From VIB‑to VB‑Group Transition Metal Disulfides:Structure Engineering Modulation for Superior Electromagnetic Wave Absorption

The laminated transition metal disulfides(TMDs),which are well known as typical two-dimensional(2D)semiconductive materials,possess a unique layered structure,leading to their wide-spread applications in various fields,such as catalysis,energy storage,sensing,etc.In recent years,a lot of research work on TMDs based functional materials in the fields of electromagnetic wave absorption(EMA)has been carried out.Therefore,it is of great significance to elaborate the influence of TMDs on EMA in time to speed up the application.In this review,recent advances in the development of electromagnetic wave(EMW)absorbers based on TMDs,ranging from the VIB group to the VB group are summarized.Their compositions,microstructures,electronic properties,and synthesis methods are presented in detail.Particularly,the modulation of structure engineering from the aspects of heterostructures,defects,morphologies and phases are systematically summarized,focusing on optimizing impedance matching and increasing dielectric and magnetic losses in the EMA materials with tunable EMW absorption performance.Milestones as well as the challenges are also identified to guide the design of new TMDs based dielectric EMA materials with high performance.

Dynamics of exciton energy renormalization in monolayer transition metal disulfides

Fundamental understandings on the dynamics of charge carriers and excitonic quasiparticles in semiconductors are of central importance for both many-body physics and promising optoelectronic and photonic applications.Here,we investigated the carrier dynamics and many-body interactions in two-dimensional(2D)transition metal dichalcogenides(TMDs),using monolayer WS2 as an example,by employing femtosecond broadband pump-probe spectroscopy.Three time regimes for the exciton energy renormalization are unambiguously revealed with a distinct red-blue-red shift upon above-bandgap optical excitations.We attribute the dominant physical process in the three typical regimes to free carrier screening effect,Coulombic exciton-exciton interactions and Auger photocarrier generation,respectively,which show distinct dependence on the optical excitation wavelength,pump fluences and/or lattice temperature.An intrinsic exciton radiative lifetime of about 1.2 picoseconds(ps)in monolayer WS2 is unraveled at low temperature,and surprisingly the efficient Auger nonradiative decay of both bright and dark excitons puts the system in a nonequilibrium state at the nanosecond timescale.In addition,the dynamics of trions at low temperature is observed to be significantly different from that of excitons,e.g.,a long radiative lifetime of^108.7 ps at low excitation densities and the evolution of trion energy as a function of delay times.Our findings elucidate the dynamics of excitonic quasiparticles and the intricate many-body physics in 2D semiconductors,underpinning the future development of photonics,valleytronics and optoelectronics based on 2D semiconductors.

D-band frontier: A new hydrogen evolution reaction activity descriptor of Pt single-atom catalysts

Hydrogen evolution reaction(HER) is crucial for achieving sustainable development and carbon neutrality, and thus demands efficient catalysts, which necessitates fundamental theory to relieve trial-and-error experiment. To fast screen HER candidates, most studies focus on d-band center(ε)associated with the Gibbs energy of H* adsorption(ΔG). Unfortunately, εrule is not applicable to Pt single atoms on transition metal disulfides(Pt_(1)/TMDs) because of the additional contributions from p states of S atom. Here, we propose a new HER descriptor — d-band frontier(d) by defining the weight of d-band in the energy range of [-1.0 eV, 1.0 eV] of Pt single atoms. This dis exactly correlated with the ΔGof Pt_(1)/TMDs, and thus perfectly describes the structure–activity relationship, as validated by systematical experimental evidences. Moreover, this ddescriptor can be extended to Pt single atoms anchored on other supports(e.g., CN, C, MoO, and CoO), indicating its promising generality.

