Xingxing Zhang

A cloud‐based epidermal gas sensor with high sensitivity and fast response based on a gold‐assisted exfoliated large area MoSe₂ nanosheet is realized. The device–smartphone IoT interface was specially designed with wireless connectivity in order to effectively upload the sensing data to a cloud‐based terminal for further research, sharing, recording, and to provide timely warnings.

Abstract

Toxic gases such as NO₂ and irritant gases such as NH₃ are two of the harshest aspects that trigger the exacerbation of the respiratory system for asthma patients. Monitoring and recording high‐risk gases are very important for tracking disease and alerting patients because humans can be exposed to a vulnerable environment with inconspicuous gases. Current detectors suffer from lack of portability and cannot provide daily real‐time detection. This work develops a light, inexpensive epidermal gas sensor based on ultralarge MoSe₂ nanosheets. MoSe₂ nanosheets are obtained using a gold‐assisted exfoliation method and the electrical and optical properties of the film are characterized. A high‐performance gas sensor for NO₂ and NH₃, which can be integrated onto human skin, is fabricated and shows great stability with up to 30% tensile strain. In particular, the device is able to detect down to 1 part per million with fast response (<200 s). The system is effective in providing timely warnings and the sensing data are uploaded to a cloud‐based terminal so that a medical institute can easily access them and provide a more accurate diagnosis.

Transport evidence of asymmetric spin–orbit coupling in few-layer superconducting 1T_d-MoTe₂

Transport evidence of asymmetric spin–orbit coupling in few-layer superconducting 1<i>T</i>_d-MoTe₂, Published online: 03 May 2019; doi:10.1038/s41467-019-09995-0

Two-dimensional transition metal dichalcogenides with peculiar spin–orbit coupling may lead to exotic phenomena. Here, the authors report a large in-plane upper critical field with a two-fold symmetry, suggesting a novel asymmetric spin–orbit coupling in few-layer 1Td-MoTe2.

PtSe 2 , an emerging 2D group-10 transition metal dichalcogenide (TMD), has aroused significant attention recently due to its intriguing physical properties. Here, the optical properties of chemical vapor deposition-grown PtSe 2 films with different thicknesses were characterized with nondestructive spectroscopic ellipsometry and Fourier transform infrared spectroscopy. The polarized optical microscopy reveals the isotropic in-plane optical response of the continuous PtSe 2 films in a scale size of at least as small as 143  ×  108 µ m 2 . The electrical transport characterization and UV-mid infrared absorption spectra reveal the coexistence of both semiconducting and metallic contents in these PtSe 2 films, making PtSe 2 quite different among the 2D material family. The effective refractive index ( n ) and the extinction coefficient ( k ) over a spectra range of 360–1700 nm were obtained. In contrast to ot...

The Schottky–Mott limit is studied in a dual‐gated graphene–WSe₂ heterojunction. Nearly ideal Schottky diode characteristics with extremely large gate tunability are demonstrated. The graphene–WSe₂ Schottky barrier height at each gate voltage is determined, showing one‐to‐one modulation, following the Schottky–Mott rule. These results have broad implications in contact engineering for 2D materials and optoelectronic devices.

Abstract

Metal–semiconductor interfaces, known as Schottky junctions, have long been hindered by defects and impurities. Such imperfections dominate the electrical characteristics of the junction by pinning the metal Fermi energy. Here, a graphene–WSe₂ p‐type Schottky junction, which exhibits a lack of Fermi level pinning, is studied. The Schottky junction displays near‐ideal diode characteristics with large gate tunability and small leakage currents. Using a gate electrostatically coupled to the WSe₂ channel to tune the Schottky barrier height, the Schottky–Mott limit is probed in a single device. As a special manifestation of the tunable Schottky barrier, a diode with a dynamically controlled ideality factor is demonstrated.

Incorporation of rare‐earth elements into the layers of ultrathin 2D nanomaterials yields an emerging class of functional materials with unique optical, magnetic, and catalytic properties. All families of ultrathin 2D rare‐earth nanomaterials are reviewed, focusing on their compositions, syntheses, and applications.

Abstract

Ultrathin 2D nanomaterials possess promising properties due to electron confinement within single or a few atom layers. As an emerging class of functional materials, ultrathin 2D rare‐earth nanomaterials may incorporate the unique optical, magnetic, and catalytic behaviors of rare‐earth elements into layers, exhibiting great potential in various applications such as optoelectronics, magnetic devices, transistors, high‐efficiency catalysts, etc. Despite its importance, reviews on ultrathin 2D rare‐earth nanomaterials or related topics are rare and only focus on a certain family of ultrathin 2D rare‐earth nanomaterials. This work is the first comprehensive review in this impressive field, which covers all families of ultrathin 2D rare‐earth nanomaterials, illustrating their compositions, syntheses, and applications. After summarizing the current achievements, the challenges and opportunities of future research on ultrathin 2D rare‐earth nanomaterials are evaluated.

