Growth and characterization of monolayer Sb-doped MoS 2 nanosheets

For the growth of MoS 2 and Sb-doped MoS 2 nanosheets, a facile CVD process was used. As illustrated in Supplementary Figure S1a, two ceramic boats loaded with molybdenum trioxide powder and antimony powder were placed at the high-temperature zone of the furnace, respectively; and another boat loaded with sulfur was placed at the inlet of the tube. The SiO 2 /Si (the thickness of SiO 2 is 300 nm) wafer was selected as the substrate, which was placed on the top of the ceramic boat loaded with MoO 3 powder. Before heating, high-purity Ar carrier gas was first introduced into the tube flow at 150 sccm for 30 min to eliminate air in growth system, then the flow of Ar gas decrease to 5 sccm. The furnace was then heated from room temperature to 955 K at a rate of 36 K/min, and the temperature was kept at 955 K for 1 min. After 1 min of growth at 955 K, open the upper cover of the furnace to let it naturally cooled to room temperature. The detailed changes of temperature at different stages for growth process were summarized in Supplementary Figure S1b. The optical micrograph (OM) and atomic force microscopy (AFM) images of an as-synthesized representative MoS 2 sample fabricated with a growth time of 1 min show in Supplementary Figure S2.

Figure 1a is an OM of an as-synthesized representative Sb-doped MoS 2 sample fabricated with a growth time of 1 min, which shows that the crystals are triangle-shaped nanosheets with the edge lengths of 30–60 μm. The achieved triangle-shaped nanosheets have a smooth surface, regular structure, and homogeneous contrast, as shown in Fig. 1b (a magnified OM of a typical Sb-doped MoS 2 nanosheet). Meanwhile, a typical AFM image also exhibits homogeneous contrast, indicating Sb-doped MoS 2 nanosheet has a highly smooth surface. The height profile (Fig. 1c) shows the thickness of the typical nanosheet is about 0.79 nm, which corresponds to monolayer MoS 2 . In the growth process, we found that the growth time directly affected the thickness and size of the final achieved crystals. Supplementary Figure S3a-c show typical OM of three typical samples after growing 30 s, 2 min, and 5 min, respectively. It is noting that their lateral size increases with growth time. After growth 5 min, the proportion of multilayer crystals increased significantly. The increase of the lateral size of crystals is mainly due to the dangling bonds exist on the edge of the crystals. The increase of the thickness of the as-prepared crystals with prolonging the growth time is due to the nuclei easier formed on the surface of crystals. Then the nuclei grow up to form new crystals, resulting in the forming of multilayer-structured crystals. In addition, we found the carrier gas flow is also an important parameter in controlling the morphology of the final crystals. Under the small carrier gas flow (<5 sccm), the vapor pressure of sulfur can’t be effectively formed in the substrate area. That is to say, there is not enough sulfur vapor to move to the substrate area in 1 min under a small carrier gas flow. It results in the formation of rhombic MoOS crystals (see Supplementary Figure S4a). However, when the flow carrier gas is larger than 40 sccm, some irregular crystals formed on the substrate (see Supplementary Figure S4b). This is mainly due to the sulfur molecules have a large movement rate and no effective residence time on the substrate zone.

Fig. 1 Growth and characterization of prepared crystals. a Optical image of monolayer Sb-doped MoS 2 crystals at low magnification. Scale bar = 40 μm. b Magnified optical image of a Sb-doped MoS 2 crystal. Scale bar = 20 μm. c A typical AFM image of a Sb-doped MoS 2 crystal, the thickness is 0.79 nm. d STEM-ADF image of a typical monolayer Sb-doped MoS 2 nanosheet, the Sb atoms have been marked. Scale bar = 2 nm e Intensity profile of the selected line (bright yellow) in d. f Magnified image of the small area in d. Scale bar = 0.5 nm g The corresponding atomic model of d. h The SAED pattern of this typical monolayer Sb-doped MoS 2 nanosheet. Scale bar = 10 1/nm i STEM-ADF image of a typical monolayer MoS 2 . Scale bar = 2 nm Full size image

