Two-dimensional HfS2-ZrS2 lateral heterojunction FETs with high rectification and photocurrent.

Multifunctional devices are an indispensable choice to fulfil the increasing demand for miniaturized and integrated circuit systems. However, bulk material-based devices encounter the challenge of miniaturized all-in-one systems with multiple functions. In this study, we designed a field effect transistor (FET) based on a monolayer HfS2-ZrS2 lateral heterojunction. It possesses simultaneous and obvious rectifying behavior and photodetection characteristics in the visible light region, such as the rectification ratio of ∼1012, photocurrent density of 13.3 nA m-1, responsivity of 57 mA W-1, and extinction ratio of 108. Notably, the rectification ratio of the single-gate FET is larger than that of the dual-gate FET under the negative gate voltage. These results indicate that monolayer lateral heterojunction-based FETs can provide an effective route to integrate rectifying and photodetection functions in single optoelectronic nanodevices.

Temperature-dependent magnetic properties of the room-temperature ferromagnetic Janus monolayer Fe2XY (X, Y = I, Br, Cl; X ≠ Y).

Excellent magnetic properties at room temperature are crucial for the application of ferromagnets in spintronic and topological quantum devices. Using first-principles calculations and atomistic spin model simulations, we investigate the temperature-dependent magnetic properties of the Janus monolayer Fe2XY (X, Y = I, Br, Cl; X ≠ Y), as well as the effects of different magnetic interactions within the next-nearest-neighbor shell on the Curie temperature (TC). A large isotropic exchange interaction between one Fe atom and its next-nearest-neighbor counterparts can significantly increase the TC, while an antisymmetric exchange interaction decreases it. More importantly, we employ the temperature rescaling method, which can obtain temperature-dependent magnetic properties quantitatively consistent with experimental values, and find that the effective uniaxial anisotropy constant and coercive field decrease with increasing temperature. Moreover, at room temperature, Fe2IY is a rectangular-loop magnetic material with a giant coercive field up to ∼8 T, demonstrating its potential for application in room-temperature memory devices. Our findings can advance the application of these Janus monolayers in room-temperature spintronic devices and through heat-assisted techniques.

Strong interlayer coupling in p-Te/n-CdSe van der Waals heterojunction for self-powered photodetectors with fast speed and high responsivity.

Self-driven photodetectors, which can detect optical signals without external voltage bias, are highly attractive in the field of low-power wearable electronics and internet of things. However, currently reported self-driven photodetectors based on van der Waals heterojunctions (vdWHs) are generally limited by low responsivity due to poor light absorption and insufficient photogain. Here, we report p-Te/n-CdSe vdWHs utilizing non-layered CdSe nanobelts as efficient light absorption layer and high mobility Te as ultrafast hole transporting layer. Benefiting from strong interlayer coupling, the Te/CdSe vdWHs exhibit stable and excellent self-powered characteristics, including ultrahigh responsivity of 0.94 A W-1, remarkable detectivity of 8.36 × 1012 Jones at optical power density of 1.18 mW cm-2 under illumination of 405 nm laser, fast response speed of 24 µs, large light on/off ratio exceeding 105, as well as broadband photoresponse (405-1064 nm), which surpass most of the reported vdWHs photodetectors. In addition, the devices display superior photovoltaic characteristics under 532 nm illumination, such as large Voc of 0.55 V, and ultrahigh Isc of 2.73 µA. These results demonstrate the construction of 2D/non-layered semiconductor vdWHs with strong interlayer coupling is a promising strategy for high-performance and low-power consumption devices.

Promising ultra-short channel transistors based on OM2S (M = Ga, In) monolayers for high performance and low power consumption.

