Vertically stacked van der Waals (vdW) heterojunctions based on two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted a great deal of attention and have created a powerful new material platform for novel, high-performance electronic and optoelectronic devices. Here, we report the construction of multilayer p-MoTe2/n-MoS2 vdW heterostructures with remarkable rectification behavior, self-powered photoresponse and distinct photosensitivity at different laser wavelengths and power densities. Field effect transistors (FETs) fabricated by MoTe2/MoS2 heterojunctions exhibit excellent gate-tunable rectification behavior and p-n junction transport characteristics, with the n-type dominating. The MoTe2/MoS2 heterojunction devices generate a self-powered photocurrent at zero bias voltage with a considerable on-off ratio reaching ∼780 and achieve a stable and fast photoresponse, due to the type-II band alignment facilitating efficient electron-hole separation. Utilizing the advantages of a p-n junction with type-II band alignment, this MoTe2/MoS2 vdW heterostructure provides more opportunities for future electronic and optoelectronic applications.
Synthesis of large-scale highly crystalline two-dimensional alloys is significant for revealing properties. Here, we have investigated the vapor growth process of high-quality bilayer CoxMo1-xS2 (x = 0.16) hexagonal nanosheets systematically. As the initial loading of the sulfur increases, the morphology of the CoxMo1-xS2 (0 < x ≤ 1) nanosheets becomes hexagons from David stars step by step at 680 °C. We find that Co atoms mainly distribute at the edge of nanosheets. When the temperature increases from 680 to 750 °C, high-quality cubic pyrite-type crystal structure CoS2 grows on the surface of CoxMo1-xS2 nanosheet gradually and forms hexagonal film induced by the nanosheet. Electrical transport measurements reveal that the CoxMo1-xS2 nanosheets and CoS2 films exhibit n-type semiconducting transport behavior and half-metallic behavior, respectively. Theoretical calculations of their band structures agree well with the experimental results.
Two-dimensional (2D) layered inorganic nanomaterials have attracted huge attention due to their unique electronic structures, as well as extraordinary physical and chemical properties for use in electronics, optoelectronics, spintronics, catalysts, energy generation and storage, and chemical sensors. Graphene and related layered inorganic analogues have shown great potential for gas-sensing applications because of their large specific surface areas and strong surface activities. This review aims to discuss the latest advancements in the 2D layered inorganic materials for gas sensors. We first elaborate the gas-sensing mechanisms and introduce various types of gas-sensing devices. Then, we describe the basic parameters and influence factors of the gas sensors to further enhance their performance. Moreover, we systematically present the current gas-sensing applications based on graphene, graphene oxide (GO), reduced graphene oxide (rGO), functionalized GO or rGO, transition metal dichalcogenides, layered III-VI semiconductors, layered metal oxides, phosphorene, hexagonal boron nitride, etc. Finally, we conclude the future prospects of these layered inorganic materials in gas-sensing applications.
Recent studies have shown that tellurium-based two-dimensional (2D) crystals undergo dramatic structural, physical, and chemical changes under ambient conditions, which adversely impact their much desired properties. Here, we introduce a diazonium molecule functionalization-based surface engineering route that greatly enhances their environmental stability without sacrificing their much desired properties. Spectroscopy and microscopy results show that diazonium groups significantly slow down the surface reactions, and consequently, gallium telluride (GaTe), zirconium telluride (ZrTe3), and molybdenum ditelluride (MoTe2) gain strong resistance to surface transformation in air or when immersed under water. Density functional theory calculations show that functionalizing molecules reduce surface reactivity of Te-containing 2D surfaces by chemical binding followed by an electron withdrawal process. While pristine surfaces structurally decompose because of strong reactivity of Te surface atoms, passivated functionalized surfaces retain their structural anisotropy, optical band gap, and emission characteristics as evidenced by our conductive atomic force microscopy, photoluminescence, and absorption spectroscopy measurements. Overall, our findings offer an effective method to increase the stability of these environmentally sensitive materials without impacting much of their physical properties.
