"Some of our catalysis colleagues recently asked us if we could make very small 'islands' of titania surrounded by silicon," explained team leader Jillian Buriak. "We took up this challenge. The idea was that if we could put gold nanoparticles selectively on the islands, then the particles would be trapped because they would not be able to migrate over the silicon to meld with gold particles on other titania islands."

The researchers, from the National Institute for Nanotechnology in Alberta and the University of Alberta, found that the surface chemistry of silicon was critical in their experiments and that "parallel functionalization" on the surface was possible. "We could do chemistry either inside or outside the pores on silicon previously etched with nanoscale patterns," said Buriak. "This is a versatile technique."

Unsocial block copolymers
The Alberta team used long polymers, called block copolymers, that have two different strands of polymers forced to "hold hands" with each other through covalent links. These polymers "do not get along", explained Buriak, so, under certain conditions, they will separate out to form nanoscale structures and spontaneously self-assemble into complex patterns – in this case hexagonal arrays.

Since the block polymers employed are made up of two polymer strands that have different properties, this means that the silicon surface is contacted by two different blocks. One of the blocks can transport reagents – in this case, hydrofluoric acid (HF) – to the surface of the silicon, so etching it.

Total control
The chemically functionalized structures have regular shapes, depending on the orientation of the starting silicon. Their size can be controlled too by changing etch time and HF concentration. "What's more the chemical properties of the structures inside the pores are different from the flat top silicon surface, which means that you can do reactions inside the pores, or on top, or in both areas at the same time," Buriak told nanotechweb.org. "You have total control over what you can do, on the nanoscale, which is great."

While catalysis was their initial goal in this work, the researchers believe that tissue engineering and interfacing is going to be the most important application for this kind of nanopatterning technique. "A growing body of research shows that cells/tissues have nanopatterns of receptors on their exterior and therefore 'prefer' a nanopatterned surface with a very specific structure," said Buriak. "Titania can be used in devices like bone implants, suggesting a possible use for nanopatterned titania, but the silicon itself could be useful for interfacing neurons too, for instance."

The team now plans to thoroughly investigate the range of shapes, patterns and nanostructures that can be made. "Thankfully, so many block copolymers are commercially available, so we can study many different shapes, sizes and structures to make new materials," added Buriak The work was reported in ACS Nano.