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Programmable assembly of hybrid nanoclusters Songbo Ni, Heiko Wolf, and Lucio Isa Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03944 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 28, 2018

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Programmable assembly of hybrid nanoclusters Songbo Ni†, ‡, Heiko Wolf‡* and Lucio Isa†* †

Laboratory for Interfaces, Soft Matter, and Assembly, Department of Materials, ETH Zurich,

Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland. ‡

IBM Research – Zurich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland.

*Corresponding authors: [email protected], [email protected] Abstract Hybrid nanoparticle clusters (often metallic) are interesting plasmonic materials with tunable resonances and a near-field electromagnetic enhancement at interparticle junctions. Therefore, in recent years, we have witnessed a surge in both the interest in these materials and the efforts to obtain them. However, a versatile fabrication of hybrid nanoclusters, i.e., combining more than one material, still remains an open challenge. Current lithographical or self-assembly methods are limited to the preparation of hybrid clusters with up to two different materials, and typically to the fabrication of hybrid dimers. Here, we provide a novel strategy to deposit and align not only hybrid dimers, but also hybrid nanoclusters possessing more complex shapes and compositions. Our strategy is based on the downscaling of sequential capillarity-assisted particle assembly (sCAPA) over topographical templates. As a proof of concept, we demonstrate dimers, linear trimers, and 2D nanoclusters with programmable compositions from a range of metallic nanoparticles. Our process does not rely on any specific chemistry and can be extended to a large

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variety of particles and shapes. The template also simultaneously aligns the hybrid (often anisotropic) nanoclusters, which could facilitate device integration, e.g., for optical readout after transfer to other substrates by a printing step. We envisage that this new fabrication route will enable the assembly and positioning of complex hybrid nanoclusters of different functional nanoparticles to study coupling effects not only locally, but also at larger scales for new nanoscale optical devices. Keywords: nanoparticles, hybrid clusters, capillary assembly, plasmonics, programmable assembly

Introduction Metallic nanoparticles have long been known to have sharp scattering and absorption spectral peaks as well as a near-field electromagnetic enhancement upon excitation by light due to collective electron oscillations at the resonance wavelength1. This collective, but geometrically highly confined electronic phenomenon is termed localized surface plasmon resonance (LSPR). Moreover, clusters of metallic nanoparticles in close proximity can enable a strong coupling of the excited surface plasmons2. Coupled surface plasmons are of great interest for both practical and fundamental purposes mainly because they feature (i) a strong and tunable resonance shift3, (ii) a high near-field electromagnetic enhancement in the inter-particle gap4, and (iii) a broad range of tunable resonances5. The plasmon resonance in a nanocluster is sensitive to the composition and morphology of the individual nanoparticles, the gap between them, their arrangement, and the dielectric environment6,7. Such plasmonic nanoclusters have been widely


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used in applications of surface-enhanced Raman scattering (SERS)8–11, in nanoscale metrology3, and for single-molecule fluorescence12,13. Currently, plasmonic nanoclusters are mostly fabricated either by lithographical methods (topdown)14 or by particle assembly (bottom-up)6,9,11,15. The former methods offer very precise control of the size of the nanoparticles, the inter-particle gap and the orientation, but suffer from poor crystal quality (polycrystallinity), limited material choices and typically a larger gap distance. The latter strategies afford more flexibility as they are suitable for a broader range of materials with high crystal quality thanks to the immense recent progress in nanoparticle synthesis. Nevertheless, current particle-assembly approaches afford relatively poor control over the morphology of the nanoclusters16. In general, both approaches are mainly used to fabricate nanoparticle clusters of the same material, i.e., compositionally homogeneous nanoclusters5,7,14. In contrast, compositionally heterogeneous nanoclusters are far less explored because of the challenges in their fabrication, but offer new interesting fundamental properties and applications that are not found for homogeneous systems. For example, hybrid nanoclusters are model systems to extend plasmonic hybridization theory17. Also, a number of novel plasmonic-assisted nanodevices can be realized by coupling different functional nanoparticles, e.g., for color routing18, photon upconversion19 and gas sensing20,21. Recently, progress has been made in fabricating hybrid nanoclusters by hole-mask evaporation18,20,22,23, e-beam lithography24,25, DNA-assisted particle assembly17,26,27, directed28 and electrostatically assisted assembly16. However, the top-down methods listed above require sophisticated alignment, and the fabricated structures again have a relatively poor crystalline quality and often a large gap distance18,24. Bottom-up methods using DNA or electrostatic interactions in bulk require fine-tuning the surface chemistry, and most often lead to a broad range of aggregates6. Finally, the state of the


