The ferroelectric polymer project started a
decade ago with two of the most powerful tools of science - coincidence and
opportunism. Ducharme and Prof. Vladimir Fridkin of the Institute of Crystallography,
Russian Academy of Sciences, Moscow, had just received a grant from the National
Science Foundation (NSF) to develop the PVDF copolymers for use in photorefractive
nonlinear optics. Meanwhile, Fridkin’s associates Kira Verkhovskaya
and Alexander Bune, along with Serguei Palto from the LB group of Lev Blinov,
decided, against the recommendation of their senior colleagues, to make LB
films from these polymers. Now this was a ‘bad idea,’ because
vinylidene fluoride is not a good amphiphile, has a hydrophilic polar component,
but no oily hydrophobic component that would stabilize it at the water surface.
The results, however, were spectacular.
Alexander Bune, Fridkin’s PhD student, came to Lincoln in 1995 with
some of the first ferroelectric LB films in his pocket. The data from Moscow
and Lincoln convinced the three teams that they had something special. Therefore,
with the NSF’s permission, they immediately abandoned the original plan
and dove right in to study the unique ultrathin ferroelectric crystals. Among
the discoveries reported by Bune et al., were the discovery of two-dimensional
ferroelectricity and the first measurements of the intrinsic ferroelectric
coercive field and intrinsic switching dynamics. This work was cited in Physics
Uspekhi in 1999 as one of 30 “especially important and interesting problems”
in physics and astrophysics on the verge of the 21st century,” by editor
V. L. Ginzburg, who had developed the original mean-field theory of ferroelectric
with L. D. Landau in 1946.
“Based on my experience and wisdom, I often counsel students and junior
colleagues that this measurement is spurious, or that experiment won’t
work,” said Ducharme, “but our most exciting discoveries have
proved the contrary.” This contrariness was evident in the discoveries
of Bune, Choi, Borca, and, more recently, Mengjun Bai (PhD 2002). Mengjun
showed a mottled-looking atomic force microscope (AFM) image to Ducharme,
who pronounced it “junk”, some mistake during annealing usually
smooth LB films. Bai not only showed that the results were not junk, but that
they were natural mesa formations, 10 nm high by 100 nm in diameter, that
form spontaneously, by plastic crystalline flow in the paraelectric phase,
during annealing of only the thinnest LB films. These ‘nanomesas’
turned out to be highly crystalline, oriented and ferroelectric, with properties
nearly identical to the bulk.
The nanomesas are the basis of novel nanomechanics theories of NCMN member
Jiangyu Li (Engineering Mechanics), who has shown that careful structuring
of piezoelectric nanocomposites can lead to electromechanical response dramatically
higher than any of the individual components. Li shares an NSF nanomanufacturing
grant with Ducharme and is joined in the study of nanoscale ferroic composites.
Li and Ducharme are joined by John Belot and Takacs, Yongfeng Lu (Electrical
Engineering), and Mei, as well as Jerry Bernholc (N. Carolina State) and Simon
Phillpot (U. Florida) in several new proposals seeded by a Research Cluster
Grant from the UNL Vice Chancellor for Research.
Several other groups joined in to study the properties of these unique films.
NCMN member Peter Dowben’s group in Physics made several key discoveries
of their own—a surface metallicity transition by Jaewu Choi (PhD 1998)
and a bulk stiffening transition by Camelia Borca (PhD 2001), and STM imaging
and nanoscale polarization manipulation by Jiandi Zhang (a former post-doc)
at Florida International University. Neutron and X-ray diffraction studies
led by NCMN member Shireen Adenwalla (Physics and Astronomy) have advanced
our understanding of the structural transitions (like the stiffening transition),
the interplay among polarization, structure, and electric field (Matt Poulsen,
BS 2000), and coupling in multilayers (Jihee Kim, PhD candidate). Wai-Ning
Mei (UNO Physics) and his group have made considerable progress in ab-initio
calculations that compare well with Choi’s band-structure studies and
Bai’s precision IR-VIS-UV ellipsometry studies done in collaboration
with the J. A. Woollam Company. Jim Takacs and his group in Chemistry have
been synthesizing analogues of PVDF that Poulsen has shown make better LB
films, yet remain good ferroelectrics.
Applied research with the ferroelectric polymer LB films includes the demonstration
of a working nonvolatile memory element (Tim Reece, MS 2002) and development
of a laser imaging technique, scanning pyroelectric microscopy (Brad Peterson,
BS 2004). Also critical to the memory applications, Christina Othon’s
PhD research focuses on the study and control of extrinsic polarization switching.
The NCMN and the Nebraska Research Initiative have been very supportive of
the ferroelectric polymer research, enabling many important discoveries and
seeding several new projects that are already bearing fruit. The work has
been reported widely, in major journals like Nature, Physical Review Letters,
and Applied Physics Letters, and at numerous international conferences.
External grant funding has been provided by the NSF, the Office of Naval Research,
the Air Force Office of Scientific Research, the Petroleum Research Fund,
the J. A. Woollam Company, and the Hewlett Packard Corporation.
Ducharme, a native of central Massachusetts, earned a PhD in Physics in 1986
with Jack Feinberg at the University of Southern California. He was an early
recruit to the NCMN, joining the faculty in 1991 after two years with IBM,
where he and W. E. Moerner (now at Stanford) developed the first photorefractive
polymers. Ducharme is now a full Professor and Vice Chair of the UNL Department
of Physics and Astronomy.
(Fall 2004)
