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Charge density waves (CDWs) are pervasive orders in solids that usually enhance the effective mass (m*) and reduce the Fermi velocity ( v F ) of carriers. Here, we report on the inverse - a reduced m* and an enhanced v F correlated with the growth of the CDW order in CuTe with gapped, practically linearly dispersing bands - reminiscent of emergent CDW-gapped topological semimetals. Using momentum-dependent electron energy-loss spectroscopy (q-EELS), we simultaneously capture m* and v F of the CDW-related, practically linearly dispersing electrons by plasmon dispersions across the transition (335 K, T CDW ), with m* of 0.28 m 0 (m 0 , the electron rest mass) and v F of ~ 0.005c (c, the speed of light) at 300 K. With the growth of the CDW order-parameter strength toward 100 K, the electrons become lighter and move faster by ~ 20%. Thorough inspection below T CDW unveils the essential role of the increasing opening of the CDW gap. CuTe is a rich platform for the exploration of CDW/correlation physics with q-EELS established as a useful probe for this type of physics., (© 2024. The Author(s).)

We present structural properties at different temperatures and high-pressure (HP) of La 3 Rh 4 Sn 13 which is one of the interesting systems in the Remika phase RE 3 Rh 4 Sn 13 (RE=Sr, Ca, La, Pr, Ce) quasi-skutterudite series using synchrotron diffraction. Data at ambient conditions revealed the presence of several weak reflections, which could be accounted only with a superlattice I* structure (I4 1 32) with lattice parameter a~19.457 Å. However, above 350 K, a complete suppression of the weak superlattice reflections of the I* structure is observed. Data at higher temperatures is found to be well described by the I structure (Pm-3n) having half the lattice parameter compared to the I* structure. HP-XRPD at ambient temperature showed that pressures greater than 7.5 GPa result in similar suppression of the weak I* superlattice reflections. Data at higher pressures is found to be well described by the I structure (Pm-3n), similar to the high-temperature phase. HP Raman measurements demonstrated changes that seem to be consistent with a locally more ordered structure as in the case of the I*→I transition. Our findings on La 3 Rh 4 Sn 13 open up new avenues to study unexplored HP phenomena, especially the superconductivity in these Remika phase quasi-skutterudites., (© 2024 Wiley-VCH GmbH.)

Charge density waves (CDWs) involved with electronic and phononic subsystems simultaneously are a common quantum state in solid-state physics, especially in low-dimensional materials. However, CDW phase dynamics in various dimensions are yet to be studied, and their phase transition mechanism is currently moot. Here we show that using the distinct temperature evolution of orientation-dependent ultrafast electron and phonon dynamics, different dimensional CDW phases are verified in CuTe. When the temperature decreases, the shrinking of c-axis length accompanied with the appearance of interchain and interlayer interactions causes the quantum fluctuations (QF) of the CDW phase until 220 K. At T < 220 K, the CDWs on the different ab-planes are finally locked with each other in anti-phase to form a CDW phase along the c-axis. This study shows the dimension evolution of CDW phases in one CDW system and their stabilized mechanisms in different temperature regimes., (© 2024. The Author(s).)

Transition metal dichalcogenides, by virtue of their two-dimensional structures, could provide the largest active surface for reactions with minimal materials consumed, which has long been pursued in the design of ideal catalysts. Nevertheless, their structurally perfect basal planes are typically inert; their surface defects, such as under-coordinated atoms at the surfaces or edges, can instead serve as catalytically active centers. Here we show a reaction probability > 90 % for adsorbed methanol (CH 3 OH) on under-coordinated Pt sites at surface Te vacancies, produced with Ar + bombardment, on layered PtTe 2 - approximately 60 % of the methanol decompose to surface intermediates CH x O (x = 2, 3) and 35 % to CH x (x = 1, 2), and an ultimate production of gaseous molecular hydrogen, methane, water and formaldehyde. The characteristic reactivity is attributed to both the triangular positioning and varied degrees of oxidation of the under-coordinated Pt at Te vacancies., (© 2024. The Author(s).)

Silicon (Si), the dominant semiconductor in microelectronics yet lacking optoelectronic functionalities in UV regions, has been researched extensively to make revolutionary changes. In this study, the inherent drawback of Si on optoelectronic functionalities in UV regions is potentially overcome through heterostructure coupling of delaminated p-type MnPS 3 , having bulk, multiple-layer, and few-layer features, with n-type Si. By artificially mimicking the architectures of shrubs with unique UV shading phenomena, the revolutionary multiple-layer MnPS 3 structures with staggered stacking configurations trigger outstanding UV photosensing performances, displaying an average EQE value of 1.1 × 10 3 %, average photoresponsivity of 3.1 × 10 2 A/W, average detectivity of 1.9 × 10 14 cm Hz 1 /2W 1- , and average on/off ratio of 1.8 × 10 3 under 365 nm light. To the best of our knowledge, this is the first attempt toward realizing gate-free MnPS 3 -based UV photodetectors, while all of the photodetection outcomes are better than those of more sophisticated field-effect transistor (FET) designs, which have remarkable impacts on the practicality and functionality of next-generation UV optoelectronics.

