Self-assembled nanocells function as non-volatile memory

First use of disordered nanowires, organic molecules as programmable memory

Chemists at Rice University have demonstrated that disordered assemblies of gold nanowires and conductive organic molecules can function as non-volatile memory, one of the key components of computer chips.

“A large part of the cost associated with creating integrated circuits comes from the painstaking precision required to ensure that each of the millions of circuits on the chip are placed in exactly the right spot,” said lead researcher Jim Tour, an organic chemist at Rice. “Our research shows that ordered precision isn’t a prerequisite for computing. It is possible to make memory circuits out of disordered systems.”

The research appears in the Oct. 29 issue of the Journal of the American Chemical Society. It marks the first time that a self-assembled ensemble of molecular electronic components has been used to create complex devices that carry out basic computing functions. Dubbed NanoCells, the devices were shown to function as re-programmable memory with memory states that hold for more than a week at room temperature, and probably far longer. Present-day dynamic random access memory, or DRAM, only holds its memory state for about one hundredth of a second and must be refreshed every thousandth of a second.

In previous experiments, Tour, the Chao Professor of Chemistry and professor of computer science, mechanical engineering and materials science, has used single molecules as switches, memory devices, resistors, diodes, junctions and wires. The creation of the prototype NanoCell marks the first time such molecules have been used to form a working microelectronic device.

The NanoCell consists of a set of discontinuous islands of gold film that are vapor-deposited onto a flat rectangle of silicon dioxide measuring about 40 microns by 10 microns. By submersing the sliver of silicon dioxide into a liquid solution of precisely synthesized organic molecules and gold nanowires, Tour is able to create conductive links between the islands of gold foil. Ten gold leads spaced five microns apart around the perimeter of the NanoCell carry electronic signals to and from the device. The size of the host platform is not critical, so the technology can scale down to much smaller sizes.

Compared to metal-oxide semiconductor technology, molecular electronic devices like NanoCells, offer the potential to reduce device size and fabrication costs by several orders of magnitude. With the NanoCell architecture, Tour hopes to address the nanoscale via the microscale, taking advantage of the ultrasmall molecules using current lithographic tools.

In addition to memory, Tour’s group is actively studying how NanoCells can be used to as logic gates. Since the precise placement of components is disordered, the NanoCells can’t be programmed like today’s computers. Instead, they must be trained to carry out specific logical functions. Even if this process is only a few percent efficient in the use of molecular devices, it could result in very high logic densities, making it possible for computer makers to create much more powerful chips.

The JACS paper, titled “NanoCell Electronic Memories,” was co-authored by Tour, postdoctoral researchers Long Cheng and Yuxing Yao, graduate student Austen Flatt, Penn State chemist Thomas Mallouk and his graduate student Sarah St. Angelo, and North Carolina State electrical engineer Paul Franzon and his graduate student David Nackashi.

The research was sponsored by the Defense Advanced Research Projects Agency, the Office of Naval Research and Molecular Electronics Corp.