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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.

Electronic band structure and spin texture of NiTe2: We predict NiTe2 to be a type-II Dirac semimetal based on ab initio calculations and explore its bulk and spin-polarized surface states using spin- and angle-resolved photoemission spectroscopy (spin-ARPES). Our results show that, unlike PtTe2, PtSe2, and PdTe2, the Dirac node in NiTe2 is located in close vicinity to the Fermi energy. Additionally, NiTe2 also hosts a pair of band inversions below the Fermi level along the -A high-symmetry direction, with one of them leading to a Dirac cone in the surface states. The bulk Dirac nodes and the ladder of band inversions in NiTe2 support unique topological surface states with chiral spin texture over a wide range of energies. Our work paves the way for the exploitation of the low-energy type-II Dirac fermions in NiTe2 in the fields of spintronics, infrared plasmonics, and ultrafast optoelectronics.

Electronic band structure and spin texture of NiTe2:We predict NiTe2 to be a type-II Dirac semimetal based on ab initio calculations and explore its bulk and spin-polarized surface states using spin- and angle-resolved photoemission spectroscopy (spin-ARPES). Our results show that, unlike PtTe2, PtSe2, and PdTe2, the Dirac node in NiTe2 is located in close vicinity to the Fermi energy. Additionally, NiTe2 also hosts a pair of band inversions below the Fermi level along the -A high-symmetry direction, with one of them leading to a Dirac cone in the surface states. The bulk Dirac nodes and the ladder of band inversions in NiTe2 support unique topological surface states with chiral spin texture over a wide range of energies. Our work paves the way for the exploitation of the low-energy type-II Dirac fermions in NiTe2 in the fields of spintronics, infrared plasmonics, and ultrafast optoelectronics.

Charge density waves (CDWs) involved with both electronic and phononic subsystems simultaneously are a common quantum state in solid-state physics, especially in low-dimensional materials. However, there is no complete CDW phase diagram in various dimensions and the phase transition mechanism is currently moot. Using the distinct temperature evolution of orientation-dependent ultrafast electron and phonon dynamics, variously dimensional CDW phases are verified in CuTe and the electronic subsystem in CuTe is also demonstrated to drive the formation of one-dimensional (1D) CDW chain phase at Tc of 335 K. At T=280 K, electron-phonon coupling creates collective modes along the a-axis, which synchronize via an interchain interaction to establish a two-dimensional (2D) CDW phase on the ab-plane while T

We present CoTe2 as a new type-II Dirac semimetal supporting Lorentz symmetry violating Dirac fermions in the vicinity of the Fermi energy. By combining first principle ab-initio calculations with experimental angle-resolved photo-emission spectroscopy results, we show the CoTe2 hosts a pair of type-II Dirac fermions around 90 meV above the Fermi energy. In addition to the bulk Dirac fermions, we find several topological band inversions in bulk CoTe2, which gives rise to a ladder of spin-polarized surface states over a wide range of energies. In contrast to the surface states which typically display Rashba-type in-plane spin splitting, we find that CoTe2 hosts novel out-of-plane spin polarization as well. Our work establishes CoTe2 as a potential candidate for the exploration of Dirac fermiology and applications in spintronic devices, infrared plasmonics, and ultrafast optoelectronics., Comment: 9 pages,7 figures, Accepted in Phys. Rev. B

Exotic magnetotransport and magnetization properties of Pt3Te4 single crystals were investigated to probe the topological properties of the Pt3Te4 semimetal. Pt3Te4 reveals a strongly correlated electronic feature as well as a non-trivial magnetoresistance (MR) characteristic. The signature of helical spin texture from the topological surface state and the chiral anomaly associated with a linear-like energy dispersion of electronic states were detected. At low temperatures, the negative longitudinal magnetoresistance in the low-field region could be explained with the transport formula containing the chiral-anomaly effect as well as the weak antilocalization transport. Moreover, the high-field transverse magnetoresistance at temperatures below 60 K showed a non-saturating linear-like behavior, which was examined with the theory of Abrikosov’s quantum MR, indicating a Dirac-cone-like dispersion in Pt3Te4 at low temperatures. This work reveals the drastic impact of the concept that the magnetotransport in Pt3Te4 can be dominated by the surface electrons in a Dirac fermion state, thus inferring that Pt3Te4 is a new candidate of Dirac semimetal.

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

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

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.

