Hierarchical Micro- and Nanoscale Structures on Surfaces Produced

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Hierarchical Micro- and Nanoscale Structures on Surfaces Produced Using a One-Step Pattern Transfer Process Jie-Ren Li, Nai-Ning Yin, and Gang-yu Liu* Department of Chemistry, University of California, Davis, California 95616, United States

ABSTRACT A one-step pattern transfer process was developed to produce arrays of hierarchical micro- and nanostructures of organosilanes. The method is based on vapor deposition through polydimethylsiloxane stamps coated with closepacked nanospheres and, as such, creating hierarchical microscale and nanoscale templates, respectively. This method offers intrinsic advantages of simplicity to be used in any laboratory environment and high throughput, that is, a 1 in. wafer can be covered in 6 h. In addition, the size and geometry can be controlled via knowledge of microcontact printing and particle lithography. Finally, the approach is generic in nature and may be utilized to produce designed functionalities. SECTION Nanoparticles and Nanostructures

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ierarchical structures with diverse functionalities ranging from nano-, micro-, to macroscale are frequently encountered in nature.1,2 Examples include DNA molecules to chromosomes, amyloid proteins to fibers, cellulose fibrils, and the hierarchy of composite structures in bones to carry out mechanical integrity and control of the body symmetry in morphogenesis.3-6 Therefore, artificial hierarchical micro- and nanoscale structures have recently attracted more attention in order to mimic nature and to explore potential applications of these structures in electronics and optoelectronic devices.7,8 Current state-of-the-art applications in producing artificial hierarchical structures include multistep lithography9-13 and microlithography in conjunction with self-assembly.14-16 Hierarchical structures formed via self-assembly and micropatterning have been successfully demonstrated, as in the case of block copolymers, taking advantage of the phase behavior of vaporized block copolymers at the area of contact underneath of the wedge.14-16 This method has the advantage of simplicity but a limitation of the phase diagram of the specific polymer systems. For nonpolymer systems, nanoparticles may be directly printed on surfaces using microlithography stamps. In this case, nanostructures must be preformed into nanoparticles, and long-range order is achieved by the closepacking of nanoparticles. Therefore, this approach is not sufficiently generic.17-20 For the methods using lithography alone, multiple steps for pattern transfer are required, such as microlithography followed by nanolithography.9-13 The limitations here are the requirements of multiple stamps and multiple pattern transfer steps. The nanoscale stamps are not trivial to make and maintain. Each step of pattern transfer could introduce complications in spatial fidelity. To overcome limitations of prior approaches and develop a simple technique that any laboratory can easily implement, we have developed a new fabrication strategy to produce hierarchical structures using only one step in the pattern transfer.

r 2011 American Chemical Society

We introduce a new method to fabricate hierarchical structures, which is generic and simple. Our method combines microcontact printing (μCP)21-26 and particle lithography27-33 to produce hierarchical micro- and nanoscale structures, respectively. The combination is achieved by incorporating close-packed silica nanoparticles onto the protruding microstructures of polydimethylsiloxane (PDMS) stamps. The key step of pattern transfer is accomplished by a one-step process of vapor deposition of organosilane through PDMS stamps coated with colloidal masks. In comparison to prior approaches, this method has the following intrinsic advantages: (a) simplicity of use in any laboratory environment; (b) generic production of designed chemical functionalities; (c) high fidelity because there is only one pattern transfer step; and (d) high throughput. The main steps involved in this method to fabricate organosilane hierarchical micro- and nanostructures are illustrated in Figure 1. Close-packed crystalline films of sizesorted monodisperse silica spheres (250 nm) are formed on Si(111) surfaces following protocols reported previously.34-37 The detailed procedure for preparing silica films is also described in the Experimental Methods section. The PDMS stamps with arrays of microlines are prepared using compact discs (CDs) as masters based on previously established protocols.38,39 Specifics for stamp preparation will be provided in the Experimental Methods section. The PDMS stamp is then brought into contact with the surface of the silica sphere crystalline films and applied with gentle pressure to ensure conformal contact (Figure 1A). The entire assembly of the PDMS stamp with silica films is heated at 100 °C to enhance the adhesion of silica particles to the PDMS stamp. Received Date: November 29, 2010 Accepted Date: January 18, 2011 Published on Web Date: January 24, 2011

