New Nanopatterning Technique Driven by Light
Dennis M. Callahan
Most gardeners can tell you that certain plants will grow in the direction of a source of illumination, such as the sun. This phenomenon is known as phototropism and results from an interaction between the light and light-absorbing compounds within the plant. Researchers in the Light-Material Interactions in Energy Conversion (LMI) EFRC have discovered that a certain inorganic material not only grows in the direction of incident light, but also changes its shape dynamically as the illumination parameters are changed. This allows a rich variety of complex nanofeatures to be grown from a single system by simply changing external illumination conditions.
The LMI researchers used an alloy of selenium and tellurium as the growth material in their experiment. The alloy belongs to a class of materials typically used for optics applications such as optical fibers and photodetectors, as well as in rewritable CDs and DVDs.
If a solution of positively charged selenium and tellurium ions is placed in an electrochemical cell and a voltage is applied across two electrodes, a selenium-tellurium alloy film will deposit on the negatively charged electrodes. The researchers took one of these electrochemical cells and shined light on it while the film was being deposited. They found that with illumination the deposited selenium-tellurium was nanostructured differently than it was without any illumination.
Even more striking, when they changed the illumination conditions, such as the incident angle, wavelength, or polarization, the deposited films showed a different, controllable nanostructure. For example, if the incident light was sent in at an angle, the films grew as tilted columns oriented toward the illumination. If the wavelength of the incident light was changed, the periodic nanostructure of the films changed in accordance with the wavelength. To make things even more interesting, if the incident light was changed during the growth, the morphology of the films would change dynamically. This means that a nanostructure could start out with one angle, periodicity, or size and change as it grew.
As complex as the process must be on the molecular level, the researchers modeled and very accurately predicted the growth morphologies using a combination of intensive computer simulations. This gave the researchers an effective way to study the feedback between the local electric field intensity from the illumination and the nucleation and dynamic growth of the nanostructure.
This technique could be used to develop a novel type of nanopatterning that could influence nanotechnology fields, such as nano optics for more efficient solar energy harvesting or more capable imaging technologies, and nanoelectronics for faster computers.
"These results pave the way for a new type of photolithographic patterning where complex 3D structures can be designed through dynamic feedback between optical excitation and the evolving nanoscale morphology," said lead author Bryce Sadtler.
This new nanopatterning method developed at the LMI adds to the array of current nanofabrication techniques, with the benefit of its simplicity, tunability and ability to create complex 3D patterns with ease.
Sadtler B, SP Burgos, NA Batara, JA Beardslee, HA Atwater, and NS Lewis. 2013. "Phototropic Growth Control of Nanoscale Pattern Formation in Photoelectrodeposited Se-Te Films." Proceedings of the National Academy of Sciences 110:19707-19712. DOI: 10.1073/pnas.1315539110