Researchers eager to use individual molecules as the components of ultra-small electronic circuits and computers have put a new spin on their ambitious goal.
They take advantage of a hitherto unexploited property of electric currents, called spin, to make molecular devices that operate under new rules. This fledgling form of electronics, called spintronics, could lead to computers that don’t forget anything when their power is turned off, and perhaps even to that ultra-powerful device, the quantum computer.
Jan Hendrik Schön of Bell Laboratories in New Jersey and co-workers have made a prototype spintronics device called a spin valve, in which the electrical current passes from one terminal to the other through individual carbon-based molecules1.
Previous spin valves were made from slabs of semiconductor, much as conventional transistors are made from silicon. But made from single molecules they could be much smaller than today’s miniaturized transistors on silicon chips. Circuits could then be more densely packed with devices and therefore more powerful.
Molecular electronics will probably complement rather than replace conventional semiconductor-based microelectronics. Making devices as small as single molecules will be very difficult. The electrical contacts for these devices "will always be larger than the dimensions of the molecules themselves," Schön cautions. This could limit the amount of miniaturization that is possible.
Up and down
Conventional devices such as transistors use electric fields to control how many electrons pass through them - in other words, how big the electric current is. A spin valve controls the current using magnetism.
It manipulates a property of every electron called spin. Spin takes one of two values: ’up’ or ’down’, and makes an electron magnetic
In a spin valve, layers of magnetic material act as a filter, letting through electrons with one spin orientation (up, say), and blocking those with oppositely oriented spins (down).
So information encoded in the electrons’ spins can be manipulated to perform computational tasks. The up/down orientation of spins is equivalent to the 1 and 0 of binary logic that computers use.
In a spin
To make their molecular spin valve, Schön and colleagues laid down a one-molecule-thick carpet of a substance called pentanethiol on top of a nickel film. The pentanethiol molecules stick out like bristles from the metal surface. A few bristles of a different molecule, benzene-1,4-thiol (BDT), conduct electrical current.
They then deposited a patchwork of thin nickel films on top, so the molecules were sandwiched between two layers of metal, which acted as electrical contacts.
These nickel films cover just a hundred thousand or so molecules each. On average, only one of these is a BDT molecule: this single molecule provides an electrical connection between the two layers of nickel. Because nickel is magnetic, it acts on a current via the electrons’ spins.
The researchers found that switching the direction in which the magnetic fields point in the top and bottom nickel layers alters the current. When the two fields are aligned, a lot of current passes through a single BDT molecule; when the fields point in opposite directions, the current drops because some electrons with the wrong spins are filtered out.
A team in Karlsruhe, Germany, led by Heiko Weber, have meanwhile shown that similar single-molecule ’wires’ spanning a tiny gap between two metal terminals act as weird wires. They conduct better in one direction than the other2.
These molecular wires are wedge shaped. In a normal metal wire this wouldn’t make any difference, showing how molecular-scale circuits could be designed using new principles.
PHILIP BALL | © Nature News Service
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