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Nanotubes Are Still the Best Bet. 
In The Age of Spiritual Machines, I cited nanotubes—using molecules organized 
in three dimensions to store memory bits and to act as logic gates—as the most likely technology to usher in the era of 
three-dimensional molecular computing. Nanotubes, first synthesized in 1991, are tubes made up of a hexagonal 
network of carbon atoms that have been rolled up to make a seamless cylinder.
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Nanotubes are very small: single-wall 
nanotubes are only one nanometer in diameter, so they can achieve high densities. 
They are also potentially very fast. Peter Burke and his colleagues at the University of California at Irvine recently 
demonstrated nanotube circuits operating at 2.5 gigahertz (GHz). However, in 
Nano Letters
, a peer-reviewed journal 
of the American Chemical Society, Burke says the theoretical speed limit for these nanotube transistors "should be 
terahertz (1 THz = 1,000 GHz), which is about 1,000 times faster than modern computer speeds."
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One cubic inch of 
nanotube circuitry, once fully developed, would be up to one hundred million times more powerful than the human 
brain.
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Nanotube circuitry was controversial when I discussed it in 1999, but there has been dramatic progress in the 
technology over the past six years. Two major strides were made in 2001. A nanotube-based transistor (with 
dimensions of one by twenty nanometers), operating at room temperature and using only a single electron to switch 

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between on and off states, was reported in the July 6, 2001, issue of 
Science
.
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Around the same time, IBM also 
demonstrated an integrated circuit with one thousand nanotube-based transistors.
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More recently, we have seen the first working models of nanotube-based circuitry. In January 2004 researchers at 
the University of California at Berkeley and Stanford University created an integrated memory circuit based on 
nanotubes.
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One of the challenges in using this technology is that some nanotubes are conductive (that is, simply 
transmit electricity), while others act like semiconductors (that is, are capable of switching and able to implement logic 
gates). The difference in capability is based on subtle structural features. Until recently, sorting them out required 
manual operations, which would not be practical for building large-scale circuits. The Berkeley and Stanford scientists 
addressed this issue by developing a fully automated method of sorting and discarding the nonsemiconductor 
nanotubes. 
Lining up nanotubes is another challenge with nanotube circuits, since they tend to grow in every direction. In 
2001 IBM scientists demonstrated that nanotube transistors could be grown in bulk, similar to silicon transistors. They 
used a process called "constructive destruction," which destroys defective nanotubes right on the wafer instead of 
sorting them out manually. Thomas Theis, director of physical sciences at IBM's Thomas J. Watson Research Center, 
said at the time, "We believe that IBM has now passed a major milestone on the road toward molecular-scale chips....If 
we are ultimately successful, then carbon nanotubes will enable us to indefinitely maintain Moore's Law in terms of 
density, because there is very little doubt in my mind that these can be made smaller than any future silicon 
transistor.
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In May 2003 Nantero, a small company in Woburn, Massachusetts, cofounded by Harvard University 
researcher Thomas Rueckes, took the process a step further when it demonstrated a single-chip wafer with ten billion 
nanotube junctions, all aligned in the proper direction. The Nantero technology involves using standard lithography 
equipment to remove automatically the nanotubes that are incorrectly aligned. Nantero's use of standard equipment has 
excited industry observers because the technology would not require expensive new fabrication machines. The 
Nantero design provides random access as well as nonvolatility (data is retained when the power is off), meaning that 
it could potentially replace all of the primary forms of memory: RAM, flash, and disk.