Volkmar Dierolf

Lehigh and four other universities have received a grant from the National Science Foundation (NSF) and the German Research Foundation (DFG) to study ferroelectric materials, which are used in electronic and optical devices, at a scale of length at which they have not previously been investigated.

The three-year Materials World Network grant, which totals $1.2 million, is led by Volkmar Dierolf, associate professor of physics. It will support research, workshops and student exchanges involving Lehigh, the University of Florida, Penn State University and the Universities of Bonn and Paderborn in Germany.

Dierolf said the project was made possible by Lehigh’s Center for Optical Technologies (COT), which has received more than $60 million in funding since opening in 2001.

“This grant is very significant for the COT,” Dierolf says. “It is a good example of the center’s growing prestige.”

The Materials World Network grant, which is administered by NSF’s Division of Materials Research, is highly competitive. Applicants are evaluated twice—first by NSF and then by the research funding agency in the partner country, which in this case is Germany’s DFG.

“We ended up as highly recommended by both research agencies,” says Dierolf, “which was fortunate, as the DFG was not able to fund all the American groups that had received first-round approval from NSF.”

The grant will enable undergraduate majors in physics and materials science and engineering to take part in international internships. It will also enable graduate students and post-doctoral researchers to travel and do research.

Studying domain walls

Ferroelectric materials are found in a wide variety of applications, ranging from barcode readers in supermarkets to high-speed electro-optic modulators to devices that power the information superhighway.

In the new project, which is titled “Nanoscale Structure and Shaping of Ferroelectric Domains,” Dierolf and his colleagues 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, Dierolf says. An electric dipole is a molecular unit with a positive charge on one end and a negative charge on the other. Ferroelectric materials have a built-in asymmetry, Dierolf says, which causes all dipoles to point in one direction, either up or down. The domain wall separates regions (domains) of opposite dipole orientation.

“A material’s nonlinear, acoustic, electric and other properties depend on the direction of this dipole, and the dipole can be manipulated and controlled in order to engineer devices,” Dierolf says.

In his research, 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. One micron is equal to one one-millionth of a meter. Dierolf and his students seek to learn how small the 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 has previously led similar international collaborations. In the new project, he is using a technique called near-field optical spectroscopy to study the domains, or regions of a ferroelectric material with the same dipole orientation, and the domain walls, which separate one domain from another.

“Ordinary optical microscopy resolves to the width of a wavelength,” Dierolf says. “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 (1 nanometer is a billionth of a meter, and a thousandth of a micron). 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 (large) 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 [bulk] 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.”

--Kurt Pfitzer

Posted on Thursday, November 16, 2006