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Layered van der Waals crystals exhibit unique properties making them attractive for applications in nanoelectronics, optoelectronics, and sensing.

The methanol oxidation reaction is particularly efficient with a Ni3Sn4 catalyst, and also has satisfactory durability.

Solution-processed In2Se3 nanosheets exhibit exceptional selectivity and sensitivity to NO2 gas, making them a promising candidate for gas detection systems.

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

Recently, several researchers have claimed the existence of superior catalytic activity associated with topological materials belonging to the class of Dirac/Weyl semimetals, owing to the high electron conductivity and charge carrier mobility in these topological materials. By means of X-ray photoelectron spectroscopy, electrocatalytic tests, and density functional theory, we have investigated the chemical reactivity (chemisorption of ambient gases), ambient stability, and catalytic properties of PdSn4, a topological semimetal showing Dirac node arcs. We find a Tafel slope of 83 mV in the hydrogen evolution reaction (HER) dec-1with an overpotential of 50 mV, with performances resembling those of pure Pd, regardless of its limited amount in the alloy, with a subsequent reduction in the cost of raw materials by ∼80%. Remarkably, the PdSn4-based electrode shows superior robustness to CO compared to pure Pd and Pt and high stability in water media, although the PdSn4surface is prone to oxidation with the formation of a sub-nanometric SnO2skin. Moreover, we also assessed the significance of the role of topological electronic states in the observed catalytic properties. Actually, the peculiar atomic structure of oxidized PdSn4enables the migration of hydrogen atoms through the Sn-O layer with a barrier comparable with the energy cost of the Heyrovsky step of HER over Pt(111) in acidic media (0.1 eV). On the other hand, the topological properties play a minor role, if existing, contrarily to the recent reports overestimating their contribution in catalytic properties. © 2021 The Authors. Published by American Chemical Society D.W.B. acknowledges the support from the Ministry of Science and Higher Education of the Russian Federation (through the basic part of the government mandate, project no. FEUZ-2020-0060 and Jiangsu Innovative and Entrepreneurial Talents Project). A.M., J.F., and F.V. acknowledge the Italian Ministry of University and Research MUR by the PRIN 2017 (no. 2017YH9MRK) and MISE FISR 2019 AMPERE (FISR2019_01294) projects for the financial support.

We have determined the electron–phonon interaction in type II Dirac semimetallic 1T-PdTe2 by means of helium atom scattering. While 1T-PdTe2 is isostructural with 1T-PtTe2, only the former is superconductor. The difference can be traced to the substantially larger value of the electron–phonon coupling in 1T-PdTe2, λ = 0.58, obtained from the Debye-Waller attenuation of the He specular peak. With this value and the surface Debye temperature, ΘD = 106.2 K, we have figured out the superconducting critical temperature, Tc = 1.83 K given by the BCS theory, which is in good agreement with Tc = (1.95 ± 0.03) K obtained with low-temperature scanning tunneling microscopy. The value of the effective mass related to ΘD indicates that the large electron–phonon coupling in 1T-PdTe2 is due to coupling, not only with the zone-center optical mode O2 at 9.2 meV, as proposed in a recent theoretical study, but also with the zone-boundary acoustic mode LA. Our results suggest that the topological states of a Dirac cone play a negligible role on the onset of superconductivity.

By means of surface-science experiments, electrochemical tests and density functional theory, we unveil the mechanisms ruling the catalytic activity of PtSn4. Specifically, through an investigation of the surface chemical reactivity toward CO, H2O, and O2 molecules at room temperature and, moreover, of surface stability in air, we show that the catalytic activity of PtSn4 is determined by the atomic tin layer constituting its surface termination. The PtSn4 surface is not affected by CO poisoning, although it evolves into a tin-oxide skin in the ambient atmosphere. We demonstrate that the hydrogen evolution reaction on PtSn4 can be modelled by the combination of two steps, i.e. Volmer and Tafel reactions. Surprisingly, surface oxidation induces a reduction of the energy barrier for the Tafel reaction, so that oxidized PtSn4 behaves similarly to Pt(111), in spite of the reduced amount of Pt in the alloy and without available over-surface Pt sites. Correspondingly, we observe in electrochemical experiments a Tafel slope of 86 mV dec−1 and an onset potential similar to that of pure Pt. However, PtSn4 contains only 29% wt of Pt, with a reduction of the economic cost by 70%. Our results indicate PtSn4 as a promising novel material for electrocatalytic reactions, whose performance could be further tuned by surface treatments.

