Bumpy surface stores data

By Kimberly Patch, Technology Research News

Cramming more data into a given storage device is all about making bits that are extremely small and consistently spaced.

Using individual molecules to store bits would be a tremendous leap forward. One molecule gaining researchers' attention is rotaxane, which contains a ringed portion that can move along a long, thread-like portion of the molecule. Moving the ring from one end of the thread to the other could be a way to change a bit from a 1 to a 0.

Perfecting a method to make storage devices out of individual molecules is a tall order. But it turns out that the rotaxane molecule's unique shape could be used for data storage in a different way.

When researchers from universities in Italy and Edinburgh found that drawing the tip of an atomic force microscope across a thin film of rotaxane molecules resulted in perfectly-spaced strings of dots, they realized the phenomenon could be used for storing data. "Once you have the capability to write a dot and to fix its position... you immediately think about writing bits," said Fabio Biscarini, head scientist of nanotechnology of multifunction materials research at the National Council of Research Institute for the Study of Nanostructured Materials (CNR-ISMN) in Italy.

The method could allow for a super-capacity DVD-like media that could be written to extremely quickly. It could also provide a different way to form biochips, or labs-on-a chip that provide tiny channels to move and mix biological substances.

The force imparted during repeated passes of the microscope tip along a line works the rotaxane thin film into a series of bumps, much like repeat passes of a truck causes washboard-like bumps to form in a dirt road. "We discovered that we had the capability to write collectively as many dots [as] we wanted by simply adjusting the length of the line scan," said Biscarini.

The rotaxane dots are a few molecules high, 20 to 40 nanometers wide and 100 nanometers apart. A nanometer is one millionth of a millimeter, or the span of 10 hydrogen atoms. The exact size of the dots depend on the thickness of the film, and their spacing is consistent, said Biscarini.

The researchers found they could write parallel strings of dots spaced closely enough to fit 100 billion of them per square inch of film. Today's DVD's hold 4.7 gigabytes of information, which works out to 2.77 billion bits, or gigabits per square inch. Today's highest-density magnetic disk drives hold 50 gigabits, and lab prototypes have reached 100 gigabits per square inch.

In theory, data could be written to the rotaxane film bits in parallel, using a stamp or an array of probe tips. This would allow information to be pressed extremely quickly into CD or DVD-like media that would hold much more information. "Parallel writing would enable high-throughput permanent information writing, as in CDs or DVD's, but with a considerably larger area density," said Biscarini.

The key to the method is carefully controlled, gentle pressure on the tip. "We had to control carefully the force acting between the tip and the rotaxane film; the transformation into dots occurs only in a very narrow range of forces," said Biscarini. "At smaller forces nothing happens; at larger forces the film is scratched and destroyed," he said.

The structure of the molecules makes the change possible, said Biscarini. The molecules' rings can rotate with relative ease even when they are part of a solid, depending on the energy states of the hydrogen bonds within and among the molecules. "This makes molecules quite mobile even in [a] solid state, if appropriate energy is provided -- [this] molecular mobility is the key ingredient for achieving a transformation in the solid state," said Biscarini.

The scanning tip provides enough energy to the rotaxane molecules along the line so that they form the nuclei of the dots, which then grow larger as the energy diffuses to nearby molecules.

The researchers also found that the process could be stopped and started. "The fascinating feature of the phenomenon is that... the film rupture could be stopped simply by decreasing, and restarted by restoring the caressing force," said Biscarini. "The evolution of the dots could be slowed down at our convenience, and we could take a sequence of snapshots," he said.

The researchers are perfecting a method of writing data to the dots in parallel, according to Biscarini. They are also working on a method to erase data, he said.

The researchers are also working on ways to quickly retrieve information stored in the rotaxane bits. "The big problem is a method to read information fast on bits one nanometer high, a few nanometers wide, and less than 100 nanometers apart," said Biscarini. Current data-reading methods measure differences in magnetic, electrical or optical properties on portions of flat films rather than just a second raised structures. However, "it is reasonable to think that the dots will have different properties with respect to the unperturbed film", he said.

A method to write to ultra-high-density rotaxane thin film bits could be ready in two years, said Biscarini. Perfecting readout and erasing methods could easily take as long as five years, however, he said.

The method could also be used to form patterned surfaces that change their structures at precise locations when stimulated, said Biscarini. "It could then become a new way to make biochips, sensors [and] pressure transducers," he said.

Biscarini's research colleagues were Massimiliano Cavallini of CNR-ISMN, Salvador León and Francesco Zerbetto of the University of Bologna in Italy, and Giovanni Bottari and David A. Lee of the University of Edinburgh in Scotland. They published the research in the January 24, 2003 issue of Science. The research was funded by the European Union.

Timeline:   2-5 years
Funding:   Government
TRN Categories:  Data Storage Technology; Materials Science and Engineering; Chemistry
Story Type:   News
Related Elements:  Technical paper, "Information Storage Using Supramolecular Surface Patterns," Science, January 24, 2003.


January 29/February 5, 2003

Page One

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Bumpy surface stores data

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Tiny hole guides atoms against tide


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