Ferroelectrics fabricated directly on plastic

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Researchers have developed a new way to fabricate nanometre scale ferroelectric structures directly on flexible plastic substrates that would be unable to withstand the processing temperatures normally required to create such nanostructures.

The technique is known as thermochemical nanolithography (tcnl) and uses a heated atomic force microscope (afm) tip to produce patterns. According to a team from the Georgia Institute of Technology, it could facilitate high density, low cost production of complex ferroelectric structures for energy harvesting arrays, sensors and actuators in nano electromechanical systems and micro electromechanical systems (mems). The research was reported July 15 in the journal Advanced Materials. "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," said Nazanin Bassiri-Gharb, co author of the paper and an assistant professor in the School of Mechanical Engineering at the 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." The researchers have produced wires approximately 30nm wide and spheres with diameters of approximately 10nm using the patterning technique. Spheres with potential application as ferroelectric memory were fabricated at densities exceeding 200GB/in2, currently the record for this perovskite type ferroelectric material. Ferroelectric materials exhibit charge generating piezoelectric responses an order of magnitude larger than those of materials such as aluminium nitride or zinc oxide. The polarisation 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°C for crystallisation. 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 challenges 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 localised 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 crystallised 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 crystallises the amorphous precursor to make the structure and the patterns are formed only when crystallisation 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°C to drive off the organic solvents. The researchers have used polyimide, glass and silicon substrates, but in principle, any material able to withstand the heating step could be used. Structures have been made from Pb(ZrTi)O3, known as PZT, and PbTiO3, known as PTO. The researchers still heated the precursor at the temperatures required to crystallise the structure, but the heating was so localised that it did not affect the substrate. 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. Ultimately, arrays of afm tips under computer control could produce complete devices, providing an alternative to current fabrication techniques.