Excited state absorption induced optical limiting action of MoS2-rGO nanocomposites

Abstract

Abstract Molybdenum sulphide-reduced graphene oxide nanocomposite made of microsphere MoS2 with varying concentration of layered (15, 30, 45\xa0wt%) rGO was prepared by hydrothermal method. XRD confirms the molecular formation of mixed phase MoS2 having rich semiconducting hexagonal phase with traces of metallic tetragonal phase (1T@2H MoS2) and reduced graphene oxide (2θ\xa0=\xa025.7°, (0\xa00\xa02)). Further EDS also ascertains the reduction of Mo and S with increase in C content with respect to rGO concentration. As the rGO concentration increases (15–45\xa0wt%), strengthening of characteristic Raman peaks of rGO (G and D band) and weakening of MoS2 ( E 1 g , E 2 g 1 and A 1 g modes) in composite was observed. Estimated ID/IG (1.04) and I2D/IG (0.62) ratio suggest the formation of 2–3 layers of graphene. SEM depicts layered morphology of rGO, floret like microstructure of MoS2 and floret microspheres MoS2 decorated rGO layers of MoS2 – rGO nanocomposite. Optical studies show absorption peaks at 266\xa0nm (π-π* transition of the C C aromatic bonds of graphene), 370\xa0nm (and n-π* transition of the C O bond of graphene) and 330\xa0nm (direct transitions in valence to conduction band of MoS2). Liquid dispersion of both pristine and composite depicted strong absorption in UV–Visible region and available electronic state favours the opportunity for two-photon absorption. Intensity dependent Z-Scan with nano-pulsed green laser excitation shows the existence of excited state absorption involved two-photon absorption. Variation of saturation intensity and nonlinear absorption coefficient with on-axis peak intensity (620 –2160 GW/cm2) confirms the involvement of sequential 2PA process (1PA\xa0+\xa0ESA). MoS2-(30\xa0wt%)rGO nanocomposite exhibited higher 2PA coefficient (9.42\xa0×\xa010−10 m/W) and lower onset optical limiting threshold (3.15\xa0×\xa01012 W/m2) than its pristine counterparts and other well-known TMDC’s and graphene derivatives. Enhanced NLO response is due to synergistic effects of rGO (higher defect induced states, extended conjugation for charge transfer) and MoS2 (floret morphology, uniform decoration and strong visible absorption). Observed excited state absorption induced energy absorbing optical limiting action of MoS2-rGO nanocomposite with augmented NLO coefficients open up potential horizon for developing laser goggles against most sensitive and widely used nanopulsed green laser.