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Metal nanocrystals (NCs), particularly for plasmonic metal NCs with specific morphology and size, can strongly interact with ultraviolet-visible or even near-infrared photons to generate energetic charge carriers, localized heating, and electric field enhancement. These unique properties offer a promising opportunity for maneuvering solar-to-chemical energy conversion through different mechanisms. As distinct from previous works, in this review, recent advances of various characterization techniques in probing and monitoring the photophysical/photochemical processes, as well as the reaction mechanisms of plasmon-mediated photoredox catalysis are thoroughly summarized. Understanding how to distinguish and track these reaction mechanisms would furnish basic guidelines to design next-generation photocatalysts for plasmon-enhanced catalysis.

The nonlinear Hall effect (NLHE), the phenomenon in which a transverse voltage can be produced without a magnetic field, provides a potential alternative for rectification or frequency doubling1,2. However, the low-temperature detection of the NLHE limits its applications3,4. Here, we report the room-temperature NLHE in a type-II Weyl semimetal TaIrTe4, which hosts a robust NLHE due to broken inversion symmetry and large band overlapping at the Fermi level. We also observe a temperature-induced sign inversion of the NLHE in TaIrTe4. Our theoretical calculations suggest that the observed sign inversion is a result of a temperature-induced shift in the chemical potential, indicating a direct correlation of the NLHE with the electronic structure at the Fermi surface. Finally, on the basis of the observed room-temperature NLHE in TaIrTe4 we demonstrate the wireless radiofrequency (RF) rectification with zero external bias and magnetic field. This work opens a door to realizing room-temperature applications based on the NLHE in Weyl semimetals. Broken inversion symmetry in a type-II Weyl semimetal TaIrTe4 enables observation of the room-temperature nonlinear Hall effect as well as wireless radiofrequency rectification.

The nonlinear Hall effect due to Berry curvature dipole (BCD) induces frequency doubling, which was recently observed in time-reversal-invariant materials. Here we report novel electric frequency doubling in the absence of BCD on a surface of the topological insulator Bi2Se3 under zero magnetic field. We observe that the frequency-doubling voltage transverse to the applied ac current shows a threefold rotational symmetry, whereas it forbids BCD. One of the mechanisms compatible with the symmetry is skew scattering, arising from the inherent chirality of the topological surface state. We introduce the Berry curvature triple, a high-order moment of the Berry curvature, to explain skew scattering under the threefold rotational symmetry. Our work paves the way to obtain a giant second-order nonlinear electric effect in high mobility quantum materials, as the skew scattering surpasses other mechanisms in the clean limit., Berry curvature dipole (BCD) leads to the nonlinear Hall effect manifested as a frequency doubling in topological materials. Here, the authors report electric frequency doubling in the absence of BCD and magnetic field on a surface of Bi2Se3 due to skew scattering arising from inherent chirality of the topological surface states.

With the increasing importance of human–machine interactions, stretchability, biocompatibility, and self-healing behavior will play a more significant role in next-generation touch panels so as to allow integration with the human body. However, profound challenges remain because the electrodes of most touch panel are constructed by stiff and brittle materials lacking a self-healing capacity. Herein, we demonstrate a semiconductive touch panel that adopts a capacitive touch-sensing system, based on a poly(N,N′-dimethylacrylamide)–titanium dioxide nanocomposite hydrogel as a transparent conductor. The panel exhibited high stretchability, softness, low parasitic capacitance, high resolution, fast response, and instant functionality recovery upon damage. The panel was stretched to more than 1100% areal strain and could still be operated as an input device without sacrificing its functionalities. Furthermore, the distortion near the edge of the panel, owing to the nonlinearity of the resistance proved by the electrical finite element method simulation, became weaker after optimizing with a four-terminal pattern through 3D printing. An epidermal touch panel adhered to the human skin, and demonstrated strong performance and self-healing ability by being able to write words, play chess, and play games.

Artificial photosynthesis has attracted increasing attention due to recent environmental and energy concerns. Metal halide perovskites (MHPs) demonstrating excellent optoelectronic properties have currently emerged as novel and efficient photocatalytic materials. Herein, the structural features of MHPs that are responsible for the photoinduced charge separation and charge migration properties are briefly introduced, and then important and necessary photophysical and photochemical aspects of MHPs related to photoredox catalysis are summarized. Subsequently, the applications of MHPs for solar energy harvesting and photocatalytic conversion, including H2 evolution, CO2 reduction, degradation of organic pollutants, and photoredox organic synthesis, are extensively demonstrated, with a focus on strategies for improving the performance (e.g., selectivity, activity, stability, recyclability, and environmental compatibility) of these MHP-based photocatalytic systems. To conclude, existing challenges and prospects on the future development of MHP-based materials towards photoredox catalysis applications are detailed.

