Monday, November 17, 2008

Storing bits of memory in nanotube switches

By Todd Morton
Published: November 17, 2008 - 08:40AM CT

The world of computer memory has been approaching an interesting crossroads. Most people are aware that we are rapidly approaching fundamental limits with both magnetic storage mediums like the hard drive, and in the fabrication of transistors through photolithography, which yields RAM and flash memory. Several areas of research, including fields like phase change memory, may provide the opportunity to move away from both magnetic domains and transistors. To explore a different route to future memory systems, researchers went high-tech and put multiwalled carbon nanotubes (MWCNTs) to use—only to discover that they work through a surprisingly retro mechanism.

The researchers fabricated a device that was rather simple compared to the usual carbon nanotube fare that we cover in Nobel Intent—a single MWCNT was spread across a silicon substrate between two platinum electrodes. Although producing these devices is delicate work, it is a far cry from depositing the multiple layers of exotic materials that make up today's field-effect transistors. When a voltage was swept from negative to positive across the nanocable device, a clear transition between conductive and nonconductive states was observed. This transition proved to be nonvolatile; that is, they didn't have to apply a constant voltage in order to maintain the conductive or non-conductive state.

Obviously, this binary state behavior could act as the foundation for computer memory, so the researchers went to work exploring the properties of the device. Read/write operations were stable over thousands of cycles, and the state of the nanocable could read without changing it, meaning the storage of a bit was stable. non-conductive state.

Moving into other performance metrics, they found that the MWCNT device was stable at ionizing radiations that cause normal electrical devices to fail, pointing to potential applications in extreme environments, like space. They remained stable over the course of several weeks in both vacuum and atmospheric conditions, and operated at temperatures that, if present in your laptop, would sear your favorite OEM's logo into your flesh.

An in-depth characterization of the devices revealed the mechanism behind the behavior, which turned out to be a throwback to the mechanical switches of the first computers. The MWCNT structure can be approximated as a sheet of graphene wrapped around a solid core, which provides mechanical stability. As the initial voltage is applied to one of these devices, the outer sheath of graphene will physically break at a defect site, which explains the significant changes in current flow. Although this mechanical process is not reversible in an absolute sense, applying a voltage in the other direction will cause enough electrostatic attraction to reconnect the two broken pieces.

The authors pointed out that similar behavior can be observed in graphene, which offers several possibilities for the actual fabrication of a real-world device based on this phenomenon. The combination of a relatively simple device fabrication technique, a high on/off current ratio at reasonable voltages, quick switching (as fast as 1 microsecond), non-volatility, and an apparently robust device makes for a formidable contender in the race for the memory systems of the future.

Nature Materials, 2008. DOI: 10.1038/nmat2331


Article abstract
Nature Materials

Published online: 16 November 2008 | doi:10.1038/nmat2331

Electronic two-terminal bistable graphitic memories

Yubao Li1,3, Alexander Sinitskii1,3 & James M. Tour1,2


Transistors are the basis for electronic switching and memory devices as they exhibit extreme reliabilities with on/off ratios of 104–105, and billions of these three-terminal devices can be fabricated on single planar substrates. On the other hand, two-terminal devices coupled with a nonlinear current–voltage response can be considered as alternatives provided they have large and reliable on/off ratios and that they can be fabricated on a large scale using conventional or easily accessible methods. Here, we report that two-terminal devices consisting of discontinuous 5–10 nm thin films of graphitic sheets grown by chemical vapour deposition on either nanowires or atop planar silicon oxide exhibit enormous and sharp room-temperature bistable current–voltage behaviour possessing stable, rewritable, non-volatile and non-destructive read memories with on/off ratios of up to 107 and switching times of up to 1 mus (tested limit). A nanoelectromechanical mechanism is proposed for the unusually pronounced switching behaviour in the devices.

  1. Departments of Chemistry; Computer Science and Mechanical Engineering and Materials Science and the Smalley Institute for Nanoscale Science and Technology, Rice University, MS 222, 6100 Main Street, Houston, Texas 77005, USA
  2. These authors contributed equally to this work

Correspondence to: James M. Tour1,2 e-mail: