  Quasi free standing monolayer graphene (QFMLG) grown on SiC by selective Si
evaporation from the Si-rich SiC(0001) face and H intercalation displays
irregularities in STM and AFM analysis, appearing as localized features, which
we previously identified as vacancies in the H layer coverage [Y Murata, et al.
Nano Res, in press, DOI: 10.1007/s12274-017-1697-x]. The size, shape,
brightness, location, and concentration of these features, however, are
variable, depending on the hydrogenation conditions. In order to shed light on
the nature of these features, in this work we perform a systematic Density
Functional Theory study on the structural and electronic properties of QFMLG
with defects in the H coverage arranged in different configurations including
up to 13 vacant H atoms, and show that these generate localized electronic
states with specific electronic structure. Based on the comparison of simulated
and measured STM images we are able to associate different vacancies of large
size (7-13 missing H) to the different observed features. The presence of large
vacancies is in agreement with the tendency of single H vacancies to aggregate,
as demonstrated here by DFT results. This gives some hints into the
hydrogenation process. Our work unravels the structural diversity of defects of
H coverage in QFMLG and provides operative ways to interpret the variety in the
STM images. The energy of the localized states generated by these vacancies is
tunable by means of their size and shape, suggesting applications in nano- and
opto-electronics.
