Self-Assembly of Nanowires at - Stevens Institute of Technology

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Nanoscience and Nanotechnology Letters Vol. 2, 150–156, 2010

Self-Assembly of Nanowires at Three-Phase Contact Lines on Superhydrophobic Surfaces Yao-Tsan Tsai, Wei Xu, Eui-Hyeok Yang, and Chang-Hwan Choi∗ Department of Mechanical Engineering, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, NJ 07030, USA This paper reports on a novel self-assembly method of nanowires using a superhydrophobic surface as a template. Well-defined superhydrophobic structures on a template surface can configure three-phase (liquid–solid–gas) contact lines at the structures’ tips and direct the site-specific self-assembly of nanowires when the colloidal droplet of nanowires recedes in evaporation. Highaspect-ratio microstructures with tapered tips were fabricatedto: by deep reactive ion etching (DRIE) Delivered by Ingenta and coated with a thin layer Stevens of Teflon for hydrophobicity. Nickel nanowires were synthesized by Institute of Technology electrodeposition through a porous alumina membrane. A uniformly dispersed nanowire suspenIP : 71.187.86.132 sion was dispensed and evaporated on the superhydrophobic template surface at normal room Wed, 05 Jan 2011 15:41:14 conditions. After complete evaporation, the assembly of nanowires on the tip structures was measured by microscopy. The results show that nanowires are mostly deposited on the structural tips because the air layer retained between the hydrophobic surface structures prevent the liquid meniscus from reaching to the bottom trenches during evaporation. The assembly rate on each tip and the alignment tendency along the surface pattern vary depending on the template surface parameters and the nanowires colloidal states, requiring further systematic studies on the effects. It is envisioned that well-tailored superhydrophobic surfaces can serve as a novel template for highly-ordered and site-specific self-assembly of functional nanomaterials in simple drying processes, significantly enhancing the capability to realize future nanomaterial-based devices and systems.

Keywords: Nanowires, Self-Assembly, Evaporation, Superhydrophobic.

1. INTRODUCTION Low-dimensional materials such as carbon nanotubes and metal/metal-oxide nanowires are promising building blocks for nanostructures and nanodevices with low power consumption, and superior electrical and mechanical properties. Practical applications, e.g., nanoelectronics, require a precise arrangement of such nanomaterials into hierarchical orders to construct the desired geometry with controllable shape, location, and direction on a large scale.1 Several techniques of nanomaterial assembly have been explored: biomolecule patterning2 and microscale contact stamping3 have high assembly resolution, but only limited biomaterials or polymers can be used; magnetic field,4 5 electric field,6 and dielectrophoresis7 enable a high throughput assembly, however the arrangement is often limited to one direction along the gradient of the field. The exploration of more efficient assembly techniques is demanded. Extended from previous studies,8 this paper ∗

Author to whom correspondence should be addressed.

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reports on a new self-assembly mechanism for nanowires based on interfacial phenomena at the three-phase contact lines configured on a micro-patterned superhydrophobic template surface.

2. ASSEMBLY PROCESS The basic concept of the self-assembly process studied in the paper is illustrated in Figure 1. A colloidal droplet containing well-dispersed nanowires is dispensed on a structured superhydrophobic template surface (Fig. 1(a)). Due to low surface tension and de-wetting of hydrophobic structures, the colloidal droplet sits on the top ridges of microstructures with an air layer trapped underneath the droplet.9–11 As the droplet evaporates, the three-phase contact line recedes and drags the nanowires along the contact line. When the dragged nanowires contact the structural tips, the adhesion and capillary forces between the nanowires and the tip surfaces will retard the movement of the nanowires. If they are greater than the interfacial drag force to the receding direction, the nanowires 1941-4900/2010/2/150/007

doi:10.1166/nnl.2010.1073

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Self-Assembly of Nanowires at Three-Phase Contact Lines on Superhydrophobic Surfaces

(a) Nanowires

W

Water

L

Air Hydrophobic Microstructure

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(b) Receding direction of droplet P Fig. 2. Surface parameters controlled for tests.