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art is limited in the number of species that can be integrated into nanoclusters. A facile and programmable method for the bottom-up fabrication of compositionally hybrid nanoclusters that allows the integration of a variety of nanoparticle types in a broad range of architectures is currently lacking. In this work, we address this challenge by exploring the downscaling of a process termed “sequential capillarity-assisted particle assembly” (sCAPA). sCAPA is a particle-assembly process on topographically structured templates that was originally developed in our lab to fabricate microscale colloidal clusters29,30. The geometry of the template determines the shape and the orientation of the clusters, and the composition of the clusters can be programmed by sequentially filling the traps with different particles. More than two particles can be sequentially and deterministically filled into the template, leading to colloidal clusters with complex, but programmable compositions. The downscaling of sCAPA to the nanoscale for the assembly of hybrid nanoclusters poses significant challenges. We start by discussing the experimental conditions for successful nanoscale sCAPA, and proceed by demonstrating the fabrication of dimers and linear trimers assembled using sub-100-nm metallic nanoparticles, with exact programmability in each lobe. The yield of the process, the optical characterization of the metallic hybrid nanoclusters, and the possible applicability to other 2D geometries are also discussed, before we conclude the paper. Results and Discussion CAPA vs. sCAPA Capillarity-assisted particle assembly (CAPA) was originally developed to pattern an array of single particles or homogeneous particle clusters on a surface31–34. It works by actively moving


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the meniscus of an evaporating droplet of a particle suspension over a topographically structured template (e.g., a poly(dimethylsiloxane) stamp). Viscous drag from an evaporation-induced convective flow brings the particles from the droplet bulk to the front of the meniscus. Particles are accumulated and densely pack near the contact line when the viscous drag outweighs opposing particle transport due to diffusion and recirculation flows of the moving meniscus33. This zone with densely packed particles is often called the accumulation zone (AZ), and its formation is a prerequisite for successful particle assembly33,35. In the AZ, minimization of the free energy directs the particles into the traps already before the meniscus moves over them29,35. As the meniscus moves over the traps, capillary forces press the particles down and assemble them. The method crucially relies on the fact that capillary forces are significantly stronger than other inter-particle forces, e.g. electrostatic and Van der Waals interactions, making it a robust patterning tool to deposit a broad range of materials without the need to match specific physicalchemical parameters. Essentially, the particles are only required to be colloidally stable at the high volume fractions found in the AZ, which is typically ensured by the addition of surfactants (which are also used to control the surface tension of the droplet. see Methods for details). To ensure a high assembly yield, the trap depth is usually close to the diameter of the particles. In contrast, sequential capillarity-assisted particle assembly, (sCAPA) was developed to enable the patterning of an array of compositionally heterogeneous particle clusters on surfaces29,30. The working principle is based on the traditional CAPA with very simple, but crucial modifications in the geometry of the traps and the surface tension of the suspensions. Figure 1a schematically illustrates the mechanism of sCAPA, where particles A and B in a rectangular trap do not interact simultaneously with the moving meniscus, i.e., the front particle, A, interacts with the meniscus earlier than the rear particle B. Only particle A thus experiences a downward capillary


