Highly Tunable Near-Room Temperature Ferromagnetism in Cr-Doped Layered Td-WTe2

The discovery of type-II Weyl semimetal states in layered transition metal dichalcogenides Td-WTe₂ indicates great potential for novel electronic, spintronic, and quantum devices. Theoretically, the interaction between the topological states and the magnetic ordered states of Td-WTe₂ enables the modulation of Weyl semimetal states by an external magnetic field. However, currently, ferromagnetism in layered Td-WTe₂ is still elusive and rarely observed. In this research, ferromagnetic order into WTe₂ using magnetic chromium (Cr) doping with a two-step Te flux strategy is introduced. The Curie temperature (T_c) and the ferromagnetic moment are well tuned with a Cr doping concentration. The T_c of the Cr-doped layered Td-WTe₂ could be regulated from 182 to 283 K, which is close to room temperature. The saturation magnetic moment could be changed from 2.26 to 4.20 emu g^–1, which is stronger than most values reported for these materials. Most intriguingly, the Cr-doped layered Td-WTe₂ single crystals still exhibit semimetallic behavior and they possess very large magnetoresistance with obvious Shubnikov de Haas (SdH) quantum oscillations and an anomalous Hall effect. The findings offer feasible ways to induce and tune ferromagnetic orders in layered Td-WTe₂ and thus to control its topological phase with external magnetic fields.     

Scalable Synthesis of Ultrathin Mn3N2 Exhibiting Room‐Temperature Antiferromagnetism

Ultrathin and 2D magnetic materials have attracted a great deal of attention recently due to their potential applications in spintronics. Only a handful of stable ultrathin magnetic materials have been reported, but their high‐yield synthesis remains a challenge. Transition metal (e.g., manganese) nitrides are attractive candidates for spintronics due to their predicted high magnetic transition temperatures. Here, a lattice matching synthesis of ultrathin Mn₃N₂ is employed. Taking advantage of the lattice match between a KCl salt template and Mn₃N₂, this method yields the first ultrathin magnetic metal nitride via a solution‐based route. Mn₃N₂ flakes show intrinsic magnetic behavior even at 300 K, enabling potential room‐temperature applications. This synthesis procedure offers an approach to the discovery of other ultrathin or 2D metal nitrides.     

High Current Density Electrical Breakdown of TiS3 Nanoribbon‐Based Field‐Effect Transistors

The high field transport characteristics of nanostructured transistors based on layered materials are not only important from a device physics perspective but also for possible applications in next generation electronics. With the growing promise of layered materials as replacements to conventional silicon technology, the high current density properties of the layered material titanium trisulfide (TiS₃) are studied here. The high breakdown current densities of up to 1.7 × 10⁶ A cm^?2 are observed in TiS₃ nanoribbon‐based field‐effect transistors, which are among the highest found in semiconducting nanomaterials. Investigating the mechanisms responsible for current breakdown, a thermogravimetric analysis of bulk TiS₃ is performed and the results with density functional theory and kinetic Monte Carlo calculations are compared. In conclusion, the oxidation of TiS₃ and subsequent desorption of sulfur atoms play an important role in the electrical breakdown of the material in ambient conditions. The results show that TiS₃ is an attractive material for high power applications and lend insight into the thermal and defect activated mechanisms responsible for electrical breakdown in nanostructured devices.     

A Quaternary van der Waals Ferromagnetic Semiconductor AgVP2Se6

The recent realization of 2D magnetism in van der Waals (vdWs) magnets holds promise for future information technology. However, the vdWs semiconducting ferromagnets, which remain rare, are especially important in developing 2D magnetic devices with new functionalities due to the possibility of simultaneous control of the carrier charge and spin. Metal thiophosphate (MTP), a multifunctional vdWs material system that combines the sought‐after properties of complex oxides, is a promising vdWs magnet system. Here, single crystals of a novel vdWs ferromagnetic semiconductor MTP AgVP₂Se₆ with a room‐temperature resistivity of 1 Ω m are successfully synthesized. Due to the nature of vdWs bonding along the c‐axis, the magnetic properties of the few‐layer AgVP₂Se₆ with different thicknesses are characterized on the exfoliated samples. The AgVP₂Se₆ flakes exhibit significant thickness‐dependent magnetic properties, and a rectangular hysteresis loop with a large coercive field of 750 Oe at 2 K and an undiminished Curie temperature of 19 K are observed in the 6.7 nm AgVP₂Se₆ flake. The discovered vdWs ferromagnet AgVP₂Se₆ with semiconducting behavior will provide alternative platforms for exploring 2D magnetism and potential applications in spintronic devices.     

