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Preparation of Large Scale Monolayers of Gold Nanoparticles on Modified Silicon Substrates Using a Controlled Pulling Method Richard D. Tilley* and Satoshi Saito Advanced Materials & Devices Laboratory, Toshiba Corporate Research and Development Center, 1, Komukai Toshiba-Cho, Saiwai-ku, Kawasaki 212-8582, Japan Received December 13, 2002. In Final Form: March 26, 2003 The preparation of large scale monolayers of gold nanoparticles on modified silicon substrates for use in storage media is reported. Gold nanoparticles were synthesized in reverse microemulsions using the surfactant cetyltrimethylammonium bromide (CTAB). Dodecanethiol was then attached to the surface of the particles, which were subsequently purified by liquid phase extraction. The formation of nanoparticle monolayers was accomplished by vertically pulling various modified silicon substrates at controlled speeds from nanoparticle solutions. The silicon substrates used were 1 cm2 in size, had silanol and hydrogen terminated surfaces, had surfaces chemically modified by (CH3)3SiNHSi(CH3)3 and (CH3O)3Si(CH2)2(CF2)7CF3, and one was coated with amorphous carbon. All of these substrates have different surface polarities. The effects of the substrate pulling speed and solvent were also investigated. The results assessed using atomic force microscopy (AFM) indicated that the surface modification was the most significant factor. Silanol terminated silicon substrates were found to have very low coverage, and the carbon fluoride modified substrates, very high monolayer coverage. It was also found that lowering the substrate pulling speed and using higher boiling point solvents favored monolayer formation, and by optimizing these factors, a 95% monolayer coverage on the carbon fluoride modified substrate has been achieved. Conclusions from the results about the mechanism of the monolayer formation are also discussed.
Introduction There is much current interest in the use of nonmagnetic and magnetic nanoparticles for the next generation of data storage disks, principally because they offer the possibilities of recording densities of up to 10 Tbit/in.2 However, if the potential of nanoparticles is to be utilized for storage media applications, it is essential that a monolayer of nanoparticles can be assembled in an ordered array over a 2.5 in. disk with minimal bilayers or vacant areas. The current research reported here has focused on assembling alkanethiol coated gold nanoparticles for two applications. First, it is expected that magnetic Fe,1 Co,2-4 FePt, CoPt, and FeCoPt5-7 particles capped with groups containing long alkane chains may be self-assembled in an identical manner and that the results and conclusions of these experiments can be extended to these particles if desired. Second, gold particles in an ordered monolayer array over a magnetic substrate may possibly be used as a mask which, followed by etching to the undercoated magnetic film, could produce a data storage disk. An advantage of this second method when compared to using magnetic nanoparticles is that there is no need to align the magnetization axis of each particle. In this study alkanethiol capped gold particles were chosen because their synthesis in reverse micelles and microemulsions is well-known and documented8-11 and * Corresponding author. E-mail:
[email protected]. (1) Wilcoxon, J. P.; Provencio, P. J. Phys. Chem. B 1999, 103, 9809. (2) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (3) Sun, S.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325. (4) Legrand, J.; Petit, C.; Pileni, M. P. J. Phys. Chem. B 2001, 105, 5643. (5) Murray C. B.; Sun, S.; Doyle, H.; Betley, T. MRS Bull. 2001, 26, 981-1019. (6) Sun, S.; Murray C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (7) Chen, M.; Nikles, D. E. Nano Lett. 2002, 2, 211.
