pubs.acs.org/Langmuir © 2009 American Chemical Society
Convective Assembly and Dry Transfer of Nanoparticles Using Hydrophobic/Hydrophilic Monolayer Templates Nam-Goo Cha,† Yolanda Echegoyen,† Tae-Hoon Kim,† Jin-Goo Park,‡ and Ahmed A. Busnaina*,† †
NSF Nanoscale Science and Engineering Center for High-Rate Nanomanufacturing, Northeastern University, Boston, Massachusetts 02115, and ‡Department of Bio-Nano Engineering, Hanyang University, Ansan 426-791, Korea Received April 27, 2009. Revised Manuscript Received June 16, 2009
A convective directed-assembly process on a flat substrate that does not require motion and is followed by a drytransfer process of nanoparticles is presented. The convective assembly process was achieved using Au nanoparticles on hydrophobic/hydrophilic-surface-patterned Si substrates as functions of temperature, gap height, and particle size. An investigation of the particle assembly mechanism showed that the effects of temperature, gap height, and particle size were responsible for controlling the evaporation time, the evaporation length, and the assembly speed, respectively. To ensure conformal contact during the dry-transfer process, chemically patterned hybrid templates with elastic and flexible properties were fabricated and used. The hybrid templates provided conformal contact with a target silicon substrate coated with MPTMS (3-mercaptopropyltrimethoxysilane) and successfully transferred Au particles to the target substrates.
I. Introduction Nanoparticles have many interesting properties, which are difficult to attain in bulk materials. To take advantage of these special properties for future devices, precise particle assembly and transfer techniques need to be developed. Conventional top-down fabrication methods could be time-consuming and unsuitable for directly handling nanoparticles. A bottom-up approach, which can be defined as the autonomous organization of objects into ordered structures, is an efficient approach to ordering large numbers of small particles on surfaces.1 The approach relies on the interaction forces between particles and/or particles and surfaces to drive the formation of ordered arrangements. By using ordered metal nanoparticle layers, many innovative applications have been reported such as dye-sensitized solar cell,2 single-electron transistors,3 biosensor,4 and so forth. Convective assembly is one of the simplest self-assembly methods for the fabrication of ordered particle structures. The process can be scaled up to produce ordered coatings on large areas. Convective assembly can be utilized on wetted substrates with contact angles below 20°. The assembly mechanism is based on the convective flow of a colloidal suspension induced by evaporation and lateral capillary forces between particles. In a hydrophilic area, the assembly starts when the thickness of the solvent layer becomes equal to the particle diameter, and the lateral capillary force helps to drive the particles to make close and dense patterns. In a hydrophobic area, however, the thickness of the solvent layer never approaches the particle diameter, and the *Corresponding author. E-mail:
[email protected]. (1) Mijatovic, D.; Eijkel, J. C. T.; Berg, A. Lab Chip 2005, 5, 492–500. (2) He, J. A.; Mosurkal, R.; Samuelson, L. A.; Li, L.; Kumar, J. Langmuir 2003, 19, 2169–2174. (3) Feldheim, D. L.; Keating, C. D. Chem. Soc. Rev. 1998, 27, 1–12. (4) Liu, J.; Lu, Y. J. Am. Chem. Soc. 2003, 125, 6642–6643. (5) Malaquin, L.; Kraus, T.; Schmid, H.; Delamarche, E.; Wolf, H. Langmuir 2007, 23, 11513–21. (6) Kralchevsky, P. A.; Denkov, N. D. Curr. Opin. Colloid Interface Sci. 2001, 6, 383–401. (7) Qin, D.; Xia, Y.; Xu, B.; Yang, H.; Zhu, C. Adv. Mater. 1999, 11, 1433–1437.
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horizontal force produced by the liquid meniscus prevents particles from depositing and forming patterns (Figure 1).5-9 The assembly of nanoparticles with a diameter of 10 nm has been achieved on a patterned substrate prepared by top-down approaches.10,11 Following the assembly, however, the transfer of assembled nanoparticles in trench templates is not easy because the particles are trapped inside the trenches, preventing particle contacting with a target surface. Therefore, an assembly process on a flat, patterned surface without using trenches or other structures is very useful in transferring nanoelements. Recently, nanoparticle printing with single-particle resolution was reported.12 However, this method utilized physically patterned PDMS (polydimethylsiloxane) substrate and a mechanical stage with a stepping motor to move the liquid meniscus of colloidal suspensions. It also utilized a plasma treatment to modify the surface properties of PDMS from hydrophobic to hydrophilic and is a relatively slow process. For the transfer process, the approach utilized a thin PMMA (polymethyl methacrylate) as an adhesion layer, which was removed by oxygen plasma. In this article, we investigated a method for convective assembly without moving parts and utilizing a Si-compliant template, which is patterned by using self-assembled monolayers (SAM). After Au particles are assembled on hybrid templates with elastic and flexible properties, a dry-transfer process is conducted on SAM-treated Si substrates.
