Chiral molecule induced valley polarization enhancement of MoS2.

Monolayers of transition metal dichalcogenides (TMDCs) are atomically thin direct-bandgap semiconductors with potential applications in nanoelectronics, opto-electronics, and electrochemical sensing. Recent theoretical and experimental results have suggested that they are ideal systems for exploiting the valley degrees of freedom of Bloch electrons. Here, we report detailed studies of the opto-valleytronic properties of a chiral histidine molecule embedded in monolayer MoS2 single crystals grown via chemical vapor deposition. By irradiating MoS2 with circularly polarized light and measuring the resulting spatially resolved circularly polarized emission, we find the existence of a significantly increased circular polarization for D-histidine doped MoS2. The increased valley contrast is attributed to the selective enhancement of both the excitation and emission rates having one particular handedness of the circular polarization. These results provide a promising pathway to enhance the valley contrast for monolayer TMDCs at room temperature.

Ultra-Small 2D PbS Nanoplatelets: Liquid-Phase Exfoliation and Emerging Applications for Photo-Electrochemical Photodetectors.

2D PbS nanoplatelets (NPLs) form an emerging class of photoactive materials and have been proposed as robust materials for high-performance optoelectronic devices. However, the main drawback of PbS NPLs is the large lateral size, which inhibits their further investigations and practical applications. In this work, ultra-small 2D PbS NPLs with uniform lateral size (11.2 ± 1.7 nm) and thickness (3.7 ± 0.9 nm, ≈6 layers) have been successfully fabricated by a facile liquid-phase exfoliation approach. Their transient optical response and photo-response behavior are evaluated by femtosecond-resolved transient absorption and photo-electrochemical (PEC) measurements. It is shown that the NPLs-based photodetectors (PDs) exhibit excellent photo-response performance from UV to the visible range, showing extremely high photo-responsivity (27.81 mA W-1 ) and remarkable detectivity (3.96 × 1010 Jones), which are figures of merit outperforming currently reported PEC-type PDs. The outstanding properties are further analyzed based on the results of first-principle calculations, including electronic band structure and free energies for the oxygen evolution reaction process. This work highlights promising applications of ultra-small 2D PbS NPLs with the potential for breakthrough developments also in other fields of optoelectronic devices.

Broadband Nonlinear Photonics in Few-Layer Borophene.

In this paper, 2D borophene is synthesized through a liquid-phase exfoliation. The morphology and structure of as-prepared borophene are systemically analyzed, and the Z-scan is used to measure the nonlinear optical properties. It is found that the saturable absorber (SA) properties of borophene make it serve as an excellent broadband optical switch, which is strongly used for mode-locking in near- and mid-infrared laser systems. Ultrastable pulses with durations as short as 792 and 693 fs are successfully delivered at the central wavelengths of 1063 and 1560 nm, respectively. Furthermore, stable pulses at a wavelength of 1878 nm are demonstrated from a thulium mode-locked fiber laser based on the same borophene SA. This research reveals a significant potential for borophene used in lasers helping extending the frontiers of photonic technologies.

Recent Progress of Fluxgate Magnetic Sensors: Basic Research and Application.

Fluxgate magnetic sensors are especially important in detecting weak magnetic fields. The mechanism of a fluxgate magnetic sensor is based on Faraday's law of electromagnetic induction. The structure of a fluxgate magnetic sensor mainly consists of excitation windings, core and sensing windings, similar to the structure of a transformer. To date, they have been applied to many fields such as geophysics and astro-observations, wearable electronic devices and non-destructive testing. In this review, we report the recent progress in both the basic research and applications of fluxgate magnetic sensors, especially in the past two years. Regarding the basic research, we focus on the progress in lowering the noise, better calibration methods and increasing the sensitivity. Concerning applications, we introduce recent work about fluxgate magnetometers on spacecraft, unmanned aerial vehicles, wearable electronic devices and defect detection in coiled tubing. Based on the above work, we hope that we can have a clearer prospect about the future research direction of fluxgate magnetic sensor.

Monte Carlo simulation of magnetic domain structure and magnetic properties near the morphotropic phase boundary.

