Self-organization of large gold nanoparticle arrays.

The synthesis of nanostructured materials with tunable properties is central to developments in nanoscale science and technology. Nonlithographic approaches based on thermodynamically driven self-organization processes are especially appealing because of their potential for low overhead in large-scale production. The spontaneous organization of monolayer-protected metal nanoparticles into periodic two-dimensional (2D) arrays is archetypal of this approach, with many of these arrays demonstrating novel optical or electronic properties as a function of particle size or interparticle spacing.1 Hydrophobic surfactants such as alkanethiols are often used to drive the nanoparticles toward selforganization at the aqueous interface; however, 2D array formation by this methodology has so far been successful only for small (<10 nm) metal particles.2,3 Stabilized metal particles beyond this size threshold become increasingly prone to multilayer or threedimensional aggregate formation, which can be attributed to the rapid increase in van der Waals attraction and the loss of surfactant chain mobility on the planar facets of the nanoparticles as a function of size.4 In this communication we describe conditions that enable large (16-170 nm) gold particles to self-organize at the air-water interface into monoparticulate films, which can subsequently be transferred onto substrates as 2D hexagonal close-packed (hcp) arrays. The choice of surfactant is crucial in the stabilization of these nanoparticle ensembles: the surfactant layer is required to be highly repulsive at close range but thin enough to maintain minimal interparticle separations, a critical parameter in the electromagnetic properties of metal nanoparticle assemblies.1,5 Short-range repulsion can be enhanced by creating a surfactant layer with hydrophobic chains at intermediate packing densities6 and an appreciable surface charge density for electrostatic doublelayer repulsion.7 These features also render the encapsulated nanoparticles amphiphilic and promote self-organization at the air-water interface. We recently demonstrated that calix[4]resorcinarenes are excellent surfactants for steric stabilization, and we have used them to maintain dispersions of gold nanoparticles up to 20 nm in hydrocarbon solutions.8 We now report that resorcinarene tetrathiol 1 is ideal for promoting the formation of 2D gold nanoparticle arrays with periodicities up to 170 nm, a length scale comparable to the resolution limits of conventional photolithography. Resorcinarene-encapsulated gold nanoparticles of low size dispersity were prepared by treating aqueous suspensions of citrate-stabilized gold particles with a solution of 1 in THF, followed by several washes with toluene.9 The adsorption of the tetrathiols onto the nanoparticle surfaces was expected to be robust, given the multivalent nature of the surfactants10 and the low rates of desorption of aryl monothiols from gold surfaces.11 The amphiphilic nanoparticles were confined to the solvent interface and could be carefully collected from the bulk solution using silanized pipets. Concentrated suspensions of nanoparticles were spread onto air-water interfaces and transferred as films in the absence of applied surface compression onto surfaces by horizontal (Langmuir-Schaefer) deposition or by slow vertical retraction of immersed substrates. Either method produced essentially monoparticulate films with hcp order as determined by transmission electron microscopy (TEM), demonstrating that the encapsulated nanoparticles organized spontaneously into 2D superlattices at the air-water interface (see Figure 1). The degree of order within the close-packed domains suggests that the uniformity of the arrays is determined largely by the size and shape dispersity of the nanoparticles themselves.12 Careful inspection of the TEM images reveals an inverse correlation between array periodicity and interparticle spacing, an indication of the increase in van der Waals attraction with unit particle size (see Figure 2).9 Periodicities obtained by Fourier transform analysis were not sufficiently precise to provide meaningful estimates for average interparticle separations, which ranged from 6 to less than 1% of the average particle diameter (1) For a recent review, see: Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. ReV. Phys. Chem. 1998, 49, 371-404. (2) Osifchin, R. G. PhD Thesis, Purdue University, 1994. (3) To the best of our knowledge, the largest unit size reported for a closepacked 2D gold nanoparticle array is 18.5 nm, prepared by electrophoretic deposition: Giersig, M.; Mulvaney, P. J. Phys. Chem. 1993, 97, 6334-36. (4) Badia, A.; Cuccia, L.; Demers, L.; Morin, F.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682-92. (5) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978-81. (6) The importance of chain packing density on conformational entropy has been discussed for amphiphile monolayers at the air-water interface: Szleifer, I.; Ben-Shaul, A.; Gelbart, W. M. J. Phys. Chem. 1990, 94, 508189. (7) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1992. (8) Stavens, K. B.; Pusztay, S. V.; Zou, S.; Andres, R. P.; Wei, A. Langmuir 1999, 15, 8337-39. (9) See Supporting Information for further details. (10) Surfactants with several well-spaced thiol groups have been suggested for enhanced adsorption to gold surfaces because of cooperative binding as well as low probability of desorption via disulfide formation: Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528-36. (11) Mohri, N.; Matsushita, S.; Inoue, M. Langmuir 1998, 14, 2343-47. (12) It has been noted that low size dispersity (5-10% relative standard deviation from the mean) is an important factor in the preparation of sterically stabilized colloidal crystals: Oxtoby, D. W. Nature 1990, 347, 725-30. Figure 1. Transmission electron micrograph (Philips EM-400, 80 keV) of self-organized 2D array of 70 ( 5 nm gold particles encapsulated by 1.The array was transferred onto Formvar-coated Cu grid (300 mesh) by manual Langmuir-Schaefer deposition in the absence of applied lateral compression. (Inset) Fourier transform of 70-nm particle array. 7955 J. Am. Chem. Soc. 2001, 123, 7955-7956