Observation of Room-Temperature Magnetoresistance in Monolayer MoS2 by Ferromagnetic Gating

Enhanced Magnetism and Anomalous Hall Transport through Two-Dimensional Tungsten Disulfide Interfaces

The magnetic proximity effect (MPE) has recently been explored to manipulate interfacial properties of two-dimensional (2D) transition metal dichalcogenide (TMD)/ferromagnet heterostructures for use in spintronics and valleytronics. However, a full understanding of the MPE and its temperature and magnetic field evolution in these systems is lacking. In this study, the MPE has been probed in Pt/WS₂/BPIO (biphase iron oxide, Fe₃O₄ and α-Fe₂O₃) heterostructures through a comprehensive investigation of their magnetic and transport properties using magnetometry, four-probe resistivity, and anomalous Hall effect (AHE) measurements. Density functional theory (DFT) calculations are performed to complement the experimental findings. We found that the presence of monolayer WS₂ flakes reduces the magnetization of BPIO and hence the total magnetization of Pt/WS₂/BPIO at T > ~120 K-the Verwey transition temperature of Fe₃O₄ (T_V). However, an enhanced magnetization is achieved at T < T_V. In the latter case, a comparative analysis of the transport properties of Pt/WS₂/BPIO and Pt/BPIO from AHE measurements reveals ferromagnetic coupling at the WS₂/BPIO interface. Our study forms the foundation for understanding MPE-mediated interfacial properties and paves a new pathway for designing 2D TMD/magnet heterostructures for applications in spintronics, opto-spincaloritronics, and valleytronics.

Strain-Induced Performance Enhancement of a Monolayer Photodetector via Patterned Substrate Engineering

Two-dimensional (2D) materials exhibit tremendous potential for applications in next-generation photodetectors. Currently, approaches aiming at enhancing the device's performance are limited, mainly relying on complex hybrid systems such as heterostructures and sensitization. Here, we propose a new strategy by constructing patterned nanostructures compatible with the conventional silicon substrate. Using CVD-grown monolayer MoS₂ on the periodical nanocone arrays, we demonstrate a high-performance MoS₂ photodetector via manipulating strain distribution engineered by the substrate at the nanoscale. Compared to the pristine MoS₂ counterpart, the strained MoS₂ photodetector exhibits a much enhanced performance, including a high signal-to-noise ratio over 10⁵ and large responsivity of 3.2 × 10⁴ A W^-1. The physical mechanism responsible for the enhancement is discussed by combining Kelvin probe force microscopy with theoretical simulation. The enhanced performances can be attributed to the improved light absorption, the fast separation of photo-excited carriers, and the suppression of dark currents induced by the designed periodical nanocone arrays. This work depicts an alternative method to achieve high-performance optoelectronic devices based on 2D materials integrated with semiconductor circuits.

Recent Progress in Fabrication and Physical Properties of 2D TMDC-Based Multilayered Vertical Heterostructures

Two-dimensional (2D) vertical heterojunctions (HSs), which are usually fabricated by vertically stacking two layers of transition metal dichalcogenide (TMDC), have been intensively researched during the past years. However, it is still an enormous challenge to achieve controllable preparation of the TMDC trilayer or multilayered van der Waals (vdWs) HSs, which have important effects on physical properties and device performance. In this review, we will introduce fundamental features and various fabrication methods of diverse TMDC-based multilayered vdWs HSs. This review focuses on four fabrication methods of TMDC-based multilayered vdWs HSs, such as exfoliation, chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD), and pulsed laser deposition (PLD). The latest progress in vdWs HS-related novel physical phenomena are summarized, including interlayer excitons, long photocarrier lifetimes, upconversion photoluminescence, and improved photoelectrochemical catalysis. At last, current challenges and prospects in this research field are provided.

Current-Induced Magnetization Switching Across a Nearly Room-Temperature Compensation Point in an Insulating Compensated Ferrimagnet

Insulating compensated ferrimagnets, especially hosting room-temperature compensation points, are considered promising candidates for developing ultra-high-density and ultrafast magnonic devices owing to combining the characteristics of both ferromagnets and antiferromagnets. These intriguing features become outstanding close to their compensation points. However, their spin-orbit torque (SOT)-induced magnetization switching, particularly in the vicinity of the compensation points, remains unclear. Herein, we systematically investigated the SOT in insulating compensated ferrimagnetic Gd₃Fe₅O₁₂/Pt heterostructures with perpendicular magnetic anisotropy. A nearly room-temperature compensation point (T_comp ∼ 297 K) was consistently identified by the magnetization curves, spin Hall-induced anomalous Hall effect, and spin Hall magnetoresistance measurements. Moreover, using 100 ns duration pulsed current, deterministic current-induced magnetization switching below and above T_comp, even at 294 and 301 K, was achieved with opposite switching polarity. It is found that a large current is required to switch the magnetization in the vicinity of T_comp, although the effective SOT field increases close to T_comp. Our finding provides alternative opportunities for exploring ultrafast room-temperature magnon-based devices.