Methanol (CH 3 OH) oxidation offers a promising avenue for transitioning to clean energy, particularly in the field of direct methanol fuel cells (DMFCs). However, the development of efficient and cost-effective catalysts for the methanol oxidation reaction (MOR) remains a critical challenge. Herein, we report the exceptional electrocatalytic activity and stability of Ni 3 Sn 4 toward MOR in acidic media, achieving a performance comparable to that of commercial Pt/C catalysts. Our catalyst design incorporates Earth-abundant Ni and Sn elements, resulting in a material that is 1800 times more cost-effective than Pt/C. Density functional theory (DFT) modeling substantiates our experimental findings, shedding light on the favorable reaction mechanisms and kinetics on the Ni 3 Sn 4 surface. Additionally, the as-synthesized Ni 3 Sn 4 electrocatalyst demonstrates commendable durability, maintaining its electrocatalytic activity even after prolonged exposure to harsh acidic conditions.

In recent years, the correlation between the existence of topological electronic states in materials and their catalytic activity has gained increasing attention, due to the exceptional electron conductivity and charge carrier mobility exhibited by quantum materials. However, the physicochemical mechanisms ruling catalysis with quantum materials are not fully understood. Here, we investigate the chemical reactivity, ambient stability, and catalytic activity of the topological nodal-line semimetal AuSn 4 . Our findings reveal that the surface of AuSn 4 is prone to oxidation, resulting in the formation of a nanometric SnO 2 skin. This surface oxidation significantly enhances the material's performance as a catalyst for the hydrogen evolution reaction in acidic environments. We demonstrate that the peculiar atomic structure of oxidized AuSn 4 enables the migration of hydrogen atoms through the Sn-O layer with a minimal energy barrier of only 0.19 eV. Furthermore, the Volmer step becomes exothermic in the presence of Sn vacancies or tin-oxide skin, as opposed to being hindered in the pristine sample, with energy values of -0.62 and -1.66 eV, respectively, compared to the +0.46 eV energy barrier in the pristine sample. Our model also suggests that oxidized AuSn 4 can serve as a catalyst for the hydrogen evolution reaction in alkali media. Additionally, we evaluate the material's suitability for the carbon dioxide reduction reaction, finding that the presence of topologically protected electronic states enhances the migration of hydrogen atoms adsorbed on the catalyst to carbon dioxide.

Layered transition metal dichalcogenides (TMDs) are two-dimensional materials exhibiting a variety of unique features with great potential for electronic and optoelectronic applications. The performance of devices fabricated with mono or few-layer TMD materials, nevertheless, is significantly affected by surface defects in the TMD materials. Recent efforts have been focused on delicate control of growth conditions to reduce the defect density, whereas the preparation of a defect-free surface remains challenging. Here, we show a counterintuitive approach to decrease surface defects on layered TMDs: a two-step process including Ar ion bombardment and subsequent annealing. With this approach, the defects, mainly Te vacancies, on the as-cleaved PtTe 2 and PdTe 2 surfaces were decreased by more than 99%, giving a defect density <1.0 × 10 10 cm -2 , which cannot be achieved solely with annealing. We also attempt to propose a mechanism behind the processes.

