Valleytronics. The valley Hall effect in MoS₂ transistors

Tailoring Valley Polarization of Interlayer Excitons in van der Waals Heterostructure toward Optical Communication

Controlling the valley polarization of interlayer excitons in van der Waals heterostructures is essential for developing valleytronic devices. Here, we demonstrate the manipulation of valley polarization of interlayer excitons in a monolayer WSe2/WS2 heterostructure using a dual-handedness metasurface. This metasurface, composed of left- and right-handed gold nanoantenna arrays, exhibits distinct chiral optical responses. Introducing the metasurface into heterostructures significantly enhances the degree of interlayer exciton valley polarization, which quantitatively corresponds to the circular dichroism (CD) of interlayer excitons, via chiral Purcell effects. A negative CD of -16% is yielded via the left-handed metasurface, and a positive CD of +25% is harvested via the right-handed metasurface under σ+ excitation of 532 nm at 77 K. Moreover, the CD is tunable with excitation wavelength, reaching a maximum of 38% at 620 nm under σ- excitation, the highest value from interlayer excitons reported so far without applying external fields. Further, we demonstrate that this platform enables direct and byte-invert arithmetic operations for information transmission. This capability highlights its potential for error detection in valleytronics-based optical communication systems.

Topological acoustofluidics

The complex interaction of spin, valley and lattice degrees of freedom allows natural materials to create exotic topological phenomena. The interplay between topological wave materials and hydrodynamics could offer promising opportunities for visualizing topological physics and manipulating bioparticle unconventionally. Here we present topological acoustofluidic chips to illustrate the complex interaction between elastic valley spin and nonlinear fluid dynamics. We created valley streaming vortices and chiral swirling patterns for backward-immune particle transport. Using tracer particles, we observed arrays of clockwise and anticlockwise valley vortices due to an increase in elastic spin density. Moreover, we discovered exotic topological pressure wells in fluids, creating nanoscale trapping fields for manipulating DNA molecules. We also found a 93.2% modulation in the bandwidth of edge states, dependent on the orientation of the substrate's crystallographic structure. Our study sets the stage for uncovering topological acoustofluidic phenomena and visualizing elastic valley spin, revealing the potential for topological-material applications in life sciences.

Spectra-Orthogonal Optical Anisotropy in Wafer-Scale Molecular Crystal Monolayers

Controlling the spectral and polarization response of two-dimensional (2D) crystals is vital for developing ultrathin platforms for compact optoelectronic devices. However, independently tuning optical anisotropy and spectral response remains challenging in conventional semiconductors due to the intertwined nature of their lattice and electronic structures. Here, we report spectra-orthogonal optical anisotropy─where polarization anisotropy is tuned independently of spectral response─in wafer-scale, one-atom-thick 2D molecular crystal (2DMC) monolayers synthesized on monolayer transition-metal dichalcogenide (TMD) crystals. Utilizing the concomitant spectral consistency and structural tunability of perylene derivatives, we demonstrate tunable optical polarization anisotropy in 2DMCs with similar spectral profiles, as confirmed by room-temperature scanning tunneling microscopy and cross-polarized reflectance microscopy. Additional angle-dependent analysis of the single-crystal and polycrystalline molecular domains reveals an epitaxial relationship between the 2DMC and TMD. Our results establish a scalable, molecule-based 2D crystalline platform for unique and tunable functionalities unattainable in covalent 2D solids.

Acoustic Valley Filter, Valve, and Diverter

The discovery of valley degrees of freedom in electronic and classical waves opened the field of valleytronics and offered the prospect for new devices based on valleys. However, the implementation of valley-based devices remains challenging in practice. Here, by taking advantage of the flexibility of phononic crystals in design and fabrication, the realizations of valley devices, or filters, valves, and diverters for acoustic waves are reported. All the devices are configured as the structures of input and output ports bridged by channels. The phononic crystals serving as ports allow the propagation of both valley polarizations, whereas the phononic crystals serving as channels, as they are narrow, only allow the propagation of single polarizations. For valley filters that achieve valley-polarized currents, the bridge channel is simply a straight single phononic crystal, but for valley valves that can turn off the valley-polarized currents, the channel consists of two parts, allowing the propagation of opposite valley polarizations. The valley diverters have one input port, and two output ports, and thus a branched channel, and the three parts in the channel allow the propagation of the same valley polarizations, so that the energy flow can be partitioned. The results may serve as a basis for developing advanced acoustic valley devices.

Chiral light detection with centrosymmetric-metamaterial-assisted valleytronics

The full-range, high-sensitivity and integratable detection of circularly polarized light (CPL) is critically important for quantum information processing, advanced imaging systems and optical sensing technologies. However, mainstream CPL detectors rely on chiral absorptive materials, and thus suffer from limited response wavelengths, low responsivity and poor discrimination ratios. Here we present a chiral light detector by utilizing valley materials to observe the spin angular momentum carried by chiral light. Delicately designed centrosymmetric metamaterials that can preserve the sign of optical spin angular momentum and greatly enhance its intensity in the near field are harnessed as a medium to inject polarized electrons into valley materials, which are then detected by the valley Hall effect. This enables high-sensitivity infrared CPL detection at room temperature by valleytronic transistors, and the detection wavelength is extended to the infrared. This approach opens pathways for chiral light detection and provides insights into potential applications of valleytronics in optoelectronic sensing.

Exciton-Polariton Valley Hall Effect in Monolayer Semiconductors on Plasmonic Metasurface

Excitons in monolayer transition metal dichalcogenides (TMDs) possess the valley degree of freedom (DOF), which is regarded as a pseudospin (in addition to charge and spin DOF) and can be addressed optically by using polarized light. Incorporating monolayer TMDs into an optical microcavity in the strong coupling regime further enables the formation of valley polaritons that are half-light and half-matter quasiparticles with addressable spin and momentum through the spin-orbit interactions of light, in analogy with the spin-Hall effect in electronic systems. By placing monolayer TMDs on a plasmonic metasurface to enable strong coupling between excitons and surface plasmon polaritons (SPPs), we report here the observation of valley resolved polaritons in momentum space and a large separation in real space. The directional coupling of valley polaritons originated from the intrinsic spin-momentum locking associated with SPPs, resembling a photonic version of the valley Hall effect for polaritons. The spatially routed valley polaritons provide a unique pathway for transporting and detecting the valley DOF through circular polarization of light for valleytronic applications.