because of their strong tendency to self-assemble into ordered hexagonal arrays.12,13 Many previous studies forming gold nanoparticle arrays have used the Langmuir-Blodgett method;14-16 however, this can be laborious and requires specific apparatus. In the experiments reported here, a substrate is immersed in a solution of gold nanoparticles and then slowly withdrawn at a controlled speed using a piezoelectric motor to produce a gold nanoparticle monolayer on the substrate. The main advantages of this method are that it is easy to perform and that it enables the effect of different substrate surfaces on the formation of gold particle assemblies to be investigated. The substrates used were designed to physisorb the particles through van der Waals attractions17 rather than covalent bonds or electrostatic interactions used in other approaches.18-21 In these experiments four primary variables were investigated, these being the (8) Wilcoxon, J. P.; Williamson, R. L.; Baughman, R. J. Phys. Chem. 1993, 98, 9933. (9) Martin, J. E.; Wilcoxon, J. P.; Odinek, J.; Provencio, P. J. Phys. Chem. B 2000, 104, 9475. (10) Lin, J.; Zhou, W.; O’Connor, C. J. Mater. Lett. 2001, 49, 282. (11) Brust, M.; Walker, M.; Bethall, D.; Schriffen, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1994, 801. (12) Lin, X. M.; Jaeger, H. M.; Sorenson, C. M..; Klaubunde, K. J. J. Phys. Chem. B 2001, 105, 3353. (13) Gutierrez-Wing, C.; Santiago, P.; Ascencio, J. A.; Camacho, A.; Jose-Yacaman, M. Appl. Phys. A 2000, 71, 237. (14) Brown, J. J.; Porter, J. A.; Daghlian, C. P.; Gibson, U. J. Langmuir 2001, 17, 7966. (15) Yonezawa, T.; Onoue, S.; Kimizuka, N. Langmuir 2001, 17, 2291. (16) Huang, S.; Tsutsui, G.; Sakaue, H.; Takahagi, T. J. Vac. Sci. Technol., B 2001, 19, 2045. (17) Resch, R.; Meltzer, S.; Vallant, T.; Hoffmann, H.; Koel, B.; Madhukar, A.; Requicha, A.; Will, P. Langmuir 2001, 17, 5666. (18) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313. (19) Wei, L.; Huo, L.; Wang, D.; Zeng, G.; Xi, S.; Zhao, B.; Zhu, J.; Wang, J.; Shen, Y.; Zuhong, L. Colloids Surf., A 2000, 175, 217. (20) Grabar, K.; Smith, P.; Musick, M.; Davis, J.; Walter, D.; Jackson, M.; Guthrie, A.; Natan, M. J. Am. Chem. Soc. 1996, 118, 1148. (21) Dorogi, M.; Gomez, J.; Osifchin, R.; Andres, R.; Reifenberger, R. Phys. Rev. B 1995, 52, 9071.
10.1021/la026993r CCC: $25.00 © 2003 American Chemical Society Published on Web 05/14/2003
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Table 1. Contact Angle Values for the Modified Silicon Substrates Measured with Water substrate contact angle with H2O
Si-OH 8-10
Si-C 52-56
Si-H 53-60
Si-CH3 75-76
Si-CF 106-108
substrate surface, the pulling speed, the solvent, and the solution concentration. Experimental Section Nanoparticle Preparation. Gold nanocrystals were synthesized using the reverse microemulsion method by the reduction of HAuCl4(H2O)4 using NaBH4. In initial experiments many surfactants were used; however, cetyltrimethylammonium bromide (CTAB) was found to be the most successful due to its solubility in octane/water and the fact that it is readily removed after the experiment by washing with n-methylformamide. In a typical experiment, solution A was made by dissolving 0.0824 g of gold salt and 2.023 g of CTAB in 16.4 mL of octane, 3.4 mL of 1-butanol, and 2 mL of water. Solution B was made with identical proportions and with the same reagents except that 0.03024 g of NaBH4 replaced the gold salt. After 1 h of homogenizing, solution B was added dropwise to solution A to produce a wine red gold colloid. The solution was then left to react for at least 1 h before 0.0002 mol of dodecanethiol, C12H25SH, was added from a 0.5 M octane solution. After the experiment the surfactant was then removed by washing three times with 200 mL of n-methylformamide. The advantage of this procedure is that at all times the gold particles are kept in solution.9 At this point the particles were observed by high-resolution transmission electron microscopy, HRTEM, and were found to have an average diameter of 3.5 nm and a 20% size distribution. To decrease the size distribution, 1 mL of the solution was reflux with a further 0.02 mL of thiol for 40 min. The subsequent TEM observations of this refluxed sample showed that no improvement in the size distribution had occurred. However, the size distribution was improved by letting this refluxed sample stand for 3 days, allowing the larger particles to separate to the bottom of the sample bottle. When a small part of the solution was removed by microsyringe and placed onto a grid, it was observed by HRTEM that the size distribution had been reduced to 5%. All the chemicals were purchased from Kanto Chemicals Co., Japan, and used without further purification. Substrate Preparation. Five different modified silicon substrates were investigated. A silanol terminated silicon surface was prepared by heating a silicon wafer at 150 °C to remove any water followed by exposure to UV light for 15 min to remove any carbon containing contaminants that might be on the surface, denoted by Si-OH in the text. A silicon substrate coated with 10 nm of amorphous carbon was prepared by sputtering and is denoted by Si-C. A hydrogen terminated surface was prepared by washing a silicon wafer in diluted HF for 20 min and then rinsing with water and is denoted by Si-H in the text. A methyl modified silicon surface was prepared by exposing a silicon wafer to (CH3)3SiNHSi(CH3)3 (Tokyo Oka, Japan) vapor for 12 h, denoted by Si-CH3. A modified silicon surface was also prepared by exposing a silicon wafer to (CH3O)3Si(CH2)2(CF2)7CF3 (GE Toshiba Silicone, Japan) vapor for 72 h, denoted by Si-CF. Before the last two silicon substrates were exposed to the respective vapors, they were heated to 100 °C for 30 min to remove any water from the silicon surface and UV treated for 15 min. The contact angles of the modified silicon substrates were measured and are listed in Table 1. The values were used as a reference to the surfaces’ wettability and polarity, with high contact angles signifying nonpolar, hydrophobic character and low contact angles signifying polar, hydrophilic character. Particle Self-assembly. The gold particle monolayers were prepared with controlled pulling speeds by attaching a substrate to a rod which could be made to move up or down with a piezoelectric motor, as shown in Figure 1. The 1 cm2 substrate was then immersed lengthwise in the gold nanoparticle solution and then slowly withdrawn from the solution using the motor. For all experiments the solution surface was exposed to the atmosphere and not enclosed. This aided evaporation of the solvent, so that the substrate dried at the substrate meniscus interface. The experiments were performed at room temperature.