The crystal structure, crystal quality, and chemical composition of the prepared Sb-doped MoS 2 crystals were investigated by AC-STEM, transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected-area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDS). Supplementary Figure S5a shows the morphology of a typical triangle Sb-doped MoS 2 crystal, and all corresponding studies in this section based on this crystal. Atomic-resolution Z-contrast imaging was carried out to clarify the arrangement of the doped Sb in the MoS 2 crystal. An as-recorded scanning transmission electron microscope annular dark field (STEM-ADF) image is shown in Fig. 1d. The corresponding atomic model is shown in Fig. 1g. Due to the difference of atomic number in Mo, Sb, and S, the atoms show the different Z-contrast in STEM–ADF image. From STEM–ADF image, some different brightness spots (which depends on the Z-contrast, the brighter spot represents the atom has a larger number) can be clearly distinguished. These all spots with different intensities correspond to Sb, Mo, and S atoms, respectively, which can be further confirmed by the experimental intensity profile (Fig. 1e). All atoms are marked in Fig. 1d. The HRTEM of this nanosheet is shown in Supplementary Figure S6. The lattice spacing along different directions are 2.82 and 1.61 Å, which are corresponding to the {100} and {110} planes of 2H-MoS 2 , respectively. The crystallinity and crystallographic orientation of Sb-doped MoS 2 crystal are further confirmed by the SAED patterns. As shown in Fig. 1h, the SAED pattern taken from [001] zone axis shows a single set of diffraction spots with six-fold symmetry, which confirm a high quality and hexagonal structured single crystal. Meanwhile, the STEM-ADF image (Fig. 1i) of purity monolayer MoS 2 which synthesized under the same conditions with the Sb doped MoS 2 , show that the MoS 2 has good crystal quality. Furthermore, according to Figs. 1d, i, the doping of Sb atoms in MoS 2 does not make significant changes in lattice spacing of STEM-ADF patterns. The raw EDS spectrum collected from the nanosheet marked a white rectangle in Supplementary Figure S5a is shown in Supplementary Figure S7. It can be seen that the crystal is composed of Mo, Sb, and S (the detected C and Cu elements originate from the copper grid), and the stoichiometric ratio of Mo, Sb and S is about 0.91:0.09:2, indicating the composition of the nanosheet crystal as Mo 0.91 Sb 0.09 S 2 . The EDS mapping was performed to characterize the distribution of Mo, Sb, and S in nanosheet. Supplementary Figure S5b-d shows the corresponding elemental mapping for S, Sb, and Mo in a selected region of a typical crystal which marked with a white rectangle in Supplementary Figure S5a. It is shown that the S, Sb, and Mo elements are homogeneously distributed across the whole collected region; indicating that the Sb atom can be uniformly doped into the crystal. Supplementary Figure S8 is the X-ray photoelectron spectroscopy (XPS) result of the as-prepared sample. It can be seen that the Sb 3d core-level binding energy peaks occur in the prepared sample. The Sb 3d 5/2 and Sb 3d 3/2 signatures are located at 530.5 and 539.8 eV, which are similar to the energy peaks of Sb3+.38 The TEM and XPS results provide clear evidences that Sb atoms are doped into MoS 2 lattice.

Optical properties of monolayer Sb-doped MoS 2 nanosheets

The successful doping of Sb atom into the MoS 2 system was further confirmed by Raman spectroscopy and photoluminescence (PL) technology. Previous studies have proved that Raman spectroscopy technology is a powerful tool in determining the doping of TMDCs.25,26,27,28,30,31,32,35 Fig. 2a is the optical image of as-prepared monolayer Sb-doped MoS 2 nanosheet on the SiO 2 /Si substrate. Figure 2b shows the corresponding Raman spectrum. Three Raman peaks located at 148, 383, and 401.7 cm−1 are detected. Among them, 383 and 401.7 cm–1 are assigned to in-plane \({\mathrm{E}}_{2{\mathrm{g}}}^1\) mode and out-of-plane A 1g mode of MoS 2 . However, these two vibration modes show slightly shifts when compared with the purity monolayer MoS 2 nanosheet.3 The Raman spectroscopy of purity monolayer MoS 2 nanosheet is also given in Fig. 2b, two typical vibration modes located at 383.6 and 401 cm−1, respectively. The shift of these two Raman peaks is mainly due to the doping of Sb atoms affect the original lattice vibration states. In addition, the peak located at 148 cm−1 may be related to Sb x S y .39

