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Hierarchical Nanoparticle Assemblies Formed via One-Step Catalytic Stamp Pattern Transfer Tzu-Yi Hung, Jessica An-Chieh Liu, Wen-Hsiu Lee, and Jie-Ren Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19807 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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ACS Applied Materials & Interfaces

Hierarchical Nanoparticle Assemblies Formed via One-Step Catalytic Stamp Pattern Transfer Tzu-Yi Hung, Jessica An-Chieh Liu, Wen-Hsiu Lee and Jie-Ren Li* Department of Chemistry, National Cheng Kung University, No.1 College Road, Tainan 70101, Taiwan KEYWORDS: catalytic stamp, one-step pattern transfer process, in situ nanoparticle synthesis, nanoparticle assemblies, hierarchical structures, surface-enhanced Raman scattering

ABSTRACT

The one-step catalytic stamp pattern transfer process is described for producing arrays of hierarchical nanoparticle assemblies. The method simply combines in situ nanoparticle synthesis triggered by free residual Si-H groups on PMDS stamps and lift-off pattern transfer technique. No additional nanoparticle synthesis procedure is required before pattern transfer process. Exquisitely uniform and precisely spaced hierarchical nanoparticle assemblies with designed geometry can be rapidly produced using catalytic stamp pattern transfer process. Sequential catalytic stamp pattern transfer also is described to generate multilayered, hierarchical nanoparticle assemblies with various geometries. The hierarchical nanoparticle assemblies catalytically transferred onto the surface is not just nanoparticles, but nanoparticle-PDMS residue composites. The in situ synthesized nanoparticles still remain optical properties. The hierarchical nanoparticle assemblies

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with precisely controlled geometry further show potential in the application of surface-enhanced Raman scattering. Capability of the one-step catalytic stamp pattern transfer allows the scalable and reproducible fabrication of well-defined hierarchical nanoparticle assemblies.

INTRODUCTION Hierarchical micro- and nanoscale structures discovered in nature, such as DNA molecules to chromosomes,1-2 amyloid proteins to fibers,3-5 collagen fibers in bones6-7 and extracellular matrix proteins in basement membrane8-9 have recently attracted more attention. A number of artificial hierarchical structures were designed to mimic nature and to explore potential applications in tissue engineering,10-11 electronics12-13 and optoelectronic applications.14-15 However, precise control of the relative position and molecular orientation in two-dimension or three-dimension still remain a current challenge in fabrication of artificial, functional hierarchical structures. Current state-of-the-art fabrication approaches to produce artificial hierarchical structures include sequential multistep lithography,16-20 lithography combined with molecular selfassembly,21-25 and layer-by-layer (LbL) assembly techniques.26-30 Prior approaches require a series of steps, including fabrication processes to define surface chemistry and pattern geometry, synthesis and modification of nanomaterials, and multiple pattern transfer or self-assembly steps to control the relative position, in order to achieve production of artificial hierarchical structures. Each step in process could introduce limitation in structure fabrication and complications in pattern fidelity. Microcontact printing (μCP) that utilizes polydimethylsiloxane (PDMS) for stamp production has been widely used to transfer organic, inorganic and biological materials on various solid substrates.31-35 Even for nanoparticles (NPs), μCP-based transfer techniques have been used


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to pattern gold nanocrystals (Au NCs),36 gold nanorod (Au NRs) assemblies,37 or quantum dot films.38 In addition to transferring molecules or nanoparticles, μCP has been demonstrated that it can induce surface chemical reactions.39-41 The catalytic stamps, the PDMS-based stamps integrated with catalysts, have been applied to trigger chemical reactions on surface.42-48 A catalytic stamp embedded with palladium nanoparticles (Pd NPs) was designed and used for catalytic hydrosilylation of alkenes/alkynes/aldehydes on H-terminated Si surfaces.42 A plasmonic stamp with patterned Au NPs was used to directly functionalize silicon surfaces using the localized surface plasmon resonance (LSPR) of the Au nanostructures, with a sub-100 nm pattern resolution.47-48 In addition to transition metal catalysts, enzymes can be immobilized on PDMS stamps for biocatalytic reactions on surface.49-52 A trypsin-immobilized PDMS stamp was prepared for site-selective enzymatic degradation of poly-L-lysine (PLL) layer on solid substrates.50 Catalytic stamp lithography provides advantages: (a) exquisite site-selectivity of surface reactions, (b) generic production of designed surface functionalities; (c) resolution limit superior to conventional μCP; and (d) high-throughput capabilities. However, the most successful examples for catalytic stamp lithography still limited in site-selective surface reactions of selfassembled monolayers (SAMs). More applications using catalytic stamp lithography will require significant investments to develop. We introduce a new one-step process, based on catalytic stamp technique, to fabricate hierarchical gold nanoparticle (Au NP) or silver nanoparticle (Ag NP) assemblies, which is generic and simple. The method presented simply combines a catalytic stamp and the μCP-based pattern transfer technique. The key step of one-step catalytic stamp pattern transfer is accomplished by simultaneous combination of in situ nanoparticle synthesis through PDMS stamps and lift-off pattern transfer technique to produce hierarchical Au or Ag NP assemblies. No additional reducing


