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The vibrational and electronic properties of the Pb-doped dichalcogenide PbxTaSe2 (x = 0, 0.25, 0.33, 0.5, 0.75, and 1) have been investigated using Raman scattering experiments. A marked variation of the main vibrational modes with Pb concentration x is observed. The concentration dependence of the vibrational modes resembles the dependence of the vibrational modes in TaSe2 on the number of crystallographic layers along the c-axis direction. The temperature and polarization dependences of Raman spectra of PbxTaSe2 revealed additional broad modes in the low-frequency regime, which are discussed in the context of the remnant charge density wave, induced disorder, or PbSe phase formed in the interface of Pb and TaSe2 layers.

Chemical vapor transport synthesis of SnTiS3 yields a self-assembled heterostructure of two distinct constituent materials, the semiconductor SnS and the semimetal TiS2. The misfit layer compound, although thermodynamically stable, is structurally complex, and precise understanding of the structure is necessary for designing nanoengineered heterojunction compound devices or for theoretical studies. In our work, we reveal the unique complexity of the quasi-periodic structure of this heterostructure by systematically investigating the misfit compound using a set of advanced electron microscopy techniques. X-ray and electron diffraction patterns along with high-resolution scanning/transmission electron microscopy images obtained from different crystallographic orientations resolve the complexity of the sublattice component layer structure and reveal the uniquely bonded alignment among interlayers and a quasi-periodic arrangement of the sublayers. Density functional theory calculations embedded with the extracted structural information provide quantitative insights into the formation of self-assembled heterojunction structures where the nonpolar van der Waals interaction is found to play a dominant role in the structural alignment over the polar interlayer interaction.

We have synthesized and studied the transport properties of FeTexSe1-x (x = 0.4, 0.46, 0.5, 0.54, and 0.58) single crystals grown by slow cooling method and found a sharp increase of Tc up to x = 0.46 followed by a moderate decrease of Tc up to x = 0.58. We present the temperature dependence of electrical resistivity under various pressures up to ~ 2.5 GPa, and the pressure dependence of Tc shows positive pressure coefficients of ~ 2.7 K/GPa and ~ 3.33 K/GPa for x = 0.46 and x = 0.54, respectively, up to 2 GPa followed by a decreasing trend with marginal variation of Tc with negative pressure coefficients of − 0.602 K GPa−1 (x = 0.46) and − 0.7 K GPa−1 (x = 0.54) at maximum pressure of 2.5 GPa. Furthermore, both normal state resistivity and activation energy decrease sharply, and it suggests that the reduction of net electron-phonon scattering and densities of states (DOS) at the Fermi surface with application of pressure leads to enhancement of metallic behavior in FeSe systems.

Transition metal-nitrogen-carbon catalysts obtained from pyrolysis of simple sources of metal and nitrogen or metal-organic frameworks are envisioned as promising replacement of Pt-based catalysts for oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. However, the lack of clarity on the active site structure is a fundamental problem in developing efficient catalysts for the complete reduction of oxygen with maximum active site density. In this study, we have synthesized a simple metal-organic complex, [ Co ( bpy ) 2 CO 3 ] NO 3 ⋅ 5 H 2 O , 1 (bpy = 2, 2′-bipyridine) and studied its ORR activity in 0.1 M KOH supporting on Ketjenblack EC-600JD, 1/C. The crystal structure of 1 was unambiguously determined using single-crystal X-ray diffraction. The onset potential of ORR, Tafel slopes, kinetic current density, the fraction of oxygens reduced to peroxyl ion ( χ HO 2 − ) and the electron transfer number (n) were compared with relevant literature and commercial 20 wt % Pt/C. Primarily, 1/C reduces O2 through the direct 4-electron pathway. ‘n’ and χ HO 2 − on 1/C were found to be 3.87 and 7% at 0.7 ViR-free vs. RHE, respectively. The turn-over frequencies for the 4- and 2-electron process were found to be 0.124 and 0.001 electrons [Co]−1 s−1 at 0.7 ViR-free vs. RHE, respectively. Density-functional theory analysis reveals that the preferential activation of the side-on mode of adsorption on 1 is responsible for high selectivity in the direct 4-electron pathway.

The crystal structure and magnetic, electronic, and thermal properties of a NdSbTe single crystal were examined by X-ray diffraction, magnetic and specific heat Cp(T) measurements, and density func...

In addition to the Ge-vacancy control of GeTe, the antimony (Sb) substitution of GeTe for the improvement of thermoelectric performance is explored for Ge1−xSbxTe with x = 0.08–0.12. The concomitant carrier concentration (n) and the aliovalent Sb ion substitution led to an optimal doping level of x = 0.10 to show ZT ∼ 2.35 near ∼800 K, which is significantly higher than those single- and multi-element substitution studies of the GeTe system reported in the literature. In addition, Ge0.9Sb0.1Te demonstrates an impressively high power factor of ∼36 μW cm−1 K−2 and a low thermal conductivity of ∼1.1 W m−1 K−1 at 800 K. The enhanced ZT level for Ge0.9Sb0.1Te is explained through a systematic investigation of micro-structural change and strain analysis from room temperature to 800 K. A significant reduction of lattice thermal conductivity (κlat) is identified and explained by the Sb substitution-introduced strained and widened domain boundaries for the herringbone domain structure of Ge0.9Sb0.1Te. The Sb substitution created multiple forms of strain near the defect centre, the herringbone domain structure, and widened tensile/compressive domain boundaries to support phonon scattering that covers a wide frequency range of the phonon spectrum to reduce lattice thermal conductivity effectively.

