Tailoring MoS2 Valley‐Polarized Photoluminescence with Super Chiral Near‐Field

Transition metal dichalcogenides with intrinsic spin–valley degrees of freedom hold great potentials for applications in spintronic and valleytronic devices. MoS₂ monolayer possesses two inequivalent valleys in the Brillouin zone, with each valley coupling selectively with circularly polarized photons. The degree of valley polarization (DVP) is a parameter to characterize the purity of valley‐polarized photoluminescence (PL) of MoS₂ monolayer. Usually, the detected values of DVP in MoS₂ monolayer show achiral property under optical excitation of opposite helicities due to reciprocal phonon‐assisted intervalley scattering process. Here, it is reported that valley‐polarized PL of MoS₂ can be tailored through near‐field interaction with plasmonic chiral metasurface. The resonant field of the chiral metasurface couples with valley‐polarized excitons, and tailors the measured PL spectra in the far‐field, resulting in observation of chiral DVP of MoS₂‐metasurface under opposite helicities excitations. Valley‐contrast PL in the chiral heterostructure is also observed when illuminated by linearly polarized light. The manipulation of valley‐polarized PL in 2D materials using chiral metasurface represents a viable route toward valley‐polaritonic devices.     

Synthesis and Broadband Photodetection of a P-Type 1D Van der Waals Semiconductor HfSnS3

1D van der Waals (vdW) materials have attracted significant interest in recent years due to their giant anisotropic and weak interlayer-coupled characters. More 1D vdW materials are urgently to be exploited for satisfying the practice requirement. Herein, the study of 1D vdW ternary HfSnS₃ high-quality single crystals grown via the chemical vapor transport technique is reported. The Raman vibration modes and band structure of HfSnS₃ are analyzed via DFT calculations. Its strong in-plane anisotropic is verified by the polarized Raman spectroscopy. The field-effect transistors (FETs) based on the HfSnS₃ nanowires demonstrate p-type semiconducting behavior as well as outstanding photoresponse in a broadband range from UV to near-infrared (NIR) with short response times of ≈0.355 ms, high responsivity of ≈11.5 A W⁻¹, detectivity of ≈8.2 × 10¹¹, external quantum efficiency of 2739%, excellent environmental stability, and repeatability. Furthermore, a typical photoconductivity effect of the photodetector is illustrated. These comprehensive characteristics can promote the application of the p-type 1D vdW material HfSnS₃ in optoelectronics.     

Birefringence‐Directed Raman Selection Rules in 2D Black Phosphorus Crystals

The incident and scattered light engaged in the Raman scattering process of low symmetry crystals always suffer from the birefringence‐induced depolarization. Therefore, for anisotropic crystals, the classical Raman selection rules should be corrected by taking the birefringence effect into consideration. The appearance of the 2D anisotropic materials provides an excellent platform to explore the birefringence‐directed Raman selection rules, due to its controllable thickness at the nanoscale that greatly simplifies the situation comparing with bulk materials. Herein, a theoretical and experimental investigation on the birefringence‐directed Raman selection rules in the anisotropic black phosphorus (BP) crystals is presented. The abnormal angle‐dependent polarized Raman scattering of the A_g modes in thin BP crystal, which deviates from the normal Raman selection rules, is successfully interpreted by the theoretical model based on birefringence. It is further confirmed by the examination of different Raman modes using different laser lines and BP samples of different thicknesses.     

