To confirm that the overall mass evolution in Fig. , resulting in a net mass gain, is truly linked with deposition of Fe, a quartz crystal used for deposition at 10 W plasma power and +40 V QCM bias voltage was analyzed. The EDS mapping (Fig. ) shows that the film contains iron, albeit with a high oxygen content. Cross-section SEM analysis shows that close to an 850 nm film was deposited on the gold coated QCM crystal (Fig. ). The cross-section micrograph also reveals some porosity, indicating that +40 V bias and 10 W plasma power is still not an optimized condition for film deposition. High resolution X-ray photoelectron spectroscopy (XPS) of a time-resolved film deposited on the gold coated QCM crystal with 10 W of plasma power and +40 V of bias, before and after sputter cleaning the surface, is given in Fig. . The Fe 2p shows peaks at 709.9-712 eV, corresponding to Fe-O. Given that the samples were porous (Fig. ) and exposed to air for several days before being analyzed by XPS, and due to the very oxyphilic nature of iron, oxidation is expected. In addition, film deposition was done in medium vacuum conditions meaning that low levels of oxygen exposure are to be expected during deposition. Therefore, the films were sputter-cleaned in the XPS chamber. After 1200 s of sputtering, shoulder peaks at 707.3 and 720.4 eV (Δ13.1 eV), likely corresponding to zero valent Fe 2p3/2 and Fe 2p1/2, respectively can be seen. The composition analysis by XPS of sputtercleaned films deposited on the gold coated QCM crystal shows 38 at. % Fe, 26 at. % C, 32 at. % O, and 4 at. % N. The presence of the C impurities is most likely Cp ligands from the metal precursors remaining on the film surface being incorporated into the film. Nitrogen is found as an impurity, likely due to the relatively high base pressure (6 Pa) or air trapped in the ferrocene evaporation chamber. To investigate the reliability of the measured mass evolution, i.e., if the total shift of the resonant frequency is significantly altered by the introduction of an electron current, a quartz crystal with iron deposited at 10 W plasma power and +40 V QCM bias, was analyzed by ERDA and RBS to estimate the areal mass density. The result was compared to the areal mass density given by the change in resonant frequency from the QCM. ERDA combined with RBS analysis showed an areal number density of 3.5×10 18 atoms/cm 2 . Assuming a flat surface of the quartz crystal in combination with the film composition values obtained from the ERDA/RBS (areal number density and relative elemental composition), the areal mass density equates to 1.25×10 -4 g/cm 2 , whereas the frequency shift in the QCM read-out inserted into equation ( ), suggested a value of 1.23×10 -4 g/cm 2 . This comparison suggests that the QCM data can be trusted even with the added bias signal. It should be noted that the QCM mass is based on a perfectly smooth surface, while we have measured a surface roughness of 290 nm on an area of 50 µm × 50 µm by AFM.