Tuning the bandgap of exfoliated InSe nanosheets by quantum confinement

Giant Tunability of Intersubband Transitions and Quantum Hall Quartets in Few-Layer InSe Quantum Wells

A two-dimensional (2D) quantum electron system is characterized by quantized energy levels, or subbands, in the out-of-plane direction. Populating higher subbands and controlling the intersubband transitions have wide technological applications such as optical modulators and quantum cascade lasers. In conventional materials, however, the tunability of intersubband spacing is limited. Here we demonstrate electrostatic population and characterization of the second subband in few-layer InSe quantum wells, with giant tunability of its energy, population, and spin-orbit coupling strength, via the control of not only layer thickness but also the out-of-plane displacement field. A modulation of as much as 350% or over 250 meV is achievable, underscoring the promise of InSe for tunable infrared and THz sources, detectors, and modulators.

Reveal long-lived hot electrons in 2D indium selenide and ferroelectric-regulated carrier dynamics of InSe/α-In2Se3/InSe heterostructure

As typical representatives of group III chalcogenides, InSe, α-In2Se3, and β'-In2Se3 have drawn considerable interest in the domain of photoelectrochemistry. However, the microscopic mechanisms of carrier dynamics in these systems remain largely unexplored. In this work, we first reveal that hot electrons in the three systems have different cooling rate stages and long-lived hot electrons, through the utilization of density functional theory calculations and nonadiabatic molecular dynamics simulations. Furthermore, the ferroelectric polarization of α-In2Se3 weakens the nonadiabatic coupling of the nonradioactive recombination, successfully competing with the narrow bandgap and slow dephasing process, and achieving both high optical absorption efficiency and long carrier lifetime. In addition, we demonstrate that the ferroelectric polarization of α-In2Se3 not only enables the formation of the double type-II band alignment in the InSe/α-In2Se3/InSe heterostructure, with the top and bottom InSe sublayers acting as acceptors and donors, respectively, but also eliminates the hindrance of the built-in electric field at the interface, facilitating an ultrafast interlayer carrier transfer in the heterojunction. This work establishes an atomic mechanism of carrier dynamics in InSe, α-In2Se3, and β'-In2Se3 and the regulatory role of the ferroelectric polarization on the charge carrier dynamics, providing a guideline for the design of photoelectronic materials.

Strong Linearly Polarized Light Emission by Coupling Out-of-Plane Exciton to Anisotropic Gap Plasmon Nanocavity

With exceptional quantum confinement, 2D monolayer semiconductors support a strong excitonic effect, making them an ideal platform for exploring light-matter interactions and as building blocks for novel optoelectronic devices. Different from the well-known in-plane excitons in transition metal dichalcogenides (TMD), the out-of-plane excitons in indium selenide (InSe) usually show weak emission, which limits their applications as light sources. Here, by embedding InSe in an anisotropic gap plasmon nanocavity, we have realized plasmon-enhanced linearly polarized photoluminescence with an anisotropic ratio up to ∼140, corresponding to degree of polarization (DoP) of ∼98.6%. Such polarization selectivity, originating from the polarization-dependent plasmonic enhancement supported by the "nanowire-on-mirror" nanocavity, can be well tuned by the InSe thickness. Moreover, we have also realized an InSe-based light-emitting diode with polarized electroluminescence. Our research highlights the role of excitonic dipole orientation in designing nanophotonic devices and paves the way for developing InSe-based optoelectronic devices with polarization control.

Prediction of intrinsic room-temperature ferromagnetism in two-dimensional CrInX2 (X = S, Se, Te) monolayers

Herein, using density functional theory, novel two-dimensional (2D) CrInX2 (X = S, Se, Te) structures are predicted to be practical ferromagnetic (FM) semiconductors. Phonon vibrations and molecular dynamics simulations verified their structural and thermodynamic stability. Sizable fully spin-polarized band gaps of 1.03 and 0.69 eV are found for CrInS2 and CrInSe2, while CrInTe2 exhibits half-metallic band nature (at 0 K with a perfect lattice). The high magnetic anisotropy energies are responsible for their long-range spin polarization. The Curie temperatures (Tc) are estimated to be 347, 397 and 447 K for CrInS2, CrInSe2 and CrInTe2, respectively, all well above the room-temperature. The high Tc originates from unusual FM direct exchange, the efficient super-exchange coupling between neighboring Cr eg-orbitals with zero virtual exchange gaps and the presence of dual Cr-X-Cr super-exchange channels. Our systematic study of the CrInX2 monolayer suggests that it could be a promising material for spintronics applications.

Colloidal stabilization of hydrophobic InSe 2D nanosheets in a model environmental aqueous solution and their impact on Shewanella oneidensis MR-1

Semiconductor InSe 2D nanomaterials have emerged as potential photoresponsive materials for broadly distributed photodetectors and wearable electronics technologies due to their high photoresponsivity and thermal stability. This paper addresses an environmental concern about the fate of InSe 2D nanosheets when disposed and released into the environment after use. Semiconducting materials are potentially reactive and often form environmentally damaging species, for example reactive oxygen and nitrogen species, when degraded. InSe nanosheets are prepared using a semi bottom-up approach which involves a reaction between indium and selenium precursors at elevated temperature in an oxygen-free environment to prevent oxidation. InSe nanosheets are formed as a stable intermediate with micrometer-sized lateral dimensions and a few monolayer thickness. The InSe 2D nanosheets are obtained when the reaction is stopped after 30 minutes by cooling. Keeping the reaction at elevated temperature for a longer period, for example 60 minutes leads to the formation of InSe 3D nanoparticles of about 5 nm in diameter, a thermodynamically more stable form of InSe. The paper focuses on the colloidal stabilization of InSe nanosheets in an aqueous solution that contains epigallocatechin gallate (EGCG), a natural organic matter (NOM) simulant. We show that EGCG coats the surface of the hydrophobic, water-insoluble InSe nanosheets via physisorption. The formed EGCG-coated InSe nanosheets are colloidally stable in aqueous solution. While unmodified semiconducting InSe nanosheets could produce reactive oxygen species (ROS) when illuminated, our study shows low levels of ROS generation by EGCG-coated InSe nanosheets under ambient light, which might be attributed to ROS quenching by EGCG. Growth-based viability (GBV) assays show that the colloidally stable EGCG-coated InSe nanosheets adversely impact the bacterial growth of Shewanella oneidensis MR-1, an environmentally relevant Gram-negative bacterium in aqueous media. The impact on bacterial growth is driven by the EGCG coating of the nanosheets. In addition, live/dead assays show insignificant membrane damage of the Shewanella oneidensis MR-1 cells by InSe nanosheets, suggesting a weak association of EGCG-coated nanosheets with the cells. It is likely that the adverse impact of EGCG-coated nanosheets on bacterial growth is the result of increasing local concentration of EGCG either when adsorbed on the nanosheets when the nanosheets interact with the cells, or when desorbed from the EGCG-coated nanosheets to interact with the bacterial cells.

