Chemistry Reference
In-Depth Information
effect,
5
spin Hall effect,
6
etc. When an infinite graphene sheet is cut into
nano-sized graphene (nanographene) sheets or semi-infinite graphene
sheets, another important issue arises related to the creation of graphene
edges, as evidenced by early theoretical
7-10
and experimental works,
12-15
which have been performed several years before the discovery of
graphene.
Depending on the direction of the cutting line of a graphene sheet,
there can be two types of edges created; that is, armchair and zigzag
edges.
15
What is important here is that the electronic structure of
nanographene depends crucially on the geometrical shape of the edges.
Indeed, according to the theoretical and experimental studies,
7-15
a
nonbonding Π-electron state called “edge state” is created along the
zigzag edges, whereas no such state is present in the armchair edges.
In connection to the presence/absence of edge state, we should remind
the issue on the aromaticity in condensed polycyclic hydrocarbon
molecules.
16-20
The nonbonding Π-electron state is created also in
these molecules with the geometrical dependence same to that in
nanographene and graphene edges. The edge state well localized in the
vicinity of the zigzag edge is therefore a kind of surface state and plays
an important role in giving electronic features that are specific to
nanographene sheet. In addition, the edge state, which is singly occupied
and strongly spin polarized, contributes to create strong spin magnetism.
Interestingly, the localized spins of the edge state are ferromagnetically
arranged through a strong ferromagnetic intra-zigzag-edge exchange
interaction (~10
3
K).
21
The circumference of a nanographene sheet can be
described in terms of a combination of zigzag and armchair edges.
Therefore, depending on the shape of a nanographene sheet, the interplay
between intra-zigzag-edge ferromagnetic interaction and inter-zigzag-
edge ferromagnetic/antiferromagnetic exchange interaction is expected to
gives rise to a variety of magnetic structures, taking into account that the
magnetic feature of edge state is sensitive to the detailed edge geometry
and chemical modifications of edges.
In this paper, we present the electronic structure of the edge state and
the magnetic properties of a disordered network of nanographite domains,
each of which consists of a stack of 3-4 nanographene sheets.