Water meniscus hanging between ridges

Delivered by Ingenta to: reactive ion etching (DRIE). A 4-inch silicon wafer was Stevens Institute of Technology first cleaned with acetone, isopropyl alcohol, and deionIP : 71.187.86.132 ized (DI) water and then dehydrated for 10 minutes Wed, 05 Jan 2011 15:41:14

Aligned nanowires (c)

Fig. 1. Evaporative self-assembly process of nanowires on a superhydrophobic template surface: (a) The suspension of nanowires is dispensed on a superhydrophobic surface. (b) Nanowires are dragged, aligned, and assembled on each tip of the surface structures when the three-phase contact line recedes in evaporation. (c) Site-specific self-assembly of nanowires is achieved on the structure tips after a complete evaporation.

will be separated from the droplet and remain on the tip surface, attaining self-ordered assembly (Fig. 1(b)). The self-assembly process will continue until the droplet completely evaporates, allowing site-specific self-alignment onto the structural ridges (Fig. 1(c)). The superhydrophobic template structures can be engineered and constructed for specific directions and patterns for desired assembly.

on a hot plate at 150 C. Adhesion promoter, Priming HMDS (KEM Chemical), was spin-coated at 2000 rpm for 1 minute and followed by spin-coating of SPR 3012 photoresist (PR) (Shipley) at 2000 rpm for 1 minute. The substrate was soft-baked on a hot plate at 115 C for 1 minute and UV-exposed by using MA-6 mask aligner (Suss MicroTec). The substrate was then developed by MF-319 (Shipley). The patterned PR thickness was measured by a profilometer to be 2.2 m. The PR-patterned silicon substrate was then etched by DRIE by using an Inductively Coupled Plasma (ICP) Etcher (Oxford Plasmalab). Figure 3 shows the scanning electron microscopy (SEM) images of the fabricated microstructures (Type I). In addition to the original microstructures, nanograss structures were also created on the bottom trench by controlling the etching parameters of DRIE. The nanograss structures formed by the so-called black silicon method12–14 is to provide the surface with more robust superhydrophobicity (i.e., de-wetting stability) by preventing the liquid meniscus from filling the air voids even though the meniscus touches the bottom trench surface due to the increased Laplace pressure of the droplet during evaporation.15–17 After fabricating the microstructures, a thin layer of Teflon was coated to make the surface superhydrophobic. Table I.

3. FABRICATION OF SUPERHYDROPHOBIC SURFACES For the superhydrophobic template surface structures, high-aspect-ratio microstructures of varying surface parameters (see Fig. 2 and Table I) were created on a silicon substrate by photolithography followed by deep Nanosci. Nanotechnol. Lett. 2, 150–156, 2010

Type Type Type Type Type

I II III IV V

Surface morphology of microstructures tested for experiments. G (m)

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W (m)

40 20 20 10 20

20 20 40 10 20

40 20 20 40 10

10 10 10 10 10

2 2 2 2 2

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Self-Assembly of Nanowires at Three-Phase Contact Lines on Superhydrophobic Surfaces (a)

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θ0 = 123°

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Fig. 4. Contact Delivered by Ingenta to: angle of a water droplet on a planar (a) and a microstructured template surface (Type II) (b) after a hydrophobic coatStevens Institute of Technology ing with Teflon. IP : 71.187.86.132 Wed, 05 Jan 2011 15:41:14

(Fig. 4(b)) and indicates that the fabricated microstructures support the composite Cassie (de-wetted) state. Fig. 3. SEM micrographs of high-aspect-ratio microstructures (a) and nano-grass structures on the bottom surface (b: a dotted square area in (a)).

