Multilevel molecular memory

In memory cells, a bit of information is either a zero or a one. One way to cram more data into a fixed region on a data-storage device is to store more than one bit in each memory cell. The current record holder is silicon flash memory, with two bits per cell; going beyond that is difficult due to device compexities. Now, a team led by Chongwu Zhou (University of Southern California) has used self-assembled molecular electronics to build working three-bit memory cells. In a nanowire transistor, they coated the 10-nm × 2-µm wire with certain molecules and manipulated parcels of charge placed in the molecules. The resulting memory cell had three different controllable bit states, with a total of 8 (2³) distinct levels. The multilevel molecular memory unit charges or discharges the molecules into different chemically reduced or oxidized (redox) states. The stored information is read out by sampling the resistance of the underlying nanowire: The attached redox molecules act like chemical gates for controlling the number of electrons in the wire. Data written this way have survived for as long as 600 hours, a new record for molecular memory, and are easily erased. The researchers suggest that data densities as high as 40 Gbits/cm² are possible. (C. Li et al., Appl. Phys. Lett. 84, 1949, 2004 http://dx.doi.org/10.1063/1.1667615 .)