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force Fc and can transmit an upward force F’c to particle B. The upward force F’c can push particle B up and make it roll out of the trap, depending on the local capillary force Fc, the opposing force (e.g., the osmotic pressure in the dense AZ) and the geometry of the trap29. On the microscale, expulsion of particle B is highly favored when the trap depth is reduced to a value close to the particle radius, without the overall trapping efficiency being compromised. Under optimal conditions, only the front particles A can be selectively trapped in each cavity, with an overall yield of almost 100%. The remaining space can be back-filled by other particles of the same or another kind, without losing the already assembled particles, which firmly adhere to the template after drying. The same strategy also applies to more complex 2D traps, and ultimately results in the formation of programmable colloidal clusters with 2D and quasi-3D geometries30.


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Figure 1. Downscaling sCAPA from the microscale to the nanoscale. (a) Schematic of the working principle of sCAPA, with the size and location of the AZ indicated. (b) SEM image showing single 80-nm gold nanoparticles assembled in rectangular traps with high yield for a relative trap depth d/2r = 1 (d: absolute trap depth, r: particle radius) and surface tension γ = 22 mN/m (c) A comparison of CAPA results at the microscale and at the nanoscale for different surface tensions (γ) of the suspension and various relative trap depths (d/2r). Note that ‘yes’ for microscale and nanoscale CAPA indicates the possibility to assemble a single particle per trap with an overall yield above 90% and 60%, respectively. ‘no*’ means that there is no appropriate window to assemble a single particle per trap and that instead more than one particle per trap is assembled. The other cases are denoted by “no”, implying assembly failure. (d) Single 80-nm gold and (e) silver nanoparticles are assembled in long linear traps, as shown in the dark-field and corresponding SEM images (assembled particles are circled in red). The red spot highlighted in (d) originates from a triangular gold nanoparticle that is a byproduct in the suspension. The scale bars are 1 µm.

sCAPA from micro to nano Downscaling sCAPA by an order of magnitude in particle size from 1-µm to sub-100-nm particles is challenging. The main challenges are associated to the enhanced particle diffusivity, which opposes both the long-range driving force (the viscous drag) and the short-range driving force (the local capillary force) of the assembly. In fact, at the nanoscale, the importance of thermal fluctuations becomes more pronounced than at the microscale35, which makes it more difficult to create a suitable accumulation zone and to keep a controlled number of particles in


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the trap during the break-off of the meniscus. There are several parameters that can be optimized to limit the detrimental effect of enhanced diffusivity: temperature, assembly speed, trap depth and surface tension of the suspension. By increasing the deposition temperature and reducing the assembly speed, one can establish a larger and denser AZ, increasing the chances of successful deposition (the details of this operation are discussed below). Increasing trap depth and reducing the surface tension have both the effect of confining the particles in the traps more strongly; i.e. either geometrically with the former or by imposing a larger downward component of the capillary force with the latter (at lower surface tension a lower contact angle with the template is also obtained). Nonetheless, capillary forces still dominate over other colloidal interactions, ensuring that the versatility and robustness of the assembly with respect to the choice of materials remains. For instance, for 100-nm particles, capillary forces are of the order of 100 pN, while electrostatic or Van der Waals interactions are around 1 pN29. This implies that successful nanoscale assembly, as shown in Figure 1b for 80-nm Au nanoparticles (see Figure S1a for the trap dimensions), can be achieved with little influence from, e.g., charge effects. For details, see the Supporting Information. The optimization of the working parameter space to assemble single nanoparticles (80 nm in diameter) in rectangular traps (trap width ~ 100 nm, trap length ~ 200 nm) is summarized in Figure 1c. The table compares the experimental results obtained by microscale CAPA (1-µm particle, trap width ~1.2 µm, trap length ~2 µm)29 versus nanoscale CAPA at different relative trap depths (d/2r) and surface tensions, γ, of the suspension. Single nanoparticles only assemble for a large trap depth (d/2r = 1) and a low surface tension of the suspension (γ = 22 mN/m), whereas single microparticles assemble for a broader range of conditions. Moreover, even for the optimal conditions of d/2r = 1 and γ = 22 mN/m, the number of nanoparticles assembled in the