The Integration of Two-Dimensional Materials and Ferroelectrics for Device Applications

In recent years, there has been growing interest in functional devices based on two-dimensional (2D) materials, which possess exotic physical properties. With an ultrathin thickness, the optoelectrical and electrical properties of 2D materials can be effectively tuned by an external field, which has stimulated considerable scientific activities. Ferroelectric fields with a nonvolatile and electrically switchable feature have exhibited enormous potential in controlling the electronic and optoelectronic properties of 2D materials, leading to an extremely fertile area of research. Here, we review the 2D materials and relevant devices integrated with ferroelectricity. This review starts to introduce the background about the concerned themes, namely 2D materials and ferroelectrics, and then presents the fundamental mechanisms, tuning strategies, as well as recent progress of the ferroelectric effect on the optical and electrical properties of 2D materials. Subsequently, the latest developments of 2D material-based electronic and optoelectronic devices integrated with ferroelectricity are summarized. Finally, the future outlook and challenges of this exciting field are suggested.

Heat Capacity of Indium or Gallium Sesqui-Chalcogenides

The chalcogenides of p-block elements constitute a significant category of materials with substantial potential for advancing the field of electronic and optoelectronic devices. This is attributed to their exceptional characteristics, including elevated carrier mobility and the ability to fine-tune band gaps through solid solution formation. These compounds exhibit diverse structures, encompassing both three-dimensional and two-dimensional configurations, the latter exemplified by the compound In2Se3. Sesqui-chalcogenides were synthesized through the direct reaction of highly pure elements within a quartz ampoule. Their single-phase composition was confirmed using X-ray diffraction, and the morphology and chemical composition were characterized using scanning electron microscopy. The compositions of all six materials were also confirmed using X-ray photoelectron spectroscopy and Raman spectroscopy. This investigation delves into the thermodynamic properties of indium and gallium sesqui-chalcogenides. It involves low-temperature heat capacity measurements to evaluate standard entropies and Tian-Calvet calorimetry to elucidate the temperature dependence of heat capacity beyond the reference temperature of 298.15 K, as well as the enthalpy of formation assessed from DFT calculations.

The future transistors

The metal-oxide-semiconductor field-effect transistor (MOSFET), a core element of complementary metal-oxide-semiconductor (CMOS) technology, represents one of the most momentous inventions since the industrial revolution. Driven by the requirements for higher speed, energy efficiency and integration density of integrated-circuit products, in the past six decades the physical gate length of MOSFETs has been scaled to sub-20 nanometres. However, the downscaling of transistors while keeping the power consumption low is increasingly challenging, even for the state-of-the-art fin field-effect transistors. Here we present a comprehensive assessment of the existing and future CMOS technologies, and discuss the challenges and opportunities for the design of FETs with sub-10-nanometre gate length based on a hierarchical framework established for FET scaling. We focus our evaluation on identifying the most promising sub-10-nanometre-gate-length MOSFETs based on the knowledge derived from previous scaling efforts, as well as the research efforts needed to make the transistors relevant to future logic integrated-circuit products. We also detail our vision of beyond-MOSFET future transistors and potential innovation opportunities. We anticipate that innovations in transistor technologies will continue to have a central role in driving future materials, device physics and topology, heterogeneous vertical and lateral integration, and computing technologies.

Giant Out-of-Plane Exciton Emission Enhancement in Two-Dimensional Indium Selenide via a Plasmonic Nanocavity

Out-of-plane (OP) exciton-based emitters in two-dimensional semiconductor materials are attractive candidates for novel photonic applications, such as radially polarized sources, integrated photonic chips, and quantum communications. However, their low quantum efficiency resulting from forbidden transitions limits their practicality. In this work, we achieve a giant enhancement of up to 34000 for OP exciton emission in indium selenide (InSe) via a designed Ag nanocube-over-Au film plasmonic nanocavity. The large photoluminescence enhancement factor (PLEF) is attributed to the induced OP local electric field (E_z) within the nanocavity, which facilitates effective OP exciton-plasmon interaction and subsequent tremendous enhancement. Moreover, the nanoantenna effect resulting from the effective interaction improves the directivity of spontaneous radiation. Our results not only reveal an effective photoluminescence enhancement approach for OP excitons but also present an avenue for designing on-chip photonic devices with an OP dipole orientation.

Lattice Thermal Conductivity of Monolayer InSe Calculated by Machine Learning Potential

The two-dimensional post-transition-metal chalcogenides, particularly indium selenide (InSe), exhibit salient carrier transport properties and evince extensive interest for broad applications. A comprehensive understanding of thermal transport is indispensable for thermal management. However, theoretical predictions on thermal transport in the InSe system are found in disagreement with experimental measurements. In this work, we utilize both the Green-Kubo approach with deep potential (GK-DP), together with the phonon Boltzmann transport equation with density functional theory (BTE-DFT) to investigate the thermal conductivity (κ) of InSe monolayer. The κ calculated by GK-DP is 9.52 W/mK at 300 K, which is in good agreement with the experimental value, while the κ predicted by BTE-DFT is 13.08 W/mK. After analyzing the scattering phase space and cumulative κ by mode-decomposed method, we found that, due to the large energy gap between lower and upper optical branches, the exclusion of four-phonon scattering in BTE-DFT underestimates the scattering phase space of lower optical branches due to large group velocities, and thus would overestimate their contribution to κ. The temperature dependence of κ calculated by GK-DP also demonstrates the effect of higher-order phonon scattering, especially at high temperatures. Our results emphasize the significant role of four-phonon scattering in InSe monolayer, suggesting that combining molecular dynamics with machine learning potential is an accurate and efficient approach to predict thermal transport.

