To enable these tailored applications, independent control of both particle size and separation is necessary. However, it is this precise parameter control that represents a barrier to most large-scale fabrication methods. Bottom-up techniques, such as nanosphere lithography, meet their limits if the final particle size and distance are strongly interrelated due to their common dependence on the diameter of the colloidal spheres used as masks.

Diblock copolymer micelle lithography (BCML), on the other hand, allows the interparticle distance to be manipulated without changing the particle size, and can tailor the particle size for a given particle separation. However, so far particle diameters have been restricted to values below approximately 15 nm. This leaves a size gap for larger particles with diameters between 15 and 100 nm – a range that is especially interesting for optical applications.

Inspirations from photography

Reporting their results in the journal Nanotechnology, researchers from the Technische Universität Dresden and the University of Ulm have demonstrated that requirements for both parameters, size and shape, can be met by their newly developed approach. For this purpose, BCML-fabricated hexagonally ordered arrays of metallic nanoparticles were used as seed particles in a photochemical growth process.

The approach was inspired by century-old findings in the field of photography. The particles were covered with a metal salt solution and irradiated with UV light. The photo-induced and catalytically enhanced reduction of the metal salt takes place only at the surface of the seed particles and thus leads to their growth. As a result, the range between 15 and 100 nm is now accessible. The team used thermal annealing to successfully control the particle shape.

Full details can be found in the journal Nanotechnology.