3D Mesoporous van der Waals Heterostructures for Trifunctional Energy Electrocatalysis

The emergence of van der Waals (vdW) heterostructures of 2D materials has opened new avenues for fundamental scientific research and technological applications. However, the current concepts and strategies of material engineering lack feasibilities to comprehensively regulate the as‐obtained extrinsic physicochemical characters together with intrinsic properties and activities for optimal performances. A 3D mesoporous vdW heterostructure of graphene and nitrogen‐doped MoS₂ via a two‐step sequential chemical vapor deposition method is constructed. Such strategy is demonstrated to offer an all‐round engineering of 2D materials including the morphology, edge, defect, interface, and electronic structure, thereby leading to robustly modified properties and greatly enhanced electrochemical activities. The hydrogen evolution is substantially accelerated on MoS₂, while the oxygen reduction and evolution are significantly improved on graphene. This work provides a powerful overall engineering strategy of 2D materials for electrocatalysis, which is also enlightening for other nanomaterials and energy‐related applications.     

0.2 λ0 Thick Adaptive Retroreflector Made of Spin‐Locked Metasurface

The metasurface concept is employed to planarize retroflectors by stacking two metasurfaces with separation that is two orders larger than the wavelength. Here, a retroreflective metasurface using subwavelength‐thick reconfigurable C‐shaped resonators (RCRs) is reported, which reduces the overall thickness from the previous record of 590 λ₀ down to only 0.2 λ₀. The geometry of RCRs could be in situ controlled to realize equal amplitude and phase modulation onto transverse magnetic (TM)‐polarized and transverse electric (TE)‐polarized incidences. With the phase gradient being engineered, an in‐plane momentum could be imparted to the incident wave, guaranteeing the spin state of the retro‐reflected wave identical to that of the incident light. Such spin‐locked metasurface is natively adaptive toward different incident angles to realize retroreflection by mechanically altering the geometry of RCRs. As a proof of concept, an ultrathin retroreflective metasurface is validated at 15 GHz, under various illumination angles at 10°, 12°, 15°, and 20°. Such adaptive spin‐locked metasurface could find promising applications in spin‐based optical devices, communication systems, remote sensing, RCS enhancement, and so on.     

3R MoS2 with Broken Inversion Symmetry: A Promising Ultrathin Nonlinear Optical Device

Nonlinear 2D layered crystals provide ideal platforms for applications and fundamental studies in ultrathin nonlinear optical (NLO) devices. However, the NLO frequency conversion efficiency constrained by lattice symmetry is still limited by layer numbers of 2D crystals. In this work, 3R MoS₂ with broken inversion symmetry structure are grown and proved to be excellent NLO 2D crystals from monolayer (0.65 nm) toward bulk‐like (300 nm) dimension. Thickness and wavelength‐dependent second harmonic generation spectra offer the selection rules of appropriate working conditions. A model comprising of bulk nonlinear contribution and interface interaction is proposed to interpret the observed nonlinear behavior. Polarization enhancement with two petals along staggered stacking direction appears in 3R MoS₂ is first observed and the robust polarization of 3R MoS₂ crystal is caused by the retained broken inversion symmetry. The results provide a new arena for realizing ultrathin NLO devices for 2D layered materials.     

Synthesis of Ultrahigh‐Quality Monolayer Molybdenum Disulfide through In Situ Defect Healing with Thiol Molecules

Monolayer transition metal dichalcogenides are 2D materials with many potential applications. Chemical vapor deposition (CVD) is a promising method to synthesize these materials. However, CVD‐grown materials generally have poorer quality than mechanically exfoliated ones and contain more defects due to the difficulties in controlling precursors' distribution and concentration during growth where solid precursors are used. Here, thiol is proposed to be used as a liquid precursor for CVD growth of high quality and uniform 2D MoS₂. Atomic‐resolved structure characterizations indicate that the concentration of sulfur vacancies in the MoS₂ grown from thiol is the lowest among all reported CVD samples. Low temperature spectroscopic characterization further reveals the ultrahigh optical quality of the grown MoS₂. Density functional theory simulations indicate that thiol molecules could interact with sulfur vacancies in MoS₂ and repair these defects during the growth of MoS₂, resulting in high‐quality MoS₂. This work provides a facile and controllable method for the growth of high‐quality 2D materials with ultralow sulfur vacancies and high optical quality, which will benefit their optoelectronic applications.     

