Layer-dependent ultrafast carrier dynamics of PdSe2 investigated by photoemission electron microscopy.

For atomically thin two-dimensional materials, variations in layer thickness can result in significant changes in the electronic energy band structure and physicochemical properties, thereby influencing the carrier dynamics and device performance. In this work, we employ time- and energy-resolved photoemission electron microscopy to reveal the ultrafast carrier dynamics of PdSe2 with different layer thicknesses. We find that for few-layer PdSe2 with a semiconductor phase, an ultrafast hot carrier cooling on a timescale of approximately 0.3 ps and an ultrafast defect trapping on a timescale of approximately 1.3 ps are unveiled, followed by a slower decay of approximately tens of picoseconds. However, for bulk PdSe2 with a semimetal phase, only an ultrafast hot carrier cooling and a slower decay of approximately tens of picoseconds are observed, while the contribution of defect trapping is suppressed with the increase of layer number. Theoretical calculations of the electronic energy band structure further confirm the transition from a semiconductor to a semimetal. Our work demonstrates that TR- and ER-PEEM with ultrahigh spatiotemporal resolution and wide-field imaging capability has great advantages in revealing the intricate details of ultrafast carrier dynamics of nanomaterials.

Wavelength-Controlled Photoconductance Polarity Switching via Harnessing Defects in Doped PdSe2 for Artificial Synaptic Features.

Optoelectronic synapses are currently drawing significant attention as fundamental building blocks of neuromorphic computing to mimic brain functions. In this study, a two-terminal synaptic device based on a doped PdSe2 flake is proposed to imitate the key neural functions in an optical pathway. Due to the wavelength-dependent desorption of oxygen clusters near the intrinsic selenide vacancy defects, the doped PdSe2 photodetector achieves a high negative photoresponsivity of -7.8 × 103 A W-1 at 473 nm and a positive photoresponsivity of 181 A W-1 at 1064 nm. This wavelength-selective bi-direction photoresponse endows an all-optical pathway to imitate the fundamental functions of artificial synapses on a device level, such as psychological learning and forgetting capability, as well as dynamic logic functions. The underpinning photoresponse is further demonstrated on a flexible platform, providing a viable technology for neuromorphic computing in wearable electronics. Furthermore, the p-type doping results in an effective increase of the channel's electrical conductivity and a significant reduction in power consumption. Such low-power-consuming optical synapses with simple device architecture and low-dimensional features demonstrate tremendous promise for building multifunctional artificial neuromorphic systems in the future.

Polarization-Resolved Near-Infrared PdSe2 p-i-n Homojunction Photodetector.

Constructing high-quality homojunctions plays a pivotal role for the advancement of two-dimensional transition metal sulfide (TMDC) based optoelectronic devices. Here, a lateral PdSe2 p-i-n homojunction is constructed by electrostatic doping. Electrical measurements reveal that the homojunction diode exhibits a strong rectifying characteristic with a rectification ratio exceeding 104 and an ideality factor approaching 1. When functioning in photovoltaic mode, the device achieves a high responsivity of 1.1 A/W under 1064 nm illumination, with a specific detectivity of 1.3 × 1011 Jones and a high linearity of 45 dB. Benefiting from the lateral p-i-n structure, the junction capacitance is significantly reduced, and an ultrafast response (3/6 µs) is obtained. Additionally, the photodiode has the capability of polarization distinction due to the unique in-plane anisotropic structure of PdSe2, exhibiting a dichroic ratio of 1.6 at a 1064 nm wavelength. This high-performance polarization-sensitive near-infrared photodetector exhibits great potential in the next-generation optoelectronic applications.

Controllable Synthesis of 2H-1T' Mox Re(1- x ) S2 Lateral Heterostructures and Their Tunable Optoelectronic Properties.

Constructing heterostructures and doping are valid ways to improve the optoelectronic properties of transition metal dichalcogenides (TMDs) and optimize the performance of TMDs-based photodetectors. Compared with transfer techniques, chemical vapor deposition (CVD) has higher efficiency in preparing heterostructures. As for the one-step CVD growth of heterostructures, cross-contamination between the two materials may occur during the growth process, which may provide the possibility of one-step simultaneous realization of controllable doping and formation of alloy-based heterostructures by finely tuning the growth dynamics. Here, 2H-1T' Mox Re(1- x ) S2 alloy-to-alloy lateral heterostructures are synthesized through this one-step CVD growth method, utilizing the cross-contamination and different growth temperatures of the two alloys. Due to the doping of a small amount of Re atoms in 2H MoS2 , 2H Mox Re(1- x ) S2 has a high response rejection ratio in the solar-blind ultraviolet (SBUV) region and exhibits a positive photoconductive (PPC) effect. While the 1T' Mox Re(1- x ) S2 formed by heavily doping Mo atoms into 1T' ReS2 will produce a negative photoconductivity (NPC) effect under UV laser irradiation. The optoelectronic property of 2H-1T' Mox Re(1- x ) S2 -based heterostructures can be modulated by gate voltage. These findings are expected to expand the functionality of traditional optoelectronic devices and have potential applications in optoelectronic logic devices.

