# Synthesis and Characterization of Highly Crystalline Vertically Aligned WSe\({}_{2}\) Nanosheets
## 3 Results and Discussion
The PL spectrum of WSe\({}_{2}\) is shown in Figure 5. Because of the semiconductor character of the monolayer WSe\({}_{2}\), the PL spectrum exhibits a strong direct transition (DT) emission of around 760 nm, due to its direct band gap [31]. When increasing the number of layers of the WSe\({}_{2}\), indirect transitions (IT) emissions also appear at a lower energy. The peak of the IT does not appear in the monolayer PL spectrum. Figure 5 shows a main peak at an 860 nm wavelength, which corresponds to a red-shift from 760 to 860 nm, indicating the indirect nature of the transitions. Furthermore, this feature--the presence of IT emissions with important wavelength shifts--demonstrates a multilayer nature, according to the literature. The value of this shift can be considered as a layer number indicator [31]. Finally, the last optical tool used to characterize the WSe\({}_{2}\) sample was ultraviolet-visible spectroscopy. The absorption spectrum is shown in Figure 6. The excitonic absorption peaks of A and B are located at wavelengths of 785 and 651 nm, respectively. These appear because of the direct transitions gap of point K. The presence of excitonic transition, red-shifted from the monolayer transition [32], demonstrates the fewer-layer characteristic of the synthesized nanosheets, according to the PL results. As s result of the superpositions of the Se p-orbitals with W d-orbitals, as well as the  
Figure 4: Raman spectrum of WSe\({}_{2}\). Figure 5: Photoluminescence spectrum of WSe\({}_{2}\). adjacent layers, the WSe\({}_{2}\) spectrum shows other absorption peaks of A' and B'. The excitonic nature of these peaks could come from the splitting of the ground and the excited states of the two transitions of A and B, because the d-electron band is perturbed at the level of the inter- and intra-layer by the Se p-orbitals [32; 33]. All of the experimental data obtained with Raman spectroscopy, PL, and UV-VIS spectroscopy, allows for concluding that WSe\({}_{2}\) materials are composed of multilayer 2H-WSe\({}_{2}\) nanosheets. The WSe\({}_{2}\) nanosheets were investigated by X-ray photoelectron spectroscopy (XPS). The XPS spectra recorded on the W 4f and Se 3d binding energy regions are shown in Figure 7. Considering the W 4f spectrum (Figure 7a), the following three main peaks can be observed: two main contributions at about 32.5 and 34.6 eV are attributed to the doublets W 4f\({}_{7/2}\) and W 4f\({}_{5/2}\), respectively, and a wide and low intense contribution of around 37.8 eV is attributed to the W 5p\({}_{3/2}\). The main contribution can be fitted with two doublets centered at 31.9 and 32.5 eV on a W 4f\({}_{7/2}\) signal, attributed to the 1T' [34] or 1T [35] phase and 2H phase, respectively. The 2H contribution (centered at 32.5 eV) corresponds to 75.0% of the total area, indicating that the sample mainly has a 2H phase. Another additional smaller contribution is observed at 35.5 eV, due to the W 4f loss. The formation of two phases is further confirmed on the Se 3d core level spectra (Figure 7b). The spectrum shows the presence of two main peaks, Se 3d\({}_{5/2}\) and Se 3d\({}_{3/2}\) doublets, at around 54.8 and 55.6 eV, respectively. Each doublet can be fitted with two components--one at 54.8 eV, corresponding  
Figure 6: UV–VIS spectrum of WSe\({}_{2}\). Figure 7: XPS spectra of the (**a**) W 4f and (**b**) Se 3d of the WSe\({}_{2}\) nanosheets. to the 2H-WSe\({}_{2}\) phase, and one corresponding to the other phase (1T or 1T'), centered at 54.3 eV. Here, again, the 2H phase represents nearly 75% of the total area (Table 1), verifying the conclusions obtained about the W 4f region. Similar results have been reported in the literature regarding WSe\({}_{2}\) crystals [34; 35], with a major 2H phase and a minor other phase assigned to the 1T' [34] or 1T [35] phases, respectively.