II. Experiments 1. Fabrication of a Hydrophobic/Hydrophilic Si Template for Convective Assembly. The hydrophobic/hydrophilicpatterned Si templates for convective assembly were fabricated as (8) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Nature 1993, 361, 26–26. (9) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183–3190. (10) Xiong, X.; Makaram, P.; Busnaina, A.; Bakhtari, K.; Somu, S.; McGruer, N.; Park, J. Appl. Phys. Lett. 2006, 89, 193108. (11) Boussert, B.; Alivisatos, A. P. Nano Lett. 2004, 4, 1093–1098. (12) Kraus, T.; Malaquin, L.; Schmid, H.; Riess, W.; Spencer, N. D.; Wolf, H. Nat. Nanotechnol. 2007, 2, 570–576.
Published on Web 07/16/2009
DOI: 10.1021/la901496s
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Figure 1. Schematic of the convective assembly on hydrophilic (top) and hydrophobic areas (bottom). The primary driving force for the convective assembly of nanoparticles is liquid evaporation. Convective assembly proceeds at a receding contact angle lower than 20° on a flat surface.5 Sufficient evaporation lengths (l) helps particles to move a longer distance and to have enough time for the lateral capillary force to attach particles to each other.
follows. Si wafers were cut into 20 mm 20 mm chips for experiments. Coupon wafers were cleaned using piranha solution (4:1 H2SO4/H2O2) for 10 min and rinsed with D.I. (deionized) water. A positive photoresist (Shipley 1805, 0.5 μm thickness) was spin-coated at 4000 rpm for 30 s and prebaked at 115 °C for 1 min. It was exposed to UV aligner (Quintel 4000, Neutronix) for 6 s with a photomask with a 3 μm line and 9 μm spacing patterns. After exposure, the wafers were developed (AZ-300 MIF), rinsed, and dried with nitrogen. A hydrophobic coating, FOTS (tridecafluoro-l,l,2,2-tetrahydrooctyltrichlorosilane, Sigma-Aldrich) was used as a precursor. The hydrophobic area was created using a vapor SAM method with a modified vacuum oven (VO11, Jeio Tech, Korea).13 A typical hydrophobic coating was achieved by an injection of 0.5 mL of FOTS into the chamber, keeping it at 10 mTorr and 150 °C for 30 min. After the hydrophobic coating, the photoresist was removed using acetone. The chips were cleaned with piranha solution for 5 min to ensure the hydrophilicity of the patterned area before conducting convective assembly. 2. Fabrication of Hybrid Templates for Dry Transfer. An elastic, flexible hybrid template was fabricated for the dry-transfer process. First, PDMS (Sylgard 184, Dow Corning) diluted with toluene (1:10) was spin-coated at 2000 rpm for 45 s on flexible, thin glass substrates (22 mm22 mm, 150 μm thickness) to make an elastic layer. It was cured using a hot plate at 115 °C for 5 min, and the measured PDMS thickness was around 35 μm. A thin Si layer (30 nm) was deposited on the PDMS layer by using a plasma sputterer (Perkin-Elmer 2400, Perkin-Elmer). The silicon layer was oxidized by dipping it in H2O2 at room temperature for 30 min. After oxidization, the hydrophobic/hydrophilic patterns were prepared and the Au particles were assembled using the approach described in section II.3. Nanoparticles were (13) Cha, , N.-G.; Park, , J.-G. In Nanomanufacturing Handbook; Busnaina, A., Ed.; CRC Press/Taylor & Francis: Boca Raton, FL, 2006; p 183.
