Probe-based "seek-and-scan" data-storage systems are ideal for making ultrahigh-density non-volatile memories. Here, a scanning probe, or array of probes, writes and reads data on media. The bit size mainly depends on the size of the probe tip and scientists have recently shown that a nanoscale-sized tip can locally reverse the polarization of a ferroelectric thin film and create domain sizes as small as nanometres across.

But there is a snag: in practical devices, the probe contacts the underlying media at very high speeds (of more than 100 µm/s), which rapidly wears down conventional scanning probe tips and thus reduces resolution. Carbon nanotubes are ideal as probes in this context thanks to their strong mechanical and wear-resistant properties, but they are prone to bending and buckling, which limits their use.

Excellent wear-tolerant properties
Yuegang Zhang and colleagues have now made carbon-nanotube probes coated with the dielectric material silicon oxide that have excellent wear-tolerant properties. The researchers estimated the buckling force in these tubes to be three orders of magnitude larger than the 5 nN force normally applied during typical read/write operations. It is also 14 times higher than that of previously made multiwalled carbon nanotube probes coated with parylene.

Zhang's team also calculated that the wear rate of a nanopencil on PZT film is 4.36 × 10–3 nm/s at a scanning speed of 50 mm/s. This means that the nanopencil could scan distances of more than 11 km without losing read/write resolution – a figure that exceeds most common data-storage application requirements.

According to the scientists, the high aspect ratio of the tubes takes advantage of the nanotubes' tiny size while avoiding the bending and buckling problems seen in polymer-coated or naked carbon-nanotube probes. Moreover, the new probes can write bit sizes as small as 6.8 nm on ferroelectric films, which would allow storage densities as high as 1 Tbit/inch2.

The researchers made their probes by first attaching a single or a bundle of nanotubes on a conductive AFM tip. Next, they coated the nanotube probes with a uniform layer of silicon oxide. Finally, they "sharpened" the nanopencil by sliding the tip on a diamond surface, which removed some of the silicon oxide coating.

Read/write operations
To use the pencil for read/write operations, Zhang and co-workers apply an electrical pulse to the nanotube tip that is contact with the surface of a ferroelectric film. The electric field can locally flip the polarisation direction in small areas of the film to write a bit. During reading, an AC signal is applied to the tip, which induces mechanical expansion and contraction of the underlying film, through a reverse piezo-electric effect, explains Zhang. The phase of the reverse piezo-electric response depends on the local polarisation direction of the bits.

The team now plans to improve its fabrication technique to make it more reproducible.

The work was published in Applied Physics Letters.