FeCp2 powder was placed in a stainless-steel evaporation chamber, mounted on the deposition system, and purged with argon gas (35 sccm), while pumping on the deposition system for 1.5 h to remove the air from the bubbler. The FeCp2 was then sublimed at 75 °C and the vapor was transported to the deposition chamber using argon (3 sccm) as carrier gas. A DC bias voltage in the range of -40 V to +40 V was applied to the QCM sensor via a second DC voltage supply (Fig. ). A multimeter was connected to the DC voltage supply to measure the electron current drawn from the plasma. The surrounding area of the QCM crystal plate and the outer walls of the QCM sensor was masked with Kapton tape to ensure that the measured electron current flows only through the top plasma-facing electrode of the QCM crystal.