Fast,Self‐Driven,Air‐Stable,and Broadband Photodetector Based on Vertically Aligned PtSe2/GaAs Heterojunction

Group‐10 layered transitional metal dichalcogenides including PtS₂, PtSe₂, and PtTe₂ are excellent potential candidates for optoelectronic devices due to their unique properties such as high carrier mobility, tunable bandgap, stability, and flexibility. Large‐area platinum diselenide (PtSe₂) with semiconducting characteristics is far scarcely investigated. Here, the development of a high‐performance photodetector based on vertically aligned PtSe₂‐GaAs heterojunction which exhibits a broadband sensitivity from deep ultraviolet to near‐infrared light, with peak sensitivity from 650 to 810 nm, is reported. The I_light/I_dark ratio and responsivity of photodetector are 3 × 10⁴ and 262 mA W^?1 measured at 808 nm under zero bias voltage. The response speed of τ_r/τ_f is 5.5/6.5 µs, which represents the best result achieved for Group‐10 TMDs based optoelectronic device thus far. According to first‐principle density functional theory, the broad photoresponse ranging from visible to near‐infrared region is associated with the semiconducting characteristics of PtSe₂ which has interstitial Se atoms within the PtSe₂ layers. It is also revealed that the PtSe₂/GaAs photodetector does not exhibit performance degradation after six weeks in air. The generality of the above good results suggests that the vertically aligned PtSe₂ is an ideal material for high‐performance optoelectronic systems in the future.     

2D Rhenium Dichalcogenides: From Fundamental Properties to Recent Advances in Photodetector Technology

Van der Waals (vdW) materials of transition metal dichalcogenides (TMD) family with semiconducting properties are currently at the forefront of research in the field of optoelectronics. The ability to couple them with one another at atomic interface precision in a synergistic way opens up unprecedented opportunities to design photodetectors of broad spectral range with excellent figures of merits not accessible to discrete materials. Recent years have seen a surge of interest in group VII TMD materials (ReS₂ and ReSe₂) due to their strong optical response from bulk to monolayer and good ambient stability. Their band gap energies spanning over visible and near-infrared ranges and the strong linear polarization sensitivity stemming from the distorted octahedral symmetry, are ideally suited for polarization-sensitive photodetectors. This review aims at providing a comprehensive understanding of the fundamental properties, optical identification of various structural features, long-debated question of band gap nature and interlayer coupling, and recent advances in the development of photodetectors based on ReS₂, ReSe₂, and their vdW heterostructures with other layered materials of practical importance. We critically review various conceptual device designs implemented based on band engineering, emphasize on the merits of these photodetectors and their potential applications, and provide an outlook for future prospects.     

All‐Layered 2D Optoelectronics: A High‐Performance UV–vis–NIR Broadband SnSe Photodetector with Bi2Te3 Topological Insulator Electrodes

