One idea at the forefront of nanoassembly is to use the properties of certain nano and microparticles to have them assemble themselves into something useful. Researchers at the Weizmann Institute recently discovered that when left to fend for themselves, long carbon nanotubes will fall in such a way as to create a semi-ordered structure. As the nanotubes themselves possess many properties favorable to these applications, this sort of simple nanoassembly could be used to quickly and easy create microscale radiators or antennas for tiny electronic devices.

This week, University of Pennsylvania researchers published results in the journal Nano Letters that outline the use of a common silicon polymer, known as polydimethylsiloxane or PDMS, to create nanoscale patterns of suspended solids. Their PDMS molds are highly durable, reusable many times over. Due to the material's flexibility and durability, it can also be used to transfer the patterns to various non-flat surfaces. To add even more value, the nanoscale patterning process is also much cheaper than standard chemical lithography.

PDMS is so common, you've probably never heard of it. But its properties are what make it an ideal substance for Penn's use. At low temperatures, the polymer acts like an elastic solid, rubber for example. If a cube of PDMS were dropped on a table, it would bounce. At the same time, it can be pulled and formed into different shapes. At higher temperatures, the material flows faster, like molasses or honey. It will flow into surface irregularities on a surface if left alone, which makes it useful for various industrial applications deal with forms.

If these properties sound familiar, it's because you've probably enjoyed something like it as a child: Silly Putty. Silly Putty is, in fact, about 4% PDMS. It can also be found in everything from cosmetics to medical devices to contact lenses to caulking to lubricant. It's even used in food additives and in treatments for head lice.

To form their nanoscale patterns, Penn researchers created a layer of PDMS gel on a grid of silicon pillars, 1 µm in diameter, each spaced 2 µm apart. In order to create the “diamond plate” pattern they used, the group harnessed known properties of these gels to swell while wetted by a solvent. The circular pores created in the material eventually deform elliptically along the same axis due to elastic interaction while the polymer is swelled under the influence of the solvent.

To create the pattern itself, the Penn group dissolved Fe3O4 nanoparticles into toluene solvent to create a solution. The PDMS gel is then bathed in the solution. The swelling mechanism creates the desired pore patterns, and the nanoparticles flow into place due to convection. Once the pattern is transferred, and the polymer removed from the solution, it returns faithfully to its original form.

“These functional nano-motifs could in turn benefit novel technologies that are sensitive to local environment change such as smart clothing, biomarkers and eco-friendly buildings. Using similar pattern transformation principles, our technique could be extended to pattern a variety of material systems such as polymers and composites, creating a new design mechanism for nanoscale manufacturing,” explained assistant professor Shu Yang of Penn's School of Engineering and Applied Science, Department of Materials Science and Engineering.

Nanopatterning could certainly benefit several fields currently being explored by nanotech researchers. Tiny electrical circuits require tiny conductors in fine patterns; plasmonics and photonics both utilize microscopic patterns to coerce particle/waves into specific modes or places, just to name a few. Anything that could benefit from smaller mechanical and electrical devices, especially in the medical field, could see an impact from technology such as Penn's cheap, reusable nanoscale patterning mechanism.