Teflon solution was prepared by dissolving 0.2 wt% of Teflon AF1600 powder (DuPont) in FC-75 (DuPont) solvent. After rolling agitation for 4 days and filtered by a Millipore filter (SLGV033RS, 0.22 m), Teflon solution was spin-coated on the microstructured surface at 1000 rpm for 30 seconds and baked at 112 C, 165 C, and 330 C for 10, 5, and 15 minutes, respectively, to give the film thickness of ∼20 nm. Figure 4 shows the contact angles of a droplet of water on a planar surface (control) and the microstructured template surface (Type II) after the Teflon coating. A theoretical contact angle of a liquid droplet on a superhydrophobic surface with a composite interface (i.e., solid and air) can be estimated by the Cassie-Baxter equation,18 cos C = f cos 0 + f − 1

(1)

where C , 0 , and f are the apparent contact angle on a composite surface, referential contact angle on a planar surface, and area fraction of a wetted solid surface, respectively. The wetting fraction of the microstructured superhydrophobic surfaces (Type II) was estimated to be f = 01, and the referential contact angle of Teflon on a planar surface was 0 = 123 (Fig. 4(a)). The theoretical contact angle on such conditions is C = 162 by the Cassie-Baxter Eq. (1), which agrees well with our measurement (∼160 ) 152

4. SYNTHESIS OF NANOWIRES Nickel (Ni) nanowires were grown by electrochemical deposition through a nanoporous anodized alumina membrane.19–22 A silver layer was thermally evaporated on one side of the alumina template membrane (Anodisc, Waterman Inc), serving as a seed layer. Ni nanowires were then electrodeposited from a nickel electroplating solution of 300 g/l NiSO4 · 6H2 O, 45 g/l NiCl2 · 6H2 O, and 45 g/l H3 BO3 at a constant current density of 2.0 mA/cm2 with a 263A galvanostat (Princeton Applied Research, Oak Ridge, TN).23 After the electrodeposition, the silver seed layer was removed by concentrated nitric acid. The alumina template membrane was selectively removed by dissolving in a 5 M NaOH solution. The nanowires were subsequently washed and transferred to DI water. Figure 5 shows the electrodeposited Ni nanowires fabricated using the anodized alumina membrane as a scaffold. Ni nanowires of 5, 10, and 20 m lengths and 200 nm diameter were synthesized and tested in this study. To make a well-dispersed suspension, the nanowires suspension was agitated by sonication (ultrasonic bath) for 30 minutes before the assembly test on the superhydrophobic template surfaces.

5. RESULTS AND DISCUSSION A planar hydrophobic and a microstructured uncoated (i.e., hydrophilic) surfaces were first tested as controls Nanosci. Nanotechnol. Lett. 2, 150–156, 2010

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The current results demonstrate that well-tailored superhydrophobic template surfaces can direct the self-assembly of nanomaterials in a simple drying process by exploiting the unique three-phase contact line dynamics on the hydrophobic structures. However, there are several challenges that remain to be overcome. The first issue is the assembly yield and the controllability of the assembly. Our current results show that only ∼10% of the surface structures are deposited with nanowires. Furthermore, the assembly rate was not uniform over the surface, i.e., higher assembly population at the initial contact boundary region. The low yield rate with non-uniform deposition suggests that the number density assembled on

(a)

(b) (a)

20 µm

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Fig. 5. SEM micrographs of Ni nanowires: (a) Nanowires in a porous anodized alumina membrane (AAO). (b) Nanowires randomly dispersed on a bare substrate after removing the membrane.

for comparison. When the colloidal droplet of nanowires evaporated on the hydrophobic planar or the hydrophilic microstructured surfaces, the nanowires were randomly distributed over the surfaces.8 While some nanowires were collected around the structural tips on the hydrophilic microstructured surface, no significant alignment along the structural pattern was measured. Instead, the majority of nanowires were accumulated on the bottom trench surface in an irregular manner. The hydrophilic microstructured surfaces exhibited little control of the assembly and alignment of the nanowires since the nanowire suspension wetted the entire surface of structures. In contrast, a wellaligned assembly of nanowires on the structural tips was observed on the microstructured superhydrophobic template surface (Fig. 6). The nanowire suspension was levitated over the bottom surface by the tips of hydrophobic surface structures. Therefore, the nanowires were floating along the liquid meniscus, and selectively collected and aligned onto the top ridge of the microstructures as the three-phase contact line of the droplet receded in evaporation. Due to the increased surface/volume ratio in small (i.e., micro and nano) scales, interfacial forces are more dominant than body forces so that inertial effects such as gravity play no significant role in this process. Nanosci. Nanotechnol. Lett. 2, 150–156, 2010

