4 research outputs found
<i>In</i><i>Situ</i> Observation of the Humidity Controlled Polymorphic Phase Transformation in Glycine Microcrystals
Glycine is a model crystal exhibiting
three polymorphic phases
and important functional properties such as piezoelectricity and ferroelectricity.
We report here <i>in situ</i> observation of the irreversible
transformation of the solution-grown glycine crystals from a β
phase into a γ phase. The slow transformation process was monitored
by piezoresponse force microscopy at room temperature. The process
of β to γ conversion was entirely controlled by the variation
of relative humidity in the sample chamber. The results show that
the rate of phase transformation in glycine is humidity dependent
with a threshold of about 25% RH. It is demonstrated that the phase
boundary is highly rugged and the transformation front propagates
inhomogeneously along the polar axis of the β phase. The mechanism
of the phase transformation is discussed
Symmetry Breaking and Electrical Frustration during Tip-Induced Polarization Switching in the Nonpolar Cut of Lithium Niobate Single Crystals
Polarization switching in ferroelectric materials is governed by a delicate interplay between bulk polarization dynamics and screening processes at surfaces and domain walls. Here we explore the mechanism of tip-induced polarization switching at nonpolar cuts of uniaxial ferroelectrics. In this case, the in-plane component of the polarization vector switches, allowing for detailed observations of the resultant domain morphologies. We observe a surprising variability of resultant domain morphologies stemming from a fundamental instability of the formed charged domain wall and associated electric frustration. In particular, we demonstrate that controlling the vertical tip position allows the polarity of the switching to be controlled. This represents a very unusual form of symmetry breaking where mechanical motion in the vertical direction controls the lateral domain growth. The implication of these studies for ferroelectric devices and domain wall electronics are discussed
Chirality-dependent growth of self-assembled diphenylalanine microtubes
The difference in the crystal structure and growth kinetics of microtubes formed from l- and d-enantiomers of diphenylalanine dipeptide is investigated both experimentally and theoretically by computer simulation. The microtubes of l- and d-enantiomers grown simultaneously and under identical experimental conditions possess different crystallographic space groups, have essential difference in sizes, and demonstrate different growth kinetics. Computer simulation by molecular mechanics methods revealed a fundamental difference in the interaction between structural units of microtubes of different chiralities. A model describing chirality-dependent growth of microtubes is proposed
Toward Ferroelectric Control of Monolayer MoS<sub>2</sub>
The
chemical vapor deposition (CVD) of molybdenum disulfide (MoS<sub>2</sub>) single-layer films onto periodically poled lithium niobate is possible
while maintaining the substrate polarization pattern. The MoS<sub>2</sub> growth exhibits a preference for the ferroelectric domains
polarized “up” with respect to the surface so that the
MoS<sub>2</sub> film may be templated by the substrate ferroelectric
polarization pattern without the need for further lithography. MoS<sub>2</sub> monolayers preserve the surface polarization of the “up”
domains, while slightly quenching the surface polarization on the
“down” domains as revealed by piezoresponse force microscopy.
Electrical transport measurements suggest changes in the dominant
carrier for CVD MoS<sub>2</sub> under application of an external voltage,
depending on the domain orientation of the ferroelectric substrate.
Such sensitivity to ferroelectric substrate polarization opens the
possibility for ferroelectric nonvolatile gating of transition metal
dichalcogenides in scalable devices fabricated free of exfoliation
and transfer