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DOI: 10.1021/jz101609a |J. Phys. Chem. Lett. 2011, 2, 289–294

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using known silane chemistry to generate designed functionalities.40-44 The functionalities of hierarchical microstructures and nanostructures presented in this Letter are 2-[methoxy(polyethyleneoxy)propyl]trichlorosilane (PEG-silane) and N-(6-aminohexyl)aminopropyltrimethoxysilane (AAPTMS). Figure 2A reveals the topographic image of PDMS stamps acquired by atomic force microscopy (AFM), showing the overall arrangement of microline arrays. The zoom-in view AFM image in Figure 2B and the corresponding cursor profile in Figure 2C indicate that the typical line width is 0.81 μm with a center-to-center separation of 1.46 μm. The height of microline arrays measures 126 ( 5 nm, in accordance with the CD masters. Figure 2D shows a topographic image of silica-particle-modified stamps. Few silica particles can be found in channels. The crystalline or close-packed silica spheres are firmly attached on the protruding surface of the PDMS stamp, as shown in Figure 2E. The cursor profile in Figure 2F reveals that the lateral dimensions of microstructures remain unchanged, which is 0.83 μm wide with a centerto-center separation of 1.51 μm. The height is 368 ( 12 nm, which is 243 ( 7 nm taller than the previous lines. The silica sphere diameter is 250 nm; thus, only a single layer of silica spheres is attached to the PDMS stamp. The cursor profile inserted reveals that the periodicity measures 254 nm, corresponding well with the diameter of silica particles. The attachment to the PDMS surface remains strong after the lift-off procedure and sustains the rinsing with ethanol. The alignment of silica spheres relative to the PDMS line is determined by the orientation of stamps with respect to the crystallization of silica spheres. In all protruding features of the PDMS stamp, the packing and the crystalline axis of silica spheres are highly consistent and follow the initial crystallinity on the surface. The particle-coated PDMS stamp is then used as a template for vapor deposition to produce arrays of organosilane hierarchical micro- and nanoscale structures. The organosilanes used for producing hierarchical structures are AAPTMS nanocircles inlaid in a PEG-silane matrix. The particle-coated PDMS stamp is attached to a clean Si(111) substrate and then placed in a reaction vessel containing PEG-silane. To generate a vapor for deposition, the reaction vessel is placed in an oven at 70 °C under ambient pressure for 6 h. During PEG-silane vapor deposition, organosilane molecules form SAMs in the void space between silica particles and the PDMS stamps. After vapor deposition is completed, the sample is sonicated in ethanol to remove adhered particles and nonreacted residues. The PEG-silane SAMs bond covalently to the substrates and are not displaced from the surface by the rinsing step. The surface, in the final step, is immersed in the AAPTMS solution (1 mM in toluene) for 2 h to produce arrays of nanocircles and then cleaned by sonication and rinsed with ethanol. The organosilane hierarchical microscale and nanoscale structures are shown in Figure 3. At the microscopic scale, the surface is covered by arrays of microlines, as displayed in Figure 3A. The bright lines are the periodic arrays of PEGsilane microstructures. The dark lines are nanocircles of AAPTMS inlaid in PEG-silane. The cursor profile in Figure 3B reveals that the typical width of the bright microlines

Figure 1. Steps for simultaneous fabrication of organosilane hierarchical microscale and nanoscale structures. (A) A PDMS stamp with designed features is brought into conformal contact with a surface covered by the close-packed silica sphere layers. (B) Upon lifting, a single layer of close-packed silica particles is transferred to the protruding areas of the PDMS stamp. (C) The particle-adhered PDMS stamp is used as a mask for vapor deposition of organosilanes. (D) Hierarchical microscale and nanoscale structures form after vapor deposition and removal of the stamp.