Excitonic circular dichroism in boron-nitrogen cluster decorated graphene

Using the first principle calculations, we propose a boron and nitrogen cluster incorporated graphene system for efficient valley polarization. The broken spatial inversion symmetry results in high Berry curvature at K and K' valleys of the hexagonal Brillouin zone in this semiconducting system. The consideration of excitonic quasiparticles within the GW approximation along with their scattering processes using the many-body Bethe-Salpeter equation gives rise to an optical gap of 1.72 eV with an excitonic binding energy of 0.65 eV. Owing to the negligible intervalley scattering, the electrons in opposite valleys are selectively excited by left- and right-handed circularly polarized light, as evident from the oscillator strength calculations. Therefore, this system can exhibit the circular-dichroism valley Hall effect in the presence of an in-plane electric field. Moreover, such excitonic qubits can be exploited for information processing.

van Hove Singularity-Induced Non-Equilibrium Anomalous Valley Hall Effect in a Two-Dimensional Lattice

van Hove singularity and valley represent two fundamental phenomena in materials science and condensed matter physics, which have recently attracted considerable interest. Here, we propose that the interplay between van Hove singularity and valley can generate a previously unreported anomalous valley Hall effect (AVHE) in a two-dimensional (2D) lattice, termed the non-equilibrium AVHE, which is characterized by the non-equilibrium transverse accumulation of valley carriers from different valleys. The physics relates to van Hove singularity-induced breaking of time-reversal symmetry and the resulting valley polarization under carrier doping, which guarantees the unique properties of non-equilibrium valley carriers. Remarkably, in contrast to typical AVHE relying on intrinsic magnetic materials, the non-equilibrium AVHE is rooted in 2D nonmagnetic systems. Using first-principles calculations, we validate the proposed mechanism and the predicted phenomena in monolayer In2Se3, known to exhibit nonmagnetic and ferroelectric natures. These findings open an avenue for exploring unconventional AVHE in 2D nonmagnetic systems.

Valley-Contrasting Linear Dichroism and Excitonic Condensation in LaOBiS2

In conjunction with the first-principles calculations, we confirm the fascinating valley-selective linear dichroism and the possibility of excitonic condensation in multiatomic-layer LaOBiS2 of a tetragonal lattice by the effective tight-binding model and many-body perturbation theory. In LaOBiS2, which indicates a spontaneous valley polarization due to crystalline symmetry reduction rather than conventional time-reversal symmetry breaking, we unravel that the excitons in X and X' valleys exclusively coupled to x- and y-linearly polarized lights, respectively. In sharp contrast to coupled quantum wells and van der Waals architectures, a new type of spatially indirect exciton is identified in single-crystal LaOBiS2, manifesting itself as an engaging platform for investigating the excitonic Bose-Einstein condensation (BEC) and superfluidity. In light of large binding energy, large radius, and small effective mass of the exciton, the transition temperature for BEC and superfluidity achieves record-high values of 325.1 and 81.3 K, respectively.

Photonic Supercoupling in Silicon Topological Waveguides

Waveguide interconnect coupling control is essential for enhancing the chip density of photonic integrated circuits to incorporate a growing number of components. However, a critical engineering challenge is to achieve both strong waveguide isolation and efficient long-range coupling on a single chip. Here, a novel photonic supercoupling phenomenon is demonstrated for waveguide coupling over separation distances from a quarter to five wavelengths (λ), leveraging the tunable mode tails and the vortex energy flow in topological valley Hall system. A supercoupled integrated chip is developed, realizing a 91% coupling ratio and a -30 dB isolation over 2.8λ waveguide separations simultaneously. Supercoupled devices are further showcased including a waveguide-cavity system with 3.2λ excitation distance, and a waveguide directional supercoupler with a compact coupling area of nearly λ2/4, which outperform conventional devices. Supercoupling provides new degrees of freedom for optimizing coupling and isolation between photonic integrated components, facilitating new applications in on-chip sensing, lasing, and telecommunications.

Two-Dimensional Nonvolatile Valley Spin Valve

A spin valve represents a well-established device concept in magnetic memory technologies, whose functionality is determined by electron transmission, controlled by the relative alignment of magnetic moments of the two ferromagnetic layers. Recently, the advent of valleytronics has conceptualized a valley spin valve (VSV)─a device that utilizes the valley degree of freedom and spin-valley locking to achieve a similar valve effect without relying on magnetism. In this study, we propose a nonvolatile VSV (n-VSV) based on a two-dimensional (2D) ferroelectric semiconductor where resistance of n-VSV is controlled by a ferroelectric domain wall between two uniformly polarized domains. Focusing on the 1T″ phase of MoS2, which is known to be ferroelectric down to a monolayer and using density functional theory combined with quantum transport calculations, we demonstrate that switching between the uniformly polarized state and the state with oppositely polarized domains separated by a domain wall results in a resistance change of as high as 107. This giant VSV effect occurs due to transmission being strongly dependent on matching (mismatching) the valley-dependent spin polarization in the two domains with the same (opposite) ferroelectric polarization orientations, when the chemical potential of 1T″-MoS2 lies within the spin-split valleys. The proposed n-VSV can be employed as a functional device for high-performance nonvolatile valleytronics.

Valley-Related Multipiezo Effect in Altermagnet Monolayer V2STeO

The multipiezo effect realizes the coupling of strain with magnetism and electricity, which provides a new way of designing multifunctional devices. In this study, monolayer V2STeO is demonstrated to be an altermagnet semiconductor with a direct band gap of 0.41 eV. The spin splittings of monolayer V2STeO are as high as 1114 and 1257 meV at the valence and conduction bands, respectively. Moreover, a pair of energy degeneracy valleys appears at X and Y points in the first Brillouin zone. The valley polarization and reversion can be achieved by applying uniaxial strains along different directions, indicating a piezovalley effect. In addition, a net magnetization coupled with uniaxial strain and hole doping can be induced in monolayer V2STeO, presenting the piezomagnetic feature. Furthermore, due to the Janus structure, the inversion symmetry of monolayer V2STeO is naturally broken, resulting in the piezoelectric property. The integration of the altermagnet, piezovalley, piezomagnetic, and piezoelectric properties make monolayer V2STeO a promising candidate for multifunctional spintronic and valleytronic devices.

