Dynamic light scattering (DLS) analyzer was used to characterize the particle size distribution of the MoS2. It can be confirmed by the DLS particle size distribution results as showin Fig. 2 that the particles have good monodispersibility, and its diameter is in the range of 3–6 nm.
It can be observed from the SEM image in Fig. 3 that the size of the uncut MoS2 bulk material particles exceeds 100nm. In Fig. 3, it can be observed that the MoS2 bulk material is very aggregated and it cannot be seen that it is in the form of particles.
According to reported [9] that the MoS2 breaking strength was is about 16 − 30 GPa. The NaCl crystallites could transfer the shear force(shear force produced by pressing shaker ) The running speed of 2500rpm is converted into a shear force of about 20-180GPa, this shear force transfer to the edge of the NaCl particles is enough to make the MoS2 bulk break into MoS2 QDs .The results indicated MoS2-QDs Successfully synthesized by using NaCl cutting method, confirmed by scanning electron microscope (SEM) micrograph show in (Fig. 4b,c). The particle size of MoS2 QD about range in 2-6nm. The square NaCl particles show in Fig. 4a have cut MoS2 QD on the surface. Figure 4b show NaCl cutting MoS2 bulk evidence. Figure 4b can prove that NaCl can use its sharp edges to cut large particles MoS2 into MoS2 QD. MoS2 bulk after cutting produces MoS2 QD attached to NaCl square surface in red cycle. In Fig. 4d, the EDS spectra of MoS2-QD respectively show two elements (including Mo, S, and) uniformly distributed.
Figure. 5a shows the high-resolution transmission electron microscope image of MoS2 QDs. The TEM image contains more than 100 MoS2 QDs. From the figure, it can be observed that the method for preparing quantum dots developed in this research can effectively prepare a large number of quantum dots. TEM measurement showed that MoS2 quantum dots size are between 3 and 10nm show in (Fig. 5a). The high-resolution TEM (HRTEM) image (Fig. 5b) shows the high crystallinity of MoS2 QDs with a lattice parameter of 0.27 nm, which is consistent with the (100) diffraction surface of MoS2. This result shows that MoS2 QDS still maintains the same crystallinity as MoS2 bulk.
Figure 6 shows the photoluminescence spectra excited at 365 nm of MoS2-QDs solution at room temperature. emission peak located at 480 nm at this excitation wavelength can be considered from the PL spectrum.The intensity of PL spectrum is arrive 3000. This data shows that this preparation method will not damage the structure of MoS2 to produce defects.
The MoS2 optical properties show in Fig. 7.Measure the optical properties of MoS2 QDS by UV-visible. The absorption wavelength of MoS2 is less than 300nm (\({\lambda }\)< 300 nm), with a tail extending into the visible range, assigned to the excitonic features of MoS2 QDs. The calculated MoS2 band gap is 4.19ev and ban gap more than single-layer MoS2(1.9ev) and bulk MoS2(1.2ev). The inset image of Fig. 7 shows that quantum dots can be very stably dispersed in ethanol solution for one month as demonstrated by the Tyndall effect in the form of a typical discernible green line resulted from light scattering when a laser beam shines through the MoS2 solution.
The atomic states of Mo and S in MoS2 QDs are analyzed by Xray photoelectron spectroscopy (XPS). The results are shown in Fig. 8a and 8b have four peaks as Mo 3d3/2(234.8 eV), Mo 3d5/2 (231.8 eV), S 2p1/2(168.5 eV) and S 2p3/2 (167.5 eV), respectively, indicating the 2H MoS2 phase in the crystal structure.Two characteristic peaks situated at 231.8 (Mo 3d5/2) and 234.8eV (Mo 3d3/2) are typical values for Mo4+ in MoS2 (Fig. 8a).