We report the temperature and composition dependence of static magnetic susceptibility and Raman spectroscopic measurements on van der Waals antiferromagnets ${\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{PS}}_{3}$. The end members ${\mathrm{NiPS}}_{3}$ and ${\mathrm{FePS}}_{3}$ feature $XY$- and Ising-like magnetism, respectively, enabling chemical tuning of magnetic anisotropy and spin correlations. ${\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{PS}}_{3}$ shows a turnover from the $XY$ to Ising anisotropy through $x\ensuremath{\approx}0.1$. Although the $XY$ anisotropy is rapidly suppressed on introducing Fe content, two-magnon scattering evidences the slow repression of short-range magnetic correlations deep inside the Fe-rich side. Counterintuitively, the two-magnon signal undergoes less renormalization of its energy with increasing $x$ despite the larger spin number and enhanced classical magnetism. The disparate static and dynamic magnetic behaviors indicate the emergence of an exotic spin state in alloy van der Waals magnets.

In two-dimensional (2D)-semiconductor-based field-effect transistors and optoelectronic devices, metal–semiconductor junctions are one of the crucial factors determining device performance. The Fermi-level (FL) pinning effect, which commonly caused by interfacial gap states, severely limits the tunability of junction characteristics, including barrier height and contact resistance. A tunneling contact scheme has been suggested to address the FL pinning issue in metal–2D-semiconductor junctions, whereas the experimental realization is still elusive. Here, we show that an oxidized-monolayer-enabled tunneling barrier can realize a pronounced FL depinning in indium selenide (InSe) transistors, exhibiting a large pinning factor of 0.5 and a highly modulated Schottky barrier height. The FL depinning can be attributed to the suppression of metal- and disorder-induced gap states as a result of the high-quality tunneling contacts. Structural characterizations indicate uniform and atomically thin-surface oxidation layer inherent from nature of van der Waals materials and atomically sharp oxide–2D-semiconductor interfaces. Moreover, by effectively lowering the Schottky barrier height, we achieve an electron mobility of 2160 cm2/Vs and a contact barrier of 65 meV in two-terminal InSe transistors. The realization of strong FL depinning in high-mobility InSe transistors with the oxidized-monolayer presents a viable strategy to exploit layered semiconductors in contact engineering for advanced electronics and optoelectronics.

We report crystal growth, AC and DC magnetic susceptibilities [χ(T, H)], magnetization [M(T, H)], and heat capacity [CP(T, H)] measurement results of GdSbTe single crystal. GdSbTe is isostructural to the confirmed nonmagnetic nodal-line semimetal ZrSiS of noncentrosymmetric tetragonal crystal structure in space group P4/nmm (No. 129), but it shows additional long-range antiferromagnetic spin ordering for the Gd spins of S = 7/2 below TN. Both χ(T, H) and CP(T, H) measurements confirm the existence of a long-range antiferromagnetic (AFM) spin ordering of Gd spins below TN ∼ 12 K, and an additional spin reorientation/recovery (sr) behavior is identified from the change of on-site spin anisotropy between Tsr1 ∼ 7 and Tsr2 ∼ 4 K. The anisotropic magnetic susceptibilities of χ(T, H) below TN clearly demonstrate that the AFM long-range spin ordering of GdSbTe has an easy axis parallel to the ab-plane direction. The field- and orientation-dependent magnetization of M(T, H) at 2 K shows two plateaus to indicate the spin-flop transition for H||ab near ∼2.1 T and a metamagnetic state near ∼5.9 T having ∼1/3 of the fully polarized magnetization by the applied field. The heat capacity measurement results yield Sommerfeld coefficient of γ ∼ 7.6(4) mJ/mol K2 and θD ∼ 195(2) K being less than half of that for the nonmagnetic ZrSiS. A three-dimensional (3D) AFM spin structure is supported by the ab initio calculations for Gd having magnetic moment of 7.1 μB and the calculated AFM band structure indicates that GdSbTe is a semimetal with bare density of states (0.36 states/eV fu) at the Fermi level, which is 10 times smaller than the measured one to suggest strong spin-fluctuation.

We report a simple preparation of novel nanocomposite for the sensitive determination of riboflavin (RF) in real samples. Nitrogen-doped carbon quantum dots/SnO2 (N-CQD/SnO2) nanocomposite was prepared using sonochemical approach and applied as a voltammetric sensor for the sensitive determination of RF for the first time. The crystallographic phase, functional groups, surface analysis, and elemental distribution were examined using XRD, FT-IR, HR-TEM and EDS spectroscopic techniques respectively. This N-CQD/SnO2 nanocomposite-modified electrode shows a fast and sensitive electrochemical response to RF sensing with a good sensitivity (2.496 μA μM−1 cm-2), wide linear range (0.05–306 μM), low detection limit (8 nM) and excellent anti-interference ability. Furthermore, the developed sensor was investigated in commercial riboflavin tablets and milk powder and obtained results are quite satisfactory.

