The Role of Substrate Wettability in Nanoparticle Transfer from

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Letter pubs.acs.org/Langmuir

The Role of Substrate Wettability in Nanoparticle Transfer from Wrinkled Elastomers: Fundamentals and Application toward Hierarchical Patterning Christoph Hanske,† Mareen B. Müller,† Vera Bieber,† Moritz Tebbe,† Sarah Jessl,† Alexander Wittemann,‡ and Andreas Fery*,† †

Physical Chemistry II, University of Bayreuth, Universitaetsstrasse 30, 95440 Bayreuth, Germany Colloid Chemistry, University of Konstanz, Universitaetsstrasse 10, 78464 Konstanz, Germany

‡

S Supporting Information *

ABSTRACT: We report on the role of surface wettability during the printing transfer of nanoparticles from wrinkled surfaces onto flat substrates. As we demonstrate, this parameter dominates the transfer process. This effect can further be utilized to transfer colloidal particles in a structured fashion, if the substrates are patterned in wettability. The resulting colloidal arrangements are highly regular over macroscopic surface areas and display distinct pattern features in both the micrometer and nanoscale regime. We study the obtained structures and discuss the potential of this approach for creating hierarchical particle assemblies of high complexity. Our findings not only contribute to a better understanding of technologically relevant colloidal assembly processes, but also open new avenues for the realization of novel materials consisting of nanoparticles. In this regard, the presented structuring method is especially interesting for the design of optically functional surface coatings.

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INTRODUCTION Surface-immobilized nanoparticles are considered excellent building blocks for novel optoelectronic devices with potential applications in high-efficiency sensors,1 low-loss optical waveguides,2 or cost-efficient solar cells.3 In all these applications, the possibility to deposit nanoparticles on surfaces in a wellordered fashion is crucial: Random particle aggregation usually coincides with a degradation or even complete loss of nanoparticle specific properties. In contrast, ordered colloidal assembly offers control over properties like plasmon resonance(s) that result from local coupling effects4 or even gives rise to completely novel collective phenomena such as optical bandgaps5 or metamaterial effects.6 With the aid of simulations, surface-immobilized colloidal arrangements could thus be tailored to generate specific optical, electronic, or magnetic responses by manipulating the crystal symmetry or by introducing hierarchical structures.7 However, many pathways common for the assembly of micrometer-sized colloids, such as sedimentation or centrifugation, offer very limited structural control or cannot be adapted to nanoscopic systems at all.8 Therefore, engineering complex colloidal assemblies with nanoscale precision over macroscopic dimensions remains a challenge for material scientists. For such tasks template-assisted self-assembly approaches appear most promising. Especially the combination of topographically structured substrates and convective assembly has proven highly efficient for aligning nanoparticles into close-packed 1D and 2D assemblies,9−12 which can be further utilized to create 3D structures by colloidal epitaxy.13 © XXXX American Chemical Society

The great versatility of this method stems from the fact that almost arbitrary surface patterns can be used to arrange particles comparable in size. However, for structures with length scales below the diffraction limit of visible light, timeconsuming and cost-intensive methods like extreme UV lithography, electron beam lithography, or focused ion beam etching become mandatory, limiting the practical range of applications severely.14 As an alternative to lithographically structured templates, we introduced the use of wrinkled substrates for template-assisted nanoparticle alignment.15 Highly regular surface wrinkles form when elastomeric materials with a hard top coating are exposed to compressive strain.16,17 Depending on the applied strain field a variety of pattern symmetries, such as parallel grooves, herringbone structures, spokelike patterns, or even more complex hierarchical wrinkles can be formed.18−20 Plasma oxidation of stretched polydimethylsiloxane (PDMS) produces permanent wrinkles with periodicities as small as 200 nm fast and reliably. The assembly of particles in these structures as well as the transfer onto various substrates like Si-wafers or glass slides have been demonstrated.21−23 While particle transfer from templates onto substrates of choice is highly important for potential applications and crucial for compatibility of the method with industrial techniques like roll-to-roll Received: October 11, 2012 Revised: November 20, 2012

