Multilevel Molecular Memory

In conventional memory cells a bit of information is either a zero or one. (In hypothetical quantum computers, a bit could be both a zero and a one at the same time, but that kind of nimble balancing is years away from exploitation and so bits continue to be bi-level.)

In the meantime one way of cramming more data into a fixed lateral region on a data storage device, other than shrinking the cell's size, is to store more than one bit in each memory cell. This is one goal of molecular electronics (or "moletronics") where, for instance, one would like to store information in the form of parcels of charge placed at several active sites around a single molecule.

A USC/NASA-Ames collaboration has taken a step in the direction of such a chemical memory by producing a memory cell with three different controllable bit states, with a total of 8 (2 raised to the 3rd power) distinct levels. This multilevel molecular memory unit works by charging or discharging "molecular wires" consisting of molecules (attached to an underlying nanowire) into different chemically reduced or oxidized (redox) states. (See the figure.)

The information stored in the unit can be read back out by sampling the resistance of the nanowire; the attached redox molecules act, in effect, as chemical gates for controlling the number of electrons in the nanowire.

In tests so far the data written this way has survived for as long as 600 hours, compared to retention times of a few hours for one-bit-per-cell molecular memories. The researchers (contact Chongwu Zhou, USC, chongwuz@usc.edu, 213-740-4708) are attempting to make more extended memory chips using the new principle. Data density rates as high as 40 Gbits/cm² are expected. (Li et al., Applied Physics Letters, 15 March 2004.)