Large-scale molecular memory

Another step toward a molecular computer

A team of UCLA and Caltech chemists has brought us a step closer to the creation of a computer operating at the molecular level. The researchers, led by Scottish nanoscience pioneer Sir Fraser Stoddart*, recently demonstrated a 160-kilobit memory device that can store information via reconfigurable molecular switches.

In a paper (subscription required) published recently in the journal Nature, the group describes the fabrication and operation of a large-scale memory device based on a series of criss-crossing nanowires arranged in a square matrix. Sitting at each crossing point of the array, and serving as the actual storage element, are some 300 bistable rotaxane molecules sandwiched between the nanowire layers.

Rotaxane structure

A rotaxane is a molecular structure in two parts which do not chemically interact, and consists of a ring and axis in a dumbbell shape. The molecule can act as a binary switch if the ring is induced to move from one end of the axis to the other via an electrical impulse.

William Dichtel, who divides his time between the Stoddart group at UCLA and that of James Heath at Caltech, views the new memory device as both a molecular electronic circuit, and a working, silicon-based circuit patterned at a density far beyond what has been achieved before.

‘There is still much work to be done,’ says Dichtel. ‘We’d like to develop molecules that can be switched many thousands of times, have higher on-off ratios, and can store information indefinitely without needing to be rewritten.’

Dichtel and his colleagues have looked at how subtle changes in the structure of the rotaxane molecule affect the characteristics of the memory device, and understanding this is one of the keys to achieving the stated goal.

The motivation behind this research was not just to build a large scale, ultra-dense memory circuit, says Dichtel, but also to learn how to work with high precision at the macromolecular scale. ‘This is a really enabling capability, and the Heath group is currently exploiting it for many other applications, including logic gates, high performance thermoelectric materials, nano-fluidics circuits for peptide sequencing, and silicon nanowire sensor devices, to name a few.’

The molecular memory device was fabricated at a density of 1011 bits cm-2, which, according to Stoddart, is that predicted for commercial memory devices in the year 2020. Density is likely to be limited by the size of the nanowires rather than the rotaxane structures. In the current study, the wires were 16 nm in diameter, with a centre-to-centre spacing of 33 nm, resulting in a memory cell size of 0.0011 µm2.

Heath’s group has fabricated arrays of 7.5 nm diameter wires spaced 15 nm apart, giving an order-of-magnitude increase in bit density over the device described in the Nature paper. But it need not stop there, and Dichtel says that it may eventually prove possible to stack a number of two dimensional arrays on top of one another.

Computer memory may be the most obvious application for the nanowire and rotaxane-based device, but such switches have potential application also in nano-mechanical devices and molecular machines. For example, compounds related to rotaxane mimic the actuation of natural muscle tissue. Dichtel adds: ‘We have also developed related molecular switches for use as drug delivery vehicles, and studied a variety of light-powered switches that might ultimately be able to convert sunlight into mechanical or chemical energy.’

There are a number of scientific and engineering challenges that need to be addressed before the type of memory device described here can become practical, but molecular electronics is clearly a field to watch very closely.

* Sir Fraser was knighted in December 2006 by Queen Elizabeth II for services to chemistry and molecular nanotechnology.

Figure: Graphical representation of the rotaxane structure used in experimental molecular memory devices. Taken from the Wikipedia page on rotaxane.

Article first published in Nanomaterials News.