Nanoelectronics is in urgent demand of exceptional device architecture with ultrathin thickness below 10 nm and dangling‐bond‐free surface to break through current physical bottleneck and achieve new record of integration level. The advance in 2D van der Waals materials endows scientists with new accessibility. This study reports an all‐layered 2D Bi₂Te₃‐SnSe‐Bi₂Te₃ photodetector, and the broadband photoresponse of the device from ultraviolet (370 nm) to near‐infrared (808 nm) is demonstrated. In addition, the optimized responsivity reaches 5.5 A W^?1, with the corresponding eternal quantum efficiency of 1833% and detectivity of 6 × 10¹⁰ cm Hz^1/2 W^?1. These figures‐of‐merits are among the best values of the reported all‐layered 2D photodetectors, which are several orders of magnitude higher than those of the previous SnSe photodetectors. The superior device performance is attributed to the synergy of highly conductive surface state of Bi₂Te₃ topological insulator, perfect band alignment between Bi₂Te₃ and SnSe as well as small interface potential fluctuation. Meanwhile, the all‐layered 2D device is further constructed onto flexible mica substrate and its photoresponse is maintained roughly unchanged upon 60 bending cycles. The findings represent a fundamental scenario for advancement of the next generation high performance and high integration level flexible optoelectronics.     

Dual Functionalization of Liquid‐Exfoliated Semiconducting 2H‐MoS2 with Lanthanide Complexes Bearing Magnetic and Luminescence Properties

Liquid exfoliated, atomically thin semiconducting transition metal dichalcogenides (TMDs), as inorganic equivalents of graphene, have attracted great interest due to their distinctive physical, optoelectronic, and chemical properties. Functionalization of 2D TMDs brings new prospects for applications in optoelectronics, quantum technologies, catalysis, and medicine. In this report, dual functionalization of 2D semiconducting 2H‐MoS₂ nanosheets through simultaneous incorporation of magnetic and luminescent properties is demonstrated. A facile method is proposed for tuning the properties of the TDM semiconductors and accessing multimodal platforms, consisting in covalent grafting of lanthanide complexes onto the surface of 2D TMDs. Dual functionalization of liquid‐exfoliated MoS₂ nanosheets is demonstrated simultaneously with both europium (III) and gadolinium (III) complexes to form a colloidally stable luminescent (with millisecond lifetimes) and paramagnetic MoS₂‐based nanohybrid material. This work is the first example of transition metal dichalcogenide nanosheets functionalized with preformed lanthanide complexes. These findings open new prospects for covalent functionalization of TMDs with molecular species bearing specific functionalities as a means to tune the optoelectronic properties of the semiconductors, in order to create advanced materials and devices with a wide range of functionalities.     

Phthalocyanine‐Based 2D Conjugated Metal‐Organic Framework Nanosheets for High‐Performance Micro‐Supercapacitors

2D conjugated metal‐organic frameworks (2D c‐MOFs) are emerging as a novel class of conductive redox‐active materials for electrochemical energy storage. However, developing 2D c‐MOFs as flexible thin‐film electrodes have been largely limited, due to the lack of capability of solution‐processing and integration into nanodevices arising from the rigid powder samples by solvothermal synthesis. Here, the synthesis of phthalocyanine‐based 2D c‐MOF (Ni₂[CuPc(NH)₈]) nanosheets through ball milling mechanical exfoliation method are reported. The nanosheets feature with average lateral size of ≈160 nm and mean thickness of ≈7 nm (≈10 layers), and exhibit high crystallinity and chemical stability as well as a p‐type semiconducting behavior with mobility of ≈1.5 cm² V^?1 s^?1 at room temperature. Benefiting from the ultrathin feature, the nanosheets allow high utilization of active sites and facile solution‐processability. Thus, micro‐supercapacitor (MSC) devices are fabricated mixing Ni₂[CuPc(NH)₈] nanosheets with exfoliated graphene, which display outstanding cycling stability and a high areal capacitance up to 18.9 mF cm^?2; the performance surpasses most of the reported conducting polymers‐based and 2D materials‐based MSCs.     

Van der Waals p–n Junction Based on an Organic–Inorganic Heterostructure

Organic–inorganic heterostructures are an emerging topic that is very interesting for optoelectronics. Here, non‐conventional p–n junctions are investigated using organic rubrene single crystal and 2D MoS₂ as the p‐ and n‐type semiconducting materials, respectively. The current‐rectifying behavior is clearly observed in the junction device. The rectification ratio can be electrically tuned by the gate voltage due to the 2D nature of the heterostructure. The devices also show good photoresponse properties with a photoresponsivity of ≈500 mA W^?1 and a fast response time. These findings suggest a new route to facilitate the design of nanoelectronic and optoelectronic devices based on layered inorganics and organics.     