Highly stable ultrathin films of large unsaturated room-temperature magnetoresistance (MR) are essential for the next-generation real-time magnetoelectric devices. A large-area, transfer-free, highly crystalline, and phase-engineered ultrathin film of Td-Mo0.27W0.71Te2.02 or 2H- & Td-Mo0.22W0.89Te1.89 on a hexagonal boron nitride (h-BN) substrate was synthesized using an atmospheric-pressure chemical vapor deposition (APCVD) method. The Td-Mo0.27W0.71Te2.02 with average mobility of 725 cm2 V−1 s−1 possesses non-saturating MR of 18% at 5 K and 11% at room temperature. Quantum correction to the magnetotransport study suggests the existence of a weak anti-localization effect dominated by the electron–electron interaction to render the non-saturating linear MR in a wide temperature range. Moreover, the spin–orbit interaction in Td-Mo0.27W0.71Te2.02 was found valid till an applied field of 0.05 T with an interaction length of 18 nm at 300 K. In this alloy system, the weak localization effect was evidenced unprecedentedly by the Te-deficient 2H- & Td-Mo0.22W0.89Te1.89 thin film with unusual co-existence of two crystal phases, which exhibit a suppressed MR caused by the recurring inelastic scattering with a reduced phase coherence length. This work explores the production of phase-engineered large-area Weyl semi-metallic 2D materials for the realization of magnetoelectrics in the near future.

The detection and control of free radical antioxidant diphenylamine cations (DPAH˙+), a typical organic industrial waste water byproduct, is important for environmental safety and protection. Diphenylamine (DPA) causes severe blood cell damage when it is in contact with living organisms. In this study, we have developed a highly sensitive electrochemical sensor with carbon interlaced nanoflake-assembled MgxNi9−xS8 microspheres for the detection of diphenylamine. This hybrid electrocatalyst MgxNi9−xS8@C material was prepared using a solvothermal co-precipitation method. The microstructural features and elemental composition of the carbon supported MgxNi9−xS8 microspheres were examined by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectral mapping analysis, and X-ray photoelectron spectroscopy (XPS). The electrochemical performance of the carbon supported MgxNi9−xS8 microspheres was investigated by cyclic voltammetry (CV) and difference pulse voltammetry (DPV). The carbon supported MgxNi9−xS8 microspheres exhibit good selectivity, a low detection limit of 16 nM, and wide linear range from 0.1 to 76 μM. This study offers the best platform upon which to explore metal sulfide–carbon matrix microstructures in industrial waste-water treatment as a potential sensing protocol for the detection of DPAH˙+.

We investigate the thermal evolution of magnetic correlations of the Co-based honeycomb lattice ${\mathrm{Na}}_{2}{\mathrm{Co}}_{2}{\mathrm{TeO}}_{6}$ with $^{23}\mathrm{Na}$ nuclear magnetic resonance and static magnetic susceptibility $\ensuremath{\chi}(T)$. The studied compound shows three-dimensional (3D) long-range magnetic ordering at ${T}_{\mathrm{N}}=26$ K. On cooling through ${T}^{*}\ensuremath{\approx}110$ K, a simple paramagnetic state undergoes a crossover to a correlated paramagnetic state featuring a power-law dependence of the nuclear spin-lattice $(1/{T}_{1})$ and spin-spin $(1/{T}_{2})$ relaxation rates as well as of the out-of-plane $\ensuremath{\chi}(T)$. The magnetic-field-direction dependence of $1/{T}_{1}$, $1/{T}_{2}$, and $\ensuremath{\chi}(T)$ uncovers anisotropic spin-spin correlations of a two-dimensional (2D) renormalized classical character. In a magnetically ordered state, we are able to identify four successive transitions or crossovers occurring at ${T}_{\mathrm{N}}=26$, ${T}_{\mathrm{N}1}=16$, ${T}_{\mathrm{N}2}=7$, and ${T}_{\mathrm{N}3}=3.5$ K. The multiple transitions and crossovers are associated with the coexistence of 2D and 3D magnetic orders or reorientation of the ordered spins. Our results suggest the presence of various types of frustrating interactions and their energy hierarchy that control complex magnetic structures and anisotropic magnetism.

The anisotropic optical response of the layered, nodal-line semimetal ZrSiS at ambient and high pressure is investigated by frequency-dependent reflectivity measurements for the polarization along and perpendicular to the layers. The highly anisotropic optical conductivity is in very good agreement with results from density functional theory calculations and confirms the anisotropic character of ZrSiS. Whereas the in-plane optical conductivity shows only modest pressure-induced changes, we found strong effects on the out-of-plane optical conductivity spectrum of ZrSiS, with the appearance of two prominent excitations. These pronounced pressure-induced effects can neither be attributed to a structural phase transition according to our single-crystal x-ray diffraction measurements, nor can they be explained by electronic correlation and electron-hole pairing effects, as revealed by theoretical calculations. Our findings are discussed in the context of the recently proposed excitonic insulator phase in ZrSiS., 6 pages, 3 figures, accepted for publication in Phys. Rev. Lett

Electron-electron interactions (EEIs) in 2D van der Waals (vdW) nanostructures is a topic of high current interest, with implications in both fundamental physics and nanoelectronics. In this Rapid Communication, we report the observation of a negative parabolic magnetoresistance (MR) in the multilayer 2D semiconductor InSe beyond the low-field weak localization/antilocalization regime, and provide evidence for the EEI origin of this MR behavior. Further, we analyze this negative parabolic MR and other observed quantum transport signatures of EEIs (temperature-dependent conductance and Hall coefficient) within the framework of Fermi-liquid theory and extract the gate voltage tunable Fermi-liquid parameter ${F}_{0}^{\ensuremath{\sigma}}$ which quantifies the electron spin-exchange interaction strength. This work opens up different directions for investigations of EEI effects in the electron transport of 2D vdW nanostructures.