Figure 1. Illustration of the apparatus used to prepare gold nanoparticle monolayers. The substrate is pulled out of the solution of gold nanoparticles at a controlled speed using a piezoelectric motor. Characterization. A Nanoscope III (Digital Instruments) was used to obtain the tapping mode atomic force microscopy (AFM) images. The percentage monolayer coverage was measured using the bearing function of the software. In general, AFM images of 1 µm and 500 nm had particle scale resolution and were used to investigate the precise nanoparticle ordering. These images confirmed the TEM observations that the thiol coated gold nanoparticles had a size of approximately 7 nm, which is in agreement with HRTEM observations, which indicated a gold core of 3.5 nm. Lower resolution images of 16 and 5 µm were used to assess the large scale coverage of the substrate.
Results and Discussion Evidence of Monolayer Formation. From SEM and AFM observations of the different substrates after pulling, it was found that for all experiments a partial nanoparticle monolayer containing vacancies had formed on the surface and not mutlilayers or supracrystals of gold particles. Evidence for this was typically taken from high-resolution AFM scans which have particle resolution, such as the one shown in Figure 2d. Figure 2d shows a typical gold particle film with vacant areas where there are no particles. Due to the resolution, it was possible to measure the surface below the nanoparticles in the gaps and vacant areas of the monolayer film. This was found to be the relatively smooth substrate and not an underlayer of nanoparticles. The SEM image in Figure 2a of the same sample also shows that a gold particle film is formed which has vacant areas where there are no particles present. The sectional analysis of the AFM image in Figure 2d (shown in Figure 2e) gives a 7.5 nm step height between the substrate and the gold particle film and between the first layer and the second layer of gold particles. This is in good agreement with the observed gold particle size and shows that a monolayer and a small amount of bilayer have been formed on the substrate. Substrate. Experiments to investigate the effect of the different substrates on the monolayer formation were performed under identical experimental conditions of a pulling speed of 4000 nm/s, hexane as the solvent, and a nanoparticle concentration of 1.2 × 1015 particles/mL. Subsequent AFM images of the different substrates showed a wide variation in the percentage coverage of each substrate, as reported in Table 2. For the carbon fluoride modified substrate it was established by taking many 16 µm AFM images, similar to that shown in Figure 2b, that the gold particles completely covered the substrate surface in one continuous, uniform monolayer. It may also be seen in the images in Figure 2 that there are many small, approximately spherical, 300 nm diameter vacancies in the monolayer
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Figure 2. SEM image (a) and low- and high-resolution AFM images (b-d) of a gold nanoparticle monolayer formed on a carbon fluoride modified silicon substrate, using hexane as the solvent, a pulling speed of 4000 nm/s, and a nanoparticle concentration of 1.2 × 1015 particles/mL. For the AFM images the gray areas are the gold nanoparticle monolayer and the black areas are the substrate. Part e is a sectional analysis of the AFM image shown in part d taken along the white line across the image. Table 2. Percentage Monolayer Coverage of Modified Silicon Substrates Using Hexane as the Solvent, a Pulling Speed of 4000 nm/s, and a Nanoparticle Concentration of 1.2 × 1015 particles/mL substrate Si-OH Si-C Si-H Si-CH3 Si-CF monolayer coverage 5% 60% 21% 26% 81% with hexane solvent
which reduced the overall coverage to 81%. Figure 2d shows that there is little ordering of the particles and that long range hexagonal arrays which were observed in TEM micrographs are not formed. This is most likely due to the roughness of the surface, which AFM images showed had 0.5 nm peaks and wells. This roughness seems to be not enough to prevent the formation of one continuous monolayer on the substrate but does prevent hexagonal ordering of the particles. Images for both the Si-H and Si-CH3 substrates were similar to those shown in Figure 2, except that the gold
