Chemistry - A Molecular Science

(Nora) #1

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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