(b)

20 µm

(c) 10 µm

Fig. 6. (a–b) Optical micrograph images of a boundary of a drying colloidal droplet of nanowires on a superhydrophobic template surface (Type III). It shows the three-phase contact line receding from the outer structure to inner one in evaporation. Arrows indicate the nanowire placed and aligned on the structural tips by the receding three-phase contact line. (c) Self-assembled nanowires after complete evaporation.

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the structural tips varies so that some tips have multiple (i.e., large P and G), the air layer disappeared and the surface became totally wetted before the evaporation comnanowires deposited onto them, while others have none. pleted. Figures 7(a and b) show the optical micrograph The assembly of nanowires on the tips is determined by images taken at the same location of the sample Type IV interfacial forces between nanowires, solvent, and contact (P = 10 m, G = 10 m) with the focus on the tip surface surface. The movement of liquid meniscus (i.e., kinetics (Fig. 7(a)) and the bottom trench (Fig. 7(b)), respectively. of three-phase contact line) can be controlled by drying Figures 7(a and b) clearly show that the nanowires are conditions such as ambient pressure, humidity, and temdeposited only on the structural tips and no nanowires are perature so as to manipulate the balance of the interfacial found at the bottom trench. The tested nanowires (20 m) forces.24–27 The deposition patterns and uniformity are also are longer than the structural gaps (P = 10 m, G = influenced by the gradients of evaporation rate and interfa10 m) and they are deposited onto multiple surface struccial tension along the droplet surface,24 25 28–30 which also tures bridging them. In contrast, Figures 7(c and d) show affect the convective flow inside of the droplet and the disthe optical micrograph images taken at the same location tribution of nanowires. Hence, further systematic studies of the sample Type V (P = 20 m, G = 20 m) with of the three-phase contact line dynamics and the dynamthe focus on the tip surface (Fig. 7(c)) and the bottom ics of nanoparticles inside of the droplet are necessary in trench (Fig. 7(d)), respectively. Figures 7(c and d) show varying evaporation conditions to understand how to reguthat few nanowires are placed on the tip surface and most late the deposition coverage and uniformity to have better nanowires sink down to the bottom trench. This result indiyield rate and controllability of the self-assembly. cates thatto: the dimensions of surface pattern are important The second issue is to maintain the de-wetted (airDelivered by Ingenta design parameters retained) state throughout the entire evaporation stage. Stevens Institute of Technology for the durable and robust self-assembly process of the colloidal droplet evaporation on superhyWhen a droplet evaporates, its radius of curvature IP : 71.187.86.132 drophobic template surfaces, e.g., a denser surface pattern decreases and thus increases the internal Laplace pressure Wed, 05 Jan 2011 15:41:14 will provide better stability and applicability.31–33 of the droplet. If the Laplace pressure exceeds a threshold If such issues are resolved through further extenpressure where the surface tension force of the structure sive studies, the new self-assembly approach explored can balance, the liquid meniscus collapses by the excessive in this study possesses several advantages than convenpressure force and the surface becomes wetted and thus tional assembly techniques, e.g., soft-lithography based loses air. In this study, it was observed that on denser strucmicrocontact printing.34 Instead of planar contact printtures (i.e., small P and G in Fig. 2), the de-wetted state was ing where the control of deposition density and selectivity maintained over the evaporation, while on wider structures (a)

(c)

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(b)

30 µm

(d)

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30 µm

Fig. 7. Optical micrograph images of nanowires deposited on superhydrophobic template surfaces, Type IV (a–b), and Type V (c–d). In (a) and (c), an optical focus was adjusted to the tip surface, and at the bottom trench in (b) and (d).