It is hoped that two-dimensional (2D) semiconductors overcome the short channel effect and continue Moore's law. However, 2D material-based ultra-short channel devices still face the challenge of simultaneously achieving high-performance (HP) and low-power (LP) consumption. Here, we theoretically designed monolayer OM2S (M = Ga, In)-based metal-oxide-semiconductor field-effect transistors (MOSFETs), considering the gate length from 1 to 5 nm, doping concentration and underlap structure. We found that in HP (LP) applications, the on-state current exceeds 1000 (500) µA µm-1 under a 1 nm (2 nm) gate length, surpassing the needs of the International Technology Roadmap for Semiconductors (ITRS) in 2028. The subthreshold swing is close to the Boltzmann tyranny (60 mV dec-1) even as the gate length shrinks to 2 nm. The energy-delay product is two orders lower than 1.02 × 10-28 J s µm-1, indicating extraordinary high-speed manipulation and low-energy expending. Therefore, monolayer OM2S has great application in ultra-short scale devices with HP and LP consumption, and can be taken as a candidate to extend Moore's Law.

Two-dimensional wide-bandgap GeSe2 vertical ultraviolet photodetectors with high responsivity and ultrafast response speed.

Germanium selenide (GeSe2), as a typical member of 2D wide bandgap semiconductors (WBSs), shows great potential in ultraviolet (UV) optoelectronics due to its excellent flexibility, superior environmental stability, competitive UV absorption coefficient, and significant spectral selectivity. However, the GeSe2-based UV photodetector suffers from high operation voltages and low photocurrent, which prevents its practical imaging applications. In this work, we report an elevated photocurrent generation in a vertical stacking graphene/GeSe2/graphene heterostructure with low operation voltage and low power consumption. Efficient collection of photoexcited carriers in GeSe2 through graphene electrodes results in outstanding UV detection properties, including a pronounced responsivity of 37.1 A W-1, a specific detectivity of 8.83 × 1011 Jones, and an ultrahigh on/off ratio (∼105) at 355 nm. In addition, building a Schottky barrier between GeSe2 and graphene and reducing the channel length can increase the photoresponse speed to ∼300 µs. These accomplishments set the stage for future optoelectronic applications of vertical 2D WBS heterostructure UV photodetectors.

Optoelectronic properties and applications of two-dimensional layered semiconductor van der Waals heterostructures: perspective from theory.

Van der Waals heterostructures (vdWHs) which combine two different materials together have attracted extensive research attentions due to the promising applications in optoelectronic and electronic devices, the investigations from theoretical simulations can not only predict the novel properties and the interfacial coupling, but also provide essential guidance for experimental verification and fabrications. This review summarizes the recent theoretical studies on electronic and optical properties of two-dimensional semiconducting vdWHs. The characteristics of different band alignments are discussed, together with the optoelectronic modulations from external fields and the promising applications in solar cells, tunneling field-effect transistors and photodetectors. At the end of the review, the further perspective and possible research problems of the vdWHs are also presented.

Effects of p-d orbital mixings on magnetocrystalline anisotropy for D3d-symmetric ferromagnetic semiconducting monolayers.

Understanding magnetic anisotropy based on electronic properties is vital for theoretical and applied research on ferromagnetic semiconductors. Here, for several representative D3d-symmetric ferromagnetic semiconducting monolayers, we investigate the effects of mixings between d-orbitals of central magnetic atoms and p-orbitals of ligands on magnetocrystalline anisotropy energy (MAE). For high-spin materials, the weakening of p-d mixing increases the electron occupation of spin-up bonding d-orbitals at the expense of the electron occupation in the corresponding spin-down orbitals, In contrast, the weakening of p-d mixing decreases the electron occupation of the spin-up antibonding d-orbitals and enhances the electron occupation in the corresponding spin-down orbitals. The weakening mixings also result in an overall shift of the spin-down band toward a higher energy with respect to the spin-up band. These changes are just the opposite in a low-spin material. More interestingly, we find that the transition point between the bonding and the antibonding spin-up bands plays a significant role in tuning the MAE. Its shift with strain is almost linearly related to the p-d bond strength and significantly affects both the electron occupation of occupied spin-up antibonding d-bands and the band shift of unoccupied spin-up d-bands. Furthermore, the correlation of these mixing-related changes in electronic structures with the MAE is qualitatively and quantitatively analyzed. Our findings can deepen the understanding of the correlation between MAE and p-d orbital mixings and provide theoretical guidance for modulating the MAE.