Strain and Interference Synergistically Modulated Optical and Electrical Properties in ReS2/Graphene Heterojunction Bubbles.

Two-dimensional (2D) material bubbles, as a straightforward method to induce strain, represent a potentially powerful platform for the modulation of different properties of 2D materials and the exploration of their strain-related applications. Here, we prepare ReS2/graphene heterojunction bubbles (ReS2/gr heterobubbles) and investigate their strain and interference synergistically modulated optical and electrical properties. We perform Raman and photoluminescence (PL) spectra to verify the continuously varying strain and the microcavity induced optical interference in ReS2/gr heterobubbles. Kelvin probe force microscopy (KPFM) is carried out to explore the photogenerated carrier transfer behavior in both strained ReS2/gr heterobubbles and ReS2/gr interfaces, as well as the oscillation of surface potential caused by optical interference under illumination conditions. Moreover, the switching of in-plane crystal orientation and the modulation of optical anisotropy of ReS2/gr heterobubbles are observed by azimuth-dependent reflectance difference microscopy (ADRDM), which can be attributed to the action of both strain effect and interference. Our study proves that the optical and electrical properties can be effectively modulated by the synergistical effect of strain and interference in a 2D material bubble.

Tuning the Exchange Bias Effect in 2D van der Waals Ferro-/Antiferromagnetic Fe3 GeTe2 /CrOCl Heterostructures.

The exchange bias effect is extremely expected in 2D van der Waals (vdW) ferromagnetic (FM)/antiferromagnetic (AFM) heterostructures due to the high-quality interface. CrOCl possesses strong magnetic anisotropy at 2D limit, and is an ideal antiferromagnet for constructing FM/AFM heterostructures to explore the exchange bias effect. Here, the exchange bias effect in Fe3 GeTe2 (FGT)/CrOCl heterostructures through both anomalous Hall effect (AHE) and reflective magnetic circular dichroism (RMCD) measurements is studied. In the AHE measurements, the exchange bias field (HEB ) at 3 K exhibits a distinct increase from ≈150 Oe to ≈450 Oe after air exposure, and such variation is attributed to the formation of an oxidized layer in FGT by analyzing the cross-sectional microstructure. The HEB is successfully tuned by changing the FGT/CrOCl thickness and the cooling field. Furthermore, a larger HEB of ≈750 Oe at 1.7 K in FGT/CrOCl heterostructure through RMCD measurements is observed, and it is proposed that the larger HEB in RMCD measurements is related to the distribution of uncompensated spins at the interface. This work reveals several intriguing phenomena of the exchange bias effect in 2D vdW magnetic systems, which paves the way for the study of related spintronic devices.

Application of SDN Network Traffic Prediction Based on Speech Recognition in Educational Information Optimization Platform.

This paper constructs a SDN network traffic prediction model based on speech recognition and applies it to the educational information optimization platform. By analyzing the influencing factors of SDN network equipment, communication links, and network traffic, this paper constructs the initial index set of SDN network traffic situation. In the data plane of SDN, the queue management algorithm is used to control the flow. On this basis, an IRS mechanism is proposed based on the advantages of SDN centralized control and the difference of transmission performance requirements between large and small streams. For the transmission of large traffic, IRS adopts greedy routing and multipath routing based on the remaining bandwidth to make the traffic evenly distributed in the network, and IRS adds the scheduling strategy based on IP addressing to avoid packet disorder. Simulation results show that the effectiveness of this algorithm can reach 95.67% at the highest, and the MSE convergence is 0.0021 at the lowest. At the same time, this method completes the quantitative evaluation of SDN network traffic situation, effectively solves the problem that SDN traffic situation labels cannot be determined, and opens a new vision of global state observation for SDN network management. This research can provide some technical support for the educational information optimization platform.

van der Waals Magnets: Material Family, Detection and Modulation of Magnetism, and Perspective in Spintronics.

van der Waals (vdW) materials exhibit great potential in spintronics, arising from their excellent spin transportation, large spin-orbit coupling, and high-quality interfaces. The recent discovery of intrinsic vdW antiferromagnets and ferromagnets has laid the foundation for the construction of all-vdW spintronic devices, and enables the study of low-dimensional magnetism, which is of both technical and scientific significance. In this review, several representative families of vdW magnets are introduced, followed by a comprehensive summary of the methods utilized in reading out the magnetic states of vdW magnets. Thereafter, it is shown that various electrical, mechanical, and chemical approaches are employed to modulate the magnetism of vdW magnets. Finally, the perspective of vdW magnets in spintronics is discussed and an outlook of future development direction in this field is also proposed.

