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“Rather than use a GaN buffer layer, we fabricate an ordered array of single-crystal sapphire nano-islands onto the sapphire substrate,” says Chan. “These modify the nucleation and growth mechanism of the GaN, resulting in a defect structure that is less detrimental to the optical properties of the device.”
Producing a high-quality array of single-crystal nano-islands would be difficult using standard etching techniques. The team adopted a novel approach, using electron beam lithography to create the nanopatterned array in a thin film of aluminum metal that had been evaporated onto the sapphire substrate. The aluminum nano-islands were oxidized at about 450 degrees C and converted into polycrystalline Al2O3, and then heated to 1,200 degrees C to induce grain growth, converting the structures into single-crystal sapphire.
“While this sounds like a long drawn-out process, it must be remembered that one of the major contributions to the cost of fabricating GaN LEDs is the time spent in the growth chamber,” says Chan. “All of our surface nanopatterning is done beforehand, which could lead to significant cost benefits.”
GaN-based LEDs grown on these nanopatterned substrates show a 24-percent improvement in output power over LEDs grown on conventional GaN templates. The increase is attributed to improvements in the device's internal quantum efficiency.
Lessons learned from fabricating GaN-based LEDs on nanopatterned sapphire substrates could contribute to the development of low-dislocation GaN material for solar cells, thermoelectric devices and smart-grid power electronics.
Fine-tuning the development of adult stem cells
Nanopatterning a polymeric surface could help scientists control the growth of adult stem cells and develop transplantation-based therapies.
Some researchers have learned that the mechanical stiffness of a flat substrate on which stem cells are placed has a profound effect on their subsequent structure and function. Nerve cells thrive on soft surfaces, while cartilage cells prefer harder surfaces.
But stem cells growing inside tissue do not encounter flat surfaces, says Sabrina Jedlicka, assistant professor of materials science and engineering and a member of the bioengineering program.
“Instead, they are confronted by topographies that vary on the nanoscale. Mimicking the cellular environment during early stem cell differentiation may provide a way to control the process and determine the type of cells that grow.”
Jedlicka and John Coulter, professor of mechanical engineering and mechanics, are trying to develop ‘off-the-shelf’ nanostructured polymeric surfaces, comprised of an ordered array of nanopillars or nanogrooves and designed to support specific types of stem cell differentiation.
“The mechanical properties of each surface will depend on the height or depth of these features and the spacing between them,” says Coulter, who chairs the International Micro/Nano Molding Technical Group of the Society of Plastics Engineers.
Coulter’s group was one of the first in the world to develop an injection molding process to fabricate nanostructures on a thermoplastic polymer.
“The trick is to create a suitable mold,” says Coulter. “In this case, we introduced the desired nanofeatures into a silicon mold using a combination of electron beam lithography and ion etching.”
To ensure that none of the thermoplastic polymer remained stuck to the mold, a thin film of plasma-polymerized ‘release’ film was then deposited onto the surface of the mold. This mold was then attached onto a micro-injection molding system.
“If you want to produce off-the-shelf nanostructured polymer surfaces at low cost, you have to produce them in large numbers,” says Coulter. “Injectionmolding in this respect is the only way to go, as the cycle time is around 15 seconds.”
