Working with the world’s most powerful transmission electron microscope, the researchers mapped the ferroelectric structural distortions in nanocrystals of germanium telluride, a semiconductor, and barium titanate, an insulator. This data was then combined with data from electron holographic polarization imaging to yield detailed information on the polarization structures and scaling limits of ferroelectric order on the nanoscale.

Ferroelectricity is the property by which materials can be electrically polarized, meaning they will be oriented in favor of either a positive or negative electrical charge. This polarization can be flipped with the application of an external electrical field, a property that could be exploited for nonvolatile data storage, similar to the use of ferromagnetic materials today but using much smaller, far more densely packed devices.

“Although much progress has been made towards understanding  nanoscale photophysical magnetic and other functional properties, understanding the basic physics of ferroelectric nanomaterials remains far less advanced,” says co-principal investigator Ramesh, who attributes contradicting reports on nanoscale ferroelectricity  in part to the lack of high-quality, nanocrystals of  ferroelectric materials that feature well-defined sizes, shapes and surfaces.

“Another problem has been the  reliance on ensemble measurements rather than single particle techniques,” he says. “Statistical-average measurement techniques tend to obscure the  physical mechanisms responsible for profound changes in ferroelectric behavior within individual nanocrystals.”

TEAM stands for “Transmission Electron Aberration-corrected Microscope.” TEAM I can resolve images of structures with dimensions as small as one half-angstrom –  less than the diameter of a single hydrogen atom.

The maps produced at TEAM I of ferroelectric distortion patterns within the highly conducting germanium telluride nanocrystals were then compared with electron holography studies of insulating nanocubes of barium titanate.

“Electron holography is an interferometry technique using coherent electron waves,” said BNL physicist. “Firing focused electron waves through the ferroelectric sample creates what’s called a phase-shift, or an interference pattern that reveals details of the targeted structure. This produces an electron hologram, which we can use to directly see local electric fields of individual ferroelectric nanoparticles.”

These combined studies enabled the independent examination of depolarizing field and surface structure influences and thereby enabled the research team to identify the fundamental factors governing the nature of the ferroelectric polarized state at finite dimensions. The results indicate that a monodomain ferroelectric state with linearly ordered polarization remains stable in these nanocrystals down to dimensions of less than 10 nanometers. Also, room-temperature polarization flipping was demonstrated down to dimensions of about five nanometers. Below this threshold, ferroelectric behavior disappeared. This indicates that five nanometers is likely a size limit for data storage applications, the authors state.

“We also showed that ferroelectric coherence is facilitated in part by control of particle morphology, which along with electrostatic boundary conditions is found to determine the spatial extent of cooperative ferroelectric distortions,” Ramesh says. “Taken together, our results provide a glimpse of the structural and electrical manifestations of ferroelectricity down to its ultimate limits.”

This story is reprinted from material from
Berkeley Lab, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.