mixtures such as Ar/CH4/H2/N2 [15,17,27], N2/CH4 [18,28–31] and
Ar/N2/CH4 [32]. In their work, N2 is supposed to effectively reduce
the crystal size and elongate the grain [15]. In this work, associated
with SEM results, the higher power and higher methane level
are indispensable for the formation of diamond nanostructures in
hydrogen and methane microwave plasmas in the absence of nitrogen.
The formation mechanism of diamond nanostructures is not
completely understood. One possible reason might be that, a higher
methane level in hydrogen plasmas produces more C2 species [33],
which promotes the growth of the dimensional diamond along certain
direction. Further work is currently under way in our group
through systematic TEM analysis.