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agents is required during pattern transfer. No additional NPs synthesis in advance is necessary. The size, shape, and spacing of hierarchical Au or Ag NP assemblies can be well-controlled by the design of PDMS stamps. Sequential catalytic stamp pattern transfer also was developed to generate multilayered, hierarchical NP assemblies with various geometries. The hierarchical metal NP assemblies catalytically transferred onto the surface is the metal NP-PDMS residue composite, instead of Ag NPs purely. The hierarchical Au NP assemblies with precisely controlled geometry still remain optical properties to serve as SERS substrates. Capability of one-step catalytic stamp pattern transfer allows the scalable and reproducible fabrication of well-defined hierarchical NP assemblies. RESULTS AND DISCUSSION Successive Steps for Fabrication of Hierarchical Nanoparticle Assemblies via One-Step Catalytic Stamp Pattern Transfer. The main steps involved in this approach to fabricate layered, hierarchical nanoparticle assemblies are illustrated in Figure 1. One-step catalytic stamp pattern transfer process begins with preparation of catalytic PDMS stamps. The PDMS stamps with arrays of micro-sized features are prepared using commercially available masters as molds based on previously established protocols.53-55 In the one-step catalytic stamp pattern transfer process, a droplet of metal precursor aqueous solutions (Au or Ag) is first added to a clean Si(111) or glass substrate, as shown in Figure 1A. The PDMS stamp with designed features is then brought into contact with the surface and the gentle pressure is applied to ensure conformal contact (Figure 1B). Most of metal precursor solutions are forced to remove through void spaces of the PDMS stamp. The remaining Au or Ag ion solutions stay between the protruding area of the PMDS and substrate. The area of contact between the substrate and the PDMS stamp also serves as a structural template to guide the formation of NP assemblies with designed geometry. During pattern transfer process,


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free residual Si-H groups of PMDS stamp surface serve as the reducing agent to gradually reduce Au or Ag ions, forming Au or Ag NPs at ambient condition (Figure 1C).56-63 After the process is completed, the PDMS stamp is carefully peeled off. A single layer of close-packed Au or Ag particles mixed with few PDMS residues is synthesized and transferred from the protruding surface of the designed PDMS stamp, simultaneously (Figure 1D). Even with rinsing copiously with deionized water, the transferred NP assemblies remain securely attached to the surface with an imprinted pattern. To fabricate multi-layered, hierarchical NP assemblies, a droplet of metal precursor solution is added to the first layer of patterned NP assemblies. Since the first layer of patterned NP assemblies increases the surface hydrophobicity, another freshly prepared PDMS stamp is immediately brought into conformal contact with the surface under certain pressure, as illustrate in Figure 1E. Surface redox reaction to generate Au or Ag NPs occurs at the interface between the PDMS stamp and the first layer of patterned NP assemblies. The protrusion feature of PDMS stamp guides the assembly of nanoparticles and determines the pattern geometry. The second layer of NP assemblies with designed geometry attaches securely on the top of the first layer, forming layered, hierarchical nanoparticle assemblies (Figure 1F). The key step of pattern transfer is accomplished by a one-step process of in situ NP synthesis through the catalytic PDMS stamps, without additional reducing agents present. The geometry of hierarchical NP assemblies corresponds to the lithographically defined feature of PDMS stamps.


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Figure 1. Steps of producing hierarchical nanoparticle assemblies via catalytic stamp pattern transfer process. (A) A droplet of Au or Ag aqueous solution is added to a clean substrate. (B) A designed PDMS stamp is brought into conformal contact with the surface containing Au or Ag aqueous solution. (C) Free residual Si-H groups on the PDMS surface react with Au or Ag ions to form Au or Ag NPs. (D) Upon lifting, a single layer of close-packed Au or Ag NPs is formed and transferred to the surface. (E) To fabricate hierarchical, multiple layers of NP assemblies, another droplet of Au or Ag aqueous solution is added to the first pattern-transferred NP assemblies, followed by placing another PDMS stamp in contact with the surface under certain pressure. (F) Upon lifting, hierarchical, multiple layers of Au or Ag structures are in situ synthesized and transferred to the surface after surface catalysis. A representative example presented in Figure 2 demonstrates that one-step catalytic stamp pattern transfer process enables to fabricate uniform, parallel sub-micron line structures of singlelayered Au NP assemblies on Si(111). The PDMS stamps with arrays of sub-micron lines are prepared using compact discs (CDs) as masters based on previously established protocols.53-55 The topographic image acquired by atomic force microscopy (AFM) reveals overall arrangement of


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sub-micron line Au NP assemblies (Figure 2A). The typical linewidth is 0.83 ± 0.05 μm with a center-to-center separation of 1.45 ± 0.06 μm, in accordance with the CD masters. The height of microline arrays measures 6.7 ± 0.6 nm. Few Au particle aggregates can be found between submicron lines. Tiny residues of Au precursor solution can be trapped inside the PDMS channels to form a local meniscus near the base of the PDMS channel wall. Free residual Si-H groups on PMDS channel wall reduce Au ions progressively to form aggregates near or inside the PDMS channels during pattern transfer process. Actually, the hierarchical Au NP assemblies catalytically transferred onto the surface is not just Au nanoparticles, but Au NP-PDMS residue composites. During pattern transfer process, free residual Si-H groups of PMDS stamp surface not only gradually reduce Au ions to form Au NPs but also produce PDMS residues as a byproduct to partially adsorb onto Au NPs, forming Au NP-PDMS residue composites. The structure height measured in Figure 2A is the apparent height of Au NP-PDMS residue composites. Estimates of the structure height as a function of pattern transfer time is plotted in Figure 2B. The estimates were made by analyzing multiple representative images of different areas of the samples. The chart demonstrates quantitatively that the height of microline arrays increases accordingly with increases in pattern transfer time. The variation in the structure height of Au particle assemblies results from increases in the pattern transfer time, ranging from 5.3 ± 0.6 nm (6 hours) to 12.6 ± 1.7 nm (72 hours). The optimized transfer time for forming Au particle assemblies measured 12 hours, experimentally. The efficiency of catalytic stamp pattern transfer is highly dependent on the concentration of metal precursor solution. Estimates of the structure height as a function of Au precursor solution concentration is plotted in Figure 2C. AFM topographic images in Figure 2C represent the Au NP microlines produced using the Au precursor solutions of 0.05 %, 0.5 %, 2 % and 5 % (wt. %), respectively. The chart demonstrates quantitatively that the height of microline