The zero-magnetic-field nonlinear Hall effect (NLHE) refers to the second-order transverse current induced by an applied alternating electric field; it indicates the topological properties of inversion-symmetry-breaking crystals. Despite several studies on the NLHE induced by the Berry-curvature dipole in Weyl semimetals, the direct current conversion by rectification is limited to very low driving frequencies and cryogenic temperatures. The nonlinear photoresponse generated by the NLHE at room temperature can be useful for numerous applications in communication, sensing, and photodetection across a high bandwidth. In this study, observations of the second-order NLHE in type-II Dirac semimetal CoTe 2 under time-reversal symmetry are reported. This is determined by the disorder-induced extrinsic contribution on the broken-inversion-symmetry surface and room-temperature terahertz rectification without the need for semiconductor junctions or bias voltage. It is shown that remarkable photoresponsivity over 0.1 A W -1 , a response time of approximately 710 ns, and a mean noise equivalent power of 1 pW Hz -1/2 can be achieved at room temperature. The results open a new pathway for low-energy photon harvesting via nonlinear rectification induced by the NLHE in strongly spin-orbit-coupled and inversion-symmetry-breaking systems, promising a considerable impact in the field of infrared/terahertz photonics., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Hydrogen production from methanol decomposition to syngas (H 2 + CO) is a promising alternative route for clean energy transition. One major challenge is related to the quest for stable, cost-effective, and selective catalysts operating below 400 °C. We illustrate an investigation of the surface reactivity of a Ni 3 Sn 4 catalyst working at 250 °C, by combining density functional theory, operando X-ray absorption spectroscopy, and high-resolution transmission electron microscopy. We discovered that the catalytic reaction is driven by surface tin-oxide phases, which protects the underlying Ni atoms from irreversible chemical modifications, increasing the catalyst durability. Moreover, we found that Sn content plays a key role in enhancing the H 2 selectivity, with respect to secondary products such as CO 2 . These findings open new perspectives for the engineering of scalable and low-cost catalysts for hydrogen production.

The exotic electronic properties of topological semimetals (TSs) have opened new pathways for innovative photonic and optoelectronic devices, especially in the highly pursuit terahertz (THz) band. However, in most cases Dirac fermions lay far above or below the Fermi level, thus hindering their successful exploitation for the low-energy photonics. Here, low-energy type-II Dirac fermions in kitkaite (NiTeSe) for ultrasensitive THz detection through metal-topological semimetal-metal heterostructures are exploited. Furthermore, a heterostructure combining two Dirac materials, namely, graphene and NiTeSe, is implemented for a novel photodetector exhibiting a responsivity as high as 1.22 A W -1 , with a response time of 0.6 µs, a noise-equivalent power of 18 pW Hz -0.5 , with outstanding stability in the ambient conditions. This work brings to fruition of Dirac fermiology in THz technology, enabling self-powered, low-power, room-temperature, and ultrafast THz detection., (© 2022 Wiley-VCH GmbH.)

Layered van der Waals crystals exhibit unique properties making them attractive for applications in nanoelectronics, optoelectronics, and sensing. The integration of two-dimensional materials with complementary metal-oxide-semiconductor (CMOS) technology requires controllable n- and p-type doping. In this work, we demonstrate the fabrication of vertical p-n heterojunctions made of p-type tin monoselenide (SnSe) and n-type tin diselenide (SnSe 2 ). The p-n heterojunction is created in a single flake by the NH 3 -plasma-assisted phase transformation from SnSe 2 to SnSe. We show that the transformation rate and crystal quality strongly depend on plasma parameters like plasma power, temperature, partial pressure, NH 3 flow, and duration of plasma treatment. With optimal plasma parameters, the full transformation of SnSe 2 flakes into SnSe is achieved within a few seconds. The crystal quality and the topography of the fabricated SnSe-SnSe 2 heterostructures are investigated using micro-Raman spectroscopy and cross-sectional transmission electron microscopy. The formation of a p-n junction is verified by current-voltage measurements., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Monolayer transition metal dichalcogenides offer an appropriate platform for developing advanced electronics beyond graphene. Similar to two-dimensional molecular frameworks, the electronic properties of such monolayers can be sensitive to perturbations from the surroundings; the implied tunability of electronic structure is of great interest. Using scanning tunneling microscopy/spectroscopy, we demonstrated a bandgap engineering technique in two monolayer materials, MoS 2 and PtTe 2 , with the tunneling current as a control parameter. The bandgap of monolayer MoS 2 decreases logarithmically by the increasing tunneling current, indicating an electric-field-induced gap renormalization effect. Monolayer PtTe 2 , by contrast, exhibits a much stronger gap reduction, and a reversible semiconductor-to-metal transition occurs at a moderate tunneling current. This unusual switching behavior of monolayer PtTe 2 , not seen in bulk semimetallic PtTe 2 , can be attributed to its surface electronic structure that can readily couple to the tunneling tip, as demonstrated by theoretical calculations.

The knowledge of the thermal expansion coefficient is of crucial importance to prevent the poor performance of devices, especially when these are made up of several layers of different materials, as in the case of 2D heterostructures. Helium atom scattering is a suitable tool for the direct measurement of the surface thermal expansion coefficient of materials. This information can be obtained directly from the position of the helium diffraction peaks, which allows determining the surface lattice constant at different temperatures by merely applying Bragg's law. We present new data for PdTe 2 which confirm a trend observed for several 2D dichalcogenides, namely, that the in-plane lattice constant remains unchanged (within experimental error) in the temperature range of interest for applications, which enables setting an upper limit for the lateral thermal expansion coefficients of these materials.

