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Effects of the Surface Pressure on the Formation of Langmuir-Blodgett Monolayer of Nanoparticles Shujuan Huang,* Kazuyuki Minami, Hiroyuki Sakaue, Shoso Shingubara, and Takayuki Takahagi Graduate School of Advanced Sciences of Matter, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8530, Japan Received June 3, 2003. In Final Form: December 17, 2003 The effects of the surface pressure on the particle arrangement of Langmuir-Blodgett (LB) monolayers of alkanethiol-capped gold nanoparticles were studied. The LB monolayers were prepared from a highly concentrated particle solution, which increases film fabrication efficiency but readily causes small particle voids in the particle array. Overcompressing the LB monolayer to a high surface pressure restructured the particles and eliminated the voids. When the gold particles capped by dodecanethiol were 8.5 nm in diameter, the particle arrangement was vastly improved and a wafer-scale LB monolayer was transferred onto a substrate at the surface pressure of 20 mN/m.
Introduction A great deal of research has focused on nanoscale particles due to their novel properties and potential applications in optical,1-3 electronic,4-6 and biosensing devices.7-9 Recently, chemical synthesis of nanoparticles, especially metal particles, has greatly advanced. For instance, gold particles with a diameter of several nanometer and very narrow size deviations have been synthesized by stabilizing the nanoparticles with functional molecules, such as alkanethiolates and dendrimers.10-12 The next step toward application to device fabrication is to manipulate the nanoparticles into functional and desired nanostructures. Of particular interest is controllable two- or three-dimensional construction. Selfassembly or self-organization of nanostructures, involving surface forces,13-15 chemical interactions,16-19 and bio* Corresponding author. E-mail:
[email protected]. (1) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463. (2) Maenosono, S.; Dushkin, C. D.; Saita, S.; Yamaguchi, Y. Jpn. J. Appl. Phys. 2000, 39, 4006. (3) Okamoto, T.; Yamaguchi, I.; Kobayashi, T. Opt. Lett. 2000, 25, 372. (4) Janes, D. B.; Kolagunta, V. R.; Osifchin, R. G.; Mahoney, W. J.; Bielefeld, J. D.; Andres, R. P.; Henderson, J. I. Superlattice Microstruct. 1995, 18, 2275. (5) Huang, S.; Tsutsui, G.; Sakaue, H.; Shingubara, S.; Takahagi, T. J. Vac. Sci. Technol. B 2000, 18, 2653. (6) Sato, T.; Ahmed, H.; Brown, D.; Johnson, B. F. G. J. Appl. Phys. 1997, 82, 696. (7) Dubertret, B.; Skourides, P.; Norris, D. J.; Noireaux, V.; Brivanlou, A. H.; Libchaber, A. Science 2002, 298, 1759. (8) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (9) Watanabe, S.; Sonobe, M.; Arai, M.; Tazume, Y.; Matsuo, T.; Nakamura, T.; Yoshida, K. Chem. Commun. 2002, 2866. (10) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (11) Kim, M.-K.; Jeon, Y.-M.; Jeon, W. S.; Kim, H.-J.; Hong, S. G.; Park, C. G.; Kim, K. Chem. Commun. 2001, 667. (12) Maye, M. M.; Zheng, W.; Leibowitz, F. L.; Ly, N. M.; Zhong, C.-J. Langmuir 2000, 16, 490. (13) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (14) Kralchevsky, P. A. Advances in Biophysics; Japan Scientific Societies: Tokyo, 1997; p 25. (15) Huang, S.; Sakaue, H.; Shingubara, S.; Takahagi, T. Jpn. J. Appl. Phys. 1998, 37, 7198. (16) Sarathy, K. V.; Thomas, P. J.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1999, 103, 399.
molecule-assisted assembly,20,21 has been extensively studied. However, all these processes share a common difficulty in forming large-scale and continuous closepacked structures. The Langmuir-Blodgett (LB) technique is one practical method for fabricating monolayers and multilayers.22,23 In our previous work,24,25 large-scale and highly ordered monolayers of alkanethiol-capped gold nanoparticles were fabricated using a relatively low particle concentration between 0.06 and 0.3 mg/mL. We have also studied the effects of particle concentration on the particle arrangement and found that low concentrations readily form highly ordered, close-packed monolayers, while concentrations 10 times greater than the above-mentioned tend to form voids in the particle arrays. The particle arrangement did not improve when the surface pressure was compressed to 10 mN/m, at which a solid monolayer was believed to form on the water surface. Using higher particle concentrations, however, can efficiently shorten the film fabrication time, which is very attractive for future applications. In this paper, the effects of compressing the monolayer on the water surface were studied, i.e., the surface pressure on gold nanoparticle arrangements using a spreading solution of a high particle concentration, 0.6 mg/mL. A void-free monolayer was fabricated at a highly compressed water surface and the film fabrication time was dramatically decreased. Experimental Section The LB instrument used in this work is a KSV minitrough with a pair of automated movable barriers, which compress the (17) Teranishi, T.; Haga, M.; Shiozawa, Y.; Miyake, M. J. Am. Chem. Soc. 2000, 122, 4237. (18) Hoeppener, S.; Maoz, R.; Cohen, S. R.; Chi, L. F.; Fuchs, H.; Sagiv, J. Adv. Mater. 2002, 14, 1036. (19) Sato, T.; Brown, D.; Johnson, B. F. G. Chem. Commun. 1997, 1007. (20) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (21) Takahagi, T.; Tsutsui, G.; Huang, S.; Sakaue, H.; Shingubara, S. Jpn. J. Appl. Phys. 2001, 40, L521. (22) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (23) Chen, S. Langmuir 2001, 17, 2878. (24) Huang, S.; Tsutsui, G.; Sakaue, H.; Shingubara, S.; Takahagi, T. J. Vac. Sci. Technol. B 2001, 19, 115. (25) Huang, S.; Tsutsui, G.; Sakaue, H.; Shingubara, S.; Takahagi, T. J. Vac. Sci. Technol. B 2001, 19, 2045.