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arrays increases accordingly with increases in metal precursor solution concentration. When the concentration of Au precursor solution was low (0.05 % and 0.5 %), irregular and partial Au nanoparticle microlines were formed, as shown in AFM images of Figure 2C. High concentration of Au precursor solutions (5 %) resulted in formation of higher Au NP microlines and more Au NP aggregates near or inside the PDMS channels during pattern transfer process (Figure 2C). The metal precursor solution of 2 % was the optimized concentration for catalytic stamp pattern transfer of Au NP hierarchical assemblies. Scanning electron microscopy (SEM) also confirms that Au NPs were in situ synthesized and transferred onto the substrate, simultaneously. Zoom-in SEM image in Figure 2D exhibits microstructures of Au NP assemblies as well as few particle aggregates between submicron lines. The bright contrast in SEM image indicates transferred Au NPs. Au particle aggregates results from the surface redox reaction of Au ions inside the PDMS micro-sized channels. The arrays of Au NP assemblies produced by catalytic stamp pattern transfer process retain characteristic optical properties. Characterization with ultraviolet-visible spectrophotometry (UVVis) enables to investigate the optical property of Au NP assemblies. For comparison, UV-vis spectra were acquired for a piece of glass with microstructures of Au NP assemblies (Figure 2E) and for a Au NPs solution (Figure 2F), respectively. A significant absorption peak was observed at 549 nm for the solution of suspended 13 nm gold nanoparticles, as the comparative reference. A red shift up to a value of 574 nm was observed in the UV-vis spectra of line arrays of gold nanoparticle assemblies on glass. The shape of Au NPs synthesized by free residual Si-H groups of PMDS stamp exhibited polygonal morphology, such as sphere, triangle, rectangle, pentagon and hexagon.56 The red spectral shift is mainly attributable to increases in Au particle polydispersity as well as changes in interparticle distance as Au NPs assemble on the confined


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areas of the surface.56, 64-65 X-ray photoelectron spectroscopy (XPS) further confirms the elemental composition of single-layered Au NP assemblies produced with one-step catalytic stamp pattern transfer process. Figure 2G exhibits the high-resolution XPS spectrum of the Au 4f core level of single-layered Au NP assemblies. Two distinct lines of the Au 4f5/2 (87.48 eV) and Au 4f7/2 (83.81 eV) separated by 3.67 eV were observed, due to the spin-orbit splitting of the Au 4f level.66

Figure 2. Characterization of periodic line microstructures of gold nanoparticle assemblies produced using catalytic stamp pattern transfer process. (A) AFM topographic image of periodic Au NP microlines. (B) Correlation between pattern height and transfer time. (C) The metal precursor concentration effect for catalytic stamp pattern transfer of Au NP microlines. AFM topographic images exhibit the Au NP microlines produced using the Au precursor solutions of


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0.05 %, 0.5 %, 2 % and 5 %, respectively. (D) SEM image to exhibit microlines of Au NP assemblies with few aggregates. (E) UV-Vis spectrum of the hierarchical Au NP microstructures (inserted image). (F) UV-Vis spectrum of the 13 nm Au NPs solution (inserted image). (G) XPS spectrum of the hierarchical Au NP microstructures. Geometry Diversity of Hierarchical Nanoparticle Assemblies Achieved Using Catalytic Stamp Pattern Transfer. Arrays of hierarchical gold nanoparticle structures with a variety of sizes and geometries can be fabricated using catalytic stamp pattern transfer. A series pf AFM images in Figure 3 demonstrate that geometries as well as sizes of hierarchical Au NP assemblies coated with residual PDMS are controllable using PDMS stamps with different surface structures for catalytic pattern transfer. A PDMS stamp of small cylindrical pillar with 2 μm in diameter and 6 μm in center-to-center separation was used to catalytically transfer Au NPs on the Si substrate (Figure 3A). Large-area AFM topography image (50 × 50 μm2) displays long-range order of ringshaped Au NP assemblies. The geometry and periodicity are consistent with the designed features of PDMS stamp. The zoom-in image further reveals that Au NP assemblies remain the ring-shaped morphology but have defect near the center area. Uneven force applied to the PDMS stamp might results in the defect formation. The corresponding cursor shows the averaged pattern height of 6.1 ± 0.5 nm and the averaged dimeter of 1.84 ± 0.17 μm. In Figure 3B, AFM topographic image (50 × 50 μm2) exhibits arrays of round-shaped Au NP assemblies produced using a PDMS stamp with large cylindrical pillar with 4 μm in diameter and 6 μm in center-to-center separation. Few Au NP aggregates can be found between sub-micron round-shaped structures. The pattern separation is consistent with the designed features of PDMS stamp, but pattern geometry slightly changes due to the meniscus formed outside the large cylindrical pillar. The zoom-in image (5 × 5 μm2) clearly reveals that Au NPs follow the PDMS protrusion to form a round-shaped structure and closely


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replicate geometry of PDMS stamp. Based on cursor analysis using multiple representative images, the pattern height measures 6.2 ± 0.4 nm and the diameter is 3.74 ± 0.22 μm. Besides line and circle patterns, square-shaped Au NP assemblies also can be fabricated using PDMS stamps with square-shaped protrusion for catalytic stamp pattern transfer. As shown in a 30 × 30 μm2AFM topography image (Figure 3C), catalytic stamp pattern transfer using a PDMS stamp of small square-shaped protrusion (1.5 μm in side length and 3 μm in center-to-center separation) enables to produce highly organized, periodic arrays of square Au NP assemblies on the Si(111) substrate. Few large Au nanoparticle aggregates can be found between or near square-shaped structures. Zoom-in AFM image indicates the structure fidelity after pattern transfer process. Cursor analysis using multiple representative images shows the pattern height of 6.5 ± 0.6 nm and the side length of 1.34 ± 0.13 μm. As stamp geometry design changed to large square-shaped protrusion (5 μm in side length and 10 μm in center-to-center separation), catalytic stamp pattern transfer still can fabricate arrays of hierarchical Au NP assemblies (Figure 3D). Few large Au NP aggregates were found between square structures. Bridge-like Au nanoparticle aggregates were also observed because formation of water bridges might occur between PDMS square protrusion during catalytic stamp pattern transfer process. Zoom-in view clearly exhibits edge mark of square shape of Au NP assemblies as well as edge-spreading structures due to meniscus. Formation of edge mark might result from lift-off process. The corresponding cursor for the line in zoom-in image shows the averaged pattern height of 6.0 ± 0.8 nm and the side length of 4.99 ± 0.35 μm. In addition to the protrusion structure, the pit structure is also applicable. A PDMS stamp with arrays of large square-shaped pits (5 μm in side length and 5 μm in edge-to-edge separation) was designed for catalytic stamp pattern transfer. The AFM image (50 × 50 μm2) in Figure 3E reveals a film of Au NPs with square-shaped areas of uncovered substrate. Few ring-shaped defects as well as Au