Sc 5 Rh 6 Sn 18 with a cage-type quasiskutterudite crystal lattice and type II superconductivity, with superconducting transition temperature T c = 4.99 K, was investigated under hydrostatic high-pressure (HP) using electrical transport, synchrotron x-ray diffraction (XRD) and Raman spectroscopy. Our data show that HP enhance the metallic nature and T c of the system. T c is found to show a continuous increase reaching to 5.24 K at 2.5 GPa. Although the system is metallic in nature, Raman spectroscopy investigations at ambient pressure revealed the presence of three weak modes at 165.97, 219.86 and 230.35 cm -1 , mostly related to the rattling atom Sc. The HP-XRD data revealed that the cage structure was stable without any structural phase transition up to ∼7 GPa. The lattice parameters and volume exhibited a smooth decrease without any anomalies as a function of pressure in this pressure range. In particular, a second order Birch-Murnaghan equation of state can describe the pressure dependence of the unit cell volume well, yielding a bulk modulus of ∼97 GPa. HP Raman investigations revealed a linear shift of all the three Raman modes to higher wavenumbers with increasing pressure up to ∼8 GPa. As the pressure enhances the bond overlap, thus inducing more electronic charges into the system, HP-XRD and Raman results may indicate the possibility of obtaining higher T c with increasing pressures in this pressure range., (© 2022 IOP Publishing Ltd.)

Despite the considerable effort, fast and highly sensitive photodetection is not widely available at the low-photon-energy range (~meV) of the electromagnetic spectrum, owing to the challenging light funneling into small active areas with efficient conversion into an electrical signal. Here, we provide an alternative strategy by efficiently integrating and manipulating at the nanoscale the optoelectronic properties of topological Dirac semimetal PtSe 2 and its van der Waals heterostructures. Explicitly, we realize strong plasmonic antenna coupling to semimetal states near the skin-depth regime (λ/10 4 ), featuring colossal photoresponse by in-plane symmetry breaking. The observed spontaneous and polarization-sensitive photocurrent are correlated to strong coupling with the nonequilibrium states in PtSe 2 Dirac semimetal, yielding efficient light absorption in the photon range below 1.24 meV with responsivity exceeding ∼0.2 A/W and noise-equivalent power (NEP) less than ~38 pW/Hz 0.5 , as well as superb ambient stability. Present results pave the way to efficient engineering of a topological semimetal for high-speed and low-energy photon harvesting in areas such as biomedical imaging, remote sensing or security applications., (© 2022. The Author(s).)

Here, we discuss the key features of electrocatalysis with mitrofanovite (Pt 3 Te 4 ), a recently discovered mineral with superb performances in hydrogen evolution reaction. Mitrofanovite is a layered topological metal with spin-polarized topological surface states with potential applications for spintronics. However, mitrofanovite is also an exceptional platform for electrocatalysis, with costs of the electrodes suppressed by 47% owing to the partial replacement of Pt with Te. Remarkably, the Tafel slope in nanostructured mitrofanovite is just 33 mV/dec, while reduced mitrofanovite has the same Tafel slope (36 mV/dec) as state-of-the-art electrodes of pure Pt. Mitrofanovite also affords surface stability and robustness to CO poisoning. Accordingly, these findings pave the way for the advent of mitrofanovite for large-scale hydrogen production.

Tin diselenide (SnSe 2 ) is a layered semiconductor with broad application capabilities in the fields of energy storage, photocatalysis, and photodetection. Here, we correlate the physicochemical properties of this van der Waals semiconductor to sensing applications for detecting chemical species (chemosensors) and millimeter waves (terahertz photodetectors) by combining experiments of high-resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy with density functional theory. The response of the pristine, defective, and oxidized SnSe 2 surface towards H 2 , H 2 O, H 2 S, NH 3 , and NO 2 analytes was investigated. Furthermore, the effects of the thickness were assessed for monolayer, bilayer, and bulk samples of SnSe 2 . The formation of a sub-nanometric SnO 2 skin over the SnSe 2 surface (self-assembled SnO 2 /SnSe 2 heterostructure) corresponds to a strong adsorption of all analytes. The formation of non-covalent bonds between SnO 2 and analytes corresponds to an increase of the magnitude of the transferred charge. The theoretical model nicely fits experimental data on gas response to analytes, validating the SnO 2 /SnSe 2 heterostructure as a suitable playground for sensing of noxious gases, with sensitivities of 0.43, 2.13, 0.11, 1.06 [ppm] -1 for H 2 , H 2 S, NH 3 , and NO 2, respectively. The corresponding limit of detection is 5 ppm, 10 ppb, 250 ppb, and 400 ppb for H 2 , H 2 S, NH 3 , and NO 2, respectively. Furthermore, SnSe 2 -based sensors are also suitable for fast large-area imaging applications at room temperature for millimeter waves in the THz range.

