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Solar-blind deep ultraviolet (DUV) photodetectors with high responsivity (R) and fast response speed are crucial for practical applications in astrophysical analysis, environmental pollution monitoring, and communication. Recently, 2D tellurium has emerged as a potential optoelectronic material because of its excellent photoelectric properties. In this study, solar-blind DUV photodetectors are demonstrated based on solution-synthesized and air-stable quasi-2D Te nanosheets (Te NSs). An R of 6.5 × 104 A/W at 261 nm and an external quantum efficiency (EQE) of higher than 2.26 × 106% were obtained, which are highest among most other 2D material-based solar-blind DUV photodetectors. Moreover, the photoelectric performance of the quasi-2D Te-based photodetector exhibited good stability even after ambient exposure for 90 days without any encapsulation. These results indicate that quasi-2D Te NSs provide a viable approach for developing solar-blind DUV photodetectors with ultrahigh R and EQE.

Two-dimensional (2D) ferromagnetism is critical for both scientific investigation and technological development owing to its low-dimensionality that brings in quantization of electronic states as well as free axes for device modulation. However, the scarcity of high-temperature 2D ferromagnets has been the obstacle of many research studies, such as the quantum anomalous Hall effect (QAHE) and thin-film spintronics. Indeed, in the case of the isotropic Heisenberg model with finite-range exchange interactions as an example, low-dimensionality is shown to be contraindicated with ferromagnetism. However, the advantages of low-dimensionality for micro-scale patterning could enhance the Curie temperature (TC) of 2D ferromagnets beyond the TC of bulk materials, opening the door for designing high-temperature ferromagnets in the 2D limit. In this paper, we review the recent advances in the field of 2D ferromagnets, including their material systems, physical properties, and potential device applications.

As a “hot” member of two-dimensional (2D) materials, black phosphorus (BP) has received growing interest in a variety of fields in physics, chemistry, materials, and biomedicine since 2014. Owing to the exclusive 2D layered structure, some fantastic properties such as tunable band structure, high charge mobility, and in-plane anisotropy provide BP with great potential for application in electronic/optoelectronic devices, energy source conversion and storage, etc. However, with in-depth investigations, the performance deficiencies of BP-based devices are exposed, originating from the poor chemical stability of BP. To address the above issues, a variety of BP/carbon hybrid composites have been constructed as an emerging type of multifunctional materials by the integration of the unique properties of carbon materials with the merits of BP into a single hybrid heterostructure, showing improved performance for applications. In this review, the enhanced stability of BP is presented based on the fundamental chemistry of BP degradation, which originates from the formation of P–C/P–O–C bonds or van der Waals interactions in BP/carbon hybrid composites. Recent research studies focusing on the utilization of BP/carbon hybrid composites for electrochemical energy storage, photocatalytic hydrogen production and optoelectronic devices are summarized, in which approaches toward the assembly of BP/carbon hybrid composites are introduced. Finally, the extant challenges and potential applications of BP/carbon hybrid materials are also proposed.

Based on the extensive investigation on graphene and graphene derivatives asymmetrically functionalized with a variety of molecular groups and the concept of Janus materials with asymmetric facial properties, Janus two-dimensional (2D) materials as derivatives of the 2D material family, including Janus graphene, Janus transition metal chalcogenides, etc., have attracted much attention in recent years. Janus 2D materials have been demonstrated experimentally and theoretically to possess unique properties such as an out-of-plane piezoelectric polarization and strong Rashba effect due to their out-of-plane asymmetry. Here, the recent progress of emerging Janus 2D materials, including Janus graphene, prediction and fabrication of various different Janus 2D materials and Janus 2D van der Waals heterostructures, is presented. The investigations on the unique properties and potential device applications of Janus 2D materials are summarized. Finally, the conclusion and prospects for the future explorations on Janus 2D materials are also provided.

Over the past decade, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted tremendous research interest for future electronics owing to their atomically thin thickness, compelling properties and various potential applications. However, interface engineering including contact optimization and channel modulations for 2D TMDCs represents fundamental challenges in ultimate performance of ultrathin electronics. This article provides a comprehensive overview of the basic understanding of contacts and channel engineering of 2D TMDCs and emerging electronics benefiting from these varying approaches. In particular, we elucidate multifarious contact engineering approaches such as edge contact, phase engineering and metal transfer to suppress the Fermi level pinning effect at the metal/TMDC interface, various channel treatment avenues such as van der Waals heterostructures, surface charge transfer doping to modulate the device properties, and as well the novel electronics constructed by interface engineering such as diodes, circuits and memories. Finally, we conclude this review by addressing the current challenges facing 2D TMDCs towards next-generation electronics and offering our insights into future directions of this field.