Electro‐Optic Modulation in Hybrid Metal Halide Perovskites

Rapid and efficient conversion of electrical signals to optical signals is needed in telecommunications and data network interconnection. The linear electro‐optic (EO) effect in noncentrosymmetric materials offers a pathway to such conversion. Conventional inorganic EO materials make on‐chip integration challenging, while organic nonlinear molecules suffer from thermodynamic molecular disordering that decreases the EO coefficient of the material. It has been posited that hybrid metal halide perovskites could potentially combine the advantages of inorganic materials (stable crystal orientation) with those of organic materials (solution processing). Here, layered metal halide perovskites are reported and investigated for in‐plane birefringence and linear electro‐optic response. Phenylmethylammonium lead chloride (PMA₂PbCl₄) crystals are grown that exhibit a noncentrosymmetric space group. Birefringence measurements and Raman spectroscopy confirm optical and structural anisotropy in the material. By applying an electric field on the crystal surface, the linear EO effect in PMA₂PbCl₄ is reported and its EO coefficient is determined to be 1.40 pm V^?1. This is the first demonstration of this effect in hybrid metal halide perovskites, materials that feature both highly ordered crystalline structures and solution processability. The in‐plane birefringence and electro‐optic response reveal that layered perovskite crystals could be further explored for potential applications in polarizing optics and EO modulation.     

Critical Angles for Reflectivity at an Isotropic-anisotropic Boundary

Abstract

The angular dependent reflectivities, and in particular sharp, critical, edges in these have been analysed for the boundary between an isotropic and a uniaxial medium. For the general uniaxial case, it is shown that for measured reflectivities of the type transverse magnetic (TM) incident to TM reflected or transverse electric (TE) incident to TE reflected there is only one sharp critical angle, the other being rounded due to TM to TE conversion. On the other hand if the TM to TE conversion reflectivity is measured (or TE to TM) then a sharp cusp occurs at the otherwise rounded critical edge. This thereby allows, from some very simple reflectivity measurements the determination of the optical tensor for the anisotropic medium. A full analytic treatment of this behaviour is presented together with numerical evaluations of the electromagnetic field distributions which illustrate how this cusp arises. The possible application of the use of this TE to TM conversion cusp measurement in the case of obliquely oriented liquid crystals is also discussed.     

Broadband MoS2 Field‐Effect Phototransistors: Ultrasensitive Visible‐Light Photoresponse and Negative Infrared Photoresponse

Inverse photoresponse is discovered from phototransistors based on molybdenum disulfide (MoS₂). The devices are capable of detecting photons with energy below the bandgap of MoS₂. Under the illumination of near‐infrared (NIR) light at 980 and 1550 nm, negative photoresponses with short response time (50 ms) are observed for the first time. Upon visible‐light illumination, the phototransistors exhibit positive photoresponse with ultrahigh responsivity on the order of 10⁴–10⁵ A W^?1 owing to the photogating effect and charge trapping mechanism. Besides, the phototransistors can detect a weak visible‐light signal with effective optical power as low as 17 picowatts (pW). A thermally induced photoresponse mechanism, the bolometric effect, is proposed as the cause of the negative photocurrent in the NIR regime. The thermal energy of the NIR radiation is transferred to the MoS₂ crystal lattice, inducing lattice heating and resistance increase. This model is experimentally confirmed by low‐temperature electrical measurements. The bolometric coefficient calculated from the measured transport current change with temperature is ?33 nA K^?1. These findings offer a new approach to develop sub‐bandgap photodetectors and other novel optoelectronic devices based on 2D layered materials.     

