Additionally, a co-flow of xenon (Xe) (99.998%) was introduced as a diffusion additive. Thin films were deposited on both CNT membranes, consisting of randomly arranged few-walled carbon nanotubes with various diameters supplied by Canatu, and polished Si (100) substrates for reference samples. The CNTs were in the form of a free-standing membrane stretched over a quartz frame giving a total area of about 25 x 25 mm. The CNT membranes were used as received. For reference experiments, 10 × 10 mm Si substrates were washed in an ultrasonic bath in acetone and ethanol for 3 minutes and then blow-dried with nitrogen gas.