The plasma CVD system used for the study has previously been described in detail and is here modified with the biased QCM system. The experimental set-up is schematically shown in Fig. . The electrical connection is based on the same principle as has been used in the case of the so-called gridless ion meter. The ground connection of the QCM oscillator was connected to the top electrode of the crystal through a 220 nF capacitor, which allows for the QCM high frequency single-ended signal to pass through and be measured while at the same time blocking the DC biasing voltage applied to the top electrode of the crystal from entering the QCM oscillator. Moreover, a 100 nF capacitor was connected in between bottom QCM electrode and the signal input of the QCM oscillator, in order to shield the QCM high frequency signal from any external interference. In this configuration, the top crystal electrode could be readily biased without any influence on the QCM operation. The QCM sensor was connected to the QCM oscillator by a coaxial cable through a BNC electrical feedthrough CF flange, which was electrically isolated from the grounded stainless-steel chamber by an isolating polytetrafluoroethylene (PTFE) vacuum break. The QCM sensor was also connected to a water-cooling/water-heating system, using electrically non-conductive PTFE hoses to electrically isolate the QCM sensor from the grounded stainless-steel chamber.