ferroelectricity ,

Hot new way to write on the nanoscale

GEORGIA TECH (US) — Researchers have developed a way to draw ferroelectric nanostructures directly on plastic using a heated atomic force microscope tip.

The technique, reported in the journal Advanced Materials, could simplify the high-density, low-cost production of complex ferroelectric structures for energy harvesting arrays, sensors, and actuators in nano-electromechanical systems and micro-electromechanical systems.

“We can directly create piezoelectric materials of the shape we want, where we want them, on flexible substrates for use in energy harvesting and other applications,” says Nazanin Bassiri-Gharb, assistant professor of mechanical engineering at Georgia Institute of Technology.

“This is the first time that structures like these have been directly grown with a CMOS-compatible process at such a small resolution.  Not only have we been able to grow these ferroelectric structures at low substrate temperatures, but we have also been able to pattern them at very small scales.”

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Using the patterning technique, researchers were able to produce wires approximately 30 nanometers wide and spheres with diameters of approximately 10 nanometers.  Spheres with potential application as ferroelectric memory were fabricated at densities exceeding 200 gigabytes per square inch—currently the record for this perovskite-type ferroelectric material, says first author Suenne Kim, a postdoctoral fellow working with Elisa Riedo, professor of physics.

Ferroelectric materials are attractive because they exhibit charge-generating piezoelectric responses an order 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, requiring temperatures greater than 600 degrees Celsius 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, doing so 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 new process 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 researchers to create patterns of crystallized material where desired.  To create energy-harvesting structures, for example, lines corresponding to ferroelectric nanowires can be drawn along the direction in which strain would be applied.

“The heat from the AFM tip crystallizes the amorphous precursor to make the structure,” Bassiri-Gharb explains.  “The patterns are formed only where 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 degrees Celsius to drive off the organic solvents.

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,” Riedo says.

The heated AFM tips were provided by William King, professor of mechanical science and engineering at the University of Illinois.

As a next step, the researchers plan to use arrays of AFM tips to produce larger patterned areas, and improve the heated AFM tips to operate for longer periods of time.  The researchers also hope to understand the basic science behind ferroelectric materials, including properties at the nanoscale.

“We need to look at the growth thermodynamics of these ferroelectric materials,” says Bassiri-Gharb.  “We also need to see how the properties change when you move from the bulk to the micron scale and then to the nanometer scale.  We need to understand what really happens to the extrinsic and intrinsic responses of the materials at these small scales.”

Ultimately, arrays of AFM tips under computer control could produce complete devices, providing an alternative to current fabrication techniques.

“Thermochemical nanolithography is a very powerful nanofabrication technique that, through heating, is like a nanoscale pen that can create nanostructures useful in a variety of applications, including protein arrays, DNA arrays, and graphene-like nanowires,” Riedo explains.

“We are really addressing the problem caused by the existing limitations of photolithography at these size scales. We can envision creating a full device based on the same fabrication technique without the requirements of costly clean rooms and vacuum-based equipment. We are moving toward a process in which multiple steps are done using the same tool to pattern at the small scale.”

Researchers from the University of Nebraska contributed to the study that was sponsored by the National Science Foundation and the U.S. Department of Energy.

More news from Georgia Tech: www.gatech.edu/newsroom/

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