Facile synthesis of MoS_2/graphite intercalated composite with enhanced electrochemical performance for sodium ion battery

MoS2 is a promising anode material for sodium ion batteries owing to its two-dimensional layered structure and high specific capacity. But it still exhibits a poor cycle stability and limited rate capability for Na＋ storage because of its poor electrical conductivity and structural instability. In this work, MoS2/graphite composite is fabricated by mechanically delaminated and restacked MoS2 and graphite to form two-dimensional composite layers. The graphite sheets will improve electrical conductivity and prevent the aggregation as well as structure collapse of the MoS2 layers during charge-discharge process. The MoS2/graphite composite exhibits excellent Na＋ storage properties. It delivers a high discharge specific capacity of 358.2 mAh/g at a current density of 100 mA]g in the first discharge process and with capacity retention of 68.1% after 800 cycles （retains 244 mAh/g）. The average discharge specific capacities retain 250.9 and 225.4 mAh/g corresponding to the current densities of 100 and 1000 mA]g, showing excellent rate capability. The improved electrochemical performance is attributed to the improved electrical conductivity and structural stability after composition of graphite sheets. The study demonstrates a new research strategy for improving sodium ion storage properties of Mo52.

Pressure-induced enhancement of optoelectronic properties in PtS2

PtS2, which is one of the group-10 transition metal dichalcogenides, attracts increasing attention due to its extraordinary properties under external modulations as predicted by theory, such as tunable bandgap and indirect-to-direct gap transition under strain; however, these properties have not been verified experimentally. Here we report the first experimental exploration of its optoelectronic properties under external pressure. We find that the photocurrent is weakly pressuredependent below 3 GPa but increases significantly in the pressure range of 3 GPa–4 GPa, with a maximum ～ 6 times higher than that at ambient pressure. X-ray diffraction data shows that no structural phase transition can be observed up to26.8 GPa, which indicates a stable lattice structure of PtS2 under high pressure. This is further supported by our Raman measurements with an observation of linear blue-shifts of the two Raman-active modes to 6.4 GPa. The pressure-enhanced photocurrent is related to the indirect-to-direct/quasi-direct bandgap transition under pressure, resembling the gap behavior under compression strain as predicted theoretically.

Vertically mounting molybdenum disulfide nanosheets on dimolybdenum carbide nanomeshes enables efficient hydrogen evolution

Designing hierarchical heterostructure to optimize the adsorption of hydrogen intermediate(H*)is impressive for hydrogen evolution reaction(HER)catalysis.Herein,we show that vertically mounting two-dimensional(2D)layered molybdenum disulfide(MoS_(2))nanosheets on 2D nonlayered dimolybdenum carbide(Mo_(2)C)nanomeshes to form a hierarchical heterostructure largely accelerates the HER kinetics in acidic electrolyte due to the weakening adsorption strength of H*on 2D Mo_(2)C nanomeshes.Our hierarchical MoS2/Mo2C heterostructure therefore gives a decrease of overpotential for up to 500 mV at-10 mA·cm^(-2)and an almost 200-fold higher kinetics current density compared with the pristine Mo2C nanomeshes and maintains robust stability with a small drop of overpotential for only 16 mV upon 5,000 cycles.We further rationalize this finding by theoretical calculations and find an optimized adsorption free energy of H*,identifying that the MoS_(2)featuring strong H*desorption plays a key role in weakening the strong binding of Mo_(2)C with H*and therefore improves the intrinsic HER activity on active C sites of Mo_(2)C.This present finding shines the light on the rational design of heterostructured catalysts with synergistic geometry.

Selective hydrogenation improves interface properties of high-k dielectrics on 2D semiconductors

The integration of high-k dielectrics with two-dimensional(2D)semiconductors is a critical step towards high-performance nanoelectronics,which however remains challenging due to the high density of interface states and the damage to the monolayer 2D semiconductors.In this study,we propose a selective hydrogenation strategy to improve the interface properties while the 2D semiconductors are not affected.Using the interface of monolayer molybdenum disulfide(MoS_(2))and silicon nitride as an example,we show substantially improved interface properties for electronic applications after the interfacial hydrogenation,as evidenced by reduced inhomogeneous charge redistribution,increased band offset,and nearly intact electronic properties of MoS_(2).Importantly,this hydrogenation process selectively occurs only at the silicon nitride surface and is compatible with the current semiconductor fabrication process.We further show that this strategy is general and applicable to other interfaces between high-k dielectrics and 2D semiconductors such as hafnium dioxide(HfO_(2))on the monolayer MoS_(2).Our results demonstrate a simple yet viable way to improve the integration of high-k dielectrics on a broad range of 2D transition metal disulfide semiconductors,shedding light on practical electronic and optoelectronic applications.