90 5 One- and Two-Dimensional Nanoparticles
neighbors. Therefore, one double bond is necessary. In other words: three of the
four valences of carbon are saturated. The fourth electron remains unbound; it is
delocalized, which means it can move freely across the graphene sheets. As these
fourth electrons are mobile, graphite shows electric conductivity within the layers.
Perpendicular to the layers, graphite is an electrical insulator. The fourth unbound
electron is also usable for functionalization. In this context, an interesting example
is graphane, where each of the carbon atoms is associated with a hydrogen atom.
This is different for boron nitride. As in boron nitride all valences are saturated,
there are no free electrons. Boron nitride is an insulator. (As will be shown in
Chapter 10, it is, strictly speaking, a wide-gap semiconductor.) Figure 5.11 displays
one layer of the structure of these compounds together with the bindings.
Within the layers depicted in Figures 5.11a,b, there are covalent bonds; inbetween the layers are weak van der Waals bonds. This allows cleaving layers of
monocrystalline material by chemical or mechanical methods. In organic chemistry these hexagonal structures are well known in aromatic compounds, derivaFigure 5.11 Structure and bindings of graphene (a) and boron nitride (b). It is important to
realize that, in contrast to graphene, boron nitride has no double bonds.
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
B
B
B
B
B
B
N
N
N
N
N
N
(a)
(b)