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nanoparticle aggregates inside the uncovered square-shaped areas are clearly observed in zoom-in AFM topography image. The height of a film of Au NPs with square-shaped pits measures 6.9 ± 0.8 nm. It is apparent that hierarchical Au NP assemblies exhibit highly consistent and reproducible geometries, in accordance with the PDMS stamps. Once the experimental conditions are optimized, dozens of samples prepared with the selected conditions exhibit identical microand nanoscale morphologies.

Figure 3. Arrays of hierarchical gold nanoparticle structures with a variety of sizes and geometries fabricated using catalytic stamp pattern transfer. Large-area AFM topography image, zoom-in view AFM image and the corresponding cursor profile for the line in zoom-in image reveal the pattern morphology and height of hierarchical Au NP assemblies fabricated using PDMS stamps of (A) small cylindrical pillar (2 μm in diameter), (B) large cylindrical pillar (4 μm in diameter),


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(C) small square-shaped protrusion (1.5 μm in side length), (D) large square-shaped protrusion (5 μm in side length), and (E) large square-shaped pit (5 μm in side length) for catalytic stamp pattern transfer process. Multilayered, Hierarchical Nanoparticle Assemblies with Various Geometries Fabricated Using Sequential Catalytic Stamp Pattern Transfer. Layer-by-layer deposition has been widely-used to pattern thin films and nanomaterials.26-30 By taking the same concept of LbL deposition, catalytic stamp pattern transfer technique enables to produce multilayered, hierarchical Au NP assemblies. The representative examples presented in Figure 4 demonstrates that arrays of multilayered, hierarchical Au NP assemblies with various geometries fabricated using sequential catalytic stamp pattern transfer processes. Figure 4A exhbits the double-layered cross microlines of Au NP assemblies with identical widths (Lw-Lw: 0.85 μm in width for each PDMS stamp). Lw represents the microline structures of the PDMS stamp with 0.85 μm in width. The large-area AFM topography (50 × 50 μm2) image clearly reveals structural uniformity of cross microline Au NP assemblies. Few defects and nanoparticle aggregates between the cross microlines still can be found. Close-view AFM 3D image clearly exhibits the height difference between layers. The line width (0.86 ± 0.05 μm) and center-to-center separation (1.47 ± 0.08 μm) are consistent with the CD masters. The water contact angle (WCA) measures 92.8 ± 2.8°, indicating that surface hydrophobicity increases due to the micro- and nanoscopic Au architecture on the surface. While using PDMS stamps with different line widths (Lw-Lt: 0.85 μm and 0.35 μm in width), two Au NP microlines overlap to form crossline structures (Figure 4B). Lt represents the microline structures of the PDMS stamp with 0.35 μm in width. The line width and center-to-center separation for each line are consistent with the CD and DVD masters, respectively. Zoom-in AFM 3D image clearly shows that thin microlines of Au NP assemblies are transferred onto wide Au


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NP microlines. The periodicity and width remain the same as PDMS stamps. Few nanoparticle aggregates are scattered between the cross microlines or on the surface of lower layer. The WCA measurement is 94.5 ± 3.2° due to the increased structural complexity of hierarchical Au NP assemblies. Different from identical geometry, sequential catalytic stamp pattern transfer enables to fabricate layered, hierarchical Au NP assemblies with various geometries. The representative images in Figure 4C reveals hierarchical Au NP assembles composed of microlines and microsized squares (Lw-Sq: 0.85 μm in width for line and 5 μm in side length for square). Sq represents the micro-sized squares of the PDMS stamp with 5 μm in side length. Microlines were transferred on the top of micro-sized squares, as shown in a large-area AFM topographic image (100 × 100 μm2) of Figure 4C. Each geometry still remains recognizable in the double-layered structures. Au nanoparticle aggregates are observed in Zoom-in AFM 3D image. The WCA measurement is 97.9 ± 2.5° due to the increase in structural complexity and Au NP aggregates. For more complicated structures, sequential catalytic stamp pattern transfer is capable of transferring and overlapping Au NP assemblies with various geometries several times to form multilayered, hierarchical Au NP assemblies. Triple-layered, hierarchical Au NP assemblies composed of two lines and one square (Lw-Lw-Sq: 0.85 μm in width for each line and 5 μm in side length for square) are presented in Figure 4D. A large-area AFM topographic image (100 × 100 μm2) clearly shows each geometry and layer structures. The line widths for each microline are 0.85 ± 0.11 μm and 0.83 ± 0.13 μm, respectively. The side length for square measures 5.12 ± 0.24 μm. However, with increasing numbers of transferred layers, few patterned areas lose the structural fidelity, as shown in zoomin view image. It is still apparent that multilayered, hierarchical structures exhibit highly consistent geometries and distinguishable layers. Because of the increase in structural complexity and height difference, the WCA measures 102.7 ± 3.1° which is higher than other multi-layered hierarchical


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structures. Taking advantage of the geometry diversity of PDMS microstructure design and the layer-by-layer characteristic of sequential catalytic pattern transfer technique, complicated hierarchical Au NP assemblies can be produced based on various combinations.