In-plane anisotropy of layered materials adds another dimension to their applications, opening up avenues in diverse angle-resolved devices. However, to fulfill a strong inherent in-plane anisotropy in layered materials still poses a significant challenge, as it often requires a low-symmetry nature of layered materials. Here, we report the fabrication of a member of layered semiconducting AIIIBVI compounds, TlSe, that possesses a low-symmetry tetragonal structure and investigate its anisotropic light–matter interactions. We first identify the in-plane Raman intensity anisotropy of thin-layer TlSe, offering unambiguous evidence that the anisotropy is sensitive to crystalline orientation. Further in-situ azimuth-dependent reflectance difference microscopy enables the direct evaluation of in-plane optical anisotropy of layered TlSe, and we demonstrate that the TlSe shows a linear dichroism under polarized absorption spectra arising from an in-plane anisotropic optical property. As a direct result of the linear dichroism, we successfully fabricate TlSe devices for polarization-sensitive photodetection. The discovery of layered TlSe with a strong in-plane anisotropy not only facilitates its applications in linear dichroic photodetection but opens up more possibilities for other functional device applications.
van der Waals (vdW) materials exhibit great potential in spintronics, arising from their excellent spin transportation, large spin-orbit coupling, and high-quality interfaces. The recent discovery of intrinsic vdW antiferromagnets and ferromagnets has laid the foundation for the construction of all-vdW spintronic devices, and enables the study of low-dimensional magnetism, which is of both technical and scientific significance. In this review, several representative families of vdW magnets are introduced, followed by a comprehensive summary of the methods utilized in reading out the magnetic states of vdW magnets. Thereafter, it is shown that various electrical, mechanical, and chemical approaches are employed to modulate the magnetism of vdW magnets. Finally, the perspective of vdW magnets in spintronics is discussed and an outlook of future development direction in this field is also proposed.
Various morphologies of polyaniline (PANI), such as microflakes stacked by 1D nanofiberes, porous microwebs, hemispheres, and nests piled by nanoparticles, were prepared through an ice-templating method, using different doping acids assisted by secondary solvent. The structure and morphology of these PANI were characterized by Fourier transform infrared (FT-IR) spectra, X-ray diffraction (XRD) patterns, and scanning electron microscope images (SEM). Although the ice-templating method has been used for preparing conducting polymer materials with unique structures in recent years, to the best our knowledge, this is the first report on synthesizing PANI doped with polyoxometalates (POMs) using the ice-templating method assisted by secondary solvent to direct the polymerization of aniline. In this paper, the effects of doping acid and the secondary solvent on the morphology of PANI were investigated in detail. All the results showed that just the doping acids, POMs with unique nucleophilic oxygen-enriched surfaces, multi-hydrogen proton, and the binding capacity with the secondary solvent play a major role in determining the formation of various morphologies of PANI. A possible mechanism for the formation of the different morphologies of PANI was proposed. The gas-responses to ammonia were examined at room temperature. Compared with PANI porous microwebs, hemispheres, or nests piled by nanoparticles, the PANI microflakes stacked by nanofibers showed the best performance in both sensitivity and time response due to their small nanofiber diameter, high surface area, and porous nature, which will have potential application in the area of chemical sensors.
Design and preparation of photosensitive materials is important for a wide range of applications in solar cells, photocatalysts, photodetectors, and other optoelectronics devices. In recent decades, many photosensitive devices based on the polymer-functionalized metal compound semiconductors have attracted extensive attention and research. These devices combine the advantages of both polymers and metal compounds, and they show tremendously enhanced photosensitive performance compared with the individual components, even exhibiting some optoelectronic properties that are absent for the constituents. In this paper, we review recent developments including mechanisms of photosensitivity, various approaches to obtaining the polymer-functionalized metal compound semiconductors, the properties that are created or enhanced through the functionalization, and some of their applications.
α-MoO3 nanosheets were synthesized by a water bath method using ammonium heptamolybdenum tetrahydrate and concentrated nitric acid as precursors. Hydrogen was doped by a chemical reduction in aqueous acidic media, with hydrazine hydrate used as the reducing agent. Temperature dependent resistance showed that the low temperature Peierls transition of H-doped MoO3 nanosheets breaks below 50 K, and its resistance is satisfied at temperatures lower than 37 K (37–10 K). This phenomenon was induced by thermal disturbance and the dominance of defects in low temperature transport, which was confirmed by photoresponse measurements taken before and after the break of the new phase.
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