UCLA and Caltech chemists report the creation of large-scale molecular memory, an important step toward building molecular computers.

The electric power industry has a keen interest in all improvements to the technology of computation. Developments augmenting the speed and fault tolerance of integrated circuits hastens the reality of the vision many of us have, of a power system capable of assessing threats to stability in almost real time, responding intelligently to changing system conditions and faults, and using local and remotely acquired data to navigate like a modern fighter jet through a field of quickly and dramatically changing end use priorities.

A team of UCLA and California Institute of Technology chemists reports in the Jan. 25 issue of the journal Nature the successful demonstration of a large-scale, "ultra-dense" memory device that stores information using reconfigurable molecular switches. This research represents an important step toward the creation of molecular computers that are much smaller and could be more powerful than today's silicon-based computers.

The 160-kilobit memory device uses interlocked molecules manufactured in the UCLA laboratory of J. Fraser Stoddart, director of the California NanoSystems Institute (CNSI).

The research published in Nature describes the fabrication and operation of a memory device. The memory is based on a series of perpendicular, crossing nanowires, similar to a tic-tac-toe board, with 400 bottom wires and another 400 crossing top wires. Sitting at each crossing of the tic-tac-toe structure and serving as the storage element are approximately 300 bistable rotaxane molecules. These molecules may be switched between two different states, and each junction of a crossbar can be addressed individually by controlling the voltages applied to the appropriate top and bottom crossing wires, forming a bit at each nanowire crossing. The 160-kilobit molecular memory was fabricated at a density of 100,000,000,000 (1011) bits per square centimeter - "a density predicted for commercial memory devices in approximately 2020," Stoddart said.

"For this commercial dream to be realized, many fundamental challenges of nano-fabrication must be solved first," Stoddart said. "The use of bistable molecules as the unit of information storage promises scalability to this density and beyond. However, there remain many questions as to how these memory devices will work over a prolonged period of time. This research is an initial step toward answering some of those questions.

"Using molecular components for memory or computation or to replace other electronic components holds tremendous promise," Stoddart said. "This research is the best example - indeed one of the only examples - of building large molecular memory in a chip at an extremely high density, testing it and working in an architecture that is practical, where it is obvious how information can be written and read.

"Our goal was to demonstrate that large-scale, working electronic circuits could be constructed at a density that is well-beyond - 10 to 15 years - where many of the most optimistic projections say is possible," said James R. Heath, Caltech's Elizabeth W. Gilloon Professor of Chemistry and a co-author of the Nature paper.

"One of the most exciting features of this research is that it moves beyond the testing of molecular electronic components in individual, non-scalable device formats and demonstrates a large, integrated array of working molecular devices," said William R. Dichtel, a researcher who is a member of both Stoddart's and Heath's research teams.

The CNSI, a joint enterprise between UCLA and the University of California, Santa Barbara, is exploring the power and potential of organizing and manipulating matter to engineer "new integrated and emergent systems and devices, by starting down at the nanoscale level," Stoddart said.

SIDE BAR  #1:
A rotaxane is a molecule in which a dumbbell-shaped component, made up of a rod section and terminated by two stoppers, is encircled by a ring. It has the potential to be a molecular abacus. The bi-stable rotaxanes behave as switches by incorporating two different recognition sites for the ring, and the ring sits preferentially at one of the two, said Stoddart, leader of the UCLA team. The molecule can act as a switch provided the ring can be induced to move from one site to the other site and then reside there for many minutes. The bistable rotaxane molecules used in the crossbar memory can be switched at very modest voltages from an "off" (low conductivity) to an "on" (high conductivity) state. The stoppers for the rotaxane molecules are designed to allow the molecules to be organized into single-molecule-thick layers, after which they are incorporated into the memory device, Stoddart said.

Fig. 1 Bi-stable rotaxane behaviour


SIDE BAR #2:
Nanosystems-related research is performed on a size-scale ranging from 1 nanometer - about one-billionth of a meter - to a few hundred nanometers. The DNA molecule is 2 nanometers wide, roughly 1,000 times smaller than a red blood cell and 10,000 times smaller than the diameter of a human hair.