Van der Waals bipolar junction transistors based on vertically stacked 2D materials (V2D‐BJT) are proposed, and experimental studies are conducted on the V2D‐BJT using an MoS₂/WSe₂/MoS₂ heterostructure in an n‐p‐n configuration. The V2D‐BJT shows excellent gas sensing performance with a low power dissipation (≈2 nW), a fast response (9 s), and a fast recovery (35 s) time.

Abstract

The majority of microelectronic devices rely on a p‐n junction. The process of making such a junction is complicated, and it is difficult to make layers that form a junction with an atomic thickness. In this study, bipolar junctions are made by using 2D atomic crystalline layers and even a single layer in which 2D layers adhere together to form a heterostructure via van der Waals forces. A vertical 2D bipolar junction transistor (V2D‐BJT) is studied for the first time. It uses an MoS₂/WSe₂/MoS₂ heterostructure and has an n‐p‐n configuration that exhibits a maximum common‐base current gain of ≈0.97 and a stable common‐emitter current gain (β) of 12 with a nanowatt power consumption. In the first attempt at gas sensing, it shows outstanding performance, exhibiting a very fast response and recovery time (9 and 35 s, respectively) with a power dissipation of only 2 nW. This study demonstrates the potential application of the V2D‐BJT in nanowatt power amplifiers as well as fast‐response and low‐power gas sensors.

Ultrahigh electrical conductivity ≈3000 times higher than that of Cu is realized in graphene embedded in metals. As a result, the corresponding graphene/Cu composites show an electrical conductivity significantly higher than that of Ag. Such graphene/metal interactions provide a unique platform to explore electron behaviors in graphene, and the results open up new opportunities for graphene's applications.

Abstract

Highly efficient conductors are strongly desired because they can lead to higher working performance and less energy consumption in their wide range applications. However, the improvements on the electrical conductivities of conventional conductors are limited, such as purification and growing single crystal of metals. Here, by embedding graphene in metals (Cu, Al, and Ag), the trade‐off between carrier mobility and carrier density is surmount in graphene, and realize high electron mobility and high electron density simultaneously through elaborate interface design and morphology control. As a result, a maximum electrical conductivity three orders of magnitude higher than the highest on record (more than 3,000 times higher than that of Cu) is obtained in such embedded graphene. As a result, using the graphene as reinforcement, an electrical conductivity as high as ≈117% of the International Annealed Copper Standard and significantly higher than that of Ag is achieved in bulk graphene/Cu composites with an extremely low graphene volume fraction of only 0.008%. The results are of significance when enhancing efficiency and saving energy in electrical and electronic applications of metals, and also of interest for fundamental researches on electron behaviors in graphene.

Bioinspired 2D MoSe₂ nanosheets with high photothermal conversion efficiency are designed to achieve efficient photothermal‐triggered cancer immunotherapy, by activating cytotoxic T lymphocytes, reprogramming of tumor‐associated macrophages to tumoricidal M1 phenotype, and inactivation of PD‐1/PD‐L1 pathway to avoid immunologic escape.

Abstract

Nonspecific absorption and clearance of nanomaterials during circulation is the major cause for treatment failure in nanomedicine‐based cancer therapy. Therefore, herein bioinspired red blood cell (RBC) membrane is employed to camouflage 2D MoSe₂ nanosheets with high photothermal conversion efficiency to achieve enhanced hemocompatibility and circulation time by preventing macrophage phagocytosis. RBC–MoSe₂‐potentiated photothermal therapy (PTT) demonstrates potent in vivo antitumor efficacy, which triggers the release of tumor‐associated antigens to activate cytotoxic T lymphocytes and inactivate the PD‐1/PD‐L1 pathway to avoid immunologic escape. Furthermore, in the ablated tumor microenvironment, the tumor‐associated macrophages are effectively reprogrammed to tumoricidal M1 phenotype to potentiate the antitumor action. Taken together, this biomimetic functionalization thus provides a substantial advance in personalized PTT‐triggered immunotherapy for clinical translation.

Here we present a size selection model for liquid-exfoliated 2D nanosheets. The ability to consistently select exfoliated nanosheets with desired properties is important for development of applications in all areas. The model presented facilitates determination of centrifugation parameters for production of dispersions with controlled size and thickness for different materials, solvents and exfoliation processes. Importantly, after accounting for the influence of viscosity on exfoliation, comparisons of different solvents are shown to be well described by the surface tension and Hansen parameter matching. This suggests that previous analyses may have overestimated the relative performance of more viscous solvents. This understanding can be extended to develop a model based on the force balance of nanosheets falling under viscous drag during centrifugation. By considering the microscopic aspect ratio relationships, this model can be both calibrated for size selection of nanosheet...