Two-dimensional metal dichalcogenide/monochalcogenide thin flakes have attracted much attention owing to their remarkable electronic and electrochemical properties; however, chemical instability limits their applications. Chemical vapor transport (CVT)-synthesized SnTiS3 thin flakes exhibit misfit heterojunction structure and are highly stable in ambient conditions, offering a great opportunity to exploit the properties of two distinct constituent materials: semiconductor SnS and semi-metal TiS2. We demonstrated that in addition to a metal-like electrical conductivity of 921 S/cm, the SnTiS3 thin flakes exhibit a strong bandgap emission at 1.9 eV, owing to the weak van der Waals interaction within the misfit-layer stackings. Our work shows that the misfit heterojunction structure preserves the electronic properties and lattice vibrations of the individual constituent monolayers and thus holds the promise to bridge the bandgap and carrier mobility discrepancy between graphene and recently established 2D transition metal dichalcogenide materials. Moreover, we also present a way to identify the top layer of SnTiS3 misfit compound layers and their related work function, which is essential for deployment of van der Waals misfit layers in future optoelectronic devices.

The quadruple perovskite CaMn$_7$O$_{12}$ is a topical multiferroic, in which the hierarchy of electronic correlations driving structural distortions, modulated magnetism, and orbital order is not well known and may vary with temperature. x-ray resonant elastic scattering (XRES) provides a momentum-resolved tool to study these phenomena, even in very small single crystals, with valuable information encoded in its polarization- and energy-dependence. We present an application of this technique to CaMn$_7$O$_{12}$. By polarization analysis, it is possible to distinguish superstructure reflections associated with magnetic order and orbital order. Given the high momentum resolution, we resolve a previously unknown splitting of an orbital order superstructure peak, associated with a distinct \textit{locked-in} phase at low temperatures. A second set of orbital order superstructure peaks can then be interpreted as a second-harmonic orbital signal. Surprisingly, the intensities of the first- and second-harmonic orbital signal show disparate temperature and polarization dependence. This orbital re-ordering may be driven by an exchange mechanism, that becomes dominant over the Jahn-Teller instability at low temperature.

This investigation reports on anisotropy in the magnetic interaction, lattice-orbital coupling and degree of phonon softening in single crystal Ni3TeO6 (NTO) using temperature- and polarization-dependent X-ray absorption spectroscopic techniques. The magnetic field-cooled and zero-field-cooled measurements and temperature-dependent Ni L3,2-edge X-ray magnetic circular dichroism spectra of NTO reveal a weak Ni-Ni ferromagnetic interaction close to ~60 K (TSO: temperature of the onset of spin ordering) with a net alignment of Ni spins (the uncompensated components of the Ni moments) along the crystallographic c-axis, which is absent from the ab-plane. Below the Néel temperature, TN~ 52 K, NTO is stable in the antiferromagnetic state with its spin axis parallel to the c-axis. The Ni L3,2-edge X-ray linear dichroism results indicate that above TSO, the Ni 3d eg electrons preferentially occupy the out-of-plane 3d3z2−r2 orbitals and switch to the in-plane 3dx2−y2 orbitals below TSO. The inherent distortion of the NiO6 octahedra and anisotropic nearest-neighbor Ni-O bond lengths between the c-axis and the ab-plane of NTO, followed by anomalous Debye-Waller factors and orbital-lattice in conjunction with spin-phonon couplings, stabilize the occupied out-of-plane (3d3z2−r2) and in-plane (3dx2−y2) Ni eg orbitals above and below TSO, respectively.

InSe, a newly rediscovered two-dimensional (2D) semiconductor, possesses superior electrical and optical properties as a direct-band-gap semiconductor with high mobility from bulk to atomically thin layers and is drastically different from transition-metal dichalcogenides, in which the direct band gap only exists at the single-layer limit. However, absorption in InSe is mostly dominated by an out-of-plane dipole contribution, which results in the limited absorption of normally incident light that can only excite the in-plane dipole at resonance. To address this challenge, we have explored a unique geometric ridge state of the 2D flake without compromising the sample quality. We observed the enhanced absorption at the ridge over a broad range of excitation frequencies from photocurrent and photoluminescence (PL) measurements. In addition, we have discovered new PL peaks at low temperatures due to defect states on the ridge, which can be as much as ∼60 times stronger than the intrinsic PL peak of InSe. Interestingly, the PL of the defects is highly tunable through an external electrical field, which can be attributed to the Stark effect of the localized defects. InSe ridges thus provide new avenues for manipulating light-matter interactions and defect engineering that are vitally crucial for novel optoelectronic devices based on 2D semiconductors.

