Engineering Relaxor Behavior in (BaTiO3 )n /(SrTiO3 )n Superlattices.

Complex-oxide superlattices provide a pathway to numerous emergent phenomena because of the juxtaposition of disparate properties and the strong interfacial interactions in these unit-cell-precise structures. This is particularly true in superlattices of ferroelectric and dielectric materials, wherein new forms of ferroelectricity, exotic dipolar textures, and distinctive domain structures can be produced. Here, relaxor-like behavior, typically associated with the chemical inhomogeneity and complexity of solid solutions, is observed in (BaTiO3 )n /(SrTiO3 )n (n = 4-20 unit cells) superlattices. Dielectric studies and subsequent Vogel-Fulcher analysis show significant frequency dispersion of the dielectric maximum across a range of periodicities, with enhanced dielectric constant and more robust relaxor behavior for smaller period n. Bond-valence molecular-dynamics simulations predict the relaxor-like behavior observed experimentally, and interpretations of the polar patterns via 2D discrete-wavelet transforms in shorter-period superlattices suggest that the relaxor behavior arises from shape variations of the dipolar configurations, in contrast to frozen antipolar stripe domains in longer-period superlattices (n = 16). Moreover, the size and shape of the dipolar configurations are tuned by superlattice periodicity, thus providing a definitive design strategy to use superlattice layering to create relaxor-like behavior which may expand the ability to control desired properties in these complex systems.

Quantifying Intrinsic, Extrinsic, Dielectric, and Secondary Pyroelectric Responses in PbZr1-xTixO3 Thin Films.

Applications such as solid-state waste-heat energy conversion, infrared sensing, and thermally-driven electron emission rely on pyroelectric materials (a subclass of dielectric piezoelectrics) which exhibit temperature-dependent changes in polarization. Although enhanced dielectric and piezoelectric responses are typically found at polarization instabilities such as temperature- and chemically induced phase boundaries, large pyroelectric effects have been primarily limited in study to temperature-induced phase boundaries. Here, we directly identify the magnitude and sign of the intrinsic, extrinsic, dielectric, and secondary pyroelectric contributions to the total pyroelectric response as a function of chemistry in thin films of the canonical ferroelectric PbZr1-xTixO3 (x = 0.40, 0.48, 0.60, and 0.80) across the morphotropic phase boundary. Using phase-sensitive frequency and applied dc-bias methods, the various pyroelectric contributions were measured. It is found that the total pyroelectric response decreases systematically as one moves from higher to lower titanium contents. This arises from a combination of decreasing intrinsic response (-232 to -97 µC m-2 K-1) and a sign inversion (+33 to -17 µC m-2 K-1) of the extrinsic contribution upon crossing the morphotropic phase boundary. Additionally, the measured secondary and dielectric contributions span between -70 and -29 and 10-115 µC m-2 K-1 under applied fields, respectively, following closely trends in the piezoelectric and dielectric susceptibility. These findings and methodologies provide novel insights into the understudied realm of pyroelectric response.

Digenite Nanosheets Synthesized by Thermolysis of Layered Copper-Alkanethiolate Frameworks.

Copper sulfide nanocrystals support localized surface plasmon resonances in the near-infrared wavelengths and have significant potential as active plasmonic nanomaterials due to the tunability of this optical response. While numerous strategies exist for synthesizing copper sulfide nanocrystals, few methods result in nanocrystals with both controlled morphological shapes and crystallinity. Here, we synthesize and characterize ultrathin (<5 nm) Cu9S5 nanosheets that are formed by solventless thermolysis, utilizing Cu alkanethiolates as single-source precursors. Layered Cu alkanethiolate precursors adopt a highly ordered structure which can be further stabilized in the presence of Cl- and also serve to template the formation of nanosheets. We show that, in the absence of Cl-, only isotropic and disk-like Cu2-xS nanocrystals form. These findings offer further insight into the use of layered metal-organic single-source precursors as templates for anisotropic nanocrystal growth.

Tamizaje de la actividad antifúngica de extractos de especies de la flora de Entre Ríos / Screening of antifungal activity of extracts present in Entre Ríos flora species

Las plantas medicinales constituyen un recurso invalorable por su potencialidad farmacológica que hace necesario estudiarlas ante la demanda de nuevos fármacos, especialmente con actividad antifúngica. La flora de Entre Ríos (Argentina) posee numerosas especies con antecedentes etnobotánicos de uso como antisépticos en heridas. Los extractos de 6 especies vegetales seleccionadas fueron evaluadas en su actividad antifúngica mediante el método de microdilución en caldo, contra cepas tipificadas de Aspergillus niger y Trichophyton rubrum. Con excepción de Castela tweedii, las demás especies ensayadas, Baccharis articulata, Blepharocalyx salicifolius, Eichhornia azurea, Ludwigia peploides y Schinus fasciculatus, mostraron actividad in vitro en alguno de sus extractos, lo que auspicia la continuidad de los estudios.

Medicinal plants are a valuable resource due to its pharmacological potentiality ant its study is necessary in the face of the demand of new drugs specially those having antifungal activity. The Entre Ríos (Argentina) has many species with ethnobotanical backgrounds used as antiseptic agent for wounds. Extracts from 6 selected vegetal species were assessed for its antifungal activity by broth microdilution to typified strains of Aspergillus nigery Trichophyton rubrum. Except for Castela tweedii, the other assayed species, Baccharis articulate, Blepharocalyx salicifolius, Eichhornia azurea, Ludwigia peploides and Schinus fasciculatus showed in vitro activity in some extracts, being necessary a continuing study.