VSi2P4/FeCl2 van der Waals heterostructure: A two-dimensional reconfigurable magnetic diode

The reconfigurable magnetic tunnel diode has recently been proposed as a promising approach to decrease the base collector leakage currents. However, conventional bulk interfaces usually suffer from strong Fermi level...

A pathway toward high-throughput quantum Monte Carlo simulations for alloys: A case study of two-dimensional (2D) GaSxSe1-x

The study of alloys using computational methods has been a difficult task due to the usually unknown stoichiometry and local atomic ordering of the different structures experimentally. In order to combat this, first-principles methods have been coupled with statistical methods such as the cluster expansion formalism in order to construct the energy hull diagram, which helps to determine if an alloyed structure can exist in nature. Traditionally, density functional theory (DFT) has been used in such workflows. In this paper, we propose to use chemically accurate many-body variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC) methods to construct the energy hull diagram of an alloy system due to the fact that such methods have a weaker dependence on the starting wavefunction and density functional, scale similarly to DFT with the number of electrons, and have had demonstrated success for a variety of materials. To carry out these simulations in a high-throughput manner, we propose a method called Jastrow sharing, which involves recycling the optimized Jastrow parameters between alloys with different stoichiometries. We show that this eliminates the need for extra VMC Jastrow optimization calculations and results in significant computational cost savings (on average 1/4 savings of total computational time). Since it is a novel post-transition metal chalcogenide alloy series that has been synthesized in its few-layer form, we used monolayer GaS_xSe_1-x as a case study for our workflow. By extensively testing our Jastrow sharing procedure for monolayer GaS_xSe_1-x and quantifying the cost savings, we demonstrate how a pathway toward chemically accurate high-throughput simulations of alloys can be achieved using many-body VMC and DMC methods.

Schottky barrier heights in two-dimensional field-effect transistors: from theory to experiment

Over the past decade, two-dimensional semiconductors (2DSCs) have aroused wide interest due to their extraordinary electronic, magnetic, optical, mechanical, and thermal properties, which hold potential in electronic, optoelectronic, thermoelectric applications, and so forth. The field-effect transistor (FET), a semiconductor gated with at least three terminals, is pervasively exploited as the device geometry for these applications. For lack of effective and stable substitutional doping techniques, direct metal contact is often used in 2DSC FETs to inject carriers. A Schottky barrier (SB) generally exists in the metal-2DSC junction, which significantly affects and even dominates the performance of most 2DSC FETs. Therefore, low SB or Ohmic contact is highly preferred for approaching the intrinsic characteristics of the 2DSC channel. In this review, we systematically introduce the recent progress made in theoretical prediction of the SB height (SBH) in the 2DSC FETs and the efforts made both in theory and experiments to achieve low SB contacts. From the comparison between the theoretical and experimentally observed SBHs, the emerging first-principles quantum transport simulation turns out to be the most powerful theoretical tool to calculate the SBH of a 2DSC FET. Finally, we conclude this review from the viewpoints of state-of-the-art electrode designs for 2DSC FETs.

Generation of a square-shaped pulse in mode-locked fiber lasers with a microfiber-based few-layer Nb2C saturable absorber

Niobium carbide (Nb₂C), a novel two-dimensional MXene material, has attracted much attention due to its outstanding electronic and optical properties. In this work, a microfiber-based few-layer Nb₂C saturable absorber (SA) is fabricated by the magnetron sputtering deposition technique. The reverse saturable absorption (RSA) response of few-layer Nb₂C nanosheets is observed with I-scan measurements. The square-wave pulses (SWPs) are generated by using the as-prepared microfiber-based few-layer Nb₂C SA in an erbium-doped fiber laser. The SWP width increases from 0.33 to 2.061 ns with the single pulse energy increases linearly up to 0.89 nJ while the amplitude remains as a constant. In addition, nonlinear polarization rotation mode-locking fiber lasers with different cavity lengths are constructed to explore the formation conditions of SWP. Our results indicate that the RSA effect of the few-layer Nb₂C nanosheets plays a decisive role in the formation of the SWP.

Device Architecture for Visible and Near-Infrared Photodetectors Based on Two-Dimensional SnSe2 and MoS2: A Review

While band gap and absorption coefficients are intrinsic properties of a material and determine its spectral range, response time is mainly controlled by the architecture of the device and electron/hole mobility. Further, 2D-layered materials such as transition metal dichalogenides (TMDCs) possess inherent and intriguing properties such as a layer-dependent band gap and are envisaged as alternative materials to replace conventional silicon (Si) and indium gallium arsenide (InGaAs) infrared photodetectors. The most researched 2D material is graphene with a response time between 50 and 100 ps and a responsivity of <10 mA/W across all wavelengths. Conventional Si photodiodes have a response time of about 50 ps with maximum responsivity of about 500 mA/W at 880 nm. Although the responsivity of TMDCs can reach beyond 10⁴ A/W, response times fall short by 3–6 orders of magnitude compared to graphene, commercial Si, and InGaAs photodiodes. Slow response times limit their application in devices requiring high frequency. Here, we highlight some of the recent developments made with visible and near-infrared photodetectors based on two dimensional SnSe₂ and MoS₂ materials and their performance with the main emphasis on the role played by the mobility of the constituency semiconductors to response/recovery times associated with the hetero-structures.

Reversible Transformation between Bipolar Memory Switching and Bidirectional Threshold Switching in 2D Layered K-Birnessite Nanosheets

Birnessite-related manganese dioxides (MnO₂) have recently been studied owing to their diverse low-dimensional layered structures and potential applications in energy devices. The birnessite MnO₂ possesses a layered structure with edge-shared MnO₆ octahedra layer stacked with interlayer of cations. The unique layered structure may provide some distinct electrical properties for the 2D layered nanosheets. In this work, layered K-birnessite MnO₂ samples are synthesized by a hydrothermal method. The resistive switching (RS) devices based on single K-birnessite MnO₂ nanosheets are fabricated by transferring the nanosheets onto SiO₂/Si substrates through a facile and feasible method of mechanical exfoliation. The device exhibits nonvolatile memory switching (MS) behaviors with high current ON/OFF ratio of ∼2 × 10⁵. And more importantly, reversible transformation between the nonvolatile MS and volatile threshold switching (TS) can be achieved in the single layered nanosheet through tuning the magnitude of compliance current (I_cc). To be more specific, a relatively high I_cc (1 mA) can trigger the nonvolatile MS behaviors, while a relatively low I_cc (≤100 μA) can generate volatile TS characteristics. This work not only demonstrates the memristor based on single birnessite-related MnO₂ nanosheet, but also offers an insight into understanding the complex resistive switching types and relevant physical mechanisms of the 2D layered oxide nanosheets.

Functionalized MXenes as ideal electrodes for Janus MoSSe

Combining functionalized MXenes and mirror asymmetric MoSSe can form ideal electrical contacts.