# Optical and Transport Properties of Ni-MoS\({}_{2}\)
## 3 Results and Discussion
\[E_{i}^{ex}(T)=E_{i}^{ex}(0)-S\left\langle\hbar\Omega\right\rangle\left[\coth \frac{\left\langle\hbar\Omega\right\rangle}{2 KT}-1\right] \tag{2}\]  
Figure 3: Temperature-dependent reflection spectra of (**a**) MoS\({}_{2}\) and (**b**) Ni-MoS\({}_{2}\) samples. where \(E_{i}{}^{ex}(0)\) is the band gap at 0 K, S is a dimensionless coupling constant, and \(\hbar\Omega\) is an average phonon energy. By fitting the [PERSON] equation (red line) and [PERSON] empirical expression (blue line), the temperature dependence of the A and B transitions could be obtained. The fitted parameters are listed in Table 1. The trend and fitted parameters shown here have a reasonable consistency with similar results of reflection of the MoS\({}_{2}\) as proposed by [PERSON] et al. [30]. Therefore, the temperature shift of excitonic-transition energies could be attributed to both the lattice variations and interactions with relevant acoustic and optical phonons. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Material** & **Feature** & **E\({}_{i}^{ex}\) (0) (eV)** & **A (meV/K)** & **B (K)** & **S** & **\textless{}fh\textgreater{} (meV)** \\ \hline \multirow{2}{*}{MoS\({}_{2}\)} & A & 1.940 \(\pm\) 0.005 & 0.353 \(\pm\) 0.05 & 125.2 \(\pm\) 100 & 2.033 \(\pm\) 0.1 & 13.1 \(\pm\) 3 \\  & B & 2.130 \(\pm\) 0.005 & 0.355 \(\pm\) 0.05 & 252.2 \(\pm\) 100 & 2.007 \(\pm\) 0.1 & 28.5 \(\pm\) 3 \\ \hline \multirow{2}{*}{Ni-MoS\({}_{2}\)} & A & 1.918 \(\pm\) 0.005 & 0.582 \(\pm\) 0.05 & 399.7 \(\pm\) 100 & 2.271 \(\pm\) 0.05 & 20.38 \(\pm\) 3 \\  & B & 2.118 \(\pm\) 0.005 & 0.517 \(\pm\) 0.05 & 236.2 \(\pm\) 100 & 2.215 \(\pm\) 0.05 & 14.67 \(\pm\) 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Values of fitting parameters of the Varshni equation and the expression proposed by [PERSON] and [PERSON] [29], which describe the temperature dependence of the transition energies of MoS\({}_{2}\) and Ni-MoS\({}_{2}\). Figure 4: Temperature-dependent excitonic transition energies of (**a**) MoS\({}_{2}\) and (**b**) Ni-MoS\({}_{2}\) samples. Room-temperature Hall effect measurements were performed for MoS\({}_{2}\) and Ni-MoS\({}_{2}\). The relevant results are shown in Table 2. The Hall effect measurement results indicate that the carrier types of the MoS\({}_{2}\) and Ni-MoS\({}_{2}\) samples are \(n\)-type. The mobilities of the MoS\({}_{2}\) and Ni-MoS\({}_{2}\) samples are 78.51 and 1.87 cm\({}^{2}\)V\({}^{-1}\)s\({}^{-1}\), respectively. The resistance was increased about 40 times when Ni was doped into MoS\({}_{2}\). Although the Hall effect measurements also indicate that the Ni-MoS\({}_{2}\) possesses a similar carrier density of \(\sim\)1.1 \(\times\) 10\({}^{13}\) cm\({}^{-2}\) as MoS\({}_{2}\) does, Ni-MoS\({}_{2}\) still has a lower mobility and higher resistance than the undoped MoS\({}_{2}\) has. These results confirm that Ni atoms could not contribute additional electrons or electron holes in MoS\({}_{2}\). On the contrary, these Ni atoms seem to form defects/traps to obstruct electron flows. Meanwhile, Ni dopants produced a band tailing effect due to slight crystal distortion, resulting in the A and B peaks for Ni-MoS\({}_{2}\) being slightly lower than those for MoS\({}_{2}\). We implemented PC measurements using a DC bias of 1 V, which records the alternating current induced by the ON/OFF illumination controlled by a mechanical chopper at a frequency of 7 Hz at different photo energies. Meanwhile, transmittance was also carried out at 20 K for comparison. The low temperature PC and transmittance spectrum measured at 20 K are shown in Figure 5. PC results indicate that the main peaks A and B were also observed for both MoS\({}_{2}\) and Ni-MoS\({}_{2}\) samples, which is consistent with previous R measurements. In additions, there is an extra signal that occurs at 1.2 eV, indicated by D. However, no obvious peak could be found at 1.2 eV for the MoS\({}_{2}\).