Due to their peculiar quasiparticle excitations, topological metals have high potential for applications in the fields of spintronics, catalysis, and superconductivity. Here, by combining spin- and angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy, and density functional theory, we discover surface-termination-dependent topological electronic states in the recently discovered mitrofanovite Pt 3 Te 4 . Mitrofanovite crystal is formed by alternating, van der Waals bound layers of Pt 2 Te 2 and PtTe 2 . Our results demonstrate that mitrofanovite is a topological metal with termination-dependent (i) electronic band structure and (ii) spin texture. Despite their distinct electronic character, both surface terminations are characterized by electronic states exhibiting strong spin polarization with a node at the Γ point and sign reversal across the Γ point, indicating their topological nature and the possibility of realizing two distinct electronic configurations (both of them with topological features) on the surface of the same material.

By means of electrocatalytic tests, surface-science techniques and density functional theory, we unveil the physicochemical mechanisms ruling the electrocatalytic activity of recently discovered mitrofanovite (Pt 3 Te 4 ) mineral. Mitrofanovite represents a very promising electrocatalyst candidate for energy-related applications, with a reduction of costs by 47% compared to pure Pt and superior robustness to CO poisoning. We show that Pt 3 Te 4 is a weak topological metal with the Z 2 invariant, exhibiting electrical conductivity (∼4 × 10 6 S/m) comparable with pure Pt. In hydrogen evolution reaction (HER), the electrode based on bulk Pt 3 Te 4 shows a very small overpotential of 46 mV at 10 mA cm -2 and a Tafel slope of 36-49 mV dec -1 associated with the Volmer-Heyrovsky mechanism. The outstanding ambient stability of Pt 3 Te 4 also provides durability of the electrode and long-term stability of its efficient catalytic performances.

Cr 5 Te 8 is a half metal with 2D van der Waals ferromagnetic structure and its magnetic properties can be tuned by changing the proportionality of Cr and Te. We report an investigation of magnetization and magnetic anisotropy near the critical transition region of a Cr 5- y Te 8 single crystal with the static and dynamic probes, to unravel the nature of field-dependent spin-spin interactions. The magnetic transition temperature T C increasing from 255 K (at near zero-field) to 279 K (at 65 kOe along the ab -plane) has been identified. Accordingly, a phase diagram of field versus transition temperature has been established. From the analysis of the field dependence of the critical behavior, we provided evidence that the scenario of 2D Heisenberg-type interactions can be employed to interpret the field-dependent magnetic transitions in Te-rich Cr 4.8 Te 8 . The precise picture for the field-reduced spin-spin interaction range has been obtained. The conclusion drawn from the present study demonstrated that Cr 4.8 Te 8 is a promising candidate for the spintronic applications with a tunable magnetic transition temperature., (© 2021 IOP Publishing Ltd.)

The advent of topological semimetals enables the exploitation of symmetry-protected topological phenomena and quantized transport. Here, we present homogeneous rectifiers, converting high-frequency electromagnetic energy into direct current, based on low-energy Dirac fermions of topological semimetal-NiTe 2 , with state-of-the-art efficiency already in the first implementation. Explicitly, these devices display room-temperature photosensitivity as high as 251 mA W -1 at 0.3 THz in an unbiased mode, with a photocurrent anisotropy ratio of 22, originating from the interplay between the spin-polarized surface and bulk states. Device performances in terms of broadband operation, high dynamic range, as well as their high sensitivity, validate the immense potential and unique advantages associated to the control of nonequilibrium gapless topological states via built-in electric field, electromagnetic polarization and symmetry breaking in topological semimetals. These findings pave the way for the exploitation of topological phase of matter for high-frequency operations in polarization-sensitive sensing, communications and imaging.

Tin diselenide (SnSe 2 ) is a van der Waals semiconductor, which spontaneously forms a subnanometric SnO 2 skin once exposed to air. Here, by means of surface-science spectroscopies and density functional theory, we have investigated the charge redistribution at the SnO 2 -SnSe 2 heterojunction in both oxidative and humid environments. Explicitly, we find that the work function of the pristine SnSe 2 surface increases by 0.23 and 0.40 eV upon exposure to O 2 and air, respectively, with a charge transfer reaching 0.56 e - /SnO 2 between the underlying SnSe 2 and the SnO 2 skin. Remarkably, both pristine SnSe 2 and defective SnSe 2 display chemical inertness toward water, in contrast to other metal chalcogenides. Conversely, the SnO 2 -SnSe 2 interface formed upon surface oxidation is highly reactive toward water, with subsequent implications for SnSe 2 -based devices working in ambient humidity, including chemical sensors. Our findings also imply that recent reports on humidity sensing with SnSe 2 should be reinterpreted, considering the pivotal role of the oxide skin in the interaction with water molecules.

