Page 109 - Nanotechnology an introduction

Chapter Contents

9.1 Graphene 191
9.2 Carbon Nanotubes 191
9.3 Carbon Nanoparticles (Fullerenes) 195
9.4 Materials Applications 195
9.5 Device Components and Devices 196
9.6 Summary 197
9.7 Further Reading 197
Buckminsterfullerene, the single-walled carbon nanotube and grapheme epitomize, respectively, nanoparticles, nanofibers and nanoplates. The full potential of their remarkable properties, especially electronic ones, in devices will, however,
have to await the development of viable nano-object manipulation technologies.
Keywords: fullerene, carbon nanotube, graphene, materials, devices
Carbon-based materials and devices are dealt with in this separate chapter because of their unique importance and versatility. They represent the
epitome of nanotechnology.

Carbon has long been an intriguing material because of its two very well known allotropes, diamond and graphite, which are so different from one
another that if one did not happen to know they have the same elemental composition one might imagine that they are different elements. Although
researchers working on carbon have long been aware of other forms, typically dubbed “carbon filaments” and “atypical char” (along with
“amorphous carbon”), usually they were regarded as a nuisance and discarded if their formation could not be avoided. The importance of the
recent “discoveries” of fullerenes (soluble carbon) and carbon nanotubes (carbon filaments) resides in the fact that their structures were elucidated
for the first time. Even nowadays, with ever improving methods of industrial synthesis, yields of carbon nanotubes (CNT) and fullerenes are
significantly less than 100%; the desired product is embedded in a gangue of uncharacterized or defective material that still merits the name
“amorphous carbon”. The most recent “discovery” is that of graphene, which is simply one of the many parallel sheets constituting graphite; here the
ingenuity resided in the preparation of a single isolated sheet, which opened the possibility of examining experimentally what was already a well-
studied material theoretically.
2
1
2
The electron configuration of carbon is 1s 2s 2p . Its four outer shell electron orbitals are 2s, 2p , 2p and 2p ; the four valence electrons may
y
z
x
1
3
2
hybridize them into sp , sp and sp , corresponding to a carbon atom bound to 2, 3 and 4 neighboring atoms, respectively. Diamond is composed
3
entirely of sp orbitals: it is ultrahard, a wide band-gap dielectric, has good thermal conductivity and is rather inert chemically. Graphite is
2
composed entirely of sp orbitals: it is soft, and an electrical conductor, has highly anisotropic thermal conductivity and is very reactive chemically.
The three new materials, graphene, carbon nanotubes and fullerenes, can be called “nanocarbon”. Like graphite, they are composed entirely of sp 2
3
orbitals, but fullerenes contain 12 pentagons and have some sp character. They correspond to nanoplates, nanofibers and nanoparticles,
respectively (see Figure 6.2). Rolling up graphene (Figure 9.1) into the smallest possible tube makes a single-walled carbon nanotube, and curling
it up into the smallest possible sphere makes a fullerene—conceptually, that is; the materials are not actually fabricated that way. These
nanomaterials have no bulk equivalents, discounting the fact that graphite is made up of endlessly repeating stacks of graphene.

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Figure 9.1 Part of a graphene sheet, showing the sp chemical bonds. There is a carbon atom at each intersection. The length of each bond is about 0.3 nm.
Carbon nanotubes have hitherto attracted most commercial interest. Table 9.1 summarizes some of the remarkable properties of these materials.
At present, they are most commonly commercially exploited by embedding them in a matrix to form some kind of composite.
Table 9.1 Some properties of bulk and nanoscale carbon materials a
Property Unit Diamond Graphite CNT
Young's modulus N m −2 10 9 10 10 10 12
−1
Thermal conductivity W m K −1 2000 20 3000
Electrical resistivity Ω m 10 12 10 −5 <10 −6
a
The given values are only approximate, in order to enable a rough comparative idea of the material properties to be formed. Actual measured values still depend on many experimental details.
9.1. Graphene
Inspired by learning about naphthalene and anthracene, countless school children have doubtless doodled endless fused polyaromatic rings. It has
long been known that graphite is composed of stacks of such polyaromatic sheets, which are called graphene. Due to convincing theoretical work,
it was however long believed that two-dimensional crystals cannot exist. The ostensive demonstration of their existence (graphene sheets) have,
post hoc, led to the explanation that their stability is due to undulations of the sheet.
The graphene lamellae stacked to make bulk graphite were known from the ease of their detachment (as shown, e.g., by writing with graphite on
paper) to be only weakly bound to each other. Individual sheets of graphene can actually be peeled off graphite using adhesive tape. Alternatively, a
crystal of silicon carbide can be heated under vacuum to 1300 °C; the silicon evaporates and the remaining carbon slowly reorganizes to form