## 2 Materials and Methods
### XPS Characterization  
X-ray photoelectron spectroscopy (XPS) technique is widely used for evaluating the chemical state of the near surface elements revealed on a material, through the determination of the chemical shift occurring for peak positions in relation to the species surrounding the atoms under investigation. Generally, the analysis performed is only qualitative and in better cases semiquantitative, but in this way it does not express its powerful potentiality. The possibility of extracting quantitative information from surface materials, with an accuracy of \(\pm\)1%, gives XPS analysis great potential. Of course, the intention to perform quantitative analyses should be clear to the analysts before operating and setting the experimental acquisition parameters. Calibration procedures and deep knowledge of the apparatus ensure a high degree of reliability, repeatability, and reproducibility of data at any time. In the present work a quite recent PHI VersaProbell 5000 multi-equipped spectrometer was employed for XPS spectrum acquisitions. A monochromatic Al\(\alpha\) X-ray source with energy 1486.6 eV, power 12.5 W and 50 \(\upmu\)m lateral resolution was employed for the acquisitions of all data sets. The polychromatic photoelectrons coming from the samples' surface were collected using a hemispherical analyzer set in FAT mode, whose operating parameters were chosen to optimize the acquisition of high-resolution spectra. In some particular cases, depth profiling along the whole thickness of the specimens under investigation could prove a fruitful method for overcoming the limits of the resolution depth, typical of XPS. Therefore, the performances of a sputter monoatomic Ar+ ion gun and cluster Ar+\({}_{2500}\) ion gun with maximum tension voltage of 20 kV were compared before acquiring depth profiling. The monoatomic Ar+ ion gun was chosen for sputtering all samples, taking a film of Ta\({}_{2}\)O\({}_{3}\) as reference sample for calibrating sputtering time and thickness of eroded material according to working conditions. In the literature it is underlined that gas cluster sputtering guns are widely accepted for depth profiling of polymers samples, avoiding chemical damage or crosslinking but surprisingly in inorganic samples, and in particular on the commercial standard Ta\({}_{2}\)O\({}_{5}\) (BCR-261T), preliminary tests showed that (i) the lower penetration depth does not lead to improved depth resolution when Ta\({}_{2}\)O\({}_{5}\) is profiled, and (ii) even though cluster guns present a less preferential sputtering of the lighter elements in the first sputtering cycles, the use of 6 kV Ar+1000 ions onto Ta\({}_{2}\)O\({}_{5}\) showed a gradual decrease in the O concentration [15]. This progressive loss of oxygen atoms as a function of depth was further enhanced when the experimental conditions \begin{table}
\begin{tabular}{c c c c c c} \hline
**SAMPLE** & **Duty** & **Power** & **T(Pulse)** & **Thickness** & **Frequency** \\
**Label** & **Cycle** & **(kW)** & **μs** & **μm** & **(Hz)** \\ \hline ZrN A & 50-50 & 1.5 & 12.5 & 2.94 & 80 \\ ZrN B & 33-33 & 1.5 & 8.25 & 2.50 & 80 \\ ZrN C & 25-25 & 1.5 & 6.25 & 2.70 & 80 \\ ZrN D & 20-20 & 1.5 & 5.0 & 1.97 & 80 \\ ZrN E & 50-50 & 2.0 & 12.5 & 2.5 & 80 \\ ZrN F & 50-50 & 1.5 & 12.5 & 2.40 & 80 \\ ZrN G & 50-50 & 1.0 & 12.5 & 2.20 & 80 \\ ZrN H & 50-50 & 0.5 & 12.5 & 2.50 & 80 \\ ZrN I & 50-50 & 1.0 & 12.5 & 2.0 & 80 \\ ZrN L & 33-33 & 1.0 & 8.25 & 2.0 & 80 \\ ZrN M & 25-25 & 1.0 & 6.25 & 2.0 & 80 \\ ZrN N & 20-20 & 1.0 & 5.0 & 2.0 & 80 \\ \hline \end{tabular}
\end{table} 
Table 1: All the main deposition parameters are resumed for the ten samples deposited using BPDMS. changed (higher tension and/or introducing sample rotation). However, an exhaustive comment for the choice of the mono ion gun rather than the cluster gun, for sputtering purposes, will be object of the section entitled \"Discussions\".