Emergent topological Dirac semimetals afford fresh pathways for optoelectronics, although device implementation has been elusive to date. Specifically, palladium ditelluride (PdTe 2 ) combines the capabilities provided by its peculiar band structure, with topologically protected electronic states, with advantages related to the occurrence of high-mobility charge carriers and ambient stability. Here, we demonstrate large photogalvanic effects with high anisotropy at terahertz frequency in PdTe 2 -based devices. A responsivity of 10 A/W and a noise-equivalent power lower than 2 pW/Hz 0.5 are achieved at room temperature, validating the suitability of PdTe 2 -based devices for applications in photosensing, polarization-sensitive detection, and large-area fast imaging. Our findings open opportunities for exploring uncooled and sensitive photoelectronic devices based on topological semimetals, especially in the highly pursuit terahertz band., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

By means of experiments and theory, the gas-sensing properties of tin diselenide (SnSe 2 ) were elucidated. We discover that, while the stoichiometric single crystal is chemically inert even in air, the nonstoichiometric sample assumes a subnanometric SnO 2 surface oxide layer once exposed to ambient atmosphere. The presence of Se vacancies induces the formation of a metastable SeO 2 -like layer, which is finally transformed into a SnO 2 skin. Remarkably, the self-assembled SnO 2 /SnSe 2-x heterostructure is particularly efficient in gas sensing, whereas the stoichiometric SnSe 2 sample does not show sensing properties. Congruently with the theoretical model, direct sensing tests carried out on SnO 2 /SnSe 2- x at an operational temperature of 150 °C provided sensitivities of (1.06 ± 0.03) and (0.43 ± 0.02) [ppm] -1 for NO 2 and H 2 , respectively, in dry air. The corresponding calculated limits of detection are (0.36 ± 0.01) and (3.6 ± 0.1) ppm for NO 2 and H 2 , respectively. No detectable changes in gas-sensing performances are observed in a time period extended above six months. Our results pave the way for a novel generation of ambient-stable gas sensor based on self-assembled heterostructures formed taking advantage on the natural interaction of substoichiometric van der Waals semiconductors with air.

Recent years have witnessed rapid progresses made in the photoelectric performance of two-dimensional materials represented by graphene, black phosphorus, and transition metal dichalcogenides. Despite significant efforts, a photodetection technique capable for longer wavelength, higher working temperature as well as fast responsivity, is still facing huge challenges due to a lack of best among bandgap, dark current, and absorption ability. Exploring topological materials with nontrivial band transport leads to peculiar properties of quantized phenomena such as chiral anomaly, and magnetic-optical effect, which enables a novel feasibility for an advanced optoelectronic device working at longer wavelength. In this work, the direct generation of photocurrent at low energy terahertz (THz) band at room temperature is implemented in a planar metal-PtTe 2 -metal structure. The results show that the THz photodetector based on PtTe 2 with bow-tie-type planar contacts possesses a high photoresponsivity (1.6 A W -1 without bias voltage) with a response time less than 20 µs, while the PtTe 2 -graphene heterostructure-based detector can reach responsivity above 1.4 kV W -1 and a response time shorter than 9 µs. Remarkably, it is already exploitable for large area imaging applications. These results suggest that topological semimetals such as PtTe 2 can be ideal materials for implementation in a high-performing photodetection system at THz band., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Transition-metal dichalcogenides showing type-II Dirac fermions are emerging as innovative materials for nanoelectronics. However, their excitation spectrum is mostly unexplored yet. By means of high-resolution electron energy loss spectroscopy and density functional theory, here, we identify the collective excitations of type-II Dirac fermions (3D Dirac plasmons) in PtTe&#95;{2} single crystals. The observed plasmon energy in the long-wavelength limit is ∼0.5  eV, which makes PtTe&#95;{2} suitable for near-infrared optoelectronic applications. We also demonstrate that interband transitions between the two Dirac bands in PtTe&#95;{2} give rise to additional excitations at ∼1 and ∼1.4  eV. Our results are crucial to bringing to fruition type-II Dirac semimetals in optoelectronics.