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rectangular traps depends on the details of the AZ, as will be discussed later. In analogy to microscale sCAPA, the possibility of trapping a single nanoparticle in a large trap serves as the first and essential step of nanoscale sCAPA. Figures 1d and 1e show single 80-nm Au and Ag nanoparticles assembled in one step in long linear traps (design dimensions are given in Figure S1c). The plasmonic properties of these metallic nanoparticles enable a simple assessment of the deposition yield by examining the scattered light in a dark-field microscope. 80-nm Au and Ag nanoparticles scatter green and blue light, respectively, and the observed dark-field images perfectly match the filling of the traps shown in the corresponding SEM images. The Au nanoparticles used here have a broader distribution in both size and shape than the Ag ones, and thus display a broader range of scattered colors because of the different resonance wavelengths.

Figure 2. Operation of nanoscale sCAPA. (a) Three regimes in sCAPA are qualitatively defined by the size of the AZ, which grows over time from left to right (dashed arrows indicate the time evolution). The meniscus moves from the right to the left. (b – d) Dark-field micrographs of the assembly results and the menisci (upper insets) during assembly for (b) regime 1, (c) regime 2 and (d) regime 3, corresponding to three different locations on the same substrate reached by the


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moving meniscus at different times. The different colors of the suspension in the three regimes reflect the different volume fractions of particles close to the meniscus. The SEM images (lower insets) reveal the most frequent outcome from each regime. The scale bars are 1 µm (b – d), 5 µm (optical insets in b – d), and 500 nm (SEM insets in b – d).

As mentioned, the number of nanoparticles assembled in the rectangular traps also depends on the details of the AZ. Figure 2a schematically illustrates three regimes of the AZ during its development. As the droplet moves over the template, the AZ grows as a function of time. Correspondingly, a general increase of the number of trapped particles with the size of the AZ is observed. Figures 2b to 2d show the assembly results for different ranges of the AZ (with the same trap design as used in Figure 1d). The results are obtained by moving the meniscus with a speed of 0.2 µm/s at a temperature of 40 K above the dew point (50 ̊C) over a distance of 1.2 mm. Initially, there is no evidence of an AZ, and hardly any particles are assembled (Figure 2b). At a later stage, the AZ features a thin strip (~ 1 µm) composed of small crystallites at the front, visible as colorful patches in the optical microscopy image (upper inset of Figure 2c). In this regime, single 80-nm Ag nanoparticles are successfully assembled (Figure 2c, Figure S2 and SEM inset of Figure 2c). At an even later stage, the AZ shows a large and highly polycrystalline strip (≥ 3 µm) at the front (optical microscopy image, upper inset of Figure 2d), and multiple particles are assembled (SEM inset of Figure 2d). Modifying the initial concentration of the nanoparticles can have the effect of anticipating or delaying the transition between the various regimes, without changing the qualitative picture. Clustering of Ag nanoparticles red-shifts the resonance wavelength, and multiple occupancies per trap are indicated by the red spots shown in Figure 2d and Figure S3. Note that even though a trap filled with multiple nanoparticles prevents


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subsequent sCAPA steps, these working conditions are well suitable for conventional CAPA of homogeneous particle clusters.15,31

Figure 3. Programmable assembly of hybrid dimers. (a, b) Dark-field micrographs (a) after the first assembly of 80-nm Ag particles and (b) after the second assembly using 70-nm Pt particles. The images have the same white balance. (c) SEM image of the dimers in the traps corresponding to the image in (b). (d) SEM image of an array of Ag-Pt dimers printed on a silicon substrate. (e) SEM image of an individual Ag-Pt dimer. (f) EDX mapping of a Ag-Pt dimer with its corresponding SEM image in the inset. The scale bars are 1 µm (a, b), 500 nm (c, d) and 100 nm (e, f). Based on these observations, we carried out a systematic study to identify the optimal processing window of nanoscale sCAPA. The details of the systematic study and the discussions regarding future improvements can be found in Supporting Information and in Figures S4 to S6. Currently, the maximum yield of trapping a single nanoparticle in the linear traps reaches ~70% at the optimal conditions corresponding to Regime 2 in Figure 2 with d/2r = 1 and γ = 22 mN/m, (see Figure S5c for full experimental details).


