Molecular Magic

Progress in the development of music technology depends on the continuing improvement of computer circuitry, which has more or less followed Moore's Law
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Progress in the development of music technology depends on the continuing improvement of computer circuitry, which has more or less followed Moore's Law for the past 40 years. Unfortunately, the number of transistor elements in an integrated circuit can't continue to double every 18 to 24 months forever; in fact, many experts believe this pace of miniaturization will become cost-prohibitive by 2015. In addition, the size of these elements will soon reach a threshold established by the need to dissipate heat and control the leakage of electrons within the circuit.

If the size of these circuit elements is to continue shrinking beyond that point (with a corresponding increase in processing speed and memory capacity), a fundamentally different technology must be developed. One promising approach seems to be the use of individual molecules as electronic switches and memory elements. Recently, research teams led by Mark Reed at Yale and James Tour at Rice University announced that they had developed just such a molecular switch and memory cell, giving this fledgling technology a big boost forward.

One of the main difficulties with this technology is controlling the flow of electrons through individual molecules. At this size scale, electrons are constrained to specific, discrete energy levels, as described by the laws of quantum physics. These energy levels are manifested as "clouds" called orbitals, which surround the nuclei of the molecule's constituent atoms. A molecule must have empty orbitals that overlap in certain ways in order to conduct electrons. If the orbitals overlap in different ways, no electrons can pass through the molecule.

This concept is the basis of the molecular devices constructed by the Yale and Rice teams. Their switch consists of a molecule called nitroamine benzenethiol sandwiched between two gold contacts. When a certain voltage is applied to the contacts, the resulting electric field twists the molecule, changing the overlapping structure of its orbitals and preventing electrons from flowing through it. When the voltage is reduced below the threshold value, the molecule snaps back to its original shape, and current can flow.

Interestingly, this switch can be easily modified to act as a binary memory cell by storing a charge within a specific section of the molecule. (A stored charge blocks conduction, representing a binary 0; without a stored charge, conduction is high, representing a binary 1.) In experiments with this memory cell, the stored charge could be retained for nearly ten minutes, unlike with conventional DRAM, which must be refreshed every few milliseconds.

In another experiment at Yale, a single benzene molecule was held between two tips of a modified scanning tunneling microscope (see "Tech Page: Nanocomputers" in the June 1996 issue of EM). The resistance of this molecule was measured in the tens of millions of ohms, and it could sustain a current of 0.2 microampere at 5 volts (see Fig. 1), which translates to roughly 1012 electrons per second traveling through the molecule. This is a much higher current than anyone expected, especially considering that the electrons must pass through the molecule one at a time.

So far, only molecular switches with two terminals have been devised. If these devices are to become the building blocks of electronic components in the future, a three-terminal switch must be built to replace the trusty transistor, in which one terminal controls the current between the other two. In addition, there are currently no molecular devices that can amplify current, which transistors can do. However, if these and other hurdles can be overcome, electronics of all types (including music technology) will take a quantum leap forward, jumping far beyond the exponential curve of Moore's Law and providing nearly unlimited processing power and memory capacity in the musical tools of tomorrow.