Power/Performance Bits: July 19

Atomic storage; nanowires from bacteria; estuary power.


Atomic storage

In the search for ever-smaller storage, a team of scientists at Delft University in the Netherlands built a 1 kilobyte memory where each bit is represented by the position of one single chlorine atom.

“In theory, this storage density would allow all books ever created by humans to be written on a single post stamp,” said lead scientist Sander Otte. They reached a storage density of 500 terabits per square inch, a 500-fold increase over the best commercial hard disk currently available.

The team used a scanning tunneling microscope, in which a sharp needle probes the atoms of a surface, one by one, enabling them to both see the atoms and push them around.

“You could compare it to a sliding puzzle,” said Otte. “Every bit consists of two positions on a surface of copper atoms, and one chlorine atom that we can slide back and forth between these two positions. If the chlorine atom is in the top position, there is a hole beneath it — we call this a 1. If the hole is in the top position and the chlorine atom is therefore on the bottom, then the bit is a 0.” Because the chlorine atoms are surrounded by other chlorine atoms, except near the holes, they keep each other in place.

STM scan (96 nm wide, 126 nm tall) of the 1 kB memory, written to a section of Richard Feynman's lecture There's Plenty of Room at the Bottom (with text markup). (Source: TU Delft/Ottelab)

STM scan (96 nm wide, 126 nm tall) of the 1 kB memory, written to a section of Richard Feynman’s lecture There’s Plenty of Room at the Bottom (with text markup). (Source: TU Delft/Ottelab)

The researchers organized their memory in blocks of 8 bytes. Each block has a marker, made of the same type of ‘holes’ as the raster of chlorine atoms, which carry information about the precise location of the block on the copper layer. The code will also indicate if a block is damaged, for instance due to some local contaminant or an error in the surface.

While the team believes their approach would allow the memory to be scaled up easily to very big sizes even if the copper surface is not entirely perfect, Otte cautions, “In its current form the memory can operate only in very clean vacuum conditions and at liquid nitrogen temperature (77 K), so the actual storage of data on an atomic scale is still some way off. But through this achievement we have certainly come a big step closer.”

Nanowires from bacteria

Scientists at the University of Massachusetts Amherst genetically designed a new strain of bacteria that spins out extremely thin and highly conductive wires made up solely of non-toxic amino acids.

This work began a decade ago with the discovery that Geobacter, a common soil microorganism, could produce “microbial nanowires,” electrically conductive protein filaments that help the microbe grow on the iron minerals abundant in soil. These microbial nanowires were conductive enough to meet the bacterium’s needs, but their conductivity was well below the conductivities of organic wires that chemists could synthesize.

“As we learned more about how the microbial nanowires worked we realized that it might be possible to improve on Nature’s design,” said Derek Lovley, professor of microbiology at Amherst. “We knew that one class of amino acids was important for the conductivity, so we rearranged these amino acids to produce a synthetic nanowire that we thought might be more conductive.”

Synthetic biowire making an electrical connection between two electrodes. (Source: UMass Amherst)

Synthetic biowire making an electrical connection between two electrodes. (Source: UMass Amherst)

The results greatly exceeded the scientists’ expectations. They genetically engineered a strain of Geobacter and manufactured large quantities of the synthetic nanowires, or ‘biowires’, which were 2000 times more conductive than the natural biological product with a diameter of just 1.5 nanometers. The conductivity of the biowire exceeds that of many types of chemically produced organic nanowires with similar diameters.

“Geobacter can be grown on cheap renewable organic feedstocks so it is a very ‘green’ process,” said Lovley. And although the biowire is made out of protein, it is extremely durable. In fact, the lab had to work for months to establish a method to break it down.

The researchers recently produced more than 20 other Geobacter strains, each producing a distinct biowire variant with new amino acid combinations. Proposed applications include biocompatible sensors, computing devices, and components of solar panels.

Estuary power

Researchers at EPFL’s Laboratory of Nanoscale Biology developed an osmotic power generation system that could provide higher than ever yields.

For the past several years, scientists around the world have been developing systems that leverage osmotic power to create electricity. Pilot projects have sprung up in places such as Norway, the Netherlands, Japan, and the United States to generate energy at estuaries, where rivers flow into the sea. For now, the membranes used in most systems are organic and fragile, and deliver low yields.

The concept behind osmotic power is fairly simple. A semipermeable membrane separates two fluids with different salt concentrations. Salt ions travel through the membrane until the salt concentrations in the two fluids reach equilibrium.

If the system is used with seawater and fresh water, salt ions in the seawater pass through the membrane into the fresh water until both fluids have the same salt concentration. And since an ion is simply an atom with an electrical charge, the movement of the salt ions can be harnessed to generate electricity.

EPFL’s system consists of two liquid-filled compartments separated by a thin membrane made of molybdenum disulfide. The membrane has a tiny hole, or nanopore, through which seawater ions pass into the fresh water until the two fluids’ salt concentrations are equal. As the ions pass through the nanopore, their electrons are transferred to an electrode – which is what is used to generate an electric current.

The membrane allows positively-charged ions to pass through, while pushing away most of the negatively-charged ones. That creates voltage between the two liquids as one builds up a positive charge and the other a negative charge. This voltage is what causes the current generated by the transfer of ions to flow.

“We had to first fabricate and then investigate the optimal size of the nanopore. If it’s too big, negative ions can pass through and the resulting voltage would be too low. If it’s too small, not enough ions can pass through and the current would be too weak,” said Jiandong Feng, lead author of the research.

In these types of systems, the current increases with a thinner membrane. EPFL’s advance is a membrane just three atoms thick. Plus, molybdenum disulfide is ideal for generating an osmotic current.

According to their calculations, a 1m² membrane with 30% of its surface covered by nanopores should be able to produce 1MW of electricity – or enough to power 50,000 standard energy-saving light bulbs. And since molybdenum disulfide (MoS2) is easily found in nature or can be grown by chemical vapor deposition, the system could feasibly be ramped up for large-scale power generation. The major challenge in scaling-up this process is finding out how to make relatively uniform pores.

Once systems become more robust, osmotic power could play a major role in the generation of renewable energy – provided there’s an estuary nearby.

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