After silica nanoparticles are attached well onto the surface of the PDMS, the stamp is then carefully peeled off. A single layer of close-packed silica particles remains attached to the protruding surface of the PDMS stamp, as shown in Figure 1B. The particle-adhered PDMS stamp is then used as a mask for vapor deposition of organosilanes, which is the key to transferring the hierarchical structures of stamps into organosilane patterns, as illustrated in Figure 1C. During vapor deposition, organosilane molecules self-assemble in the void space at the interface. The area of contact between the substrate and the base of the spheres serves as a template to guide the formation of nanostructures with circular geometry. The void spaces defined by PDMS stamps are fully covered with organosilane to form arrays of microlines. After vapor deposition is completed, the particle-coated PDMS stamp is removed, and the hierarchical structures of silane can be characterized (Figure 1D). Further chemical modification can be performed


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Figure 2. The stamps used for fabrication of organosilane hierarchical structures. (A) The representative topographic image of the PDMS stamp produced with commercially available masters. (B) Zoom-in view of the PDMS stamp. (C) The corresponding cursor profile of the PDMS stamp. (D) The AFM image of the particle-adhered PDMS stamp. (E) Close-up view to reveal the organization of particles on the protruding features of the PDMS stamp. (F) The cursor profiles to show the particle-adhered PDMS stamp and arrangement of silica spheres (inserted cursor).

vapor deposition process, PEG-silane molecules undergo hydrosilylation with free hydroxyl groups on Si(111). Each organosilane molecule can form siloxane bonds to anchor to the surface and to also connect to neighboring molecules to form close-packed layers. The thickness of PEGsilane layers is determined by the height of deposition sites. The channels of the PDMS stamp are more spacious for the organosilane deposition than silica spheres, so that the height of the PEG-silane layers in the bright lines is taller than that of layers in the dark regions. The height of the PEG bright microline is 6.1 ( 0.2 nm, and the dark PEG lines measure 1.4 ( 0.2 nm. The zoom-in AFM image of the dark region in Figure 3C (green square) clearly reveals circular nanostructures of AAPTMS SAMs surrounded by PEG-silane layers. The circular AAPTMS patterns form a hexagonal arrangement, which precisely replicates geometries of silica particle masks. The cursor profile in Figure 3D reveals that the diameter of AAPTMS nanocircles is 86 nm with a separation of 255 nm. The diameter of circles is determined by contact areas between the silica particle and the surface. The periodicity of the AAPTMS nanocircles is 252 ( 7 nm, defined by the diameter of the silica spheres. The height difference between AAPTMS nanocircles and surrounding PEG-silane layers in the dark region is 1.4 ( 0.2 nm. It is apparent that organosilane hierarchical structures exhibit highly consistent and reproducible geometries with defined surface functionalities. Once the experimental conditions are optimized, dozens of samples prepared with the selected conditions exhibit identical microscale and nanoscale morphologies. Taking advantage of the variety in geometries for PDMS microstructures and diameters for silica spheres, complicated hierarchical structures can be produced based on various combinations.

Figure 3. Patterned surfaces with hierarchical structures fabricated using vapor deposition of organosilane through PDMS stamps coated with colloidal masks. (A) The AFM image to exhibit the line micropatterns of PEG-silane with AAPTMS nanostructures inlaid in PEG-silane layers. (B) The cursor profile to reveal the micropatterns. (C) The zoom-in image to reveal the hexagonal arrangement of AAPTMS nanostructures in PEG-silane layers. (D) The corresponding cursor profile for the line in C.

measures 0.73 μm with a center-to-center separation of 1.49 μm. The overall width of the dark lines is 0.85 ( 0.07 μm with a center-to-center separation of 1.51 ( 0.09 μm. The dimension of microlines corresponds well with the PDMS stamp. The PEG-silane structures are defined by the channels of PDMS stamps, where the PEG-silane vapor can access and deposit onto the uncover surfaces. During the