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

Topological materials such as Dirac or Weyl semimetals are new states of matter characterized by symmetry-protected surface states responsible for exotic low-temperature magnetotransport properties. Here, transport measurements on AuSn4 single crystals, a topological nodal-line semimetal candidate, reveal the presence of two-dimensional superconductivity with a transition temperature Tc ~ 2.40 K. The two-dimensional nature of superconductivity is verified by a Berezinsky–Kosterlitz–Thouless transition, Bose-metal phase, and vortex dynamics interpreted in terms of thermally-assisted flux motion in two dimensions. The normal-state magnetoconductivity at low temperatures is found to be well described by the weak-antilocalization transport formula, which has been commonly observed in topological materials, strongly supporting the scenario that normal-state magnetotransport in AuSn4 is dominated by the surface electrons of topological Dirac-cone states. The entire results are summarized in a phase diagram in the temperature–magnetic field plane, which displays different regimes of transport. The combination of two-dimensional superconductivity and surface-driven magnetotransport suggests the topological nature of superconductivity in AuSn4. Surface states of topological semimetals may give rise to unusual transport properties and topological superconductivity. Here, the H-T phase diagram of AuSn4 is experimentally established, displaying 2D superconductivity, Bose metal behavior, and normal-state magnetotransport driven by surface states.

Tin diselenide (SnSe2) 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 SnSe2 surface towards H2, H2O, H2S, NH3, and NO2 analytes was investigated. Furthermore, the effects of the thickness were assessed for monolayer, bilayer, and bulk samples of SnSe2. The formation of a sub-nanometric SnO2 skin over the SnSe2 surface (self-assembled SnO2/SnSe2 heterostructure) corresponds to a strong adsorption of all analytes. The formation of non-covalent bonds between SnO2 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 SnO2/SnSe2 heterostructure as a suitable playground for sensing of noxious gases, with sensitivities of 0.43, 2.13, 0.11, 1.06 [ppm]−1 for H2, H2S, NH3, and NO2, respectively. The corresponding limit of detection is 5 ppm, 10 ppb, 250 ppb, and 400 ppb for H2, H2S, NH3, and NO2, respectively. Furthermore, SnSe2-based sensors are also suitable for fast large-area imaging applications at room temperature for millimeter waves in the THz range.

Sc

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 PtSe2 and its van der Waals heterostructures. Explicitly, we realize strong plasmonic antenna coupling to semimetal states near the skin-depth regime (λ/104), 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 PtSe2 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/Hz0.5, 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.

Here, we discuss the key features of electrocatalysis with mitrofanovite (Pt

Tin diselenide (SnSe

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 Pt3Te4. Mitrofanovite crystal is formed by alternating, van der Waals bound layers of Pt2Te2 and PtTe2. 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 (Pt3Te4) 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 Pt3Te4 is a weak topological metal with the invariant, exhibiting electrical conductivity (∼4 × 106 S/m) comparable with pure Pt. In hydrogen evolution reaction (HER), the electrode based on bulk Pt3Te4 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 Pt3Te4 also provides durability of the electrode and long-term stability of its efficient catalytic performances.

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.

Tin sulfide (SnS) is one of the promising materials for the applications of optoelectronics and photovoltaics. This study determines the nematic dynamics of photoexcited electrons and phonons in SnS single crystals using polarization-dependent pump–probe spectroscopy at various temperatures. As well as the fast (0.21–1.38 ps) and slow (>5 ps) relaxation processes, a 36–41 GHz coherent acoustic phonon with a sound velocity of 4883 m/s that is generated by the thermoelastic effect is also observed in the transient reflectivity change (Δ R/ R) spectra. Electrons and coherent acoustic phonons show significant in-plane anisotropy from 330 to 430 K due to strong electron–phonon coupling. However, this in-plane anisotropy weakens dramatically in the low-temperature (430 K) phases. These results add to the knowledge about the anisotropy of electrons and coherent acoustic phonons that give SnS applications in photovoltaic or optoelectronic devices.

Dirac semimetals (DSM) host linear bulk bands and topologically protected surface states, giving rise to exotic and robust properties. Platinum ditelluride (PtTe2) belongs to this interesting group of topological materials. Here, we employ scanning tunneling microscopy (STM) in combination with first-principles calculations to visualize and identify the native defects at the surface of a freshly cleaved PtTe2 crystal. Around these defects, short-wavelength electron density oscillations are observed. Fourier transform analysis of the energy-dependent quasiparticle interference patterns is in good agreement with our calculated joint density of states, demonstrating the singular properties of the surface of this type-II DSM. Our results evidence the power of STM in understanding the surface of topological materials.

