Major Areas of Opportunity - Implications of Emerging Micro-and Nanotechnologies - page 29

nanotube arrays on surfaces for efficient current emitting elements is of great inter-
est for low-power field emission displays, and a number of companies are racing to
develop flat panel displays for next generation television and computer screens.
Computing
Further down the road, computer memory and logic concepts based on carbon
nanotubes are being explored. Transistors made from carbon nanotubes a few
nanometers in diameter—a hundred times smaller than the 130-nanometer tran-
sistor gates now found on computer chips—have been demonstrated. 1 Also dem-
onstrated are collections of nanotube transistors working together as simple logic
gates, the fundamental computer component that transforms electrical signals into
meaningful ones and zeros. If nanotubes or related nanowires could be used as
tiny electronic switches or transistors, computer designers could, in principle, cram
billions of devices onto a chip (the Pentium 4 has only 55 million transistors). 2 The
real challenge in this or any other alternative nanotechnology approach to fabricat-
ing computer chips is to design and connect up many millions or billions of such
components in a highly manufacturable and reliable architecture.
Mechanical Properties
As a result of their seamless cylindrical structure, carbon nanotubes have low
density, high stiffness, and high axial strength. Theoretical studies and recent ex-
perimental measurements suggest that the Young's modulus and breaking
strength of single-wall carbon nanotubes are exceptionally high. 3 Carbon fibers
with a tensile strength up to 6 GPa are commercially available, while initial exper-
imental measurements on 4-mm-long single-wall carbon nanotube (SWNT) “ropes”
consisting of tens to hundreds of individual SWNTs have yielded values up to 45
GPa. 4 The hope is that millimeter-long SWNTs can be formed into longer fibers or
dispersed into a composite matrix while still maintaining a significant fraction of this
observed improvement over conventional carbon fibers. The major challenge is
retaining the strength of nanofibers and assemblies of nanofibers in conjunction
with a matrix material so these properties can be controlled, optimized, and made
practical.
Energy Storage
Carbon nanotubes could be used to improve batteries. They can in principle
store twice as much energy density as graphite, the form of carbon currently used
as an electrode in many rechargeable lithium batteries. Conventional graphite elec-
trodes can reversibly store one lithium ion for every six carbon atoms. Tiny straws
of carbon tubes reversibly store one charged ion for every three carbon atoms,
double the capacity of graphite. 5 Carbon nanotubes are also being investigated for
hydrogen storage. They may be capable of storing amounts comparable to or
exceeding the U.S. Department of Energy target of 6.5 percent of their own weight
in hydrogen, a level considered necessary to be practical for fuel cell electric vehi-
cles. 6
The carbon nanotube is a now-classic example of a well-defined nanostruc-
ture, and exploring ways to exploit its unique properties for possible nanotechnol-
ogy-based applications remains a subject of intense interest. A general issue is the
ability to reproducibly obtain large quantities of selective configurations of single-
or multiple-wall nanotubes. Methods for synthesizing nanotubes, controlling orien-