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-NiTe2, 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., High-frequency rectifiers at terahertz regime are pivotal components in modern communication, whereas the drawbacks in semiconductor junctions-based devices inhibit their usages. Here, the authors report electromagnetic rectification with high signal-to-noise ratio driven by chiral Bloch-electrons in type-II Dirac semimetal NiTe2-based device allowing for efficient THz detection.

In this study, a Co-thin film was deposited on the van der Waals compound of FePS 3 for the investigation of interfacial magnetic coupling, which is crucial to the application in spintronic devices. As characterized by atomic force microscopy, the exfoliated FePS 3 surface was composed of defects within ± 1 monolayer height. The Co thin film covered the FePS 3 substrate uniformly with a roughness within ± 0.5 nm. The 2 nm-Pd/7 nm-Co/FePS 3 exhibited isotropic magnetism in the surface plane and the magnetic coercivity drastically decreased by more than 50% when the temperature was elevated from 85 K to 110–120 K, which is nearly the Neel temperature of FePS 3 . This observation indicates the interfacial magnetic coupling between Co and FePS 3 . The Co/FePS 3 magnetic coupling is robust even after annealing up to 200 °C. Furthermore, the measurement of X -ray magnetic circular dichroism confirmed the presence of non-compensated Fe moment along the in-plane direction is parallel to the Co magnetization direction. The net Fe-moment is supposed to play an essential role in mediating the magnetic coupling between the in-plane ferromagnetic Co and the perpendicular antiferromagnetic FePS 3 .

Bulk PtSn4 has recently attracted the interest of the scientific community for the presence of electronic states exhibiting Dirac node arcs, enabling possible applications in nanoelectronics. Here, by means of surface-science experiments and density functional theory, we assess its suitability for catalysis by studying the chemical reactivity of the (0 1 0)-oriented PtSn4 surface toward CO, H2O, O2 molecules at room temperature and, moreover, its stability in air. We demonstrate that the catalytic activity of PtSn4 is determined by the composition of the outermost atomic layer. Specifically, we find that the surface termination for PtSn4 crystals cleaved in vacuum is an atomic Sn layer, which is totally free from any CO poisoning. In oxygen-rich environment, as well as in ambient atmosphere, the surface termination is a SnOx skin including SnO and SnO2 in comparable amount. However, valence-band states, including those forming Dirac node arcs, are only slightly affected by surface modifications. The astonishingly beneficial influence of surface oxidation on catalytic activity has been demonstrated by electrocatalytic tests evidencing a reduction of the Tafel slope, from 442 down to 86 mV dec−1, whose origin has been explained by our theoretical model. The use of surface-science tools to tune the chemical reactivity of PtSn4 opens the way toward its effective use in catalysis, especially for hydrogen evolution reaction and oxygen evolution reaction.

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-PtTe2 -metal structure. The results show that the THz photodetector based on PtTe2 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 PtTe2 -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 PtTe2 can be ideal materials for implementation in a high-performing photodetection system at THz band.

We report the results of a 51 V nuclear magnetic resonance (NMR) study on the single crystal BaCo 2 V 2 O 8 at temperatures between 3 and 300 K. The NMR features for fields both parallel and perpendicular to the c -axis have been identified. We found no substantial changes in the NMR shift and line width at low temperatures as a constant field of 7.06 T was applied along the c -axis. The NMR observations thus indicate the absence of magnetic long-range ordering under this field, consistent with the specific heat result which showed the disappearance of the magnetic transition beyond an external field of 4 T in BaCo 2 V 2 O 8 .