## 3 Results
The presence of such bands, indicative of an oxide phase, could be due to the presence of a tungsten phase originating from disproportion, as already discussed in relation to the XRD results. Metallic nanoparticles oxidize easily. Figure 8 presents a comparison of FTIR reflectance before and after the two annealing steps for all films. The R% decrease is markedly more pronounced in the 500 W and 300 W samples that in the 700 W one. FTIR reflectance in the higher-power film remained almost the same, while in both the other films it decreased after annealing. This took place because the content of W\({}_{2}\)N increases after annealing and the metallic amorphous tungsten nitride clusters crystallize, joining to the prevalent W\({}_{2}\)N phase lattice, which has a lower FTIR reflectance with respect to an amorphous phase without grain boundaries (which are able to scatter and reduce reflectance). In the high-power case, amorphous WN\({}_{x}\) crystallizes as W\({}_{2}\)N, which could diminish reflectance, but the concomitant formation of metallic tungsten, as also shown by the XRD spectra, balances this decrease by virtue of the higher FTIR reflectance of the metal, denoting an overall effect of reflectance invariance. All the films contain simple and multiple tungsten-nitrogen bonds. For a trigonal bi-pyramidal geometry around tungsten, nitrogen ions acting as a ligands can occupy equatorial or axial sites, so the W-N bonds show short (equatorial) and long (axial) distances. The intense bands from 850 cm\({}^{-1}\) to 1200 cm\({}^{-1}\) reflect the stretching of the W-N bond, centered at 1097 cm\({}^{-1}\) in the 300 W and 500 W films and 1086 cm\({}^{-1}\) in the 700 W film. According to [PERSON] and [PERSON] [22], the signal at 1097 cm\({}^{-1}\) can be attributed to the W-N bond stretching in the complex (N-N)\({}_{x}\)WN, where there is a triple bond W-N [23]. The characteristic signature of the W\({}_{2}\)N phase is stretching at 870 cm\({}^{-1}\), which was not perceived in our film joining in the aforementioned band. Of particular interest are the stretching frequencies of the bridging tungsten bonds evidenced in the inset of Figure 9b. All the films before annealing had a characteristic strong band around 600 cm\({}^{-1}\) attributable to N-W-N stretching in W complexes with hosted nitrogen molecules, which is less pronounced after annealing, testifying to the evolution toward different N-W-N geometries and types of bond. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Acoustic Band Raman Shift** & **Acoustic Band Raman Shift** \\  & **First Maximum Position (cm\({}^{-1}\))** & **Second Maximum Position (cm\({}^{-1}\))** \\ \hline
300 W & 154 & 199 \\
500 W & 157 & 206 \\
700 W & 161 & 201 \\  & 187 & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Acoustic band splitting of different samples after annealing. Figure 8: FTIR Reflectance before and after annealing for 300 W, 500 W and 700 W samples. Bands located at 647 cm\({}^{-1}\) and 693 cm\({}^{-1}\), attributed to the bridging nitrogen W-N-W [24], remained almost unchanged after annealing. In the 300 W and 500 W films at 1585 cm\({}^{-1}\), there was an N-H bending band. Moreover, at 1390 cm\({}^{-1}\), in all the films a scissoring vibration of N-H appeared. The three N-H stretching vibration bands, located at 3089 cm\({}^{-1}\), 3128 cm\({}^{-1}\) and 3253 cm\({}^{-1}\) in the 300 W, 500 W and 700 W films, respectively, did not change meaningfully after annealing. It is important to note that W hydrazinium complexes, of the kind W--N-NH\({}_{\rm x}\), can play a role in the reduction pathway of N\({}_{2}\) to form nitrides. FTIR analysis allows a general consideration of the tungsten nitridation process that takes place in plasma inside a sputtering chamber. Nitrogen reduction to form nitrides (breaking the strong N\(\equiv\)N and forming the azide ion [N=N=N]\({}^{-}\)), from the chemical point of view, requires extreme conditions.