  We studied, by scanning tunneling microscopy, the morphology of nanopits of
monolayer depth created at graphite surfaces by hydrogen plasma etching under
various conditions such as H$_2$ pressure, temperature, etching time, and RF
power of the plasma generation. In addition to the known pressure-induced
transition of the nanopit morphology, we found a sharp temperature-induced
transition from many small rather round nanopits of ~150 nm size to few large
hexagonal ones of 300-600 nm within a narrow temperature range. The remote and
direct plasma modes switching mechanism, which was proposed to explain the
pressure-induced transition, is not directly applicable to this newly found
transition. Scanning tunneling spectroscopy (STS) measurements of edges of the
hexagonal nanopits fabricated at graphite surfaces by this method show clear
signatures of the peculiar electronic state localized at the zigzag edge (edge
state), i.e., a prominent peak near the Fermi energy accompanied by
suppressions on either side in the local density of states. These observations
indicate that the hexagonal nanopits consist of a high density of zigzag edges.
The STS data also revealed a domain structure of the edge state in which the
electronic state varies over a length scale of ~3 nm along the edge. The
present study will pave the way for microscopic understanding of the
anisotropic etching mechanism and of spin polarization in zigzag nanoribbons
which are promising key elements for future graphene nanoelectronics.
