Large In-Plane and Vertical Piezoelectricity in Janus Transition Metal Dichalchogenides

Piezoelectricity in 2D van der Waals materials has received considerable interest because of potential applications in nanoscale energy harvesting, sensors, and actuators. However, in all the systems studied to date, strain and electric polarization are confined to the basal plane, limiting the operation of piezoelectric devices. In this paper, based on <i>ab initio</i> calculations, we report a 2D materials system, namely, the recently synthesized Janus MXY (M = Mo or W, X/Y = S, Se, or Te) monolayer and multilayer structures, with large out-of-plane piezoelectric polarization. For MXY monolayers, both strong in-plane and much weaker out-of-plane piezoelectric polarizations can be induced by a uniaxial strain in the basal plane. For multilayer MXY, we obtain a very strong out-of-plane piezoelectric polarization when strained transverse to the basal plane, regardless of the stacking sequence. The out-of-plane piezoelectric coefficient <i>d</i>₃₃ is found to be strongest in multilayer MoSTe (5.7–13.5 pm/V depending on the stacking sequence), which is larger than that of the commonly used 3D piezoelectric material AlN (<i>d</i>₃₃ = 5.6 pm/V); <i>d</i>₃₃ in other multilayer MXY structures are a bit smaller, but still comparable. Our study reveals the potential for utilizing piezoelectric 2D materials and their van der Waals multilayers in device applications

‘Unzipping’ of twin lamella in nanotwinned nickel nanowires under flexural bending

<p>We report the fabrication of nickel nanowires with parallel growth-twin structures (‘twin lamella’ along the wire axis) by electrochemical deposition, and demonstrate an interesting twin ‘unzipping’ phenomenon in such nanotwinned nanowires under bending. Through <i>in situ</i> TEM, we found that ‘unzipping’ of twin lamella was achieved by gradually increasing twin spacing along the wire axis via a layer-by-layer twin boundary migration process. Molecular dynamics simulations suggest that partial dislocation slip is responsible for activating the ‘unzipping’, with a multi-step-process involving dislocation loop initiation, expansion and partially annihilation. Our work could provide new insights into the deformation mechanisms of nanotwinned 1-D metallic nanostructures.</p> <p><b>IMPACT STATEMENT</b></p> <p>Nickel nanowires with parallel-twin structures were fabricated and demonstrated an interesting twin lamella ‘unzipping’ behavior upon flexural bending, which provides new insights into the deformation mechanisms of nanotwinned metallic materials.</p

Porous Spinel Zn_<i>x</i>Co_3–<i>x</i>O₄ Hollow Polyhedra Templated for High-Rate Lithium-Ion Batteries

Nanostructured metal oxides with both anisotropic texture and hollow structures have attracted considerable attention with respect to improved electrochemical energy storage and enhanced catalytic activity. While synthetic strategies for the preparation of binary metal oxide hollow structures are well-established, the rational design and fabrication of complex ternary metal oxide with nonspherical hollow features is still a challenge. Herein, we report a simple and scalable strategy to fabricate highly symmetric porous ternary Zn_<i>x</i>Co_3–<i>x</i>O₄ hollow polyhedra composed of nanosized building blocks, which involves a morphology-inherited and thermolysis-induced transformation of heterobimetallic zeolitic imidazolate frameworks. When tested as anode materials for lithium-ion batteries, these hollow polyhedra have exhibited excellent electrochemical performance with high reversible capacity, excellent cycling stability, and good rate capability

Plasmonic Pumping of Excitonic Photoluminescence in Hybrid MoS₂–Au Nanostructures

We report on the fabrication of monolayer MoS₂-coated gold nanoantennas combining chemical vapor deposition, e-beam lithography surface patterning, and a soft lift-off/transfer technique. The optical properties of these hybrid plasmonic–excitonic nanostructures are investigated using spatially resolved photoluminescence spectroscopy. Off- and in-resonance plasmonic pumping of the MoS₂ excitonic luminescence showed distinct behaviors. For plasmonically mediated pumping, we found a significant enhancement (∼65%) of the photoluminescence intensity, clear evidence that the optical properties of the MoS₂ monolayer are strongly influenced by the nanoantenna surface plasmons. In addition, a systematic photoluminescence broadening and red-shift in nanoantenna locations is observed which is interpreted in terms of plasmonic enhanced optical absorption and subsequent heating of the MoS₂ monolayers. Using a temperature calibration procedure based on photoluminescence spectral characteristics, we were able to estimate the local temperature changes. We found that the plasmonically induced MoS₂ temperature increase is nearly four times larger than in the MoS₂ reference temperatures. This study shines light on the plasmonic–excitonic interaction in these hybrid metal/semiconductor nanostructures and provides a unique approach for the engineering of optoelectronic devices based on the light-to-current conversion