Synthesis of low-symmetry 2D Ge(1-x)SnxSe2 alloy flakes with anisotropic optical response and birefringence.

Low-symmetry two-dimensional (2D) materials with unique in-plane anisotropy can promote both fundamental science and practical applications in optics, optoelectronics, electronics, and polarization detection. As a member of 2D materials, doping/alloying material systems have gained great attention owing to the tunable bandgap and special properties. However, the in-plane anisotropic optical and electrical properties of these 2D alloy materials have rarely been reported. In this work, low-symmetry 2D Ge(1-x)SnxSe2 (x = 0-1.0) alloy flakes have been synthesized by chemical vapor deposition (CVD) with a bandgap varying from 1.55 eV (GeSe2, x = 0) to 1.90 eV (SnSe2, x = 1.0). Angle-resolved polarized Raman spectroscopy (ARPRS) is used to confirm the in-plane vibrational anisotropy, and azimuth-dependent reflectance difference microscopy (ADRDM) is applied to visualize the in-plane optical anisotropy. Polarization-dependent transmission spectroscopy (PDTS) is carried out to reflect the in-plane absorptional anisotropy and linear dichroism, and birefringence characteristics are also found in the subsequent studies. All of the results indicate the unique in-plane optical anisotropy and birefringence characteristics of the 2D Ge(1-x)SnxSe2 alloy flakes, providing new opportunities for polarization-controlled devices, optical wave plates, and polarizers.

Magnetism and Optical Anisotropy in van der Waals Antiferromagnetic Insulator CrOCl.

van der Waals (vdW) magnetic insulators are of significance in both fundamental research and technological application, but most two-dimensional (2D) vdW magnetic systems are unstable and of high lattice symmetry. Stable 2D vdW magnetic insulators with anisotropic structure are needed to modulate the properties and unlock potential applications. Here we present a stable vdW antiferromagnetic material, CrOCl, with low-symmetry orthorhombic structure, and investigate systematically its magnetism, phase transition behavior, and optical anisotropy. Spin-phonon coupling effects are uncovered by the abnormal frequency shifts of Raman-active modes, suggesting the formation of a magnetic superstructure. The sizable abnormal change of interplanar spacing indicates the presence of a structural transition at around 27 K. Further in-plane vibrational, reflectional, and absorptional anisotropic properties are explored both experimentally and theoretically, revealing a highly polarization sensitive characteristic and linear dichroism in 2D CrOCl. Meanwhile, the particularly high polarization dependency of the second-harmonic generation and the nonlinear susceptibility of ∼2.24 × 10-11 m/V make it suitable in the field of polarization-dependent nonlinear optics. The findings on the intricate properties of 2D CrOCl lay foundations for future applications of low-symmetry vdW magnets in spin-dependent electronic and optoelectronic devices.

Highly Sensitive Polarization Photodetection Using a Pseudo-One-Dimensional Nb(1- x)Ti xS3 Alloy.

Low-symmetry layered two-dimensional (2D) materials with strong in-plane optical anisotropy can potentially be applied for polarization photodetection. This is especially true for those 2D materials with a direct band gap, which can efficiently absorb light with specific axial polarization. However, discovering such new anisotropic 2D materials with a direct band structure is still extremely challenging. Here, we fabricate a photodetector using a pseudo-one-dimensional (pseudo-1D) Nb(1- x)Ti xS3 alloy device and demonstrate that it is highly sensitive to the polarized light because of the strong in-plane optical anisotropy and direct band gap of the alloy by combining angle-resolved polarization Raman spectroscopy, azimuth-dependent reflectance difference microscopy, polarization-dependent absorption spectroscopy, and hybrid functional theory calculations. As a consequence, the polarization photodetector of the Nb(1- x)Ti xS3 alloy shows a large photocurrent anisotropic ratio and a high photoresponse. The choice of a low-symmetry layered pseudo-1D Nb(1- x)Ti xS3 alloy in polarization photodetection might open up new functionalities for novel optoelectronic device applications.

In-Plane Optical Anisotropy and Linear Dichroism in Low-Symmetry Layered TlSe.