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We then proceeded to the fabrication of the hybrid nanoclusters. As a proof of concept, both hybrid dimers of Au-Ag and Ag-Pt were assembled. To achieve a high assembly yield, Ag (80nm) and Pt (70-nm) nanoparticles were used, as they offer better uniformity in both shape and size, as shown in Figure 3. Additional details on Au-Ag dimers are in the SI. Figure 3a and 3b show the dark-field images after the first (80-nm Ag) and second deposition step (70-nm Pt), respectively. Crucially, as described in previous work on sCAPA at the microscale30, the particles deposited in the first step, after drying, remain in the trap during the second deposition step, thus enabling a sequential filling of the template. The color of the scattered light slightly red-shifts after the placement of the Pt next to the Ag nanoparticle. Figure 3c shows the SEM image of the final structures in the traps corresponding to Figure 3b (the trap dimensions are given in Figure S1b). An offset angle is often observed between the long axis of the dimer and the trap, which can be reduced by finely matching the particle size to the trap dimension. After the second deposition step, capillary forces push the nanoparticles into close contact during drying (see Figure 3c and S7a). The inter-particle gap, which strongly affects the cluster’s plasmonic properties, is then determined by the presence of a layer of surfactant molecules, which decorate the nanoparticles in the AZ. In order to determine the thickness of the surfactant layer, we have compared the measured spectra from the Au-Ag dimers (due to their better optical response in visible light) with numerical simulations carried out for a varying inter-particle spacing. The results give a consistent separation between 2 to 4 nm, compatible with to the thickness of the surfactant layer (see Figure S7b, S8b and S9c for more details). The assembled nanoclusters can be printed onto other substrates, such as a silicon wafer, as shown in Figure 3d and the close-up in Figure 3e. The material contrast of the lobes of the hybrid dimers is clearly visible in SEM images, and the composition of the dimer is further confirmed by the EDX


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mapping shown in Figure 3f. The overall yield of the Ag-Pt dimers over a large area of 200 µm × 200 µm reaches ~50%, which can be expected based on the yield of a single sCAPA step as discussed in Figure S5. As we will discuss later, the values of the yield are mostly defined by the polydispersity of the particles and the traps. More details and a larger view are given in Figure S10. The protocol presented here does not depend on any specific chemistry of the particles, thus it offers a universal way to produce arrays of hybrid dimers composed of different functional particles.

Figure 4. Programmable assembly of linear trimers. (a) Schematic of the sequential assembly process. (b – c) A linear trimer with the programmed sequence Au-Ag-Au, as confirmed by SEM


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(b) and EDX mapping (c). (d) SEM image of an array of the assembled trimers printed on a silicon substrate. (e) Evolution of the scattered light in dark-field microscopy from the same location during the sequential assembly process. (f) SEM image of the final structure of the nanoclusters on the PDMS template after the third assembly run. The area marked by dashed boxes in (e) shows the change of the scattered light during the growth of the Au-Ag-Au trimers, whose final structure is shown in the dashed box of the SEM image (f). (g – i) Scattering spectra of (g) single Au and Ag nanoparticles, (h) a Au-Ag dimer (h) and (i) a Au-Ag-Au trimer. The light is polarized in the longitudinal direction along the long axis of the linear nanoclusters. The SEM and dark-field images show the corresponding structures and the scattered light, respectively. The scale bars are 100 nm (b, c), 500 nm (d – f), and 100 nm (g – i).

