Exploring the Pb1- x Srx HfO3 System and Potential for High Capacitive Energy Storage Density and Efficiency

The hafnate perovskites PbHfO₃ (antiferroelectric) and SrHfO₃ ("potential" ferroelectric) are studied as epitaxial thin films on SrTiO₃ (001) substrates with the added opportunity of observing a morphotropic phase boundary (MPB) in the Pb_{1- x} Sr_x HfO₃ system. The resulting (240)-oriented PbHfO₃ (Pba2) films exhibited antiferroelectric switching with a saturation polarization ≈53 µC cm^-2 at 1.6 MV cm^-1 , weak-field dielectric constant ≈186 at 298 K, and an antiferroelectric-to-paraelectric phase transition at ≈518 K. (002)-oriented SrHfO₃ films exhibited neither ferroelectric behavior nor evidence of a polar P4mm phase . Instead, the SrHfO₃ films exhibited a weak-field dielectric constant ≈25 at 298 K and no signs of a structural transition to a polar phase as a function of temperature (77-623 K) and electric field (-3 to 3 MV cm^-1 ). While the lack of ferroelectric order in SrHfO₃ removes the potential for MPB, structural and property evolution of the Pb_{1- x} Sr_x HfO₃ (0 ≤ x < 1) system is explored. Strontium alloying increased the electric-breakdown strength (E_B ) and decreased hysteresis loss, thus enhancing the capacitive energy storage density (U_r ) and efficiency (η). The composition, Pb_0.5 Sr_0.5 HfO₃ produced the best combination of E_B  = 5.12 ± 0.5 MV cm^-1 , U_r  = 77 ± 5 J cm^-3 , and η = 97 ± 2%, well out-performing PbHfO₃ and other antiferroelectric oxides.

High-k Polymer Nanocomposite Materials for Technological Applications

Understanding the properties of small molecules or monomers is decidedly important. The efforts of synthetic chemists and material engineers must be appreciated because of their knowledge of how utilize the properties of synthetic fragments in constructing long-chain macromolecules. Scientists active in this area of macromolecular science have shared their knowledge of catalysts, monomers and a variety of designed nanoparticles in synthetic techniques that create all sorts of nanocomposite polymer stuffs. Such materials are now an integral part of the contemporary world. Polymer nanocomposites with high dielectric constant (high-k) properties are widely applicable in the technological sectors including gate dielectrics, actuators, infrared detectors, tunable capacitors, electro optic devices, organic field-effect transistors (OFETs), and sensors. In this short colloquy, we provided an overview of a few remarkable high-k polymer nanocomposites of material science interest from recent decades.

Synthesis, structure and magnetic properties of multipod-shaped cobalt ferrite nanocrystals

A non-aqueous sol–gel route followed by oriented attachment to make multi-pod CoFe₂O₄ nanocrystals showing large room temperature saturation magnetization.

One-Step Synthesis, Structure, and Band Gap Properties of SnO2 Nanoparticles Made by a Low Temperature Nonaqueous Sol-Gel Technique

Because of its electrically conducting properties combined with excellent thermal stability and transparency throughout the visible spectrum, tin oxide (SnO₂) is extremely attractive as a transparent conducting material for applications in low-emission window coatings and solar cells, as well as in lithium-ion batteries and gas sensors. It is also an important catalyst and catalyst support for oxidation reactions. Here, we describe a novel nonaqueous sol-gel synthesis approach to produce tin oxide nanoparticles (NPs) with a low NP size dispersion. The success of this method lies in the nonhydrolytic pathway that involves the reaction between tin chloride and an oxygen donor, 1-hexanol, without the need for a surfactant or subsequent thermal treatment. This one-pot procedure is carried out at relatively low temperatures in the 160-260 °C range, compatible with coating processes on flexible plastic supports. The NP size distribution, shape, and dislocation density were studied by powder X-ray powder diffraction analyzed using the method of whole powder pattern modeling, as well as high-resolution transmission electron microscopy. The SnO₂ NPs were determined to have particle sizes between 3.4 and 7.7 nm. The reaction products were characterized using liquid-state ¹³C and ¹H nuclear magnetic resonance (NMR) that confirmed the formation of dihexyl ether and 1-chlorohexane. The NPs were studied by a combination of ¹³C, ¹H, and ¹¹⁹Sn solid-state NMR as well as Fourier transform infrared (FTIR) and Raman spectroscopy. The ¹³C SSNMR, FTIR, and Raman data showed the presence of organic species derived from the 1-hexanol reactant remaining within the samples. The optical absorption, studied using UV-visible spectroscopy, indicated that the band gap (E _g) shifted systematically to lower energy with decreasing NP sizes. This unusual result could be due to mechanical strains present within the smallest NPs perhaps associated with the organic ligands decorating the NP surface. As the size increased, we observed a correlation with an increased density of screw dislocations present within the NPs that could indicate relaxation of the stress. We suggest that this could provide a useful method for band gap control within SnO₂ NPs in the absence of chemical dopants.