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Patterned Substrates. Microscope slides (Menzel) were cut into pieces of 15 mm × 26 mm and cleaned by the RCA method.25 Microcontact prints were fabricated with flat PDMS stripes or structured PDMS stamps cast from lithographically structured silicon wafers (GeSiM) following the protocol in ref.26 The stamps were inked with a 10 mM OTS solution in anhydrous toluene, dried by spin coating for 60 s at 4000 rpm and then brought into contact with the glass substrates for 60 s. Particle Assembly/Transfer. Directly before spin coating, the wrinkled substrates were cut into 10 mm × 20 mm stripes, cleaned by 1 min of ultrasonication (Elmasonic S30H) in purified water and rehydrophilized by 2 min of plasma treatment (0.2 mbar O2, 100 W). The particles were assembled into the wrinkles by spin coating (P6700, Specialty Coating Systems) 50 μL of a 0.75 wt % particle solution at 3000 rpm for 60 s. For the transfer, 10 μL of water was applied to the freshly prepared glass substrates and the particle filled wrinkles were placed onto the droplet. After drying overnight, the wrinkles were lifted from the substrate. The actual drying time ranged from 2 to 4 h depending on the ambient conditions. Shorter processing times could be achieved by utilizing smaller amounts of water or condensation of air humidity instead of applying a droplet. Characterization. Imaging was performed using either a Dimension IIIa or a Dimension V (Bruker) atomic force microscope (AFM) operated in tapping mode using stiff cantilevers (42 N/m, 300 kHz, OMCL-AC160TS-W2, Olympus). Scanning electron microscope (SEM) micrographs were recorded with a LEO Gemini microscope (Zeiss) operated at 2 kV and a working distance of 4 mm. The specimens were sputter coated (Cressington 208HR) with a 1.3 nm layer of Pt.

transfer, it is rather ill-understood and has not been systematically investigated so far. In the present Letter, we focus on the role of surface wettability for the transfer process. It turns out that this parameter plays a dominating role. Building up on this result, we show that particle transfer can be performed in a patterned fashion, if the substrates are in a first step patterned in wettability. We study the resulting structures and discuss the potential of this approach for creating hierarchical particle assemblies of higher complexity than so far possible by wrinkleassisted assembly.

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EXPERIMENTAL SECTION

Materials. Polydimethylsiloxane (PDMS) was prepared by mixing the prepolymer Sylgard 184 (Dow Corning) with the supplied curing agent at a ratio of 10:1. For surface cleaning analytical grade chemicals (isopropanol, NH3, H2O2, from VWR) were used as received. Octadecyltrichlorosilane (OTS, 97%, ABCR) for microcontact printing was diluted in dry toluene (anhydrous, 99.8%, Aldrich). Water was purified via a Milli-Q system (Millipore). Particles. A detailed description of the synthesis and characterization of nanoparticles coated with poly(acrylic acid) (PAA) brushes is given in ref 24. Briefly, narrowly dispersed polystyrene (PS) particles of 100 nm in diameter were prepared by emulsion polymerization using styrene as the monomer, sodium dodecyl sulfate (SDS) as the emulsifier, and potassium persulfate (KPS) as the initiator. The reaction was carried out in a batch process at 80 °C. In the final stage of particle growth, a copolymerizable photoinitiator was added under starved conditions at 70 °C. At this stage of the polymerization, the particles were still swollen with residual styrene. Copolymerization of styrene and photoinitiator at the surface gave rise to covalently anchored photoinitiator groups at the particle surface. Measurements of the size distribution by differential centrifugal sedimentation (DCS) revealed a polydispersity index (PDI) given as the weight-average diameter divided by the number-average diameter of 1.01. The particles were purified by ultrafiltration prior to grafting PAA brushes from their surface. Photopolymerization of the water-soluble monomer acrylic acid was carried out in a high performance UV reactor setup (Laboclean LC Forschungsreaktor, a.c.k. aqua concept). The hydrodynamic thickness of the PAA brush is 112 nm in water as measured by dynamic light scattering (DLS). This corresponds to a hydrodynamic diameter of 324 nm for the core−shell particles. A second type of brush coated particles (see Figure S4, Supporting Information) was prepared along similar lines, using PS core particles with a diameter of 192 nm according to DLS and a PDI of 1.00 as determined by DCS. Photoemulsion polymerization of acrylic acid was conducted with a submersible UV lamp (TQ 150, Heraeus Noblelight) yielding a PAA shell of 165 nm hydrodynamic thickness and a total hydrodynamic particle diameter of 522 nm. The synthesis of carboxylated PS latex particles (see Figure S3, Supporting Information) was accomplished by emulsion polymerization of styrene with acrylic acid (5.2 mol % relative to styrene) as the comonomer, SDS as the emulsifier, and KPS as the initiator. The particles with an average diameter of 110 nm as determined by DLS had a narrow size distribution (PDI = 1.02 as determined by differential centrifugal sedimentation). Carrying surface bound charges the carboxylated particles exhibit a zeta potential of −55 ± 5 mV, which is sufficient to get stable suspensions. All particle types were cleaned by exhaustive ultrafiltration against deionized water. Wrinkled Substrates. A flat PDMS plate of 2 mm thickness was prepared by casting the PDMS prepolymer/cross-linker mixture onto a leveled glass dish, gelation at r.t. overnight and subsequent curing at 80 °C for 5 h. Stripes of 10 mm × 40 mm were cut out, elongated by 15% in a custom built stretching stage and oxidized in a plasma cleaner (Flecto 10, Plasma Technology) with an O2 pressure of 0.2 mbar. After a 5 min treatment at 100 W the PDMS stripes were cooled to r.t. and relaxed.