Even more interesting for applications is that sCAPA at the nanoscale also allows more than two sequential deposition steps, enabling the encoding of more complex compositions. As an example, here we assembled and spectroscopically characterized a linear trimer with a predefined composition: Au-Ag-Au. Figure 4a schematically shows the sequential assembly process. Figures 4b and 4c show the SEM image of a trimer and its corresponding EDX mapping, respectively. Figure 4d shows an array of trimers after they have been printed onto a silicon substrate from the PDMS template (some of the trimers may have been separated by shear forces during printing). The yield of trimers is limited by the yield of trapping a single particle in each step. Therefore, the maximum yield of trimers is expected to reach ~30% (0.73) using the current optimum experimental conditions, so that an array of mixed nanostructures is obtained. Figure 4e shows a typical three-step sequential process for the sCAPA of Au-Ag-Au trimers. The dark-field images are all taken at the same location, and the final morphology is


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shown in the SEM image of Figure 4f. The change in the scattered light from a single Au nanoparticle via the Au-Ag dimer to the Au-Ag-Au trimer can be tracked with both nonpolarized (see highlighted spots in Figure 4e) and polarized light. The color of each cluster can vary owing to both the polydispersity of the nanoparticles (Figure S11) and the orientation of the clusters. In a small number of specific cases, a larger separation between the assembled nanoparticles can be observed, giving dark-field images with elongated scattering patterns and some color mixing. These cases can be detected and excluded a posteriori by comparing the dark-field and the corresponding SEM images (see Figure S12). The optical properties of the single nanoparticles, the dimer and the trimer clusters were characterized by dark-field microscopy. Figures 4g to 4i show the spectroscopic change from Au and Ag single nanoparticles (Figure 4g) to a Au-Ag dimer (Figure 4h) and to a Au-Ag-Au trimer (Figure 4i), with the light polarized in the longitudinal direction. The spectra with transverse polarization can be found in Figure S8. The SEM images of the nanoclusters and the corresponding dark-field images are presented in the insets of Figures 4g to 4i. The spectroscopic development with the longitudinal polarization is clearly visible (Figures 4g - 4i): the spectra red-shift and exhibit an increased magnitude of the plasmonic peak as the nanocluster grows. The measurements also correlate with numerical simulations (Figure S8). As previously discussed (Figure S7), the bright plasmonic peak in Au-Ag dimers at longitudinal polarization is always found between 650 nm and 690 nm (Figure S9c), indicating an inter-particle gap of about 2 to 4 nm, in good agreement with the simulations, where peaks at 650 nm and 690 nm were found for gaps of 4 nm and 2 nm, respectively.


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Figure 5. Programmable sequential assembly of complex hybrid 2D nanoclusters. (a) A pair of Ag nanoparticles is assembled first, followed by the assembly of a pair of Pt nanoparticles. (b) SEM image of a square-like Ag-Pt cluster printed onto a Si substrate. (c) First, a chain of Pt nanoparticles is assembled, followed by the assembly of two Au nanoparticles in the trap arms. (d) SEM image of a U-shaped Pt-Au cluster printed onto a Si substrate. The arrow above the figure indicates the assembly direction for both experiments. Scale bars are 100 nm.

Finally, we applied our technique to demonstrate a proof-of-principle realization of hybrid nanoparticle clusters with complex 2D geometries. Figure 5 shows hybrid nanoclusters in a square trap as well as in a U-shaped trap (details of the traps are given in Figures S1d and S1e, respectively). In the square trap, a pair of 80-nm Ag nanoparticles is assembled first, followed by a pair of 70-nm Pt nanoparticles (Figure 5a). In the U-shaped trap, a chain of three 70-nm Pt nanoparticles is assembled first, and then two 70-nm Au nanoparticles are deposited in the arms of the trap (Figure 5c). The material difference can be clearly recognized in the SEM images


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(Figures 5b and 5d). More complex designs require more control in the local movement of the meniscus with respect to the trap as well as a lower tolerance in the trap dimensions and particle polydispersity. Moreover, the current yield of such structures is significantly lower than that of the linear clusters described in Figures 3 and 4, but these demonstrations mark the starting point of future developments.