Nonvolatile charge trap memory is an important part of the continuous development of information technology. As a 2-dimensional (2D) material with fantastic physical characteristics, molybdenum disulfide (MoS 2 ) has been receiving extensive attention for its potential applications in electronic devices. However, while various attempts have been made to devise its charge-trap gate stack, it’s still impossible to avoid a certain performance degradation. Here, a MoS 2 -based nonvolatile charge trapping memory device with a charge-trap gate stack formed by implanting N ions into SiO 2 is reported. The fabricated N-implanted memory devices with the energy of 6.5 keV and the dose of 1  ×  10 15 ions cm −2 exhibit a high on/off current ratio up to 10 7 , a large memory window of 9.1 V, and a high program/erase speed of 10/100 µ s. Moreover, the memory device shows an excellent cycling endurance of more than 10 4 ...

Perpendicular magnetic anisotropy (PMA) and spin‐orbit torque (SOT) efficiency are greatly enhanced by MoS₂. First‐principles calculation and X‐ray absorption reveal that MoS₂ results in the modification of orbital hybridization at the Pt/Co interface. These findings may pave a new way to engineer the PMA and SOT efficiency by 2D materials.

Abstract

2D transition metal dichalcogenides have attracted much attention in the field of spintronics due to their rich spin‐dependent properties. The promise of highly compact and low‐energy‐consumption spin‐orbit torque (SOT) devices motivates the search for structures and materials that can satisfy the requirements of giant perpendicular magnetic anisotropy (PMA) and large SOT simultaneously in SOT‐based magnetic memory. Here, it is demonstrated that PMA and SOT in a heavy metal/transition metal ferromagnet structure, Pt/[Co/Ni]₂, can be greatly enhanced by introducing a molybdenum disulfide (MoS₂) underlayer. According to first‐principles calculation and X‐ray absorption spectroscopy (XAS), the enhancement of the PMA is ascribed to the modification of the orbital hybridization at the interface of Pt/Co due to MoS₂. The enhancement of SOT by the role played by MoS₂ is explained, which is strongly supported by the identical behavior of SOT and PMA as a function of Pt thickness. This work provides new possibilities to integrate 2D materials into promising spintronics devices.

The distorted octahedral structure of 1T′‐MoS₂ is confirmed, and the in‐plane optical and electrical anisotropies of metastable 1T′‐MoS₂ layers are investigated. Importantly, the dependence of electrocatalytic activity on the anisotropic charge transport in 1T′‐MoS₂ layers is demonstrated.

Abstract

Crystal phases play a key role in determining the physicochemical properties of a material. To date, many phases of transition metal dichalcogenides have been discovered, such as octahedral (1T), distorted octahedral (1T′), and trigonal prismatic (2H) phases. Among these, the 1T′ phase offers unique properties and advantages in various applications. Moreover, the 1T′ phase consists of unique zigzag chains of the transition metals, giving rise to interesting in‐plane anisotropic properties. Herein, the in‐plane optical and electrical anisotropies of metastable 1T′‐MoS₂ layers are investigated by the angle‐resolved Raman spectroscopy and electrical measurements, respectively. The deconvolution of J₁ and J₂ peaks in the angle‐resolved Raman spectra is a key characteristic of high‐quality 1T′‐MoS₂ crystal. Moreover, it is found that its electrocatalytic performance may be affected by the crystal orientation of anisotropic material due to the anisotropic charge transport.

In situ observations of the growth of a van der Waals (vdW) heterostructure are presented. The antimonene–graphene vdW growth dynamics are elucidated based on real‐time low‐energy electron microscopy measurements in an effort to develop a deeper understanding of the growth mechanisms of this new class of materials and facilitate their integration in innovative technologies.

Abstract

Van der Waals (vdW) heterostructures have recently been introduced as versatile building blocks for a variety of novel nanoscale and quantum technologies. Harnessing the unique properties of these heterostructures requires a deep understanding of the involved interfacial interactions and a meticulous control of the growth of 2D materials on weakly interacting surfaces. Although several epitaxial vdW heterostructures have been achieved experimentally, the mechanisms governing their synthesis are still nebulous. With this perspective, herein, the growth dynamics of antimonene on graphene are investigated in real time. In situ low‐energy electron microscopy reveals that nucleation predominantly occurs on 3D nuclei followed by a self‐limiting lateral growth with morphology sensitive to the deposition rate. Large 2D layers are observed at high deposition rates, whereas lower growth rates trigger an increased multilayer nucleation at the edges as they become aligned with the Z2 orientation leading to atoll‐like islands with thicker, well‐defined bands. This complexity of the vdW growth is elucidated based on the interplay between the growth rate, surface diffusion, and edges orientation. This understanding lays the groundwork for a better control of the growth of vdW heterostructures, which is critical to their large‐scale integration.