Memristor Based on Inorganic and Organic Two-Dimensional Materials: Mechanisms, Performance, and Synaptic Applications

A memristor is a two-terminal device with nonvolatile resistive switching (RS) behaviors. Recently, memristors have been highly desirable for both fundamental research and technological applications because of their great potential in the development of high-density memory technology and neuromorphic computing. Benefiting from the unique two-dimensional (2D) layered structure and outstanding properties, 2D materials have proven to be good candidates for use in gate-tunable, highly reliable, heterojunction-compatible, and low-power memristive devices. More intriguing, stable and reliable nonvolatile RS behaviors can be achieved in multi- and even monolayer 2D materials, which seems unlikely to be achieved in traditional oxides with thicknesses less than a few nanometers because of the leakage currents. Moreover, such two-terminal devices show a series of synaptic functionalities, suggesting applications in simulating a biological synapse in the neural network. In this review article, we summarize the recent progress in memristors based on inorganic and organic 2D materials, from the material synthesis, device structure and fabrication, and physical mechanism to some versatile memristors based on diverse 2D materials with good RS properties and memristor-based synaptic applications. The development prospects and challenges at the current stage are then highlighted, which is expected to inspire further advancements and new insights into the fields of information storage and neuromorphic computing.

Recent progress in the CVD growth of 2D vertical heterostructures based on transition-metal dichalcogenides

This review summarizes recent advances in the controllable CVD growth of 2D TMDC vertical heterostructures under four different strategies.

High-Temperature Anomalous Hall Effect in a Transition Metal Dichalcogenide Ferromagnetic Insulator Heterostructure

Integration of transition metal dichalcogenides (TMDs) on ferromagnetic materials (FM) may yield fascinating physics and promise for electronics and spintronic applications. In this work, high-temperature anomalous Hall effect (AHE) in the TMD ZrTe₂ thin film using a heterostructure approach by depositing it on a ferrimagnetic insulator YIG (Y₃Fe₅O₁₂, yttrium iron garnet) is demonstrated. In this heterostructure, significant anomalous Hall effect can be observed at temperatures up to at least 400 K, which is a record high temperature for the observation of AHE in TMDs, and the large R_AHE is more than 1 order of magnitude larger than those previously reported values in topological insulators or TMD-based heterostructures. A complicated interface with additional ZrO₂ and amorphous YIG layers is actually observed between ZrTe₂ and YIG. The magnetization of interfacial reaction-induced ZrO₂ and YIG is believed to play a crucial role in the induced high-temperature AHE in the ZrTe₂. These results present a promising system for the spintronic device applications, and it may shed light on the designing approach to introduce magnetism to TMDs at room temperature.

Peculiar piezoelectricity of atomically thin planar structures

The emergence of piezoelectricity in two-dimensional (2D) materials has represented a milestone towards employing low-dimensional structures for future technologies. 2D piezoelectric materials possess unique and unprecedented characteristics that cannot be found in other morphologies; therefore, the applications of piezoelectricity can be substantially extended. By reducing the thickness into the 2D realm, piezoelectricity might be induced in otherwise non-piezoelectric materials. The origin of the enhanced piezoelectricity in such thin planes is attributed to the loss of centrosymmetry, altered carrier concentration, and change in local polarization and can be efficiently tailored via surface modifications. Access to such materials is important from a fundamental research point of view, to observe the extraordinary interactions between free charge carriers, phonons and photons, and also with respect to device development, for which planar structures provide the required compatibility with the large-scale fabrication technologies of integrated circuits. The existence of piezoelectricity in 2D materials presents great opportunities for applications in various fields of electronics, optoelectronics, energy harvesting, sensors, actuators and biotechnology. Additionally, 2D flexible nanostructures with superior piezoelectric properties are distinctive candidates for integration into nano-scale electromechanical systems. Here we fundamentally review the state of the art of 2D piezoelectric materials from both experimental and theoretical aspects and report the recent achievements in the synthesis, characterization and applications of these materials.

Controllable growth of transition metal dichalcogenide multilayer flakes with kirigami structures

2D TMDCs with triangular-shaped and hexagonal-shaped kirigami structures are grown on amorphous SiO₂ substrates by chemical vapor deposition (CVD).

Spin polarization and tunable valley degeneracy in a MoS2 monolayer via proximity coupling to a Cr2O3 substrate

Introducing magnetism in two-dimensional materials is of particular importance for both fundamental research and practical applications in nanoscale spintronics. Herein, we report the lifting of valley degeneracy in a MoS₂ monolayer via magnetic proximity coupling to an insulating antiferromagnetic Cr₂O₃ substrate and the gate-voltage tunability of the MoS₂/Cr₂O₃ heterojunction on the basis of first-principles calculations. Our calculations suggest that there is a large Zeeman splitting of 23.4 meV in the MoS₂ monolayer due to strong spin-orbit coupling, corresponding to a magnetic exchange field of 100 T. Both spin and valley indices flip when the magnetic ordering of Cr₂O₃ is reversed. More interestingly, the charge transfer, magnetic moment, band gap and Schottky barrier of the heterojunction can be tuned continually by applying an external out-of-plane gate voltage, resulting in variable valley Zeeman splitting ranging from 11.3 to 34.5 meV. These findings demonstrate great potential applications of the Cr₂O₃/MoS₂ heterojunction in nanoscale spintronics.