A charge density wave (CDW) is one of the fundamental instabilities of the Fermi surface occurring in a wide range of quantum materials. In dimensions higher than one, where Fermi surface nesting can play only a limited role, the selection of the particular wave vector and geometry of an emerging CDW should in principle be susceptible to controllable manipulation. In this work, we implement a simple method for straining materials compatible with low-temperature scanning tunneling microscopy/spectroscopy (STM/S), and use it to strain-engineer new CDWs in 2H-NbSe2. Our STM/S measurements combined with theory reveal how small strain-induced changes in the electronic band structure and phonon dispersion lead to dramatic changes in the CDW ordering wave vector and geometry. Our work unveils the microscopic mechanism of a CDW formation in this system, and can serve as a general tool compatible with a range of spectroscopic techniques to engineer novel electronic states in any material where local strain or lattice symmetry breaking plays a role., to appear in PNAS (2018)

The search for proximity-induced superconductivity in topological materials has generated widespread interest in the condensed matter physics community. The superconducting states inheriting nontrivial topology at interfaces are expected to exhibit exotic phenomena such as topological superconductivity and Majorana zero modes, which hold promise for applications in quantum computation. However, a practical realization of such hybrid structures based on topological semimetals and superconductors has hitherto been limited. Here, we report the strong proximity-induced superconductivity in type-II Weyl semimetal WTe2, in a van der Waals hybrid structure obtained by mechanically transferring NbSe2 onto various thicknesses of WTe2. When the WTe2 thickness (tWTe2) reaches 21 nm, the superconducting transition occurs around the critical temperature (Tc) of NbSe2 with a gap amplitude (Δp) of 0.38 meV and an unexpected ultralong proximity length (lp) up to 7 μm. With the thicker 42 nm WTe2 layer, however, the proxi...

Organic–inorganic hybrid two-dimensional (2D) perovskites have recently attracted great attention in optical and optoelectronic applications due to their inherent natural quantum-well structure. We report the growth of high-quality millimeter-sized single crystals belonging to homologous two-dimensional (2D) hybrid organic–inorganic Ruddelsden–Popper perovskites (RPPs) of (BA)2(MA)n−1PbnI3n+1 (n = 1, 2, and 3) by a slow evaporation at a constant-temperature (SECT) solution-growth strategy. The as-grown 2D hybrid perovskite single crystals exhibit excellent crystallinity, phase purity, and spectral uniformity. Low-threshold lasing behaviors with different emission wavelengths at room temperature have been observed from the homologous 2D hybrid RPP single crystals. Our result demonstrates that solution-growth homologous organic–inorganic hybrid 2D perovskite single crystals open up a new window as a promising candidate for optical gain media.

We report the magnetic behaviour of ball-milled nanoparticles of antiferromagnetic spin-1 Li2Ni(WO4)2 system by magnetic susceptibility and neutron diffraction measurements. Bulk sample shows indicators of two successive three dimensional (3D) antiferromagnetic (AF) ordering at T N 1 ∼ 18 K and T N 2 ∼ 13 K in χ (T). The magnetic susceptibility and neutron diffraction measurements indicate that no long-range magnetic order is detected when bulk sample is reduced to nano size particles, despite of the identical crystal structures. The super-super exchange interaction is proposed being disrupted as a result of cell volume expansion in nanoparticles. The Zero-Field-Cooled (ZFC) and Field-Cooled (FC) curve bifurcation and M(H) curve of nanoparticle reveal that the weak ferromagnetic behavior exists at low temperature because of spin disorder on the surface of nanoparticle.

We report the single crystal growth and superconducting properties of a misfit layered (SnS)1.15(TaS2) compound. The transport, magnetic, and thermodynamic properties revealed the superconducting transition with an onset temperature of Tc ∼ 3.01 K. The high resolution transmission electron microscopy (HRTEM) image clearly shows the misfit stacking of SnS and TaS2 layers. Based on the Werthamer–Helfand–Hohenberg (WHH) formula and Ginzburg–Landau theory, the upper critical fields are Hc2(0) = 0.64 ± 0.06 T and 0.22 ± 0.02 T with coherence lengths of ξ = 22.67 and 38.68 nm for field applied perpendicular (H⊥) and parallel (H//) to the plane, respectively. On the basis of the specific heat measurement data analysis of derived parameters including Sommerfeld coefficient γ = 5.831 ± 0.012 mJ mol–1 K–2, Debye temperature ΘD = 151 K, specific heat jump ΔCe/γTc = 0.812, and electron–phonon coupling constant λel–ph ∼ 0.724, all indicate the weak-coupling nature for (SnS)1.15(TaS2) as a misfit layered superconductor...

Solution-processable 2D metal-halide perovskites are highly promising for cost-effective optoelectronic applications due to their intrinsic multiquantum well structure. However, the lack of stability is still a major obstacle in the use of this class of materials in practical devices. Here, the authors demonstrate the stable optoelectronic properties using formamidinium (FA)-based centimeter-long 2D perovskite (BA)2FAPb2I7 high-quality single-crystal controlled by the thickness of two perovskite layers. The large area single-crystal exhibits good crystallinity, phase purity, and spectral uniformity. Moreover, the (BA)2FAPb2I7 single-crystal shows excellent stability at open atmospheric conditions when compared to methylammonium (MA)-based (BA)2MAPb2I7 counterparts. The photodetectors fabricated using 2D perovskite single-crystal on the rigid Si/SiO2 substrate reveal high photoresponsivity (Rλ)(≈5 A W−1), the fast response time (