Probing Anisotropic Deformation and Near-Infrared Emission Tuning in Thin-Layered InSe Crystal under High Pressure

Indium selenide (InSe) exhibits high lattice compressibility and an extraordinary capability of tailoring the optical band gap under pressure beyond other 2D materials. Herein, by applying hydrostatic pressure via a diamond anvil cell, we revealed an anisotropic deformation dynamic and efficient manipulation of near-infrared light emission in thin-layered InSe strongly correlated to layer numbers (N = 5-30). As N > 20, the InSe lattice is compressed in all directions, and the intralayer compression leads to widening of the band gap, resulting in an emission blue shift (∼120 meV at 1.5 GPa). In contrast, as N ≤ 15, an efficient emission red shift is observed from band gap shrinkage (rate of 100 meV GPa^-1), which is attributed to the predominant uniaxial interlayer compression because of the high strain resistance along the InSe-diamond interface. These findings advance the understanding of pressure-induced lattice deformation and optical transition evolution in InSe and could be applied to other 2D materials.

Coupling between Pyroelectricity and Built-In Electric Field Enabled Highly Sensitive Infrared Phototransistor Based on InSe/WSe2/P(VDF-TrFE) Heterostructure

The assorted utilization of infrared detectors induces the demand for more comprehensive and high-performance electronic devices that work at room temperature. The intricacy of the fabrication process with bulk material limits the exploration in this field. However, two-dimensional (2D) materials with a narrow band gap opening aid in infrared (IR) detection relatively, but the photodetection range is narrowed due to the inherent band gap. In this study, we report an unprecedented attempt at the coordinated use of both 2D heterostructure (InSe/WSe₂) and the dielectric polymer (poly(vinylidene fluoride-trifluoroethylene), P(VDF-TrFE)) for both visible and IR photodetection in a single device. The remnant polarization due to the ferroelectric effect of the polymer dielectric enhances the photocarrier separation in the visible range, resulting in high photoresponsivity. On the other hand, the pyroelectric effect of the polymer dielectric causes a change in the device current due to the increased temperature induced by the localized heating effect of the IR irradiation, which results in the change of ferroelectric polarization and induces the redistribution of charge carriers. In turn, it changes the built-in electric field, the depletion width, and the band alignment across the p-n heterojunction interface. Consequently, the charge carrier separation and the photosensitivity are therefore enhanced. Through the coupling between pyroelectricity and built-in electric field across the heterojunction, the specific detectivity for the photon energy below the band gap of the constituent 2D materials can reach up to 10¹¹ Jones, which is better than all reported pyroelectric IR detectors. The proposed approach combining the ferroelectric and pyroelectric effects of the dielectric as well as exceptional properties of the 2D heterostructures can spark the design of advanced and not-yet realized optoelectronic devices.

First-principles study on the electronic structure and photocatalytic property of a novel two-dimensional ZrS2/InSe heterojunction

Photocatalytic water cracking technology provides a broad prospect for solving the current energy crisis using solar energy and water resources. In this paper, a two-dimensional ZrS₂/InSe heterojunction for accelerating the process of hydrogen production from water decomposition was constructed, and its electronic structure and photocatalytic property were studied using first-principles calculation. The results show that the lattice mismatch rate of the heterojunction from monolayer ZrS₂ and monolayer InSe is 2.48%, and its binding energy is -1.696 eV, indicating that the structure of the heterojunction is stable. The ZrS₂/InSe heterojunction is an indirect bandgap with a bandgap value of 1.41 eV and a typical type-II band arrangement. Importantly, the ZrS₂/InSe heterostructure has a Z-scheme structure, which is beneficial to the separation of photogenerated electron hole pairs. Moreover, the ZrS₂/InSe heterojunction has a strong absorption ability for visible light (up to 3.84 × 10⁵ cm^-1), which is helpful for improving its photocatalytic efficiency. The two-dimensional ZrS₂/InSe heterojunction is a very promising photocatalyst, as concluded from the above studies.

Subnanometer-Wide Indium Selenide Nanoribbons

Indium selenides (In_xSe_y) have been shown to retain several desirable properties, such as ferroelectricity, tunable photoluminescence through temperature-controlled phase changes, and high electron mobility when confined to two dimensions (2D). In this work we synthesize single-layer, ultrathin, subnanometer-wide In_xSe_y by templated growth inside single-walled carbon nanotubes (SWCNTs). Despite the complex polymorphism of In_xSe_y we show that the phase of the encapsulated material can be identified through comparison of experimental aberration-corrected transmission electron microscopy (AC-TEM) images and AC-TEM simulations of known structures of In_xSe_y. We show that, by altering synthesis conditions, one of two different stoichiometries of sub-nm In_xSe_y, namely InSe or β-In₂Se₃, can be prepared. Additionally, in situ AC-TEM heating experiments reveal that encapsulated β-In₂Se₃ undergoes a phase change to γ-In₂Se₃ above 400 °C. Further analysis of the encapsulated species is performed using X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), energy dispersive X-ray analysis (EDX), and Raman spectroscopy, corroborating the identities of the encapsulated species. These materials could provide a platform for ultrathin, subnanometer-wide phase-change nanoribbons with applications as nanoelectronic components.

Nanosized indium selenide saturable absorber for multiple solitons operation in Er3+-doped fiber laser

With the advances in the field of ultrafast photonics occurring so fast, the demand for optical modulation devices with high performance and soliton lasers which can realize the evolution of multiple soliton pulses is gradually increasing. Nevertheless, saturable absorbers (SAs) with appropriate parameters and pulsed fiber lasers which can output abundant mode-locking states still need to be further explored. Due to the special band gap energy values of few-layer indium selenide (InSe) nanosheets, we have prepared a SA based on InSe on a microfiber by optical deposition. In addition, we demonstrate that our prepared SA possesses a modulation depth and saturable absorption intensity about 6.87% and 15.83 MW/cm², respectively. Then, multiple soliton states are obtained by dispersion management techniques, including regular solitons, and second-order harmonic mode-locking solitons. Meanwhile, we have obtained multi-pulse bound state solitons. We also provide theoretical basis for the existence of these solitons. The results of the experiment show that the InSe has the potential to be an excellent optical modulator because of its excellent saturable absorption properties. This work also is important for improving the understanding and knowledge of InSe and the output performance of fiber lasers.

Revealing the origin of PL evolution of InSe flake induced by laser irradiation

Two-dimensional InSe has been considered as a promising candidate for novel optoelectronic devices owing to large electron mobility and a near-infrared optical band gap. However, its widespread applications suffer from environmental instability. A lot of theoretical studies on the degradation mechanism of InSe have been reported whereas the experimental proofs are few. Meanwhile, the role of the extrinsic environment is still obscure during the degradation. As a common technique of studying the degradation mechanism of 2D materials, laser irradiation exhibits many unique advantages, such as being fast, convenient, and offering in situ compatibility. Here, we have developed a laser-treated method, which involves performing repeated measurements at the same point while monitoring the evolution of the resulting PL, to systematically study the photo-induced degradation process of InSe. Interestingly, we observe different evolution behavior of PL intensity under weak irradiation and strong irradiation. Our experimental results indicate the vacancy passivation and degrading effect simultaneously occurring in InSe under a weak laser irradiation, resulting in the PL increasing first and then decreasing during the measurement. Meanwhile we also notice that the passivation has a stronger effect on the PL than the degrading effect of weak oxidation. In contrast, under a strong laser irradiation, the InSe suffers serious destruction caused by excess heating and intense oxidation. This leads to a direct decrease of PL and corresponding oxidative products. Our work provides a reliable experimental supplement to the photo oxidation study of InSe and opens up a new avenue to regulate the PL of InSe.