In-plane anisotropy of layered materials adds another dimension to their applications, opening up avenues in diverse angle-resolved devices. However, to fulfill a strong inherent in-plane anisotropy in layered materials still poses a significant challenge, as it often requires a low-symmetry nature of layered materials. Here, we report the fabrication of a member of layered semiconducting AIIIBVI compounds, TlSe, that possesses a low-symmetry tetragonal structure and investigate its anisotropic light-matter interactions. We first identify the in-plane Raman intensity anisotropy of thin-layer TlSe, offering unambiguous evidence that the anisotropy is sensitive to crystalline orientation. Further in-situ azimuth-dependent reflectance difference microscopy enables the direct evaluation of in-plane optical anisotropy of layered TlSe, and we demonstrate that the TlSe shows a linear dichroism under polarized absorption spectra arising from an in-plane anisotropic optical property. As a direct result of the linear dichroism, we successfully fabricate TlSe devices for polarization-sensitive photodetection. The discovery of layered TlSe with a strong in-plane anisotropy not only facilitates its applications in linear dichroic photodetection but opens up more possibilities for other functional device applications.

Novel Surface Molecular Functionalization Route To Enhance Environmental Stability of Tellurium-Containing 2D Layers.

Recent studies have shown that tellurium-based two-dimensional (2D) crystals undergo dramatic structural, physical, and chemical changes under ambient conditions, which adversely impact their much desired properties. Here, we introduce a diazonium molecule functionalization-based surface engineering route that greatly enhances their environmental stability without sacrificing their much desired properties. Spectroscopy and microscopy results show that diazonium groups significantly slow down the surface reactions, and consequently, gallium telluride (GaTe), zirconium telluride (ZrTe3), and molybdenum ditelluride (MoTe2) gain strong resistance to surface transformation in air or when immersed under water. Density functional theory calculations show that functionalizing molecules reduce surface reactivity of Te-containing 2D surfaces by chemical binding followed by an electron withdrawal process. While pristine surfaces structurally decompose because of strong reactivity of Te surface atoms, passivated functionalized surfaces retain their structural anisotropy, optical band gap, and emission characteristics as evidenced by our conductive atomic force microscopy, photoluminescence, and absorption spectroscopy measurements. Overall, our findings offer an effective method to increase the stability of these environmentally sensitive materials without impacting much of their physical properties.

Enhanced Electrical and Optoelectronic Characteristics of Few-Layer Type-II SnSe/MoS2 van der Waals Heterojunctions.

van der Waals heterojunctions formed by stacking various two-dimensional (2D) materials have a series of attractive physical properties, thus offering an ideal platform for versatile electronic and optoelectronic applications. Here, we report few-layer SnSe/MoS2 van der Waals heterojunctions and study their electrical and optoelectronic characteristics. The new heterojunctions present excellent electrical transport characteristics with a distinct rectification effect and a high current on/off ratio (∼1 × 105). Such type-II heterostructures also generate a self-powered photocurrent with a fast response time (<10 ms) and exhibit high photoresponsivity of 100 A W-1, together with high external quantum efficiency of 23.3 × 103% under illumination by 532 nm light. Photoswitching characteristics of the heterojunctions can be modulated by bias voltage, light wavelength, and power density. The designed novel type-II van der Waals heterojunctions are formed from a combination of a transition-metal dichalcogenide and a group IV-VI layered 2D material, thereby expanding the library of ultrathin flexible 2D semiconducting devices.

Enhanced current rectification and self-powered photoresponse in multilayer p-MoTe2/n-MoS2 van der Waals heterojunctions.

Vertically stacked van der Waals (vdW) heterojunctions based on two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted a great deal of attention and have created a powerful new material platform for novel, high-performance electronic and optoelectronic devices. Here, we report the construction of multilayer p-MoTe2/n-MoS2 vdW heterostructures with remarkable rectification behavior, self-powered photoresponse and distinct photosensitivity at different laser wavelengths and power densities. Field effect transistors (FETs) fabricated by MoTe2/MoS2 heterojunctions exhibit excellent gate-tunable rectification behavior and p-n junction transport characteristics, with the n-type dominating. The MoTe2/MoS2 heterojunction devices generate a self-powered photocurrent at zero bias voltage with a considerable on-off ratio reaching ∼780 and achieve a stable and fast photoresponse, due to the type-II band alignment facilitating efficient electron-hole separation. Utilizing the advantages of a p-n junction with type-II band alignment, this MoTe2/MoS2 vdW heterostructure provides more opportunities for future electronic and optoelectronic applications.

Gate-tunable diode-like current rectification and ambipolar transport in multilayer van der Waals ReSe2/WS2 p-n heterojunctions.