Figure 4. Arrays of multilayered, hierarchical nanoparticle assemblies with various geometries fabricated using sequential catalytic stamp pattern transfer processes. Large-area plus zoom-in AFM topography images and water contact angle measurements reveal the pattern morphology and surface hydrophobicity of layered, hierarchical NP assemblies with various geometric shapes of (A) cross lines with identical widths (Lw-Lw: 0.85 μm in width for each line), (B) cross lines with different widths (Lw-Lt: 0.85 μm and 0.35 μm in width, respectively), (C) lines overlapped with squares (Lw-Sq: 0.85 μm in width for line and 5 μm in side length for square), and (D) cross


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lines overlapped with squares (Lw-Lw-Sq: 0.85 μm in width for each line and 5 μm in side length for square). The design of PDMS stamps and the numbers of pattern transfer determine the geometry and the layers of Au NP assemblies. Surface structures of Au NP assemblies are highly relative to the surface hydrophobicity and roughness. The changes in water contact angle and surface roughness were evaluated and compared for multilayered, hierarchical Au NP assemblies, as shown in Figure 5. By comparing Lw-Lw and Lw-Lt, Au NP assemblies with identical line geometry but different line width, surface roughness of Lw-Lt assemblies (12.5 ± 1.1 nm) is slightly higher than Lw-Lw assemblies (11.9 ± 1.3 nm). Such small difference in surface roughness results in difference in water contact measurements. As comparing Au NP assemblies with the same layers (Lw-Lw, Lw-Lt and Lw-Sq), Lw-Sq hierarchical assemblies exhibits higher surface roughness (22.1 ± 1.5 nm) and water contact angle (97.9 ± 2.5°) among three structures. In the condition of the same layer numbers, variation of local structures in hierarchical assemblies directly leads to changes in surface roughness as well as surface hydrophobicity. As hierarchical Au NP assemblies reach multiple layers (n ≥ 3), as Lw-Lw-Sq structures shown in Figure 4D, geometry diversity and structural complexity dominate the surface roughness of local structures. Changes in vertical and lateral dimensions of hierarchical Au NP assemblies result in local geometry complexity and a dramatic increase in surface roughness (24.7 ± 1.8 nm for Lw-Lw-Sq structures), compared to simple linear Au NP assemblies (4.1 ± 0.6 nm for Lw structures). The WCA measurement further reaches to 102.7 ± 3.2°. By analyzing the surface roughness and water contact angle, the results suggest that surface hydrophobicity of hierarchical Au NP assemblies is tunable with the critical factors of the layer number and local geometry complexity.


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Figure 5. Analysis of the effect of layer number and structure geometry on (A) water contact angle and (B) surface roughness. Applicability of Catalytic Stamp Pattern Transfer for Different Metals. Catalytic stamp pattern transfer was also applied successfully for fabricating hierarchical silver nanoparticle assemblies. A representative example presented in Figure 6 demonstrates that one-step catalytic stamp pattern transfer process enables to fabricate periodic, single-layered or multi-layered Ag NP microline structures on Si(111). The optimized concentration of Ag precursor solution was 2 %, the same as the concentration used for catalytically pattern transferring Au NP assemblies. The topographic image (60 × 100 μm2) acquired by AFM reveals large-area arrays of parallel sub-


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micron line Ag NP assemblies (Figure 6A). The AFM topography image clearly reveals nonuniformity in structure height of Ag NP assemblies. Few Ag particle aggregates and pattern defects can still be found between or on sub-micron lines. The inserted image in Figure 6A exhibits zoomin view of Ag NP microlines with few defects. The typical linewidth is 0.84 ± 0.07 μm with a center-to-center separation of 1.47 ± 0.05 μm, which corresponds to the CD masters. The height of microline arrays measures 6.3 ± 0.9 nm. Like Au NP assemblies, Ag NPs catalytically transferred onto the surface is not just Ag nanoparticles, but Ag NP-PDMS residue composites. As Ag ions gradually were reduced to form Ag NPs, PDMS residues were produced as a byproduct to partially adsorb onto Ag NPs, forming Ag NP-PDMS residue composites. The structure height measurements in Figure 6 are the apparent height of Ag NP-PDMS residue composites, instead of Ag NP height purely. Unlike Au microline arrays, catalytic stamp pattern transfer was unable to produce uniform, single-layered microlines of Ag NP assemblies on Si(111). Large Ag nanoparticle aggregates can be produced and transferred onto the surface via surface redox reaction with free residual Si-H groups on PMDS, leading to increases in surface roughness and pattern defects. Figure 6B exhbits the double-layered cross microlines of Ag NP assemblies with identical widths (Lt-Lt: 0.35 μm in width for each line). The large-area AFM topography (50 × 50 μm2) image clearly reveals structural non-uniformity of cross microline Ag NP assemblies with longrange order. Close-view AFM 3D image inserted in Figure 6B clearly exhibits the height difference between layers. Few defects and Ag NP aggregates between the cross microlines also can be found. The line width and center-to-center separation are consistent with the DVD masters. Instead of patterning Ag microlines with identical width, PDMS stamps with different line widths (Lw-Lt: 0.85 μm and 0.35 μm in width for line width, respectively) were used to fabricate the overlapped,