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RESULTS AND DISCUSSION For all experiments discussed in this paper, we used polystyrene (PS) particles with a diameter of 100 nm, from which a dense shell of poly(acrylic acid) was grafted. Due to their well-defined architecture, such model systems have been utilized earlier in assembly experiments.27 Details on the particle preparation and their properties can be found in ref 28. Still, we want to point out that the concepts presented here are of a general nature and can be transferred to other hydrophilic colloidal systems. As a proof, additional experiments with carboxylated PS particles were conducted and are included in the Supporting Information (Figure S3) of this paper. The procedure for aligning particles on topographically structured templates and transferring them onto flat substrates is illustrated in Scheme 1. First a suspension droplet containing the nanoparticles is placed on top of a freshly hydrophilized wrinkled PDMS stamp. During spin coating, excess suspension quickly evaporates and the remaining particles are driven into Scheme 1. Colloidal Deposition Procedure Consisting of Particle Alignment in Wrinkles and Subsequent Wet Transfer to a Flat Substrate

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the grooves of the stamp by attractive capillary forces. Hereby, a preceding hydrophilization step by short plasma treatment is crucial to obtain well-ordered linear particle assemblies over macroscopic length scales. For a subsequent release of the aligned particles onto a flat surface, a small droplet of water is deposited onto the substrate, and the prefilled wrinkled PDMS stamp is placed on top. During the following drying step, the particles are released from the confining template and transferred onto the substrate. Complete drying is necessary to facilitate particle transfer and it is reasonable to assume that capillary forces govern the process. Regarding the underlying mechanism the presented wet transfer method clearly differs from classical adhesion based transfer techniques.29 While the established lift-off and contact printing methods require conformal contact between the particles and the target substrate, our approach is applicable even when the particles are located deep inside the grooves of the topographically structured PDMS stamp (see Supporting Information Figure S3). Indeed a big challenge for wrinkle-assisted particle assembly arises, because the lateral and spatial dimensions of wrinkles in general cannot be adjusted independently. Thus, for certain wrinkle geometries, direct contact between the particles and the substrate could only be established through extensive deformation of the stamp. A wet particle transfer process based on capillary forces is therefore advantageous as it grants access to a wider range of patterns than conventional contact printing and lift-off methods. To investigate the influence of substrate wettability on the particle transfer systematically, we first cleaned substrates via the RCA method, which is known to produce silanol groups on glass surfaces rendering them strongly hydrophilic. A series of substrates was then permanently hydrophobized by applying a monolayer of octadecyltrichlorosilane (OTS) via contact printing with a flat PDMS stamp, while a second series was left unmodified and used within few hours after cleaning. Figure 1 displays surface morphologies typically obtained when hydrophilic (Figure 1A) or hydrophobic (Figure 1C) substrates were employed in the subsequent particle transfer step. Strikingly, the particles were almost exclusively transferred onto the hydrophilic substrate, resulting in the formation of regular linear assemblies. Whereas the majority of particles was transferred upon contact with a hydrophilic substrate, most of the particles remained inside the wrinkles when the offered surface was strongly hydrophobic (see Figure 1B/D). By counting the number of deposited particles on hydrophilic and hydrophobic substrates, we found the transfer selectivity to be approximately 1000:1. Since closed layers of OTS sufficiently suppress the particle attachment, chemical structuring should allow one to control the particle transfer locally as depicted in Figure 2A. Consequently, we patterned glass surfaces by microcontact printing (μCP) of OTS with a topographically structured PDMS stamp. As shown in the AFM micrograph in Figure 2B, the stamping process resulted in a smooth OTS layer surrounding 10 μm wide quadratic regions with rounded edges. The typical height difference of 2−3 nm between the contact area and the bare substrate regions agrees well with thickness measurements of OTS monolayers. Despite the substrates being macroscopically hydrophobic, the noncontact regions remained locally wettable (see Supporting Information Figure S1). Figure 2C shows the surface of an identically patterned glass slide after a wrinkle-assisted particle transfer. We found that the particles were transferred exclusively onto

