Transparent Spheres Simplify 3-D Nanolithography

A photolithography technique using nanospheres could make fabrication of 3-D structures easier, and at lower costs.

The new technique, developed by a team from North Carolina State University and based on oblique illumination of colloidal particles, employs nanoscale spheres and UV light to produce hollow-core 3-D nanostructures with complex symmetry, according to a study. This method could be used for a variety of biomedical, electronics and photonics applications.

Nanospheres used to develop 3-D structures bend and scatter the light that passes through them according to the angle of incidence. Images courtesy of Xu Zhang/North Carolina State University.

This new avenue has already proved significantly less expensive and time-consuming than conventional lithography methods, the researchers said. Existing procedures use a variety of expensive techniques that require specialized lenses, mechanical probes, electron beams or lasers to focus light on a photosensitive film when creating 2-D patterns. To subsequently create 3-D structures, numerous 2-D patterns must be printed on top of each other.

“Our approach reduces the cost of nanolithography to the point where it could be done in your garage,” said professor Dr. Chih-Hao Chang.

The researchers found that by placing nanoscale polystyrene spheres on the surface of photosensitive film, the pattern of light can be shaped to create nanostructures in three dimensions.

The nanospheres are transparent but can bend and scatter the light based on its angle of incidence. The researchers controlled the nanolithography by altering the size of the nanospheres and the duration of light exposures, as well as the angle, wavelength and polarization of light.

A variety of nanostructure designs are possible, too, as single or multiple beams of light can be used. The nanospheres self-assemble in a regularly spaced array, which could allow the creation of uniform patterns of 3-D nanostructures.

A variety of asymmetric 3-D nanostructures can be fabricated by shining light on nanoparticles.

“This could be used to create an array of nanoneedles for use in drug delivery or other applications,” said doctoral candidate Xu Zhang.

The researchers are now looking at using the new technique to create nanoscale inkjet printers for printing electronics or biological cells, or to create antennas or photonic components. They are also studying the potential use of other materials to create the 3-D nanostructures.

“We’re exploring the use of nanosphere materials other than polystyrene, as well as nanoparticle shapes other than spheres,” Chang said. “And ultimately we want to look at ways of controlling the placement of particles on the photosensitive film in patterns other than uniform arrays.”

The work was funded by NASA and the North Carolina State Nanosystems Engineering Research Center for Advanced Self-Powered Systems of Integrated Sensors and Technologies.

The research was published in Small (doi: 10.1002/smll.201402750).

For more information, visit www.ncsu.edu.