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cross microlines of Ag NPs (Figure 6C). The line width and center-to-center separation for each line are consistent with the CD and DVD masters. The zoom-in AFM 3D image clearly reveals that two microlines of Ag NP assembles overlapped to each other, forming double-layered cross microlines. The periodicity and width remain the same as PDMS stamps. Few nanoparticle aggregates are scattered between the cross microlines or on the surface of lower layer. Estimates of the structure height as a function of pattern transfer time is plotted in Figure 6D. Multiple representative images of different areas of the samples were analyzed to generate the estimates. The chart demonstrates quantitatively that the height of Ag microline arrays increases accordingly with increases in pattern transfer time. As the pattern transfer time increases, the variation in the structure height of Ag particle assemblies ranges from 5.6 ± 0.7 nm (6 hours) to 9.5 ± 0.9 nm (72 hours). Compared to Au particle assemblies, the structure height of Ag particle assemblies is slightly less due to less reactivity of Ag ions with free residual Si-H groups on PMDS. Numbers of Ag NP aggregates with different sizes also increase apparently with time, leading to the pattern non-uniformity. Thus, in order to avoid large Ag NP aggregates but to remain pattern integrity, the optimized transfer time for forming Ag particle assemblies measured 12 hours, experimentally. To investigate the optical property and elemental composition of Ag NPs produced during catalytic stamp pattern transfer process, the arrays of Ag NP assemblies were characterized using ultraviolet-visible spectrophotometry and X-ray photoelectron spectroscopy. UV-vis spectra were acquired for a piece of glass with microstructures of Ag NP assemblies, as shown in Figure 6E. A significant absorption peak was observed at 412 nm for Ag NPs synthesized during catalytic stamp pattern transfer. Ag NPs of 15 nm in aqueous solution were synthesized as the comparative reference. UV-vis spectra in Figure 6F exhibits a significant absorption peak at 401 nm. The red


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spectral shift observed in transferred Ag NPs is attributable to increases in Ag particle sizes as well as changes in interparticle distance as Ag NPs assemble on the confined areas of the surface. A significant peak broadening with an increase in the full width at half maximum (FWHM) also indicates particle polydispersity and large particle size of transferred Ag NPs.67 The highresolution XPS spectrum in Figure 6G exhibits the Ag 3d core level of single-layered Au NP assemblies. Two distinct lines of the Au 3d3/2 (374.3 eV) and Au 3d5/2 (368.5 eV) were observed, due to the spin-orbit splitting of the Au 3d level.68 The results demonstrate that catalytic stamp pattern transfer process is a high-throughput fabrication approach to pattern different types of NP assemblies. The transferred hierarchical nanoparticle assemblies still maintain pattern fidelity and optical property.

Figure 6. Morphological and property characterization of single-layered and two-layered line microstructures of hierarchical Ag nanoparticle assemblies fabricated using catalytic stamp pattern transfer process. (A) Large area AFM topographic image with inserted zoom-in image to reveal periodic Ag NP line microstructures. (B) Wide-area AFM image with inserted zoom-in view of Ag NP cross line microstructures with identical widths (Lt-Lt: 0.35 μm in width for each line). (C)


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Large area AFM topography with inserted zoom-in view to exhibit Ag NP cross line microstructures with different widths (Lw-Lt: 0.85 μm and 0.35 μm in width, respectively). (D) Correlation between pattern height and transfer time. (E) UV-Vis spectrum of the hierarchical Ag NP microstructures (inserted image). (F) UV-Vis spectrum of the 15 nm Ag NP solution (inserted image). (G) XPS spectrum of the hierarchical Ag NP microstructures. Hierarchical Gold Nanoparticle Assemblies for Surface-Enhanced Raman Spectroscopy Application. Preparation of plasmonic nanoparticle structures is essential for surface-enhanced Raman spectroscopy (SERS) applications.69-71 Catalytic stamp pattern transfer can be considered as a bottom-up self-assembly method to produce plasmonic nanoparticle structures with precisely controlled geometry, which can be utilized as SERS substrates. Figure 7 exhibits the Raman spectra of 4-aminothiophenol (4-ATP) using the micron-sized square arrays of Au NP assemblies on glass as a SERS substrate. Two strong characteristic peaks at 1081 cm-1 and 1588 cm-1 can be observed for the 4-ATP molecules on the micron-sized square of Au NP assemblies, which correspond to the a1 modes of the 4-ATP molecule (red arrows in Figure 7).72 A very weak peak observed at 1179 cm-1 indicates the a1 modes of the 4-ATP molecule. (green arrows in Figure 7).72 In addition to the characteristic peaks of 4-ATP, the characteristic peaks of PDMS also were identified in Raman spectra. A peak with medium intensity observed at 492 cm-1 and a very weak peak observed at 2886 cm-1 represent the Si-O-Si stretch and CH3 symmetric stretch of PDMS, respectively (black arrows in Figure 7).73 It is apparent that free residual Si-H groups on PMDS not only reduce Au ions to form Au NPs but also generate few PDMS residues as the adhesive during catalytic stamp pattern transfer. The representative Raman spectra demonstrates that the Au NP assemblies with precisely controlled geometry produced via catalytic stamp pattern transfer enable to serve as SERS substrates. However, partially adsorbed PDMS residues could cause


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chemical interferences in spectra acquisition, which might limit the utility of plasmonic NPs in other applications. To remove more surface-bound PDMS residues, oxygen plasma treatment or immersion in the dilute TBAF solution might be feasible approaches to improve the accessibility of the plasmonic NPs.

Figure 7. The Raman spectra of 4-aminothiophenol using the micron-sized square arrays of gold nanoparticle assemblies on glass (inserted image) as a SERS substrate. The red dashed line highlights SERS signal of 4-aminothiophenol. The red and green arrows in the Raman spectra represent the characteristic peaks of 4-aminothiophenol, and the black arrows indicate PDMS residues. Proposed Mechanism for Catalytic Stamp Pattern Transfer. Based on the evidence of structural and spectroscopic characterization, a proposed mechanism is deduced for formation of hierarchical nanoparticle assemblies via one-step catalytic stamp pattern transfer, as illustrated in Figure 8A. During the pattern transfer process, few tiny residues of metal precursor aqueous solution stay between the designed PDMS stamp and substrate, and form a local meniscus near the base of the PDMS structures. Free residual Si-H groups on PMDS reduce metal ions gradually to form NPs without additional reducing agents present. The contact areas between the substrate and the protrusion feature of PDMS stamp spatially confine the arrangement of NPs, guiding NPs to


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form hierarchical structures with designed geometry. Meanwhile, the surface redox reaction of free residual Si-H groups with metal ions also generate PDMS residues as the byproduct. Interestingly, the PDMS residues serve as the adhesive to connect the synthesized NPs and further to stabilize the entire layered, hierarchical NP assemblies. To prove the PDMS residues serve as the adhesive, a tetrabutylammonium fluoride (TBAF) solution (1M in THF) was to dissolve PDMS residues. After immersion in TBAF solution for 30 min, hierarchical Au NP assemblies were fully dissolved, and only few Au NPs and PDMS residues remained on surface (Figure 8B). Without the PDMS residues as the adhesive, the layered, hierarchical NP assemblies result in structural collapse. The PDMS residues only cover the NP surface partially, which allows molecules, such as 4-ATP, to adsorb on nanoparticles for SERS detection.