The electrochemical nitrogen reduction reaction (eNRR) provides a sustainable way to generate ammonia (NH3) but its Faradaic efficiency (FE) is relatively low and could be further improved, especially on the highly active molybdenum carbide (Mo2C) electrocatalyst. Our theoretical calculations suggest that unlike substitute doping model, interstitial Fe doping into Mo2C is highly eNRR-selective due to decreasing hydrogen evolution activity and unique surface-hydrogenation mechanism on negatively charged Mo sites around Fe atoms. Inspired by this prediction, we successfully developed a novel interstitial Fe doped Mo2C electrocatalyst by the pyrolysis of Fe-doped Mo/Zn bimetallic imidazolate frameworks (Fe-Mo/Zn BIFs) as a precursor where Fe atoms substitute Zn sites to obtain the spatial confinement effect. When evaluated in eNRR catalysis, Fe/Mo2C catalyst therefore exhibits a much higher FE of 20.1% at −0.45 V vs. RHE (reversible hydrogen electrode), which is almost double that of pristine Mo2C and substitute type. Moreover, the Fe/Mo2C catalyst also exhibits high ammonia yield rate with excellent durability over the six continuous cycles.

Great success in 2D van der Waals (vdW) heterostructures based photodetectors is obtained owing to the unique electronic and optoelectronic properties of 2D materials. Performance of photodetectors based 2D vdW heterojunctions at atomic scale is more sensitive to the nanointerface of the heterojunction than conventional bulk heterojunction. Here, a nanoengineered heterostructure for the first‐time demonstration of a nanointerface using an inserted graphene layer between black phosphorus (BP) and InSe which inhibits interlayer recombination and greatly improves photodetection performances is presented. In addition, a transition of the transport characteristics of the device is induced by graphene, from diffusion motion of minority carriers to drift motion of majority carriers. These two reasons together with an internal photoemission effect make the BP/G/InSe‐based photodetector have ultrahigh specific detectivity at room temperature. The results demonstrate that high‐performance vdW heterostructure photodetectors can be achieved through simple structural manipulation of the heterojunction interface on nanoscale., A nanoscale interface engineering in which few‐layer graphene (G) is inserted into the interlayer of black phosphorus (BP)/InSe, forming a 2D BP/G/InSe van der Waals heterojunction, is introduced. Effective electron transfer from BP to InSe is facilitated by the insertion of G, which induces a better photoresponse performance of the BP/G/InSe device.

Low-symmetry layered materials such as black phosphorus (BP) have been revived recently due to their high intrinsic mobility and in-plane anisotropic properties, which can be used in anisotropic electronic and optoelectronic devices. Since the anisotropic properties have a close relationship with their anisotropic structural characters, especially for materials with low-symmetry, exploring new low-symmetry layered materials and investigating their anisotropic properties have inspired numerous research efforts. In this paper, we review the recent experimental progresses on low-symmetry layered materials and their corresponding anisotropic electrical transport, magneto-transport, optoelectronic, thermoelectric, ferroelectric, and piezoelectric properties. The boom of new low-symmetry layered materials with high anisotropy could open up considerable possibilities for next-generation anisotropic multifunctional electronic devices.

Density functional theory calculations of the layer (L)-dependent electronic band structure, work function and optical properties of &beta, InSe have been reported. Owing to the quantum size effects (QSEs) in &beta, InSe, the band structures exhibit direct-to-indirect transitions from bulk &beta, InSe to few-layer &beta, InSe. The work functions decrease monotonically from 5.22 eV (1 L) to 5.0 eV (6 L) and then remain constant at 4.99 eV for 7 L and 8 L and drop down to 4.77 eV (bulk &beta, InSe). For optical properties, the imaginary part of the dielectric function has a strong dependence on the thickness variation. Layer control in two-dimensional layered materials provides an effective strategy to modulate the layer-dependent properties which have potential applications in the next-generation high performance electronic and optoelectronic devices.

2D layered materials turn out to be the most attractive hotspot in materials for their unique physical and chemical properties. A special class of 2D layered material refers to materials exhibiting phase transition based on environment variables. Among these materials, transition metal dichalcogenides (TMDs) act as a promising alternative for their unique combination of atomic‐scale thickness, direct bandgap, significant spin–orbit coupling and prominent electronic and mechanical properties, enabling them to be applied for fundamental studies as catalyst materials. Metal phosphorous trichalcogenides (MPTs), as another potential catalytic 2D phase transition material, have been employed for their unusual intercalation behavior and electrochemical properties, which act as a secondary electrode in lithium batteries. The preparation of 2D TMD and MPT materials has been extensively conducted by engineering their intrinsic structures at the atomic scale. In this study, advanced synthesis methods of preparing 2D TMD and MPT materials are tested, and their properties are investigated, with stress placed on their phase transition. The surge of this type of report is associated with water‐splitting catalysis and other catalytic purposes. This study aims to be a guideline to explore the mentioned 2D TMD and MPT materials for their catalytic applications., 2D phase transition materials, including transition metal dichalcogenides (TMDs) and metal phosphorous trichalcogenides (MPTs), attract researchers' interests for their unique combination of large surface area, direct bandgap, intrigue chemical, and mechanical properties. Great efforts are devoted to preparing 2D TMD and MPT materials by engineering their intrinsic structures at the atomic scale, aiming to achieve active catalyst toward catalytic purposes.