Figure 1. SEM images displaying typical substrates (left column) and wrinkles (right column) obtained by particle transfer. While colloidal deposition onto hydrophilic surfaces (A) resulted in a complete particle transfer leaving empty wrinkles (B), most of the particles remained inside the wrinkles (D) when the contacting substrate (C) had been hydrophobized with OTS. The colloidal assemblies consisted of highly regular particle double lines (insets).

the square regions thereby recreating the printed micropattern with excellent fidelity. On the corresponding wrinkled PDMS stamp (Figure 2D) the release procedure creates the inverse structure appearing as nonfilled square arrays. In Figure 2E/F, the fine structures of the obtained particle arrays are displayed. Measuring 680 nm, the average particle line spacing on the glass substrate is consistent with the typical periodicity of the wrinkles determined as 670 nm. The enlarged AFM images together with the corresponding cross sections (Figure 2G/H) clearly show that the linear colloidal assemblies produced by spin coating are retained during the transfer. Under the preparative conditions applied in this study, we obtained welldefined particle double lines. It must however be noted that various other morphologies such as single particle or 3D lines are accessible by simple adjustments of the wrinkle geometry and the spin coating conditions. In the presented AFM images, two types of defects are visible: cracks running perpendicular to the wrinkles and Y-shaped junctions. Both types are intrinsic to the system and form during the relaxation process of the plasma oxidized PDMS. Since the area covered by these defects is small compared to the regularly structured regions, their influence on the macroscopic optical properties of the colloidal assemblies is in fact negligible. Considering the different characteristic dimensions of the particles, wrinkles and micropatterns, it is evident that the presented combination of template-assisted colloidal assembly and chemical prestructuring is a powerful toolset for the production of hierarchically organized structures. However, one particular advantage of our method arises from the fact that wrinkling results from a macroscopic deformation, thus allowing easy upscaling. This is demonstrated by the photograph in Figure 3A which shows a sample with hydrophilic, C

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Figure 2. Particle transfer onto chemically patterned substrates (A). Patterned films of OTS were prepared by μCP leading to hydrophilic squares surrounded by a hydrophobic matrix (B). The particles were exclusively transferred onto the hydrophilic regions forming linear particle assemblies bundled into square arrays (C). AFM imaging of the corresponding wrinkled stamp used for aligning the particles revealed the inverse structure (D). Magnified AFM images of the particle decorated surface (E) and the applied wrinkle (F) show the fine structure of the obtained assemblies. The linear arrangement of the colloidal particles originates from the submicrometer periodicity of the wrinkled PDMS stamp. Each line consists of particle doublets stacked together to form parallel wires. The lines are roughly 200 nm in width and have their centers spaced less 700 nm apart from each other as shown by the cross sections (green lines) of the substrate (G) and the stamp (H).

