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PUBLISHED ONLINE: 10 JUNE 2012 | DOI: 10.1038/NNANO.2012.83

Self-orienting nanocubes for the assembly of plasmonic nanojunctions

Bo Gao, Gaurav Arya and Andrea R. Tao*

Plasmonic hot spots are formed when metal surfaces with high curvature are separated by nanoscale gaps and an electromagnetic field is localized within the gaps. These hot spots are responsible for phenomena such as subwavelength focusing1,2, surface-enhanced Raman spectroscopy3 and electromagnetic transparency4, and depend on the geometry of the nanojunctions between the metal surfaces5. Direct-write techniques such as electron-beam lithography can create complex nanostructures with impressive spatial control6 but struggle to fabricate gaps on the order of a few nanometres or manufacture arrays of nanojunctions in a scalable manner. Self-assembly methods, in contrast, can be carried out on a massively parallel scale using metal nanoparticle building blocks of specific shape7,8. Here, we show that polymer-grafted metal nanocubes can be self-assembled into arrays of one-dimensional strings that have well-defined interparticle orientations and tunable electromagnetic properties. The nanocubes are assembled within a polymer thin film and we observe unique superstructures derived from edge–edge or face–face interactions between the nanocubes. The assembly process is strongly dependent on parameters such as polymer chain length, rigidity or grafting density, and can be predicted by free energy calculations. A significant challenge in the self-assembly of shaped nanoparticles is the formation of non-close-packed nanoparticle groupings that adopt specific interparticle orientations, which precludes the use of assembly methods that involve colloidal crystallization or jamming. Several strategies have been developed for achieving programmable assembly through anisotropic chemical modification of the nanoparticle surface, including the use of DNA linkers9,10, block co-polymers11,12 or patchy...