## 3 Results  
### Characterization  
#### 3.1.1 NbSe\({}_{2}\) Material Characterization  
The NbSe\({}_{2}\) flake was characterized by Raman spectroscopy at room temperature under a 0.4-mW laser power and 532-nm wavelength excitation laser (XperRam200, Nano base) (Figure 1d). The spatially resolved Raman mapping images are shown according to A\({}_{\text{1g}}\) and E\({}_{\text{2g}}\) modes, as shown in Figure 1d and Figure S2, respectively. As a result, the intensity of both peaks (A\({}_{\text{1g}}\) and E\({}_{\text{2g}}\)) was distributed uniformly. It suggests that we successfully grew high uniformity of the NbSe\({}_{2}\) flake using the CVD method. In the middle part of the NbSe\({}_{2}\) flake, there was a damage point, which was attributed to the laser focus during the measurement process. In Figure 1e, it is clearly shown that the Raman peak of our NbSe\({}_{2}\) flake is located at about 225 cm\({}^{-1}\) and about 250 cm\({}^{-1}\) of NbSe\({}_{2}\), which are assigned to the out-of-plane mode A\({}_{\text{1g}}\) peak and the in-plane mode E\({}_{\text{2g}}\) peak in NbSe\({}_{2}\) flake [27]. To further optimize our CVD-grown NbSe\({}_{2}\) flake, we conducted experiments to meticulously explore possibly contributing to the NbSe\({}_{2}\) thickness as a function of Se source distance (d\({}_{\text{Se}}\)) from the center of the quartz. Figure 2 shows the NbSe\({}_{2}\) flake grown with different distances between the Se solid source and center of the quartz, ranging from 17 to 18.5 cm while fixing all other parameters. It is noted that the one-zone CVD chamber was used for synthesis. The temperature set for the growth (800 \({}^{\circ}\)C for this experiment) can be kept well at the center of the quartz tube but gradually declines as it goes toward the edge of the quartz tube. Thus, controlling the loading position of Se (d\({}_{\text{Se}}\)) in the quartz tube can be interpreted as controlling the heating temperature of the Se source. Base on that, the NbSe\({}_{2}\) flake thickness concerning the d\({}_{\text{Se}}\) for 17-18.5 cm was separately measured in Figure 2a-h. At a long d\({}_{\text{Se}}\) (18.5 cm), the NbSe\({}_{2}\) flake showed a thickness of 2.4 nm as confirmed by atomic force microscopy (AFM) height profile function (Figure 2a,b). With decreasing the d\({}_{\text{Se}}\) to 18 cm, the NbSe\({}_{2}\) flake thickness was increased to 4.8 nm (Figure 2c,d). Furthermore, with decreasing the d\({}_{\text{Se}}\) to 17.5, 17 cm, the thickness of the NbSe\({}_{2}\) flake increased up to 7.2 and 12 nm, respectively (Figure 2e-h). These tendencies can be explained as follows. The different heating temperatures of Se induce different flow rates of the selenium vapor by affecting the vaporizing speed of Se and consequently affect the selenization amount of Nb\({}_{2}\)O\({}_{5}\)[27]. The selenization amount during growth can heavily determine the thickness of the NbSe\({}_{2}\), because the NbSe\({}_{2}\) form a stacking layer over the grown flakes rather than expanding towards the SiO\({}_{2}\) surface, which can form a larger flake. Note that the SiO\({}_{2}\) surface is not flat and has many dangling bonds, which result in high energy barrier energy and low diffusion NbSe\({}_{0,1,2}\) radicals [28]. Thus, NbSe\({}_{2}\) growth at the SiO\({}_{2}\) surface is remarkably limited and, therefore, the staking of NbSe\({}_{2}\) is perpendicularly formed. Figure 2i summarizes the thickness of NbSe\({}_{2}\) flake corresponding to the d\({}_{\text{Se}}\) (for 17-18.5 cm). The error bar indicates \(\pm\)10 \({}^{\circ}\)C and \(\pm\)2 nm for the \(x\) and \(y\)-axis, respectively. From the data, with increasing the d\({}_{\text{Se}}\), the thickness of NbSe\({}_{2}\) flake decreased due to the selenization process as described above.