The development of infrared photodetectors is mainly limited by the choice of available materials and the intricate crystal growth process. Moreover, thermally activated carriers in traditional III-V and II-VI semiconductors enforce low operating temperatures in the infrared photodetectors. Here we demonstrate infrared photodetection enabled by interlayer excitons (ILEs) generated between tungsten and hafnium disulfide, WS2/HfS2. The photodetector operates at room temperature and shows an even higher performance at higher temperatures owing to the large exciton binding energy and phonon-assisted optical transition. The unique band alignment in the WS2/HfS2 heterostructure allows interlayer bandgap tuning from the mid- to long-wave infrared spectrum. We postulate that the sizeable charge delocalization and ILE accumulation at the interface result in a greatly enhanced oscillator strength of the ILEs and a high responsivity of the photodetector. The sensitivity of ILEs to the thickness of two-dimensional materials and the external field provides an excellent platform to realize robust tunable room temperature infrared photodetectors. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) National Research Foundation (NRF) Accepted version The work is supported by the Agency for Science, Technology and Research (A*STAR) under the 2D Materials Pharos Program (grant no. 152 700014 and grant no. 152 700017). Q.J.W. acknowledges funding from the National Research Foundation Competitive Research Program (NRF-CRP18-2017-02 and NRF–CRP19–2017–01). K.S.T. acknowledges support from the Center for Nanostructured Graphene (CNG) under the Danish National Research Foundation (project DNRF103) and from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant no. 773122, LIMA). G.L. acknowledges the supported under the grant MOE2017-T2-1-114. H.L. acknowledges the support by the Ministry of Science and Technology (MOST) in Taiwan under grant no. MOST 109-2112-M-001-014-MY3. J.T. and S. Lukman thank C. W. Lee, A. Ngo and M. Zhao for their valuable inputs and K Hippalgaonkar for sharing tools in the device fabrication.

The observation of chirality is ubiquitous in nature. Contrary to intuition, the population of opposite chiralities is surprisingly asymmetric at fundamental levels. Examples range from parity violation in the subatomic weak force to the homochirality in essential biomolecules. The ability to achieve chirality-selective synthesis (chiral induction) is of great importance in stereochemistry, molecular biology and pharmacology. In condensed matter physics, a crystalline electronic system is geometrically chiral when it lacks any mirror plane, space inversion center or roto-inversion axis. Typically, the geometrical chirality is predefined by a material's chiral lattice structure, which is fixed upon the formation of the crystal. By contrast, a particularly unconventional scenario is the gyrotropic order, where chirality spontaneously emerges across a phase transition as the electron system breaks the relevant symmetries of an originally achiral lattice. Such a gyrotropic order, proposed as the quantum analogue of the cholesteric liquid crystals, has attracted significant interest. However, to date, a clear observation and manipulation of the gyrotropic order remain challenging. We report the realization of optical chiral induction and the observation of a gyrotropically ordered phase in the transition-metal dichalcogenide semimetal $1T$-TiSe$_2$. We show that shining mid-infrared circularly polarized light near the critical temperature leads to the preferential formation of one chiral domain. As a result, we are able to observe an out-of-plane circular photogalvanic current, whose direction depends on the optical induction. Our study provides compelling evidence for the spontaneous emergence of chirality in the correlated semimetal TiSe$_2$. Such chiral induction provides a new way of optical control over novel orders in quantum materials.

The spin Hall effect (SHE) is usually observed as a bulk effect in high-symmetry crystals with substantial spin–orbit coupling (SOC), where the symmetric spin–orbit field imposes a widely encountered trade-off between spin Hall angle (θSH) and spin diffusion length (Lsf), and spin polarization, spin current and charge current are constrained to be mutually orthogonal. Here, we report a large θSH of 0.32 accompanied by a long Lsf of 2.2 μm at room temperature in a low-symmetry few-layered semimetal MoTe2, thus identifying it as an excellent candidate for simultaneous spin generation, transport and detection. In addition, we report that longitudinal spin current with out-of-plane polarization can be generated by both transverse and vertical charge current, due to the conventional and a newly observed planar SHE, respectively. Our study suggests that manipulation of crystalline symmetries and strong SOC opens access to new charge-spin interconversion configurations and spin–orbit torques for spintronic applications. A large spin Hall angle and long spin diffusion length are found in the low-symmetry, few-layer semimetal MoTe2 at room temperature, thus identifying this material as an excellent candidate for simultaneous spin generation, transport and detection.

Aggregation-induced emission luminogens (AIEgens) provide a new approach to address the aggregation-induced quenching (ACQ) defect prevalent in traditional fluorescent dyes. Since there are only two dispersion forms of single molecule (monomer state, weak fluorescence) and aggregates (multimer state, strong fluorescence) in aqueous medium, most of the reported AIEgens exhibit a single emission band and therefore cannot be used for dual-channel fluorescence imaging in living cells. In this work, the “monomer-dimer-multimer” dispersion mechanism was innovatively introduced into the design of dual-emission AIEgens. Under the guidance of this concept, two cyanovinyl-pyrene dual-emission AIEgens with twisted molecular configuration have been readily synthesized and studied by theoretical calculations, UV–visible absorption, steady-state fluorescence, time-resolved fluorescence, dynamic laser scanning (DLS) and laser confocal imaging. These AIEgens emit weak fluorescence in monomer state due to intramolecular C C rotation. With gradual aggregation, they form dimer state to emit blue fluorescence centered at 460 nm, and then convert into multimer state to emit red fluorescence around 570 nm, simultaneously exhibiting AIE and dual-emission characteristics. They also have excellent photostability, good membrane permeability and low cytotoxicity. Confocal imaging tests demonstrate that AIEgens composed of cyanovinyl-pyrene and benzimidazole moieties is available for dual-channel imaging in Hela cells and macrophages cells, and shows great potential in designing ratiometric nanoprobe. Our design mechanism will provide a versatile platform for the development of dual-emission AIEgens.