10.1021/la0302293 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/30/2004
Effects of the Surface Pressure on Monolayers
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Figure 1. SEM images of internal particle arrays of the islandlike domains of dodecanethiol-capped gold particles transferred at the surface pressure of (a) 0 mN/m, (b) 15 mN/m, and (c) 20 mN/m. nanoparticles that are spread on a water surface, and a surface pressure sensor, which controls the barriers. Gold particles were synthesized by reducing chloroauric acid (HAuCl4) using a mixture of trisodium citrate and tannic acid.26,27 The nanoparticles were then capped with dodecanethiol molecules using a method reported elsewhere25 and dissolved in chloroform.28 LB monolayers of gold particles, which had a diameter of 8.5 nm, were prepared using a spreading solution that had a particle concentration of 0.6 mg/mL. The sample was spread by carefully casting minute droplets (∼3 µL) of the dodecanethiol-capped gold particle solution onto the surface of pure water in the LB trough at intervals of 30 s. After the solvent evaporated, the hydrophobic dodecanethiolcapped gold particles remained on the water’s surface and were then compressed by moving the barriers at a speed of 5 mm/min. The surface pressure isotherm was recorded throughout the compression. A horizontal lifting method transferred the gold nanoparticles to a substrate. To study the effects of surface pressure on the particle arrangement, the particles were transferred at various surface pressures, 0, 5, 10, 15, 20, 25 mN/ m. Approximately 1 h was required to completely prepare the LB monolayer. It is known that for realizing a devise application, it is very important to fabricate a particle construction on a solid substrate; therefore, we used a silicon substrate modified with hydrophobic hexamethyldisilizane in this work. The temperature of the pure water in the trough was about 24 °C. Scanning electron microscopy (SEM) observations of the transferred gold particles were performed on a high-resolution microscope Hitachi S-5000, at 30 kV.
Results and Discussion Our previous work revealed that after a particle solution was spread and evaporated, thiol-capped gold particles formed many two-dimensional islandlike arrays or domains that floated on the water’s surface without compressing the barriers.24,25 We believe that the attractive forces between the particles, which were induced by the surface tension while the solvent evaporated, caused the particle domains to form. For low particle concentrations of the spread solution between 0.06 and 0.3 mg/mL, the particles showed highly ordered arrangements within the domains. At a high particle concentration of 0.6 mg/mL, however, many voids formed within the particle domains. Moving the barriers compressed these islandlike domains into a solid monolayer on the water’s surface at a surface pressure of 10 mN/m. Unfortunately, the particle voids remained within the LB monolayer when the high particle concentration solution was used. (26) Slot, J. W.; Geuze, H. J. Eur. J. Cell Biol. 1985, 38, 87. (27) Tsutsui, G.; Huang, S.; Sakaue, H.; Shingubara, S.; Takahagi, T. Jpn. J. Appl. Phys. 2001, 40, 346. (28) Huang, S.; Minami, K.; Sakaue, H.; Shingubara, S.; Takahagi, T. J. Appl. Phys. 2002, 92, 7486.
Figure 2. Particle void percentage of the islandlike domains as a function of the surface pressure in the LB trough.