Asymmetric Ferroelectric-Gated Two-Dimensional Transistor Integrating Self-Rectifying Photoelectric Memory and Artificial Synapse.

Ferroelectric field-effect transistors (Fe-FET) are promising candidates for future information devices. However, they suffer from low endurance and short retention time, which retards the application of processing memory in the same physical processes. Here, inspired by the ferroelectric proximity effects, we design a reconfigurable two-dimensional (2D) MoS2 transistor featuring with asymmetric ferroelectric gate, exhibiting high memory and logic ability with a program/erase ratio of over 106 and a self-rectifying ratio of 103. Interestingly, the robust electric and optic cycling are obtained with a large switching ratio of 106 and nine distinct resistance states upon optical excitation with excellent nonvolatile characteristics. Meanwhile, the operation of memory mimics the synapse behavior in response to light spikes with different intensity and number. This design realizes an integration of robust processing memory in one single device, which demonstrates a considerable potential of an asymmetric ferroelectric gate in the development of Fe-FETs for logic processing and nonvolatile memory applications.

Phase-controlled synthesis of SnS2and SnS flakes and photodetection properties.

Two-dimensional (2D) layered tin sulfide compounds including SnS2and SnS have attracted increasing attention due to their great potential application in the fields of optoelectronics and energy storage. However, device development has been delayed by the lack of capabilities to synthesize large-scale and high-quality 2D tin sulfide. Here, a phase-controlled synthesis of SnS2and SnS flakes with lateral size over 100 µm was successfully realized via a facile chemical vapor deposition method. The lateral size of flakes and phase transformation of SnS2to SnS can be tuned via changing the synthesis temperature. Compared to the formation of the SnS2phase at relative low temperature (<750 °C), the SnS phase is favorable at higher temperature. The phototransistor based on the as-prepared SnS2and SnS exhibits excellent photoresponse to 405 nm laser, including a high responsivity (1.7 × 106mA W-1), fast response rates (rise/decay time of 13/51 ms), an outstanding external quantum efficiency (5.3 × 105%), and a remarkable detectivity (6.24 × 1012Jones) for SnS2-based phototransistor, and these values are superior to the most reported SnS2based photodetectors. Although the responsivity (3390 mA W-1) and detectivity (1.1 × 1010Jones) of SnS-based device is lower than that of the SnS2phototransistor, it has a faster rise/decay time of 3.10/1.59 ms. This work provides a means of tuning the size and phase of 2D layered tin sulfide, and promotes the application of SnS2in high-performance optoelectronic devices.

Magnetoelectric coupling effects on the band alignments of multiferroic In2Se3-CrI3 trilayer heterostructures.

Due to unique magnetoelectric coupling effects, two-dimensional (2D) multiferroic van der Waals heterostructures (vdWHs) are promising for next-generation information processing and storage devices. Here, we design theoretically multiferroic In2Se3/CrI3 trilayer vdWHs with different stacking patterns. For the CrI3/In2Se3/CrI3 trilayer vdWHs, whether ferroelectric upward or downward polarization, type-I and type-II band alignments are formed for spin-up and spin-down channels. However, for the CrI3/In2Se3/In2Se3 trilayer vdWHs, downward polarization induces the type-III band alignment, which is typical for spin-tunnel transistors. Moreover, nonvolatile ferroelectric polarization and stacking patterns can induce the conversion between a unipolar semiconductor and a bipolar (unipolar) half-metal. These results provide a possible route to realize nanoscale multifunctional spintronic devices based on 2D multiferroic systems.

Band offset trends in IV-VI layered semiconductor heterojunctions.