Differences in the Toxicological Potential of 2D versus Aggregated Molybdenum Disulfide in the Lung

2D molybdenum disulfide (MoS₂) has distinct optical and electronic properties compared to aggregated MoS₂, enabling wide use of these materials for electronic and biomedical applications. However, the hazard potential of MoS₂ has not been studied extensively. Here, a comprehensive analysis of the pulmonary hazard potential of three aqueous suspended forms of MoS₂—aggregated MoS₂ (Agg‐MoS₂), MoS₂ exfoliated by lithiation (Lit‐MoS₂), and MoS₂ dispersed by Pluronic F87 (PF87‐MoS₂)—is presented. No cytotoxicity is detected in THP‐1 and BEAS‐2B cell lines. However, Agg‐MoS₂ induces strong proinflammatory and profibrogenic responses in vitro. In contrast, Lit‐ and PF87‐MoS₂ have little or no effect. In an acute toxicity study in mice, Agg‐MoS₂ induces acute lung inflammation, while Lit‐MoS₂ and PF87‐MoS₂ have little or no effect. In a subchronic study, there is no evidence of pulmonary fibrosis in response to all forms of MoS₂. These data suggest that exfoliation attenuates the toxicity of Agg‐MoS₂, which is an important consideration toward the safety evaluation and use of nanoscale MoS₂ materials for industrial and biological applications.     

SnSe/MoS2 van der Waals Heterostructure Junction Field‐Effect Transistors with Nearly Ideal Subthreshold Slope

The minimization of the subthreshold swing (SS) in transistors is essential for low‐voltage operation and lower power consumption, both critical for mobile devices and internet of things (IoT) devices. The conventional metal‐oxide‐semiconductor field‐effect transistor requires sophisticated dielectric engineering to achieve nearly ideal SS (60 mV dec^?1 at room temperature). However, another type of transistor, the junction field‐effect transistor (JFET) is free of dielectric layer and can reach the theoretical SS limit without complicated dielectric engineering. The construction of a 2D SnSe/MoS₂ van der Waals (vdW) heterostructure‐based JFET with nearly ideal SS is reported. It is shown that the SnSe/MoS₂ vdW heterostructure exhibits excellent p–n diode rectifying characteristics with low saturate current. Using the SnSe as the gate and MoS₂ as the channel, the SnSe/MoS₂ vdW heterostructure exhibit well‐behavioured n‐channel JFET characteristics with a small pinch‐off voltage V_P of ?0.25 V, nearly ideal subthreshold swing SS of 60.3 mV dec^?1 and high ON/OFF ratio over 10⁶, demonstrating excellent electronic performance especially in the subthreshold regime.     

Negative birefraction of acoustic waves in a sonic crystal

Optical birefringence and dichroism are classical and important effects originating from two independent polarizations of optical waves in anisotropic crystals. Furthermore, the distinct dispersion relations of transverse electric and transverse magnetic polarized electromagnetic waves in photonic crystals can lead to birefringence more easily. However, it is impossible for acoustic waves in the fluid to show such a birefringence because only the longitudinal mode exists. The emergence of an artificial sonic crystal (SC) has significantly broadened the range of acoustic materials in nature that can give rise to acoustic bandgaps and be used to control the propagation of acoustic waves. Recently, negative refraction has attracted a lot of attention and has been demonstrated in both left-handed materials and photonic crystals. Similar to left-handed materials and photonic crystals, negative refractions have also been found in SCs. Here we report, for the first time, the acoustic negative-birefraction phenomenon in a two-dimensional SC, even with the same frequency and the same 'polarization' state. By means of this feature, double focusing images of a point source have been realized. This birefraction concept may be extended to other periodic systems corresponding to other forms of waves, showing great impacts on both fundamental physics and device applications.     