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transferred via a dry process from the template to a 20 mm20 mm silicon chip coated with MPTMS (3-mercaptopropyltrimethoxysilane, Sigma-Aldrich). The MPTMS was deposited on Si wafers using a vapor SAM approach under the same conditions as for FOTS. After Au nanoparticle assembly on a hybrid template, it was sandwiched with an MPTMS-coated Si wafer where pressure and temperature are applied. The dry-transfer process was conducted using a nanoimprint tool (NX2000, Nanonex) at 100 psi and 150 °C for 5 min and separation at room temperature (Figure 2). 3. Convective Assembly Process. Different aqueous suspensions of gold nanoparticles (5, 10, 20, 50, and 100 nm diameter, 5.6 109 particles/mL) were used (British Biocell International, U.K.) All experiments were carried out with the original concentration of the suspension. A 50 μL portion of the Au nanoparticle solution was deposited on the hydrophobic/ hydrophilic-patterned substrate using a micropipet and was covered with a hydrophobic top-glass using different gap heights (t). The cover top-glass thickness was 1 mm in order to prevent bending during evaporation. The top-glass cover was supported by glass spacers used to control the gap. The spacers were positioned at the edge of the top-glass cover and bonded with instant adhesive. This hydrophobic top glass cover was functionalized using the vapor SAM method with FOTS. The gap height between the hydrophobic glass and the Si substrate was controlled using 150-, 300-, and 450((20)-μm-thick spacers. The sandwiched substrates were dried at 30, 50, and 70 °C while maintaining less than 20% relative humidity (RH). The diameters of the circular water layer (D) covered with a top-glass were calculated by using the volume of liquid and the gap height. Calculated diameters for 150, 300, and 450 μm gap heights with a 50 μL Au suspension were 20.6, 14.5, and 11.9 mm, respectively. Figure 3 shows a schematic of the experimental procedure for convective assembly on hydrophobic/hydrophilic-patterned substrates. 4. Characterization. The surface contact angle variation was measured before and after coating by a contact angle analyzer (Phoenix 300, SEO, Korea) using DI water. Advancing and receding angles were obtained by increasing or decreasing the drop volume until the three-phase boundary moved over the surface.14 The thicknesses of the sputtered Si and the vapor-SAMdeposited monolayer were measured by AFM (XE150, PSIA, Korea) and an ellipsometer (M2000, J. A. Woollam), respectively. After convective assembly, the samples were imaged using SEM (JSM-6360, JEOL, Japan) and AFM.
III. Results and Discussion 1. Convective Assembly. The mechanism of convective assembly using a colloidal suspension droplet has been described in several papers.15-17 The primary driving force for the convective assembly of nanoparticles is liquid evaporation. The evaporation creates a continuous flow from the bulk to the thin film of liquid in the hydrophilic area. This continuous flow carries the suspended particles toward the surface where they assemble on the hydrophilic areas. The assembly process starts when the thickness of the (14) Drelich, J.; Miller, J. D.; Good, R. J. J. Colloid Interface Sci. 1996, 179, 37– 50. (15) Prevo, B. G.; Velev, O. D. Langmuir 2004, 20, 2099–2107. (16) Kim, M. H.; Im, S. H.; Park, O. O. Adv. Funct. Mater. 2005, 15, 1329–1335. (17) Ormonde, A. D.; Hicks, E. C.; Castillo, J.; Van Duyne, R. P. Langmuir 2004, 20, 6927–6931.
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Figure 2. Schematic of the experimental procedure for dry transfer using an elastic, flexible hybrid template.
Figure 3. Schematic of the experimental procedure for convective assembly on hydrophobic/hydrophilic-patterned substrates.
liquid layer is less than or equal to the particle diameter at the contact line.18 Particles are assembled to make arrays or (18) Adachi, E.; Dimitrov, A. S.; Nagayama, K. Langmuir 1995, 11, 1057–1060.
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layers during liquid evaporation after the formation of the contact line by the lateral capillary force between particles, which is influenced by the vapor pressure, liquid pressure, and surface tension along the three-phase contact line on the DOI: 10.1021/la901496s
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particles.19 The lateral capillary force between the particles is one of the key driving forces in the self-assembly of nanoparticles. It is reported that the viscosity of water in glass capillaries of 80 nm diameter is elevated by approximately 40%.20,21 If the withdrawal velocity of the liquid is equal to the particle assembly speed, then a continuous, homogeneous monolayer will grow. However, the high contact angles in the hydrophobic area do not allow the thin liquid film to form and the convective flow is limited. Consequently, no deposition occurs in the hydrophobic area. For steady-state assembly, a simple equation for balancing the volumetric fluxes of the liquid and assembly of particles was proposed by Dimitrov and Nagayama.22 vc ¼
βje lφ hð1 -εÞð1 -φÞ
ð1Þ
Equation 1 shows the relationship among the growth velocity of the layer, vc, the local evaporation rate, je, the evaporation length, l, the particle volume fraction, φ, the thickness of the particle array, h, and the porosity of the array, ε, where β (0