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is not efficiently attainable, discrete droplets can be 3. E. Menard, M. A. Meitl, Y. Sun, J. U. Park, D. J. L. Shir, Y. S. Nam, S. Jeon, and J. A. Rogers, Chem. Rev. 107, 1117 selectively dispensed on arbitrary locations of the template (2007). surfaces with regulated droplet sizes and nanowire com4. C. M. Hangarter, Y. Rheem, B. Yoo, E. H. Yang, and N. V. Myung, positions (e.g., sizes and concentrations), which will allow Nanotechnology 18, 205305 (2007). great flexibility of pattern design and device architectures. 5. Y. Rheem, C. M. Hangarter, E. H. Yang, D. Y. Park, N. V. It has recently been demonstrated that the distribution and Myung, and B. Yoo, IEEE Transactions on Nanotechnology 7, 251 (2008). deposition of nanomaterials on superhydrophobic surfaces 6. M. Senthil Kumar, T. H. Kim, S. H. Lee, S. M. Song, J. W. can be manipulated by control of surface patterns and the Yang, K. S. Nahm, and E. K. Suh, Chem. Phys. Lett. 383, 235 35–38 Hence, in addisizes/concentrations of nanoparticles. (2004). tion to the evaporative conditions such as humidity and 7. H. W. Seo, C. S. Han, D. G. Choi, K. S. Kim, and Y. H. Lee, temperature, the physical/mechanical/chemical conditions Microelectron. Eng. 81, 83 (2005). of the template surface structures as well as the nanowire 8. Y. T. Tsai, W. Xu, E.-H. Yang, and C.-H. Choi, Interfacial-tensiondirected self-assembly of nanowires on superhydrophobic surfaces, compositions in the colloidal droplet can be conveniently Proceedings of the ASME International Mechanical Engineering modulated to allow greater controllability in the selfCongress and Exposition Boston, MA, November (2008). assembly process. The droplet-based drying process also 9. D. Quere, Nat. Mater. 1, 14 (2002). allows multiple layer-by-layer depositions. After the first 10. D. Quere, Annual Review of Materials Research 38, 71 layer of assembly is made, the second layer assembly can (2008). 11. M. Nosonovsky and B. Bhushan, Curr. Opin. Colloid Interface Sci. be applied on top of the first layer by dispensing new col14, 270 (2009). loidal droplets with a variety of nanomaterial compositions Delivered by Ingenta to: 12. H. Jansen, M. De Boer, R. Legtenberg, and M. Elwenspoek, Journal and evaporation modes. As such layer-by-layer assembly is of Technology Stevens Institute of Micromechanics and Microengineering 5, 115 (1995). repeated, the high-aspect-ratio 3-D nanostructuresIPof: 71.187.86.132 cus13. H. Jansen, M. de Boer, J. Burger, R. Legtenberg, and tomized nanomaterial compositions and functionalities can 2011 15:41:14 M. Elwenspoek, Microelectron. Eng. 27, 475 (1995). Wed, 05 Jan also be achieved along the vertical direction. 14. H. Jansen, M. De Boer, H. Wensink, B. Kloeck, and M. Elwenspoek,

6. CONCLUSIONS In this study, we explored a new nanowire self-assembly mechanism based on interfacial phenomena configurable at the three-phase contact line of a superhydrophobic surface. A superhydrophobic template surface enabled the site-specific self-assembly of nanowires on the structural tips as a colloidal droplet of nanowires receded in a simple evaporation process. The primary understanding acquired from this study provides an insight into the essential design parameters of superhydrophobic template surfaces for an efficient self-assembly. An optimization of superhydrophobic template surfaces and evaporative processes will enable successful development of a simple and efficient self-assembly scheme for a wide range of functional nanomaterials. Acknowledgments: The authors would like to thank Johanna Heureaux for her assistance. This work has been partially supported by Stevens Startup Funds. In addition, the research has been carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DEAC02-98CH10886.

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Received: 15 April 2010; Accepted: 4 June 2010

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