Conductivity in transparent anatase TiO2 films epitaxially grown by reactive sputtering deposition

The synthesis of semiconducting TiO₂ thin films deposited by reactive sputtering is discussed. In particular, defect doping of the anatase polymorph that is epitaxial stabilized on (0 0 1) LaAlO₃ was explored using either oxygen or water vapor as the oxidizing species. For films grown in oxygen, a transition from insulating to metallic conductivity of the films is observed as the O₂ pressure is reduced. X-ray diffraction measurements show the presence of the Ti_nO_2n−1 phase when the oxygen pressure is reduced sufficiently to induce conductive behavior. Hall measurements indicate that these materials are p-type. In contrast, the use of water vapor as the oxidizing species enabled the formation of n-type semiconducting TiO₂ with carrier density on the order of 10¹⁸ cm⁻³ and mobility of 10–15 cm²/V s.     

Fast Photoresponse from 1T Tin Diselenide Atomic Layers

Atomically layered 2D crystals such as transitional metal dichalcogenides (TMDs) provide an enchanting landscape for optoelectronic applications due to their unique atomic structures. They have been most intensively studied with 2H phase for easy fabrication and manipulation. 1T phase material could possess better electrocatalytic and photocatalytic properties, while they are difficult to fabricate. Herein, for the first time, the atomically layered 1T phase tin diselenides (SnSe₂, III‐IV compound) are successfully exfoliated by the method of mechanical exfoliation from bulk single crystals, grown via the chemical vapor transport method without transport gas. More attractively, the high performance atomically layered SnSe₂ photodetector has been first successfully fabricated, which displays a good responsivity of 0.5 A W^?1 and a fast photoresponse down to ≈2 ms at room temperature, one of the fastest response times among all types of 2D photodetectors. It makes SnSe₂ a promising candidate for high performance optoelectronic devices. Moreover, high performance bilayered SnSe₂ field‐effect transistors are also demonstrated with a mobility of ≈4 cm² V^?1 s^?1 and an on/off ratio of 10³ at room temperature. The results demonstrate that few layered 1T TMD materials are relatively stable in air and can be exploited for various electrical and optical applications.     

Synthesis of Large‐Area Atomically Thin BiOI Crystals with Highly Sensitive and Controllable Photodetection

Compared to the most studied 2D elements and binary compounds, ternary layered compounds with more adjustable physical and chemical properties have exhibited potential applications in electronic and optoelectronic devices. Here, 2D ternary layered BiOI crystals are synthesized first with a domain size up to 100 µm via space‐confined chemical vapor deposition. The photodetectors based on the as‐grown BiOI nanosheets demonstrate high sensitivity to 473 nm light. The I_on/I_off ratio and detectivity of BiOI photodetectors can reach up to 1 × 10⁵ and 8.2 × 10¹¹ Jones at 473 nm, respectively. Particularly, the contact and dark current of the photodetectors can be controlled by 254 nm ultraviolet light irradiation due to the introduction of oxygen vacancies. The facile synthesis of large‐area atomically thin BiOI and its controllable performance by ultraviolet light irradiation suggest that 2D BiOI crystal is a promising material for fundamental investigations and optoelectronic applications.     

Few‐Layered PtS2 Phototransistor on h‐BN with High Gain

The very recently rediscovered group‐10 transition metal dichalcogenides (TMDs) such as PtS₂ and PtSe₂, have joined the 2D material family as potentially promising candidates for electronic and optoeletronic applications due to their theoretically high carrier mobility, widely tunable bandgap, and ultrastability. Here, the first exploration of optoelectronic application based on few‐layered PtS₂ using h‐BN as substrate is presented. The phototransistor exhibits high responsivity up to 1.56 × 10³ A W^?1 and detectivity of 2.9 × 10¹¹ Jones. Additionally, an ultrahigh photogain ≈2 × 10⁶ is obtained at a gate voltage V _g = 30 V, one of the highest gain among 2D photodetectors, which is attributed to the existence of trap states. More interestingly, the few‐layered PtS₂ phototransistor shows a back gate modulated photocurrent generation mechanism, that is, from the photoconductive effect dominant to photogating effect dominant via tuning the gate voltage from the OFF state to the ON state. Such good properties combined with gate‐controlled photoresponse of PtS₂ make it a competitive candidate for future 2D optoelectronic applications.     