From the group-IIIA (borophene), to the group-IVA (silicene, germanene and stanine), as well as the group-VA (phosphorene, arsenene, antimonene and bismuthene) and even the group-VIA (selenene and tellurene nanosheets), these two-dimensional (2D) monoelemental materials are highly similar to graphene in lattice structure, and they can also be collectively referred to as Xenes. However, the unique specific atomic arrangement of these materials gives rise to a vast diversity of properties, and these single element 2D materials as graphene analogues are the most chemically tractable materials for synthetic exploration, thus attracting widespread attention and research. Intrinsic defects, as an defects inevitably introduced in the of experimental preparation, and extrinsic defects, as an artificially introduced and controllable way, both of them can effectively improve the environmental stability of single element materials and regulate the properties of Xenes. Herein, a review of the historical work about the properties and application of defective 2D monoelemental materials is presented. Based on the structure and properties of original single element 2D materials, the change of electronic and optoelectronic properties of these materials from different main groups under the action of different defects are classified and compared. In addition, the existing methods of realizing defects of 2D Xenes-based materials are summarized, and the application of the defects engineering in the field of optoelectronic devices, energy storage and conversion, catalysts as well are also comprehensively searched and reviewed. Finally, this review provides the prospect and challenge for the preparation and application of defective 2D monoelemental materials on the basis of the current research and development.

2D crystals are emerging new materials in multidisciplinary fields including condensed state physics, electronics, energy, environmental engineering, and biomedicine. To employ 2D crystals for practical applications, these nanoscale crystals need to be processed into macroscale materials, such as suspensions, fibers, films, and 3D macrostructures. Among these macromaterials, fibers are flexible, knittable, and easy to use, which can fully reflect the advantages of the structure and properties of 2D crystals. Therefore, the fabrication and application of 2D crystal-based fibers is of great importance for expanding the impact of 2D crystals. In this Review, 2D crystals that are successfully prepared are overviewed based on their composition of elements. Subsequently, methods for preparing 2D crystals, 2D crystals dispersions, and 2D crystal-based fibers are systematically introduced. Then, the applications of 2D crystal-based fibers, such as flexible electronic devices, high-efficiency catalysis, and adsorption, are also discussed. Finally, the status-of-quo, perspectives, and future challenges of 2D crystal-based fibers are summarized. This Review provides directions and guidelines for developing new 2D crystal-based fibers and exploring their potentials in the fields of smart wearable devices.

Owing to its thickness-modulated direct energy band gap, relatively strong light–matter interaction, and unique nonlinear optical response at a long wavelength, few-layer black phosphorus, or phosphorene, becomes very attractive in ultrafast photonics applications. Herein, we synthesized a graphene/phosphorene nano-heterojunction using a liquid phase-stripping method. Tiny lattice distortions in graphene and phosphorene suggest the formation of a nano-heterojunction between graphene and phosphorene nanosheets. In addition, we systematically investigate their nonlinear optical responses at different wavelength regimes. Our experiments indicate that the combined advantages of ultrafast relaxation, broadband response in graphene, and the strong light–matter interaction in phosphorene can be combined together by nano-heterojunction. We have further fabricated two-dimensional (2D) nano-heterojunction based optical saturable absorbers and integrated them into an erbium-doped fiber laser to demonstrate the generation of a stable ultrashort pulse down to 148 fs. Our results indicate that a graphene/phosphorene nano-heterojunction can operate as a promising saturable absorber for ultrafast laser systems with ultrahigh pulse energy and ultranarrow pulse duration. We believe this work opens up a new approach to designing 2D heterointerfaces for applications in ultrafast photonics and other research. The fabrication of a 2D nano-heterojunction assembled from stacking different 2D materials, via this facile and scalable growth approach, paves the way for the formation and tuning of new 2D materials with desirable photonic properties and applications.

Studies of the nonlinear optical phenomena that describe the light-matter interactions in 2D crystalline materials have promoted a diverse range of photonic applications. MXene, as a recently developed new 2D material, has attracted considerable attention because of its graphene-like but highly tunable and tailorable electronic/optical properties. In this study, we systematically characterize the nonlinear optical response of MXene Ti3C2Tx nanosheets over the spectral range of 800 nm to 1800 nm. A large effective nonlinear absorption coefficient (βeff∼-10−21 m2/V2) due to saturable absorption is observed for all of the testing wavelengths. The contribution of saturable absorption is two orders of magnitude higher than other lossy nonlinear absorption processes, and the amplitude of βeff strongly depends on the light bleaching level. A negative nonlinear refractive index (n2∼-10−20 m2/W) with value comparable to that of the intensively studied graphene was demonstrated for the first time. These results demonstrate the efficient broadband light signal manipulating capabilities of Ti3C2Tx, which is only one member of the large MXene family. The capability of an efficient broadband optical switch is strongly confirmed using Ti3C2Tx as saturable absorbers for mode-locking operation at 1066 nm and 1555 nm, respectively. A highly stable femtosecond laser with pulse duration as short as 159 fs in the telecommunication window is readily obtained. Considering the diversity of the MXene family, this study may open a new avenue to advanced photonic devices.