monolayer was not entirely continuous and the vacancy areas covered far more of these substrates. The Si-OH substrate had only a 5% monolayer coverage, which was composed of approximately spherical, 500 nm diameter nanoparticle domains. It can be seen that, excepting the amorphous carbon coated substrate, the percentage of monolayer coverage correlates to the contact angles of the substrates as listed in Table 1. The low-contact-angle, polar, silanol terminated silicon substrate has a very low coverage, and the highcontact-angle, nonpolar carbon fluoride modified substrate has the greatest coverage. This is in line with expectations with the nonpolar thiol coated gold particles forming a larger monolayer on the most nonpolar surfaces. Figure 3a shows a typical image for the amorphous carbon coated substrate; it may be seen that the gold monolayer does not form a continuous film but a far less ordered arrangement of particles. This is further high-
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Tilley and Saito Table 4. Percentage Monolayer Coverage of Modified Silicon Substrates Using Octane as the Solvent, Pulling Speeds of 4000 and 800 nm/s and a Nanoparticle Concentration of 1.2 × 1015 particles/mL and a Pulling Speed of 40 nm/s and a Nanoparticle Concentration of 1.2 × 1014 particles/mL
Figure 3. Low- and high-resolution AFM images of the gold monolayer formed on an amorphous carbon coated silicon substrate, using hexane as the solvent, a pulling speed of 4000 nm/s, and a nanoparticle concentration of 1.2 × 1015 particles/ mL. The gray areas in the AFM images are the gold nanoparticle monolayer, and the black areas are the substrate.
lighted in Figure 3b and c, in which it may be observed that the particles have little ordering, with the monolayer being composed of discrete nanoparticles and very small clusters of particles. The relatively high surface coverage
pulling speed (nm/s)
Si-OH
4000 800 40
6% 5% 10%
monolayer coverage Si-C Si-H Si-CH3 71% 77% 91%
27% 31% 70%
32% 44% 75%
Si-CF 85% 87% 95%
and the discontinuous monolayer formation were attributed to the greater surface roughness of the substrate. AMF analysis of the bare amorphous carbon coated substrate showed that the surface was covered in 2 nm peaks and wells. This roughness is significantly greater when compared to those of the other smoother surfaces and may help the particles attach to the surface, but it also appears to prevent the formation of long range particle ordering and also disrupt the formation of a continuous monolayer of particles. Solvent. The second set of experiments investigated the effect of using different solvents. For each substrate, experimental conditions of a pulling speed of 4000 nm/s and a particle concentration of 1.2 × 1015 particles/mL were maintained with the gold particles dispersed in the longer chain alkane solvents, octane and decane. The results summarized in Table 3 showed for all substrates, except Si-OH, that the percentage of monolayer coverage using decane was greater than the monolayer formed using octane, which in turn was greater than the monolayer formed using hexane. This increase in coverage may be attributed to the higher boiling points and lower evaporation rates of octane and decane and indicates that a slower evaporation rate improves the surface coverage. Pulling Speed. The third set of experiments investigated the effect of pulling speed on the substrate coverage; these used octane as the solvent, as hexane was found to appreciably evaporate for the experiments which had a long length of time. By using the piezoelectric motor, two pulling speeds, 4000 and 800 nm/s, could be compared. The 40 nm/s speed was obtained by allowing the octane solution to slowly evaporate over 3 days. For the evaporation experiments, the concentration of particles had to be reduced to 1.2 × 1014 particles/mL to limit bilayer formation. Despite this, all of the substrates, except SiOH, still had between 5% and 7% of the bilayer which formed in small 100 nm diameter domains on the monolayer. During the course of the evaporation experiment, the concentration could be expected to increase as the amount of solvent decreased; this effect was minimized by using a larger, 30 cm3, starting volume of solution. The results as shown in Table 4 indicate the percentage coverage increase for all substrates as the pulling speed decreases. This trend is illustrated in Figure 4, which shows the AFM images of the Si-CH3 substrate with different pulling speeds of 4000, 800, and 40 nm/s. It can be seen in this figure how the monolayer coverage increases as the pulling speed is reduced, with the 40 nm/s pulling
Table 3. Percentage Monolayer Coverage of Modified Silicon Substrates Using Hexane, Octane, and Decane Solvents, a Pulling Speed of 4000 nm/s, and a Nanoparticle Concentration of 1.2 × 1015 particles/mL substrate
Si-OH
Si-C
Si-H
Si-CH3
Si-CF
monolayer coverage with hexane solvent monolayer coverage with octane solvent monolayer coverage with decane solvent
5% 6% 5%
60% 71% 79%
21% 27% 37%
26% 32% 40%
81% 85% 88%
