Results and Discussions The X-ray diffraction (XRD) results show that the diffraction peak of the (100) crystal plane is the strongest, which indicates that all MoS₂ QDs are 2H phase MoS₂. The as-prepared MoS₂ QDs-1 has better crystallinity and smaller average grain size of 3.88 nm than MoS₂ QDs-2 (Fig. 2). In addition, the number of layers for MoS₂ QDs-1 is about 6, and that for MoS₂ QDs-2 is about 13 (Fig. 3). These results indicate that MoS₂ QDs-1 has fewer layers. Since the average grain size and the number of layers for MoS₂ QDs-1 are better than those of MoS₂ QDs-2, the band gap and photoluminescence properties of MoS₂ QDs-1 are better than those of MoS₂ QDs-2 by the quantum confinement effect. The ultraviolet-visible (UV-vis) absorption spectra show that the optical band gap of MoS₂ QDs-1 is 3.65 eV [Fig. 5(a)], and the fluorescence photoluminescence (PL) spectra reveal that the fluorescence intensity of MoS₂ QDs-1 is stronger than that of MoS₂ QDs-2 [Figs. 6(a) and 6(b)]. When the excitation wavelength is increased from 270 nm to 360 nm, the positions of the emission peaks of MoS₂ QDs-1 and MoS₂ QDs-2 show an obvious red-shift phenomenon. The luminescence intensity of MoS₂ QDs-1 and MoS₂ QDs-2 are the highest when excited at wavelength of 310 nm, and the corresponding emission peak positions are at 430 nm and 420 nm, respectively. This red-shifted emission peaks may originate from the thermal fluorescence properties and polydispersity of K-points in the Brillouin zone of MoS₂ QDs, or some changes occurred during the fluorescence excitation process in the MoS₂ QDs themselves, such as agglomeration. The 1931 CIE images of MoS₂ QDs-1 and MoS₂ QDs-2 show their strongest emission domains at (0.16, 0.15) and (0.16, 0.16), respectively [Figs. 6(c) and 6(d)]. In addition, we use quinine sulfate as the reference material, and the fluorescence yield of MoS₂ QDs-1 (10.8%) is significantly higher than that of MoS₂ QDs-2 (7.2%) through the calculation formula of fluorescence quantum yield.

Molybdenum disulfide quantum dots (MoS₂ QDs) have potential applications in the fields of sensing, fluorescence detection, and photocatalysis due to their excellent physicochemical properties such as controllable size and strong quantum confinement effect. The performance of MoS₂ QDs is closely related to their size and number of layers. How to obtain MoS₂ QDs with controllable size and number of layers is still a difficult problem. In this study, the MoS₂ QDs with a small average grain size and few layers are synthesized by a facile and energy-intensive hydrothermal method. The effects of different sulfur sources (glutathione and L-cysteine) on the photoluminescence properties of MoS₂ QDs are systematically studied. The MoS₂ QDs prepared with glutathione as the sulfur source have a smaller average grain size, fewer layers, and better photoluminescence in comparison to L-cysteine-based MoS₂ QDs. We hope that our basic strategy and findings can be helpful on the design of high-quality MoS₂ QDs.

Firstly, 0.0468 g of (NH₄)₆Mo₇O₂₄·4H₂O is dissolved in 2.5 mL of deionized water, and its pH value is adjusted to 6.5 with 10% mass fraction of ammonia water. Then, 0.254 g of glutathione and the above solution are added to 10 mL of ionized water (molar ratio of Mo∶S=1∶3) and stirred for 8 min until complete dissolution. Next, the mixed solution is transferred to a polytetrafluoroethylene stainless steel autoclave with a size of 50 mL and placed in an oven at 200 ℃ for 24 h. Then, the solution obtained from the reaction is placed in a sand core filter (0.22 μm) to filter out suspended particles, and the solution supernatant is collected after centrifugation at 4 ℃ and 10000 r/min for 15 min. Finally, the supernatant is dialyzed in a dialysis bag (the interception molecular weight of the dialysis bag is 10000 u) for 24 h, and the solution is collected and stored in a refrigerator at 4 ℃ and labeled as MoS₂ QDs-1. Similarly, we weighes 0.0983 g ammonium molybdate as molybdenum source and 0.200 g L-cysteine as sulfur source (molar ratio of Mo∶S=1∶3) to prepare MoS₂ QDs-2.

In this study, homogeneous dispersed MoS₂ QDs are successfully obtained by a one-step hydrothermal method using glutathione and L-cysteine as sulfur sources respectively. Among them, the MoS₂ QDs-1 sample has a smaller average size (3.88 nm), a lower average height (4.75 nm), a smaller optical band gap (3.65 eV), and a higher fluorescence quantum yield (10.8%) in comparison to MoS₂ QDs-2 sample. Therefore, the structural and optical properties of the MoS₂ QDs-1 sample are better under these experimental conditions. The carbon chain of glutathione (C₁₀H₁₇N₃O₆S) is longer than that of L-cysteine (C₃H₇NO₂S), which is beneficial to the nucleation of nanocrystals. In addition to providing a sulfur source, glutathione can also act as a surfactant to inhibit the growth of crystal nuclei. Consequently, compared with L-cysteine, MoS₂ QDs with a smaller average size and lower average height are more easily obtained from glutathione as a sulfur source, and the optical properties and photoluminescence properties of MoS₂ QDs are affected by their sizes and number of layers. The average size of MoS₂ QDs-1 is smaller than that of MoS₂ QDs-2. Meanwhile, MoS₂ QDs-1 has fewer layers. Therefore, MoS₂ QDs-1 has a better optical band gap and higher fluorescence quantum yield.