Two-Dimensional Transition Metal Dichalcogenides: A Theory and Simulation Perspective

Two-dimensional transition metal dichalcogenides (2D TMDs) are a promising class of functional materials for fundamental physics explorations and applications in next-generation electronics, catalysis, quantum technologies, and energy-related fields. Theory and simulations have played a pivotal role in recent advancements, from understanding physical properties and discovering new materials to elucidating synthesis processes and designing novel devices. The key has been developments in ab initio theory, deep learning, molecular dynamics, high-throughput computations, and multiscale methods. This review focuses on how theory and simulations have contributed to recent progress in 2D TMDs research, particularly in understanding properties of twisted moiré-based TMDs, predicting exotic quantum phases in TMD monolayers and heterostructures, understanding nucleation and growth processes in TMD synthesis, and comprehending electron transport and characteristics of different contacts in potential devices based on TMD heterostructures. The notable achievements provided by theory and simulations are highlighted, along with the challenges that need to be addressed. Although 2D TMDs have demonstrated potential and prototype devices have been created, we conclude by highlighting research areas that demand the most attention and how theory and simulation might address them and aid in attaining the true potential of 2D TMDs toward commercial device realizations.

Edge-Energy-Driven Growth of Monolayer MnI2 Islands on Ag(111): High-Resolution Imaging and Theoretical Analysis

The reduced dimensionality of thin transition metal dihalide films on single-crystal surfaces unlocks a diverse range of magnetic and electronic properties. However, achieving stoichiometric monolayer islands requires precise control over the growth conditions. In this study, we employ scanning probe microscopy to investigate the growth of MnI2 on Ag(111) via single-crucible evaporation. The catalytic properties of the Ag(111) surface facilitate MnI2 dehalogenation, leading to the formation of a reconstructed iodine adlayer that acts as a buffer layer for the growth of truncated hexagonal MnI2 islands. These islands exhibit alternating edge lengths and distinct Kelvin potentials, as revealed by Kelvin probe force microscopy. Density functional theory (DFT) calculations support the experimentally observed island heights and lattice parameters and provide insights into the formation energies of both pristine and reconstructed edges. The asymmetry in edge lengths is attributed to differences in edge formation energies, driven by the position (up or down) of edge iodine atoms, as confirmed by DFT. This structural difference accounts for the observed variation in the Kelvin potential between the two types of island edge terminations.

High transmission in 120-degree sharp bends of inversion-symmetric and inversion-asymmetric photonic crystal waveguides

Bending loss is one of the serious problems for constructing nanophotonic integrated circuits. Recently, many works reported that valley photonic crystals (VPhCs) enable significantly high transmission via 120-degree sharp bends. However, it is unclear whether the high bend-transmission results directly from the valley-photonic effects, which are based on the breaking of inversion symmetry. In this study, we conduct a series of comparative numerical and experimental investigations of bend-transmission in various triangular PhCs with and without inversion symmetry and reveal that the high bend-transmission is solely determined by the domain-wall configuration and independent of the existence of the inversion symmetry. Preliminary analysis of the polarization distribution indicates that high bend-transmissions are closely related to the appearance of local topological polarization singularities near the bending section. Our work demonstrates that high transmission can be achieved in a much wider family of PhC waveguides, which may provide novel designs for low-loss nanophotonic integrated circuits with enhanced flexibility and a new understanding of the nature of valley-photonics.

Wearable Biodevices Based on Two-Dimensional Materials: From Flexible Sensors to Smart Integrated Systems

The proliferation of wearable biodevices has boosted the development of soft, innovative, and multifunctional materials for human health monitoring. The integration of wearable sensors with intelligent systems is an overwhelming tendency, providing powerful tools for remote health monitoring and personal health management. Among many candidates, two-dimensional (2D) materials stand out due to several exotic mechanical, electrical, optical, and chemical properties that can be efficiently integrated into atomic-thin films. While previous reviews on 2D materials for biodevices primarily focus on conventional configurations and materials like graphene, the rapid development of new 2D materials with exotic properties has opened up novel applications, particularly in smart interaction and integrated functionalities. This review aims to consolidate recent progress, highlight the unique advantages of 2D materials, and guide future research by discussing existing challenges and opportunities in applying 2D materials for smart wearable biodevices. We begin with an in-depth analysis of the advantages, sensing mechanisms, and potential applications of 2D materials in wearable biodevice fabrication. Following this, we systematically discuss state-of-the-art biodevices based on 2D materials for monitoring various physiological signals within the human body. Special attention is given to showcasing the integration of multi-functionality in 2D smart devices, mainly including self-power supply, integrated diagnosis/treatment, and human-machine interaction. Finally, the review concludes with a concise summary of existing challenges and prospective solutions concerning the utilization of 2D materials for advanced biodevices.

Quantum Anomalous Layer Hall Effect in Realistic van der Waals Heterobilayers

The quantum anomalous layer Hall effect (QALHE), characterized by the precise control of the quantum anomalous Hall effect on different layers due to spin-layer-chirality coupling in van der Waals (vdW) layered materials, is of great importance in both fundamental physics and nanodevices. In this work, through the analysis of a low-energy effective model for vdW heterobilayers under biaxial strain, we propose the QALHE in valleytronic materials for the first time. The spin-layer-locked edge states and Chern numbers in heterobilayers give rise to dissipationless currents localized in specific layers, realizing the long-sought QALHE in heterobilayers. The switch of the chirality of edge states and Chern numbers in heterobilayer systems can be achieved by applying a biaxial strain. We have validated this mechanism in a series of realistic valleytronic materials, including VSi2N4/VSiCN4 and RuCl2/FeCl2 heterobilayers. Our work reveals a new mechanism for achieving the QALHE with promising applications in spintronics and quantum layertronics.

Two-Periodic MoS2-Type Metal-Organic Frameworks with Intrinsic Intralayer Porosity for High-Capacity Water Sorption

2D metal-organic frameworks (2D-MOFs) are an important class of functional porous materials. However, the low porosity and surface area of 2D-MOFs have greatly limited their functionalities and applications. Herein, the rational synthesis of a class of mos-MOFs with molybdenum disulfide (mos) net based on the assembly of trinuclear metal clusters and 3-connected tripodal organic ligands is reported. The non-crystallographic (3,6)-connected mos net, different from the 3-connected hcb net of graphene, offers abundant intralayer voids courtesy of the split of one node into two. Indeed, mos-MOFs exhibit high apparent Brunauer-Emmett-Teller surface areas, significantly superior to those of other 2D-MOF analogs. Markedly, hydrolytically stable Cr-mos-MOF-1 displays an impressive water vapor uptake of 0.75 g g-1 at 298 K and P/P0 = 0.9, among the highest in 2D-MOFs. The combined water adsorption and X-ray diffraction study reveal the water adsorption mechanisms, suggesting the importance of intralayer porosities of mos-MOFs for high-performance water capture. This study paves the way for a reliable approach to synthesizing 2D-MOFs with high porosity and surface areas for diverse applications.