We report the single-crystal synthesis and detailed investigations of the cage-type superconductor Sc 5 Ru 6 Sn 18 , using powder x-ray diffraction (XRD), magnetization, specific-heat and muon-spin relaxation (µSR) measurements. Sc 5 Ru 6 Sn 18 crystallizes in a tetragonal structure (space group I4 1 /acd) with lattice parameters a  =  1.387(3) nm and c  =  2.641(5) nm. Both DC and AC magnetization measurements prove the type-II superconductivity in Sc 5 Ru 6 Sn 18 with T c   ≈  3.5(1) K, a lower critical field [Formula: see text]  =  157(9) Oe and an upper critical field, [Formula: see text]  =  26(1) kOe. The zero-field electronic specific-heat data are well fitted using a single-gap BCS model, with [Formula: see text]  =  0.64(1) meV. The Sommerfeld constant γ varies linearly with the applied magnetic field, indicating s-wave superconductivity in Sc 5 Ru 6 Sn 18 . Specific-heat and transverse-field (TF) µSR measurements reveal that Sc 5 Ru 6 Sn 18 is a superconductor with strong electron-phonon coupling, with TF-µSR also suggesting a single-gap s-wave character of the superconductivity. Furthermore, zero-field µSR measurements do not detect spontaneous magnetic fields below T c , hence implying that time-reversal symmetry is preserved in Sc 5 Ru 6 Sn 18 .

Specific heat, magnetic susceptibility, and neutron scattering have been used to investigate the nature of the spin system in the antiferromagnet Nd 3 Co 4 Sn 13 . At room temperature Nd 3 Co 4 Sn 13 has a cubic, Pm-3n structure similar to Yb 3 Rh 4 Sn 13 . Antiferromagnetic interactions between, Nd 3+ ions dominate the magnetic character of this sample and at 2.4 K the Nd spins enter a long range order state with a magnetic propagation vector q  =  (0 0 0) with an ordered moment of 1.78(2) µ B at 1.5 K. The magnetic Bragg intensity grows very slowly below 1 K, reaching ~2.4 µ B at 350 mK. The average magnetic Nd 3+ configuration corresponds to the 3D irreducible representation Γ 7 . This magnetic structure can be viewed as three sublattices of antiferromagnetic spin chains coupled with each other in the 120°-configuration. A well-defined magnetic excitation was measured around the 1 1 1 zone centre and the resulting dispersion curve is appropriate for an antiferromagnet with a gap of 0.20(1) meV.

X-ray scattering (XRS), x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopic techniques were used to study the electronic and atomic structures of the high-quality Sr 3 Ir 4 Sn 13 (SIS) single crystal below and above the transition temperature (T *  ≈ 147 K). The evolution of a series of modulated satellite peaks below the transition temperature in the XRS experiment indicated the formation of a possible charge density wave (CDW) in the (110) plane. The EXAFS phase derivative analysis supports the CDW-like formation by revealing different bond distances [Sn 1(2) -Sn 2 ] below and above T * in the (110) plane. XANES spectra at the Ir L 3 -edge and Sn K-edge demonstrated an increase (decrease) in the unoccupied (occupied) density of Ir 5d-derived states and a nearly constant density of Sn 5p-derived states at temperatures T < T * in the (110) plane. These observations clearly suggest that the Ir 5d-derived states are closely related to the anomalous resistivity transition. Accordingly, a close relationship exists between local electronic and atomic structures and the CDW-like phase in the SIS single crystal.

Thermoelectric properties of alkaline-earth-metal disilicides are strongly dependent on their electronic band structure in the vicinity of the Fermi level. In particular, the strontium disilicide, SrSi2 with a narrow band gap of about few tens of meV is composed of non-toxic, naturally abundant elements, and its thermoelectric properties are very sensitive to the substitution/alloying with third elements. In this article, we summarize the thermoelectric performance of substituted and Sr-deficient/Sr-rich SrSi2 alloys to realize the high thermoelectric figure-of-merit (ZT) for practical applications in the electronic and thermoelectric aspects, and also to explore the alternative routes to further improve its ZT value.

To investigate the electronic structure of the intermetallic compound CeCoGe2, we performed electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) measurements in a temperature range of 10-300 K. For comparison, the non-magnetic counterpart LaCoGe2 is also studied. It is found that CeCoGe2 exhibits a broad maximum in the S(T) near 75 K, at which the sudden drop in the ρ(T) is observed. Temperature-dependent electrical resistivity and the Seebeck coefficient of CeCoGe2 can be described well by a two-band model, which reveals the signature of Kondo scattering in CeCoGe2. On the other hand, a typical metallic-like behavior is seen in the non-magnetic LaCoGe2 from the ρ(T) and S(T) studies. Analysis of the thermal conductivity indicates that the electronic contribution dominates thermal transport above 100 K in both CeCoGe2 and LaCoGe2. In addition, it is found that the variation in low-temperature lattice thermal conductivity of CeCoGe2 as compared to that of LaCoGe2 is most likely due to the phonon-point-defect scattering.

