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Balke N.,Center for Nanophase Materials science | Balke N.,Institute for Functional Imaging of Materials | Maksymovych P.,Center for Nanophase Materials science | Maksymovych P.,Institute for Functional Imaging of Materials | And 11 more authors.
ACS Nano

Ferroelectricity in functional materials remains one of the most fascinating areas of modern science in the past several decades. In the last several years, the rapid development of piezoresponse force microscopy (PFM) and spectroscopy revealed the presence of electromechanical hysteresis loops and bias-induced remnant polar states in a broad variety of materials including many inorganic oxides, polymers, and biosystems. In many cases, this behavior was interpreted as the ample evidence for ferroelectric nature of the system. Here, we systematically analyze PFM responses on ferroelectric and nonferroelectric materials and demonstrate that mechanisms unrelated to ferroelectricity can induce ferroelectric-like characteristics through charge injection and electrostatic forces on the tip. We will focus on similarities and differences in various PFM measurement characteristics to provide an experimental guideline to differentiate between ferroelectric material properties and charge injection. In the end, we apply the developed measurement protocols to an unknown ferroelectric material. © 2015 American Chemical Society. Source

He K.,University of Houston | Babaye Khorasani F.,University of Houston | Retterer S.T.,Center for Nanophase Materials science | Thomas D.K.,Oak Ridge National Laboratory | And 2 more authors.
ACS Nano

The diffusive dynamics of dilute dispersions of nanoparticles of diameter 200-400 nm were studied in microfabricated arrays of nanoposts using differential dynamic microscopy and single particle tracking. Posts of diameter 500 nm and height 10 μm were spaced by 1.2-10 μm on a square lattice. As the spacing between posts was decreased, the dynamics of the nanoparticles slowed. Moreover, the dynamics at all length scales were best represented by a stretched exponential rather than a simple exponential. Both the relative diffusivity and the stretching exponent decreased linearly with increased confinement and, equivalently, with decreased void volume. The slowing of the overall diffusive dynamics and the broadening distribution of nanoparticle displacements with increased confinement are consistent with the onset of dynamic heterogeneity and the approach to vitrification. © 2013 American Chemical Society. Source

Boreyko J.B.,Center for Nanophase Materials science | Mruetusatorn P.,University of Tennessee at Knoxville | Sarles S.A.,University of Tennessee at Knoxville | Retterer S.T.,Center for Nanophase Materials science | And 2 more authors.
Journal of the American Chemical Society

Droplet interface bilayers (DIBs) are a robust platform for studying synthetic cellular membranes; however, to date no DIBs have been produced at cellular length scales. Here, we create microscale droplet interface bilayers (μDIBs) at the interface between aqueous femtoliter-volume droplets within an oil-filled microfluidic channel. The uniquely large area-to-volume ratio of the droplets results in strong evaporation effects, causing the system to transition through three distinct regimes. First, the two adjacent droplets shrink into the shape of a single spherical droplet, where an augmented lipid bilayer partitions two hemispherical volumes. In the second regime, the combined effects of the shrinking monolayers and growing bilayer force the confined bilayer to buckle to conserve its mass. Finally, at a critical bending moment, the buckling bilayer fissions a vesicle to regulate its shape and mass. The μDIBs produced here enable evaporation-induced bilayer dynamics reminiscent of endo-and exocytosis in cells. © 2013 American Chemical Society. Source

Wang Y.,Griffith University | Sumpter B.G.,Center for Nanophase Materials science | Huang J.,Center for Nanophase Materials science | Zhang H.,Griffith University | And 4 more authors.
Journal of Physical Chemistry C

Organic/inorganic hybrid perovskite materials are highly attractive for dye-sensitized solar cells as demonstrated by their rapid advances in energy conversion efficiency. In this work, the structures, energetics, and electronic properties for a range of stoichiometric surfaces of the orthorhombic perovskite CH3NH3PbI3 are theoretically studied using density functional theory. Various possible spatially and constitutionally isomeric surfaces are considered by diversifying the spatial orientations and connectivities of surface Pb-I bonds. The comparison of surface energies for the most stable configurations identified for all surfaces shows that the stabilities of stoichiometric surfaces are mainly dictated by the coordination numbers of surface atoms, which are directly correlated with the number of broken bonds. Additionally, Coulombic interactions between I anions and organic countercations on the surface also contribute to the stabilization. Electronic properties are compared between the most stable (100) surface and the bulk phase, showing generally similar features except for the lifted band degeneracy and the enhanced bandgap energy for the surface. These studies on the stoichiometric surfaces serve as a first step toward gaining a fundamental understanding of the interfacial properties in the current structural design of perovskite based solar cells, in order to help facilitate further breakthroughs in solar conversion efficiencies. (Figure Presented). © 2014 American Chemical Society. Source

Ievlev A.V.,Center for Nanophase Materials science | Ievlev A.V.,Oak Ridge National Laboratory | Alikin D.O.,Ural Federal University | Morozovska A.N.,Ukrainian Academy of Sciences | And 6 more authors.
ACS Nano

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. © 2014 American Chemical Society. Source

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