Chemistry - A Molecular Science

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.

© by

North

Carolina

State

University