Carrier and phonon transport in 2D InSe and its Janus structures

Recently, two-dimensional (2D) Indium Selenide (InSe) has been receiving much attention in the scientific community due to its reduced size, extraordinary physical properties, and potential applications in various fields. In this review, we discussed the recent research advancement in the carrier and phonon transport properties of 2D InSe and its related Janus structures. We first introduced the progress in the synthesis of 2D InSe. We summarized the recent experimental and theoretical works on the carrier mobility, thermal conductivity, and thermoelectric characteristics of 2D InSe. Based on the Boltzmann transport equation (BTE), the mechanisms underlying carrier or phonon scattering of 2D InSe were discussed in detail. Moreover, the structural and transport properties of Janus structures based on InSe were also presented, with an emphasis on the theoretical simulations. At last, we discussed the prospects for continued research of 2D InSe.

Highly Stable InSe-FET Biosensor for Ultra-Sensitive Detection of Breast Cancer Biomarker CA125

Two-dimensional materials-based field-effect transistors (FETs) are promising biosensors because of their outstanding electrical properties, tunable band gap, high specific surface area, label-free detection, and potential miniaturization for portable diagnostic products. However, it is crucial for FET biosensors to have a high electrical performance and stability degradation in liquid environments for their practical application. Here, a high-performance InSe-FET biosensor is developed and demonstrated for the detection of the CA125 biomarker in clinical samples. The InSe-FET is integrated with a homemade microfluidic channel, exhibiting good electrical stability during the liquid channel process because of the passivation effect on the InSe channel. The InSe-FET biosensor is capable of the quantitative detection of the CA125 biomarker in breast cancer in the range of 0.01-1000 U/mL, with a detection time of 20 min. This work provides a universal detection tool for protein biomarker sensing. The detection results of the clinical samples demonstrate its promising application in early screenings of major diseases.

Interfacial Charge Transfer and Ultrafast Photonics Application of 2D Graphene/InSe Heterostructure

Interface interactions in 2D vertically stacked heterostructures play an important role in optoelectronic applications, and photodetectors based on graphene/InSe heterostructures show promising performance nowadays. However, nonlinear optical property studies based on the graphene/InSe heterostructure are insufficient. Here, we fabricated a graphene/InSe heterostructure by mechanical exfoliation and investigated the optically induced charge transfer between graphene/InSe heterostructures by taking photoluminescence and pump-probe measurements. The large built-in electric field at the interface was confirmed by Kelvin probe force microscopy. Furthermore, due to the efficient interfacial carrier transfer driven by the built-in electric potential (~286 meV) and broadband nonlinear absorption, the application of the graphene/InSe heterostructure in a mode-locked laser was realized. Our work not only provides a deeper understanding of the dipole orientation-related interface interactions on the photoexcited charge transfer of graphene/InSe heterostructures, but also enriches the saturable absorber family for ultrafast photonics application.

Tunable Gain SnS2/InSe Van der Waals Heterostructure Photodetector

Due to the favorable properties of two-dimensional materials such as SnS_2, with an energy gap in the visible light spectrum, and InSe, with high electron mobility, the combination of them can create a novel platform for electronic and optical devices. Herein, we study a tunable gain SnS₂/InSe Van der Waals heterostructure photodetector. SnS₂ crystals were synthesized by chemical vapor transport method and characterized using X-ray diffraction and Raman spectroscopy. The exfoliated SnS₂ and InSe layers were transferred on the substrate. This photodetector presents photoresponsivity from 14 mA/W up to 740 mA/W and detectivity from 2.2 × 10⁸ Jones up to 3.35 × 10⁹ Jones by gate modulation from 0 V to +70 V. Light absorption and the charge carrier generation mechanism were studied by the Silvaco TCAD software and the results were confirmed by our experimental observations. The rather high responsivity and visible spectrum response makes the SnS₂/InSe heterojunction a potential candidate for commercial visible image sensors.

Optical determination of layered-materials InSe thickness via RGB contrast method and regression analysis

Indium selenide (InSe) features intriguing thickness-dependent optoelectronic properties, and a simple, and precise way to identify the thickness is essential for the rapid development of InSe research. Here, a red, green, and blue (RGB) color contrast method with regression analysis for quantitative correlation of three optical contrasts from RGB channels with the InSe thickness (1-35 nm), is demonstrated. The lower accuracy of the thickness identification obtained from the individual channels was discussed. Moreover, the effective refractive indices in the three RGB regions can be extracted from the Fresnel equation and numerical analysis by finding the best fit to the experimental optical contrast. After further consideration of the wavelength-dependent refractive indices, the slope of the regression line between the estimated thickness and that obtained from the atomic force microscope was improved from 1.59 ± 0.05 to 0.97 ± 0.02. The complex refractive index spectra of InSe (1-10 layers) generated fromab initionumerical calculation results were also adopted to identify the InSe thickness. Compared to dispersion, the evolution of the band structure had less effect on thickness identification. This work could be extended to other layered materials, facilitate the thickness-dependent study of layered materials, and expedite the realization of their practical applications.

Nearly Ideal Two-Dimensional Electron Gas Hosted by Multiple Quantized Kronig-Penney States Observed in Few-Layer InSe

A theoretical ideal two-dimensional electron gas (2DEG) was characterized by a flat density of states independent of energy. Compared with conventional two-dimensional free-electron systems in semiconductor heterojunctions and noble metal surfaces, we report here the achievement of ideal 2DEG with multiple quantized states in few-layer InSe films. The multiple quantum well states (QWSs) in few-layer InSe films are found, and the number of QWSs is strictly equal to the number of atomic layers. The multiple stair-like DOS as well as multiple bands with parabolic dispersion both characterize ideal 2DEG features in these QWSs. Density functional theory calculations and numerical simulations based on quasi-bounded square potential wells described as the Kronig-Penney model provide a consistent explanation of 2DEG in the QWSs. Our work demonstrates that 2D van der Waals materials are ideal systems for realizing 2DEG hosted by multiple quantized Kronig-Penney states, and the semiconducting nature of the material provides a better chance for construction of high-performance electronic devices utilizing these states, for example, superlattice devices with negative differential resistance.

