Luminescent MoS2 quantum dots with reverse saturable absorption prepared by pulsed laser ablation

Effects of quantum confinement, focusing on upconversion and downconversion photoluminescence and reverse saturable absorption, on MoS2 quantum dots produced by nanosecond UV laser ablations is not widely reported. The effect of wavelength and pulse energy of ablating laser on the size, colloidal stability, upconversion and downconversion photoluminescence and non-linear properties of MoS2 quantum dots (QDs) are reported here. MoS2 QDs are grown by pulsed laser ablation (PLA) from bulk MoS2 pellets in deionized (DI) water with UV (355 nm), visible (532 nm) and IR (1064 nm) laser beams from a nanosecond Nd-YAG laser source and ablation is repeated with varying pulse energy. The target MoS2 pellet prepared for ablation was sintered at 800 °C in nitrogen ambience for 8 h. Hexagonal structure of MoS2 pellet is confirmed by XRD and Raman spectroscopy before and after sintering. The exfoliated MoS2 QDs dispersion show appreciable photon upconversion (UPC) and down conversion (DC) photoluminescence (PL). The downconversion PL emission shows multiple peaks. QDs size is found to vary with the wavelength of the ablating laser beam. The mean particle size, measured by dynamic light scattering measurements and TEM analysis, is in the range of 2-30 nm. Absorbance measurement shows high energy peaks around 5.88 eV (211 nm) with shoulder peak at 3.79 eV (327 nm) corresponding to MoS2 QDs. MoS2 QDs dispersion show moderate dispersion stability as per zeta potential analysis. Raman analysis of spin coated few layer MoS2 QDs shows characteristic in-plane (E12g) and out-of-plane (A1g) vibrational modes of MoS2. Nonlinear optical studies of colloidal MoS2 show reverse saturable absorption (RSA) due to two photon absorption (TPA) process.