pubs.acs.org/Langmuir © 2010 American Chemical Society
Characterization of Gas-Expanded Liquid-Deposited Gold Nanoparticle Films on Substrates of Varying Surface Energy Kendall M. Hurst,* Christopher B. Roberts, and W. Robert Ashurst Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849, United States Received October 15, 2010. Revised Manuscript Received December 8, 2010 Dodecanethiol-stabilized gold nanoparticles (AuNPs) were deposited via a gas-expanded liquid (GXL) technique utilizing CO2-expanded hexane onto substrates of different surface energy. The different surface energies were achieved by coating silicon (100) substrates with various organic self-assembled monolayers (SAMs). Following the deposition of AuNP films, the films were characterized to determine the effect of substrate surface energy on nanoparticle film deposition and growth. Interestingly, the critical surface tension of a given substrate does not directly describe nanoparticle film morphology. However, the results in this study indicate a shift between layer-by-layer and island film growth based on the critical surface tension of the capping ligand. Additionally, the fraction of surface area covered by the AuNP film decreases as the oleophobic nature of the surfaces increases. On the basis of this information, the potential exists to engineer nanoparticle films with desired morphologies and characteristics.
Introduction Self-assembled monolayers (SAMs) have been studied extensively due to their importance in surface modification and their potential applications in nanotechnology.1-3 Ultrathin, molecular films deposited on metallic or semiconducting surfaces have been demonstrated to dramatically change the surface properties. In particular, SAMs effectively control the wetting, lubrication, and adhesion of surfaces and interfaces.1-6 One of the most prominent examples for the application of SAMs involves the ability to significantly reduce adhesion in microelectromechanical systems (MEMS) by reducing the very high surface energy associated with silicon-based materials to a suitable, much lower energy surface.7,8 Such monomolecular films, which act as a passivation layer, directly bond to silicon surfaces, alleviating capillary forces and reducing attractive electrostatic and van der Waals forces which can both detrimentally affect MEMS.9-11 Typically, alkyltrichlorosilane precursors are used to create SAMs for reducing adhesion in silicon-based MEMS. The most suitable and widely studied precursors include octadecyltrichlorosilane (OTS)12,13 and perfluorodecyltrichlorosilane (FDTS).14 *Corresponding author. E-mail:
[email protected]. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Maboudian, R. Mater. Res. Soc. Bull. 1998, 23, 47. (3) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (4) Kulkarni, S. A.; Mirji, S. A.; Mandale, A. B.; Gupta, R. P.; Vijayamohanan, K. P. Mater. Lett. 2005, 59, 3890. (5) Vossmeyer, T.; Jia, S.; Delonno, E.; Diehl, M.; Kim, R.; Peng, X.; Alivisatos, A. P.; Heath, G. R. J. Appl. Phys. 1998, 84, 3664. (6) Liu, J.; Casavant, M. J.; Cox, M.; Walters, D. A.; Boul, P.; Lu, W.; Rimberg, A. J.; Smith, K. A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1999, 303, 125. (7) Srinivasan, U.; Houston, M. R.; Howe, R. T.; Maboudian, R. J. Microelectromech. Syst. 1998, 2, 252. (8) Ashurst, W. R.; Carraro, C.; Maboudian, R.; Frey, W. Sens. Actuators, A 2003, 104, 213. (9) Tas, N.; Sonnenberg, T.; Jansen, H.; Legtenberg, R.; Elwenspoek, M. J. Micromech. Microeng. 1996, 6, 385. (10) Rymuza, Z. Microsyst. Technol. 1999, 5, 173. (11) Guckel, H.; Sniegowski, J. J.; Christenson, T. R.; Mohney, S.; Kelly, T. F. Sens. Actuators, A 1989, 20, 117. (12) Ashurst, W. R.; Yau, C.; Carraro, C.; Maboudian, R.; Dugger, M. T. J. Microelectromech. Syst. 2001, 10, 41. (13) Ashurst, W. R.; Yau, C.; Carraro, C.; Lee, C.; Kluth, G. J.; Howe, R. T.; Maboudian, R. Sens. Actuators, A 2001, 91, 239. (14) Frechette, J.; Maboudian, R.; Carraro, C. J. Microelectromech. Syst. 2006, 15, 737.