hydrophobic, and wettability patterned areas after particle transfer. The structured surface area roughly measures 9 mm × 23 mm and can be divided into three regions with distinct particle structures: a large area with strong iridescent coloring, a weakly colored square region in the center and a transparent Lshaped area dividing both. The iridescent colors are caused by the particle lines, which due to their spacing can act as a diffraction grating for visible light. This effect is most pronounced in the unstamped surface area (Figure 3B),

where unhindered particle transfer occurred. The patterned region (Figure 3C/E) bearing approximately 75% less particles show the same angle dependent coloring, albeit at a significantly lower intensity. Finally, the L-shaped area resulting from the flat rim (Figure 3D) of the PDMS stamp used for microcontact printing almost completely suppressed colloidal attachment and therefore remained transparent. Thus, the optical properties can indeed be tailored by adjustment of the assembled colloidal structures. D

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Figure 3. Large-scale parallel surface structuring with colloidal particles. The photograph (A) displays a glass slide consisting of three distinct regions produced by μCP of OTS and subsequent particle deposition. Continuous particle lines are obtained in the hydrophilic noncontact region (B), whereas microarrays are found on the chemically patterned area (C). A closed layer of OTS resulting from the unstructured rim of the PDMS stamp prevents colloidal attachment (D). The insets (B−D) are each 14 μm in width; however, faithful pattern reproduction was achieved over much larger areas (E). With decreasing amounts of deposited particles, the intensity of the iridescent colors caused by the submicrometer particle lines is reduced.

micrometer squares, thereby proving the versatility and robustness of the procedure.

In an attempt to further increase the complexity of the structures, we repeated the particle transfer experiment under perpendicular orientation. The results of this experiment are presented in Figure 4. As shown by the AFM images, the

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CONCLUSIONS

In summary, we show in this Letter that substrate wettability plays a dominant role in the transfer of particles from wrinkletemplates onto flat substrates. Building up on this fundamental finding, we demonstrate that wettability patterning opens a pathway for the assembly of nanoparticles into hierarchical structures: While wrinkled substrates can be used to arrange colloidal particles into highly ordered linear assemblies with submicrometer spacing, local wettability contrast was found to be sufficient for the construction of a micrometer sized superlattice via selective transfer. Being based on capillary forces rather than adhesion, our method allows transferring particles in cases when direct contact with the target substrate cannot be established easily. The process is robust enough to conduct multiple deposition steps and allows us to produce highly complex nanoparticle assemblies over macroscopic lengths. From a general point of view, the possibility to control the symmetry of colloidal assemblies over multiple length scales is of great relevance for the creation of high efficiency plasmonic structures, waveguides, or metamaterials. Considering that both topographical templating and the chemical patterning can be achieved without any lithographic steps,30 our work opens exciting avenues for the technological realization of novel functional materials. Printing processes, such as the one introduced here, are as well a first step toward integration of this approach into industrial processes like roll-to-roll printing and further upscaling.

Figure 4. Rectangular nanowells obtained by dual particle transfer. The assembly process is robust enough to facilitate multiple particle deposition. Colloids applied in the second step were transferred onto the originally deposited particle lines at a tilt angle of 90° (A). The enlarged image displays a single microarray with a gridlike substructure consisting of particles (B).

colloidal assemblies created during the first deposition step are mostly unaffected by the repeated procedure. In addition to attractive van der Waals and hydrophilic interactions, physical confinement due to the wrinkled PDMS stamp placed on top of the initially deposited particles may contribute to the fixation of the colloidal pattern. The hydrophilicity of the line patterned square arrays is sufficient to facilitate attachment of additional particles in the second step. Through the repeated particle transfer, we created rectangular submicrometer wells inside the E

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ASSOCIATED CONTENT

S Supporting Information *

Comparison of the macroscopic and microscopic wettability of OTS patterned substrates; SEM images of a particle decorated glass substrate and the corresponding wrinkled PDMS stamp after site-selective transfer; AFM micrographs of colloidal assemblies of carboxylated PS particles; examples of different surface morphologies obtained with line patterned substrates. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: (+49) 921-55-2753. Fax: (+49) 0921552059. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been conducted within the special collaborative research project SFB 840 (project B5) funded by the German Science Foundation (DFG). The authors would like to acknowledge Beate Förster for her guidance during SEM imaging as well as Markus Hund for his assistance with the AFM measurements. We also thank Bernhard Glatz for helping with the sample preparations.

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REFERENCES

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