MoS 2 /graphene nanocomposite films are fabricated by vacuum filtering with liquid-phase exfoliated MoS 2 / graphene suspension. The nanocomposite films are characterized by Raman spectroscopy, UV−vis spectroscopy, and atomic force microscopy, indicating the optical films with a large scale and high optical homogeneity. The enhanced saturable absorption of MoS 2 /graphene nanocomposite films compared with pristine MoS 2 film and graphene film is investigated using an open-aperture Z-scan technique with a femtosecond laser at 800 nm. The nonlinear absorption coefficient of MoS 2 /graphene nanocomposite film is ∼ −1217.8 cm/GW, which is larger than that of MoS 2 film (∼ −136.1 cm/GW) and graphene film (∼ −961.6 cm/GW) at the same condition. The imaginary part of the third-order nonlinear optical susceptibility of the nanocomposite film can reach Imχ (3) ∼ 10 −9 esu with a figure of merit ∼10 −14 esu cm, low saturable intensity (∼157.0 GW/cm 2 ), and high modulation length (∼32%). A coupling model is considered in order to understand the nonlinear absorption properties of MoS 2 /graphene nanocomposite films, which suggest the enhancement can be attributed to charge transfer between MoS 2 and graphene. The results pave the way for the design of nonlinear optical properties with two-dimensional materials for good performance of optical switches or mode lockers based on saturable absorbers.

No abstract

Band alignment is a key issue for the optoelectronics based on 2D layered transition metal dichalcogenides (TMDs) heterostructures. Herein, band alignment of MoTe2/MoS2 mixed heterostructure is measured with high‐resolution X‐ray photoelectron spectroscopy. The MoTe2/MoS2 heterostructure belongs to type‐II heterostructure with the conduction band offset of 0.46 eV and the valence band offset of 0.9 eV. The stronger saturable absorption is observed in MoTe2/MoS2 heterostructure film compared with that of pure MoTe2 and MoS2 nanofilms at the same condition. An energy‐level model combined with Runge–Kutta algorithm is used to understand the enhancement mechanism. It is found that the interlayer transition from MoTe2/MoS2 heterojunction plays an important role in the nonlinear optical enhancement. Meanwhile, band structure of MoTe2/MoS2 heterostructure is calculated by the first principles. The contributions of the MoTe2 and MoS2 to the heterojunction are almost equal and the valence band maximum and conduction band minimum exist in MoTe2 and MoS2 separately. This structure can form the interlayer carriers easily. The results suggest that the band alignment of TMDs paves the way for the type‐II heterostructure for enhanced nonlinear response in the development of optical modulator, ultrafast laser mode lockers, saturable absorbers, and optical switches.

No abstract

Band alignment based on mixed van der Waals heterostructures is vital to design new types of photoanodes based on transition metal dichalcogenides. Herein, different mixed ratios of WS2/MoS2 photoanodes are made using a liquid‐phase exfoliation method. The WS2/MoS2 photoanode is characterized by high‐resolution X‐ray photoelectron spectroscopy, which suggests type‐II heterostructure with a conduction band offset of 0.54 eV and a valence band offset of 0.55 eV. Herein, it is observed that the maximum photocurrent density at the ratio WS2/MoS2 (1:1) is eight times and four times larger than pure WS2 and MoS2, respectively. The amperometric I–t, Mott–Schottky, and Nyquist measurements are used to analyze the photoelectric enhancement, which suggests efficient charge separation in the photoelectrodes and low electrode–electrolyte interface resistance. Raman spectroscopy and X‐ray diffraction spectroscopy with the mode shift confirm the charge transfer and lattice change in the mixed heterostructure. The charge density difference of WS2/MoS2 also confirms charge redistribution after heterostructure formation to accelerate charge transfer at the heterostructure interface efficiently. Herein, a cheap and easy way to design WS2/MoS2 heterojunction‐based photoanodes for photoelectrochemical devices is explained.