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Monolayer two-dimensional transition-metal dichalcogenides, such as tungsten disulfide (WS2), are regarded as promising candidates for optoelectronic and electronic applications. Although theoretical calculations have predicted outstanding electronic properties of WS2, the performance of WS2-based electronic devices is still limited by the relatively high Schottky barrier and low carrier mobility. In this work, low-energy argon (Ar+) plasma treatment was used as a nondestructive preconditioning technique to tailor the electrical properties of the WS2monolayer grown by chemical vapor deposition. Photoluminescence and Raman spectroscopy were used to monitor the modified optical properties of WS2with increasing plasma treatment time. An improved electrical conductivity was observed after a short-time plasma treatment. The physical mechanism was further revealed by a comparative study between top-electrode and bottom-electrode devices and simulation based on the density functional theory. It is concluded that mild Ar+plasma treatment can effectively lower the Schottky barrier height and the effective mass of carriers, which reduces the turn-on voltage and enhances the mobility, respectively. However, if the processing time is too long, the WS2lattice structure will be destroyed. This work has provided an effective method for manipulating the Schottky barrier and mobility of monolayer WS2transistors and paves the way for developing high-performance electronic devices based on 2D semiconductors.

Among large numbers of transition metal dichalcogenides (TMDCs), monolayer rhenium disulfide (ReS2) is of particular interest due to its unique structural anisotropy, which opens up unprecedented opportunities in dichroic atomical electronics. Understanding the domain structure and controlling the anisotropic evolution of ReS2during the growth is considered critical for increasing the domain size toward a large-scale growth of monolayer ReS2. Herein, by employing angle-resolved Raman spectroscopy, we reveal that the hexagonal ReS2domain is constructed by six well-defined subdomains with each b-axis parallel to the diagonal of the hexagon. By further combining the first-principles calculations and the transmission electron microscopy (TEM) characterization, a dislocation-involved anisotropic evolution is proposed to explain the formation of the domain structures and understand the limitation of the domain size. Based on these findings, growth rates of different crystal planes are well controlled to enlarge the domain size, and moreover, single-crystal domains with a triangle shape are obtained. With the improved domain size, large-scale uniform, strictly monolayer ReS2films are grown further. Scalable field-effect transistor (FET) arrays are constructed, which show good electrical performances comparable or even superior to that of the single domains reported at room temperature. This work not only sheds light on comprehending the novel growth mechanism of ReS2but also offers a robust and controllable strategy for the synthesis of large-area and high-quality two-dimensional materials with low structural symmetry.

Two-dimensional transition-metal dichalcogenides (TMDCs) possess unique electronic and optical properties, which open up a new opportunity for atomically thin optoelectronic devices. Synthesizing large-scale monolayer TMDCs on the SiO2/Si substrate is crucial for practical applications, however, it remains a big challenge. In this work, a method which combines chemical vapor deposition (CVD) and thermal evaporation was employed to grow monolayer tungsten disulfide (WS2) crystals. Through controlling the density and the distribution of W precursors, a wafer-scale continuous uniform WS2film was achieved, with the structural and spectral characterizations confirming a monolayer configuration and a high crystalline quality. Wafer-scale field-effect transistor arrays based on the monolayer WS2were fabricated. The devices show superior electrical performances, and the maximal mobility is almost 1 order of magnitude higher than those of CVD-grown large-scale TMDC devices reported so far.

We report a fully enclosed cylindrical single-electrode-based triboelectric nanogenerator (S-TENG) consisting of a perfluoroalkoxy (PFA) ball with surface-etched nanowires, a floating latex balloon, and an Al electrode at the end of the balloon. The mechanism of the S-TENG includes two independent processes: contact-induced electrification between the PFA ball and the balloon and electrostatic induction between the charged PFA ball and the Al electrode. The relationships between the electrical outputs and the sliding distance of the PFA ball were systematically investigated by combining experimental results with finite-element calculations. The S-TENG delivers an output voltage up to 236 V and a short-circuit current of 4.8 μA, which can be used as a direct power source for driving tens of green light-emitting diodes (LEDs). The S-TENG is a potential power source for gas-flow harvesters, air navigation, and environmental monitoring.

Transition-metal dichalcogenides with intrinsic spin-valley degree of freedom have enabled great potentials for valleytronic and optoelectronic applications. However, the degree of valley polarization is usually low under nonresonant excitation at room temperature due to the phonon-assisted intervalley scattering. Here, achiral and chiral Au arrays are designed to enhance the optical response and valley polarization in monolayer and bilayer WS2. A considerable band edge emission with 7 times increment is realized under the resonant coupling with Au dimer-prism arrays. Valley polarization enhancement is quantitatively predicted by the inherent mechanisms from elevated electromagnetic field intensity and radiation efficiency and further realized in polarized photoluminescence. A tunable valley polarization up to 30.0% is achieved in bilayer WS2under a nonresonant excitation at room temperature. All of these results provide a promising route toward the development of room-temperature valley-dependent optoelectronic devices.