Ferroelectric materials, particularly when realized as thin films, always held a fascination for me, perhaps because my adopted home town of Colorado Springs is the site of many ferro materials specialists, including Ramtron International Corp. and Symetrix Corp.
Initially, the nonlinear ways that ferroelectrics could assume an electrical charge was deemed ideal for nonvolatile memories, and for capacitor arrays in wireless applications.
Ferroelectric thin films have been used in some high-volume RF and digital camera applications from vendors such as Panasonic, but the ferroelectric RAM, or FeRAM, never had as broad a base as Ramtron and its licensees anticipated.
However, the class of ferroelectric ceramics known as perovskites always showed potential beyond logic and memory. Piezoelectric capabilities suggested ferroelectrics could be used in pressure-sensing MEMS (micro-electro-mechanical system) applications. More recently, some research labs and large OEMs had experimented with integrating ferroelectric layers in amorphous silicon substrates used in nanostructures for solar photovoltaics.
A team from Georgia Tech, with support from the University of Nebraska, Lincoln, and the University of Illinois, Urbana Champaign, has identified a realm of energy harvesting where ferroelectrics could again come in handy. But this effort involves a new methodology that the authors of the Georgia Tech study call thermochemical nanolithography.
The goal was to produce thin-film nanostructures at a very small size, at low temperatures, using a process compatible with CMOS semiconductors. The team used an atomic force microscope to produce wires 30nm wide and spheres with a diameter of 10nm. The latter spheres also could be used in building memory devices that would exceed the density record of 200 Gbytes/sq. in., the current limit in traditional ferroelectric nonvolatile memory. Rather than opt for memory replacements, however, the team sees the most immediate use for the nanolithography method in developing the sensors and actuators in large energy arrays.
The atomic force microscope (AFM) was used to implement extremely localized heating, though any effort to scale up the lithography of nanostructures would have to demonstrate cost-effective use of production-level AFM. The microscope lithography method already has been used with two of the most common perovskite films -- lead zirconium titanate, commonly known as PZT, and lead titanate, commonly known as PTO.
The reason the team under assistant professor Nazanin Bassiri-Gharb emphasized the rather generic term of "energy harvesting" is that ferroelectric thin films have been well explored in realms of amorphous thin-film and silicon structures used in photovoltaic harvesting. But energy harvesting implies using a mix of methods, including solar PV, wind energy, and piezoelectric harvesting, to continuously power ambient systems such as sensor networks. In fact, the Georgia Tech work could be used in conjunction with earlier ferro and MEMS efforts in solar photovoltaics.
Ferro thin films and nanostructures could be used in PV cells of large surface area, to increase the efficiency of traditional solar energy sources. Thermochemical nanolithography could be used as a small-scale gap filler to power wearable electronics, autonomous vehicles, and distributed sensor networks. The key is in making the AFM lithography tip viable for high-volume production.
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