Linear Dichroism and Nondestructive Crystalline Identification of Anisotropic Semimetal Few‐Layer MoTe2

Semimetal 1T′ MoTe₂ crystals have attracted tremendous attention owing to their anisotropic optical properties, Weyl semimetal, phase transition, and so on. However, the complex refractive indices (n‐ik) of the anisotropic semimetal 1T′ MoTe₂ still are not revealed yet, which is important to applications such as polarized wide spectrum detectors, polarized surface plasmonics, and nonlinear optics. Here, the linear dichroism of as‐grown trilayer 1T′ MoTe₂ single crystals is investigated. Trilayer 1T′ MoTe₂ shows obvious anisotropic optical absorption due to the intraband transition of d_z² orbits for Mo atoms and p_x orbits for Te atoms. The anisotropic complex refractive indices of few‐layer 1T′ MoTe₂ are experimentally obtained for the first time by using the Pinier equation analysis. Based on the linear dichroism of 1T′ MoTe₂, angle‐resolved polarized optical microscopy is developed to visualize the grain boundary and identify the crystal orientation of 1T′ MoTe₂ crystals, which is also an excellent tool toward the investigation of the optical absorption properties in the visible range for anisotropic 2D transition metal chalcogenides. This work provides a universal and nondestructive method to identify the crystal orientation of anisotropic 2D materials, which opens up an opportunity to investigate the optical application of anisotropic semimetal 2D materials.     

Wrap‐Around Core–Shell Heterostructures of Layered Crystals

Engineered heterostructures create new functionality by integrating dissimilar materials. Combining different 2D crystals naturally produces two distinct classes of heterostructures, vertical van der Waals (vdW) stacks or 2D sheets bonded laterally by covalent line interfaces. When joining thicker layered crystals, the arising structural and topological conflicts can result in more complex geometries. Phase separation during one‐pot synthesis of layered tin chalcogenides spontaneously creates core–shell structures in which large orthorhombic SnS crystals are enclosed in a wrap‐around shell of trigonal SnS₂, forcing the coexistence of parallel vdW layering along with unconventional, orthogonally layered core–shell interfaces. Measurements of the optoelectronic properties establish anisotropic carrier separation near type II core–shell interfaces and extended long‐wavelength light harvesting via spatially indirect interfacial absorption, making multifunctional layered core–shell structures attractive for energy‐conversion applications.     

Highly Confined and Tunable Hyperbolic Phonon Polaritons in Van Der Waals Semiconducting Transition Metal Oxides

2D van der Waals (vdW) layered polar crystals sustaining phonon polaritons (PhPs) have opened up new avenues for fundamental research and optoelectronic applications in the mid‐infrared to terahertz ranges. To date, 2D vdW crystals with PhPs are only experimentally demonstrated in hexagonal boron nitride (hBN) slabs. For optoelectronic and active photonic applications, semiconductors with tunable charges, finite conductivity, and moderate bandgaps are preferred. Here, PhPs are demonstrated with low loss and ultrahigh electromagnetic field confinements in semiconducting vdW α‐MoO₃. The α‐MoO₃ supports strong hyperbolic PhPs in the mid‐infrared range, with a damping rate as low as 0.08. The electromagnetic confinements can reach ≈λ₀/120, which can be tailored by altering the thicknesses of the α‐MoO₃ 2D flakes. Furthermore, spatial control over the PhPs is achieved with a metal‐ion‐intercalation strategy. The results demonstrate α‐MoO₃ as a new platform for studying hyperbolic PhPs with tunability, which enable switchable mid‐infrared nanophotonic devices.     

Pulsed laser deposition optical waveguiding Bi3TiNbO9 thin films on fused silica

The layered-perovskite ferroelectric Bi₃TiNbO₉ (BTN) optical waveguiding thin films have been prepared on fused silica substrates by pulsed laser deposition (PLD). X-ray θ-2θ scans revealed that the films are single-phase perovskite and highly (00l) textured. The wavelength dependence of the transmittance of the films was determined. We obtained an average transmittance of 75% in the wavelength range of 400-1100 nm and the band gap E_g=3.55 eV. The optical waveguiding properties of the films were characterized by using prism-coupling method. The distinct m lines of the guided transverse magnetic (TM) and transverse electric (TE) modes of the BTN films waveguide have been observed. The cross sectional morphology of the film was studied by scanning electron microscopy (SEM).     

