© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Bin Su, Yuchen Wu, Yue Tang, Yi Chen, Wenlong Cheng,* and Lei Jiang Free-Standing 1D Assemblies of Plasmonic Nanoparticles Dr. B. Su, Y. Tang, Y. Chen, Prof. W. L. Cheng, Prof. L. Jiang Department of Chemical Engineering Monash University Clayton, Victoria 3800, Australia E-mail: wenlong.cheng@monash.edu Dr. B. Su, Dr. Y. C. Wu, Prof. L. Jiang Beijing National Laboratory for Molecular Sciences (BNLMS) Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences (ICCAS) Beijing, 100190, P. R. China Y. Tang, Y. Chen, Prof. W. L. Cheng The Melbourne Centre for Nanofabrication Clayton, Victoria 3168, Australia DOI: 10.1002/adma.201301003 The ability to assemble elementary plas- monic nanoparticle building blocks into structurally well-defined nanoarchitectures [1] has implications in many frontier research areas ranging from miniaturized optoelec- tronics, [2,3] novel biosensing, [4,5] to energy harvesting devices. [6] These well-defined plasmonic nanoarchitectures may include “molecule-like” assemblies [7] and one- dimensional (1D) “polymer-like” ordered assemblies [8] as well as two-dimensional ordered assemblies (2D superlattices) [9] and three-dimensional ordered assemblies (supercrystals). [10] Over the past 20 or more years of development, significant progress has been made in synthesizing plasmonic nanoparticle building blocks. [1] However, the well-defined ordered assembly of these pre-synthesized building blocks remains in the embryonic stage of development due to the difficulties in controlling interactions among nanoparticles. As for 1D assemblies of nanoparticles, they are potential elements in future nano- photonic circuits due to their ability to guide and switch lights at the nanometer scale without diffraction limits. [2,3] Strong near-field coupling [11] and field enhancement effects [12] between plasmonic nanoparticles can lead to unidirectional optical transport along 1D nanopar- ticle chains at a subwavelength scale. [13] 1D assemblies of plas- monic nanoparticles can be fabricated by top-down lithography such as electron beam lithography, however, it is an expen- sive and time consuming process. Bottom-up self-assembly of pre-synthesized plasmonic nanoparticles can overcome these challenges. To date, biomolecule-directed assembly, [14] step polymerization, [15] chain polymerization, [8b] nanopore-assisted assembly [16] or carbon nanotube-templated assembly, [17] and combined top-down and bottom-up approaches [8a,18] have been developed for generating 1D assemblies of plasmonic nano- particles. However, these 1D assemblies are formed either in bulk solution or on solid surfaces. To the best of our knowl- edge, free-standing 1D assemblies of plasmonic nanoparticles, which are desired for designing plasmonic circuits free of elec- tromagnetic interference from substrates or solvents, have not yet been reported. Herein we demonstrate a simple yet effective method to gen- erate free-standing 1D assemblies of gold nanoparticles (GNPs) by a combined top-down and bottom-up approach in conjunc- tion with superhydrophobicity-directed fluid drying. Briefly, a droplet of plasmonic nanoparticle solution is positioned onto silicon micropillar arrays fabricated by top-down lithography. These micropillars serve as wetting defects to control the rup- ture of colloidal droplets, [19] generating a microscale liquid bridge between neighboring micropillars ( Scheme 1). Such a liquid bridge provides a confined space for guiding nanopar- ticle assembly, leading to the formation of free-standing 1D assemblies of GNPs after water evaporation. The free-standing GNP plasmonic nanowires can be as long as 30 μm and as thin as a single-particle width. Furthermore, we show 1D assemblies of GNPs can exhibit plasmonic waveguiding properties at the nanometer scale. We believe the combined top-down/bottom-up Scheme 1. Illustration of formation process of free-standing 1D assemblies of gold nanopar- ticles (GNPs) and their plasmonic waveguide properties. a) A droplet of GNP/calcein solution is carefully placed upon a superhydrophobic spindle-pillar structured surface. The concentra- tions of GNPs and calcein are 0.01% and 0.01% (w/w), respectively. b) Following unidirec- tional motion of the droplet, liquid films break forming microscale liquid bridges between neighboring micropillars. c) The liquid bridges are physically confined environments, leading to one-dimensional self-assembly of nanoparticles after water evaporation. 1D assemblies are robust and remain suspended across two micropillars. d) Multiple free-standing 1D assem- blies can form by the superhydrophobic micropillar-directed fluid drying process. e) Plasmonic waveguide based on a GNP linear assembly. Photons are coupled with electron oscillation of the assembled GNPs and localized surface plasmon resonance (LSPR) propagates along the particle chain. f ) Schematic illustration of how LSPR propagates. GNPs are smaller than the wavelength of light and the free electrons can oscillate collectively in the presence of an incident light beam, forming so-called LSPR. The LSPR of the first particle can induce electron oscillation of adjacent particles, therefore, lead to plasmon propagation along the chain. Adv. Mater. 2013, DOI: 10.1002/adma.201301003