Coherent optical spin Hall transport for polaritonics at room temperature

Spin or valley degrees of freedom hold promise for next-generation spintronics. Nonetheless, the macroscopic coherent spin current formations are still hindered by rapid dephasing due to electron scattering, specifically at room temperature. Exciton polaritons offer excellent platforms for spin-optronic devices via the optical spin Hall effect. However, this effect could neither be unequivocally observed at room temperature nor be exploited for practical spintronic devices due to the presence of strong thermal fluctuations or large linear spin splitting. Here we report the observation of room-temperature optical spin Hall effect of exciton polaritons, with the spin current flow over 60 μm in a formamidinium lead bromide perovskite microcavity. We provide direct evidence of long-range coherence in the flow of polaritons and the spin current carried by them. Leveraging the spin Hall transport of polaritons, we further demonstrate two polaritonic devices, namely, a NOT gate and a spin-polarized beamsplitter, advancing the frontier of room-temperature polaritonics in perovskite microcavities.

Edge Dependence of Nonlocal Transport in Gapped Bilayer Graphene

The topological properties of gapped graphene have been explored for valleytronics applications. Prior transport experiments indicated their topological nature through large nonlocal resistance in Hall-bar devices, but the origin of this resistance was unclear. This study focused on dual-gate bilayer graphene (BLG) devices with naturally cleaved edges, examining how edge-etching with an oxygen plasma process affects electron transport. Before etching, local resistance at the charge neutral point increased exponentially with the displacement field and nonlocal resistance was well explained by ohmic contribution, which is typical of gapped BLG. After-etching, however, local resistance saturated with increasing displacement field, and nonlocal resistance deviated by 2 orders of magnitude from ohmic contribution. We suggest that these significant changes in local and nonlocal resistance arise from the formation of edge conducting pathways after the edge-etching, rather than from a topological property of gapped BLG that has been claimed in previous literatures.

Single layer TlX (X = Cl/Br/I) with a ferroelectric-valley coupling potential for an electrically tunable polarizer

Ferrovalley materials are generally hexagonal lattice systems with a ferromagnetism-valley coupling, in which the intrinsic ferromagnetism can induce valley polarization. However, as of now the number of ferrovalleys found is still limited. In this article, TlX (X = Cl/Br/I) single-layers (SLs) are proposed with a tetragonal lattice structure as well as ferroelectricity-valley coupling. Furthermore, they possess good mechanical and dynamic stability, and are expected to be synthesized in experiments. When applying an in-plane electric field or uniaxial strain, the SL-TlX (X = Cl/Br/I) show a robust valley polarization. Because the direction of polarization of SL-TlX (X = Cl/Br/I) can be controlled with a rapid reversal of the electric field direction, SL-TlX (X = Cl/Br/I) are potential valleytronic materials with an electrically controllable valley polarization. The SL-TlX (X = Cl/Br/I) have linear optical selection rules, different from those of traditional hexagonal lattice systems, giving them potential for manufacturing electrically controllable optical filters.

Boosting Light-Matter Interactions in Plasmonic Nanogaps

Plasmonic nanogaps in strongly coupled metal nanostructures can confine light to nanoscale regions, leading to huge electric field enhancement. This unique capability makes plasmonic nanogaps powerful platforms for boosting light-matter interactions, thereby enabling the rapid development of novel phenomena and applications. This review traces the progress of nanogap systems characterized by well-defined morphologies, controllable optical responses, and a focus on achieving extreme performance. The properties of plasmonic gap modes in far-field resonance and near-field enhancement are explored and a detailed comparative analysis of nanogap fabrication techniques down to sub-nanometer scales is provided, including bottom-up, top-down, and their combined approaches. Additionally, recent advancements and applications across various frontier research areas are highlighted, including surface-enhanced spectroscopy, plasmon-exciton strong coupling, nonlinear optics, optoelectronic devices, and other applications beyond photonics. Finally, the challenges and promising emerging directions in the field are discussed, such as light-driven atomic effects, molecular optomechanics, and alternative new materials.

Thermoelectric Transport Driven by the Hilbert-Schmidt Distance

The geometric characteristics of Bloch wavefunctions play crucial roles in the properties of electronic transport. Within the Boltzmann equation framework, we demonstrate that the thermoelectric performance of materials is significantly influenced by the Hilbert-Schmidt distance of Bloch wavefunctions. The connection between the distribution of quantum distance on the Fermi surface and the electronic transport scattering rate is established in the presence of magnetic and nonmagnetic impurities. The general formulation is applied to isotropic quadratic band-touching semimetals, where one can concentrate on the role of quantum geometric effects other than the Berry curvature. It is verified that the thermoelectric power factor can be succinctly expressed in terms of the maximum quantum distance, dmax. Specifically, when dmax reaches one, the power factor doubles compared to the case with trivial geometry (dmax = 0). These findings highlight the significance of quantum geometry in understanding and improving the performance of thermoelectric devices.

Ferrovalleytricity in a two-dimensional antiferromagnetic lattice

Control over and manipulation of valley physics via ferrovalleytricity is highly desirable for advancing valleytronics. Current research focuses primarily on two-dimensional ferromagnetic systems, while antiferromagnetic counterparts are seldom explored. Here, we present a general mechanism for extending the ferrovalleytricity paradigm to antiferromagnetic lattices to achieve spin control over valley physics. Our symmetry analysis and k·p model reveal that by introducing a Zeeman field aroused by the proximity effect, spin-switchable non-uniform potential is imposed on the two sublattices of an antiferromagnetic lattice. This enables spin control over the anomalous valley Hall effect, thereby realizing ferrovalleytricity. This mechanism is confirmed in a CrBr3-MnPSe3-CrBr3 heterotrilayer from first principles, where the spin-switchable non-uniform Zeeman effect is exerted on two Mn sublattices when the antiferromagnetic MnPSe3 layer is sandwiched between ferromagnetic CrBr3 layers. Such a non-uniform Zeeman effect combined with valley physics guarantees spin control over the anomalous valley Hall effect, i.e., ferrovalleytricity, in the MnPSe3 layer. Our work will shed light on potential applications of valley physics in antiferromagnetic systems.

