Five universities in the U.S. and Germany have received a grant from the National Science Foundation (NSF) and the German Research Foundation (DFG) to study ferroelectric materials at a scale of length at which they have not previously been investigated.
The three-year, $1.2-million Materials World Network grant will support research and student exchanges involving Lehigh, Florida and Penn State and the Universities of Bonn and Paderborn in Germany. It is led by Volkmar Dierolf, associate professor of physics at Lehigh.
Ferroelectric materials are used in many electronic and optical applications, ranging from barcode readers in supermarkets to high-speed electro-optic modulators to devices that power the information superhighway.
In the new project, "Nanoscale Structure and Shaping of Ferroelectric Domains," researchers will conduct lab experiments and computer simulations to study the properties of ferroelectric domain walls in ferroelectric materials.
Ferroelectric materials have intrinsic microscopic electric dipoles, or molecular units with a positive charge on one end and a negative charge on the other. The materials have a built-in asymmetry, says Dierolf, which causes all dipoles to point in one direction, either up or down. Ferroelectric domains are regions of a ferroelectric material with the same dipole orientation. Ferroelectric domain walls separate domains with opposite dipole orientations.
"A material's nonlinear, acoustic, electric and other properties depend on the direction of this dipole," says Dierolf, "and the dipole can be manipulated and controlled in order to engineer devices."
Dierolf develops ion probes that can be monitored by their emission spectrum to examine how defects – both intentional and unintentional – affect the properties of materials at the sub-micron scale. Dierolf and his students are seeking to learn how small the ferroelectric domain can be.
"As with soup bubbles," says Dierolf, "the bigger domains tend to grow at the expense of the smaller ones. Many exciting devices could be realized if we learned to reduce the domain size."
Dierolf uses a technique called near-field optical spectroscopy to study the ferroelectric domains and ferroelectric domain walls.
"Ordinary optical microscopy resolves to the width of a wavelength," says Dierolf. "When we use near-field optical microscopy, we insert a tip with an opening measuring one-tenth of a micron into the focused light to increase the microscope's resolution. One of our specialties is to observe domain inversion in the microscope in real time. For that, we apply an electric field through the tip to make the domains switch while we monitor the emission of our probes."
The goal of the researchers, says Dierolf, is not only to image the ferroelectric materials but also to learn to control their domains at the nanoscale. Using near-field optical spectroscopy, Dierolf has imaged the domain wall regions with a resolution of 70 nanometers, and has made local electric field measurements along that length, obtaining instant feedback about domain growth and the shape of domains and domain walls.
Dierolf's group is the first to use near-field optical spectroscopy to image domain-wall structures.
The other university researchers in the international collaboration are engaged in a variety of tasks related to ferroelectric materials. Some are attempting to simulate the behaviors of the domain walls, while others are seeking to fabricate new integrated optical devices.
"This is an integrated approach that bridges the gulf between fundamental scientific research and the pursuit of actual applications," says Dierolf, whose previous research has been reported in the journals Applied Physics Letters, Photonics Spectra and Laser Focus World.
"We are attempting to span the gamut from theory to control of materials to fabrication of devices."
Until now, says Dierolf, much of the research into ferroelectric materials has been conducted at either the bulk or atomic scale.
The U.S.-German collaborators are charting new territory.
"The range at which we are studying these materials falls between the bulk and atomic scales," Dierolf says. "The collective phenomenon that occurs when a group of atoms switches their dipoles involves a few hundred atoms. This lies between what the atomic physicists have learned by studying actual individual atoms and what materials scientists have learned by studying much larger assemblies of thousands or tens of thousands of atoms.
"This is the nature of research into ferroelectric materials at the nanoscale: We look at things that are neither individual atoms or defects nor bulk materials, but rather are in between those two."