The ongoing quest for defect-free thin films systems that are apt for being used as spin filtering materials for spintronic applications did yet not deliver satisfying results regarding materials that would be up to the pertinent requirements. Using soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Co-L2,3 and Fe-L2,3 absorption edges, we have investigated the magnetic properties of a nanostructured thin film with self-assembled CoFe2O4 nanopillars embedded in BiFeO3, the latter being a well-known system for its combined multiferroic and spintronic properties. In this BiFeO3-CoFe2O4 heterostructure we observed a significant XMCD signal at the Co-L2,3 edges which turns out to be the largest among the presently reported for Co ions at room temperature. A quantitative analysis of the Co-L2,3 spectra unveils that such a large Co-L2,3 XMCD signal stems from the impeccable fully inverted spinel ordering of the A- and B-sites in antiphase-free CoFe2O4 nanopillars. This twofold perfect CoFe2O4 ordering feature yields an unprecedented optimization within a multifunctional ferrimagnetic-multiferroic thin film system highly relevant for spintronic applications, also resulting in an equally unprecedented macroscopic magnetic moment for such material as compared to its pure form as well as to technologically relevant thin film compound systems.

Large size (∼2 cm) single crystals of layered MoTe2 in both 2H- and 1T′-types were synthsized using TeBr4 as the source of Br2 transport agent in chemical vapor transport growth. The crystal structures of the as-grown single crystals were fully characterized by X-ray diffraction, Raman spectroscopy, scanning transmission electron microscopy, scanning tunneling microscopy (STM), and electrical resistivity (ρ) measurements. The resistivity ρ(T), magnetic susceptibility χ(T), and heat capacity Cp(T) measurement results reveal a first order structural phase transition near ∼240 K for 1T′-MoTe2, which has been identified to be the orthorhombic Td-phase of MoTe2 as a candidate of Weyl semimetal. The STM study revealed different local defect geometries found on the surface of 2H- and Td-types of MoTe6 units in trigonal prismatic and distorted octahedral coordination, respectively.

We studied the magnetoresistivity and the Hall effect of the type-I Weyl semimetal TaAs to address the controversy surrounding its anomalous transport properties in relation to its bulk topological character. For fields and currents along the basal plane, we observe a very pronounced planar Hall effect (PHE) upon field rotation with respect to the crystallographic axes at temperatures as high as $T=100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Parametric plots of the PHE signal as a function of the longitudinal magnetoresistivity (LMR) collected at $T=10\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ lead to concentric traces as reported for ${\mathrm{Na}}_{3}\mathrm{Bi}$ and GdBiPt. This would suggest that the negative LMR and the PHE observed in TaAs are intrinsically associated with the axial anomaly among its Weyl nodes. For fields nearly along the $a$ axis we observe hysteresis as one surpasses the quantum limit, where the magnetic torque indicates a change in regime as the field increases, i.e., from paramagnetism and diamagnetism due to Weyl fermions above and below the Weyl node(s), respectively, to a paramagnetic one associated with the field-independent $n=0$ Landau level. Hysteresis coupled to the overall behavior of the torque would be consistent with a topological phase transition associated with the suppression of the Weyl dispersion at the quantum limit. This transition leads to the suppression of the negative LMR confirming that it is intrinsically associated with the Weyl dispersion. The Hall effect for fields along the $c$ axis reveals two successive changes in slope, or two successive decrements in carrier mobility, one at the quantum limit and a second one at the critical field where a phase transition toward an insulating state was recently reported. This suggests the possibility of two successive phase transitions as function of the field with the higher-field one involving solely the $n=0$ Landau level. Finally, for both field orientations we observe Shubnikov--de Haas like oscillations beyond the quantum limit hence involving quasiparticles at fractional filling factors.

We report inelastic He atom scattering measurements of the (001) surface phonon dispersion of the topological crystalline insulator ${\mathrm{Pb}}_{0.7}{\mathrm{Sn}}_{0.3}\mathrm{Se}$. This material exhibits a temperature-dependent topological transition, so we measure the surface dispersion curves in both the trivial and nontrivial phases. We identify that, peculiarly, most surface modes are resonances, rather than pure surface states. We find that a shear vertical surface resonance branch around 9.0 meV dramatically changes on going from the trivial to the topological phase. We associate this remarkable change with the emergence of surface Dirac fermions. We use the measured dispersion of this resonance branch to determine the corresponding mode-dependent electron-phonon coupling ${\ensuremath{\lambda}}_{\ensuremath{\nu}}(\mathbf{q})$.

Cadmium arsenide (Cd3As2) has recently attracted considerable interest for the presence of 3D massless Dirac fermions with ultrahigh mobility and magnetoresistance. However, its surface properties are currently largely unexplored both theoretically and experimentally, due to the very large unit cell and the challenging growth of single-crystal samples, respectively. Here, by combining ab initio calculations with surface-science spectroscopic experiments, the presence of a surface reconstruction is unveiled in centimeter-scale (112)-oriented Cd3As2 single-crystal foils produced by the self-selecting vapor growth. Outermost Cd atoms descend into the As-sublayer with a subsequent self-passivation of the dangling bonds with As atoms, forming the triangle lattice previously imaged by scanning tunneling microscopy. Moreover, the oxidation mechanism of this reconstructed surface, dominated by the formation of As?O?Cd bonds, is revealed. Interestingly, it is found that the band structure of the reconstructed surface of Cd3As2 is quite robust against surface oxidation. Both computational and experimental findings point to a successful exploitation in technology of Cd3As2 single crystals.