Exploring the valleytronic, optical, and piezoelectric properties of Janus MoBXY2 (X = N, P; Y = S, Se, Te) monolayers for multifunctional applications

Inspired by recent studies on MoS2 and MoSi2N4, we propose and investigate Janus MoBXY2 (X = N, P; Y = S, Se, Te) monolayers, which exhibit robust dynamic and thermal stabilities. Our findings reveal that all these monolayers are semiconductors, with MoBNS2 and MoBPTe2 exhibiting direct band gaps at the K/K' points, resulting in degenerate valleys and significant valley spin splitting (VSS) in the valence band. Notably, Berry curvatures at K and K' points, with opposite signs, suggest potential for inducing the valley Hall effect (VHE). Furthermore, MoBNS2 and MoBPTe2 demonstrate pronounced optical absorption in the visible light region and high carrier mobility, especially MoBNS2 with a hole mobility reaching up to 2.0 × 103 cm2 V-1 s-1 along the zig-zag direction. Attributed to their Janus structure, these monolayers exhibit strong in-plane and out-of-plane piezoelectric responses, with d11 values ranging from 1.268 to 4.754 pm V-1 and d31 values ranging from 0.053 to 0.137 pm V-1. Investigation of strain effects highlights possibilities for indirect-to-direct band gap transitions and manipulation of band gaps and VSS. This study not only enriches the understanding of MoBXY2 monolayer properties but also suggests their promising applications in valleytronics, photovoltaics, and piezoelectric devices.

Valley manipulation by external fields in two-dimensional materials and their hybrid systems

Investigating two-dimensional (2D) valleytronic materials opens a new chapter in physics and facilitates the emergence of pioneering technologies. Nevertheless, this nascent field faces substantial challenges, primarily attributed to the inherent issue of valley energy degeneracy and the manipulation of valley properties. To break these constraints, the application of external fields has become pivotal for both generating and manipulating the valley properties of 2D systems. This paper takes a close look at the latest progress in modulating the valley properties of 2D valleytronic materials using external fields, covering a wide array of configurations from monolayers and bilayers to intricate heterostructures. We hope that this overview will inspire more exciting discoveries and significantly propel the evolution of valleytronics within the realm of 2D material research.

Tunable valley polarization and high Curie temperature in two-dimensional GdF2/WSe2 van der Waals heterojunctions

Two-dimensional (2D) van der Waals (vdW) heterojunctions have potential applications in spintronic devices owing to their unique electronic structure and properties. The 2D ferromagnetic material GdF2, formed by the rare earth element (Gd) with 4f electrons and fluorine, exhibits spontaneous valley polarization, perpendicular magnetic anisotropy and other excellent properties. Monolayer WSe2 has a similar structure to monolayer GdF2 and can be used to construct a vdW heterojunction. The heterojunction not only retains the original excellent properties but also generates new physical properties due to interfacial charge transfer and coupling. Therefore, this work investigates the electronic structure, magnetic anisotropy energy and Curie temperature (Tc) of the GdF2/WSe2 heterojunction. The GdF2/WSe2 heterojunction exhibits spontaneous valley polarization and can be modulated by biaxial strain. Additionally, valley polarization can be regulated by applying an external electric field and changing interface spacing. These results indicate that the GdF2/WSe2 heterojunction can be used as a promising platform for the study of spintronic and valleytronic devices and provide ideas for the development of new electronic devices.

Pressure-Dependent Shape and Edge Configurations of MoS2 by Kinetic Monte Carlo Simulation

Understanding the influence of precursor pressures is crucial for optimizing the properties of MoS2 grown through the chemical vapor deposition (CVD) process. In this study, we use kinetic Monte Carlo (KMC) simulations to investigate how varying the pressures of molybdenum (PMo) and sulfur (PS) impacts the structural properties of MoS2, such as grain shape and edge configurations. The simulations differentiate three distinct regimes─growth, steady-state, and etching─each defined by specific PMo, PS, and the most probable atomic sites for filling or etching. We further explore how these regimes influence the atomic configuration of MoS2, particularly the formation of different edge structures like sulfur zigzag (ZZS), molybdenum zigzag (ZZMo), and their respective derivatives. A pressure diagram based on the equations of state and most probable atomic sites was constructed for each regime and validated by comparing predicted ZZ-derived edges to experimental observations. Additionally, the study examines the impact of etching on various line defects, providing insights into the evolution of the MoS2 edges during the CVD process. These findings underscore the importance of controlling both growth and cessation phases in the CVD process to customize edge configurations, with significant implications for chemical functionalization, catalysis, and the electronic properties of transition metal dichalcogenides.

Indirect-To-Direct Bandgap Crossover and Room-Temperature Valley Polarization of Multilayer MoS2 Achieved by Electrochemical Intercalation

Monolayer (1L) group VI transition metal dichalcogenides (TMDs) exhibit broken inversion symmetry and strong spin-orbit coupling, offering promising applications in optoelectronics and valleytronics. Despite their direct bandgap, high absorption coefficient, and spin-valley locking in K or K' valleys, the ultra-short valley lifetime limits their room-temperature applications. In contrast, multilayer TMDs, with more absorptive layers, sacrifice the direct bandgap and valley polarization upon gaining inversion symmetry from the bilayer structure. Here, we demonstrate that multilayer molybdenum disulfide (MoS2) can maintain 1) a structure with broken inversion symmetry and strong spin-orbit coupling, 2) a direct bandgap with high photoluminescence (PL) intensity, and 3) stable valley polarization up to room temperature. Through the intercalation of organic 1-ethyl-3-methylimidazolium (EMIM+) ions, multilayer MoS2 not only exhibits layer decoupling but also benefits from an electron doping effect. This results in a hundredfold increase in PL intensity and stable valley polarization, achieving 55% and 16% degrees of valley polarization at 3 K and room temperature, respectively. The persistent valley polarization at room temperature, due to interlayer decoupling and trion dominance facilitated by a gate-free method, opens up potential applications in valley-selective optoelectronics and valley transistors.

Low-Defect-Density Monolayer MoS2 Wafer by Oxygen-Assisted Growth-Repair Strategy

Atomic chalcogen vacancy is the most commonly observed defect category in two dimensional (2D) transition-metal dichalcogenides, which can be detrimental to the intrinsic properties and device performance. Here a low-defect density, high-uniform, wafer-scale single crystal epitaxial technology by in situ oxygen-incorporated "growth-repair" strategy is reported. For the first time, the oxygen-repairing efficiency on MoS2 monolayers at atomic scale is quantitatively evaluated. The sulfur defect density is greatly reduced from (2.71 ± 0.65) × 1013 down to (4.28 ± 0.27) × 1012 cm-2, which is one order of magnitude lower than reported as-grown MoS2. Such prominent defect deduction is owing to the kinetically more favorable configuration of oxygen substitution and an increase in sulfur vacancy formation energy around oxygen-incorporated sites by the first-principle calculations. Furthermore, the sulfur vacancies induced donor defect states is largely eliminated confirmed by quenched defect-related emission. The devices exhibit improved carrier mobility by more than three times up to 65.2 cm2 V-1 s-1 and lower Schottky barrier height reduced by half (less than 20 meV), originating from the suppressed Fermi-level pinning effect from disorder-induced gap state. The work provides an effective route toward engineering the intrinsic defect density and electronic states through modulating synthesis kinetics of 2D materials.

