Interfacial-Strain-Controlled Ferroelectricity in Self-Assembled BiFeO3 Nanostructures

Self-assembled BiFeO₃-CoFe₂O₄ (BFO-CFO) vertically aligned nanocomposites are promising for logic, memory, and multiferroic applications, primarily due to the tunability enabled by strain engineering at the prodigious epitaxial vertical interfaces. However, local investigations directly revealing functional properties in the vicinity of such critical interfaces are often hampered by the size, geometry, microstructure, and concomitant experimental artifacts. Ferroelectric switching in the presence of lateral distributions of vertical strain thus remains relatively unexplored, with broader implications for all strain-engineered functional devices. By implementing tomographic atomic force microscopy, 3D domain orientation mapping, and spatially-resolved ferroelectric switching movies, local tensile strain significantly impacts the ferroelectric switching, principally by retarding domain nucleation in the BFO nearest to the vertically epitaxial tensile-strained interfaces. The relaxed centers of the BFO pillars become preferred domain nucleation and growth sites for low biases, with up to an order of magnitude change in the edge:center switching ratio for high biases. The new, multi-dimensional imaging approach—and its corresponding insights especially for directly strained interface effects on local properties—thereby advances the fundamental understanding of polarization switching and provides design principles for optimizing functional response in confined nanoferroic systems.     

Structure and Dielectric Properties of NiTiO3 Powders Synthesized by the Modified Sol–Gel Method

This study reports the synthesis of nickel titanate (NiTiO₃) powders by using the modified sol‐gel method, with nickel acetate tetrahydrate as the nickel source, titanium isopropoxide as the titanium source, and 2‐methoxyethanol as the solvent, followed by post‐heat treatment in air at temperatures ranging from 500 °C to 900 °C. The characteristics of powders were determined by X‐ray diffraction (XRD), FT‐infrared spectroscopy (FT‐IR), ultraviolet/visible spectroscopy (UV/Vis), and Raman spectroscopy. The particle size and surface area of the powders were also measured. The results indicated that single‐phase NiTiO₃ can be prepared using the modified sol‐gel method, followed by post‐heat treatment at the relatively low temperature of 550 °C. The crystallite sizes and particle sizes of NiTiO₃ powders increase in conjunction with the post‐heat treatment temperatures. However, the surface area of the powders shrinks as the post‐heat treatment temperatures increase. The dielectric constants of NiTiO₃ powders, based on the capacitance‐voltage analysis, are within a range of 13.2 to 17.8.     

Synthesis of unsymmetrical sulfamides and polysulfamides via SuFEx click chemistry

As hydrogen-bond donors and acceptors, N,N′-disubstituted sulfamides have been used in a range of applications from medicinal chemistry to anion-binding catalysis. However, compared to ureas or thioureas, the utilization of this unique moiety remains marginal, in part because of a lack of general synthetic methods to access unsymmetrical sulfamides. Specifically, polysulfamides are a virtually unknown type of polymer despite their potential utility in non-covalent dynamic networks, an intense area of research in materials science. We report herein a practical and efficient process to prepare unsymmetrical sulfamides via Sulfur(vi)-Fluoride Exchange (SuFEx) click chemistry. This process was then applied to synthesize polysulfamides. Thermal analysis showed that this family of polymers possess high thermal stability and tunable glass transition temperatures. Finally, hydrolysis studies indicated that aromatic polysulfamides could be recycled back to their constituting monomers at the end of their life cycle.

A general, practical, and efficient synthesis of N,N′-disubstituted sulfamides has been developed and applied to the preparation of polysulfamides, a virtually unknown class of polymers.     

The Cyclic Hydrogen‐Bonded 6‐Azaindole Trimer and its Prominent Excited‐State Triple‐Proton‐Transfer Reaction

The compound 6‐azaindole undergoes self‐assembly by formation of N(1)?H???N(6) hydrogen bonds (H bonds), forming a cyclic, triply H‐bonded trimer. The formation phenomenon is visualized by scanning tunneling microscopy. Remarkably, the H‐bonded trimer undergoes excited‐state triple proton transfer (ESTPT), resulting in a proton‐transfer tautomer emission maximized at 435 nm (325 nm of the normal emission) in cyclohexane. Computational approaches affirm the thermodynamically favorable H‐bonded trimer formation and the associated ESTPT reaction. Thus, nearly half a century after Michael Kasha discovered the double H‐bonded dimer of 7‐azaindole and its associated excited‐state double‐proton‐transfer reaction, the triply H‐bonded trimer formation of 6‐azaindole and its ESTPT reaction are demonstrated.     

Activating Basal Planes of NiPS3 for Hydrogen Evolution by Nonmetal Heteroatom Doping

NiPS₃, one of the most promising catalysts among transition metal trichalcogenidophosphates (MTPs) in hydrogen evolution reaction (HER) electrocatalysis, is still inhibited by its unsatisfactory activity originating from its semiconducting nature and inert basal plane. Here, it is proposed, for the first time, to engineer the basal surface activity of NiPS₃ by nonmetal heteroatom doping, and predict that the degree to which the valance band of NiPS₃ is filled dominates not only the electrical conductivity of the catalyst, but also the strength of hydrogen adsorption at its surface. Direct experimental evidence is offered that in all the single nonmetal doping samples, C‐doped NiPS₃ exhibits the optimum activity owing to its moderate filled state of valance band and that C, N codoping even shows Pt‐like activity with an ultralow overpotential of 53.2 mV to afford 10 mA cm^?2 current density and a high exchange current density of 0.7 mA cm^?2 in 1 m KOH. The findings that less valance electrons of dopants than substitutional atoms are of pivotal importance for improving HER activity of NiPS₃ catalyst pave the way for readily designing novel MTPs of ever high performance to replace the incumbent Pt‐based catalysts.     

Application of auditory masking in improved multiband excitation model

Modifications to improve the performance of the improved multiband excitation (IMBE) model in coding narrow-band speech are presented in this paper. The first modification is based on the phenomenon of auditory masking which helps the model to focus on the perceptual aspect of the coder by eliminating components that are inaudible to human ears during the analysis process of the IMBE model. In addition, a spectral amplitude enhancement stage is added. Preliminary results indicate that the proposed modifications improve the objective and subjective performances of the IMBE model in coding narrow-band speech at 4.15 kbps.