Gold nanorods (Au NRs) have received extensive attention owing to their extremely attractive applications in photoredox catalysis, plasmon-enhanced spectroscopy, biomedical technologies and optoelectronic devices. Enabled by the unique and tunable surface plasmon resonance (SPR), anisotropic Au NRs can interact with and harvest incident light covering the much of the solar spectrum. As such, they may serve as unusual media to supply energetic hot charge carriers, generate heat, and provide strong local electric field and reactive site for redox reactions through different mechanisms. In this review, we present a comprehensive overview on the burgeoning field of Au NRs-based materials for solar energy conversion. We firstly provide a detailed elucidation of the key underpinning science for plasmonic Au NRs and Au NRs-mediated catalysis. The possible charge transfer processes and corresponding roles of Au NRs played in different photoredox catalysis systems are demonstrated, followed by introducing the latest advances in constructing Au NRs-based hybrids with tailored structure. The applications of the hybrids in photoinduced catalysis, photothermal catalysis and photoelectrochemical catalysis are then discussed. Particularly, the process-intensified engineering strategies to maximize solar energy conversion efficiency are further highlighted based on some typical examples. Finally, the perspectives on future research trends and challenges in rational design and deliberate construction of Au NRs-mediated photoredox catalysis systems in a smart configuration are proposed.

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUG Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research. Ministerio de Ciencia e Innovación | Ref. RYC2018-026103-I Agencia Estatal de Investigación | Ref. CTQ2017-82711-P Ministerio de Economía y Competitividad | Ref. MDM-2015-0538 Agencia Estatal de Investigación | Ref. PID2019-107314RB-I00

The resistance bottleneck in metal-interconnect scaling calls for new interconnect materials. This paper explores topological semimetals as a potential solution. After reviewing the desirable properties of topological semimetals for back-end-of-line (BEOL) interconnects, we use CoSi as an example to demonstrate the decreasing resistance-area product with scaling and provide material-search guidelines., 4 pages, 12 figures

The spin polarization induced by the spin Hall effect (SHE) in thin films typically points out of the plane. This is rooted on the specific symmetries of traditionally studied systems, not in a fundamental constraint. Recently, experiments on few-layer ${\rm MoTe}_2$ and ${\rm WTe}_2$ showed that the reduced symmetry of these strong spin-orbit coupling materials enables a new form of {\it canted} spin Hall effect, characterized by concurrent in-plane and out-of-plane spin polarizations. Here, through quantum transport calculations on realistic device geometries, including disorder, we predict a very large gate-tunable SHE figure of merit $\lambda_s\theta_{xy}\sim 1\text{--}50$ nm in ${\rm MoTe}_2$ and ${\rm WTe}_2$ monolayers that significantly exceeds values of conventional SHE materials. This stems from a concurrent long spin diffusion length ($\lambda_s$) and charge-to-spin interconversion efficiency as large as $\theta_{xy} \approx 80$\%, originating from momentum-invariant (persistent) spin textures together with large spin Berry curvature along the Fermi contour, respectively. Generalization to other materials and specific guidelines for unambiguous experimental confirmation are proposed, paving the way towards exploiting such phenomena in spintronic devices. These findings vividly emphasize how crystal symmetry and electronic topology can govern the intrinsic SHE and spin relaxation, and how they may be exploited to broaden the range and efficiency of spintronic materials and functionalities., Comment: Published version, Phys. Rev. Reserach

Summary Metal-based nanostructures have received great research attention for constructing photoredox catalysts because of their tunable electronic and optical properties, as typically demonstrated by plasmonic metals. In this review, we begin by briefly presenting the development history, basic concept, and influential factors of surface plasmon resonance of metal nanostructures. Then, we describe the mechanisms that have been proposed to explain the metal-plasmon-enhanced photoactivity of semiconductors and their characteristics. Dedicated emphasis is put on the function-oriented engineering strategies for maximizing the efficacy of these mechanisms for improving the performance of metal-semiconductor photocatalysts with theoretical explanations and some representative examples. In addition, we also discuss the latest advances in a non-plasmonic metal that offers a scattered photon-harvesting channel for photoredox catalysis. Finally, we present our vision of the future development in this field. The rational design and deliberate synthesis of metal-semiconductor hybrids in a smart configuration will afford more possibilities toward solar energy conversion.

Considerable effort has been devoted to manipulating the optical absorption of metal nanostructures for diverse applications. However, it still remains a challenge to develop a general and flexible method to promote broadband absorption of metal nanostructures without changing their size and shape. Here, we report a new strategy of hybridizing two conceptually different optical models to realize broadband absorption enhancement of metal nanoparticles (NPs), which is enabled by constructing a core–shell heterostructure, consisting of a spherical dielectric core covered by a metal NPs interlayer and tunable semiconductor shell. This approach integrates the interfacial photon management, photoexcitation of metal NPs and injection of hot charge carriers into the semiconductor shell, and results in distinctly enhanced hot charge carrier generation and transfer, thereby boosting the broad-spectrum light driven catalysis. The structure–plasmon–catalysis interplay of the heterostructure is comprehensively studied and optimized. This proof-of-concept proves to be generally feasible by varying the type of both metal NPs and support medium, opening a new avenue to control the optoelectronic properties of materials.