The morphotropic phase boundary (MPB), which is the boundary separating a tetragonal phase from a rhombohedral phase by varying the composition or mechanical pressure in ferroelectrics, has been studied extensively for decades because it can lead to strong enhancement of piezoelectricity. Recently, a parallel ferromagnetic MPB was experimentally reported in the TbCo2-DyCo2 ferromagnetic system and this discovery proposes a new way to develop potential materials with giant magnetostriction. However, the role of magnetic domain switching and spin reorientation near the MPB region is still unclear. For the first time, we combine micromagnetic theory with Monte Carlo simulation to investigate the evolution of magnetic domain structures and the corresponding magnetization properties near the MPB region. It is demonstrated that the magnetic domain structure and the corresponding magnetization properties are determined by the interplay among anisotropy energy, magnetostatic energy and exchange energy. If the anisotropy energy barrier is large compared with the magnetostatic energy barrier and the exchange energy barrier, the MPB region is a T and R mixed structure and magnetic domain switching is the dominant mechanism. If the anisotropy energy barrier is small, the MPB region will also contain M phases and spin reorientation is the dominant mechanism. Our work could provide a guide for the design of advanced ferromagnetic materials with enhanced magnetostriction.

Porphyrin-based porous organic polymers synthesized using the Alder-Longo method: the most traditional synthetic strategy with exceptional capacity.

Porphyrin is a typical tetrapyrrole chromophore-based pigment with a special electronic structure and functionalities, which is frequently introduced into various porous organic polymers (POPs). Porphyrin-based POPs are widely used in various fields ranging from environmental and energy to biomedicine-related fields. Currently, most porphyrin-based POPs are prepared via the copolymerization of specific-group-functionalized porphyrins with other building blocks, in which the tedious and inefficient synthesis procedure for the porphyrin greatly hinders the development of such materials. This review aimed to summarize information on porphyrin-based POPs synthesized using the Alder-Longo method, thereby skipping the complex synthesis of porphyrin-bearing monomers, in which the porphyrin macrocycles are formed directly via the cyclic tetramerization of pyrrole with monomers containing multiple aldehyde groups during the polymerization process. The representative applications of porphyrin-based POPs derived using the Alder-Longo method are finally introduced, which pinpoints a clear relationship between the structure and function from the aspect of the building blocks used and porous structures. This review is therefore valuable for the rational design of efficient porphyrin-based porous organic polymer systems that may be utilized in various fields from energy-related conversion/storage technologies to biomedical science.

One-dimensional TeSe nano-heterojunction: formation, calculations, carrier dynamics, and application in broad-spectrum photodetectors.

Seawater contains many electrolytes, is abundant in nature, environmentally friendly, and chemically stable, and exhibits substantial potential for replacement of traditional inorganic electrolytes in photoelectrochemical-type photodetectors (PDs). Herein, one-dimensional semiconductor TeSe nanorods (NRs) with core-shell nanostructures were reported, and their morphology, optical behavior, electronic structure, and photoinduced carrier dynamics were systematically investigated. As photosensitizers, the as-resultant TeSe NRs were assembled into PDs, and the influence of the bias potential, light wavelength and intensity, and the concentration of seawater on the photo-response of TeSe NR-based PDs was evaluated. These PDs exhibited favorable photo-response performance upon illumination with light in the ultraviolet-visible-near-infrared (UV-Vis-NIR) range and even simulated sunlight. Moreover, the TeSe NR-based PDs also exhibited a long duration and cycling stability of its on-off switching and might be useful in marine monitoring.

Study on SO2 and Cl2 sensor application of 2D PbSe based on first principles calculations.

In this paper, we use 2D PbSe to design a gas sensor to monitor the presence of SO2 and Cl2. We use first principles to verify the feasibility of this material, such as atomic structure, band gap, differential charge density and Bader charge. The results show that 2D PbSe can distinctly adsorb SO2 and Cl2. Furthermore, the adsorption of SO2 and Cl2 will affect the electronic structure of 2D PbSe, and some electrons in the PbSe are transferred to gas atoms. The band gap of the system after adsorption is smaller than that of the PbSe before adsorption. The band gap of single layer PbSe decreases by 41.92% after SO2 adsorption and 60.61% after Cl2 adsorption. The band gap of multi-layer PbSe decreases by 72.97% after SO2 adsorption and 43.24% after Cl2 adsorption. This shows that single layer PbSe is more sensitive to Cl2 and multi-layer PbSe is more sensitive to SO2. It provides a potential possibility for designing gas sensors for SO2 and Cl2 based on 2D PbSe.

Applications of Few-Layer Nb2C MXene: Narrow-Band Photodetectors and Femtosecond Mode-Locked Fiber Lasers.