Modified Martin-Puplett interferometer for magneto-optical Kerr effect measurements at sub-THz frequencies

We present a magneto-optical Kerr effect (MOKE) spectrometer based on a modified Martin-Puplett interferometer, utilizing continuous wave sub-THz low-power radiation in a broad frequency range. This spectrometer is capable of measuring the frequency dependence of the MOKE response function, both the Kerr rotation and ellipticity, simultaneously, with accuracy limited by a sub-milliradian threshold, without the need for a reference measurement. The instrument's versatility allows it to be coupled to a cryostat with optical windows, enabling studies of a variety of quantum materials such as unconventional superconductors, two-dimensional electron gas systems, quantum magnets, and other systems showing optical Hall response at sub-Kelvin temperatures and in high magnetic fields. We demonstrate the functionality of the MOKE spectrometer using an undoped InSb wafer as a test sample.

Circular photocurrents in centrosymmetric semiconductors with hidden spin polarization

Centrosymmetric materials with site inversion asymmetries possess hidden spin polarization, which remains challenging to be converted into spin currents because the global inversion symmetry is still conserved. This study demonstrates the spin-polarized circular photocurrents in centrosymmetric transition metal dichalcogenide semiconductors at normal incidence without applying electric bias. The global inversion symmetry is broken by using a spatially-varying circularly polarized light beam, which could generate spin gradient owing to the hidden spin polarization. The dependence of the circular photocurrents on electrode configuration, illumination position, and beam spot size indicates an emergence of circulating electric current under spatially inhomogeneous light, which is associated with the deflection of spin-polarized current through the inverse spin Hall effect. The circular photocurrents is subsequently utilized to probe the spin polarization and the inverse spin Hall effect under different excitation wavelengths and temperatures. The results of this study demonstrate the feasibility of using centrosymmetric materials with hidden spin polarization and non-vanishing Berry curvature for spintronic device applications.

Prediction of the two-dimensional ferromagnetic semiconductor Janus 2H-ZrTeI monolayer with large valley and piezoelectric polarizations

Two-dimensional room-temperature Janus ferrovalley semiconductors with valley polarization and piezoelectric polarization offer new perspectives for designing multifunctional nanodevices. Herein, using first-principles calculations, we predict that the Janus 2H-ZrTeI monolayer is an intrinsic ferromagnetic semiconductor with in-plane magnetic anisotropy and a Curie temperature of 111 K. The Janus ZrTeI monolayer possesses a significant valley polarization of 141 meV due to time-reversal and inversion symmetry breaking. Based on the valley-contrasting Berry curvature, the anomalous valley Hall effect can be observed under an in-plane electric field. Meanwhile, the breaking of the inversion symmetry and mirror symmetry results in large longitudinal and transverse piezoelectric coefficients. By applying biaxial strain, the Janus 2H-ZrTeI monolayer can also be transformed into a Weyl nodal line semimetal. Furthermore, bilayers of ZrTeI with AB and BA stacking configurations allow the coexistence of valley polarization and ferroelectricity, enabling the manipulation of magnetism, ferroelectric polarization, and valley polarization through interlayer sliding. Our work provides a platform for studying valley polarization, piezoelectricity, and multiferroic coupling, which is significant for the application of multifunctional devices.

Opto-twistronic Hall effect in a three-dimensional spiral lattice

Studies of moiré systems have explained the effect of superlattice modulations on their properties, demonstrating new correlated phases1. However, most experimental studies have focused on a few layers in two-dimensional systems. Extending twistronics to three dimensions, in which the twist extends into the third dimension, remains underexplored because of the challenges associated with the manual stacking of layers. Here we study three-dimensional twistronics using a self-assembled twisted spiral superlattice of multilayered WS2. Our findings show an opto-twistronic Hall effect driven by structural chirality and coherence length, modulated by the moiré potential of the spiral superlattice. This is an experimental manifestation of the noncommutative geometry of the system. We observe enhanced light-matter interactions and an altered dependence of the Hall coefficient on photon momentum. Our model suggests contributions from higher-order quantum geometric quantities to this observation, providing opportunities for designing quantum-materials-based optoelectronic lattices with large nonlinearities.

Easy-to-configure zero-dimensional valley-chiral modes in a graphene point junction

The valley degree of freedom in two-dimensional (2D) materials can be manipulated for low-dissipation quantum electronics called valleytronics. At the boundary between two regions of bilayer graphene with different atomic or electrostatic configuration, valley-polarized current has been realized. However, the demanding fabrication and operation requirements limit device reproducibility and scalability toward more advanced valleytronics circuits. We demonstrate a device architecture of a point junction where a valley-chiral 0D PN junction is easily configured, switchable, and capable of carrying valley current with an estimated polarization of ~80%. This work provides a building block in manipulating valley quantum numbers and scalable valleytronics.

Three-dimensional valley-contrasting sound

Spin and valley are two fundamental properties of electrons in crystals. The similarity between them is well understood in valley-contrasting physics established decades ago in two-dimensional (2D) materials like graphene-with broken inversion symmetry, the two valleys in graphene exhibit opposite orbital magnetic moments, similar to the spin-1/2 behaviors of electrons, and opposite Berry curvature that leads to a half topological charge. However, valley-contrasting physics has never been explored in 3D crystals. Here, we develop a 3D acoustic crystal exhibiting 3D valley-contrasting physics. Unlike spin that is fundamentally binary, valley in 3D can take six different values, each carrying a vortex in a distinct direction. The topological valley transport is generalized from the edge states of 2D materials to the surface states of 3D materials, with interesting features including robust propagation, topological refraction, and valley-cavity localization. Our results open a new route for wave manipulation in 3D space.

Large-Area Epitaxial Growth of Transition Metal Dichalcogenides

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.

