SEM images of the PZT nanotube arrays, all with outer diameters of approximately 100 nanometers. The hexagonal patterns are approximately three microns wide. Credit: Ashley Bernal and Nazanin Bassiri-Gharb.

Investigators at the Georgia Institute of Technology have demonstrated a promising new manufacturing process to form piezoelectrically active ferroelectric nanotubes, based on lead zirconate titanate, and other nanostructures. Most importantly, the process can deliver custom-defined shapes, locations and pattern variation across the same substrate. Their work was confirmed by nanocharacterization specialists at the Oak Ridge National Lab.

The Georgia Tech researchers call their technique “soft template infiltration” and say that it could represent a big leap in making nanoelectromechanical systems and devices. The breakthrough comes from using a “soft” polymeric template that decomposes when the ferroelectric material crystallizes.

According to a news release from the school, the ability to tailor the properties and dimensions of these free-standing structures, plus their strong piezoelectric response, “could ultimately lead to production of actively-tunable photonic and phononic crystals, terahertz emitters, energy harvesters, micromotors, micropumps and nanoelectromechanical sensors, actuators and transducers — all made from the PZT material.”

“We are using a new nanomanufacturing method for creating three-dimensional nanostructures with high aspect ratios in ferroelectric materials that have attractive piezoelectric properties,” says Nazanin Bassiri-Gharb in the release. She is an assistant professor in Georgia Tech’s Woodruff School of Mechanical Engineering. Bassiri-Gharb and others report on this work in the paper, “Free-Standing Ferroelectric Nanotubes Processed via Soft-Template Infiltration” in Advanced Materials (doi:10.1002/adma.201103993).

In the paper, the authors say they can control the formation of nanotubes with outer diameters in the range of 100–200 nanometers, wall thickness ranges of 5-25 nanometers and aspect ratios as high as 5:1.

Previously, somewhat similar structures have been fabricated using what are grouped into either “top-down” and “bottom-up” processes. According to Bassiri-Gharb, her team looked for a more agile alternative process because top-down methods can cause surface damage (that degrades the ferroelectric and piezoelectric performance) and the existing bottom-up approaches, heretofore, have not been able to combine precise location and high aspect ratios.

“This technique gives us a degree of control over the three-dimensional process that we’ve not had before,” she says. “When we did the characterization, we saw a size effect that until now had been observed only in thin films of this material at much larger size scales. …These are truly smart materials, which means they respond to external stimuli such as applied electric fields, thermal fields or stress fields. Devices made from these materials could be fine tuned to respond to a different wavelength or to emit at a different wavelength during operation.”

The soft template infiltration technique begins with making the template. In the group’s report, they discuss using a silicon substrate that has been spin-coated with a polymeric negative electron-beam resist material, followed by electron-beam lithography. (The group notes that a variety of other patterning techniques could be used to create the template.)

Composite SEM image of PZT nanotube arrays and their piezoelectric response as measured by band-excitation PFM (BE-PFM). Credit: Ashley Bernal and Nazanin Bassiri-Gharb.

Next, a thin layer of aluminum oxide is added on top of template, which is then immersed in a bath containing a sol-gel precursor solution for PZT. After being pyrolyzed, the material is annealed in a two-step heat treating process, which crystallizes the material and dissipates the polymer substrate. What remains are free-standing PZT nanotubes connected by the aluminum oxide layer. Alternatively, nanorods or nanowires, instead of nanotubes, can be produced simply by increasing the amount of chemical infiltration.

Making the innovative nanostructures is one thing, but characterizing them is another. Bassiri-Gharb says her group “leveraged a new characterization method available through Oak Ridge to study the piezoelectric response of these nanostructures on the substrate where they were produced.”

As it turns out ORNL researchers, such as coauthors Sergei Kalinin and Alexander Tselev, had a novel technique of their own. The duo, who work at the lab’s Center for Nanophase Materials Sciences, tested a new method that permits high-accuracy, in-situ measurements of the nanoscale piezoelectric properties of the structures. (Kalinin received ACerS’ Ross Coffin Purdy Award in 2003 and is an expert on band excitation scanning force microscopy and functional imaging on the nanoscale.)

The characterization techniques and analysis—worthy of a separate story—made use of what has been called band-excitation piezoresponse force microscopy. The BE-PFM approach, which is based on atomic force microscopy, allowed Kalinin and Tselev to precisely separate the properties of the AFM tip from those of the PZT sample. According to the news release, the BE-PFM delivered sufficient detail to detect the size-scale piezoelectric effects.

What did they find? Bassiri-Gharb says, “One of our most important observations is that these piezoelectric nanomaterials allow us to generate a four- to six-factor increase in the extrinsic piezoelectric response compared to the use of thin films. This would be a huge advantage in terms of manufacturing because it means we could get the same response from much smaller structures than we would have had to otherwise use.”

Besides smaller structures, the researchers note in their paper two other big advantages to soft template infiltration. First, “the approach can be tailored for a variety of ceramic materials by appropriate choice of sol-gel precursors and processing temperatures.” They continue, “this nanomanufacturing method is easily scaled for larger patterned areas by use of nanoimprint lithography instead of EBL for creation of the soft templates.”