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Ordered Arrays of Amphiphilic Gold Nanoparticles in Langmuir Monolayers Joseph J. Brown, Jessica A. Porter,† Charles P. Daghlian, and Ursula J. Gibson* Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire 03755 Received May 8, 2001. In Final Form: August 23, 2001 Large ordered arrays of gold nanoparticles with 1.4 nm cores were created in Langmuir monolayers from an amphiphilic starting material, Palmitoyl Nanogold. Transfer of the films to transmission electron microscope grids by the Langmuir-Schaeffer method resulted in observation of highly ordered hexagonal arrays larger than 250 × 350 nm2. The application of sustained pressure increased the size of ordered regions over those observed immediately after compression. Our work demonstrates ordered arrays with particles that are smaller than those that have been successfully ordered previously in Langmuir layers. The amphiphilic nature of the particles assists in maintaining a monolayer during compression.
Introduction Metal nanoparticles have attracted a great deal of interest in recent years because of their transport and optical properties; in particular, the observations of Coulomb blockade effects1,2 and of a metal-insulator transition3,4 in films of these materials have highlighted their unique electrical properties. In particles of 1-2 nm diameter, there is a possibility of making room-temperature single-electron devices.2 At present, most transport measurements are quasi one-dimensional, made perpendicular to the nanoparticle layer. However, in the future planar devices will be important. If two-dimensional systems are to be explored, it is desirable to form ordered as well as random arrays of these materials in order to clarify the role of disorder and noise in their performance. The technique described here should be useful for deposition of such arrays onto electrodes for electrical characterization. There are numerous reports of Langmuir-Blodgett (L-B) films and spontaneous assembly of two-dimensional ordered arrays in particles with core diameters of 3-10 nm5-11 but many fewer on particles in the 1-2 nm diameter range.12 Reports of ordering in alkanethiolate- and boro* Corresponding author. E-mail:
[email protected]. Phone: 603-646-3243. Fax: 603-646-3856. † Colby College. (1) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (2) Wang, B.; Wang, H. Q.; Li, H. X.; Zeng, C. G.; Hou, J. G.; Xiao, X. D. Phys. Rev. B 2001, 6303, 5403. (3) Markovich, G.; Collier, C. P.; Heath, J. R. Phys. Rev. Lett. 1998, 80, 3807. (4) Henrichs, S.; Collier, C. P.; Saykally, R. J.; Shen, Y. R.; Heath, J. R. J. Am. Chem. Soc. 2000, 122, 4077. (5) Ohara, P. C.; Heath, J. R.; Gelbart, W. M. Angew. Chem. 1997, 36, 1078. (6) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (7) Gelbart, W. M.; Sear, R. P.; Heath, J. R. Faraday Discuss. 1999, 112, 299. (8) Sear, R. P.; Chung, S. W.; Markovich, G. Phys. Rev. E 1999, 59, R6255. (9) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T. Acc. Chem. Res. 1999, 32, 397. (10) Gutierrez-Wing, C.; Santiago, P.; Ascencio, J. A.; Camacho, A.; Jose-Yacaman, M. Appl. Phys. A 2000, 71, 237. (11) Ohara, P. C.; Leff, D. V.; Heath, J. R.; Gelbart, W. M. Phys. Rev. Lett. 1995, 75, 3466. (12) Chi, L. F.; Rakers, S.; Hartig, M.; Gleiche, M.; Fuchs, H.; Schmid, G.
hydride-passivated clusters13-15 in both uniform and bimodal distributions indicate the strong driving force for 2-D crystallization in 6-10 nm clusters, with high-quality arrays forming. Langmuir-Blodgett films of alkanethiolate-passivated particles have been studied as a function of size distribution, alkane chain length, and surface pressure,6 and it is found that the ratio of ligand length to core diameter is crucial in determining whether ordered arrays or lacy networks form. This is in accord with reports on spontaneous ordering in three dimensions.9 Facecentered cubic (fcc) arrays are observed in systems with large (core diameter/interparticle separation) ratios for alkanethiolate-coated gold. The transition from fcc to lower density crystals is in good agreement with the molecular dynamics calculations of Landman et al.13 Several contributions, including entropy and van der Waals forces between cores,11 and steric interactions6,13 between the passivating ligands are important in determining the degree of order possible in a given system. For larger particles and short alkanethiol ligands, the van der Waals forces dominate, and the particles order spontaneously either on a water subphase or a TEM grid as the solvent evaporates. In systems where spontaneous ordering is limited, there are also reports of using both pressure17 and solvent-based10 “annealing” techniques to improve order in these arrays. Even when ligand interactions favor ordering, spontaneous ordering may be short-range or may nucleate in multiple sites, and an external driving force will be required for the formation of large arrays. As reported by Sear et al.,18 with 4-8 nm particles coated by short-chain ligands, small spontaneously ordered rafts form. Upon increasing the surface density of particles, either by increased concentration or compression, filamentary and then hexagonal array structures emerge. Longer range ordering may also be induced during solvent evaporation by the changing shape of the meniscus, which provides a local driving force for organization.5 More rigid ligands have also been explored in the attempt to order smaller clusters. Chi et al.12 used triphenylphosphine and T8OOS(13) Cleveland, C. L.; Landman, U.; Schaaff, T. G. Phys. Rev. Lett. 1997, 79, 1873. (14) Kiely, C. J.; Fink, J.; Zheng, J. G. Adv. Mater. 2000, 12, 640. (15) Brust, M.; Bethell, D.; Kiely, C. J. Langmuir 1998, 14, 5425. (16) Fink, J.; Kiely, C. J.; Bethell, D. Chem. Mater. 1998, 922. (17) Chen, X. Y.; Li, J. R.; Jiang, L. Nanotechnology 2000, 11, 108. (18) Sear, R. P.; Chung, S. W.; Markovich, G. Phys. Rev. E 1999, 59, R6255.
