Others are exploring various approaches to make flexible nanoscale memory that could be scaled down beyond silicon. Memory elements based on transistors made from ferroelectric-coated graphene, carbon nanotube, and other nanowires could all eventually be made flexible. Some groups have made flexible memory using organic materials, as well as two-electrode devices based on thin films of titanium dioxide and graphene.
Compared to the zinc oxide transistors, which have three electrodes, two-terminal devices can be packed more densely and potentially even in 3-D, says Curt Richter a researcher at the National Institute of Standards and Technology. Richter has made flexible titanium dioxide memory. But since the electronic read-and-write circuits in today’s flash memory are designed for silicon transistors, “the advantage of nanowire transistors is you wouldn’t have to change the control logic and architecture so much. You could tweak it a little bit and plug the device in.”
A lot of work remains to be done to make the nanowire memory devices practical. Right now, they only retain their conductance state for over 11 hours about 70 times–flash memory can withstand about 100,000 writing cycles. And the nanowires are currently 100 nanometers wide and two micrometers long, although Sohn says they can potentially be scaled down to smaller than flash memory.
The researchers will also have to prove that the memory devices are fast, says James Tour, a chemistry professor at Rice University who is working on ultradense memory using graphite and silicon oxide. In the paper, the researchers rewrite the device every second. Flash memory, by comparison, has a writing speed of microseconds. “At one second, it is not even on the table for being interesting to device folks,” Tour says. “They must increase the speed one million-fold to begin to catch their attention.” Because it is hard to make large batches of nanowires that work uniformly and to align them on surfaces, Tour says, “doing anything on nanowire electronics would not be feasible in mass-production.”
But Georgia Tech materials scientist Zhong Lin Wang, who has made nanogenerators and sensors from zinc oxide nanowires, says that the new memory could be integrated with those devices, paving the way for a completely new kind of electronics technology based on zinc oxide. “This paper demonstrates an exciting application of zinc oxide nanowires as nonvolatile memory, which is a key component for future flexible electronics,” he says.