Hydrostatic pressure response of $\mathrm{LaAu}{\mathrm{Sb}}_{2}$ charge density wave (CDW) system is investigated using electrical transport, x-ray diffraction (XRD), and density-functional theory (DFT). Resistivity data at ambient pressure evidence a clear CDW transition at 83 K. X-ray diffraction at ambient pressure reveals that in plane and out of plane axes show opposite behavior with decreasing temperature, in particular, out of plane $c$ axis develops a distinct change at \ensuremath{\sim}250 K, much above the observed CDW transition at 83 K. The CDW transition shifts to low temperature with increasing pressure. Our resistivity data indicate a complete suppression of the CDW transition at \ensuremath{\sim}3.6 GPa. High-pressure XRD revealed a change from the linear trend for the out of plane ($c$) and the in plane ($a$) lattice parameters for pressure above 3.8 GPa. With compression, DFT indicated an anomaly in the c/a ratio around 8 GPa. The calculated electronic structure also indicated minor changes in the band structure in this pressure range. In addition, high-pressure DFT structural investigations reveal the $\mathrm{LaAu}{\mathrm{Sb}}_{2}$ system to be stable up to pressures as high as 150 GPa.

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.

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 (Pt3Te4) 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 Pt3Te4 is a weak topological metal with the Z2 invariant, exhibiting electrical conductivity (∼4 × 106 S/m) comparable with pure Pt. In hydrogen evolution reaction (HER), the electrode based on bulk Pt3Te4 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 Pt3Te4 also provides durability of the electrode and long-term stability of its efficient catalytic performances. © 2021 American Chemical Society. Deutsche Forschungsgemeinschaft, DFG: SFB 1083; National Natural Science Foundation of China, NSFC: 21775078, 22075159, tsqn202103058; Ministry of Education and Science of the Russian Federation, Minobrnauka: FEUZ-2020-0060 L.Z. acknowledges funding by the National Natural Science Foundation of China (Nos. 21775078 and 22075159) and Taishan Scholar Program (No. tsqn202103058). F.C.B. acknowledges funding by the DFG through the SFB 1083 Structure and Dynamics of Internal Interfaces (Project A 12). D.W.B. acknowledged support 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. I.V. and J.F. thank NFFA-Trieste.

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 Pt3Te4. Mitrofanovite crystal is formed by alternating, van der Waals bound layers of Pt2Te2 and PtTe2. 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.

PdTe2-based photodetectors reveal anisotropic and highly efficient intraband photocurrents at long wavelength., Emergent topological Dirac semimetals afford fresh pathways for optoelectronics, although device implementation has been elusive to date. Specifically, palladium ditelluride (PdTe2) 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 PdTe2-based devices. A responsivity of 10 A/W and a noise-equivalent power lower than 2 pW/Hz0.5 are achieved at room temperature, validating the suitability of PdTe2-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.

By means of experiments and theory, the gas-sensing properties of tin diselenide (SnSe

Sc5Rh6Sn18 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.

Here, we discuss the key features of electrocatalysis with mitrofanovite (Pt3 Te4 ), 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. © 2022 by the authors. Licensee MDPI, Basel, Switzerland. Funding: D.W.B. acknowledges 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 the Jiangsu Innovative and Entrepreneurial Talents Project. This work has been partially supported by the Spanish Ministerio de Ciencia e Innovación under Project PID2019–109525RB-I00. D.F. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the “Maríade Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M). I.V., J. F., and P.T. thank NFFA-Trieste.

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 .