The band offsets between semiconductors are significantly associated with the optoelectronic characteristics and devices design. Here, we investigate the band offset trends of few-layer and bulk IV-VI semiconductors MX and MX2(M = Ge, Sn; X = S, Se, Te). For common-cation (anion) systems, as the atomic number increases, the valence band offset of MX decreases, while that of MX2has no distinct change, and the physical origin can be interpreted using band coupling mechanism and atomic potential trend. The band edges of GeX2system straddle redox potentials of water, making them competitive candidates for photocatalyst. Moreover, layer number modulation can induce the band offset of GeSe/SnS and GeSe2/GeS2heterojunction undergoing a transition from type I to type II, which makes them suitable for optoelectronic applications.

Mechano-ferroelectric coupling: stabilization enhancement and polarization switching in bent AgBiP2Se6 monolayers.

Two-dimensional (2D) van der Waals (vdW) ferroelectrics are core candidates for the development of next-generation non-volatile storage devices, which rely highly on ferroelectric stability and feasible approaches to manipulate the ferroelectric polarization and domain. Here, based on density functional theory calculations, we demonstrate that the bending deformation can not only manipulate the polarization direction and domain size of AgBiP2Se6 monolayers but also significantly improve the ferroelectric stability. The ordered polarization in the bent AgBiP2Se6 monolayers can be well maintained at a temperature of 200 K in molecular dynamics simulations; by contrast, it is broken at only 100 K for their freestanding counterparts. These phenomena can be attributed to synergic effects from the asymmetric strain energy induced by a strain gradient and a reduced migration barrier of Ag ions from convex to concave surfaces. More interestingly, a ferroelectric bubble can be induced in the monolayer under biaxial compression strain. This mechano-ferroelectric coupling represents a new mechanism and feasible route towards stabilization and polarization flip in 2D ferroelectrics.

In-plane ferroelectricity in few-layered GeS and its van der Waals ferroelectric diodes.

Two-dimensional ferroelectric semiconductors (2DFeSs) have been attracting extensive research attention on account of their unique properties and versatile applications in random-access memory, digital signal processors, and neuromorphic computing. Germanium sulfide (GeS) is predicted as a typical 2DFeS with a large spontaneous polarization of 484 pC m-1. Furthermore, the moderate band gap equivalent to 1.63 eV of GeS provides it with significant potential to create a strong bulk photovoltage in the visible light range. However, the fabrication of chemically stable few-to-monolayer GeS has not been reported so far, owing to the strong interlayer force and high chemical reactivity of the surface. Herein we demonstrate a new method for fabricating high quality, air-stable, ultrathin GeS nanoflakes. The electrical characterization confirms the formation of few-layered GeS with a remarkable in-plane ferroelectric hysteresis, which is forbidden by the inversion symmetry in bulk GeS crystals. After applying a coercive field of about 18.1 kV cm-1, a switchable shift current can also be observed in the polarized GeS nanoflakes under light irradiation. To further enhance the photoresponsivity, few-layered InSe was transferred onto the GeS nanoflakes to form van der Waals ferroelectric diodes. The interfacial perturbation breaking the inversion symmetry results in the enhancement of robust dipoles in the GeS side along the interface, which can be tuned by the in-plane electric field. Overall, this work opens the door for exploring the low-dimensional ferroelectric memory and energy conversion applications based on 2D GeS nanoflakes and provides a deeper understanding of the photovoltaic mechanism with in-plane 2D ferroelectric diodes.

Self-driven SnS1-xSex alloy/GaAs heterostructure based unique polarization sensitive photodetectors.