Highly Uniform Trilayer Molybdenum Disulfide for Wafer‐Scale Device Fabrication

Molybdenum disulfide (MoS₂) is a layered semiconducting material with a tunable bandgap that is promising for the next generation nanoelectronics as a substitute for graphene or silicon. Despite recent progress, the synthesis of high‐quality and highly uniform MoS₂ on a large scale is still a challenge. In this work, a temperature‐dependent synthesis study of large‐area MoS₂ by direct sulfurization of evaporated Mo thin films on SiO₂ is presented. A variety of physical characterization techniques is employed to investigate the structural quality of the material. The film quality is shown to be similar to geological MoS₂, if synthesized at sufficiently high temperatures (1050 °C). In addition, a highly uniform growth of trilayer MoS₂ with an unprecedented uniformity of ±0.07 nm over a large area (> 10 cm²) is achieved. These films are used to fabricate field‐effect transistors following a straightforward wafer‐scale UV lithography process. The intrinsic field‐effect mobility is estimated to be about cm² V^–1 s^–1 and compared to previous studies. These results represent a significant step towards application of MoS₂ in nanoelectronics and sensing.     

Electron Field Emission of Geometrically Modulated Monolayer Semiconductors

Electron field emission, electrons emitted from solid surfaces under high electric field, offers significant scientific interests in materials sciences and potential optoelectronics applications. 2D atomic layers, such as MoS₂, exhibit fascinating properties for diverse applications in next‐generation nanodevices and rich physical phenomena for fundamental research. However, the study on field emission of semiconducting monolayers is lacking owing to its low efficiency and stability of electron emission. Here, electron field emission of the geometrically modulated monolayer semiconductors suspended with 1D nanoarrays is demonstrated. Chemical vapor deposition synthesis of two prototype monolayers of transition metal dichalcogenides (TMD), MoS₂ and MoSe₂, is presented and their diverse band structures offer an ideal platform to explore the fundamental process of the electron emission in the TMD. Geometrical modulation and charge transfer of the semiconducting monolayers can be significantly tuned with the structural suspension with the 1D ZnO nanoarrays. Possible mechanisms on the enhanced electron emission of the 2D monolayers are discussed. With geometrical control of the monolayers, a highly efficient and stable electron emission of TMD monolayers is achieved in low turn‐on electric fields, enabling applications on electrons sources and opening a new avenue toward geometrically tuned atomic layers.     

New Frontiers on van der Waals Layered Metal Phosphorous Trichalcogenides

The exponentially growing works on 2D materials have resulted in both high scientific interest and huge potential applications in nanocatalysis, optoelectronics, and spintronics. Of especial note is that the newly emerged and promising family of metal phosphorus trichalcogenides (MPX₃) contains semiconductors, metals, and insulators with intriguing layered structures and architectures. The bandgaps of the members in this family range from 1.3 to 3.5 eV, significantly enriching the application of 2D materials in the broad wavelength spectrum. In this review, emphasizing their remarkable structural, physicochemical, and magnetic properties, as well as the numerous applications in various fields, the innovative progress on layered MPX₃ crystals is summarized. Different from other layered materials, these crystals will advance a fascinating frontier in magnetism and spintronic devices with their especially featured atomic layered nanosheets. Thus, their crystal and electronic structures, along with some related researches in magnetism, are discussed in detail. The assortments of growth methods are then summarized. Considering their potential applications, the prominent utilization of these 2D MPX₃ nanoscrystals in catalysis, batteries, and optoelectronics is also discussed. Finally, the outlook of these kinds of layered nanomaterials is provided.     