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However, other candidates exist with varying terminal groups which could have advantages over alkyltrichlorosilanes such as mercaptopropyltrimethoxysilane (MPTS)15-18 and aminophenyltrimethoxysilane (APhTS).19 Aside from molecular SAMs, other surface modification methods have been examined to reduce adhesion in MEMS, namely the reduction of real contact surface area between contacting components by increasing surface roughness.20-23 Previous studies have signified SAMs as the favored technique due to only moderate reductions in adhesion due to surface roughening methods.23 More recently, DelRio et al.24 re-examined roughness modifications on polysilicon MEMS surfaces. Their results indicated that 20-50 nm silicon carbide particulates formed in situ during MEMS fabrication strongly influenced the adhesion of microstructures by increasing the average separation distance between two surfaces. Our previous studies25 have also investigated the effect of rough films produced using 5 nm gold nanoparticles on the adhesion of MEMS components. These smaller particles were shown to dramatically reduce adhesion by up to 2 orders of magnitude. Therefore, the potential exists to combine the effects of both chemical and physical surface modifications by depositing rough films of small nanoparticles on top of liquid-phase deposited SAMs. Additionally, the deposition of nanoparticle films is not limited to applications in MEMS. Other potential (15) Grabar, K. C.; Allison, K. A.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Dolan, C. M.; Greeman, R. G.; Fox, A. P.; Musick, M. D.; Nathan, M. J. Langmuir 1996, 12, 2353. (16) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Nathan, M. J. J. Am. Chem. Soc. 1996, 118, 1148. (17) Harnisch, J. A.; Pris, A. D.; Porter, M. D. J. Am. Chem. Soc. 2001, 123, 5829. (18) Vakarelski, I. U.; McNamee, C. E.; Higashitani, K. Colloids Surf., A 2007, 295, 16. (19) Zhang, F.; Srinivasan, M. P. Langmuir 2004, 20, 2309. (20) Fan, L. S.; Tai, Y. C.; Muller, R. S. Sens. Actuators, A 1989, 20, 41. (21) Alley, R. L.; Mai, P.; Komvopoulos, K.; Howe, R. T. Solid-State Sens. Actuators 1993, 288. (22) Houston, M. R.; Maboudian, R.; Howe, R. T. Solid-State Sens. Actuators 1995, 210. (23) Yee, Y.; Chun, K.; Lee, J. D.; Kim, C. J. Sens. Actuators, A 1996, 52, 145. (24) DelRio, F. W.; Dunn, M. L.; Boyce, B. L.; Corwin, A. D.; de Boer, M. P. J. Appl. Phys. 2006, 99, 104304. (25) Hurst, K. M.; Roberts, C. B.; Ashurst, W. R. Nanotechnology 2009, 20, 185303.
Published on Web 12/21/2010
DOI: 10.1021/la1041629
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applications exist in the areas of catalysis (microreactors) and energy (photovoltaics), among others. However, little is known about how nanoparticles will deposit and behave upon functionalized monomolecular films. In the present work, we investigate the characteristics of small gold nanoparticle (AuNP) films on SAMs of varying surface energies. The surface energy of the coated substrates were quantified as the critical surface tension determined by Zisman plots. AuNPs of 4.5 ( 1.2 nm in diameter, stabilized in an organic solvent by 1-dodecanethiol capping ligands, were deposited onto the substrates by a CO2 gas-expanded hexane (GXL) deposition process.25 This process effectively deposits conformal films of AuNPs and supercritically dries the AuNP films in order to avoid dewetting effects which can be detrimental.25-28 Following nanoparticle deposition and supercritical drying, the nanoparticle films are characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The microscopy results are analyzed by simple image analysis techniques. The AuNP films are then characterized with respect to critical surface tension and oleophobicity of the substrates.
Materials and Methods Substrates were prepared from Si (100) wafers (University Wafers) diced into 1 1 cm2 pieces. The substrate pieces were then ultrasonicated in 2-propanol (Acros Organics) and acetone (Fisher Scientific) for 5 min each to remove debris before being dried under a stream of nitrogen. To ensure that the silicon oxide (SiO2) layers were thin and smooth, the substrates were immersed in concentrated hydrofluoric acid (HF, 49 wt %, Fisher Scientific) for 10 min to etch away the natural SiO2 layers. After a copious rinse in deionized ultrafiltered (DIUF) water and drying with nitrogen, the substrates were exposed to an RF oxygen plasma at 25 W and 260 mTorr O2 pressure for 10 min to facilitate the formation of a thin, uniform SiO2 layer. The HF/plasma steps were repeated once more to ensure that the oxidized silicon was as smooth and uniform as possible. SAMs were deposited onto cleaned Si (100) substrates following similar liquid-phase procedures. n-Octadecyltrichlorosilane (OTS, Gelest, Inc.) monolayers were formed by preparing a 1 mM solution in n-hexane (Sigma-Aldrich). After allowing OTS molecules to hydrolyze for 45 min, the substrates were immersed into the OTS/organic solution for 45 min before a copious rinse in n-hexane. 