Highly Sensitive NO2 Gas Sensors Based on MoS2@MoO3 Magnetic Heterostructure.

Recently, two-dimensional (2D) materials and their heterostructures have attracted considerable attention in gas sensing applications. In this work, we synthesized 2D MoS2@MoO3 heterostructures through post-sulfurization of α-MoO3 nanoribbons grown via vapor phase transport (VPT) and demonstrated highly sensitive NO2 gas sensors based on the hybrid heterostructures. The morphological, structural, and compositional properties of the MoS2@MoO3 hybrids were studied by a combination of advanced characterization techniques revealing a core-shell structure with the coexistence of 2H-MoS2 multilayers and intermediate molybdenum oxysulfides on the surface of α-MoO3. The MoS2@MoO3 hybrids also exhibit room-temperature ferromagnetism, revealed by vibrating sample magnetometry (VSM), as a result of the sulfurization process. The MoS2@MoO3 gas sensors display a p-type-like response towards NO2 with a detection limit of 0.15 ppm at a working temperature of 125 °C, as well as superb selectivity and reversibility. This p-type-like sensing behavior is attributed to the heterointerface of MoS2-MoO3 where interfacial charge transfer leads to a p-type inversion layer in MoS2, and is enhanced by magnetic dipole interactions between the paramagnetic NO2 and the ferromagnetic sensing layer. Our study demonstrates the promising application of 2D molybdenum hybrid compounds in gas sensing applications with a unique combination of electronic and magnetic properties.

THz/Far infrared synchrotron observations of superlattice frequencies in MgB2.

THz/Far Infrared synchrotron absorption experiments on pure and doped MgB2 samples show that the absorption spectral weight at low wavenumber (i.e., <110 cm-1) evolves as the temperature is reduced to 10 K. Distinct spectral peak intensities increase as the temperature of MgB2 and doped MgB2 approaches, and then crosses, the superconducting transition temperature. These experimental data suggest a strong link to superconductivity induced by subtle shifts in structural symmetry. Significant increases in absorption are observed at frequencies that correspond to the superconducting gaps for doped and pure MgB2, and at fractions of these frequency (or energy) values. This low wavenumber spectral transition is consistent with the notion that superlattice frequencies contribute to the optic modes of the MgB2 phonon dispersion and are critical to the superconducting transition for this structure. Key integer ratios are identified in real and reciprocal spaces that link bonding character, Fermi vectors and Fermi surfaces as well as phonon properties with geometric parameters and specific superlattice symmetries for MgB2. Similarly consistent spectral data at low wavenumber are also obtained for carbon doped Mg11B2. Density Functional Theory calculations of superlattice phonon dispersions result in folded mode frequencies that match these observed low wavenumber experiments. These results show that symmetry reductions, largely electronic in character although coupled to vibrations, occur with change in temperature and imply strong links to superconductivity mechanisms.

Biowaste-Derived, Self-Organized Arrays of High-Performance 2D Carbon Emitters for Organic Light-Emitting Diodes.

Low-cost flexible organic light-emitting diodes (OLEDs) with nanoemitter material from waste open up new opportunities for sustainable technology. The common emitter materials generated from waste are carbon dots (CDs). However, these have poor luminescent properties. Further solid-state emission quenching makes application in display devices challenging. Here, flexible and rigid OLED devices are demonstrated using self-assembled 2D arrays of CDs derived from waste material, viz., human hair. High-performance CDs with a quantum yield (QY) of 87%, self-assembled into 2D arrays, are achieved by improving the crystallinity and decreasing the CDs' size distribution. The CD island array exhibits ultrahigh hole mobility (≈10-1 cm2 V-1 s-1 ) and significant reduction in solid-state emission quenching compared to pristine CDs; hence, it is used here as an emitting layer in both indium tin oxide (ITO)-coated glass and ITO-coated flexible poly(ethylene terephthalate) (PET) substrate OLED devices, without any hole-injection layer. The flexible OLED device exhibits a stable, voltage-independent blue/cyan emission with a record maximum luminescence of 350 cd m-2 , whereas the OLED device based on the rigid glass substrate shows a maximum luminescence of 700 cd m-2 . This work sets up a platform to develop next-generation OLED displays using CD emitters derived from the biowaste material.

Exploring the Role of Manganese on the Microstructure, Mechanical Properties, Biodegradability, and Biocompatibility of Porous Iron-Based Scaffolds.

In this work, the role that manganese plays in determining the structure and performance of sintered biodegradable porous Fe-Mn alloys is described. Powder metallurgy processing was employed to produce a series of biodegradable porous Fe-xMn (x = 20, 30, and 35 wt %) alloys suitable for bone scaffold applications. Increasing manganese content increased the porosity volume in the sintered alloys and influenced the ensuing properties of the metal. The Fe-35Mn alloy possessed optimum properties for orthopedic application. X-ray diffraction analysis and magnetic characterization confirmed the predominance of the antiferromagnetic austenitic phase and ensured the magnetic resonance imaging (MRI) compatibility of this alloy. The porous Fe-35Mn alloy possessed mechanical properties (tensile strength of 144 MPa, elastic modulus of 53.3 GPa) comparable to human cortical bone. The alloy exhibited high degradation rates (0.306 mm year-1) in simulated physiological fluid, likely due to its considerable Mn content and the high surface area inherent to its porous structures, while cytotoxicity and morphometry tests using mammalian preosteoblast cells (MC3T3-E1) indicated good cell viability in the Fe-35Mn alloy.

Single Step Process for Crystalline Ni-B Compounds.

Crystalline Ni2B, Ni3B, and Ni4B3 are synthesized by a single-step method using autogenous pressure from the reaction of NaBH4 and Ni precursors. The effect of reaction temperature, pressure, time, and starting materials on the composition of synthesized products, particle morphologies, and magnetic properties is demonstrated. High yields of Ni2B (>98%) are achieved at 2.3⁻3.4 MPa and ~670 °C over five hours. Crystalline Ni3B or Ni4B3 form in conjunction with Ni2B at higher temperature or higher autogenous pressure in proportions influenced by the ratios of initial reactants. For the same starting ratios of reactants, a longer reaction time or higher pressure shifts equilibria to lower yields of Ni2B. Using this approach, yields of ~88% Ni4B3 (single phase orthorhombic) and ~72% Ni3B are obtained for conditions 1.9 MPa < Pmax < 4.9 MPa and 670 °C < Tmax < 725 °C. Gas-solid reaction is the dominant transformation mechanism that results in formation of Ni2B at lower temperatures than conventional solid-state methods.

Synthesis of Magnesium Nickel Boride Aggregates via Borohydride Autogenous Pressure.

We demonstrate synthesis of the ternary intermetallic MgNi3B2 using autogenous pressure from the reaction of NaBH4 with Mg and Ni metal powder. The decomposition of NaBH4 to H2 and B2H6 commences at low temperatures in the presence of Mg and/or Ni and promotes formation of Ni-borides and MgNi3B2 with the increase in temperature. MgNi3B2 aggregates with Ni-boride cores are formed when the reaction temperature is >670 °C and autogenous pressure is >1.7 MPa. Morphologies and microstructures suggest that solid-gas and liquid-gas reactions are dominant mechanisms and that Ni-borides form at a lower temperature than MgNi3B2. Magnetic measurements of the core-shell MgNi3B2 aggregates are consistent with ferromagnetic behaviour in contrast to stoichiometric MgNi3B2 which is diamagnetic at room temperature.