Despite polymers have great advantage in improving the energy density of hydrogen storage containers due to their lightweight characteristic, the production of high gas barrier polymers in a low-cost and high-efficiency manner remains a challenge. Here, polyethylene/graphene flakes (PE/GFs) composite films with ultra-high gas barrier properties were produced by no-boundary constraints hot-pressing method. Theoretical analysis showed that an extensional-shear coupled flow was generated inside the polymer melts and the shear-flow force near the polymer melts surface is always higher than that inside the melts during this process, which is responsible for the alignment of GFs and PE molecules. The DSC results indicated that the crystallinity of the composites after hot-pressing is increased by about 10%. The quantitative results of positron annihilation lifetime spectrum (PALS) showed that the fractional free volume of composites is reduced from 6.52 vol% to 5.43 vol%. As a consequence of the synergistic effect of the above aspects, the barrier performance of the composites has been significantly improved, and the helium leak rate of the composites reached an astonishing 3.92 × 10−9 Pa•m3/s with only 0.5 wt% GFs, outperforming unoriented pure PE with a decrease of 98.6%. Moreover, the tensile strength and Young's modulus of the oriented composites were increased by 31.4% and 21.1%, respectively. The work presented here demonstrates that the simple, fast, and economical preparation method can be readily used to produce composites with superior barrier performance, which will provide great reference for the processing technology of hydrogen storage containers.

Black phosphorus (BP) has been proposed as the channel material in the next generation ultrascaled CMOS devices. In order to gain insight into the current characteristics in 2-D layered materials, the current distribution of a few-layer BP Schottky barrier FET is investigated via state-of-the-art quantum device simulations. Approximately 40% of the total current was found to be concentrated in the top layer when the device was switched on, with the remaining current distributed among the other layers. In comparison, ~80% of the current concentrated below the surface in a Si device with the same structure. These features are related to the strength of the intra/interlayer interaction in few-layer BP and are unique to 2-D layered materials. Moreover, the current distribution and the device performance were different for the top- and side-contacted devices, with the side-contacted devices yielding lower resistance compared with the top-contacted devices.

Semiconductor-based heterogeneous photocatalysis has received considerable attention as an alternative promising approach to alleviate the globally increasing energy demand and environmental concerns. As promising candidates, one dimensional (1D) CdS nanostructures have gained growing interest due to their unique advantages, including a narrow band gap, suitable band edge positions, high aspect ratio structure with low reflectance, and excellent electronic charge transfer. This review summarizes a panorama of the latest progress related to the design and construction of 1D CdS based photocatalysts with boosted performance, involving constructing 1D solid solution, loading cocatalysts, coupling with semiconductors, hybridizing with carbon-based materials and functionalization with biocatalysts. Finally, some challenges and opportunities for future exploration of 1D CdS based photocatalysts are critically discussed. It is anticipated that this review will afford enriched information on the rational utilization of 1D CdS materials with ameliorated performances by harnessing their inherent merits, and particularly stimulate further developments of low dimensional heterostructures towards various artificial photoredox reactions.

The nature of Fermi surface defines the physical properties of conductors and many physical phenomena can be traced to its shape. Although the recent discovery of a current-dependent nonlinear magnetoresistance in spin-polarized non-magnetic materials has attracted considerable attention in spintronics, correlations between this phenomenon and the underlying fermiology remain unexplored. Here, we report the observation of nonlinear magnetoresistance at room temperature in a semimetal WTe2, with an interesting temperature-driven inversion. Theoretical calculations reproduce the nonlinear transport measurements and allow us to attribute the inversion to temperature-induced changes in Fermi surface convexity. We also report a large anisotropy of nonlinear magnetoresistance in WTe2, due to its low symmetry of Fermi surfaces. The good agreement between experiments and theoretical modeling reveals the critical role of Fermi surface topology and convexity on the nonlinear magneto-response. These results lay a new path to explore ramifications of distinct fermiology for nonlinear transport in condensed-matter., The nature of non-saturating magnetoresistance (MR) in topological materials is an important issue in condensed matter research but remains elusive. The authors here report the nonlinear MR at room temperature in WTe2 with temperature-driven inversion due to the temperature-induced changes in Fermi surface convexity.

Graphene as an electron-conductive medium has been widely used to construct efficient photocatalysts for solar energy conversion, while the discovery of photosensitizer role of graphene further enriches our understanding of the multifunctional facets of graphene in graphene-based composite photocatalysis. However, the photosensitive efficiency of graphene remains relatively low, and the photosensitization mechanism has yet to be elucidated. Herein, we report a facile wet chemistry strategy to enhance the photosensitive efficiency of graphene via oxidation treatment of graphene oxide and particularly reveal the underlying origin of such boosted photosensitive efficiency of graphene. Graphene derived from graphene oxide with enhanced oxidation degree shows remarkably improved photosensitive efficiency for wide-bandgap ZnO toward visible-light-driven photoreduction process. Controlled experiments disclose that the residual content of oxygenated functional groups is the main factor affecting the photosensitiv...

Recent years have seen the rocket rise of graphene as the unique two-dimensional carbon nanosheets and its outstanding promise in materials science. In particular, because of its diverse, tunable structural and electronic properties, graphene has been well recognized to be an ideal co-catalyst to optimize the photocatalytic performance of semiconductors. Given that the conductive, optical, chemical and mechanical performances of graphene are closely linked to its structural diversity, tremendous efforts have been devoted to designing and tailoring the graphene nanosheets to construct the desirable architecture. The tailored graphene materials (GMs), such as zero-dimensional graphene quantum dots, one-dimensional graphene nanoribbons and three-dimensional graphene frameworks show a variety of fascinating features, thereby offering a fertile and flexible ground for the further development of GMs-enhanced photocatalysis. This review aims to provide an overview on the structural diversity, tunable properties, and synthetic strategies of these GMs, followed by highlighting their multi-functionality in heterogeneous photocatalysis. Finally, the perspectives on future research trends and challenges in constructing more efficient GMs-enhanced systems for solar energy conversion are presented. The integral comprehension of GMs with all dimensions would further guide the fundamental processing-structure-properties-applications relationships of GMs.