Conclusion In this work, we have demonstrated the downscaling of sequential capillarity-assisted particle assembly (sCAPA) from sub-1-µm to sub-100-nm particles by optimizing the trap geometry and the working conditions. Different hybrid nanoclusters can be assembled in different geometries and compositions, including dimers, linear trimers and 2D clusters. The shape of the nanoclusters is determined by the geometry of the template, whereas their composition is determined by the programming of the assembly sequence of the different nanoparticles. A key advantage is that the cluster-assembly process relies solely on capillary forces, so that multiple materials can be incorporated into the nanostructures without the need of specific chemical functionality. Here, we have limited the demonstrations to metallic Au, Ag and Pt nanoparticles and to binary systems, but our results hold the promise for nanoscale sCAPA to be extended to a much broader range of materials as well as to ternary or even more complex systems, e.g., A-B-C or A-B-C-D linear nanoclusters. In terms of potential applications, the universality of the process enables the realization of different plasmonic nanodevices, such as Pt-Au hydrogen sensors20. The simultaneous assembly and alignment of the anisotropic nanoclusters on the template further facilitate the integration


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and optical readout of the nanodevices, both at the individual nanocluster level and for largescale arrays. The current yield may still appear low for large-scale applications, but the strength of the method lies in the possibility realizing versatile materials’ combinations. For instance, nanoscale sCAPA has been already successfully demonstrated to produce BaTiO3–Au nanodimers of 100 nm/80 nm. Here, the process was designed to couple the two nanoparticles, so that the gold one could act as a plasmonic nanoantenna to enhance the second-harmonicgeneration properties of the barium titanate one36. The limitations that we encountered in highyield large-area depositions are anyway not intrinsic limitations of the technique, but are instead to be ascribed to shortcomings in the properties of the materials we used. Polydispersity, both in particle size and trap geometry is the main hindering factor. Concerning the former, we already showed a 10% yield increase in the first deposition step by using Ag nanoparticles instead of Au ones because of their narrower size distribution. Since controlling the properties of the AZ holds the key to a successful sCAPA, we envisage that future processes, with an active feedback between deposition yield and AZ control will further improve the method. The idea is also not limited to spherical nanoparticles of the sizes we employed here. The fact that capillary forces overcome other interactions makes in fact capillary assembly a very robust tool down to sub-10 nm objects37. Whether this limit can also be reached in sCAPA remains to be seen in relation to the challenges just described above. Clusters of smaller nanoparticles have also already been used to produce complex color pixels38, and mixing different materials in prescribed sequences may open new routes for additional combinations. Additionally, anisotropic nanoparticles have been patterned using capillary assembly39,40, also exploiting sequential depositions in shapespecific traps19. The extension of nanoscale sCAPA to create unique 2D structures with controlled composition is likely to trigger further studies that can help understand the complex


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coupling effects among different materials in 2D geometries and pave the way for many more future applications. Among those, the programmable design of asymmetric active nanoparticles for sensing, delivery and remediation applications holds particular interest41. Summarizing, we envision nano-sCAPA to become a powerful tool to couple and pattern nanoclusters made from different functional nanoparticles, which will be useful for the exploration of novel physical phenomena as well as for the fabrication of new nanoscale devices. Methods Template fabrication: The poly(dimethylsiloxane) (PDMS) template was replicated from a patterned master. The master was a silicon wafer patterned by electron-beam lithography using a layer of resist with hydrogen silsesquioxane (HSQ, Dow Corning). The dose and step size used in the e-beam exposure were 7700 µC/cm2 and 10 nm, respectively (Figure S1 gives the dimensions and structures of the different designs). The patterned and cured HSQ on the silicon wafer was used to replicate the PDMS template. The composition of the pre-polymer mixture for the PDMS molding, the surface treatment of the master, and the subsequent replication process were adopted from ref. 29. Preparation of colloidal suspensions: Gold (Au) colloidal suspensions (80 nm ± 5 nm, 100 nm ± 5 nm, polydispersity < 15%) were purchased from BBI Solution. Silver (Ag, 80 nm ± 4 nm) and platinum (Pt, 70 nm ± 4 nm) colloidal suspensions (polydispersity < 15%) were purchased from Nanocomposix. The original solvent was replaced by an aqueous mixture of Triton X-45 (Fluka Chemie AG, 0.1 wt%) and sodium dodecyl sulfate (SDS, Fluka, Chemie AG, 1 mM). Optimization of the surfactant mixtures was carried out in previous studies33. The concentrations


