Power/Performance Bits: Jan. 3

Paper-based bacteria battery

Researchers at Binghamton University, State University of New York have created a bacteria-powered battery on a single sheet of paper that can power disposable electronics. The manufacturing technique reduces fabrication time and cost, and the design could revolutionize the use of bio-batteries as a power source in remote, dangerous and resource-limited areas.

“Papertronics have recently emerged as a simple and low-cost way to power disposable point-of-care diagnostic sensors,” said Seokheun “Sean” Choi, assistant professor in the electrical and computer engineering department at Binghamton. “Stand-alone and self-sustained, paper-based, point-of-care devices are essential to providing effective and life-saving treatments in resource-limited settings.”

On one half of a piece of chromatography paper, the team placed a ribbon of silver nitrate underneath a thin layer of wax to create a cathode. The pair then made a reservoir out of a conductive polymer on the other half of the paper, which acted as the anode. Once properly folded and a few drops of bacteria-filled liquid are added, the microbes’ cellular respiration powers the battery.

The foldable, bacteria-powered battery. (Source: Seokheun “Sean” Choi/ Binghamton)

“The device requires layers to include components, such as the anode, cathode and PEM (proton exchange membrane),” said Choi. “[The final battery] demands manual assembly, and there are potential issues such as misalignment of paper layers and vertical discontinuity between layers, which ultimately decrease power generation.”

Different folding and stacking methods can significantly improve power and current outputs. Scientists were able to generate 31.51 microwatts at 125.53 microamps with six batteries in three parallel series and 44.85 microwatts at 105.89 microamps in a 6×6 configuration.

While it would take millions of paper batteries to power a common 40-watt light bulb, there was enough power to run biosensors that monitor glucose levels in diabetes patients or detect pathogens in a body.

Building nanowires with tiny diamonds

Scientists at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory have discovered a way to use diamondoids – the smallest possible bits of diamond – to assemble atoms into the thinnest possible electrical wires, just three atoms wide.

The new technique could potentially be used to build tiny wires for a wide range of applications, including fabrics that generate electricity, optoelectronic devices that employ both electricity and light, and superconducting materials that conduct electricity without any loss.

Although there are other ways to get materials to self-assemble, this is the first one shown to make a nanowire with a solid, crystalline core that has good electronic properties, said Nicholas Melosh, an associate professor at SLAC and Stanford.

The needle-like wires have a semiconducting core – a combination of copper and sulfur known as a chalcogenide – surrounded by the attached diamondoids, which form an insulating shell.

They started with the smallest possible diamondoids – single cages that contain just 10 carbon atoms – and attached a sulfur atom to each. Floating in a solution, each sulfur atom bonded with a single copper ion. This created the basic nanowire building block.

The building blocks then drifted toward each other, drawn by the van der Waals attraction between the diamondoids, and attached to the growing tip of the nanowire.

This animation shows molecular building blocks joining the tip of a growing nanowire. Each block consists of a diamondoid – the smallest possible bit of diamond – attached to sulfur and copper atoms (yellow and brown spheres). Like LEGO blocks, they only fit together in certain ways that are determined by their size and shape. The copper and sulfur atoms form a conductive wire in the middle, and the diamondoids form an insulating outer shell. (Source: SLAC National Accelerator Laboratory)

“Much like LEGO blocks, they only fit together in certain ways that are determined by their size and shape,” said Stanford graduate student Fei Hua Li. “The copper and sulfur atoms of each building block wound up in the middle, forming the conductive core of the wire, and the bulkier diamondoids wound up on the outside, forming the insulating shell.”

The team has already used diamondoids to make one-dimensional nanowires based on cadmium, zinc, iron and silver, including some that grew long enough to see without a microscope. The cadmium-based wires are similar to materials used in optoelectronics, and the zinc-based ones are like those used in solar applications and in piezoelectric energy generators.

New multiferroic material

Scientists at the Tokyo Institute of Technology, Kyushu University, and the Nagoya Institute of Technology demonstrated the multiferroic nature of a thin film of BiFe1-xCoxO3 (BFCO).

Multiferroic materials exhibit both ferromagnetism and ferroelectricity. These are expected to be used as multiple-state memory devices. Furthermore, if the two orders are strongly coupled and the magnetization can be reversed by applying an external electric field, the material should work as a form of low power consumption magnetic memory.

Portions of the BiFeO3 lattice of cycloidal and collinear phases with only Fe ions are shown at left and right, respectively. The arrows indicate the Fe3+ moment direction. The ground state of BiFeO3 had a cycloidal spin structure, which is destabilized by substitution of Co for Fe and at higher temperatures. The spin magnetic moments compensate with each other in the left panel, but canting between neighboring spins leads to the appearance of weak ferromagnetism in the left panel. (Source: Tokyo Institute of Technology)

It was speculated that ferroelectric BFO thin film, a close relative of BFCO, might be ferromagnetic as well, but previous attempts were thwarted by the presence of magnetic impurity. This team successfully synthesized pure, thin films of BFCO by using pulsed laser deposition to perform epitaxial growth on a SrTiO3 (STO) substrate.

They then conducted a series of tests to show that BFCO is both ferroelectric and ferromagnetic at room temperature. They manipulated the direction of ferroelectric polarization by applying an electric field, and showed that the low-temperature cychloidal spin structure, essentially the same as that of BiFeO3, changes to a collinear one with ferromagnetism at room temperature.

In the future, the scientists hope to realize electrical control of ferromagnetism, which could be applied in low power consumption, non-volatile memory devices.