10.1021/la010691s CCC: $20.00 © 2001 American Chemical Society Published on Web 11/17/2001
Letters
Langmuir, Vol. 17, No. 26, 2001 7967
Figure 1. Maximum grain size of ∼30 nm. This film was transferred and imaged 2 h after spreading and immediately after the third compression. The maximum surface pressure used was 35 mN/m.
stabilized Au55 clusters on a Langmuir-Blodgett tank with a conventional dipping technique. These ligands should minimize steric interactions, but in their work, only very small ordered regions composed of 10-20 clusters were observed. In this paper, we use Au55 particles (1.4 nm) with similar stiff ligand coatings to minimize steric interactions, but which are also amphiphilic in order to resist film collapse. We apply surface pressures higher than those Chi et al. used, for long time periods. The applied pressure drives the coalescence of smaller ordered regions into coherent arrays of micron dimensions. Some defects are observed within the arrays, but lattice orientation, as determined by a Fourier transform of the image, can be maintained over hundreds of particle diameters. Experiment Langmuir monolayers of gold nanoparticles were created by depositing an alcohol solution of nanoparticles onto deionized water (18 MΩ, pH ) 7.0) at room temperature (23 °C) in a JoyceLoebl Langmuir trough. All trough surfaces were cleaned prior to film spreading by sequential wiping with water, 2-propanol, ethanol, methanol, and cyclohexane. A fresh filter paper Wilhelmy plate was used for each run. The films reported here were deposited from Nanoprobes Palmitoyl Nanogold. These 1.4 nm diameter particles are coated with triphenyl phosphine groups,19 some of which have p-Nmethyl-carboxamidophenylphosphine groups in place of the triphenyl phosphine. On average, one of the p-N-methylcarboxamidophenylphosphine groups on each particle has a reactive aminopropyl group in place of the methyl group attached to an amide. An amide linkage ties the amine part of the aminopropyl group to palmitic acid (C14H29COOH). Several nitrile groups may also be present in the coating.20 The overall effect is amphiphilic: a weakly polar particle with one nonpolar alkane tail and an otherwise stiff coating. Particles were stored dry in a sealed container in a freezer at -13 °C. These particles degraded to larger agglomerates when stored for longer than 1 month before film formation. Despite the instability of the starting material, (19) Powell, R. Nanoprobes, Inc. Personal communication. “In addition to the palmitoyl ligand, Nanogold is also passivated with a mixture of triphenylphosphine and a N-methyl-carboxamidophenylphosphine similar to those described for the undecagold (Hainfeld, in Colloidal Gold: Principles, Methods and Applications, M. A. Hayat (Ed.), Academic Press, San Diego, CA, 1989; Vol. 2, 413.). The palmitoyl moiety is actually coordinated via a cross-linking reaction to a similar coordinated phosphine which bears a reactive substituent.” (20) From Nanoprobes, Inc., Yaphank, NY.
Figure 2. Maximum grain size of ∼150 nm. This image shows a film 2.5 h after it was last compressed and 19.5 h from the time it was created. This film was compressed five times; the maximum surface pressure was 35 mN/m. This is the same film seen in Figure 1, 17.5 h and two compressions later. degradation was not observed in films stored at room temperature after deposition onto electron microscope grids, even after 2-3 months; it is possible that oxidation or unintentional cross-linking during handling stabilizes the material. Palmitoyl Nanogold20 (10-20 nmol) was dissolved in anhydrous ethanol or methanol and 2-propanol 50%/50 vol %, at concentrations of 1 nmol/10 µL and 1 nmol/25 µL. With the surface compression barrier retracted to give the largest surface area (∼100 cm2), the solution was deposited dropwise onto the surface with a microsyringe. Small (