Oxygen Evolution Reaction on 2D Ferromagnetic Fe3GeTe2: Boosting the Reactivity by the Self‐Reduction of Surface Hydroxyl

Fe₃GeTe₂ is a water‐ and air‐stable, metallic, and layered material. Very recently, few‐layer and single‐layer Fe₃GeTe₂ have been successfully exfoliated from its bulk and revealed as 2D ferromagnets (Nature 2018 , 563, 94; Nat. Mater. 2018 , 17, 778). Here, the basal plane of Fe₃GeTe₂ is demonstrated to be of high electrocatalytic activity towards oxygen evolution reaction (OER) without resorting to any chemical modifications, by means of systematic density functional theory computations. The Fe₃GeTe₂ nanosheet preserves the metallic character of the bulk, and its 2D layered structure provides abundant exposed active sites to catalyze OER. All these unique characteristics suggest that the Fe₃GeTe₂ nanosheet may be an excellent catalyst for electrochemical OER. More importantly, it is found that the self‐reduction of surface hydroxyl into water can significantly reduce the overpotential for OER, which greatly boosts the OER activity. This work not only reveals new mechanisms for OER but also opens the door for the application of emerging 2D ferromagnets in the field of energy storage and conversion.     

Controlled Vapor–Solid Deposition of Millimeter‐Size Single Crystal 2D Bi2O2Se for High‐Performance Phototransistors

Atomically thin 2D materials have received intense interest both scientifically and technologically. Bismuth oxyselenide (Bi₂O₂Se) is a semiconducting 2D material with high electron mobility and good stability, making it promising for high‐performance electronics and optoelectronics. Here, an ambient‐pressure vapor–solid (VS) deposition approach for the growth of millimeter‐size 2D Bi₂O₂Se single crystal domains with thicknesses down to one monolayer is reported. The VS‐grown 2D Bi₂O₂Se has good crystalline quality, chemical uniformity, and stoichiometry. Field‐effect transistors (FETs) are fabricated using this material and they show a small contact resistivity of 55.2 Ω cm measured by a transfer line method. Upon light irradiation, a phototransistor based on the Bi₂O₂Se FETs exhibits a maximum responsivity of 22 100 AW^?1, which is a record among currently reported 2D semiconductors and approximately two orders of magnitude higher than monolayer MoS₂. The Bi₂O₂Se phototransistor shows a gate tunable photodetectivity up to 3.4 × 10¹⁵ Jones and an on/off ratio up to ≈10⁹, both of which are much higher than phototransistors based on other 2D materials reported so far. The results of this study indicate a method to grow large 2D Bi₂O₂Se single crystals that have great potential for use in optoelectronic applications.     

XMCD and XMCD‐PEEM Studies on Magnetic‐Field‐Assisted Self‐Assembled 1D Nanochains of Spherical Ferrite Particles

Quasi‐1D nanochains of spherical magnetic ferrite particles with a homogeneous particle size of ≈200 nm and a micrometer‐sized chain length are fabricated via a self‐assembly method under an external magnetic field. This assisting magnetic field (H_assist), applied during synthesis, significantly modifies the distribution of the Fe²⁺O_h, Fe³⁺T_d, and Fe³⁺O_h cations in the chains, as demonstrated by X‐ray magnetic circular dichroism (XMCD) combined with theoretical analysis. This provides direct evidence of the nontrivial role of external synthetic conditions for defining the crystal chemistry of nanoscale ferrites and in turn their magnetic properties, providing an extra degree of freedom for intentional control over the performances of 1D magnetic nanodevices for various applications. Magnetic imaging, performed via XMCD in photoemission electron microscopy, further shows the possibility of creating and trapping a series of adjacent magnetic domain walls in a single chain, suggesting that there is great application potential for these nanochains in 1D magnetic nanodevices, as determined by field‐ or current‐driven domain wall motions. Practical control over the magnetic properties of the nanochains is also achieved by extrinsic dopants of cobalt and zinc, which are observed to occupy the ferrite ionic sites in a selective manner.