Using first-principles calculations, we investigate the geometric and electronic structures of organic–inorganic hybrid perovskite, MAPbX 3 (MA = CH 3 NH 3 + , X = Cl, Br, I), the key materials for the highest efficiency solid-state solar cells. The different halide elements in perovskite compound control the electronic characteristics of the materials, such as their orbitals, density of states and spatial distribution of the charges. We identify the orbitals consisting of the conduction and valence bands. Furthermore, we show that MAPbI 3 can produce and transfer more electrons than other hybrid perovskite solar cells do.

The expected depletion of fossil fuel reserves and its serious environmental impact have emphasised the issue of sustainable development of the human society. Solar hydrogen by photocatalytic water splitting is a promising alternative to conventional fossil fuels, which is of great potential to relieve the energy and environmental issues and bring an energy revolution in a clean and sustainable manner. This review is going to make a brief introduction of the basic principles of photocatalytic water splitting and the concept of different kinds of water splitting systems. Various engineering strategies for searching higher efficiency of water splitting based on the photocatalytic processes, including light harvesting, charge carriers separation and co-catalysts loading, have been outlined and discussed with selected typical examples on some elaborately designed semiconductor-based photocatalytic systems. Moreover, recent impressive progresses and advancements for photocatalytic water splitting with some pro...

The current rapid industrial development generates a high need for alternative sustainable sources of energy. Artificial photosynthesis, which can directly convert solar energy into usable or storable energy resources, provides such a promising alternative. Semiconductor nanowires have gained growing interest in artificial photosynthesis due to their unique geometrical and electronic characteristics, which can provide large aspect-ratios, direct pathways for charge transport, decoupling the direction of charge carrier collection, and low reflectance induced by light scattering and trapping. In particular, heterostructured semiconductor nanowire arrays (NWAs) have recently been of great interest in artificial photosynthesis because of their unique structural and physicochemical properties. Nanowire structure can lower the electrochemical over-potential while the heterojunctions can enhance light absorption and charge separation, which thus increase the artificial photosynthesis efficiency of heterostructured semiconductor NWAs. In this review, we will highlight recent advances in the application of heterostructured semiconductor NWAs to artificial photosynthesis. An emphasis will be placed on the unique characteristics and benefits of using heterostructured semiconductor NWAs with different heterojunctions for artificial photosynthesis, as well as the associated challenges and directions for future research in this area.

The ability to switch magnetic elements by spin-orbit-induced torques has recently attracted much attention for a path toward high-performance, nonvolatile memories with low power consumption. Realizing efficient spin-orbit-based switching requires the harnessing of both new materials and novel physics to obtain high charge-to-spin conversion efficiencies, thus making the choice of spin source crucial. Here, the observation of spin-orbit torque switching in bilayers consisting of a semimetallic film of 1T'-MoTe2 adjacent to permalloy is reported. Deterministic switching is achieved without external magnetic fields at room temperature, and the switching occurs with currents one order of magnitude smaller than those typical in devices using the best-performing heavy metals. The thickness-dependence can be understood if the interfacial spin-orbit contribution is considered in addition to the bulk spin Hall effect. Further threefold reduction in the switching current is demonstrated with resort to dumbbell-shaped magnetic elements. These findings foretell exciting prospects of using MoTe2 for low-power semimetal-material-based spin devices.

Topological crystalline insulators (TCI) are insulating electronic phases of matter with nontrivial topology originating from crystalline symmetries. Recent theoretical advances have provided powerful guidelines to search for TCIs in real materials. Using density functional theory, we identify a class of new TCI states in the tetragonal lattice of the Ca$_2$As material family. On both top and side surfaces, we observe topological surface states protected independently by rotational and mirror symmetries. We show that a particular lattice distortion can single out the newly proposed topological protection by the rotational symmetry. As a result, the Dirac points of the topological surface states are moved to generic locations in momentum space away from any high symmetry lines. Such topological surface states have not been seen before. Moreover, the other family members, including Ca$_2$Sb, Ca$_2$Bi and Sr$_2$Sb, feature different topological surface states due to their distinct topological invariants. We thus further propose topological phase transitions in the pseudo-binary systems such as (Ca$_{1-x}$Sr$_x$)$_2$As and Ca$_2$As$_x$Sb$_{1-x}$. Our work reveals rich and exotic TCI physics across the Ca$_2$As family of materials, and suggests the feasibility of materials database search methods to discover new TCIs., 5 pages, 4 figures

We propose that the quasi-one-dimensional molybdenum selenide compound Tl2-xMo6Se6 is a time-reversal-invariant topological superconductor induced by inter-sublattice pairing, even in the absence of spin-orbit coupling (SOC). No noticeable change in superconductivity is observed in Tl-deficient (0

Weyl semimetals are novel topological conductors that host Weyl fermions as emergent quasiparticles. In this Rapid Communication, we propose a new type of Weyl semimetal state that breaks both time-reversal symmetry and inversion symmetry in the $R\mathrm{AlGe}$ ($R=\mathrm{rare}\ensuremath{-}\mathrm{earth}$) family. Compared to previous predictions of magnetic Weyl semimetal candidates, the prediction of Weyl nodes in $R\mathrm{AlGe}$ is more robust and less dependent on the details of the magnetism because the Weyl nodes are generated already by the inversion breaking and the ferromagnetism acts as a simple Zeeman coupling that shifts the Weyl nodes in $k$ space. Moreover, $R\mathrm{AlGe}$ offers remarkable tunability, which covers all varieties of Weyl semimetals including type I, type II, inversion breaking, and time-reversal breaking, depending on a suitable choice of the rare-earth elements. Furthermore, the unique noncentrosymmetric and ferromagnetic Weyl semimetal state in $R\mathrm{AlGe}$ enables the generation of spin currents.

