The optical response of ring-shaped gold nanoparticles prepared by colloidal lithography is investigated. Compared to solid gold particles of similar size, nanorings exhibit a redshifted localized surface plasmon that can be tuned over an extended wavelength range by varying the ratio of the ring thickness to its radius. The measured wavelength variation is well reproduced by numerical calculations and interpreted as originating from coupling of dipolar modes at the inner and outer surfaces of the nanorings. The electric field associated with these plasmons exhibits uniform enhancement and polarization in the ring cavity, suggesting applications in near-infrared surface-enhanced spectroscopy and sensing.

Self-organization of colloidal particles on surfaces to form 2D or 3D nanofabrication templates has been explored actively in the past decade as an effective bottom-up method to produce a plethora of nanoarchitectures with diverse functionalities. Specifically, several elegant approaches to pattern surfaces with large-scale 2D arrays of nanosized structures through lateral self-assembly of colloidal spheres have been developed. These methods are commonly termed colloidal lithography (CL). A frequently used version of CL, nanosphere lithography (NSL) employs organized 2D colloidal crystals with a hexagonal close-packed motif as an evaporation mask, often in combination with reactive ion etching. Evaporation through the holes between close-packed nanospheres defines the resulting pattern, and in many applications material deposition conditions such as evaporation angle or specific deposition technique (e.g., sputtering, thermal deposition) are used to vary the achieved patterns. With this method facile production of vast planar arrays of diverse nanostructures has been accomplished. [1][2][3][4][5][6][7][8] In an alternative approach, referred to here as sparse colloidal lithography (SCL), charged colloidal beads are utilized in a similar manner as in NSL. [9,10] This method, developed in our group, enables facile production of large areas (several cm 2 ) of nanoscopic features like holes in thin films, disc-, ringand crescent-shaped structures with overall sizes currently down to 20 nm and which occupy 10 to 50 % of the total surface area. [11][12][13][14][15][16] The size distribution of SCL-fabricated nanostructures is largely determined by the size dispersions of the masking colloids and is typically less than 5 % for colloids with average diameters > 100 nm and up to 10 % for smaller colloids. In contrast to NSL, a sparse monolayer of colloidal particles defines the evaporation/etch mask in SCL. The convenience of this technique, employing charged polystyrene (PS) nanoparticles as etch and/or evaporation mask, has recently been demonstrated in a variety of applications such as investigation of fibroblast response to nanotopography, [17] model catalysts of Pt/alumina and Pt/ceria [18] and in the study of optical properties of macroscopic arrays of supported metallic nanostructures like discs, crescents, or rings or nanoholes in optically thin films. [11,13,14,16] In spite of the general advantage of facile bottom-up nanofabrication and a large variety of possible nanostructural motifs, SCL has so far been subject to limitations in producing nanostructures composed of materials with unfavourable etching selectivity, that is, where the substrate or polystyrene etch rates compete with the etch rate of the actual materials of the nanostructure. Examples of such systems are Pt on Au or Au-silica hybrid structures on glass. Another disadvantage of the method is the necessity of the reactive oxygen treatment for the PS mask removal so that nanostructures composed of the materials prone to oxidation (like Ag or Ru) rap...

The optical responses of 75-150 nm diameter gold nanorings to changes in local refractive index have been quantified by near-infrared extinction spectroscopy and compared to DDA calculations and an analytical approach. The "bulk" refractive index sensitivities of gold nanorings are substantially (>5 times) larger than those of nanodisks with similar diameters. Nanorings retain a significantly larger sensitivity than nanodisks at the same spectral position, demonstrating a clear shape dependence that may correlate to a systematic difference in the influence of the dielectric substrate. The nanoring bulk refractive index sensitivity scales linearly with plasmon peak position. The spectral sensitivity to thin films of alkanethiols gave a shift of approximately 5.2 nm/CH2 unit while bulk sensitivities as high as 880 nm/RIU were observed, the highest such reported sensitivities. Both bulk and thin dielectric film sensitivities correlated well with theory. Real-time label-free monitoring of protein binding via molecular recognition was demonstrated.

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