Figure 8. Proposed mechanism of catalytic stamp pattern transfer. (A) Schematic diagram of proposed mechanism. (B) AFM topographic images and corresponding cursor profiles reveal the structural collapse of hierarchical nanoparticle assemblies after immersion in tetrabutylammonium fluoride solution for 30 min.


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CONCLUSION The one-step catalytic stamp pattern transfer process was developed to produce arrays of hierarchical NP assemblies. The method simply combines in situ nanoparticle synthesis and liftoff pattern transfer technique to simultaneously produce hierarchical microscale nanoparticle structures. Compared to prior approaches, this method has the following intrinsic advantages: (a) simplicity of use in any laboratory; (b) no additional reducing agents required during pattern transfer process; (c) generic production of hierarchical Au or Ag NP assemblies; (d) controllability of pattern geometry and local structure complexity; (e) high pattern fidelity due to one pattern transfer step; and (f) high throughput similar to microcontact printing and lift-off pattern transfer techniques. Besides, sequential catalytic stamp pattern transfer also was developed to generate multilayered, hierarchical Au or Ag NP assemblies with various geometries. The hierarchical metal NP structures catalytically transferred onto the surface is not just metal nanoparticles, but metal NP-PDMS residue composites. The in situ synthesized Au or Ag NP still remain optical properties. The hierarchical Au NP assemblies with precisely controlled geometry produced via catalytic stamp pattern transfer enable to serve as SERS substrates. Work in progress focuses on developing SERS-based sensors utilizing the hierarchical Au NP assemblies. EXPERIMENTAL SECTION Chemical and Materials. Hydrogen tetrachloroaurate(III) trihydrate (ACS reagent), silver nitrate (ACS reagent), sodium citrate dihydrate (ACS reagent), sodium borohydride (ACS reagent), 4aminothiophenol (ACS reagent), sulfuric acid, ammonium hydroxide, hydrogen peroxide solution (30 wt. %) and tetrabutylammonium fluoride solution (1M in THF) were purchased from SigmaAldrich (St. Louis, MO, USA) and used without further purification. The silicone elastomer kits


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(PDMS, Sylgard 184) purchased from Dow Chemical (Midland, MI, USA) were used to replicate the feature from masters. Pure ethanol (ACS grade) was acquired from Pharmco-Aaper (Hurst, TX, USA). Hexane (ACS grade) was acquired from JT Baker Chemicals at Fisher Scientific (Pittsburgh, PA, USA). Hexane and ethanol were utilized to clean the PDMS stamps. Single-side polished silicon (111) wafers doped with boron (Mustec Corp., Hsinchu, Taiwan) and microscope cover glasses (Paul Marienfeld GmbH & Co., Lauda-Königshofen, Germany) were used as substrates for the catalytic stamp pattern transfer experiments. Deionized water (18 MΩ · cm, PURELAB Classic, ELGA, High Wycombe, United Kingdom) was used for rinsing samples. Catalytic Stamp Pattern Transfer Process. One-step catalytic stamp pattern transfer process begins with preparation of catalytic PDMS stamps. The commercially available masters used to generate catalytic PDMS stamps include a compact disc (Sony CD-R), a digital video disc (Sony DVD-R), and AFM calibration references (Bruker, Santa Barbara, CA, USA; Ted Pella, Redding, CA USA). By mixing Sylgard 184 curing agents at a ratio of 1:10, the mixture was poured onto the masters and then cured in the oven at 70 °C for 6 h. The PMDS stamps were cleaned with hexane and sonicated in pure ethanol for 30 min to remove any unreacted low-molecular-weight oligomers. Before use, the Si(111) and microscope cover glass substrates were cleaned by immersion in piranha solution, a mixture of sulfuric acid and hydrogen peroxide with a volume ratio of 3:1. Substrates were then rinsed copiously with deionized water and dried with nitrogen. The metal precursors used for catalytic stamp pattern transfer process were hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 ⋅ 3H2O) and silver nitrate (AgNO3). A hydrogen tetrachloroaurate(III) trihydrate solution (0.127 mM, 2 wt. %) and a silver nitrate solution (11.8 mM, 2 wt. %) were prepared in deionized water, respectively, before experiments. In a typical lift-off pattern transfer process, a 10 μL droplet of Au or Ag aqueous solution was first added to a