Fig. 2 Raman and photoluminescence properties. a Optical image of the typical Sb-doped MoS 2 nanosheets collected from Raman spectra equipment CCD detector. Scale bar = 20 μm. b, and c Raman, and photoluminescence spectra of monolayer Sb-doped MoS 2 and MoS 2 crystals. d-f Raman peak intensity mappings at 148, 383, and 401.7 cm−1, which collected from the marked square area in a. Scale bars = 10 μm Full size image

Figures 2d-f show the Raman peak intensity mapping images of the peak centered at 148 cm−1 (the mode related to Sb x S y ), 383 cm−1 (\({\mathrm{E}}_{2{\mathrm{g}}}^1\) mode of MoS 2 ), and 401.7 cm−1 (A 1g mode of MoS 2 ), respectively. It can be seen that three modes have the strong intensity and uniform color contrast. Therefore, the prepared nanosheet is a uniform Sb-doped MoS 2 crystal. Figure 2c is the PL results of monolayer Sb-doped MoS 2 nanosheet and purity monolayer MoS 2 nanosheet. Only A exciton is observed in PL results. The typical PL emission of MoS 2 located at 666 nm, which corresponding to the A exciton. When Sb atoms doped into MoS 2 to form Mo 0.91 Sb 0.09 S 2 crystals, the A exciton peak is red-shift from 666 to 687 nm. Meanwhile, the A exciton intensity becomes weaker and full width at half maximum becomes larger, respectively. The changes of A exciton are mainly due to the effect of Sb atoms to electronic band structure. The observed red-shift in A exciton for the Sb-doped MoS 2 samples mainly origins from the decrease of electronic bandgap. When the foreign Sb doped in the MoS 2 , the impurity energy levels will form in the electronic band. As a result, it decreases the electronic bandgap of MoS 2 . So, the combination of excited electron-hole pairs will produce photons with smaller energy. More detailed discussion has been given in the theory section.

The characterization of A and B excitons by reflection magnetic circular dichroism (RMCD) spectroscopy

Studies have shown that the RMCD spectroscopy is an effective tool to investigate the electronic band structure and magnetic properties of crystals.40,41,42 More recently, the A and B excitons of MoS 2 crystals are studied in detail by the RMCD.42 The effect of Sb doping on MoS 2 were further confirmed by the RMCD results. The strong spin-orbit coupling in MoS 2 causes the spin degeneracy of electronic structures. The maximum splitting for the valence band appears at the K and K’ points; while, at these points, the splitting of conduction band is very small (Fig. 3b). As a result, we will observe A and B excitons in PL, optical absorption spectrum, and optical reflection spectrum by some specific measurements.43 As shown in Fig. 3a, the room-temperature RMCD spectrum from monolayer MoS 2 and Sb-doped MoS 2 single crystals display a similar shape, with A and B excitons observed in both crystals. For monolayer Sb-doped MoS 2 , compared with MoS 2 , the A exciton red shift about 21 nm and the B exciton red shift about 12 nm, respectively. The room-temperature RMCD results are very similar with the PL results.

Fig. 3 The characterization of A and B excitons. a Reflection magnetic circular dichroism (RMCD) spectra of monolayer Sb-doped MoS 2 and pristine MoS 2 crystals. The RMCD were performed at room temperature and the external field is −3T. b Schematic view of the splitting of electronic band structure at the K and K’ points under the different applied magnetic fields (\(B\parallel \pm \hat z\)) due to the strong spin-orbit coupling. c Temperature-dependent RMCD spectra under −3T magnetic field, and d low-temperature (70 K) RMCD spectra under different magnetic fields of monolayer Sb-doped MoS 2 Full size image