Nanotube nonvolatile addressable memory

from the snap-to-it dept.
Researchers have used the Van der Waals attraction between two closely spaced perpendicular nanotubes to give the pair two stable mechanical states. In one state the tubes are well separated and unstrained, while in the second they are in close contact and the attraction between them bends the upper tube, holding them together. They were able to both switch between the states and sense them electronically. They calculate potential switching rates of 100GHz and densities of 1012 bits/cm2 Thomas Rueckes, Kyoungha Kim, Ernesto Joselevich, Greg Y. Tseng, Chin-Li Cheung, and Charles M. Lieber, writing in [Science 289:94-97 7July2000], describe calculations about and experiments with these crossed nanotube storage elements. These devices remind me of mercury-wetted reed relay contacts, with similar hysteresis from contact forces.

The authors' memory array design has two perpendicular arrays of nanotubes, one array above the other, with a crossbar of memory devices formed by their intersections. If the design contained only ohmic switches between the layers, then the bits could not be sensed independently. For example, if bits are on, forming connections, at positions {3,10}, {12, 10}, and {12, 17} in the array, then there would appear to be a connection at {3, 17}, even if that bit is supposed to be off. The "standard solution to this problem is the incorporation of diodes at each cross point." This prevents the backwards flow of current that would otherwise cause the unwanted connections. The authors consider the ideal solution to be using contacts between metallic nanotubes in one layer with semiconducting nanotubes in the other to effectively form Schottky diodes at each memory element. They also suggest some other approaches.

The authors calculated the properties of their memory element using (10,10) nanotubes. Their calculated densities are set by the balance between the bending stiffness of the nanotubes and the strength of the attraction between them. The minimum size needed to make a weak enough spring for bistability is 10 nm on hard substrates and 5 nm on soft ones. The 100GHz switching speed includes time for both mechanical switching and for RC charging of the array.

Experimentally, the authors built junctions from 50 nm diameter nanotube ropes using a micromanipulator. One junction could be switched on by 10 volts between the tubes, and off by 40 volts applied to both tubes. The on resistance was 140 megohms, while the off resistance was 1.4 gigohms. This device could be switched repeatedly for several days. Another device had an on resistance of 112 kilohms. The assembly of an array of these devices has not yet been shown experimentally.

This memory device is a high performance application of nanotechnology. It is potentially comparable in density to some of the mechanically scanned AFM memory proposals, but has a much higher potential speed. It would also put a very high density of independent mechanical actuators under electronic control, which might be useful in driving nanoscale robotics.

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