On-chip photonics and optoelectronics with a van der Waals material dielectric platform

During the last few decades, photonic integrated circuits have increased dramatically, facilitating many high-performance applications, such as on-chip sensing, data processing, and inter-chip communications. The currently dominating material platforms (i.e., silicon, silicon nitride, lithium niobate, and indium phosphide), which have exhibited great application successes, however, suffer from their own disadvantages, such as the indirect bandgap of silicon for efficient light emission, and the compatibility challenges of indium phosphide with the silicon industry. Here, we report a new dielectric platform using nanostructured bulk van der Waals materials. On-chip light propagation, emission, and detection are demonstrated by taking advantage of different van der Waals materials. Low-loss passive waveguides with MoS2 and on-chip light sources and photodetectors with InSe have been realised. Our proof-of-concept demonstration of passive and active on-chip photonic components endorses van der Waals materials for offering a new dielectric platform with a large material-selection degree of freedom and unique properties toward close-to-atomic scale manufacture of on-chip photonic and optoelectronic devices.

Engineering the Dipole Orientation and Symmetry Breaking with Mixed-Dimensional Heterostructures

Engineering of the dipole and the symmetry of materials plays an important role in fundamental research and technical applications. Here, a novel morphological manipulation strategy to engineer the dipole orientation and symmetry of 2D layered materials by integrating them with 1D nanowires (NWs) is reported. This 2D InSe -1D AlGaAs NW heterostructure example shows that the in-plane dipole moments in InSe can be engineered in the mixed-dimensional heterostructure to significantly enhance linear and nonlinear optical responses (e.g., photoluminescence, Raman, and second harmonic generation) with an enhancement factor of up to ≈12. Further, the 1D NW can break the threefold rotational symmetry of 2D InSe, leading to a strong optical anisotropy of up to ≈65%. These results of engineering dipole orientation and symmetry breaking with the mixed-dimensional heterostructures open a new path for photonic and optoelectronic applications.

Plasmon-enhanced Raman spectroscopy of two-dimensional semiconductors

Two-dimensional (2D) semiconductors have grown fast into an extraordinary research field due to their unique physical properties compared to other semiconducting materials. The class of materials proved extremely fertile for both fundamental studies and a wide range of applications from electronics/spintronics/optoelectronics to photocatalysis and CO₂reduction. 2D materials are highly confined in the out-of-plane direction and often possess very good environmental stability. Therefore, they have also become a popular material system for the manipulation of optoelectronic properties via numerous external parameters. Being a versatile characterization technique, Raman spectroscopy is used extensively to study and characterize various physical properties of 2D materials. However, weak signals and low spatial resolution hinder its application in more advanced systems where decoding local information plays an important role in advancing our understanding of these materials for nanotechnology applications. In this regard, plasmon-enhanced Raman spectroscopy has been introduced in recent time to investigate local heterogeneous information of 2D semiconductors. In this review, we summarize the recent progress of plasmon-enhanced Raman spectroscopy of 2D semiconductors. We discuss the current state-of-art and provide future perspectives on this specific branch of Raman spectroscopy applied to 2D semiconductors.

Microscopic optical nonlinearities and transient carrier dynamics in indium selenide nanosheet

This work systematically investigates the third-order nonlinear optical (NLO) properties and ultrafast carrier dynamics of layered indium selenide (InSe) obtained by mechanical exfoliation (ME). The two-photon absorption (TPA) effect of layered InSe was tested using micro-Z/I-scan techniques. The results indicate that InSe flakes undergo the TPA response under the excitation of both 520 nm and 1040 nm fs pulses, and that InSe is more likely to achieve TPA saturation under visible light excitation. Furthermore, ultrafast carrier dynamics revealed that InSe flakes in the visible region undergo a transition from photoinduced absorption to photobleaching and exhibit a fast recombination time of ∼0.4-1ps, suggesting a high optical modulation speed as high as ∼1-2.5 THz.

Engineering Near-Infrared Light Emission in Mechanically Exfoliated InSe Platelets through Hydrostatic Pressure for Multicolor Microlasing

γ-indium selenide (InSe) is a van der Waals semiconductor and holds great potentials for low-energy-consumption electronic and optoelectronic devices. Herein, we investigated the hydrostatic pressure engineered near-infrared (NIR) light emission of mechanically exfoliated γ-InSe crystals using the diamond anvil cell (DAC) technique. A record-wide spectral tuning range of 185 nm and a large linear pressure coefficient of 40 nm GPa^-1 were achieved for spontaneous emissions, leading to ultrabroadband microlasing spectrally ranging from 1022 to 911 nm. This high emission tunability can be attributed to the compression of the soft intralayer In-Se bonds under high pressure, which suppressed the band gap shrinkage by increasing the interlayer interaction. Furthermore, two band gap crossovers of valence (direct-to-indirect) and conduction bands were resolved at approximately 4.0 and 7.0 GPa, respectively, resulting in pressure-sensitive emission lifetime and intensity. These findings pave the pathways for pressure-sensitive InSe-based NIR light sources, sensors and so on.

Layer Number-Dependent Raman Spectra of γ-InSe

The two-dimensional layered semiconductor InSe, with its high carrier mobility, chemical stability, and strong charge transfer ability, plays a crucial role in optoelectronic devices. The number of InSe layers (L) has an important influence on its band structure and optoelectronic properties. Herein we present systematic investigations on few-layer (1L-7L) γ-InSe by optical contrast and Raman spectroscopy. We propose three quantified formulas to quickly identify the layer number using optical contrast, the frequency difference of two A₁ modes, and ultralow-frequency Raman spectroscopy, respectively. Moreover, angle-resolved polarization Raman spectra show that γ-InSe is isotropic in the a-b plane. Furthermore, using Raman mapping, we find that the relative strength of the low-frequency interlayer shear modes is particularly sensitive to the interaction between the sample and the substrate.

An analysis of Schottky barrier in silicene/Ga2SeS heterostructures by employing electric field and strain

Two-dimensional materials are leading the way in nanodevice applications thanks to their various advantages. Although two-dimensional materials show promise for many applications, they have certain limitations. In the last decade, the increasing demand for the applications of novel two-dimensional materials has accelerated heterostructure studies in this field. Hence, restoring the combination of two-dimensional heterostructured materials has been reported. In this paper, we show that the effect of the external electric field and biaxial strain on the silicene/Ga₂SeS heterostructure has a critical impact on the tuning of the Schottky barrier height. The findings such as the variation of the electronic band gap, interlayer charge transfer, total dipole moment, and n-type/p-type Schottky barrier transitions of the silicene/Ga₂SeS heterostructure under external effects imply that the device performance can be adjusted with Janus 2D materials.