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A one-step pattern transfer process was developed to produce arrays of hierarchical structures of organosilanes. The method simply combines microcontact printing and particle lithography to simultaneously produce hierarchical micro- and nanoscale structures, respectively. Compared to prior approaches, this method is simple and can be implemented by any laboratory. The final structure should have high fidelity as there is only one step in the pattern transfer. The method has high throughput, similar to microcontact printing. More importantly, this approach is generic and, in principle, can be used to produce hierarchical micro- and nanostructures with designed functionalities, taking advantage of our rich knowledge of silane chemistry.40-44 Work is in progress to improve the sharpness of edges at the nanoscale and to produce various other functionalities.

adhered onto the surface of PDMS, the PDMS stamp was carefully peeled off. A single layer of close-packed silica particles was transferred to the protruding surface of the PDMS stamp. The organosilanes used for fabricating hierarchical structures were purchased from Gelest (Morrisville, PA) and used without further purification. To generate an organosilane vapor, the reaction vessel was placed in an oven and heated at 70 °C for 6 h under ambient pressure.31,33 Atomic force microscope (MFP-3D-SA, Asylum Research, Santa Barbra, CA) was used for the characterization of organosilane hierarchical micro- and nanostructures and PDMS stamps. All images were acquired using contact mode in air with the typical force of less than 1 nN. Sharpened Si3N4 microlevers (Veeco Metrology Group, Santa Barbara, CA) with a force constant of 0.1 N/m, measured by the thermal noise method,48 were used for the characterization. Images were processed using Gwyddion open source software, which is freely available on the Internet and supported by the Czech Metrology Institute.

EXPERIMENTAL METHODS The silica particles (250 nm in diameter) were purchased from Fiber Optic Center Inc. (New Bedford, MA). The substrate used for fabrication was Si(111) (Virginia Semiconductor Inc., Fredericksburg, VA). Before use, the Si(111) substrates were cleaned by immersion in piranha solution, which is a mixture of sulfuric acid and hydrogen peroxide (EMD Chemicals, Gibbstown, NJ) with a (v/v) ratio of 3:1. Substrates were then rinsed copiously with deionized water and dried with nitrogen. The silica particles were initially washed with deionized water by resuspension and sonication, followed by centrifugation. The particle pellet at the bottom of a microcentrifuge tube was resuspended in deionized water to reach a concentration of 1% (w/w). A drop of the solution was deposited on a freshly cleaned Si(111) substrate and dried at room temperature. As water evaporates during drying, capillary forces pull the mesospheres together to form organized crystalline layers on flat surfaces.34-37 The master used to create PDMS stamps is a compact disc (Sony CD-R). Utilizing CDs as masters for microcontact printing stamps has been described previously.38,39 The PDMS elastomer kits (Sylgard 184) from Dow Corning (Midland, MI) were used to replicate the feature from masters. PDMS stamps were prepared by mixing Sylgard 184 curing agent at a ratio of 1:10. The mixture was poured onto the CD master and then cured in the oven at 70 °C for 4 h. The stamps were cleaned using hexane (Fisher Scientific, Pittsburgh, PA) and sonicated in ethanol (Gold Shield Chemical Co., Hayward, CA) for 20 min to remove any unreacted low-molecular-weight oligomers. Application of the PDMS stamps to transfer particles has been described previously.45-47 In a typical lift-off process, a PDMS stamp with designed features was brought into conformal contact with the surface of the silica sphere crystalline films under a certain pressure. The procedure started from bringing the PDMS stamp into contact with the surface of the silica sphere crystalline films and then applying gentle pressure to ensure conformal contact by hand. The entire assembly was sandwiched between two glass slides and then fastened with paper clamps to provide stable pressure while transferring silica particles to the PDMS stamp. The entire assembly of the PDMS stamp with silica films was heated at 100 °C for 3 h. After silica nanoparticles were well-


AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. Tel: (530) 7549678. Fax: (530) 754-8557. E-mail: [email protected].

ACKNOWLEDGMENT The authors gratefully acknowledge support from the University of California, Davis, CCRC research grant, and the National Science Foundation (CHE-0809977).

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