With the fast development of semiconductor technology, self-driven devices have become an indispensable part of modern electronic and optoelectronic components. In this field, in addition to traditional Schottky and p-n junction devices, hybrid 2D/3D semiconductor heterostructures provide an alternative platform for optoelectronic applications. Herein we report the growth of 2D SnS1-xSex (x = 0, 0.5, 1) nanosheets and the construction of a hybrid SnS0.5Se0.5/GaAs heterostructure based self-driven photodetector. The strong anisotropy of 2D SnS1-xSex is demonstrated theoretically and experimentally. The self-driven photodetector shows high sensitivity to incident light from the visible to near-infrared regime. At the wavelength of 405 nm, the device enables maximum responsivity of 10.2 A W-1, high detectivity of 4.8 × 1012 Jones and fast response speed of 0.5/3.47 ms. Impressively, such a heterostructure device exhibits anisotropic photodetection characteristics with the dichroic ratio of ∼1.25 at 405 nm and ∼1.45 at 635 nm. These remarkable features can be attributed to the high-quality built-in potential at the SnS0.5Se0.5/GaAs interface and the alloy engineering, which effectively separates the photogenerated carriers and suppresses the deep-level defects, respectively. These results imply the great potential of our SnS0.5Se0.5/GaAs heterostructure for high-performance photodetection devices.

Photovoltaic Field-Effect Photodiodes Based on Double van der Waals Heterojunctions.

High performance photodetectors based on van der Waals heterostructures (vdWHs) are crucial to developing micro-nano-optoelectronic devices. However, reports show that it is difficult to balance fast response and high sensitivity. In this work, we design a photovoltaic field-effect photodiode (PVFED) based on the WSe2/MoS2/WSe2 double vdWHs, where the photovoltage that originated from one vdWH modulates the optoelectronic characteristics of another vdWH. The proposed photodiode exhibits an excellent self-powered ability with a high responsivity of 715 mA·W-1 and fast response time of 45 µs. This work demonstrates an efficient method that optimizes the photoelectric performance of vdWH by introducing the photovoltaic field effect.

Cross-Substitution Promoted Ultrawide Bandgap up to 4.5 eV in a 2D Semiconductor: Gallium Thiophosphate.

Exploring 2D ultrawide bandgap semiconductors (UWBSs) will be conductive to the development of next-generation nanodevices, such as deep-ultraviolet photodetectors, single-photon emitters, and high-power flexible electronic devices. However, a gap still remains between the theoretical prediction of novel 2D UWBSs and the experimental realization of the corresponding materials. The cross-substitution process is an effective way to construct novel semiconductors with the favorable parent characteristics (e.g., structure) and the better physicochemical properties (e.g., bandgap). Herein, a simple case is offered for rational design and syntheses of 2D UWBS GaPS4 by employing state-of-the-art GeS2 as a similar structural model. Benefiting from the cosubstitution of Ge with lighter Ga and P, the GaPS4 crystals exhibit sharply enlarged optical bandgaps (few-layer: 3.94 eV and monolayer: 4.50 eV) and superior detection performances with high responsivity (4.89 A W-1 ), high detectivity (1.98 × 1012 Jones), and high quantum efficiency (2.39 × 103 %) in the solar-blind ultraviolet region. Moreover, the GaPS4 -based photodetector exhibits polarization-sensitive photoresponse with a linear dichroic ratio of 1.85 at 254 nm, benefitting from its in-plane structural anisotropy. These results provide a pathway for the discovery and fabrication of 2D UWBS anisotropic materials, which become promising candidates for future solar-blind ultraviolet and polarization-sensitive sensors.

Reversible Half Wave Rectifier Based on 2D InSe/GeSe Heterostructure with Near-Broken Band Alignment.

2D van der Waals heterostructures (vdWHs) offer tremendous opportunities in designing multifunctional electronic devices. Due to the ultrathin nature of 2D materials, the gate-induced change in charge density makes amplitude control possible, creating a new programmable unilateral rectifier. The study of 2D vdWHs-based reversible unilateral rectifier is lacking, although it can give rise to a new degree of freedom for modulating the output state. Here, a InSe/GeSe vdWH-FET is constructed as a gate-controllable half wave rectifier. The device exhibits stepless adjustment from forward to backward rectifying performance, leading to multiple operation states of output level. Near-broken band alignment in the InSe/GeSe vdWH-FET is a crucial feature for high-performance reversible rectifier, which is shown to have backward and forward rectification ratio of 1:38 and 963:1, respectively. Being further explored as a new bridge rectifier, the InSe/GeSe device has great potential in future gate-controllable alternating current/direct current convertor. These results indicate that 2D vdWHs with near-broken band alignment can offer a pathway to simplify the commutating circuit and regulating speed circuit.