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both similar in structure to the Si-CH3 substrate. In particular, for the Si-CF substrate a very high, 95% monolayer coverage was found. The concentration and sizes of vacant areas in the monolayer for this substrate were greatly reduced, with a typical vacancy diameter of 35 nm observed. Due to the fact that the previous experiments suggested that both slow pulling times and high boiling point solvents increased the monolayer coverage, experiments with the carbon fluoride modified substrate, decane and hexadecane solutions of gold nanoparticles were left to slowly evaporate. In the case of the hexadecane solution, this took 10 days. It was found for both solvents that only a 10% monolayer coverage was present on the substrate. This suggests that there are possible limits to how high a boiling point solvent may be used and by how much the pulling speed may be reduced. Concentration. Two experiments were performed to investigate the effect of concentration on the carbon fluoride modified substrate using an octane solution and a pulling speed of 4000 nm/s. It was found that increasing the particle concentration by 10 times to 1.2 × 1016 particles/mL induced 3-D growth and the formation of a bilayer of nanoparticles on the substrate. The bilayer formed as small, 100 nm diameter sized domains and covered 12% of the substrate, leading to a decrease in the total area covered by only a monolayer. Reducing the concentration by 10 times to 1.2 × 1014 particles/mL led to a small decrease in the coverage to 75% with only a 2% bilayer coverage formed. Mechanism Discussion. From the above results several conclusion may be drawn about the mechanism of monolayer formation. The results of the first experiment clearly indicate that the substrate surface is the most important factor and that for nonpolar surfaces the particles must be either positively attracted to the surface or at least not repelled. The opposite is true in the case of the polar Si-OH surface, for which it is clearly more favorable for the gold nanoparticles to stay in solution, solvated by the organic solvent, rather than attach to the substrate surface. To investigate whether there was water on the Si-OH substrate surface, which might be repelling the thiol coated particles, experiments for this substrate were repeated in an argon atmosphere in a glovebox. The substrate was heated in the glovebox at 150 °C for 15 min prior to the start of the experiment. The results remained unchanged, indicating that surface water is probably not responsible for the particles not attaching to the surface. The results that both a slow pulling speed and a lower boiling point solvent increase the monolayer coverage for the Si-C, Si-H, Si-CH3, and Si-CF substrates indicate that the monolayer formation is a thermodynamically stable process and that the longer time the particles have to assemble on the surface, the greater the coverage obtained. Figure 4. AFM images (5 µm) of the gold monolayer formed on a methyl modified silicon substrate, using octane as the solvent and (a) a pulling speed of 4000 nm/s and (b) a pulling speed of 800 nm/s with a nanoparticle concentration of 1.2 × 1015 particles/mL. In the AFM images the gray and the white areas are the gold nanoparticle monolayer and the black areas are the substrate. Part c uses a pulling speed of 40 nm/s and a nanoparticle concentration of 1.2 × 1014 particles/mL. In the image the gray areas are the gold nanoparticle monolayer, the black areas are the substrate, and the white areas are bilayers or nanoparticle aggregates.
speed showing a dramatic increase in substrate coverage. The monolayers on the Si-H and Si-CF substrates were
Conclusions The preparation of large scale monolayers of gold nanoparticles on modified silicon substrates has been achieved by vertically pulling modified silicon substrates at controlled speeds from nanoparticle solutions. The results showed that the surface modification of the substrate was the most significant factor with the least polar, carbon fluoride modified silicon substrate having the highest monolayer coverage. Lowering the substrate pulling speed and using higher boiling point solvents was also found to favor monolayer formation. By optimiz-
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ing these three factors, a 95% gold nanoparticle monolayer coverage on the least polar substrate has been achieved. Significantly, these experiments show that large scale nanoparticle monolayers can be produced easily and cheaply, thus paving the way for the use of nanoparticles in numerous applications, including data storage media.
Tilley and Saito
Acknowledgment. R.D.T. thanks Toshiba Corporation for funding this research and for financial support through the Toshiba Fellowship program. We thank Dr. Hiroyuki Fujimori of Toshiba Ceramics Co. Ltd. for TEM observations. LA026993R