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clean substrate. The PDMS stamp with designed features was brought into conformal contact with the substrate surface by applying gentle pressure by hand. The gentle pressure applied was to exclude the air bubbles and excessive metal precursor solution trapped between the stamp and the substrate. A sheet of Kimwipes disposable wiper was used to accelerate the drainage of excessive metal precursor solution through void spaces of the PDMS stamp. The drainage of excessive metal precursor solution not only controls the solution quantity in the conformal contact areas but also prevents the formation of large Au or Ag NP aggregates between hierarchical NP assemblies. Afterward, the weight of PDMS stamp (1.50 ± 0.25 g for each stamp) itself became the major source of force to maintain steady pressure. No additional pressure was applied to PDMS stamp during pattern transfer process, avoiding the pattern distortion of hierarchical NP assemblies. The pattern transfer process was carried out for 6~72 hours at ambient condition. After the process is completed, the PDMS stamp was carefully peeled off. A single layer of close-packed Au or Ag NPs was simultaneously synthesized and transferred from the protruding surface of the designed PDMS stamp to the substrates. The samples were rinsed with deionized water and dried with nitrogen for further characterization. To produce multiple layers of Au or Ag NP assemblies, a droplet of Au or Ag aqueous solution (10 μL) was added to the first patterned nanoparticle assemblies. Another freshly prepared PDMS stamp was then brought into conformal contact with the surface under certain pressure. Once pattern transfer process is completed, layered, hierarchical NP assemblies with various geometric shapes were rinsed with deionized water and dried with nitrogen for further characterization. Preparation of Gold Nanoparticles (Au NPs) and Silver Nanoparticles (Ag NPs). Gold and silver nanoparticles were synthesized according to previously reported procedures.67,

74

For

synthesis of Au NPs, a solution of hydrogen tetrachloroaurate(III) trihydrate (0.25 mM in


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deionized water, 50 mL) was prepared and added to a 100 mL round-bottom flask with a magnetic stir bar on a stirring hot plate. The solution was heated to a rolling boil. To the rapidly-stirred boiling solution, 1.5 mL of a sodium citrate dihydrate solution (38.8 mM in deionized water) was quickly added. The solution color began to change from pale yellow to red. The solution continued heating for 10 min and then was removed from heat when the solution color has turned deep red. Ag NPs were synthesized using sodium borohydride as the primary reductant and sodium citrate as the secondary reductant as well as a stabilizing agent. A 100 mL round-bottom flask containing 19 mL deionized water and a magnetic stir bar was placed on a stirring hot plate and stirred rapidly. A silver nitrate aqueous solution (0.01 M, 0.5 mL), a sodium citrate dihydrate aqueous solution (0.01 M, 0.5 mL) and a sodium borohydride aqueous solution (0.05 M, 0.12 mL) were slowly added to the round-bottom flask in order, with the rate of one drop per 1 sec. A silver nitrate and sodium citrate dihydrate mixture solution revealed colorless. As the first drop of sodium borohydride aqueous solution was added, the solution color began to change from colorless to pale yellow, indicating that Ag NPs were formed. The solution was kept in the dark with vigorous stirring for 2 hours. Morphological Characterization of Hierarchical Nanoparticle Assemblies. Pattern morphology analysis and local height measurements of hierarchical NP assemblies were performed using BioScope Catalyst and MultiMode 8 atomic force microscopes (AFM) with Nanoscope-V controller (Bruker, Santa Barbara, CA, USA) at ambient conditions. All images were acquired using tapping mode in air with a typical force less than 1 nN. Rectangular silicon cantilevers with a sharpened probe (tip radius of less than 7 nm), force constant of 7.4 N/m and an average resonance frequency of 160 kHz (PPP-NCSTR, Nanosensors, Neuchatel, Switzerland) were used for the AFM imaging and structure height measurements. Digital images, cursor profile,


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and pattern morphology analysis were processed with NanoScope Analysis (version 1.9, Bruker, Santa Barbara, CA, USA) and Gwyddion, an open source software which is freely available on the internet and supported by the Czech Metrology Institute. In addition to AFM, morphology of hierarchical NP assemblies was also confirmed with the ultra-high resolution scanning electron microscope (HR-SEM, SU8000, Hitachi, Japan). Composition and Optical Property Analysis of Hierarchical Nanoparticle Assemblies. To identify chemical composition and physical property of hierarchical NP assemblies, the analysis was carried out using ultraviolet-visible spectroscopy (UV-Vis), X-ray photoelectron spectroscopy (XPS), water contact angle (WCA) measurements and Raman spectroscopy. Hierarchical Au/Ag NP assemblies were produced on microscope cover glasses or silicon wafer. UV-Vis spectra of the hierarchical Au or Ag NP assemblies on microscope cover glasses and Au or Ag NPs solutions were recorded with a UV-Vis spectrophotometer (U-0080D Hitachi, Tokyo, Japan). The chemical composition and states of hierarchical Au or Ag NP assemblies were characterized by XPS (PHI 5000 VersaProbe, ULVAC-PHI, Inc., Japan). Equilibrium WCA measurements were recorded on hierarchical Au or Ag NP assemblies by using a DSA100 contact angle meter (KRÜSS GmbH, Hamburg, Germany) at room temperature. All of the WCA measurements (n ≥ 5 per sample) were obtained using water droplets (10 mL per drop) at different locations on each sample surface. 4Aminothiophenol (4-ATP) was used as a Raman standard to evaluate the feasibility of using hierarchical Au NP assemblies for SERS application. A 20 μL droplet of 50 mM 4-ATP in ethanol was added onto hierarchical Au NP assemblies on glass and dried slowly inside a covered petri dish at ambient condition as the sample for the following Raman experiments. Raman spectra were recorded by a iHR550 Raman spectrometer (Horiba, Japan) connected with an inverted microscope (Olympus IX73, Japan) and a HeNe laser (2 mW, excitation wavelength of 632.8 nm). The data


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acquisition time was 10 s per spectrum. The sample stage position and the optical focus were adjusted manually to probe different positions on each sample surface during the course of the measurement. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Jie-Ren Li: 0000-0001-8336-0092 Author Contributions T. Y. H. and J. A. C. L. designed and conducted the pattern transfer experiments and sample characterization. W. H. L. contributed to the Raman experiments. J. R. L. wrote the manuscript. T. Y. H., J. A. C. L. and W. H. L. contributed to the discussion and commented on the manuscript. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge support from Ministry of Science and Technology (MOST 104-2113-M-006-006-MY2), Taiwan. The authors also thank the Instrument Center of NCKU for technical assistance in acquisition of XPS spectrum and SEM images. REFERENCES


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Hierarchical Nanoparticle Assemblies Formed via ... - ACS Publications

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