Investigations of two-dimensional transition-metal chalcogenides (TMCs) have recently revealed interesting physical phenomena, including the quantum spin Hall effect1,2, valley polarization3,4 and two-dimensional superconductivity5, suggesting potential applications for functional devices6,7,8,9,10. However, of the numerous compounds available, only a handful, such as Mo- and W-based TMCs, have been synthesized, typically via sulfurization11,12,13,14,15, selenization16,17 and tellurization18 of metals and metal compounds. Many TMCs are difficult to produce because of the high melting points of their metal and metal oxide precursors. Molten-salt-assisted methods have been used to produce ceramic powders at relatively low temperature19 and this approach20 was recently employed to facilitate the growth of monolayer WS2 and WSe2. Here we demonstrate that molten-salt-assisted chemical vapour deposition can be broadly applied for the synthesis of a wide variety of two-dimensional (atomically thin) TMCs. We synthesized 47 compounds, including 32 binary compounds (based on the transition metals Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Pt, Pd and Fe), 13 alloys (including 11 ternary, one quaternary and one quinary), and two heterostructured compounds. We elaborate how the salt decreases the melting point of the reactants and facilitates the formation of intermediate products, increasing the overall reaction rate. Most of the synthesized materials in our library are useful, as supported by evidence of superconductivity in our monolayer NbSe2 and MoTe2 samples21,22 and of high mobilities in MoS2 and ReS2. Although the quality of some of the materials still requires development, our work opens up opportunities for studying the properties and potential application of a wide variety of two-dimensional TMCs. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

According to the single vector hydrophone positioning theory, some experiments were done in the pool. The noncoherent sources positioning and coherent sources positioning were tested respectively. The experimental results agree with the theoretical results, which validate the feasibility of the academic and provides experimental basis for engineering application.

The recent proposal of the type-II Weyl semimetal state has attracted significant interest. In this Letter, we propose the concept of the three-dimensional type-II Dirac fermion and theoretically identify this new symmetry-protected topological state in the large family of transition-metal icosagenides, MA_{3} (M=V, Nb, Ta; A=Al, Ga, In). We show that the VAl_{3} family features a pair of strongly Lorentz-violating type-II Dirac nodes and that each Dirac node can be split into four type-II Weyl nodes with chiral charge ±1 via symmetry breaking. Furthermore, we predict that the Landau level spectrum arising from the type-II Dirac fermions in VAl_{3} is distinct from that of known Dirac or Weyl semimetals. We also demonstrate a topological phase transition from a type-II Dirac semimetal to a quadratic Weyl semimetal or a topological crystalline insulator via crystalline distortions.

Charge density wave (CDW) phases are symmetry-reduced states of matter in which a periodic modulation of the electronic charge frequently leads to drastic changes of the electronic spectrum, including the emergence of energy gaps. We analyze the CDW state in a 1T-TiSe$_2$ monolayer within a density functional theory framework and show that, similarly to its bulk counterpart, the monolayer is unstable towards a commensurate $2{\times}2$ periodic lattice distortion (PLD) and CDW at low temperatures. Analysis of the electron and phonon spectrum establishes the PLD as the stable $T=0$ K configuration with a narrow bandgap, whereas the undistorted and semi-metalic state is stable only above a threshold temperature. The lattice distortions as well as the unfolded and reconstructed band structure in the CDW phase agree well with experimental results. We also address evidence in our results for the role of electron-electron interactions in the CDW instability of 1T-TiSe$_2$ monolayers., Comment: 5 pages, 4 figures

We introduce the first multiorbital effective tight-binding model to describe the effect of electron-electron interactions in this system. Upon fixing all the effective hopping parameters in the normal state against an ab initio band structure, and with only the overall scale of the interactions as the sole adjustable parameter, we find that a self-consistent Hartree-Fock solution reproduces extremely well the experimental behavior of the charge density wave (CDW) order parameter in the full range $0lTl{T}_{c}$, as well as the precise reciprocal space locations of the partial gap opening and Fermi arc development. The interaction strengths extracted from fitting to the experimental CDW gap are consistent with those derived from an independent Stoner-type analysis.

Topological metals and semimetals (TMs) have recently drawn significant interest. These materials give rise to condensed matter realizations of many important concepts in high-energy physics, leading to wide-ranging protected properties in transport and spectroscopic experiments. The most studied TMs, i.e., Weyl and Dirac semimetals, feature quasiparticles that are direct analogues of the textbook elementary particles. Moreover, the TMs known so far can be characterized based on the dimensionality of the band crossing. While Weyl and Dirac semimetals feature zero-dimensional points, the band crossing of nodal-line semimetals forms a one-dimensional closed loop. In this paper, we identify a TM which breaks the above paradigms. Firstly, the TM features triply-degenerate band crossing in a symmorphic lattice, hence realizing emergent fermionic quasiparticles not present in quantum field theory. Secondly, the band crossing is neither 0D nor 1D. Instead, it consists of two isolated triply-degenerate nodes interconnected by multi-segments of lines with two-fold degeneracy. We present materials candidates. We further show that triplydegenerate band crossings in symmorphic crystals give rise to a Landau level spectrum distinct from the known TMs, suggesting novel magneto-transport responses. Our results open the door for realizing new topological phenomena and fermions including transport anomalies and spectroscopic responses in metallic crystals with nontrivial topology beyond the Weyl/Dirac paradigm.

