The oxygen plasma treatment was done in a plasma tool (FEMTO DIENER PLASMA), with a stimulating plasma frequency of 2.45 GHz and maximum output power of 100 W. The chamber was evacuated and then filled with O2, until a pressure 0.6 mbar was reached. The plasma was ignited with the plasma generator set to 10% of its maximum power output (i.e., 10 W). The plasma process duration was from 12 to 36 secs.