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nanotimes Research

terial, said Suenne Kim, the paper’s first author and a postdoctoral fellow in laboratory of Professor Elisa Riedo in Georgia Tech’s School of Physics.

Ferroelectric materials are attractive because they ex- hibit charge-generating piezoelectric responses an or- der of magnitude larger than those of materials such as aluminum nitride or zinc oxide. The polarization of the materials can be easily and rapidly changed, giving them potential application as random access memory elements.

But the materials can be difficult to fabricate, requi- ring temperatures greater than 600° Celsius (1,112° F) for crystallization. Chemical etching techniques produce grain sizes as large as the nanoscale features researchers would like to produce, while physical etching processes damage the structures and reduce their attractive properties. Until now, these challen- ges required that ferroelectric structures be grown on a single-crystal substrate compatible with high temperatures, then transferred to a flexible substrate for use in energy-harvesting. The thermochemical nanolithography process, which was developed at Georgia Tech in 2007, addresses those challenges by using extremely localized heating to form structures only where the resistively-heated AFM tip contacts a precursor material. A computer controls the AFM writing, allowing the researchers to create patterns of crystallized material where desired. To create energy-harvesting structures, for example, lines cor- responding to ferroelectric nanowires can be drawn along the direction in which strain would be applied.

“The heat from the AFM tip crystallizes the amor- phous precursor to make the structure,” Bassiri-Gh- arb explained. “The patterns are formed only where

Image shows the topography (by atomic force microscope) of a ferroelectric PTO line array crystallized on a 360-nm thick precursor film on polyimide. The scale bar corre- sponds to one micron.

© Suenne Kim/Gatech

11-06/07 :: June/July 2011

the crystallization occurs.” To begin the fabrication, the sol-gel precursor material is first applied to a substrate with a standard spin-coating method, then briefly heated to approximately 250° Celsius (482° F) to drive off the organic solvents. The researchers have used polyimide, glass and silicon substrates, but in principle, any material able to withstand the 250-degree heating step could be used. Structures have been made from Pb(ZrTi)O3 – known as PZT, and PbTiO3 – known as PTO.

“We still heat the precursor at the temperatures required to crystallize the structure, but the heating is so localized that it does not affect the substrate,” explained Riedo, a co-author of the paper and an associate professor in the Georgia Tech School of Physics.