High-performance position-sensitive detector based on the lateral photovoltaic effect in MoSe2/p-Si junctions

Optoelectronic position-sensitive detectors (PSDs) based on the lateral photovoltaic effect (LPE) have been a focus of research due to their ability to detect very small displacements. In this paper, we investigate the LPE properties of MoSe₂/p-Si junctions prepared using pulsed laser deposition. The LPE shows a good linear dependence with the position of the laser spot. A large positional sensitivity and a fast optical relaxation time of 563  mV mm^-1 and 2 μs, respectively, were observed in the MoSe₂ (10 nm)/p-Si junction. The influence of the laser power and the wavelength on the LPE suggests that the observed response originates from the photoelectric effect. The large positional sensitivity and fast relaxation time of the LPE make the MoSe₂/p-Si junction a promising candidate for PSDs.

Lanthanide Yb/Er co-doped semiconductor layered WSe2 nanosheets with near-infrared luminescence at telecommunication wavelengths

Atomically thin layers of transition metal dichalcogenides (TMDs) have recently drawn great attention. However, doping strategies and controlled synthesis for wafer-scale TMDs are still in their early stages, greatly hindering the construction of devices and further basic studies. In this work, we develop the fast deposition of wafer-scale layered lanthanide ion Yb/Er co-doped WSe2 using pulsed laser deposition. WSe2 nanosheets were chosen as the host, while Yb3+ and Er3+ ions served as the sensitizer and activator, respectively. The obtained Yb/Er co-doped WSe2 layers exhibit good uniformity and high crystallinity with highly textured features. Under the excitation of a diode laser at 980 nm, down-conversion emission is observed at around 1540 nm, assigned to the emission transition between the 4I13/2 and 4I15/2 states of Er3+. Considering the significance of 1540 nm luminescence in the application of photonic technologies, this observation in the WSe2:Yb/Er nanosheets down to the monolayer provides a new opportunity for developing photonic devices at the 2D limit. Our work not only offers a general method to prepare wafer-scale lanthanide doped TMDs, but also to widely modulate the luminescence of atomically layered TMDs by introducing lanthanide ions.

Valley Polarization of Trions and Magnetoresistance in Heterostructures of MoS2 and Yttrium Iron Garnet

Manipulation of spin degree of freedom (DOF) of electrons is the fundamental aspect of spintronic and valleytronic devices. Two-dimensional transition metal dichalcogenides (2D TMDCs) exhibit an emerging valley pseudospin, in which spin-up (-down) electrons are distributed in a +K (-K) valley. This valley polarization gives a DOF for spintronic and valleytronic devices. Recently, magnetic exchange interactions between graphene and magnetic insulator yttrium iron garnet (YIG) have been exploited. However, the physics of 2D TMDCs with YIG have not been shown before. Here we demonstrate strong many-body effects in a heterostructure geometry comprising a MoS₂ monolayer and YIG. High-order trions are directly identified by mapping absorption and photoluminescence at 12 K. The electron doping density is up to ∼10¹³ cm^-2, resulting in a large splitting of ∼40 meV between trions and excitons. The trions exhibit a high circular polarization of ∼80% under optical pumping by circularly polarized light at ∼1.96 eV; it is confirmed experimentally that both phonon scattering and electron-hole exchange interaction contribute to the valley depolarization with temperature; importantly, a magnetoresistance (MR) behavior in the MoS₂ monolayer was observed, and a giant MR ratio of ∼30% is achieved, which is 1 order of magnitude larger than the reported ratio in MoS₂/CoFe₂O₄ heterostructures. Our experimental results confirm that the giant MR behaviors are attributed to the interfacial spin accumulation due to YIG substrates. Our work provides an insight into spin manipulation in a heterostructure of monolayer materials and magnetic substrates.

Effects of rhenium dopants on photocarrier dynamics and optical properties of monolayer, few-layer, and bulk MoS2

We report a comprehensive study on the effects of rhenium doping on optical properties and photocarrier dynamics of MoS₂ monolayer, few-layer, and bulk samples. Monolayer and few-layer samples of Re-doped (0.6%) and undoped MoS₂ were fabricated by mechanical exfoliation, and were studied by Raman spectroscopy, optical absorption, photoluminescence, and time-resolved differential reflection measurements. Similar Raman, absorption, and photoluminescence spectra were obtained from doped and undoped samples, indicating that the Re doping at this level does not significantly alter the lattice and electronic structures. Red-shift and broadening of the two phonon Raman modes were observed, showing the lattice strain and carrier doping induced by Re. The photoluminescence yield of the doped monolayer is about 15 times lower than that of the undoped sample, while the photocarrier lifetime is about 20 times shorter in the doped monolayer. Both observations can be attributed to diffusion-limited Auger nonradiative recombination of photocarriers at Re dopants. These results provide useful information for developing a doping strategy of MoS₂ for optoelectronic applications.