Power/Performance Bits: March 14

Magnetic storage on one atom

Scientists at IBM Research created a single-atom magnet and were able to store one bit of data on it, making it the world’s smallest magnetic storage device.

Using electrical current, the researchers showed that two magnetic atoms could be written and read independently even when they were separated by just one nanometer. This tight spacing could, the team hopes, eventually yield magnetic storage that is 1,000 times denser than today’s hard disk drives and solid state memory chips.

The device uses the rare earth element holmium on a magnesium oxide film held at 5 kelvin (-450°F).

Holmium is particularly suitable for single-atom storage because it has many unpaired electrons that create a strong magnetic field, and they sit in an orbit close to the atom’s center where they are shielded from the environment. This gives holmium both a large and stable field, said Fabian Natterer, a physicist at Swiss Federal Institute of Technology (EPFL) who also works with IBM Research. But the shielding has a drawback: it makes the holmium notoriously difficult to interact with. And until now, many physicists doubted whether it was possible to reliably determine the atom’s state.

To write the data onto a single holmium atom, the team used a pulse of electric current from the magnetized tip of scanning tunneling microscope, which could flip the orientation of the atom’s field between a 0 or 1. In tests the magnets proved stable, each retaining their data for several hours, with the team never seeing one flip unintentionally. They used the same microscope to read out the bit — with different flows of current revealing the atom’s magnetic state.

Scanning tunneling microscope view of a single atom of Holmium, an element used as a magnet to store one bit of data. (Source: IBM Research – Almaden)

“To demonstrate independent reading and writing, we built an atomic-scale structure with two holmium bits, to which we write the four possible states and which we read out both magnetoresistively and remotely by electron spin resonance. The high magnetic stability combined with electrical reading and writing shows that single-atom magnetic memory is indeed possible,” said the researchers in the published study.

The team plans to observe three mini-magnets that are oriented so their fields are in competition with each other, so they continually flip. “You can now play around with these single-atom magnets, using them like Legos, to build up magnetic structures from scratch,” Natterer says.

Recycling silicon for batteries

A team from Tohoku University and Osaka University found a way to recycle waste silicon as a high-performance anode for lithium-ion batteries.

Producing high purity silicon wafers is an expensive, energy-intensive process. Yet the researchers estimate that 45–55% of the high-quality silicon is discarded as industrial waste in the final wafer cutting process. This waste is about 90,000 metric tons a year worldwide, an amount large enough to meet the global demands for anode materials for lithium-ion batteries.

To turn waste silicon sawdust into anodes, the team first had to purify the silicon sludge of contaminants introduced by the wafer manufacturing process, then mold the sawdust into nanostructures appropriate for use as an anode.

Production process from silicon sawdust to lithium battery anode. (Source: Tohoku University)

The team found that by pulverizing the silicon sawdust into silicon nanoflakes, about 16 nm in thickness, repeated expanding and contracting to turn the silicon into a porous framework resembling wrinkled papers, and coating the nanoflakes with carbon was effective in fabricating high capacity and durable anodes. So far, a test half-cell has achieved a constant capacity of 1200 mAh/g over 800 cycles, a capacity 3.3 times as large as that of conventional graphite (ca. 360 mAh/g).

According to the researchers, this method of material recycling is applicable for the mass production of high-performance lithium-ion battery anode materials at a reasonably low cost. The team expects it to have practical use in the battery industry.

DNA switch

A team from Arizona State University developed the first controllable DNA switch to regulate the flow of electricity within a single, atomic-sized molecule.

“It has been established that charge transport is possible in DNA, but for a useful device, one wants to be able to turn the charge transport on and off. We achieved this goal by chemically modifying DNA,” said Nongjian Tao, who directs the Biodesign Center for Bioelectronics and Biosensors and is a professor of engineering at ASU. “Not only that, but we can also adapt the modified DNA as a probe to measure reactions at the single-molecule level. This provides a unique way for studying important reactions implicated in disease, or photosynthesis reactions for novel renewable energy applications.”

The group modified one of DNA’s double helix chemical letters, abbreviated as A, C, T or G, with another chemical group, called anthraquinone (Aq). Anthraquinone is a three-ringed carbon structure that can be inserted in between DNA base pairs but contains a redox group (short for reduction, or gaining electrons or oxidation, losing electrons).

How anthraquinone fits into DNA’s double helix. (Source: ASU)

The modified Aq-DNA helix was slipped in between the rungs that make up the ladder of the DNA helix, providing the ability to reversibly gain or lose electrons.

“We found the electron transport mechanism in the present anthraquinone-DNA system favors electron “hopping” via anthraquinone and stacked DNA bases,” said Tao. In addition, they found they could reversibly control the conductance states to make the DNA switch on (high-conductance) or switch-off (low conductance). When anthraquinone has gained the most electrons (its most-reduced state), it is far more conductive.

Beyond potential future applications in DNA nano-electronics, the team sees the method as a new tool to examine redox reaction kinetics and thermodynamics at the single molecule level.