Topological semimetals (TSMs) including Weyl semimetals and nodal-line semimetals are expected to open the next frontier of condensed matter and materials science. Although the first inversion breaking Weyl semimetal was recently discovered in TaAs, its magnetic counterparts, i.e., the time-reversal breaking Weyl and nodal line semimetals, remain elusive. They are predicted to exhibit exotic properties distinct from the inversion breaking TSMs including TaAs. In this paper, we identify the magnetic topological semimetal states in the ferromagnetic half-metal compounds Co2TiX (X = Si, Ge, or Sn) with Curie temperatures higher than 350 K. Our first-principles band structure calculations show that, in the absence of spin-orbit coupling, Co2TiX features three topological nodal lines. The inclusion of spin-orbit coupling gives rise to Weyl nodes, whose momentum space locations can be controlled as a function of the magnetization direction. Our results not only open the door for the experimental realization of topological semimetal states in magnetic materials at room temperature, but also suggest potential applications such as unusual anomalous Hall effect in engineered monolayers of the Co2TiX compounds at high temperature.

Due to the intriguing optical and electronic properties, 2D materials have attracted a lot of interest for the electronic and optoelectronic applications. Identifying new promising 2D materials will be rewarding toward the development of next generation 2D electronics. Here, palladium diselenide (PdSe2), a noble-transition metal dichalcogenide (TMDC), is introduced as a promising high mobility 2D material into the fast growing 2D community. Field-effect transistors (FETs) based on ultrathin PdSe2 show intrinsic ambipolar characteristic. The polarity of the FET can be tuned. After vacuum annealing, the authors find PdSe2 to exhibit electron-dominated transport with high mobility (µe (max) = 216 cm2 V−1 s−1) and on/off ratio up to 103. Hole-dominated-transport PdSe2 can be obtained by molecular doping using F4-TCNQ. This pioneer work on PdSe2 will spark interests in the less explored regime of noble-TMDCs. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

Surface plasmon resonance (SPR)-mediated photocatalysis without the bandgap limitations of traditional semiconductor has aroused significant attention in solar-to-chemical energy conversion. However, the photocatalytic efficiency barely initiated by the SPR effects is still challenged by the low concentration and ineffective extraction of energetic hot electrons, slow charge migration rates, random charge diffusion directions, and the lack of highly active sites for redox reactions. Here, the tunable, progressive harvesting of visible-to-near infrared light (vis-NIR, λ > 570 nm) by designing plasmonic Au nanorods and metal (Au, Ag, or Pt) nanoparticle codecorated 1D CdS nanowire (1D CdS NW) ensemble is reported. The intimate integration of these metal nanostructures with 1D CdS NWs promotes the extraction and manipulated directional separation and migration of hot charge carriers in a more effective manner. Such cooperative synergy with tunable control of interfacial interaction, morphology optimization, and cocatalyst strategy results in the distinctly boosted performance for vis-NIR-driven plasmonic photocatalysis. This work highlights the significance of rationally progressive design of plasmonic metal-semiconductor-based composite system for boosting the regulated directional flow of hot charge carrier and thus the more efficient use of broad-spectrum solar energy conversion.

Atomically thin transitional metal ditellurides like WTe2 and MoTe2 have triggered tremendous research interests because of their intrinsic nontrivial band structure. They are also predicted to be 2D topological insulators and type-II Weyl semimetals. However, most of the studies on ditelluride atomic layers so far rely on the low-yield and time-consuming mechanical exfoliation method. Direct synthesis of large-scale monolayer ditellurides has not yet been achieved. Here, using the chemical vapor deposition (CVD) method, we demonstrate controlled synthesis of high-quality and atom-thin tellurides with lateral size over 300 {\mu}m. We found that the as-grown WTe2 maintains two different stacking sequences in the bilayer, where the atomic structure of the stacking boundary is revealed by scanning transmission electron microscope (STEM). The low-temperature transport measurements revealed a novel semimetal-to-insulator transition in WTe2 layers and an enhanced superconductivity in few-layer MoTe2. This work paves the way to the synthesis of atom-thin tellurides and also quantum spin Hall devices.

We theoretically investigate the development of unconventional superconductivity in the Sb(111) thin film when its Fermi level is tuned to near type-II Van Hove singularities (VHS), which locate at non-time-reversal invariant momenta. Via patch renormalization group analysis, we show that the leading instability is a chiral p + ip-wave superconducting order. The origin of such pairing relies on the hexagonal structure of the VHS and strong spin-orbit coupling, resulting in the anisotropy of the electron-electron scattering to provide an attractive channel. Our study hence suggests that superconducting Sb thin films originated from VHS physics may host Majorana zero modes in the magnetic vortices and provides another application perspective to such material., Comment: 7 pages, 5 figures, accepted by Physical Review B

The recent discovery of the first Weyl semimetal in TaAs provides the first observation of a Weyl fermion in nature. Such a topological semimetal features a novel type of anomalous surface state, the Fermi arc, which connects a pair of Weyl nodes through the boundary of the crystal. Here, we present theoretical calculations of the quasiparticle interference (QPI) patterns that arise from the surface states including the topological Fermi arcs in the Weyl semimetals TaAs and NbP. Most importantly, we discover that the QPI exhibits termination points that are fingerprints of the Weyl nodes in the interference pattern. Our results, for the first time, propose a universal interference signature of the topological Fermi arcs in TaAs, which is fundamental for scanning tunneling microscope (STM) measurements on this prototypical Weyl semimetal compound. More generally, our work provides critical guideline and methodology for STM studies on new Weyl semimetals. Further, the scattering channels revealed by our QPIs are broadly relevant to surface transport and device applications based on Weyl semimetals.