The present work investigates the dodecanethiol-capped gold particle arrangement of the LB monolayers, which were prepared from a particle concentration of 0.6 mg/ mL, at different surface pressures. Figure 1 shows SEM images of the internal portion of the islandlike domain transferred at various surface pressures. When the surface pressure was 0 mN/m, i.e., the barrier was not compressed, particle voids, as shown in Figure 1a, were observed between small ordered particle arrays. When the barriers were moved and the particles were compressed to a surface pressure of 15 mN/m, the voids still existed as shown in Figure 1b. However, when the particles were further compressed to 20 mN/m, most of the voids disappeared and a close-packed monolayer formed, as shown in Figure 1c. We estimated the void areas from the SEM images. In this work, particle voids on a scale of one particle or larger were accounted for in the calculation. The void percentages in the gold particle domains at various surface pressures are plotted in Figure 2. It is noted that compressing from 0 to 15 mN/m caused a slight decrease in the void percentage, but increasing the pressure to 20 mN/m dramatically decreased the void percentage from 6.5% to 0.1%. In addition, the estimated gaps between adjacent gold particles, which were determined from the average center-to-center distances in the SEM images, indicated that the gaps were constant at about 2.2 nm as the pressure increased from 0 to 25 mN/m. The value of 2.2 nm is approximately twice the thickness of a self-assembled monolayer of dodecanethiol on a gold surface.29 (29) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
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Figure 3. Large area SEM images of the LB monolayer of dodecanethiol-capped gold particles transferred at surface pressures of (a) 20 mN/m and (b) 25 mN/m. The arrows indicate particle overlap.
On the other hand, when the surface pressure was 0 mN/m, SEM observations show that the islandlike domains were isolated from each other.24 Moving the barriers congregated the isolated domains on the water’s surface. When the barriers were compressed to a surface pressure of 10 mN/m, most of the particle domains were in contact and nearly formed a solid monolayer.25 When the surface pressure was raised to 15 mN/m, the particles narrowly overlapped at the boundaries of most domains. Compressing further to 20 mN/m created a nearly continuous and void-free LB monolayer, as shown by the dark portion in Figure 3a, that had slightly wider particle overlaps. The overlapped particles are the white striplike portions, which are indicated by the arrows. The width of the overlap was less than 100 nm when the pressure is 20 mN/m. When the surface pressure rose to 25 mN/m, the width of the particle overlap increased to ∼400 nm in some places as shown in Figure 3b, but the particle arrangements did not drastically improve. Therefore, the surface pressure of 20 mN/m is the optimum for a high-quality continuous monolayer of gold nanoparticles. The above results are consistent with the following process for forming a dodecanethiol-capped gold particle LB monolayer. As we previously reported,25 the surface tension of the solvent causes numerous small particle arrays to form when a droplet of a highly concentrated particle suspension is spread and then evaporated on the water’s surface. Meanwhile, since the particle concentration is quite high, some of the arrays are in close proximately and the resulting interaction between these small particle arrays is large enough for the arrays to
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coalesce, forming the islandlike domains. Due to the poor array accommodations, many particle voids form within the islandlike domains, as shown in Figure 1a. When the barrier is moved, the islandlike domains gather. As observed by SEM, when the surface pressure is compressed to 10 mN/m, most of the domains gather to form a solid monolayer, but the particle void percentage remains high. Compressing the surface to the pressure of 15 mN/m causes the domain boundaries to overlap, but does not change the particle arrangement within the domain. As the surface pressure rose to a considerably high value, 20 mN/m, some of the compressing force was transferred to the internal particles in the domain structures and restructured the particles. Consequently, the particle voids are filled and a void-free monolayer of gold particles forms. As mentioned above, the islandlike domain structures of the gold particles are believed to form due to the attraction between the particles, which is induced by evaporating the solvent, and is very stable due to the uniform alkanethiol monolayer surrounding the gold particles.24 Therefore, it is difficult to change the particle arrangement. As a result, the particle arrangement within the domains does not improved till the surface pressure of 15 mN/m. Instead, the gold particles on the boundaries of the domains overlapped under the compressing force. On the other hand, the gap between the adjacent particles in the LB monolayer is almost twice the thickness of dodecanethiol monolayer capping the gold particles, which suggests that the dodecanethiol molecules attached to the gold particles do not interpenetrate between adjacent particles. This makes the particles in the LB monolayer mobile. Consequently, the internal particles in the domain structure are capable of forming highly ordered and voidfree arrays under a relatively high compressing force, in this work, under the surface pressure of 20 mN/m for gold particles that have a diameter of 8.5 nm. Conclusion A wafer-scale LB monolayer of dodecanethiol-capped gold nanoparticles was successfully fabricated using highly concentrated solution, which dramatically shortened the fabrication time. A detailed study was conducted on the effects of the surface pressure on the arrangement of the particles. The particle void percentage within the islandlike domains remained high as the surface pressure was compressed from 0 to 15 mN/m. Compressing the monolayer to a surface pressure of 20 mN/m, however, caused a 65-fold decrease in the void percentage and formed a nearly void-free monolayer. The results of this work demonstrate that under high surface pressure, the LB monolayers of alkanethiol-capped gold particles, which are formed on the surface of water, are capable of restructuring to form a highly ordered structure. Acknowledgment. This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO)’s “Nanotechnology Materials Program-Nanotechnology Particle Project”, based on funds provided by the Ministry of Economy, Trade and Industry, Japan (METI). LA0302293