Deliberately Designed Atomic-Level Silver-Containing Interface Results in Improved Rate Capability and Utilization of Silver Hollandite for Lithium-Ion Storage

α-MnO₂-structured materials are generally classified as semiconductors; thus, we present a strategy to increase electrochemical utilization through the design of a conductive material interface. Surface treatment of silver hollandite (Ag_xMn₈O₁₆) with Ag⁺ (Ag₂O) provides significant benefits to the resultant electrochemistry, including a decreased charge-transfer resistance and a 2-fold increase in deliverable energy density at a high rate. The improved function of this designed interface relative to conventional electrode fabrication strategies is highlighted.

First-Principles Prediction of Two-Dimensional Electron Gas Driven by Polarization Discontinuity in Nonpolar/Nonpolar AHfO3/SrTiO3 (A = Ca, Sr, and Ba) Heterostructures

By using first-principles electronic structure calculations, we explored the possibility of producing two-dimensional electron gas (2DEG) in nonpolar/nonpolar AHfO₃/SrTiO₃ (A = Ca, Sr, and Ba) heterostructures. Two types of nonpolar/nonpolar interfaces, (AO)⁰/(TiO₂)⁰ and (HfO₂)⁰/(SrO)⁰, each with AO and HfO₂ surface terminations, are modeled, respectively. The polarization domain and resulting interfacial electronic property are found to be more sensitive to the surface termination of the film rather than the interface model. As film thickness increases, an insulator-to-metal transition is found in all the heterostructures with HfO₂ surface termination: for (AO)⁰/(TiO₂)⁰ interfaces, predicted critical film thickness for an insulator-to-metal transition is about 7, 6, and 3 unit cells for CaHfO₃/SrTiO₃, SrHfO₃/SrTiO₃, and BaHfO₃/SrTiO₃, respectively; for (HfO₂)⁰/(SrO)⁰ interfaces, the critical film thickness is about 7.5, 5.5, and 4.5 unit cells, respectively. In contrast, for the heterostructures with AO surface termination, CaHfO₃/SrTiO₃ exhibits a much larger critical film thickness about 11-12 unit cells for an insulator-to-metal transition; while SrHfO₃/SrTiO₃ and BaHfO₃/SrTiO₃ do not show any polarization behavior even film thickness increases up to 20 unit cells. The strain-induced polarization behavior was well-elucidated from energy versus polarization profile. This work is expected to stimulate further experimental investigation to the interfacial conductivity in the nonpolar/nonpolar AHfO₃/SrTiO₃ HS.

Smallest Bimetallic CoPt3 Superparamagnetic Nanoparticles

We present for the first time a nonaqueous sol-gel route to produce ultrasmall (<2 nm) magnetic bimetallic CoPt₃ nanoparticles (NPs). The one-pot procedure is carried out at low temperature (180 °C) using benzyl alcohol, acting as both reducing agent and solvent. The highly monodisperse CoPt₃ NPs were investigated with innovative advanced X-ray methods (whole powder pattern modeling), HR-STEM, XPS, and SQUID magnetometry. XPS showed Co was mostly in metallic form, but with a very small amount of CoO on the NP surface. The spherical NPs had an ultrasmall diameter of 1.6 nm and could self-assemble in aligned linear chains, or nanobelts, of single NPs. They are superparamagnetic, with blocking temperature of ∼20 K and coercivity at 10 K of 27.9 kA m^-1 (∼350 Oe). However, there is evidence of a second magnetic phase (probably CoO) in the ZFC magnetization curve, which enhances their magnetization values, without significantly affecting their superparamagnetism.