Comment on 'Insight into hydrogenation of graphene: Effect of hydrogen plasma chemistry' [Appl. Phys. Lett. 105,183104 (2014)]

摘要

In a recent letter, Felten et al. reported a detailed study of monolayer graphene modification in H_2 low pressure plasmas. The originality of this work is that it attempts to correlate plasma properties (namely, energy distributions of positive ions) with the resulting modification of graphene samples. It is important to note that during plasma treatment, graphene samples were electrically floating and the authors claim that they measured energy distributions of ions arriving on a floating dialectic surface with the help of a dielectric end cap of the mass spectrometer. However, the authors seem to ignore the fact that the spectrometer itself is not electrically floating and hence the measured ion energy distributions are referenced to the laboratory ground and not to the floating potential of the end cap. The shift between the measured distributions and the distributions one would expect on a floating substrate is caused primarily by the dc offset of the plasma potential. Therefore, reported ion energy distributions do not represent ions interacting with floating graphene samples. Unfortunately, this issue cannot be simply resolved by shifting the ion energy scale by a constant value of the floating potential of the end cap because of the presence of a strong radiofrequency component of the plasma potential (as evidenced by the saddle-like shape of the measured ion energy distributions).