Free-standing, patternable nanoparticle/polymer monolayer arrays formed by evaporation induced self-assembly at a fluid interface.

Ordered nanoparticle (NP) monolayers are of fundamental interest as 2D artificial solids in which electronic, magnetic, and optical properties can be tuned through electron charging and quantum confinement of individual NPs mediated by coupling interactions with neighboring NPs.1,2 They are also of technological interest for a diverse range of applications including photovoltaics,3 sensors,4 catalysis,5 and magnetic storage.6 To date a myriad of techniques including Langmuir-Blodgett (LB) deposition,1,7,8 droplet evaporation,9-12 and interfacial assembly13 have been used to organize monosized, hydrophobic NPs into ordered 2D arrays. It is generally agreed that for all these approaches NP assembly is driven by attractive van der Waals interactions balanced by steric repulsion.14 However, under attractive conditions, depending on the competing effects of NP diffusion, convection, and solvent dewetting, a variety of patterns can emerge including ordered 2D and 3D arrays as well as fractal aggregates, ‘coffee rings’, and percolation clusters.15-18 For this reason maintaining NPs at a fluid interfaceseither in evaporating droplets13 or at a water surface19s has been successful in creating rather large scale, ordered NP monolayers. van der Waals interactions confined to NP interstices confer to these monolayers a high Young’s modulus allowing them to be prepared as free-standing membranes spanning 500-nm apertures13 and to be transferred from a water surface to a solid substrate via microcontact printing using a prepatterned poly(dimethylsiloxane) (PDMS) stamp.19,20 Here we extend the burgeoning work on NP monolayer fabrication in three substantive respects. First, we assemble hydrophobic gold NPs on a water interface from toluene containing polymethylmethacrylate (PMMA). In this case solvent evaporation concentrates the thinning film in NPs and PMMA, leading to NP selfassembly and solidification into an ordered NP/polymer nanocomposite monolayer with physical dimensions of up to 10 cm2 (see Figure 1a). Second, the NP/PMMA monolayer can be transferred to arbitrary substrates and remains stable as a freestanding membrane suspended over cm-sized holesseven with free edges (Figure 1a). Third, the PMMA serves as a photoresist enabling two modes of electron beam (e-beam) NP patterning. Lower e-beam doses direct differential NP solubility and result in NP patterns with somewhat diffuse interfaces as shown previously for e-beam patterning of 1-dodecanethiol ligated Au monolayers deposited on silcon nitride21 (Figure 2a,b). At higher e-beam doses the PMMA serves as a negative resist resulting in submicrometer patterns with edge roughness comparable to that of the NP diameter (Figure 2c,d). In combination these extensions contribute to the ability to integrate NP arrays into robust microand macroscale devices. Additionally this approach represents a new means to attain very highly loaded yet flexible particle/polymer nanocomposites,18 which have eluded most synthetic efforts to date. Monosized, 1-dodecanethiol-ligated Au nanoparticles (about 5.5 nm in diameter) were synthesized by a modified single-phase method22 (see the Supporting Information). For monolayer self† Department of Chemical and Nuclear Engineering, The University of New Mexico. ‡ Sandia National Laboratories. § Center for High Technology Materials, The University of New Mexico. Figure 1. (a) Optical image of NP/PMMA film transferred to glass, arrow shows ∼1 cm2 unsupported area; (b) SEM image of NP/PMMA film transferred to silicon; (c) GISAXS pattern of film prepared as in (b); and (d) TEM of free-standing NP/PMMA film on a holey carbon grid.