InSe:Ge-doped InSe van der Waals heterostructure to enhance photogenerated carrier separation for self-powered photoelectrochemical-type photodetectors

Two-dimensional (2D) van der Waals (vdW) materials with tunable heterostructures and superior optoelectronic properties have opened a new platform for various applications, e.g., field-effect transistors, ultrasensitive photodetectors and photocatalysts. In this work, an InSe/InSe(Ge) (germanium doped InSe) vdW heterostructure is designed to improve the photoresponse performance of sole InSe in a photoelectrochemical (PEC)-type photodetector. Photoelectrochemical measurements demonstrated that this heterostructure has excellent photoresponse characteristics, including a photocurrent density of 9.8 μA cm^-2, a photo-responsivity of 64 μA W^-1, and a response time/recovery time of 0.128 s/0.1 s. Moreover, the measurements also revealed the self-powering capability and long-term cycling stability of this heterostructure. The electronic properties of the prepared pure and Ge-doped single crystals unveiled a negative and temperature-independent thermoelectric power and temperature-activated resistivity. The negative character of dominating charge carriers was confirmed by Hall measurements, which corroborated by electrical resistivity revealed a carrier concentration below ∼10¹⁵ cm^-3 and an electron mobility of ∼500 cm² V^-1 s^-1 in Ge-doped crystals. Additionally, the Mott-Schottky model explored the mechanism of charge transfer and enhanced PEC performance. Band bending at the InSe/InSe(Ge)-electrolyte interface benefits the separation and transformation of photogenerated carriers from the heterostructure to electrolyte due to the tunable energy band alignment. These results indicate that the InSe/InSe(Ge) vdW heterostructure is promising for PEC-type photodetectors, which provide a novel way to utilize 2D vdW heterostructures in optoelectronics.

Spectral Engineering of InSe Nanobelts for Full-Color Imaging by Tailoring the Thickness

In this work, we report on the synthesis of InSe nanobelts through a catalyst-free chemical vapor deposition (CVD) growth approach. A remarkable blue shift of the peak photoresponse was observed when the thickness of the InSe nanobelt decreases from 562 to 165 nm. Silvaco Technology Computer Aided Design (TCAD) simulation reveals that such a shift in spectral response should be ascribed to the wavelength-dependent absorption coefficient of InSe, for which incident light with shorter wavelengths will be absorbed near the surface, while light with longer wavelengths will have a greater penetration depth, leading to a red shift of the absorption edge for thicker nanobelt devices. Considering the above theory, three kinds of photodetectors sensitive to blue (450 nm), green (530 nm), and red (660 nm) incident light were achieved by tailoring the thickness of the nanobelts, which can enable the spectral reconstruction of a purple "H" pattern, suggesting the potential application of 2D layered semiconductors in full-color imaging.

Phase Modulation of Self-Gating in Ionic Liquid-Functionalized InSe Field-Effect Transistors

Understanding the Coulomb interactions between two-dimensional (2D) materials and adjacent ions/impurities is essential to realizing 2D material-based hybrid devices. Electrostatic gating via ionic liquids (ILs) has been employed to study the properties of 2D materials. However, the intrinsic interactions between 2D materials and ILs are rarely addressed. This work studies the intersystem Coulomb interactions in IL-functionalized InSe field-effect transistors by displacement current measurements. We uncover a strong self-gating effect that yields a 50-fold enhancement in interfacial capacitance, reaching 550 nF/cm² in the maximum. Moreover, we reveal the IL-phase-dependent transport characteristics, including the channel current, carrier mobility, and density, substantiating the self-gating at the InSe/IL interface. The dominance of self-gating in the rubber phase is attributed to the correlation between the intra- and intersystem Coulomb interactions, further confirmed by Raman spectroscopy. This study provides insights into the capacitive coupling at the InSe/IL interface, paving the way to developing liquid/2D material hybrid devices.

Room-temperature Near-infrared Excitonic Lasing from Mechanically Exfoliated InSe Microflake

The development of chip-level near-infrared laser sources using two-dimensional semiconductors is imperative to maintain the architecture of van der Waals integrated optical interconnections. However, the established two-dimensional semiconductor lasers may have either the disadvantages of poor controllability of monolayered gain media, large optical losses on silicon, or complicated fabrication of external optical microcavities. This study demonstrates room-temperature near-infrared lasing from mechanically exfoliated γ-phase indium selenide (InSe) microflakes free from external optical microcavities at a center wavelength of ∼1030 nm. The lasing action occurs at the sub-Mott density level and is generated by exciton-exciton scattering with a high net modal optical gain of ∼1029 cm^-1. Moreover, the lasing is sustained for microdisks fabricated by a simple laser printing with a reduced threshold. These results suggest that InSe is a promising material for near-infrared microlasers and can be employed in a wide range of applications, including imaging, sensing, and optical interconnects.

Optical Inspection of 2D Materials: From Mechanical Exfoliation to Wafer-Scale Growth and Beyond

Optical inspection is a rapid and non-destructive method for characterizing the properties of two-dimensional (2D) materials. With the aid of optical inspection, in situ and scalable monitoring of the properties of 2D materials can be implemented industrially to advance the development and progress of 2D material-based devices toward mass production. This review discusses the optical inspection techniques that are available to characterize various 2D materials, including graphene, transition metal dichalcogenides (TMDCs), hexagonal boron nitride (h-BN), group-III monochalcogenides, black phosphorus (BP), and group-IV monochalcogenides. First, the authors provide an introduction to these 2D materials and the processes commonly used for their fabrication. Then they review several of the important structural properties of 2D materials, and discuss how to characterize them using appropriate optical inspection tools. The authors also describe the challenges and opportunities faced when applying optical inspection to recently developed 2D materials, from mechanically exfoliated to wafer-scale-grown 2D materials. Most importantly, the authors summarize the techniques available for largely and precisely enhancing the optical signals from 2D materials. This comprehensive review of the current status and perspective of future trends for optical inspection of the structural properties of 2D materials will facilitate the development of next-generation 2D material-based devices.

Synthesis of photo-responsive indium selenides (InSe and In2Se3) from tris(4,6-dimethyl-2-pyrimidylselenolato)indium(iii) as a molecular precursor

Facile and selective synthesis of phase pure photo-responsive InSe and In2Se3 nanostructures employing air-stable In[Sepym(Me-4,6)2]3 as a novel molecular precursor.

