Formation of heterojunctions of transition metal dichalcogenides (TMDs) stimulates wide interest in new device physics and technology by tuning optical and electronic properties of TMDs. TMDs heterojunctions are of scientific and technological interest for exploration of next generation flexible electronics. Herein, we report on a two-step epitaxial ambient-pressure CVD technique to construct in-plane MoS2-WS2 heterostructures. The technique has the potential to artificially control the shape and structure of heterostructures or even to be more potentially extendable to growth of TMD superlattice than that of one-step CVD technique. Moreover, the unique MX2 heterostructure with monolayer MoS2 core wrapped by multilayer WS2 is obtained by the technique, which is entirely different from MX2 heterostructures synthesized by existing one-step CVD technique. Transmission electron microscopy, Raman and photoluminescence mapping studies reveal that the obtained heterostructure nanosheets clearly exhibit the modulated structural and optical properties. Electrical transport studies demonstrate that the special MoS2 (monolayer)/WS2 (multilayer) heterojunctions serve as intrinsic lateral p-n diodes and unambiguously show the photovoltaic effect. On the basis of this special heterostructure, depletion-layer width and built-in potential, as well as the built-in electric field distribution, are obtained by KPFM measurement, which are the essential parameters for TMD optoelectronic devices. With further development in future studies, this growth approach is envisaged to bring about a new growth platform for two-dimensional atomic crystals and to create unprecedented architectures therefor.

Plasmon-free surface enhanced Raman scattering (SERS) based on the chemical mechanism (CM) is drawing great attention due to its capability for controllable molecular detection. However, in comparison to the conventional noble-metal-based SERS technique driven by plasmonic electromagnetic mechanism (EM), the low sensitivity in the CM-based SERS is the dominant barrier toward its practical applications. Herein, we demonstrate the 1T' transition metal telluride atomic layers (WTe and MoTe) as ultrasensitive platforms for CM-based SERS. The SERS sensitivities of analyte dyes on 1T'-W(Mo)Te reach EM-comparable ones and become even greater when it is integrated with a Bragg reflector. In addition, the dye fluorescence signals are efficiently quenched, making the SERS spectra more distinguishable. As a proof of concept, the SERS signals of analyte Rhodamine 6G (R6G) are detectable even with an ultralow concentration of 40 (400) fM on pristine 1T'-W(Mo)Te, and the corresponding Raman enhancement factor (EF) reaches 1.8 × 10 (1.6 × 10). The limit concentration of detection and the EF of R6G can be further enhanced into 4 (40) fM and 4.4 × 10 (6.2 × 10), respectively, when 1T'-W(Mo)Te is integrated on the Bragg reflector. The strong interaction between the analyte and 1T'-W(Mo)Te and the abundant density of states near the Fermi level of the semimetal 1T'-W(Mo)Te in combination gives rise to the promising SERS effects by promoting the charge transfer resonance in the analyte-telluride complex.

Lateral WS2-MoS2 heterostructures are synthesized by a shortcut one-step growth recipe with low-cost and soluble salts. The 2D spatial distributions of the built-in potential and the related electric field of the lateral WS2-MoS2 heterostructure are quantitatively analyzed by scanning Kelvin probe force microscopy revealing the fundamental attributes of the lateral heterostructure devices.

2D van der Waals (vdW) layered polar crystals sustaining phonon polaritons (PhPs) have opened up new avenues for fundamental research and optoelectronic applications in the mid-infrared to terahertz ranges. To date, 2D vdW crystals with PhPs are only experimentally demonstrated in hexagonal boron nitride (hBN) slabs. For optoelectronic and active photonic applications, semiconductors with tunable charges, finite conductivity, and moderate bandgaps are preferred. Here, PhPs are demonstrated with low loss and ultrahigh electromagnetic field confinements in semiconducting vdW α-MoO . The α-MoO supports strong hyperbolic PhPs in the mid-infrared range, with a damping rate as low as 0.08. The electromagnetic confinements can reach ≈λ /120, which can be tailored by altering the thicknesses of the α-MoO 2D flakes. Furthermore, spatial control over the PhPs is achieved with a metal-ion-intercalation strategy. The results demonstrate α-MoO as a new platform for studying hyperbolic PhPs with tunability, which enable switchable mid-infrared nanophotonic devices.

A SiO2/Si substrate modified by an octadecyltrimethoxysilane (OTMS) self‐assembled monolayer is used to obtain high‐quality graphene devices with low intrinsic doping level. The carrier mobility can reach 47 000 cm2 V−1 s−1. The findings will pave the way for approaching the intrinsic properties of supported graphene, elucidating the scattering origins, and gaining a better understanding of the mechanism of carrier transport in graphene.