# Optical and Transport Properties of Ni-MoS\({}_{2}\)
## 3 Results and Discussion
For the Ni-MoS\({}_{2}\) sample, this peak D could be attributed to Ni dopants that induce an additional Ni doping level, providing a different transition path of carriers. In addition, the whole PC spectrum of Ni-MoS\({}_{2}\) is wider than that of undoped MoS\({}_{2}\), which leads to ambiguous A and B positions in the PC spectrum of Ni-MoS\({}_{2}\). Since Ni dopants not only introduce a doping level but also some impurities or defects that cause additional deep levels in the forbidden band, these multi-levels probably result in an obvious and wide width in the PC spectrum of Ni-MoS\({}_{2}\). As for the transmittance results, we carried out transmission experiments for samples after exfoliating them several times. The normalized transmission results for MoS\({}_{2}\) and Ni-MoS\({}_{2}\) have been also included in Figure 5. The ratio of normalized transmission gradually decreased from 1.3 to 1.9 eV due to absorption. However, a low transmission still could be found above 1.9 eV, which could be attributed to some tiny cracks. These cracks were probably produced during the exfoliating process and led to samples that could not absorb the whole photoenergy. In contrast, there is no doping state in the PC spectrum except for both clear A and B excitons, which is consistent with the reflectance results. Since these electrons in the doping state transit to the conduction band, it is worth noting that the photocurrent has been enhanced up to several tens of times and the response wavelength has also been extended when Ni is doped in MoS\({}_{2}\). This result indicates that the doping is a good way to improve their applications in light responsibility for solar cells and photo detectors. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **MoS\({}_{2}\)** & **Ni-MoS\({}_{2}\)** \\ \hline Type & \(n\) & \(n\) \\ Resistance (\(\Omega\)) & 6733 & 283188 \\ Carrier density (cm\({}^{-2}\)) & 1.102 \(\times\) 10\({}^{13}\) & 1.183 \(\times\) 10\({}^{13}\) \\ Mobility(cm\({}^{2}\)/V\(\cdot\)s) & 78.5089 & 1.86593 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The results of room-temperature Hall effect measurements for MoS\({}_{2}\) and Ni-MoS\({}_{2}\). In order to understand the temperature dependence and the activation energy of the Ni-induced defect in MoS\({}_{2}\), we carried out temperature-dependent I-V measurements and analyzed the temperature-dependent conductivity of MoS\({}_{2}\) and Ni-MoS\({}_{2}\) for the calculation of activation energy. The samples were illuminated with a monochromatic light of 1.2 eV filtered from a xenon lamp. A typical semiconducting transport behavior is observed for both samples from the conductivity (\(\sigma\))-T plot shown in Figure 6. The \(\sigma\) decreases gradually while lowering the temperature from 300 to 20 K. The inset in Figure 6 shows the Arrhenius plot of both samples. It is possible to obtain information about the activation energy by analyzing the experimental data on the basis of the following relation (3):  
\[\sigma\,=\,\sigma_{0}\times\exp(-E_{a}/\text{kT}) \tag{3}\]  
where \(\sigma_{0}\) is the preexponential factor, \(E_{a}\) is the activation energy and k is the Boltzmann constant. For MoS\({}_{2}\), two different slopes can roughly be differentiated and the corresponding values of activation energy \(E_{a}\) are calculated as \(E_{a}\)1 = 84 meV and \(E_{a}\)2 = 6.7 meV, respectively. On the other hand, activation energy \(E_{a}\) of Ni-MoS\({}_{2}\) is 57 meV for \(E_{a}\)1 and 6.5 meV for \(E_{a}\)2. These results indicate the presence of shallower donors in Ni-MoS\({}_{2}\). In principle, smaller \(E_{a}\) values in Ni-MoS\({}_{2}\) could be attributed to Ni-induced donor defects or related impurities. Figure 5: **Photoconductivity** (PC) and normalized transmission results of MoS\({}_{2}\) and Ni-MoS\({}_{2}\) samples. A peak at 1.2 eV in the PC spectrum was observed only for Ni-MoS\({}_{2}\). Figure 6: The calculated conductivity versus temperature and corresponding Arrhenius plot (the inset) of the MoS\({}_{2}\) and Ni-MoS\({}_{2}\).