Power/Performance Bits: Jan. 31

Microbial nanowires

Microbiologists at the University of Massachusetts Amherst report that they have discovered a new type of microbial nanowire, the protein filaments that bacteria use to make electrical connections with other microbes or minerals.

The team was motivated by the potential for improved “green” conducting materials for electronics.

According to Derek Lovley, professor of microbiology at Amhurst, “Microbial nanowires are a revolutionary electronic material with substantial advantages over man-made materials. Chemically synthesizing nanowires in the lab requires toxic chemicals, high temperatures and/or expensive metals. The energy requirements are enormous. By contrast, natural microbial nanowires can be mass-produced at room temperature from inexpensive renewable feedstocks in bioreactors with much lower energy inputs. And the final product is free of toxic components.”

Until now the lab’s focus has been the nanowires of just one bacterium, Geobacter sulfurreducens. However, when the lab began looking at the protein filaments of other Geobacter species, they found a wide range in conductivities. For example, one species recovered from uranium-contaminated soil produced poorly conductive filaments. However, another species, Geobacter metallireducens produced nanowires 5,000 times more conductive than the G. sulfurreducens wires.

Geobacter (red) expressing electrically conductive nanowires. Such natural nanowires can be mass produced from inexpensive, renewable feedstocks with low energy costs compared to chemical synthesis with toxic chemicals and high energy requirements. (Source: UMass Amherst)

The team did not study the G. metallireducens strain directly. Instead, they took the gene for the protein that assembles into microbial nanowires from it and inserted this into G. sulfurreducens. The result is a genetically modified G. sulfurreducens that expresses the G. metallireducens protein, making nanowires much more conductive than G. sulfurreducens would naturally produce.

The researchers attribute G. metallireducens nanowires’ extraordinarily high conductivity to its greater abundance of aromatic amino acids. Closely packed aromatic rings appear to be a key component of microbial nanowire conductivity, and more aromatic rings probably means better connections for electron transfer along the protein filaments.

The high conductivity of the G. metallireducens nanowires suggests that they may be an attractive material for the construction of conductive materials, electronic devices and sensors for medical or environmental applications. The team says discovering more about the mechanisms of nanowire conductivity “provides important insight into how we might make even better wires with genes that we design ourselves.”

Conducting electricity without heat

According to a new study by scientists at the Department of Energy’s Lawrence Berkeley National Laboratory, the University of California, Berkeley, Oak Ridge National Laboratory, and Duke University, electrons in vanadium dioxide can conduct electricity without conducting heat.

In investigating the material, the researchers found that the thermal conductivity attributed to the electrons is ten times smaller than what would be expected from the Wiedemann-Franz Law, which governs the relationship between electrical and thermal conductivity.

“This was a totally unexpected finding,” said Junqiao Wu, a physicist at Berkeley Lab and a UC Berkeley professor of materials science and engineering. “It shows a drastic breakdown of a textbook law that has been known to be robust for conventional conductors. This discovery is of fundamental importance for understanding the basic electronic behavior of novel conductors.”

Vanadium dioxide (VO2) nanobeams synthesized by Berkeley researchers show exotic electrical and thermal properties. In this false-color scanning electron microscopy image, thermal conductivity was measured by transporting heat from the suspended heat source pad (red) to the sensing pad (blue). The pads are bridged by a VO2 nanobeam. (Source: Junqiao Wu/Berkeley Lab)

Metallic vanadium dioxide was already noted for its unusual ability to switch from an insulator to a metal when it reaches 67 degrees Celsius, or 152 degrees Fahrenheit. However, the amount of electricity and heat that vanadium dioxide can conduct is tunable by mixing it with other materials.

When the researchers doped single crystal vanadium dioxide samples with the metal tungsten, they lowered the phase transition temperature at which vanadium dioxide becomes metallic. At the same time, the electrons in the metallic phase became better heat conductors. This enabled the researchers to control the amount of heat that vanadium dioxide can dissipate by switching its phase from insulator to metal and vice versa, at tunable temperatures.

While there are a handful of other materials besides vanadium dioxide that can conduct electricity better than heat, those occur at temperatures hundreds of degrees below zero, making it challenging to develop into real-world applications.

Vanadium dioxide has the added benefit of being transparent below about 30 degrees Celsius (86 degrees Fahrenheit), and absorptive of infrared light above 60 degrees Celsius (140 degrees Fahrenheit).

While there are more questions that need to be answered before vanadium dioxide can be commercialized, the researchers expect it could be used to help scavenge or dissipate the heat in engines, or be developed into a window coating that improves the efficient use of energy in buildings.

New type of solar cell

Researchers from the University of Göttingen, Deutsches Elektronen-Synchrotron (DESY), the Max Planck Institute for Biophysical Chemistry, and the Technical University of Clausthal-Zellerfeld proposed the foundations for an entirely new type of photovoltaic cell. The mechanism behind the new perovskite solid-state solar cell relies on so-called polaron excitations, which combine the excitation of electrons and vibrations of the crystal lattice.

“In conventional solar cells, the interaction between the electrons and the lattice vibrations can lead to unwanted losses, causing substantial problems, whereas the polaron excitations in the perovskite solar cell can be created with a fractal structure at certain operating temperatures and last long enough for a pronounced photovoltaic effect to occur,” explained Dirk Raiser, of the Max Planck Institute for Biophysical Chemistry in Göttingen and DESY.

An experimental polaron solar cell in the lab. (Source: Dirk Raiser, MPIbC/DESY)

In order for the effect to take place, the perovskite solar cells had to be cooled in the laboratory to around minus 35 degrees Celsius. If this effect is to be used in practical applications, it will be necessary to produce ordered polaron states at higher temperatures.

“The measurements so far were made in a carefully characterised reference material, in order to demonstrate the principle of the effect. For this purpose, the low transition temperature was accepted,” said Prof. Simone Techert, Leading Scientist at DESY.

Material physicists at Göttingen are trying to modify and optimize the material in order to achieve a higher operating temperature. Another avenue the researchers are exploring is the use of additional light to produce the excitation.

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