# Optical and Transport Properties of Ni-MoS\({}_{2}\)
## 3 Results and Discussion  
The samples were formed with many stacked layers after growth. The thickness of the layered MoS\({}_{2}\) crystal is about 20 \(\upmu\)m. The three-dimensional crystal is formed by stacking the two-dimensional MoS\({}_{2}\) sheets. Since there is the interaction of a weak van der Waals force providing a natural cleavage plane of the crystal between two adjacent MoS\({}_{2}\) sheets [22], it would be easy to separate the samples into thin specimens with a smooth surface. The OM images of MoS\({}_{2}\) and Ni-MoS\({}_{2}\) are presented in Figure 2. According to previous literature, the optical micrograph image of the surface of 2H MoS\({}_{2}\) may show a hexagonal shape, while that of 3R MoS\({}_{2}\) may show a triangular shape [23; 24]. In our case, both images show the feature with hexagonal corners (\(\gamma\) = 120 degrees) on the surface, indicating that the MoS\({}_{2}\) samples are 2H single crystals. The smooth surfaces of MoS\({}_{2}\) are beneficial for performing the R measurements [25]. In order to understand exciton behavior in MoS\({}_{2}\) and Ni-MoS\({}_{2}\), temperature-dependent R spectra were carried out at a temperature range of 20 to 300 K for both MoS\({}_{2}\) and Ni-MoS\({}_{2}\) samples. The results of the R spectra of the MoS\({}_{2}\) and Ni-MoS\({}_{2}\) indicate two main resonance features existing in each spectrum for both samples, as Figure 3 shows. We analyzed these spectra using the below Equation (1) proposed by [PERSON] et al. [26]  
\[\mathrm{R}\left(\
u\right)=\mathrm{R}_{o}+R_{x}Re\left[\frac{h\
u_{x}-h_{o}+i \Gamma_{x}}{\Gamma_{x}^{2}+\left(h\
u-h\
u_{x}\right)^{2}}\right]e^{i\Theta} \tag{1}\]  
where \(\mathrm{R}_{o}\) is the background, \(R_{x}\) is the intensity, \(h\
u_{x}\) is the photon energy, \(\Gamma_{x}\) is the broadening parameter and \(\Theta\) is the phase. Afterward, two obvious peaks at 1.83 and 2.02 eV were obtained and denoted as A and B at 300 K, respectively. The energy difference between A and B excitonic transitions is 0.15 eV [23]. Furthermore, the values of features A and B are a little lower than those observed in the absorption  
Figure 1: Electron spectroscopy for chemical analysis (ESCA) results of (**a**) full spectrum and (**b**) high resolution in Ni region for Ni-MoS\({}_{2}\). Figure 2: (Optical microscope) OM images of (**a**) MoS\({}_{2}\) and (**b**) Ni-MoS\({}_{2}\) under 30\(\times\) magnification. peaks and photoluminescence peaks of atomically thin MoS\({}_{2}\) samples [9], which could explain these two features arising from direct bandgap transitions between the maxima of split valance bands (v1 and v2) and the minimum of the conduction band (c1). The valance band was split due to the combined effects including the interlayer and spin-orbit coupling [27]. Both features are gradually shifted to the slight highly 1.91 and 2.11 eV, respectively, when the temperature is ramped down to 20 K due to less thermal activation. In addition, both direct bandgap transitions A and B in the R spectra of the Ni-MoS\({}_{2}\) are very close to that of the MoS\({}_{2}\) under different temperatures. Therefore, this result could indirectly reflect that the MoS\({}_{2}\) crystal structure did not suffer from much distortion even though Ni atoms were doped. On the other hand, the interference feature could be found only in the R spectrum of Ni-MoS\({}_{2}\). This could be attributed to the poor surface roughness of the MoS\({}_{2}\) we picked up for measurement. Here we adopted the [PERSON] equation, E\({}_{\text{g}}\)(T) = E\({}_{\text{g}}\)(0) \(-\) (\(\alpha\)T\({}^{2}\)/(T + \(\beta\))), to describe the bandgap reduction with the temperature [28]. The constant \(\alpha\) is related to the electron-phonon and \(\beta\) is related to the Debye temperature. The spectral shift of the A and B features with the energy is depicted in Figure 4. Furthermore, another analysis method was also considered which was proposed by [PERSON] and [PERSON] [29]. The empirical expression is shown as below in Equation (2).