Lifting the valley degeneracy in two-dimensional transition metal dichalcogenides could promote their applications in information processing. Various external regulations, including magnetic substrate, magnetic doping, electric field, and carrier doping, have been implemented to enhance the valley splitting under the magnetic field. Here, a phase engineering strategy, through modifying the intrinsic lattice structure, is proposed to enhance the valley splitting in monolayer WSe 2 . The valley splitting in hybrid H and T phase WSe 2 is tunable by the concentration of the T phase. An obvious valley splitting of ∼4.1 meV is obtained with the T phase concentration of 31% under ±5 T magnetic fields, which corresponds to an effective Landeǵ eff factor of −14, about 3.5-fold of that in pure H-WSe 2 . Comparing the temperature and magnetic field dependent polarized photoluminescence and also combining the theoretical simulations reveal the enhanced valley splitting is dominantly attributed to exchange interaction of H phase WSe 2 with the local magnetic moments induced by the T phase. This finding provides a convenient solution for lifting the valley degeneracy of two-dimensional materials.

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A series of carbon precursors were obtained via the co-thermal dissolution of coal and wheat straw (WS) with different weight ratios. Three-dimensional hierarchical porous carbon (HPC X ) materials were prepared from the carbon precursors. Among the porous carbon materials, HPC 1/3 (WS:coal = 1:3 in weight) showed a specific capacitance of 384 F g −1 at 1 A g −1 in 6 M KOH and a good retention capability of 92% at a current density of 10 A g −1 . A symmetrical supercapacitor was further fabricated using HPC 1/3 for both electrodes. The supercapacitor exhibited an excellent cycle lifetime, retaining 98% of specific capacitance after 10 000 cycles. The excellent electrochemical performance of HPC 1/3 is closely related to the corresponding composition of the carbon precursor. Gas chromatography/mass spectrometry and Orbitrap mass spectrometry were used to reveal the detailed composition of carbon precursors at the molecular level. The N-containing and O-containing compounds in carbon precursors are helpful for the enhancement of capacity, surface polarity, and electrical conductivity. Aromatic compounds with a high unsaturation in a carbon precursor contribute to the high specific capacitance and cycling stability.

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Monolayer two-dimensional transition-metal dichalcogenides, such as tungsten disulfide (WS 2 ), are regarded as promising candidates for optoelectronic and electronic applications. Although theoretical calculations have predicted outstanding electronic properties of WS 2 , the performance of WS 2 -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 WS 2 monolayer grown by chemical vapor deposition. Photoluminescence and Raman spectroscopy were used to monitor the modified optical properties of WS 2 with 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 topelectrode 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 WS 2 lattice structure will be destroyed. This work has provided an effective method for manipulating the Schottky barrier and mobility of monolayer WS 2 transistors and paves the way for developing high-performance electronic devices based on 2D semiconductors. KEYWORDS: tungsten disulfide (WS 2 ), WS 2 field-effect transistors, Ar + plasma treatment, Schottky barrier (SB), work function