Materials exhibiting nodal-line fermions promise superb impact on technology for the prospect of dissipationless spintronic devices. Among nodal-line semimetals, the ZrSiX (X = S, Se, Te) class is the most suitable candidate for such applications. However, the surface chemical reactivity of ZrSiS and ZrSiSe has not been explored yet. Here, by combining different surface-science tools and density functional theory, it is demonstrated that the formation of ZrSiS and ZrSiSe surfaces by cleavage is accompanied by the washing up of the exotic topological bands, giving rise to the nodal line. Moreover, while the ZrSiS has a termination layer with both Zr and S atoms, in the ZrSiSe surface, reconstruction occurs with the appearance of Si surface atoms, which is particularly prone to oxidation. It is demonstrated that the chemical activity of ZrSiX compounds is mostly determined by the interaction of the Si layer with the ZrX sublayer. A suitable encapsulation for ZrSiX should not only preserve their surfaces from interaction with oxidative species, but also provide a saturation of dangling bonds with minimal distortion of the surface. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim U.S. Department of Energy, USDOE: DE-SC0014208 A.P. thanks Elettra Sincrotrone Trieste S.C.p.A. for financial support. Z.M. thanks the support by the U.S. Department of Energy under grant DE-SC0014208 for material synthesis. This work was partly performed in the framework of the Nanoscience Foundry and Fine Analysis facility (NFFA-MIUR Italy Progetti Internazionali).

We have synthesized high-quality single crystals of SnxPb1-xTe and carried out detailed studies of the magnetotransport properties of one of the samples, Sn0.05Pb0.95Te. Longitudinal magnetoresistance increases almost linearly with increasing applied field (H) and reaches ∼310% atH= 13 T. At higher fields, both longitudinal and Hall resistance show clear Shubnikov de Haas oscillations. The oscillations are smooth and periodic, and there exists only one frequency,fα∼ 57 T. However, an additional frequency,fβ∼ 69 T, appears as the angle between the field direction and the normal to the sample surface (θ) is increased. Bothfαandfβexhibitθ-dependence;fαdecreases whereasfβincreases gradually with increasingθ. The presence of two frequencies in Sn0.05Pb0.95Te indicates that there exist two Fermi surface pockets (αandβpockets). We have constructed the Landau-level fan plot and determined the Berry phase (δ) for theαpocket to beδ∼ 0.1. Thisδvalue is very close to the expected value of 0 for a topologically trivial system.

The magnetotransport properties of a hybrid InSe/monolayer graphene in a SiC system are systematically studied. Compared to those of its bare graphene counterpart, in InSe/graphene, we can effectively modify the carrier density, mobility, effective mass, and electron–electron (e–e) interactions enhanced by weak disorder. We show that in bare graphene and hybrid InSe/graphene systems, the logarithmic temperature (lnT) dependence of the Hall slope R H = δR xy /δB = δρ xy /δB can be used to probe e–e interaction effects at various temperatures even when the measured resistivity does not show a lnT dependence due to strong electron–phonon scattering. Nevertheless, one needs to be certain that the change of R H is not caused by an increase of the carrier density by checking the magnetic field position of the longitudinal resistivity minimum at different temperatures. Given the current challenges in gating graphene on SiC with a suitable dielectric layer, our results suggest that capping a van der Waals material on graphene is an effective way to modify the electronic properties of monolayer graphene on SiC.

Atomically thin vertically stacked 2D vdW heterostructures have recently emerged as a new kind of device with intriguing novel phenomena for both academic and industrial interests. However, the lack of p-type materials remains a challenging issue to create useful devices for the realization of practical applications. Here, we demonstrate the first vertically stacked few-layered p-type GaSe and n-type SnS2 vdW heterostructure for high-performance optoelectronic applications. It is found that the phototransistors based on a few-layered GaSe/SnS2 p-n junction show superior performance with the responsivity, EQE and specific detectivity as high as ~35 AW−1, 62%, and 8.2 × 1013 J, respectively, which exceed all the reported values derived from 2D materials. Also, the GaSe/SnS2 p-n junction can serve as a photovoltaic cell with a high power conversion efficiency of about ~2.84%. Moreover, the heterostructures can be deposited on flexible PET substrates with excellent performance. Through a detailed study, the underlying mechanism responsible for the high performance can be attributed to the unique type II band alignment and excellent quality of the interface. The heterojunctions presented in this work demonstrate a new illustration for the stacking of 2D materials, which is very useful for the development of next-generation novel optoelectronic devices.

The impact of induced defects such as Bi intercalants, Bi antisites, and Se vacancies on the transport properties of the Bridgman-grown Bi2Se3-based single crystals in the ab plane were investigated by means of temperature-dependent electrical resistivity, Seebeck coefficient, and thermal conductivity measurements. We found that the Bi2Se3-based crystals can be grown either along the c-axis or perpendicular to it, depending on the different Bi-Se ratios in the starting material. All grown crystals showed a weak metallic behavior with a predominant electron-phonon scattering governing their electrical transport. The absolute value of the Seebeck coefficient of the n-type crystals grown along the c-axis is higher than those grown perpendicular to the c-axis (along the ab plane) at room temperature. The Fermi energy estimated from the Seebeck coefficient data is in the range of 0.20–0.43 eV. The thermal conductivity measurement showed that the lattice phonons dominate thermal transport in these Bi2Se3-based crystals. Analyses of lattice thermal conductivity data of the crystals by the Debye-Callaway approximation revealed that both boundary and point-defect scattering of phonons have a significant effect on the size and shape of the low-temperature phonon peak. These present findings indicate that, in addition to the change in composition, the physical properties of the Bi2Se3-based crystals are also affected significantly by the induced defects. Finally, the near-stoichiometric Bi2.04Se2.99 compound has a room-temperature thermoelectric figure of merit (ZT) of ∼0.12.

