Demonic devices make densest memory

A memory device the size of a human white blood cell and a light-driven motor that transports molecules away from equilibrium like Maxwell's mythical demon: both have been created by chemists using rotaxanes, long dumbbell-shaped molecules threaded with trapped rings that can move back and forth.

Teams of scientists led by James Heath and Fraser Stoddart, from Caltech, Pasadena and the University of California in Los Angeles respectively, have made a molecular electronic device that mimics dynamic random access memory (DRAM) circuits on today's computer microchips.¹ This piece of nanoelectronic circuitry could bring molecular computers a step closer; the device's cellular dimensions are not expected to be matched by its solid-state equivalent before the year 2020.

There is a limit to how small and how many integrated circuits can be fabricated and packed onto a silicon chip. But the ultimate miniaturisation of molecular electronics could overcome this, using individual molecules to make nanoscopic analogues of electronic components in integrated circuits. For example, molecular switches can mimic the operations performed by logic gates in memory circuits. One such group of molecules are the [2]rotaxanes.

The California team's [2]rotaxane has a positively-charged ring which can sit at one of two different sites on the dumbell's axis, representing the 'zero' or 'one' of the binary system our computers use to store data. The ring normally sticks to a tetrathiafulvalene (TTF) group, which represents a low conductance state, or 'zero'. But oxidising the TTF group gives it a positive charge, repelling the ring to the alternative site. This represents a high conductance 'one' state.

In order to construct a working molecular DRAM, the California team used a monolayer of two-state [2]rotaxanes sandwiched between 400 silicon and 400 titanium nanowires arranged in a crosshatched pattern. A single bit of data can be stored at the point where a silicon and titanium nanowire cross each other, trapping about 100 [2]rotaxane molecules between them.

That means the complete electronic memory circuit contained 160 000 bits at a density of 10¹¹ bits cm^-2. A single bit is only 15 nanometres wide, or about one ten-thousandth the diameter of a human hair. By contrast, the most dense memory devices currently available are approximately 140 nanometres in width. 'Whether it's actually possible to get this new memory circuit into a laptop, I don't know,' admitted Heath. 'But we have time.'

David Leigh, who works with rotaxanes at Edinburgh University, UK, called the work 'a superb technical achievement'. Although the molecular DRAM suffers from many circuit defects, each bit operates independently so the viable ones can be identified and used in a memory device. 'The development of defect-tolerant computer architectures by Heath, along with Stanley Williams at Hewlett-Packard in the 1990s, offers a clever means of overcoming the 'mistakes' that the current, rather crude, reaction ion etching process and self-assembly methods of monolayer production introduce for organic molecular structures,' said Leigh.

'The latest rotaxane-based memory from Heath and Stoddart exploits this brilliantly, delivering an amazing organic-based 160-kilobit memory that is competitive with the smallest memories produced by photolithography,' he added. 'Surely this sort of integration of sophisticated organics with silicon is going to be the future?'

Demon ratchet

Leigh's team, meanwhile, have created their own rotaxane-based wonder: a nanomachine that works like a ratchet, transporting molecules in only one direction.² The device is unusual because it drives the molecules away from their thermodynamic equilibrium, proving that artificial devices can mimic the sophisticated biochemical machinery in every living cell.

The chemists developed their light-driven molecular motor to perform a task that was first imagined by physicist James Clerk Maxwell in 1867. He suggested a way to apparently violate the second law of thermodynamics, which insists that heat cannot flow spontaneously from a cold material to a hotter material, and that any system will naturally tend towards greater disorder. Maxwell proposed that a cunning little demon might be able to make heat flow the 'wrong' way by strategically opening and closing a gate between two boxes full of gas, so that one filled up with hot, fast-moving molecules, and the other with cold, slow molecules.

Crucially, Maxwell's demon must open the gate without expending any energy - if it could do that, the system would move away from thermodynamic equilibrium.

The proposal had scientists baffled, not least because one of the consequences was a promise of perpetual motion. But a better understanding of the link between information and entropy finally demonstrated the impossibility of this thermodynamic free lunch.

Driving any system away from equilibrium always requires the input of energy. For the Edinburgh team, this meant developing a light-activated nanomachine that could sort molecular fragments into two different 'boxes'.

The solution was another rotaxane structure (see picture): a long molecular chain, blocked at both ends, on which is threaded a large ring-shaped molecule (red) that can move back and forth between two different sites (blue and green) on the chain's axis.

Part way along the axis is a chemical group - a stilbene - that acts as a light-activated gate. Normally kinked so that the ring is trapped at one end of the dumbbell, a blast of light will straighten out the stilbene and allow the ring to move to the other end of the dumbbell. Light-absorbing molecules in solution help to close the gate again once the ring has passed over it. This effectively separates the rotaxane into two compartments. The scientists monitored the ring's precise position using NMR spectroscopy, and over time they found that more of the rotaxane's rings ending up in the compartment furthest from the stilbene gate. 'It is as if Maxwell's demon drives the whole system away from equilibrium.' said Leigh. 'This is the first example of an "information ratchet" where there is a net signalled movement of particles in one direction only.'

Lionel Milgrom