Vertically stacked van der Waals (vdW) heterojunctions of two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted a great deal of attention due to their fascinating properties. In this work, we report two important gate-tunable phenomena in new artificial vdW p-n heterojunctions created by vertically stacking p-type multilayer ReSe2 and n-type multilayer WS2: (1) well-defined strong gate-tunable diode-like current rectification across the p-n interface is observed, and the tunability of the electronic processes is attributed to the tunneling-assisted interlayer recombination induced by majority carriers across the vdW interface; (2) the distinct ambipolar behavior under gate voltage modulation both at forward and reverse bias voltages is found in the vdW ReSe2/WS2 heterojunction transistors and a corresponding transport model is proposed for the tunable polarity behaviors. The findings may provide some new opportunities for building nanoscale electronic and optoelectronic devices.

Band Engineering by Controlling vdW Epitaxy Growth Mode in 2D Gallium Chalcogenides.

Atomically thin quasi-2D GaSe flakes are synthesized via van der Waals (vdW) epitaxy on a polar Si (111) surface. The bandgap is continuously tuned from its commonly accepted value at 620 down to the 700 nm range, only attained previously by alloying Te into GaSe (GaSex Te1- x ). This is accomplished by manipulating various vdW epitaxy kinetic factors, which allows the choice bet ween screw-dislocation-driven and layer-bylayer growth, and the design of different morphologies with different material-substrate interaction (strain) energies.

Enhancing light emission efficiency without color change in post-transition metal chalcogenides.

Two-dimensional (2D) materials can take a large amount of mechanical deformation before reaching the fracture limit due to their high Young's modulus, and this in return, provides a way to tune the properties of 2D materials by strain engineering. Previous works have shown that the optical band gap of transition metal chalcogenides (TMDs) can be modulated by strain, resulting in a drift of the photoluminescence (PL) peak position and a decrease (or little change) in PL intensity. Here, we report a member of the post-transition metal chalcogenides (PTMCs), 2D-GaSe sheets, displaying vastly different phenomena under strain. Strained 2D-GaSe emits photons at almost the same wavelength as unstrained material but appears an order of magnitude brighter. In contrast to TMDs, optical spectroscopy measurements reveal that changes in the optical properties are mostly related to the colossal optical absorption anisotropy of GaSe, instead of commonly accepted strain-induced band renormalization. Results suggest that the light-matter interaction and the optical properties of 2D-GaSe can be controlled at will by manipulating the optical absorption.

Self-Driven Photodetector and Ambipolar Transistor in Atomically Thin GaTe-MoS2 p-n vdW Heterostructure.

Heterostructure engineering of atomically thin two-dimensional materials offers an exciting opportunity to fabricate atomically sharp interfaces for highly tunable electronic and optoelectronic devices. Here, we demonstrate abrupt interface between two completely dissimilar material systems, i.e, GaTe-MoS2 p-n heterojunction transistors, where the resulting device possesses unique electronic properties and self-driven photoelectric characteristics. Fabricated heterostructure transistors exhibit forward biased rectifying behavior where the transport is ambipolar with both electron and hole carriers contributing to the overall transport. Under illumination, photoexcited electron-hole pairs are readily separated by large built-in potential formed at the GaTe-MoS2 interface efficiently generating self-driven photocurrent within <10 ms. Overall results suggest that abrupt interfaces between vastly different material systems with different crystal symmetries still allow efficient charge transfer mechanisms at the interface and are attractive for photoswitch, photodetector, and photovoltaic applications because of large built-in potential at the interface.

Tuning the optical, magnetic, and electrical properties of ReSe2 by nanoscale strain engineering.

Creating materials with ultimate control over their physical properties is vital for a wide range of applications. From a traditional materials design perspective, this task often requires precise control over the atomic composition and structure. However, owing to their mechanical properties, low-dimensional layered materials can actually withstand a significant amount of strain and thus sustain elastic deformations before fracture. This, in return, presents a unique technique for tuning their physical properties by "strain engineering". Here, we find that local strain induced on ReSe2, a new member of the transition metal dichalcogenides family, greatly changes its magnetic, optical, and electrical properties. Local strain induced by generation of wrinkle (1) modulates the optical gap as evidenced by red-shifted photoluminescence peak, (2) enhances light emission, (3) induces magnetism, and (4) modulates the electrical properties. The results not only allow us to create materials with vastly different properties at the nanoscale, but also enable a wide range of applications based on 2D materials, including strain sensors, stretchable electrodes, flexible field-effect transistors, artificial-muscle actuators, solar cells, and other spintronic, electromechanical, piezoelectric, photonic devices.