Optical properties of nanometer disks, holes and rings prepared by colloidal lithography

The optical properties of nanostructures prepared by colloidal lithography are studied. Colloidal lithography is a fabrication approach based on a self-assembling pattern definition. An electrostatically self-assembled array of nanoparticles is used as a mask and combined with etching or deposition processes. The experimental conditions and procedures required to obtain well controlled particle films with short-range order are discussed. Technical issues addressed are related to either the particle adsorption from solution or drying of the resultant particle film to produce a mask. Specific parameters studied include the influence of salt concentration in the particle solution, the particle size, the particle polydispersity and the use of a binder layer. Colloidal lithography is a new, fast and relatively simple approach for fabrication of nanostructures, with potential for large area (cm2) coverage. Nanostructures can be produced with variable and well defined size, shape and interparticle distance. Optical extinction spectra for the disks, holes and rings, are measured. The spectra for disks and holes show similar peaks with positions, which is interpreted as excitation of localized surface plasmons on the disks and around the holes. The similarities in resonance energy are in agreement with electrostatic cylinder theory for complementary structures (solid cylinder / cylindrical void). The peak positions for rings are red-shifted. This can be explained as symmetric coupling between superimposed disk and hole plasmon modes, lowering the resonance energy. In the short-range ordered arrays (lacking long-range order), we find no evidence for plasmon coupling for particles, while coupling through the gold film gives a blue-shift for hole arrays. The colloidal lithography fabrication approach and the optical results have relevance for, for example, the development of miniaturized biosensors