The emergence of Dirac semimetals has stimulated growing attention, owing to the considerable technological potential arising from their peculiar exotic quantum transport related to their nontrivial topological states. Especially, materials showing type-II Dirac fermions afford novel device functionalities enabled by anisotropic optical and magnetotransport properties. Nevertheless, real technological implementation has remained elusive so far. Definitely, in most Dirac semimetals, the Dirac point lies deep below the Fermi level, limiting technological exploitation. Here, it is shown that kitkaite (NiTeSe) represents an ideal platform for type-II Dirac fermiology based on spin-resolved angle-resolved photoemission spectroscopy and density functional theory. Precisely, the existence of type-II bulk Dirac fermions is discovered in NiTeSe around the Fermi level and the presence of topological surface states with strong (≈50%) spin polarization. By means of surface-science experiments in near-ambient pressure conditions, chemical inertness towards ambient gases (oxygen and water) is also demonstrated. Correspondingly, NiTeSe-based devices without encapsulation afford long-term efficiency, as demonstrated by the direct implementation of a NiTeSe-based microwave receiver with a room-temperature photocurrent of 2.8 µA at 28 GHz and more than two orders of magnitude linear dynamic range. The findings are essential to bringing to fruition type-II Dirac fermions in photonics, spintronics, and optoelectronics. © 2021 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH. I.V., A.B.S., and L.Z. contributed equally to this work. L.W. acknowledged support from the State Key Program for Basic Research of China (No. 2017YFA0305500, 2018YFA0306204), Shanghai Municipal Science and Technology Major Project (Grant No. 2019SHZDZX01), and the Science and Technology Commission of Shanghai Municipality (21ZR1473800). A.P. thanks CERIC‐ERIC for the access to the NAP‐XPS facility. D.W.B. acknowledged 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. This work has been partly performed in the framework of the nanoscience foundry and fine analysis (NFFA‐MUR Italy Progetti Internazionali) facility. A.B, B.G, and A.A. acknowledge funding from Science and Engineering Research Board (SERB) and Department of Science and Technology (DST), government of India. A.A. thanks the HPC facility at IIT Kanpur for computational resources.

By means of experiments and theory, the gas-sensing properties of tin diselenide (SnSe2) were elucidated. We discover that, while the stoichiometric single crystal is chemically inert even in air, the nonstoichiometric sample assumes a subnanometric SnO2 surface oxide layer once exposed to ambient atmosphere. The presence of Se vacancies induces the formation of a metastable SeO2-like layer, which is finally transformed into a SnO2 skin. Remarkably, the self-assembled SnO2/SnSe2-x heterostructure is particularly efficient in gas sensing, whereas the stoichiometric SnSe2 sample does not show sensing properties. Congruently with the theoretical model, direct sensing tests carried out on SnO2/SnSe2-x at an operational temperature of 150 °C provided sensitivities of (1.06 ± 0.03) and (0.43 ± 0.02) [ppm]-1 for NO2 and H2, respectively, in dry air. The corresponding calculated limits of detection are (0.36 ± 0.01) and (3.6 ± 0.1) ppm for NO2 and H2, 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.

Tin diselenide (SnSe2) is a van der Waals semiconductor, which spontaneously forms a subnanometric SnO2 skin once exposed to air. Here, by means of surface-science spectroscopies and density functional theory, we have investigated the charge redistribution at the SnO2-SnSe2 heterojunction in both oxidative and humid environments. Explicitly, we find that the work function of the pristine SnSe2 surface increases by 0.23 and 0.40 eV upon exposure to O2 and air, respectively, with a charge transfer reaching 0.56 e-/SnO2 between the underlying SnSe2 and the SnO2 skin. Remarkably, both pristine SnSe2 and defective SnSe2 display chemical inertness toward water, in contrast to other metal chalcogenides. Conversely, the SnO2-SnSe2 interface formed upon surface oxidation is highly reactive toward water, with subsequent implications for SnSe2-based devices working in ambient humidity, including chemical sensors. Our findings also imply that recent reports on humidity sensing with SnSe2 should be reinterpreted, considering the pivotal role of the oxide skin in the interaction with water molecules. © PID2019-109525RB-I00; Horizon 2020 Framework Programme, H2020: 730872; Ministerio de Economía y Competitividad, MINECO: CEX2018-000805-M, E12H1800010001; Ministero dell’Istruzione, dell’Università e della Ricerca, MIUR; Ministry of Education and Science of the Russian Federation, Minobrnauka: FEUZ-2020-0060 This work has been partially supported by the Spanish Ministerio de Ciencia e Innovación under Project PID2019-109525RB-I00. D.F. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M). D.F. and A.A.T. acknowledge the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. A.P. and G.D. acknowledge the CERIC–ERIC Consortium for the access to the Nanospectroscopy facility and financial support. G.D. acknowledges funding of a Ph.D. fellowship from PON Ricerca e Innovazione 2014–2020 (Project E12H1800010001) by the Italian Ministry of University and Research (MIUR). D.W.B. acknowledges the support by the Ministry of Science and Higher Education of the Russian Federation (through the basic part of the government mandate, Project No. FEUZ-2020-0060).