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of Au, Ag and Pt colloidal suspensions used for the assembly were 0.05 mg/ml, 0.02 mg/ml and 0.05 mg/ml, respectively. Sequential capillarity-assisted particle assembly and pattern transfer: The assembly setup has been described in detail in ref.

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. A drop of 40-µl colloidal suspension was used for each

assembly step. The assembly temperature was adjusted to 30 – 50 K above the dew point (4060 ̊C). The moving speed of the stage was set at 0.2 µm/s. After assembly, the nanocluster pattern could be transferred by pressing the PDMS template onto other substrates (e.g., glass or silicon) with an adhesive layer33. Imaging and analysis: Assembled or printed nanoclusters were imaged by dark-field microscopy (Zeiss Axioscope) at various magnifications and by SEM (Leo 1550 Gemini, Carl Zeiss AG, or SU8000, Hitachi). EDX characterization was performed using X-max (Oxford Instruments) in a scanning electron microscope (SU8000, Hitachi). The dark-field images of the entire assembly area (200 µm × 200 µm, see the example in Figure S2) were used to calculate the assembly yield for 80-nm Ag nanoparticles. Single and multiple Ag nanoparticles in a trap scatter blueish and reddish light, respectively. The number of traps containing a single or multiple Ag nanoparticles was counted manually according to the color scattered. We measured a total of 16 samples (4 × 4, 20 µm × 20 µm for each sample) uniformly distributed over one entire assembly area. The yield for one entire assembly area was averaged over the measurements of 16 samples. Optical measurement: Dark-field scattering microscopy in reflection mode was performed to characterize the nanoclusters. A 100x objective (LD EC Epiplan-NEOFLUAR, NA = 0.75) was used for both illumination and collection of scattered light. A single nanoparticle or nanocluster in the PDMS trap with a glass backplane was illuminated with white light (HAL 100 halogen


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lamp, Carl Zeiss AG, Jena, Germany). The incident light could be polarized by inserting a linear polarizer into the light path. The polarizer was rotated manually to align with the nanocluster. A part of the scattered light went to a camera, and the remainder was reflected via a mirror and directed to a spectrometer (MSP400, J&M GmbH, Aalen, Germany) through an optical fiber. Spectra were recorded from 400 to 750 nm. Reference spectra were taken on flat parts of the sample. The scattering spectra shown were normalized by the reference spectra. The integration time was 6.5 s for each measurement, and for each spectrum 50 measurements were averaged. Acknowledgments: We thank A. Olziersky for help with the e-beam lithography; K. Xiong, A. B. Dahlin and F. Timpu for advice on simulations; C. Rawlings, C. Schwemmer, L. Czornomaz and L. Herrmann for useful discussions and help; C. Bolliger for assistance with the manuscript, and R. Allenspach and W. Riess for continuous support. L.I. and S.N. acknowledge financial support from the Swiss National Science Foundation (grant PP00P2_144646/1). AUTHOR INFORMATION Corresponding Author [email protected], [email protected] Author Contributions SN performed and characterized the experiments. SN, LI and HW discussed the results and wrote the manuscript. All authors have given approval to the final version of the manuscript. Supporting Information The Supporting Information contain additional experimental details of the fabrication process and of the optical characterization of the hybrid nanoclusters.


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