"Nanostructures have dimensions in the wavelength range of visible light, and therefore they can be used directly as active optoelectronic devices," Zhifeng Ren of Boston College told nanotechweb.org. "For example, nanotube arrays can act as optical nanoantennas capable of directly receiving or transmitting light waves."

To make the crystals, the scientists first laid down particles of nickel catalyst by allowing a monolayer of polystyrene nanospheres to self-assemble onto a 10 mm-square silicon substrate. They used the nanospheres as a mask for depositing nickel by electron-beam evaporation: removal of the polystyrene particles with toluene left a honeycomb pattern of nickel dots.

The nickel dots then acted as a catalyst for the growth of aligned carbon nanotubes by hot filament plasma-enhanced chemical vapour deposition. "We demonstrated that an inexpensive self-assembly of nanotubes into highly ordered arrays is possible," said Ren.

The resulting periodic array of aligned nanotubes appeared colourful because it diffracted visible light. "We showed for the first time that there is a strong and very rich interaction between periodic nanotube arrays and visible electromagnetic radiation," added Ren. "There is a simple interaction leading to interference and diffraction effects, as well as photonic crystal interactions that change dispersions of the photonic modes, forming photonic bands and gaps."

Ren says that the photonic crystals have applications in components such as inexpensive demultiplexers, demodulators, filters and switches. Applications that use the photonic band-gap properties of the arrays, such as high efficiency filters and lossless reflecting surfaces, could also be significant.

And what now? The team will continue to work towards improving the quality of the nanotubes and the scale-up of the self-assembled arrays. "We will also attempt to combine carbon nanotube structures with other materials by thin-film coating to form optical composites with desirable optoelectronic properties," said Ren. "Most importantly, we will continue working towards device applications using the basic optical properties of nanotube arrays."

The scientists reported their work in Nano Letters.