Highly-Tunable Intrinsic Room-Temperature Ferromagnetism in 2D van der Waals Semiconductor Crx Ga1- x Te

The combination of semiconductivity and tunable ferromagnetism is pivotal for electrical control of ferromagnetism and next-generation low-power spintronic devices. However, Curie temperatures (T_C ) for most traditional intrinsic ferromagnetic semiconductors (≤200 K) and recently discovered two-dimensional (2D) ones (<70 K) are far below room temperature. 2D van der Waals (vdW) semiconductors with intrinsic room-temperature ferromagnetism remain elusive considering the unfavored 2D long-range ferromagnetic order indicated by Mermin-Wagner theorem. Here, vdW semiconductor Cr_x Ga_{1- x} Te crystals exhibiting highly tunable above-room-temperature ferromagnetism with bandgap 1.62-1.66 eV are reported. The saturation magnetic moment (M_sat ) of Cr_x Ga_{1- x} Te crystals can be effectively regulated up to ≈5.4 times by tuning Cr content and ≈75.9 times by changing the thickness. vdW Cr_x Ga_{1- x} Te ultrathin semiconductor crystals show robust room-temperature ferromagnetism with the 2D quantum confinement effect, enabling T_C 314.9-329 K for nanosheets, record-high for intrinsic vdW 2D ferromagnetic semiconductors. This work opens an avenue to room-temperature 2D vdW ferromagnetic semiconductor for 2D electronic and spintronic devices.

Enhancement of InSe Field-Effect-Transistor Performance against Degradation of InSe Film in Air Environment

The degradation of InSe film and its impact on field effect transistors are investigated. After the exposure to atmospheric environment, 2D InSe flakes produce irreversible degradation that cannot be stopped by the passivation layer of h-BN, causing a rapid decrease for InSe FETs performance, which is attributed to the large number of traps formed by the oxidation of 2D InSe and adsorption to impurities. The residual photoresist in lithography can cause unwanted doping to the material and reduce the performance of the device. To avoid contamination, a high-performance InSe FET is achieved by a using hard shadow mask instead of the lithography process. The high-quality channel surface is manifested by the hysteresis of the transfer characteristic curve. The hysteresis of InSe FET is less than 0.1 V at V_d of 0.2, 0.5, and 1 V. And a high on/off ratio of 1.25 × 10⁸ is achieved, as well relative high I_on of 1.98 × 10⁻⁴ A and low SS of 70.4 mV/dec at V_d = 1 V are obtained, demonstrating the potential for InSe high-performance logic device.

Structural, electronic, and transport properties of Janus GaInX2(XS, Se, Te) monolayers: first-principles study

Two-dimensional Janus monolayers have outstanding electronic and transport properties due to their asymmetric atomic structures. In the present work, we systematically study the structural, electronic, and transport properties of the Janus GaInX₂(X= S, Se, Te) monolayers by using the first-principles calculations. The stability of the investigated monolayers is confirmed via the analysis of vibrational spectrum and molecular dynamics simulations. Our calculations demonstrate that while GaInS₂and GaInSe₂monolayers are direct semiconductors, GaInTe₂monolayer exhibits the characteristics of an indirect semiconductor. The band gap of GaInX₂decreases when the chalcogen elementXvaries from S to Te. Obtained results reveal that small spin-orbit splitting energy in the valence band is found around the Γ point of the Brillouin zone when the spin-orbit coupling is included. Interestingly, GaInS₂and GaInSe₂have high and directional isotropic electron mobility meanwhile the directional anisotropy of the electron mobility is found in the Janus GaInTe₂monolayer. Our findings not only present superior physical properties of GaInX₂monolayers but also show promising potential applications of these materials in nanoelectronic devices.

Revealing the tunability of electronic structures and optical properties of novel SWCNT derivatives, phenine nanotubes

Single-walled carbon nanotubes (SWCNTs) have evoked great interest for various luminescent applications, but the large emission heterogeneity resulting from the structural complexity of the samples seriously restricts their further development. Herein we theoretically explore the electronic structures and optical properties of phenine nanotubes (pNTs), which are typical luminescent SWCNT derivatives with determined molecular structures that have been synthesized recently (Z. Sun, K. Ikemoto, T. M. Fukunaga, T. Koretsune, R. Arita, S. Sato and H. Isobe, Science, 2019, 363, 151-155; K. Ikemoto, S. Yang, H. Naito, M. Kotani, S. Sato and H. Isobe, Nat. Commun., 2020, 11, 1807). Interestingly, pNTs are found to feature different semiconducting properties to SWCNTs, as indicated by a spatial separation trend in the HOMO and LUMO resulting from periodic structural vacancies. The HOMO-LUMO and optical gaps of pNTs depend inversely on their lengths and diameters, but diameter variation should be an ineffective method for property tuning due to its negligible influence. By contrast, chemical modifications via N doping or hydrogenation highly affect the HOMO-LUMO gaps and their distributions and greatly broaden the light absorption/emission range, and importantly, low-dose hydrogenation is predicted to be a feasible strategy to enhance luminescence. This work, by studying the fundamental photophysical properties of pNTs and making comparisons to SWCNTs, shows the promise of structural vacancy engineering and surface functionalization in acquiring multifunctional tube-like materials.

Influence of Ce, Nd, Eu and Tm Dopants on the Properties of InSe Monolayer: A First-Principles Study

Doping of foreign atoms may substantially alter the properties of the host materials, in particular low-dimension materials, leading to many potential functional applications. Here, we perform density functional theory calculations of two-dimensional InSe materials with substitutional doping of lanthanide atoms (Ce, Nd, Eu, Tm) and investigate systematically their structural, magnetic, electronic and optical properties. The calculated formation energy shows that the substitutional doping of these lanthanide atoms is feasible in the InSe monolayer, and such doping is more favorable under Se-rich than In-rich conditions. As for the structure, doping of lanthanide atoms induces visible outward movement of the lanthanide atom and its surrounding Se atoms. The calculated total magnetic moments are 0.973, 2.948, 7.528 and 1.945 μB for the Ce-, Nd-, Eu-, and Tm-doped systems, respectively, which are mainly derived from lanthanide atoms. Further band structure calculations reveal that the Ce-doped InSe monolayer has n-type conductivity, while the Nd-doped InSe monolayer has p-type conductivity. The Eu- and Tm-doped systems are found to be diluted magnetic semiconductors. The calculated optical response of absorption in the four doping cases shows redshift to lower energy within the infrared range compared with the host InSe monolayer. These findings suggest that doping of lanthanide atoms may open up a new way of manipulating functionalities of InSe materials for low-dimension optoelectronics and spintronics applications.