Thickness-Dependent and Magnetic-Field-Driven Suppression of Antiferromagnetic Order in Thin V₅S₈ Single Crystals

With materials approaching the 2D limit yielding many exciting systems with intriguing physical properties and promising technological functionalities, understanding and engineering magnetic order in nanoscale, layered materials is generating keen interest. One such material is V₅S₈, a metal with an antiferromagnetic ground state below the Néel temperature <i>T</i>_N ∼ 32 K and a prominent spin-flop signature in the magnetoresistance (MR) when <i>H</i>∥<i>c</i> ∼ 4.2 T. Here we study nanoscale-thickness single crystals of V₅S₈, focusing on temperatures close to <i>T</i>_N and the evolution of material properties in response to systematic reduction in crystal thickness. Transport measurements just below <i>T</i>_N reveal magnetic hysteresis that we ascribe to a metamagnetic transition, the first-order magnetic-field-driven breakdown of the ordered state. The reduction of crystal thickness to ∼10 nm coincides with systematic changes in the magnetic response: <i>T</i>_N falls, implying that antiferromagnetism is suppressed; and while the spin-flop signature remains, the hysteresis disappears, implying that the metamagnetic transition becomes second order as the thickness approaches the 2D limit. This work demonstrates that single crystals of magnetic materials with nanometer thicknesses are promising systems for future studies of magnetism in reduced dimensionality and quantum phase transitions

Prediction of Enhanced Catalytic Activity for Hydrogen Evolution Reaction in Janus Transition Metal Dichalcogenides

Significant efforts have been made in improving the hydrogen evolution reaction (HER) catalytic activity in transition metal dichalcogenides (TMDs), which are promising nonprecious catalysts. However, previous attempts have exploited possible solutions to activate the inert basal plane, with little improvement. Among them, the most successful modification requires a careful manipulation of vacancy concentration and strain simultaneously. To fully realize the promise of TMD catalysts for HER in an easier and more effective way, a new means in tuning the HER catalytic activity is needed. Herein, we propose exploiting the inherent structural asymmetry in the recently synthesized family of Janus TMDs as a new means to stimulate HER activity. We report a density functional theory (DFT) study of various Janus TMD monolayers as HER catalysts, and identify the WSSe system as a promising candidate, where the basal plane can be activated without large applied tensile strains and in the absence of significant density of vacancies. We predict that it is possible to realize a strain-free Janus TMD-based catalyst that can readily provide promising intrinsic HER catalytic performance. The calculated density of states and electronic structures reveal that the introduction of in-gap states and a shift in the Fermi level in hydrogen adsorbed systems due to Janus asymmetry is the origin of enhanced HER activity. Our results should pave the way to design high-performance and easy-accessible TMD-based HER catalysts

Survival of <i>E. coli</i> O157:H7 in the tested soils.

<p>Solid line: neutral soils; dashed line: acidic soils.</p

Stepwise multiple-linear regression analysis of soil properties and the survival time (<i>t_d</i>) of <i>E. coli</i> O157:H7 in the test soils.

<p># Survival time to reach detection limit (<i>t_d</i>), the ratio of Gram-negative bacteria phospholipid fatty acids (PLFAs) to Gram-positive bacteria PLFAs (G⁻/G⁺); exchangeable potassium (K); total soil organic carbon (OC); correlation is significant at the 0.001 probability level(***).</p

Principal component analysis (PCA) of the survival parameters (<i>δ</i>, <i>p</i> and <i>t_d</i>).

<p><i>δ</i>, <i>p</i> and <i>t_d</i> are the same as shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081178#pone-0081178-t002" target="_blank">Table 2</a>. A-S: acidic soils; N-S: neutral and slight alkaline soils.</p

Statistical measures and fitted parameter values of the Weibull model describing the survival of <i>E</i>. <i>coli</i> O157:H7 in soils.

<p># Survival time to reach detection limit (<i>t_d</i>); time needed for first decimal reduction in <i>E</i>. <i>coli</i> O157:H7 population (<i>δ</i>); shape parameter (<i>p</i>).</p>*<p>Significant differences (p < 0.05) indicated by different letters.</p