VS2nanosheet as a promising candidate of recycle and reuse NO2gas sensor and capturer: a DFT study.

We describe the utilization of VS2nanosheet as high sensing response, reuse, and thermodynamic stability at room temperature NO2and NO gas sensors by using the density functional theory method. We focus on the electronic structures and adsorption energy toward a variety of gaseous molecules (such as O2, CO, H2O, NH3, NO, and NO2) adsorbed on the VS2nanosheet. The results show that chemical interactions existed between NO/NO2molecules and VS2nanosheet due to sizable adsorption energy and strong covalent (S-N) bonds. In particular, the adsorption energies, charge transfer and electronic properties between NO2adsorbed system is significantly changed compared with the other gas molecules (CO, NO, H2O, NH3, and O2) adsorbed systems under biaxial strains, which is effective to achieve the capture or reversible release of NO2for cycling capability. Our analysis indicates that VS2nanosheet is promising as electrical devices candidate for NO2high-performance gas sensor or capturer.

Few-layer In4/3P2Se6 nanoflakes for high detectivity photodetectors.

Metal phosphorus trichalcogenides (MPX3) have attracted extensive attention as promising two-dimensional (2D) layered materials in future electronic and optoelectronic devices. Here, for the first time, few-layer In4/3P2Se6 nanoflakes have been successfully exfoliated from home-made high-quality single crystals. The In4/3P2Se6 crystal belongs to the R3 space group, and possesses a weak van der Waals force between the adjacent layers and a direct bandgap of 1.99 eV. Furthermore, the In4/3P2Se6-based photodetectors show high performances in the visible light region, such as a high responsivity (R) of 4.93 A·W-1, a high external quantum efficiency (EQE) of 1509% and a fast response time, as low as 2.1 ms. In particular, the high detectivity (D) of the devices can reach up to 4.3 × 1013 Jones (light ON/OFF ratio ≈104) under illumination from a 405 nm light at a bias voltage of 1 V, which is favoured by the ultralow dark current (∼100 fA). These excellent performances pave the way for the implementation of In4/3P2Se6 nanoflakes as promising candidates for future optoelectronic detection applications.

Self-assembly of an alkynylpyrene derivative for multi-responsive fluorescence behavior and photoswitching performance.

Highly emissive fluorophores based on polyaromatic hydrocarbons with tunable emission properties and aggregated structures play a very important role in relevant functional studies. In this study, a novel alkynylpyrene derivative 1 was synthesized, which exhibits unimolecular to excimer emission in methanol with an increasing concentration accompanied by the formation of nanovesicles via the π-π stacking, hydrogen bond and hydrophobic interaction. The self-assembly behavior as well as emission properties of 1 in aprotic polar solvents (ACN, acetone, DMF and DMSO) can also be adjusted by the volume fraction of the poor solvent H2O, which can induce 1 self-assembly to excimer state and could be applied in information transfer. Moreover, upon visible light irradiation, photoswitchable performance of nanovesicles of 1 was observed in which the emission markedly changes from yellow to blue; this is attributed to the cycloaddition reaction of alkynyl groups and singlet oxygen, which can be generated without the addition of external photosensitizers. The multi-responsive and fluorescence behavior of the alkynylpyrene derivative show that the self-assembly can be used to expand the development of this type of fluorophores, and the novel photoinduced tunability of the fluorescence emission provides an effective strategy to obtain high-performance transmitting and sensing materials.