Salt-assisted growth and ultrafast photocarrier dynamics of large-sized monolayer ReSe2

Owing to its anisotropic optical and electrical properties,rhenium diselenide(ReSe2)has garnered considerable attention recently as a candidate material for polarization-sensitive photodetectors.However,the direct and controllable synthesis of large-sized ReSe2 with a uniform thickness is still a great challenge.Herein,we have refined the synthesis method to obtain uniform monolayer ReSe2 flakes with a size of up to^106μm on sapphire via an ambient-pressure chemical vapor deposition technique using Na promoter from sodium chloride.Interestingly,optical pump-probe spectroscopy revealed a fast switching from saturable absorption(SA)to absorption enhancement(AE)in subpicosecond time scale,followed by a slower decay induced by exciton recombination.Furthermore,both AE and SA signals exhibited clear angular dependence with a periodicity of 180°,which reflected the dichroism in nonlinear absorption dynamics.In addition,the photocarrier dynamics including free-carrier transport and subpicosecond relaxation due to exciton formation or surface trapping was probed using time resolved terahertz spectroscopy.We believe that our study serves as a reference for atomically controlled synthesis of large-sized ReSe2 and provides useful insights on its optoelectronic properties for novel device applications.

Nonlinear electronic and ultrafast optical signatures in chemical vapor-deposited ultrathin PtS_(2) ribbons

The emerging two-dimensional(2D)platinum disulfide(PtS_(2))has driven increasing attentions due to its high electron mobility,good air-stability,and strong interlayer interaction which leads to a widely tunable electronic structure.However,a detailed study on its covalent-like layer-dependent properties remains infant.Herein,we demonstrate the successful production of ultrathin 1T-PtS_(2) ribbons with thickness centralized almost at monolayer 1L-4L and large domain size up to 210 μm on Au foils using chemical vapor deposition(CVD)technique,which enables macro-and microscopic study of its extraordinary layer-dependent features with precise control of the number of layers.Using electron energy loss spectroscopy(EELS)and optical pump-probe spectroscopy(OPPS),we reveal that both the electron and ultrafast optical absorption signals of the as-grown 2D PtS_(2) show strong nonlinear layer-dependent responses which manifest discriminated transition in 1L-4L PtS_(2) ribbons.The layer-dependent nonlinear response of 2D PtS_(2) can be well interpreted in the frame of calculated electron and phonon structures.These achievements offer a platform for successfully fabricating large-sized ultrathin 2D PtS_(2) and facilitating our knowledge about its electronic and optoelectronic properties.