Synthesis and characterization of MoS2/Fe@Fe3O4 nanocomposites exhibiting enhanced microwave absorption performance at normal and oblique incidences

Herein, we attempted to prepare MoS₂/Fe@Fe₃O₄ nanocomposites capable of strongly absorbing broadband incident electromagnetic (EM) radiation and probed the effects of their composition on complex permittivity and permeability at 2–18 GHz. Calculations of normal-incidence reflection losses (RLs) based on EM parameters revealed that the Fe@Fe₃O₄ to MoS₂ mass ratio strongly influenced the absorption peak intensity and bandwidth. Specifically, an RL peak of −31.8 dB@15.3 GHz and a bandwidth (RL < − 10 dB) of 4.8 GHz (13.2–18 GHz) were achieved at a thickness of 1.52 mm and a Fe@Fe₃O₄ to MoS₂ mass ratio of 60:40. Further, RL and bandwidth were investigated for oblique incidence, in which case two kinds of EM waves (TE – electric field perpendicular to plane of incidence; TM – electric field in the plane of incidence) were considered. The absorption peaks of TE and TM waves did not exceed −20 dB when the incidence angle increased to 30°, and the bandwidth (RL < − 10 dB) reached 4.2 GHz (TE wave) and 4.0 GHz (TM wave) when this angle was further increased to 40.0° and 50.4°, respectively. Finally, the mechanism of microwave absorption was discussed in detail.     

In Situ Repair of 2D Chalcogenides under Electron Beam Irradiation

Molybdenum disulfide (MoS₂) and bismuth telluride (Bi₂Te₃) are the two most common types of structures adopted by 2D chalcogenides. In view of their unique physical properties and structure, 2D chalcogenides have potential applications in various fields. However, the excellent properties of these 2D crystals depend critically on their crystal structures, where defects, cracks, holes, or even greater damage can be inevitably introduced during the preparation and transferring processes. Such defects adversely impact the performance of devices made from 2D chalcogenides and, hence, it is important to develop ways to intuitively and precisely repair these 2D crystals on the atomic scale, so as to realize high‐reliability devices from these structures. Here, an in situ study of the repair of the nanopores in MoS₂ and Bi₂Te₃ is carried out under electron beam irradiation by transmission electron microscopy. The experimental conditions allow visualization of the structural dynamics of MoS₂ and Bi₂Te₃ crystals with unprecedented resolution. Real‐time observation of the healing of defects at atomic resolution can potentially help to reproducibly fabricate and simultaneously image single‐crystalline free‐standing 2D chalcogenides. Thus, these findings demonstrate the viability of using an electron beam as an effective tool to precisely engineer materials to suit desired applications in the future.     

Rational Design of Nanoporous MoS2/VS2 Heteroarchitecture for Ultrahigh Performance Ammonia Sensors

2D transition metal dichalcogenides (TMDs) have received widespread interest by virtue of their excellent electrical, optical, and electrochemical characteristics. Recent studies on TMDs have revealed their versatile utilization as electrocatalysts, supercapacitors, battery materials, and sensors, etc. In this study, MoS₂ nanosheets are successfully assembled on the porous VS₂ (P‐VS₂) scaffold to form a MoS₂/VS₂ heterostructure. Their gas‐sensing features, such as sensitivity and selectivity, are investigated by using a quartz crystal microbalance (QCM) technique. The QCM results and density functional theory (DFT) calculations reveal the impressive affinity of the MoS₂/VS₂ heterostructure sensor toward ammonia with a higher adsorption uptake than the pristine MoS₂ or P‐VS₂ sensor. Furthermore, the adsorption kinetics of the MoS₂/VS₂ heterostructure sensor toward ammonia follow the pseudo‐first‐order kinetics model. The excellent sensing features of the MoS₂/VS₂ heterostructure render it attractive for high‐performance ammonia sensors in diverse applications.