Thermal Relaxation in Janus Transition Metal Dichalcogenide Bilayers

In this work, we employ molecular dynamics simulations with semi-empirical interatomic potentials to explore heat dissipation in Janus transition metal dichalcogenides (JTMDs). The middle atomic layer is composed of either molybdenum (Mo) or tungsten (W) atoms, and the top and bottom atomic layers consist of sulfur (S) and selenium (Se) atoms, respectively. Various nanomaterials have been investigated, including both pristine JTMDs and nanostructures incorporating inner triangular regions with a composition distinct from the outer bulk material. At the beginning of our simulations, a temperature gradient across the system is imposed by heating the central region to a high temperature while the surrounding area remains at room temperature. Once a steady state is reached, characterized by a constant energy flux, the temperature control in the central region is switched off. The heat attenuation is investigated by monitoring the characteristic relaxation time (τav) of the local temperature at the central region toward thermal equilibrium. We find that SMoSe JTMDs exhibit thermal attenuation similar to conventional TMDs (τav~10-15 ps). On the contrary, SWSe JTMDs feature relaxation times up to two times as high (τav~14-28 ps). Forming triangular lateral heterostructures in their surfaces leads to a significant slowdown in heat attenuation by up to about an order of magnitude (τav~100 ps).

Valley-selective carrier transfer in SnS-based van der Waals heterostructures

Valleytronics, i.e., use of the valley degree of freedom in semiconductors as an information carrier, is a promising alternative to conventional approaches for information processing. Transition metal dichalcogenides with degenerate K/K' valleys have received attention as prototype 2D/layered semiconductors for valleytronics, but these systems rely on exotic effects such as the valley-Hall effect for electrical readout of the valley occupancy. Non-traditional valleytronic systems hosting sets of addressable non-degenerate valleys could overcome this limitation. In the van der Waals semiconductor Sn(II) sulfide (SnS), for instance, different bandgaps and band edges may allow manipulating the population of the X- and Y-valleys via charge transfer across interfaces to other layered semiconductors. Here, we establish this concept by comparing SnS flakes and SnS-based heterostructures. Cathodoluminescence spectroscopy shows a striking reversal of the luminescence intensity of the two valleys in SnS-GeS van der Waals stacks, which stems from a selective electron transfer from the Y-valley into GeS while X-valley electrons remain confined to SnS. Our results suggest that non-traditional systems, embodied here by SnS-based van der Waals heterostructures, open avenues for valley-selective readout relying on design parameters such as heterostructure band offsets that are among the core concepts of semiconductor technology.

Strong light-matter coupling in van der Waals materials

In recent years, two-dimensional (2D) van der Waals materials have emerged as a focal point in materials research, drawing increasing attention due to their potential for isolating and synergistically combining diverse atomic layers. Atomically thin transition metal dichalcogenides (TMDs) are one of the most alluring van der Waals materials owing to their exceptional electronic and optical properties. The tightly bound excitons with giant oscillator strength render TMDs an ideal platform to investigate strong light-matter coupling when they are integrated with optical cavities, providing a wide range of possibilities for exploring novel polaritonic physics and devices. In this review, we focused on recent advances in TMD-based strong light-matter coupling. In the foremost position, we discuss the various optical structures strongly coupled to TMD materials, such as Fabry-Perot cavities, photonic crystals, and plasmonic nanocavities. We then present several intriguing properties and relevant device applications of TMD polaritons. In the end, we delineate promising future directions for the study of strong light-matter coupling in van der Waals materials.

Plasmonic Near-Infrared-Response S-Scheme ZnO/CuInS2 Photocatalyst for H2O2 Production Coupled with Glycerin Oxidation

Solar fuel synthesis is intriguing because solar energy is abundant and this method compensates for its intermittency. However, most photocatalysts can only absorb UV-to-visible light, while near-infrared (NIR) light remains unexploited. Surprisingly, the charge transfer between ZnO and CuInS2 quantum dots (QDs) can transform a NIR-inactive ZnO into a NIR-active composite. This strong response is attributed to the increased concentration of free charge carriers in the p-type semiconductor at the interface after the charge migration between ZnO and CuInS2, enhancing the localized surface plasmon resonance (LSPR) effect and the NIR response of CuInS2. As a paradigm, this ZnO/CuInS2 heterojunction is used for H2O2 production coupled with glycerin oxidation and demonstrates supreme performance, corroborating the importance of NIR response and efficient charge transfer. Mechanistic studies through contact potential difference (CPD), Hall effect test, and finite element method (FEM) calculation allow for the direct correlation between the NIR response and charge transfer. This approach bypasses the general light response issues, thereby stepping forward to the ambitious goal of harnessing the entire solar spectrum.

Tailoring of the polarization-resolved second harmonic generation in two-dimensional semiconductors

Second harmonic generation is a non-linear optical phenomenon in which coherent radiation with frequency ω interacts with a non-centrosymmetric material and produces coherent radiation at frequency 2ω. Owing to the exciting physical phenomena that take place during the non-linear optical excitation at the nanoscale, there is currently extensive research in the non-linear optical responses of nanomaterials, particularly in low-dimensional materials. Here, we review recent advancements in the polarization-resolved second harmonic generation propertied from atomically thin two-dimensional (2D) crystals and present a unified theoretical framework to account for their nonlinear optical response. Two major classes of 2D materials are particularly investigated, namely metal chalcogenides and perovskites. The first attempts to tune and control the second harmonic generation properties of such materials via the application of specific nanophotonic schemes are additionally demonstrated and discussed. Besides presenting recent advances in the field, this work also delineates existing limitations and highlights emerging possibilities and future prospects in this field.

Real-Time Observation for MoS2 Growth Kinetics and Mechanism Promoted by the Na Droplet

While the molten salt-catalyzed chemical vapor deposition (CVD) technique is recognized for its effectiveness in producing large-area transition metal chalcogenides, understanding their growth mechanisms involving alkali metals remains a challenge. Here, we investigate the kinetics and mechanism of sodium-catalyzed molybdenum disulfide (MoS2) growth and etching through image analysis conducted using an integrated CVD microscope. Sodium droplets, agglomerated via the thermal decomposition of the sodium cholate dispersant, catalyze the precipitation of supersaturated MoS2 laminates and induce growth despite fragmentation during this process. Triangular MoS2 crystals display a distinct self-exhausting exponential behavior and slow growth of thermodynamically favorable crystallographic faces, exhibiting a sulfur-dominant pressure. The growth and etching processes are facilitated by the scooting of sodium droplets along grain edges, displaying comparable rates. Leveraging these kinetics makes it possible to engineer atypical MoS2 shapes. This combined microscope not only enhances the understanding of growth mechanisms but also contributes to the facile development of next-generation nanomaterials.

