42 research outputs found
Light intensity-induced phase transitions in graphene oxide doped polyvinylidene fluoride
The coupling of light with low-frequency functionalities of dielectrics and liquid crystals and an ability to turn “on” and “off” the pyro-, piezo-, or ferro- electric properties of materials on demand by optical means leads to fascinating science and device applications. Moreover, to achieve all-optical control in nano-circuits, the coupling of the light with mechanical degrees of freedom is highly desirable and has been elusive until recently. In this work, we report on the light intensity-induced structural phase transitions in graphene oxide doped piezoelectric polyvinylidene fluoride (PVDF) film observed by micro-Raman spectroscopy. Increasing the laser power results in a steady transformation of the Raman spectrum featured piezoelectric phase to one of non-piezoelectric structure. This effect is accompanied by volumetric change of a PVDF unit cell by a factor of two, useful for a photostriction materials application. Furthermore, we observed the reversible switching of α and phases as a function of the light intensity (laser power between 5.7–31.3 mW). This opens up a new route for multi-functionality control where strain, piezoelectric constants and polarization can be modified by light
Structural Origin of Recovered Ferroelectricity in BaTiO Nanoparticles
Nanoscale BaTiO particles (~10 nm) prepared by ball-milling a mixture of
oleic acid and heptane have been reported to have an electric polarization
several times larger than that for bulk BaTiO. In this work, detailed
local, intermediate, and long-range structural studies are combined with
spectroscopic measurements to develop a model structure of these materials. The
X-ray spectroscopic measurements reveal large Ti off-centering as the key
factor producing the large spontaneous polarization in the nanoparticles.
Temperature-dependent lattice parameter changes reveal the sharpening of the
structural phase transitions in these BaTiO nanoparticles compared to the
pure nanoparticle systems. Sharp crystalline-type peaks in the barium oleate
Raman spectra suggest that this component in the composite core-shell matrix, a
product of mechanochemical synthesis, stabilizes an enhanced polar structural
phase of the BaTiO core nanoparticles.Comment: 5 figures in main text. 1 table and 3 figures in supplementary
documen
Chemotactic response and adaptation dynamics in Escherichia coli
Adaptation of the chemotaxis sensory pathway of the bacterium Escherichia
coli is integral for detecting chemicals over a wide range of background
concentrations, ultimately allowing cells to swim towards sources of attractant
and away from repellents. Its biochemical mechanism based on methylation and
demethylation of chemoreceptors has long been known. Despite the importance of
adaptation for cell memory and behavior, the dynamics of adaptation are
difficult to reconcile with current models of precise adaptation. Here, we
follow time courses of signaling in response to concentration step changes of
attractant using in vivo fluorescence resonance energy transfer measurements.
Specifically, we use a condensed representation of adaptation time courses for
efficient evaluation of different adaptation models. To quantitatively explain
the data, we finally develop a dynamic model for signaling and adaptation based
on the attractant flow in the experiment, signaling by cooperative receptor
complexes, and multiple layers of feedback regulation for adaptation. We
experimentally confirm the predicted effects of changing the enzyme-expression
level and bypassing the negative feedback for demethylation. Our data analysis
suggests significant imprecision in adaptation for large additions.
Furthermore, our model predicts highly regulated, ultrafast adaptation in
response to removal of attractant, which may be useful for fast reorientation
of the cell and noise reduction in adaptation.Comment: accepted for publication in PLoS Computational Biology; manuscript
(19 pages, 5 figures) and supplementary information; added additional
clarification on alternative adaptation models in supplementary informatio
Stoichiometric and ultra-stable nanoparticles of II-VI compound semiconductors
Nanoparticles of (CdSe)n are found extremely stable at n = 33
and 34 with structures distinctively different from the bulk fragments. They
grow exclusively in large amount with a simple solution method. The diameter
is determined as 1.5 nm. Such ultra-stable nanoparticles had been predicted
both theoretically and experimentally after the discovery of carbon
fullerenes, but not been produced macroscopically in any other element or
compound system. First-principles calculations predict that the structures of (CdSe)33
and (CdSe)34 are puckered (CdSe)28-cages accommodating
respectively (CdSe)5 an (CdSe)6 inside to form a three-dimensional
network of essentially hetero-polar sp3-bonding