We report a study of the charge-density-wave (CDW) behavior in LaAgSb(2) by means of electrical resistivity, Seebeck coefficient, thermal conductivity, specific heat, and nuclear magnetic resonance (NMR) measurements. Except for the Seebeck coefficient, apparent indications of CDW ordering at around 207 K were noticed in the physical quantities investigated. On the other hand, all measured physical properties are insensitive to the second CDW formation (∼184 K), as the transition character is considerably weaker than the high-temperature one. Further, analyses of the thermal conductivity and NMR Knight shift data revealed that the observed variations are essentially of electronic origin. The present findings are in good agreement with the previous results, indicating that the high-temperature CDW ordering is associated with a small gapping of the Fermi surface with a minor periodic displacement of the crystal lattice in LaAgSb(2).

We report the results of the electrical resistivity and Seebeck coefficient as well as thermal conductivity measurements on the stoichiometric CaAl(2)Si(2) and non-stoichiometric CaAl(1.75)Si(2.25), CaAl(1.9)Si(2.1), CaAl(2.1)Si(1.9), and CaAl(2.25)Si(1.75) compounds in the temperature range 10-300 K. It has been found that the magnitude of electrical resistivity decreases for the non-stoichiometric samples, attributed to the shift of Fermi energy from the dip of the density of states as a consequence of the changed Si/Al content. In addition, a systematic change in the magnitude of Seebeck coefficient as a function of Al/Si concentration has been observed. The results have been associated with the effect of hole/electron doping on the Fermi level density of states. A detailed analysis of the electrical resistivity and Seebeck coefficient suggests the presence of two types of charge carrier and the temperature dependent changes in their mobility. From the thermal conductivity results, we correlated the extent of disorder and Al/Si ratio with various thermal scattering mechanisms in the investigated temperature range.

The biopharmaceutics classification system (BCS) allows biowaiver for rapid dissolving immediate-release (IR) products of Class I drugs (high solubility and high permeability). The possibility of extending biowaivers to Class III high solubility and low permeability drugs is currently under scrutiny. In vivo bioequivalence data of different formulations of Class III drugs would support such an extension. The objective of this work was to demonstrate the bioequivalence of two marketed IR tablet products of a Class III drug, metformin hydrochloride, that are rapidly dissolving and have similar in vitro dissolution profiles. The effect of race on the systemic exposure of metformin was also explored. A randomized, open-label, two-period crossover study was conducted in 12 healthy Chinese male volunteers. Each subject received a single-dose of 500 mg of each product after an overnight fasting. The plasma concentrations of metformin were followed for 24 h. No significant formulation effect was found for the bioequivalence metrics: areas under concentration-time curve (AUC0-t, AUC0-infinity) and maximal concentration (Cmax). The 90% confidence intervals for the ratio of means were found within the acceptance range of 80-125% for the log-transformed data. Based on these results, it was concluded that the two IR products are bioequivalent. The pharmacokinetic parameters of metformin in Chinese for both products were similar and were in good agreement with those reported for metformin IR tablets in other ethnic populations. This study serves as an example for supporting biowaiver for BCS Class III drugs.

Extracellular phospholipase A2 (PLA2) is a proinflammatory enzyme found especially in the inflammatory exudate to modulate blood flow to areas of antigen stimulation. In this study we found that PLA2 exerted a biphasic effect on the proliferation of phytohemagglutinin (PHA)-stimulated human mononuclear cells (PHA MNC). At low concentrations range from 0.001 to 1 U/ml, PLA2 enhanced the proliferation of PHA MNC (maximal increase was 37.0 +/- 5.67%). Conversely, at concentrations over 10 U/ml, PLA2 markedly suppressed the PHA-induced MNC proliferation (maximal decrease was 88.86 +/- 2.89%). PLA2 was non-toxic to lymphocytes after three days culture, unless the concentration was higher than 100 U/ml. The membrane polarization of PHA-stimulated lymphocytes was also increased by PLA2 at a low concentration. In addition, PLA2 displayed a similar effect on the proliferation of streptokinase-streptodornase (SK/SD) or allogeneic cell stimulated lymphocytes. The change of lymphocyte proliferation by PLA2, was parallel to the change of percentage of helper T cells. Furthermore--a CD4-rich population was proved more susceptible to PLA2 effect than a CD8-rich population. Para-bromophenacyl bromide (pBPB), an irreversible inhibitor of PLA2, abrogated the biphasic effect of PLA2 on PHA MNC proliferation. These results suggest that PLA2 plays a regulatory role on immune reactions by modulating the percentage of helper T cells.