We predict planar Sb/Bi honeycomb to harbor a two-dimensional (2D) topological crystalline insulator (TCI) phase based on first-principles computations. Although buckled Sb and Bi honeycombs support 2D topological insulator (TI) phases, their structure becomes planar under tensile strain. The planar Sb/Bi honeycomb structure restores the mirror symmetry and is shown to exhibit non-zero mirror Chern numbers, indicating that the system can host topologically protected edge states. Our computations show that the electronic spectrum of a planar Sb/Bi nanoribbon with armchair or zigzag edges contains two Dirac cones within the band gap and an even number of edge bands crossing the Fermi level. Lattice constant of the planar Sb honeycomb is found to nearly match that of hexagonal-BN. The Sb nanoribbon on hexagonal-BN exhibits gapped edge states, which we show to be tunable by an out-of-the-plane electric field, providing controllable gating of edge state important for device applications.

Semimetal MoTe2 can be a type II Weyl semimetal in the bulk, but monolayer of this material is predicted to be quantum spin hall insulators. This dramatic change in electronic properties with number of layers is an excellent example of the dimensional effects of quantum transport. However, a detailed experimental study of the carrier transport and band structure of ultrathin semimetal MoTe2 is lacking so far. We performed magneto-transport measurements to study the conduction behavior and quantum phase coherence of 1T' MoTe2 as a function of its thickness. We show that due to a unique two-band transport mechanism (synergetic contribution from electron conduction and hole conduction), the conduction behavior of 1T' MoTe2 changes from metallic to p-type unipolar, and finally to ambipolar as the thickness decreases, suggesting that this effect can be used in devices by effectively controlling the thickness. Our transport studies, optical measurements and first-principles electronic structure calculations reveal that 1T' MoTe2 remains gapless down to a few (~2–3) layers. Despite being gapless, 1T' MoTe2 exhibits metal-insulator transition at 3-layer thickness, due to enhanced carrier localization effect.

The Weyl semimetal WTe2 and MoTe2 show great potential in generating large spin currents since they possess topologically protected spin-polarized states and can carry a very large current density. In addition, the intrinsic non-centrosymmetry of WTe2 and MoTe2 endows with a unique property of crystal symmetry-controlled spin-orbit torques. An important question to be answered for developing spintronic devices is how spins relax in WTe2 and MoTe2. Here, a room-temperature spin relaxation time of 1.2 ns (0.4 ns) in WTe2 (MoTe2) thin film using the time-resolved Kerr rotation (TRKR) is reported. Based on ab initio calculation, a mechanism of long-lived spin polarization resulting from a large spin splitting around the bottom of the conduction band, low electron-hole recombination rate, and suppression of backscattering required by time-reversal and lattice symmetry operation is identified. In addition, it is found that the spin polarization is firmly pinned along the strong internal out-of-plane magnetic field induced by large spin splitting. This work provides an insight into the physical origin of long-lived spin polarization in Weyl semimetals, which could be useful to manipulate spins for a long time at room temperature.

A Weyl semimetal is a new state of matter that host Weyl fermions as quasiparticle excitations. The Weyl fermions at zero energy correspond to points of bulk band degeneracy, Weyl nodes, which are separated in momentum space and are connected only through the crystal's boundary by an exotic Fermi arc surface state. We experimentally measure the spin polarization of the Fermi arcs in the first experimentally discovered Weyl semimetal TaAs. Our spin data, for the first time, reveal that the Fermi arcs' spin polarization magnitude is as large as 80% and possesses a spin texture that is completely in-plane. Moreover, we demonstrate that the chirality of the Weyl nodes in TaAs cannot be inferred by the spin texture of the Fermi arcs. The observed non-degenerate property of the Fermi arcs is important for the establishment of its exact topological nature, which reveal that spins on the arc form a novel type of 2D matter. Additionally, the nearly full spin polarization we observed (~80%) may be useful in spintronic applications., 4 figures, 15 pages

Weyl nodes are topological objects in three-dimensional metals. Their topological property can be revealed by studying the high-field transport properties of a Weyl semimetal. While the energy of the lowest Landau band (LLB) of a conventional Fermi pocket always increases with magnetic field due to the zero point energy, the LLB of Weyl cones remains at zero energy unless a strong magnetic field couples the Weyl fermions of opposite chirality. In the Weyl semimetal TaP, we achieve such a magnetic coupling between the electron-like Fermi pockets arising from the W1 Weyl fermions. As a result, their LLBs move above chemical potential, leading to a sharp sign reversal in the Hall resistivity at a specific magnetic field corresponding to the W1 Weyl node separation. By contrast, despite having almost identical carrier density, the annihilation is unobserved for the hole-like pockets because the W2 Weyl nodes are much further separated. These key findings, corroborated by other systematic analyses, reveal the nontrivial topology of Weyl fermions in high-field measurements., the Fig2 and Fig 3 are not compatible with this version

22 p. A metal-semiconductor phase transition in a ternary transition metal dichalcogenide (TMD) monolayer is achieved by alloying Te into WSe2 (WSe2(1−x)Te2x, where x equals 0% –100%). The optical bandgaps of the WSe2(1−x)Te2x monolayer can be tuned from 1.67 to 1.44 eV (2H semiconductor) and drops to 0 eV (1Td metal), which opens up an exciting opportunity in functional electronic/optoelectronic devices. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version