Observations of quasiparticle interference have been used in recent years to examine exotic carrier behavior at the surfaces of emergent materials, connecting carrier dispersion and scattering dynamics to real-space features with atomic resolution. We observe quasiparticle interference in the strongly Rashba split 2DEG-like surface band found at the tellurium termination of BiTeBr and examine two mechanisms governing quasiparticle scattering: We confirm the suppression of spin-flip scattering by comparing measured quasiparticle interference with a spin-dependent elastic scattering model applied to the calculated spectral function. We also use atomically resolved STM maps to identify point defect lattice sites and spectro-microscopy imaging to discern their varying scattering strengths, which we understand in terms of the calculated orbital characteristics of the surface band. Defects on the Bi sublattice cause the strongest scattering of the predominantly Bi 6p derived surface band, with other defects causing nearly no scattering near the conduction band minimum.

2D ternary semiconductor single crystals, an emerging class of new materials, have attracted significant interest recently owing to their great potential for academic interest and practical application. In addition to other types of metal dichalcogenides, 2D tin dichalcogenides are also important layered compounds with similar capabilities. Yet, multi-elemental single crystals enable to assist multiple degrees of freedom for dominant physical properties via ratio alteration. This study reports the growth of single crystals Se-doped SnS2 or SnSSe alloys, and demonstrates their capability for the fabrication of phototransistors with high performance. Based on exfoliation from bulk high quality single crystals, this study establishes the characteristics of few-layered SnSSe in structural, optical, and electrical properties. Moreover, few-layered SnSSe phototransistors are fabricated on both rigid (SiO2/Si) and versatile polyethylene terephthalate substrates and their optoelectronic properties are examined. SnSSe as a phototransistor is demonstrated to exhibit a high photoresponsivity of about 6000 A W−1 with ultra-high photogain (η) ≈8.8 × 105, fast response time ≈9 ms, and specific detectivity (D*) ≈8.2 × 1012 J. These unique features are much higher than those of recently published phototransistors configured with other few-layered 2D single crystals, making ultrathin SnSSe a highly qualified candidate for next-generation optoelectronic applications.

Vertically aligned LiV3O8 nanorod thin films were grown on stainless steel substrates for the first time by Pulsed Laser Deposition (PLD) method. Highly crystalline phase of LiV3O8 thin film with less than 1% of lithium deficient Li0.3V2O5 phase was obtained at a substrate temperature of 700 °C as confirmed by XRD analysis. The mixed phases of Li0.3V2O5, V2O5 and Li0.97V3O8 was obtained with the rise in substrate temperature to 800 °C. The substrate temperature was found to play a crucial role in the phase formation. The highly oriented vertical growth of the nanorods were seen in FESEM analysis of film grown at 700 °C. It is concluded that the novel nanostructured thin film LiV3O8 reported here can have its potential applications in batteries.

We report the experimental results of dc and ac magnetic susceptibility (χdc and χac) and specific heat (CP) measurements of the powdered Ni3TeO6-type oxide. Upon decreasing the grain size from micrometer (bulk) to nanometer range, the magnetic measurements have indicated multiple transitions, viz., a cusp at paramagnetic to ferrimagnetic (FIM) transition temperature (TC) ∼ 78 K, an antiferromagnetic (AFM) transition temperature (TN) ∼ 52 K, and a re-entrant spin glass (RSG) transition at Tm2 ∼ 7.49 K. The ferrimagnetic transition temperature TC is found to be highly sensitive to dc magnetic fields, where superposition of a small dc field (10 Oe) in the ac susceptibility measurement or the application of a dc magnetic field of up to 100 Oe in the dc magnetic measurement is enough to suppress the observed TC at 78 K. The CP(T) measurement also revealed a RSG state at lower temperatures for the sample with nano-sized grains. The absence of any sharp peak at TC in the CP(T) curves implies the absence of a true long-range FIM spin order in the samples. The coexistence of a RSG state with multiple magnetic transitions is understood as an effect of competitive AFM and FIM phases in the Ni3TeO6-type structure.

Cadmium arsenide is a time-honored material within condensed matter physics, with the first investigations dating back to the thirties. Nowadays, after theorists predicted a pair of symmetry-protected three-dimensional Dirac cones in its band structure, cadmium arsenide is going through an intense revival. Cadmium arsenide is now thought of as a three-dimensional analogue of graphene. Several experimental studies showed compelling evidence of conical bands in this material, revealing a number of interesting properties and phenomena. To interpret them correctly, a detailed understanding of the basic material parameters has become even more important than before. To this end, the authors extensively review the past and current knowledge of cadmium arsenide. They start with the crystal lattice properties, and continue with the technological aspects of its crystal growth. This is followed by a discussion of the theoretical and experimental results, leading to different possible views of this material's electronic bands.