Brookhaven physicists Yimei Zhu (back) and Myung-geun Han examine the breakthrough nanoscale images of ferroelectric polarizations.

As
scientists learn to manipulate little-understood nanoscale materials,
they are laying the foundation for a future of more compact, efficient,
and innovative devices. In research to be published online July 8 in the
journal Nature Materials, scientists at the U.S. Department of Energy’s
Brookhaven National Laboratory, Lawrence Berkeley National Laboratory,
and other collaborating institutions describe one such advance—a
technique revealing unprecedented details about the atomic structure and
behavior of exotic ferroelectric materials, which are uniquely equipped
to store digital information. This research could guide the scaling up
of these exciting materials and usher in a new generation of advanced
electronics.

   

Brookhaven
scientists used a technique called electron holography to capture
images of the electric fields created by the materials’ atomic
displacement with picometer precision—that’s the trillionths-of-a-meter
scale crucial to understanding these promising nanoparticles. By
applying different levels of electricity and adjusting the temperature
of the samples, researchers demonstrated a method for identifying and
describing the behavior and stability of ferroelectrics at the
smallest-ever scale, with major implications for data storage.

   

“This
kind of detail is just amazing – for the first time ever we can
actually see the positions of atoms and link them to local
ferroelectricity in nanoparticles,” said Brookhaven physicist Yimei Zhu.
“This kind of fundamental insight is not only a technical milestone,
but it also opens up new engineering possibilities.”

   

Ferroelectrics
are perhaps best understood as the mysterious cousins of more familiar
ferromagnetic materials, commonly seen in everything from refrigerator
magnets to computer hard drives. As the name suggests, ferromagnetics
have intrinsic magnetic dipole moments, meaning that they are always
oriented toward either “north” or “south.” These dipole moments tend to
align themselves on larger scales, giving rise to the magnetization
responsible for attraction and repulsion. Applying an external magnetic
field can actually flip that magnetization, allowing programmers and
engineers to manipulate the material.

   

Similarly,
ferroelectric materials also have a molecular-scale dipole moment, but
one characterized by a positive or negative electric charge rather than
magnetic polarity. This polarization can also be manipulated, but
flipping the charge requires an external electric field. This critical,
tunable characteristic comes from an internal subatomic asymmetry and
ordering phenomena, which was imaged in detail for the first time by the
transmission electron microscopes used in this new study.

   

Current
magnetic memory devices, such as the hard drives in most computers,
“write” information into ferromagnetic materials by flipping that
intrinsic dipole moment to correspond with the 1 or 0 of a computer’s
binary code. Those manipulated polarities then translate into everything
from movies to web sites. The remarkable ability of these materials to
retain information even when turned off—what’s called nonvolatile
storage—makes them an essential building block for our increasingly
digital world.

   

In
the emerging ferroelectric model of data storage, applying an electric
field toggles between that material’s two electric states, which
translates into code. When scaled up similarly to ferromagnetics, that
process can manifest on a computer as the writing or reading of digital
information. And ferroelectric materials may trump their magnetic
counterparts in ultimate efficacy.

   

“Ferroelectric
materials can retain information on a much smaller scale and with
higher density than ferromagnetics,” Zhu said. “We’re looking at moving
from micrometers (millionths of a meter) down to nanometers (billionths
of a meter). And that’s what’s really exciting, because we now know that
on the nanoscale each particle can become its own bit of information.
We knew very little about manipulating ferroelectric behavior in
nanomaterials before this.”

   

The
trick to scaling up individual ferroelectric nanoparticles into useful
devices is understanding just how tightly together they can be packed
and ordered without compromising their distinct polarizations, which
theory suggests should be extremely difficult to achieve. The electron
holography experiments conducted at Brookhaven Lab demonstrated a method
for determining those parameters under a range of conditions.

   

“Electron
holography is an interferometry technique using coherent electron
waves,” said Brookhaven physicist Myung-Geun Han. “When electron waves
pass through a ferroelectric sample, they are influenced by local
electric fields, yielding a so-called phase-shift. The interference
pattern between the electrons that pass through electric fields and
those that don’t creates what’s called an electron hologram, which
allows us to directly ‘see’ those local electric fields around
individual ferroelectric nanoparticles.”

   

Local
electric fields emanate from ferroelectric nanoparticles, and these
“fringing” fields are like the functional footprint of a particle’s
polarity. Consider the way a small magnet’s effects can be felt even at a
slight distance from its surface—a similar field exists in
ferroelectric materials. When imaged by electron holography, the
fringing field indicates the integrity of electrical polarity and the
distance required between particles before they begin to interfere with
each other.

   

The
study revealed that the electric polarity could remain stable for
individual ferroelectric materials, meaning that each nanoparticle can
be used as a data bit. But because of their fringing fields,
ferroelectrics need a little elbow room (on the order of five
nanometers) to effectively operate. Otherwise, once scaled up for
computer storage, they can’t keep code intact and the information
becomes garbled and corrupted. Understanding the atomic-scale properties
revealed in this study will help guide implementation of these exotic
particles.

   

“Properly
used, ferroelectrics could ramp up memory density and store an
unparalleled multiple terabytes of information on just one square inch
of electronics,” Han said. “This brings us closer to engineering such
devices.”

The
ferroelectric nanoparticles tested, semiconducting germanium telluride
and insulating barium titanate, were engineered at Lawrence Berkeley
National Laboratory and brought to Brookhaven Lab for the electron
holography experiments. Additional experiments using x-ray diffraction
were conducted at Argonne National Laboratory’s Advanced Photon Source.

   

The
work featured collaborators from the University of California at
Berkeley, the University of New Orleans, Central Michigan University,
Lawrence Berkeley National Lab and Brookhaven National Lab. In addition
to Zhu and Han, Brookhaven scientist Vyacheslav Volkov was also involved
in the project. The research was funded by DOE’s Office of Science.

Source: Brookhaven Laboratory

Direct polarization images of individual ferroelectric nano cubes captured with electron holography. The fringing field, or “footprint” of electric polarization, can be seen clearly in (a), but it vanishes when the material is subjected to high temperatures (b). The lower images show that no fringing field can be observed before application of electricity (c), but a clear field emanates after current is applied (d).