Nottingham nanotubes

UK researchers are taking part in a study to investigate the use of carbon nanotubes to create compact memory cells for ubiquitous electronic devices.

Computers, mobile phones, cameras and iPods are getting smaller, presenting designers with the challenge of miniaturising their memory formats.

This process requires the shrinking of silicon transistors, the devices that switch current on and off and store data as 1 and 0 binary units. However, as transistors get smaller their operation is disrupted by quantum phenomena, such as electrons tunnelling through the barriers between wires.

The race to find an alternative to silicon transistors is under way at many academic institutions including Nottingham University, where researchers are exploring ways of exploiting carbon nanotubes to create compact memory cells that use little power and write information at high speeds.

The team has designed a cell, constructed on a silicon substrate analogous to two banks separated by a river. A carbon nanotube attached to an electrode sits on the left bank, an electrode sits in the middle, and a third electrode sits on the right bank. The carbon nanotube is uniquely designed so that it can reach the other side of the bank by stretching out telescopically.

‘The carbon nanotube is like a Russian doll,’ said Elena Bichoutskaia, the researcher leading the study. The tube contains many smaller versions of itself. Each tube floats inside the tube it is inside without touching the walls.

‘The tubes float inside each other because they are responding to electrostatic, van der Waals and capillary forces,’ said Bichoutskaia.

When a known voltage is applied, the inner tubes of the carbon nanotube will slide out like a telescope until they cross the gap in the silicon substrate and touch the electrode on the other bank. When this happens, the cell is in a conducting state and attains a ‘1’ binary unit.

The cell can be returned to a non-conducting state (binary ‘0’) when another known shot of voltage is applied to the gate (middle) electrode. The tube will detach from the electrode and capillary forces, which exist between the outer wall of a carbon nanotube and its inner wall, will push the tubes back inside the carbon nanotube.

‘You can repeat it as many times as you want,’ said Bichoutskaia. ‘There is no wear or tear, there is no friction between the walls so they can essentially move in and out just by applying the voltage shot on the electrode. It could work for 1,000 years.’

Also, like a Flash drive, the carbon nanotube cell can be non-volatile, which means it can retain memory even when no voltage is applied.

Bichoutskaia said the tube would stay attached to an electrode in a conducting state even when the voltage was shut off, because of electrostatic forces created between the electrode and the inner wall of the nanotube.

Part of her research has been to predict the performance of these memory cells by modelling them on the computer, which enabled her team to determine the right amount of voltage to extend and retract the telescopic carbon nanotube.

Bichoutskaia said it would have been too expensive to experiment with real carbon nanotubes. ‘The difference between half a volt and 200 volts is crucial,’ she said. ‘Whereas 200 volts would explode the walls of the carbon nanotube, half a volt would not be enough to move the tube.’

The researchers found it requires only a few volts to move the carbon nanotubes because they are so small. ‘They are the lightest possible particle for data storage,’ said Bichoutskaia.

In addition to their low power consumption their nano-size could also benefit the future of data storage. ‘You can imagine in the future we will have so much information that has to be stored,’ said Bichoutskaia.

‘We need to go on a much smaller scale and start using molecular level of storage. A carbon nanotube is 100 times thinner than a strand of your hair. So imagine how many nanotubes can be aligned and attached to a small micrometre sized electrode.’

Commercialisation of such a technology may face many challenges and Bichoutskaia did not want to speculate on how long it would take. Yet she claimed the end is nigh for the silicon transistor.

‘The first computers had mechanical gears in them, and these were later replaced by valves,’ she said. ‘Valves were replaced by relays and now we live in transistor world. I think we will just do what we did in the past and forget about the current architecture and find something better for the nanoscale.’

Siobhan Wagner