2D ternary transitional metal dichalcogenides have been spotlighted recently. A one‐step chemical vapor deposition (CVD) method to synthesize monolayer WTe_{2 x} S_{2(1− x )} alloys is reported. By tuning the ratio of chalcogen precursors and H₂ flow rate, both semiconducting 1H and metallic 1T′ structures can be obtained. Local displacement of Te atoms from the original 1H lattice sites is also observed and studied.

Abstract

Alloying 2D transition metal dichalcogenides has opened up new opportunities for bandgap engineering and phase control. Developing a simple and scalable synthetic route is therefore essential to explore the full potential of these alloys with tunable optical and electrical properties. Here, the direct synthesis of monolayer WTe_{2 x} S_{2(1− x )} alloys via one‐step chemical vapor deposition (CVD) is demonstrated. The WTe_{2 x} S_{2(1− x )} alloys exhibit two distinct phases (1H semiconducting and 1T ′ metallic) under different chemical compositions, which can be controlled by the ratio of chalcogen precursors as well as the H₂ flow rate. Atomic‐resolution scanning transmission electron microscopy–annular dark field (STEM‐ADF) imaging reveals the atomic structure of as‐formed 1H and 1T ′ alloys. Unlike the commonly observed displacement of metal atoms in the 1T ′ phase, local displacement of Te atoms from original 1H lattice sites is discovered by combined STEM‐ADF imaging and ab initio molecular dynamics calculations. The structure distortion provides new insights into the structure formation of alloys. This generic synthetic approach is also demonstrated for other telluride‐based ternary monolayers such as WTe_{2 x} Se_{2(1− x )} single crystals.

Monolayer VSe₂ represents a unique system for exploring the interplay between charge density wave and magnetism phenomena. Evidence of spin frustration is obtained in monolayer VSe₂, which is significant toward the search for exotic low‐dimensional quantum phases and further theoretical and experimental studies of van der Waals monolayer magnets.

Abstract

Monolayer VSe₂, featuring both charge density wave and magnetism phenomena, represents a unique van der Waals magnet in the family of metallic 2D transition‐metal dichalcogenides (2D‐TMDs). Herein, by means of in situ microscopy and spectroscopic techniques, including scanning tunneling microscopy/spectroscopy, synchrotron X‐ray and angle‐resolved photoemission, and X‐ray absorption, direct spectroscopic signatures are established, that identify the metallic 1T‐phase and vanadium 3d¹ electronic configuration in monolayer VSe₂ grown on graphite by molecular‐beam epitaxy. Element‐specific X‐ray magnetic circular dichroism, complemented with magnetic susceptibility measurements, further reveals monolayer VSe₂ as a frustrated magnet, with its spins exhibiting subtle correlations, albeit in the absence of a long‐range magnetic order down to 2 K and up to a 7 T magnetic field. This observation is attributed to the relative stability of the ferromagnetic and antiferromagnetic ground states, arising from its atomic‐scale structural features, such as rotational disorders and edges. The results of this study extend the current understanding of metallic 2D‐TMDs in the search for exotic low‐dimensional quantum phenomena, and stimulate further theoretical and experimental studies on van der Waals monolayer magnets.

Heterogenous electrocatalysts based on transition metal sulfides (TMS) are being actively explored in renewable energy research because nanostructured forms support high intrinsic activities for both the hydrogen evolution reaction and oxygen evolution reaction. Ways to improve the performance of TMS‐based materials by manipulating their internal and external nanoarchitectures are described.

Abstract

Heterogenous electrocatalysts based on transition metal sulfides (TMS) are being actively explored in renewable energy research because nanostructured forms support high intrinsic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, it is described how researchers are working to improve the performance of TMS‐based materials by manipulating their internal and external nanoarchitectures. A general introduction to the water‐splitting reaction is initially provided to explain the most important parameters in accessing the catalytic performance of nanomaterials catalysts. Later, the general synthetic methods used to prepare TMS‐based materials are explained in order to delve into the various strategies being used to achieve higher electrocatalytic performance in the HER. Complementary strategies can be used to increase the OER performance of TMS, resulting in bifunctional water‐splitting electrocatalysts for both the HER and the OER. Finally, the current challenges and future opportunities of TMS materials in the context of water splitting are summarized. The aim herein is to provide insights gathered in the process of studying TMS, and describe valuable guidelines for engineering other kinds of nanomaterial catalysts for energy conversion and storage technologies.