Although the physicochemical properties of niobium carbide (Nb2C) have been widely investigated, their exploration in the field of photoelectronics is still at the infancy stage with many potential applications that remain to be exploited. Hence, it is demonstrated here that few-layer Nb2C MXene can serve as an excellent building block for both photoelectrochemical-type photodetectors (PDs) and mode-lockers. We show that the photoresponse performance can be readily adjusted by external conditions and that Nb2C NSs exhibit a great potential for narrow-band PDs. The demonstrated mechanism was further confirmed by work functions predicted by density functional theory calculations. In addition, as an optical switch for passively mode-locked fiber lasers, ultrastable pulses can be demonstrated in the telecommunication and mid-infrared regions for Nb2C MXene, and as high as the 69th harmonic order with 411 MHz at the center wavelength of 1882 nm can be achieved. These intriguing results indicate that few-layer Nb2C nanosheets can be used as building blocks for various photoelectronic devices, further broadening the application prospects of two-dimensional MXenes.

Two-Dimensional Black Arsenic Phosphorus for Ultrafast Photonics in Near- and Mid-Infrared Regimes.

Black arsenic phosphorus (b-AsP), as one kind of novel two-dimensional (2D) materials, bridges the band gap between black phosphorus and graphene. Thanks to its great advantages, including high carrier mobility, excellent in-plane anisotropy, and broad tunability band gap, b-AsP has aroused great interest in fields of photonics and photoelectronics. In this paper, ultrathin 2D b-AsP nanomaterials were fabricated by the liquid-phase exfoliation method, and their strong broadband linear and nonlinear absorptions were characterized by ultraviolet-visible-infrared and Z-scan technology. The experimental determination of the nonlinear absorption coefficient and low saturation intensity of b-AsP were -0.23 cm/GW and 3.336 GW/cm2, respectively. Based on density functional theory, the partial charge density and band structure at the conduction band minimum and valence band maximum were calculated, which further proves the excellent optical properties of 2D b-AsP. By first using 2D b-AsP as a novel saturable absorber in both erbium-doped and thulium-doped fiber lasers, mode-locked soliton pulses can stably operate at 1.5 and 2 µm. The laser pulses generated by 2D b-AsP possess higher stability to resist self-splitting than those generated by other 2D material-based mode-lockers. These experimental results highlight that 2D b-AsP has great application potential as a novel optical material in ultrafast photonics from near- to mid-infrared regimes.

Machine Learning Magnetic Parameters from Spin Configurations.

Hamiltonian parameters estimation is crucial in condensed matter physics, but is time- and cost-consuming. High-resolution images provide detailed information of underlying physics, but extracting Hamiltonian parameters from them is difficult due to the huge Hilbert space. Here, a protocol for Hamiltonian parameters estimation from images based on a machine learning (ML) architecture is provided. It consists in learning a mapping between spin configurations and Hamiltonian parameters from a small amount of simulated images, applying the trained ML model to a single unexplored experimental image to estimate its key parameters, and predicting the corresponding materials properties by a physical model. The efficiency of the approach is demonstrated by reproducing the same spin configuration as the experimental one and predicting the coercive field, the saturation field, and even the volume of the experiment specimen accurately. The proposed approach paves a way to achieve a stable and efficient parameters estimation.

Simulation study on exchange interaction and unique magnetization near ferromagnetic morphotropic phase boundary.

Extensive efforts have been made in searching enhanced functionalities near the so-called morphotropic phase boundaries (MPBs) in both ferroelectric and ferromagnetic materials. Due to the exchange anti-symmetry of the wave function of fermions, it is widely recognized that the exchange interaction plays a critical role in ferromagnetism. As a quantum effect, the exchange interaction is magnitudes larger than electric interaction, leading to a fundamental difference between ferroelectricity and ferromagnetism. In this paper, we establish an energetic model capturing the interplay among the anisotropy energy, magnetostatic energy and the exchange energy to investigate systematically the effects of the exchange energy on the behavior of the ferromagnetic MPB. For the first time, it is found that the exchange energy can narrow the width of MPB region in the composition temperature phase diagram for ferromagnetic MPB systems. As temperature increases, MPB region becomes wider because of the weakening of the exchange interaction. Our simulation results suggest that the exchange energy play a critical role on the unique behavior of ferromagnetic MPB, which is in contrast different from that of ferroelectric MPB.