## 3 Results
Thus, the highest sensitivity for the graphene modified structure when the reflectivity level was not optimized was 330 deg/RIU, but now it is 325 deg/RIU. As in the first case, the same trend was observed: where to enhance the sensitivity, the metallic layer thickness should be decreased and to reduce the FWHM, the metallic layer thickness should be increased. Accordingly, for the MoS\({}_{2}\) structure, the refractive index of the top layer that maximizes sensitivity is 3.53, and, consequently, silicon is the the best candidate to accomplish the requirements of the hypothethical material. Furthermore, considering the value of the refractive index for which the structure's sensitivity is greatly enhanced, the fourth set of optimizations was run, where the metal thickness, the semiconductor thickness, and the number of 2D material monolayers were considered as parameters. BaTiO\({}_{3}\) was selected for the graphene and WS\({}_{2}\) configurations was chosen because its refractive index was closest to the optimal levels (2.66 for graphene and 2.64 for WS\({}_{2}\)) [47], while silicon for MoS\({}_{2}\) based structure. The reflectivity curves can \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**L.N.**} & \multirow{2}{*}{**Material**} & \multicolumn{2}{c}{**Configuration**} & \multirow{2}{*}{**Sensitivity (deg/RIU)**} & \multirow{2}{*}{**FWHM (deg)**} \\  & & & **Mat. (\(\mathbf{n_{\text{semiconductor}}}\))** & \\ \hline
1 & & 40 nm–11 nm–1 L (2.66) & 325 & 7.1 \\
2 & Graphene & 47 nm–8 nm–1 L (2.64) & 200 & 4.6 \\
3 & & 37 nm–2 L (1.87) & 212 & 7 \\
4 & & 40 nm–6 nm–1 L (3.53) & 256 & 9.3 \\
5 & MoS\({}_{2}\) & 42 nm–6 nm–1 L (2.98) & 200 & 7.4 \\
6 & & 34 nm–14 nm–1 L (2.13) & 254 & 9.2 \\
7 & & 41 nm–9 nm–1 L (2.64) & 314 & 7.9 \\
8 & WS\({}_{2}\) & 48 nm–7 nm–2 L (2.51) & 208 & 4.6 \\
9 & & 43 nm–5 nm–2 L (2.69) & 212 & 7 \\ \hline \end{tabular}
\end{table} 
Table 4: Configurations with the optimal refractive index, for which the sensitivity is enhanced, FHWM is narrowed and the reflectivity level at resonance is minimised. Figure 5: Reflectivity curves for the optimized semiconductor refractive index: the black line shows the sensitivity enhanced configurations, the red line shows the FWHM narrowing structures, the blue line shows the configurations with the minimum level of reflectivity at resonance minimized, the dashed line shows the sensor’s response for \(n+\Delta n\). be seen in Figure 6 and the performance determined for each configuration are presented in Table 5. The convergence plots of these layers and objectives can be found in Figure S7 and additional configurations together with the sensitivity and FWHM can be found in Figure S8 and Table S4 in the Supplementary Information. The refractive index of silicon was calculated using the following formula [48]:  
\[n_{SI}=A+A_{1}e^{-\frac{\lambda}{t_{1}}}+A_{2}e^{-\frac{\lambda}{t_{2}}} \tag{6}\]  
where \(A\) = 0.344904, \(A_{1}\) = 2271.88813, \(A_{2}\) = 3.39538, \(t_{1}\) = 0.058304, \(t_{2}\) = 0.30384 and \(\lambda\) is the wavelength. Because it was shown that polycristaline silicon with a thickness of 10 nm would not induce a strong absorption in a visible spectrum [49], the extinction coefficient was neglected. In this case, the minimum level of reflectivity strongly influences the sensitivity. Thus, if the graphene based structure sensitivity was 330 deg/RIU with a minimum level of reflectivity at 4%, in this case, the sensitivity is decreased to 302 deg/RIU, but the reflectivity level at resonance is smaller than 0.5%. Similarly, for the WS\({}_{2}\) based configuration, sensitivity decreased from 325 deg/RIU to 302 deg/RIU and for the MoS\({}_{2}\) based configuration it decreased from 255 deg/RIU to 232 deg/RIU. It is worth mentioning that the FWHM is not improved nor altered. A quantitative comparison of the recent literature with the results obtained in this work is presented in Table 6. Only the configurations based on the BK7 prism and silver as a plasmonic platform were compared.