Making ferroelectric nanorocks with an atomic force microscope hammerPublished on February 7th, 2012 | By: Eileen De Guire
Jim O’Neil, a fellow graduate student a good while ago, liked to say, “Ceramic engineering is all about making big rocks into little rocks, and then making little rocks into big rocks.”
I’ve lost track of Jim, but a new Rapid Communication in the Feb. 2012 Journal of the American Ceramic Society reminded me of his take on our branch of materials science.
The short paper by a Korean team, Son and Jung, describes a novel method of making discrete ferroelectric particles by a method that amounts to making “little rocks out of big rocks” — but on a nanoscale!
The investigators were interested in fabricating PbTiO3 nanodots for ferroelectric random access memory, which is a promising material for a nonvolatile memory applications.
Demand for smaller devices is driving the development of high density, high performance memories, and further downsizing is starting to run into physical limitations imposed by materials properties and processing.
For example, some types of RAM materials, are susceptible to a surface effect on the memory switching mechanism as they are scaled down. In ferroelectric RAM materials, there is a critical size that is determined by the maximum size of the nonferroelectric component.
Processing, too, imposes size limitations. Several methods have been used to fabricate ferroelectric nanostructures, such as self-assembly, an anodizing aluminum oxide template process, e-beam lithography and dip-pen lithography. According to the paper, PTO nanodots have been fabricated in the 22 nm to 60 nm range by these methods. The smallest PTO nanodots (22 nm) were made by self-assembly, however, the authors note that “these nanodots did not exhibit a canonical piezoelectric hysteresis loop,” even though the critical size for PTO nanodots to exhibit ferroelectricity is a few nanometers. Dip-pen lithography can make nanodots of about 40 nm with good ferroelectric properties. The anodizing aluminum oxide process is not conducive to making dots less than 60 nm.
Son and Jung’s idea was simple: Make a nanodot, whack it with a hammer, anneal to crystallize the shattered pieces and, finally, test for ferroelectricity. Their goal was to fabricate PTO nanodots less than 10 nm diameter with ferroelectric properties.
So, using dip-pen lithography, they made a PTO nanodot that was 40 nm diameter and 25 nm thick. Using an atomic force microscope tip as a hammer, the dot was shattered after “repeated collisions,” that is, they had to beat on it. The resulting nanoparticles ranged in size from several nanometers to a few tens of nanometers, and were of “diverse sizes in both diameter and thickness.”
After crystallization, tests showed that ferroelectric properties were present in a 25 nm dia. x 11 nm thick nanodot and in one that was 10 nm dia. x 8 nm thick. The 10-nm-diameter nanodot, according to the article, is “closer to the theoretical critical size” than has been achieved in other studies. PTO nanodots that were less than about 3 nm thick proved difficult to test because of high leakage currents.
The paper is “Ferroelectric PbTiO3 Nanodots Shattered Using Atomic Force Microscopy,” Jong Yeog Son, Inhwa Jung, JACerS, Feb. 2012 (doi:10.1111/j.1551-2916.2011.05026.x).
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