Chapter 8 Solid Materials
(a)
(b)
Figure 8.18 Carbon nanotube Space-filling model of tube is viewed down its axis (a) and along its side
(b)
.
the body. Buckyball is also a superconductor,
and efforts have been made to take
advantage of this property as well. However, no practical uses of buckyball have yet been developed.
A carbon nanotube
is a rolled-up sheet of graphite (Figure 8.18) that is only
nanometers in diameter but up to a centimeter long. Nanotubes are very strong, and, depending on how the graphite sheets are rolled
(straight across or at a slight diagonal),
they are conductors or semiconductors. Si
ngle nanotubes have been used to make
molecular wires, diodes, and
transistors, and groups of nanotubes have been integrated
into logic circuits, fundamental computer components. These devices are a hundred times smaller than those on present-day computer ch
ips. Nanotubes are a basic building block in
the new field of molecular electronics. Ind
eed, a new technology, one based on devices
that measure less than a 1000 nanometers, is currently being developed. This new technology is referred to as
nanotechnology
.
Diamond
(Figure 8.19) is constructed from sp
3 hybridized carbon atoms. The
resulting three-dimensional frame
work has a much lower pack
ing efficiency (34%) than
found for metals or ionic compounds. The strength of its C-C bonds combined with its rigid structure makes diamond the hardest subs
tance known. The electrons of the covalent
bonds are delocalized over the entire structure, wh
ich results in a comple
tely filled valence
band and an empty conduction
band. Although diamond is an
insulator because its band
gap is very large, the diamond structure type
is adopted by silicon a
nd germanium, two of
the most common semiconductors.
Figure 8.19 Diamond
While differences in the structures of covalent and ionic compounds are expected,
there are definite similarities in some structures. For example, the carbon atoms in a diamond unit cell adopt the lattice positions of
a face-centered cubic cell and an additional
four positions of tetrahedral coordination that
lie totally within the cell. Remember that
there are four atoms in the face-centered cubic
unit cell, and each atom totally within the
unit cell contributes another one full atom to
the cell stoichiometry. Consequently, there
are a total of eight carbon atoms in the diamond unit cell. This unit cell type is the basis of several important covalent solids. The
zinc blende structure
results when the corners and
face-centers are occupied by one type of atom, but the four sites within the cell are occupied by another. Figure 8.20 shows the structure of ZnS, which is the prototype for the
zinc blende structure
(zinc blende is the mineral name for zinc sulfide). GaP, a
useful semiconductor, also adopts this structure
type. In fact, this is the structure type of
the most common semiconductors, including GaAs and InP. The similarity in properties
Figure 8.20 Zinc blende (ZnS) Zinc atoms are grey and sulfur atoms are yellow.
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