Chiral absorption enhancement via critically coupled resonances in atomically thin photonic crystal exciton-polaritons

Atomically thin transition metal dichalcogenides (TMDS) offer a promising route to the scaling down of optoelectronic devices to the ultimate thickness limit. But the weak light-matter interaction caused by their atomically thin nature makes them inevitably rely on external photonic structures to enhance optical absorption. Here, we report chiral absorption enhancement in atomically thin tungsten diselenide (WSe2) using chiral resonances in photonic crystal (PhC) nanostructures patterned directly in WSe2 itself. We show that the quality factors (Q factors) of the resonances grow exponentially as the PhC thickness approaches atomic limit. As such, the strong interaction of high Q factor photonic resonance with the coexisting exciton resonance in WSe2 results into self-coupled exciton-polaritons. By balancing the light coupling and absorption rates, the incident light can critically couple to chiral resonances in WSe2 PhC exciton-polaritons, leading to the theoretically limited 50% optical absorptance with over 84% circular dichroism (CD).

Rules for dissipationless topotronics

Topological systems hosting gapless boundary states have attracted huge attention as promising components for next-generation information processing, attributed to their capacity for dissipationless electronics. Nevertheless, recent theoretical and experimental inquiries have revealed the emergence of energy dissipation in precisely quantized electrical transport. Here, we present a criterion for the realization of truly no-dissipation design, characterized as Nin = Ntunl + Nbs, where Nin, Ntunl, and Nbs represent the number of modes participating in injecting, tunneling, and backscattering processes, respectively. The key lies in matching the number of injecting, tunneling, and backscattering modes, ensuring the equilibrium among all engaged modes inside the device. Among all the topological materials, we advocate for the indispensability of Chern insulators exhibiting higher Chern numbers to achieve functional devices and uphold the no-dissipation rule simultaneously. Furthermore, we design the topological current divider and collector, evading dissipation upon fulfilling the established criterion. Our work paves the path for developing the prospective topotronics.

Chirality-Dependent Dynamic Evolution for Trions in Monolayer WS2

Monolayer transition metal dichalcogenides exhibit valley-dependent excitonic characters with a large binding energy, acting as the building block for future optoelectronic functionalities. Herein, combined with pump-probe ultrafast transient transmission spectroscopy and theoretical simulations, we reveal the chirality-dependent trion dynamics in h-BN encapsulated monolayer tungsten disulfide. By resonantly pumping trions in a single valley and monitoring their temporal evolution, we identify the temperature-dependent competition between two relaxation channels driven by chirality-dependent scattering processes. At room temperature, the phonon-assisted upconversion process predominates, converting excited trions to excitons within the same valley on a sub-picosecond (ps) time scale. As temperature decreases, this process becomes less efficient, while alternative channels, notably valley depolarization process for trions, assume importance, leading to an increase of trion density in the unpumped valley within a ps time scale. Our time-resolved valley-contrast results provide a comprehensive insight into trion dynamics in 2D materials, thereby advancing the development of novel valleytronic devices.

Spin Hall effect modulated by an electric field in asymmetric two-dimensional MoSiAs2Se

Spin current generation from charge current in nonmagnetic materials promises an energy-efficient scheme for manipulating magnetization in spintronic devices. In some asymmetric two-dimensional (2D) materials, the Rashba and valley effects coexist owing to strong spin-orbit coupling (SOC), which induces the spin Hall effect due to spin-momentum locking of both effects. Herein, we propose a new Janus structure MoSiAs2Se with both valley physics and the Rashba effect and reveal an effective way to modulate the properties of this structure. The results demonstrated that applying an external electric field is an effective means to modulating the electronic properties of MoSiAs2Se, leading to both type I-II phase transitions and semiconductor-metal phase transitions. Furthermore, the coexistence of the Rashba and valley effects in monolayer MoSiAs2Se contributes to the spin Hall effect (SHE). The magnitude and direction of spin Hall conductivity can also be manipulated with an out-of-plane electric field. Our results enrich the physics and materials of the Rashba and valley systems, opening new opportunities for the applications of 2D Janus materials in spintronic devices.

Ultrathin Boundary-Less SnO2 Films with Surface-Activated Two-Dimensional Nanograins Enable Fast and Sensitive Hydrogen Gas Sensing

Fast and reliable semiconductor hydrogen sensors are crucially important for the large-scale utilization of hydrogen energy. One major challenge that hinders their practical application is the elevated temperature required, arising from undesirable surface passivation and grain-boundary-dominated electron transportation in the conventional nanocrystalline sensing layers. To address this long-standing issue, in the present work, we report a class of highly reactive and boundary-less ultrathin SnO2 films, which are fabricated by the topochemical transformation of 2D SnO transferred from liquid Sn-Bi droplets. The ultrathin SnO2 films are purposely made to consist of well-crystallized quasi-2D nanograins with in-plane grain sizes going beyond 30 nm, whereby the hydroxyl adsorption and grain boundary side-effects are effectively suppressed, giving rise to an activated (101)-dominating dangling-bond surface and a surface-controlled electrical transportation with an exceptional electron mobility of 209 cm2 V-1 s-1. Our work provides a new cost-effective strategy to disruptively improve the gas reception and transduction of SnO2. The proposed chemiresistive sensors exhibit fast, sensitive, and selective hydrogen sensing performance at a much-reduced working temperature of 60 °C. The remarkable sensing performance as well as the simple and scalable fabrication process of the ultrathin SnO2 films render the thus-developed sensors attractive for long awaited practical applications in hydrogen-related industries.

Room-Temperature Geometrical Circular Photocurrent in Few-Layer MoS2

Valleytronics, i.e., the manipulation of the valley degree of freedom, offers a promising path for energy-efficient electronics. One of the key milestones in this field is the room-temperature manipulation of the valley information in thick-layered material. Using scanning photocurrent microscopy, we achieve this milestone by observing a geometrically dependent circular photocurrent in a few-layer molybdenum disulfide (MoS2) under normal incidence. Such an observation shows that the system symmetry is lower than that of bulk MoS2 material, preserving the optical chirality-valley correspondence. Moreover, the circular photocurrent polarity can be reversed by applying electrical bias. We propose a model where the observed photocurrent results from the symmetry breaking and the built-in field at the electrode-sample interface. Our results show that the valley information is still retained even in thick-layered MoS2 at room temperature and opens up new opportunities for exploiting the valley index through interface engineering in multilayer valleytronics devices.