# Synthesis and Characterization of Highly Crystalline Vertically Aligned WSe\({}_{2}\) Nanosheets
## 3 Results and Discussion  
The sample morphology, resulting from the selenization of W film deposited on the sapphire substrate, was examined by SEM (Figure 1). The observations show that the sample is composed of well-distributed platelets over the entire substrate surfaces. These platelets are nanosheets with a well-defined shape exhibiting sharp edges. Two main contrasts, clearer or darker, can be observed in the lower magnification image (Figure 1a), and can be related to the two orientations of nanosheets to the substrate, perpendicular or parallel, respectively. The higher magnification image (Figure 1b, enlargement of a clearer contrast area) indicates that the WSe\({}_{2}\) nanosheets with a thickness between 40-50 nm are preferably grown vertically (perpendicularly to the substrate). The morphology of the nanosheets was investigated using TEM. The nanosheets were removed from the substrate by scratching, and were collected on a holey-carbon copper grid. The TEM analyses confirm the SEM observations above. A TEM image of a nanosheet with a large edge is shown in Figure 2a, which exhibits dark fringes related to the (002) planes with an interlayer spacing of 0.65 nm, valued according to the theoretical spacing of 0.648 nm. The sample was also characterized using selected-area electron diffraction. The shape of the electron diffraction pattern, given in Figure 2b, is explained by the selected area, which includes many nanosheets, without preferential orientation as a result of the TEM grid preparation. The diffraction spots are localized on the rings with the d-spacing characteristic of the WSe\({}_{2}\) material. Indeed, the two rings can be indexed by the (001) and (008) planes, with a d-spacing of 0.284 and 0.162 nm, respectively. The XRD pattern of the WSe\({}_{2}\) sample prepared on sapphire (Figure 3) reveals the presence of very intense peaks at 13.6\({}^{\circ}\), 41.6\({}^{\circ}\), and 56.7\({}^{\circ}\), in agreement with the (002), (006), and (008) diffraction planes of the hexagonal WSe\({}_{2}\), respectively (space group P6\({}_{3}\)/mmc, Joint Committee on Power Diffractions Standards (JCPDS) card (no. 38-1388)). The strong peak intensities with a narrow full width at half  
Figure 1: SEM images of tungsten diselenide (WSe\({}_{2}\)) with (**a**) low and (**b**) high magnification (\(\times\)2500 and \(\times\)60,000, respectively). maximum (FWHM) indicate that the sample is highly crystalline (the crystallite size was estimated to be higher than 400 nm using the Debye-Scherrer equation, namely, out of the range where this equation remains valid), and that the nanosheets with van der Waals planes perpendicular to the substrate surface (with a C-axis parallel to the surface) are present [14; 29], corroborating the observations made by SEM. Raman spectroscopy and photoluminescence (PL) are important tools to reveal the optical properties of TMD materials, as well as the number of layers composing the samples [30]. On a typical Raman spectrum of WSe\({}_{2}\), recorded with an argon laser excited at a 532 nm wavelength (Figure 4), two unresolved peaks at 251 and 257.6 cm\({}^{-1}\) are well-visible [30]. The observed peak at 251 cm\({}^{-1}\) corresponds to the E\({}_{\text{2g}}\)[1] mode (in-plane vibrational mode). The shoulder peak observed at 257.6 cm\({}^{-1}\), associated with the out-of-plane mode, is the A\({}_{\text{1g}}\) mode. This means that a substantial vibration along the vertical layer direction exists (bond vibration between W and Se) [30]. The presence of a Raman peak at 306 cm\({}^{-1}\) (B\({}_{\text{2g}}\)[1] mode) has been reported to be related to the interlayer interaction [22; 24]. The Raman signature of the as-grown WSe\({}_{2}\) indicates that its structure is a multilayer 2H type. Figure 3: XRD pattern of WSe\({}_{2}\). Figure 2: TEM images of a (**a**) nanosheet edge and of the (**b**) Selected Area Electron Diffraction (SAED) pattern for WSe\({}_{2}\) nanosheets.