Strain-controlled synthesis of ultrathin hexagonal GaTe/MoS2 heterostructure for sensitive photodetection

Ultrathin hexagonal GaTe, with relatively high charge density, holds great potential in the field of optoelectronic devices. However, the thermodynamical stability limits it fabrications as well as applications. Here, by introducing two-dimensional MoS2 as the substrate, we successfully realized the phase-controlled synthesis of ultrathin h-GaTe, leading to high-quality h-GaTe/MoS2 heterostructures. Theoretical calculation studies reveal that GaTe with hexagonal phase is more thermodynamically stable on MoS2 templates, which can be attributed to the strain stretching and the formation energy reduction. Based on the achieved p-n heterostructures, optoelectronic devices are designed and probed, where remarkable photoresponsivity (32.5 A/W) and fast photoresponse speed (<50 μs) are obtained, indicating well-behaved photo-sensing behaviors. The study here could offer a good reference for the controlled growth of the relevant materials, and the achieved heterostructure will find promising applications in future integrated electronic and optoelectronic devices and systems.

Liquid-Exfoliated 2D Materials for Optoelectronic Applications

Two-dimensional (2D) materials have attracted tremendous research attention in recent days due to their extraordinary and unique properties upon exfoliation from the bulk form, which are useful for many applications such as electronics, optoelectronics, catalysis, etc. Liquid exfoliation method of 2D materials offers a facile and low-cost route to produce large quantities of mono- and few-layer 2D nanosheets in a commercially viable way. Optoelectronic devices such as photodetectors fabricated from percolating networks of liquid-exfoliated 2D materials offer advantages compared to conventional devices, including low cost, less complicated process, and higher flexibility, making them more suitable for the next generation wearable devices. This review summarizes the recent progress on metal-semiconductor-metal (MSM) photodetectors fabricated from percolating network of 2D nanosheets obtained from liquid exfoliation methods. In addition, hybrids and mixtures with other photosensitive materials, such as quantum dots, nanowires, nanorods, etc. are also discussed. First, the various methods of liquid exfoliation of 2D materials, size selection methods, and photodetection mechanisms that are responsible for light detection in networks of 2D nanosheets are briefly reviewed. At the end, some potential strategies to further improve the performance the devices are proposed.

A Bi-Anti-Ambipolar Field Effect Transistor

Multistate logic is recognized as a promising approach to increase the device density of microelectronics, but current approaches are offset by limited performance and large circuit complexity. We here demonstrate a route toward increased integration density that is enabled by a mechanically tunable device concept. Bi-anti-ambipolar transistors (bi-AATs) exhibit two distinct peaks in their transconductance and can be realized by a single 2D-material heterojunction-based solid-state device. Dynamic deformation of the device reveals the co-occurrence of two conduction pathways to be the origin of this previously unobserved behavior. Initially, carrier conduction proceeds through the junction edge, but illumination and application of strain can increase the recombination rate in the junction sufficiently to support an alternative carrier conduction path through the junction area. Optical characterization reveals a tunable emission pattern and increased optoelectronic responsivity that corroborates our model. Strain control permits the optimization of the conduction efficiency through both pathways and can be employed in quaternary inverters for future multilogic applications.

Ultra-thin structures of manganese fluorides: conversion from manganese dichalcogenides by fluorination

In this study, it is predicted by density functional theory calculations that graphene-like novel ultra-thin phases of manganese fluoride crystals, that have nonlayered structures in their bulk form, can be stabilized by fluorination of manganese dichalcogenide crystals. First, it is shown that substitution of fluorine atoms with chalcogens in the manganese dichalcogenide host lattice is favorable. Among possible crystal formations, three stable ultra-thin structures of manganese fluoride, 1H-MnF2, 1T-MnF2 and MnF3, are found to be stable by total energy optimization calculations. In addition, phonon calculations and Raman activity analysis reveal that predicted novel single-layers are dynamically stable crystal structures displaying distinctive characteristic peaks in their vibrational spectrum enabling experimental determination of the corresponding phases. Differing from 1H-MnF2 antiferromagnetic (AFM) large gap semiconductor, 1T-MnF2 and MnF3 single-layers are semiconductors with ferromagnetic (FM) ground state.

Electronic, optical, and thermoelectric properties of Janus In-based monochalcogenides

Inspired by the successfully experimental synthesis of Janus structures recently, we systematically study the electronic, optical, and electronic transport properties of Janus monolayers In₂XY(X/Y= S, Se, Te withX≠Y) in the presence of a biaxial strain and electric field using density functional theory. Monolayers In₂XYare dynamically and thermally stable at room temperature. At equilibrium, both In₂STe and In₂SeTe are direct semiconductors while In₂SSe exhibits an indirect semiconducting behavior. The strain significantly alters the electronic structure of In₂XYand their photocatalytic activity. Besides, the indirect-direct gap transitions can be found due to applied strain. The effect of the electric field on optical properties of In₂XYis negligible. Meanwhile, the optical absorbance intensity of the Janus In₂XYmonolayers is remarkably increased by compressive strain. Also, In₂XYmonolayers exhibit very low lattice thermal conductivities resulting in a high figure of meritZT, which makes them potential candidates for room-temperature thermoelectric materials.

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.

Direct band gap and strong Rashba effect in van der Waals heterostructures of InSe and Sb single layers

Van der Waals heterostructures formed by stacking different types of 2D materials are attracting increasing attention due to new emergent physical properties such as interlayer excitons. Recently synthesized atomically thin indium selenide (InSe) and antimony (Sb) individually exhibit interesting electronic properties such as high electron mobility in the former and high hole mobility in the latter. In this work, we present a first-principles investigation on the stability and electronic properties of ultrathin bilayer heterostructures composed of InSe and Sb single layers. The calculated electronic band structures reveal a direct band gap semiconducting nature of the InSe/Sb heterostructures independent of stacking pattern. Taking spin-orbit coupling (SOC) into account, we find a large Rashba spin splitting at the bottom of conduction band, which originates from the atomic SOC with the symmetry breaking in the heterostructure. The strength of the Rashba spin splitting can be tuned by applying in-plane biaxial strain or an out-of-plane external electric field. The presence of large Rashba spin splitting together with a suitable band gap in InSe/Sb bilayer heterostructures make them promising candidates for spin field-effect transistor and optoelectronic device applications.