1H,1H,2H,2H-Perfluorodecyltrichlorosilane (FDTS, Gelest, Inc.) monolayers were formed following a similar procedure utilizing a 1 mM solution in n-hexane. 3-Mercaptopropyltrimethoxysilane (MPTS, Sigma-Aldrich) monolayers were formed following the method of Goss et al.18,29 Prepared Si (100) substrates were immersed into a solution of 20 mL of 2-propanol, 400 μL of DIUF water, and 400 μL of MPTS heated to 70 °C for 30 min. The substrates were then rinsed consecutively with 2-propanol, water, and n-hexane to remove any excess physisorbed molecules. p-Aminophenyltrimethoxysilane (APhTS, Gelest, Inc.) monolayers were formed following a procedure similar to Zhang et al.19 in which the substrates were first rinsed consecutively in methanol (Fisher Scientific), a 1:1 (v/v) mixture of methanol and toluene (Fisher Scientific), and pure toluene before being immersed in a 3 mM APhTS solution in toluene for 30 min. Following deposition, the substrates were ultrasonicated in toluene for 10 min to remove excess sticky APhTS molecules and then rinsed in n-hexane. Following all substrate rinsing procedures, the coated substrates were dried under a stream of nitrogen and the SAMs were annealed by heating at 120 °C for (26) McLeod, M. C.; Kitchens, C. L.; Roberts, C. B. Langmuir 2005, 21, 2414. (27) McLeod, M. C.; Anand, M.; Kitchens, C. L.; Roberts, C. B. Nano Lett. 2005, 5, 461. (28) Liu, J.; Anand, M.; Roberts, C. B. Langmuir 2006, 22, 2964. (29) Goss, C. A.; Charych, D. H.; Majda, M. Anal. Chem. 1991, 63, 85.
652 DOI: 10.1021/la1041629
Figure 1. Chemical structures of (a) octadecyltrichlorosilane (OTS), (b) perfluorodecyltrichlorosilane (FDTS), (c) mercaptopropyltrimethoxysilane (MPTS), and (d) aminophenyltrimethoxysilane (APhTS). about 1 h in ambient atmosphere. SAM deposition was confirmed by contact angle measurements and AFM analysis. Figure 1 illustrates the structures of the four molecules used for SAM formation. Gold nanoparticles (AuNPs) were synthesized by a two-phase liquid arrested precipitation method similar to that of Sigman et al.30 A 36 mL aqueous solution containing 0.38 g of hydrogen tetrachloroaurate trihydrate (Sigma-Aldrich) was combined with a solution of 2.7 g of tetraoctylammonium bromide (TOAB, Sigma-Aldrich) in 24.5 mL of toluene. After stirring for 1 h, the lower aqueous phase was removed and discarded, leaving an organic phase containing gold ions. This organic solution was then combined with a 30 mL aqueous solution containing 0.5 g of NaBH4 (Sigma-Aldrich) which reduces the gold ions to their ground state. After stirring the mixture for 8 h allowing nanoparticle growth, the lower aqueous phase was removed and discarded. 240 μL of 1-dodecanethiol (Sigma-Aldrich) was then added and slowly stirred overnight to cap the particles, cease particle growth, and stabilize the AuNP dispersion. The AuNP dispersion was then centrifuged with ethanol (Pharmco-Aaper) at 4500 rpm for 5 min to rinse excess thiol and NaBH4 molecules. The centrifugation step was repeated several times before the AuNPs were dispersed and stored in n-hexane. Transmission electron microscopy (TEM) was used to determine the average nanoparticle size of 4.5 nm in diameter. AuNPs were deposited onto SAM-coated substrates by a CO2 gas-expanded liquid (GXL) and critical point drying (CPD) process.25 Within a stainless steel high-pressure vessel, CO2 gas was slowly added above a AuNP dispersion in hexane over a period of about 24 h. This slow pressurization (up to about 60 bar) causes the partitioning of CO2 into the organic hexane phase. The desolution of the CO2 into the dispersion effectively reduces the ability of the organic solvent to solvate the stabilized AuNPs, thereby inducing precipitation. A more detailed description of this process can be found elsewhere.25-28 Following precipitation, the CO2/hexane mixture was isochorically heated to the supercritical state of the mixture (about 90 bar and 40 °C) to eliminate the liquid-vapor interface. This effectively dries the substrates without disturbing the AuNP films. The dried AuNP-coated substrates were then removed following isothermal depressurization at 40 °C. This isothermal depressurization was performed over a period of about 3 h utilizing a series of two needle valves. An RTD feedback thermocouple and heating tape were used to maintain temperature during the depressurization process to prevent a decrease in temperature due to the rapid expansion of CO2. The substrates were then heated at 120 °C for 1 h to anneal the dodecanethiol-capped AuNP films to the substrates in order to achieve more accurate static contact angle measurements during analysis. (30) Sigman, M. B.; Saunders, A. E.; Korgel, B. A. Langmuir 2004, 20, 978.
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Table 1. Contact Angles (deg) of Various Solvents on SiO2 and SAMs on Si (100)a SiO2
OTS
FDTS
MPTS
APhTS
methanol
