pale solution, as indicated in Figure 5e (see Figure S12 for detail). By contrast, at CDDAB of 8.53 × 10−5 M and 17.1 × 10−5 M ([DDAB]/[AuCl3] = 6.05 and 12.1, respectively), AuCl3 selectively etches the tips, and end-to-end connections stay dominant. As shown in Figure 5e, relative absorbance is kept nearly constant even after 9 h, which indicates that the NRs suspensions are relatively stable at these concentrations. At a higher CDDAB of 25.6 × 10−5 M ([DDAB]/ [AuCl3] = 18.2), side-by-side packing of NRs becomes predominant as AuCl3 surrounded densely by DDAB becomes less active in etching. The sidewalls are closely packed but separated by approximately 2 nm, similar to the NR assembly with no additives (Figure 5d). The average number of NR arms decrease as the relative concentration of DDAB to AuCl3 increases as indicated with filled circles in Figure 5e (see the Figure S12 for detail). The number is as high as 5 at the dense network in Figure 5a and lower than 2 due to an increased portion of dead ends at the very loose network in Figure 5c. This high controllability of network density and connectivity opens new avenues into the study of electrical conductivity and optical transparency of percolating networks. In summary, we have presented a highly controllable method of inducing 2-D end-to-end self-assembly of NRs by trapping them at the confined air/liquid interface of the dip-coating technique. This percolating network spreads all over the substrate in monolayer thickness, maintaining its uniformity. This extraordinary assembly is driven by interfacial adsorption and directional capillary attraction. The NR tips are selectively etched by AuCl3, which forms inverse micelles with DDAB in toluene. As a result, the tips become nonwettable in toluene. The change in surface property leads to spontaneous anchoring of the NRs at the air/liquid interface, which is subsequently deformed. Consequently, capillary attraction pulls neighboring NRs confined at a 2-D geometry along the end-to-end direction, forming a network structure. Dip-coating allows controlled supply of NR “feed” to the network via convective flow at the air/liquid interface and the 2-D, monolayer-thick, fractal clusters of the NRs are simultaneously transferred onto a solid substrate over a large area. Another beauty of the approach is that the density of the resulting networks can be delicately controlled by adjusting either the upward drawing rate of the substrate or the relative concentration of AuCl3 to DDAB. This interfacial assembly of NRs provides a unique means to create end-to-end networks with controlled density and thickness over a large scale, while avoiding costly and multistep processes such as high-temperature annealing or lithographic patterning. The resultant network can provide high electrical conductivity while preserving the unique properties of the NRs. Numerous charge pathways in the wide 2-D NR network are expected to serve as the most effective electrontransporting layer in film devices. Moreover, insight from our simple approach to assemble anisotropic NPs at fluid−fluid interfaces will open new possibilities for designing macroscopic materials with diverse nanostructures from nanocolloidal building blocks.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental procedure and data, including TEM images, XPS, XRD, UV/vis spectroscopy analysis, and results from a comparison experiment. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (D.C.L.) [email protected]. *E-mail: (S.-H.K.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20133030011330), and by the National Research Foundation (NRF) grant funded by the Korean government (grants NRF-2011-0030256 and NRF2014R1A2A2A01006739).

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dx.doi.org/10.1021/nl504259v | Nano Lett. XXXX, XXX, XXX−XXX