System Bits: May 9

Graphene adopts exotic electronic states
In a platform that may be used to explore avenues for quantum computing, MIT researchers have found that a flake of graphene, when brought in close proximity with two superconducting materials, can inherit some of those materials’ superconducting qualities.

They reminded that in normal conductive materials such as silver and copper, electric current flows with varying degrees of resistance, in the form of individual electrons that ping-pong off defects, dissipating energy as they go. Superconductors, by contrast, are named for their ability to conduct electricity without resistance, by means of electrons that pair up and move through a material as one, generating no friction.

In the new development, the team explained that as graphene is sandwiched between superconductors, its electronic state changes dramatically, even at its center. The researchers found that graphene’s electrons, formerly behaving as individual, scattering particles, instead pair up in “Andreev states” — a fundamental electronic configuration that allows a conventional, nonsuperconducting material to carry a “supercurrent,” an electric current that flows without dissipating energy. This is believed to be the first investigation of Andreev states due to superconductivity’s “proximity effect” in a two-dimensional material such as graphene.

MIT physicists have found that a flake of graphene, when brought in close proximity with two superconducting materials, can inherit some of those materials’ superconducting qualities. As graphene is sandwiched between superconductors, its electronic state changes dramatically, even at its center. Pictured is the experimental concept and device schematic.
(Source: MIT)

Down the road, the researchers said this graphene platform might be used to explore exotic particles, such as Majorana fermions, which are thought to arise from Andreev states and may be key particles for building powerful, error-proof quantum computers.

Landry Bretheau, a postdoc in MIT’s Department of Physics, and lead author on a new paper on this subject said, “There is a huge effort in the condensed physics community to look for exotic quantum electronic states. In particular, new particles called Majorana fermions are predicted to emerge in graphene that is connected to superconducting electrodes and exposed to large magnetic fields. Our experiment is promising, as we are unifying some of these ingredients.”

Flexible, organic, biodegradable electronics
With the potential for diverse medical and environmental applications without adding to the mounting pile of global electronic waste, Stanford University researchers have developed a new semiconductor as flexible as skin and easily degradable.

A newly developed flexible, biodegradable semiconductor developed by Stanford engineers shown on a human hair. (Source: Stanford University)

Troubled by the mounting electronic waste in the world — estimated by a United Nations Environment Program report to be almost 50 million tons in 2017 — Stanford engineer Zhenan Bao and her team are rethinking electronics.

“In my group, we have been trying to mimic the function of human skin to think about how to develop future electronic devices,” Bao said. She described how skin is stretchable, self-healable and also biodegradable – an attractive list of characteristics for electronics. “We have achieved the first two [flexible and self-healing], so the biodegradability was something we wanted to tackle.”

The team created a flexible electronic device that can easily degrade just by adding a weak acid like vinegar. This is the first example of a semiconductive polymer that can decompose.

In addition to the polymer – essentially a flexible, conductive plastic – the team developed a degradable electronic circuit and a new biodegradable substrate material for mounting the electrical components. This substrate supports the electrical components, flexing and molding to rough and smooth surfaces alike. When the electronic device is no longer needed, the whole thing can biodegrade into nontoxic components.

Making graphene from ethene
With the potential to open new applications for graphene, researchers from the Georgia Institute of Technology, Technische Universität München in Germany, and the University of St. Andrews in Scotland have developed a new way to produce single-layer graphene from a simple precursor: ethene – also known as ethylene – the smallest alkene molecule, which contains just two atoms of carbon. 

Measured and theoretically simulated images of stages in the dehydrogenation process observed in programmed surface heating experiments. The sequence starts from adsorbed ethene (at 300K), leading to self-evolved 24-carbon-atom cluster precursors (between 570K and 670 K), and culminates with graphene formed at elevated temperatures (between 770K and 970K). (Source: Georgia Tech)

By heating the ethene in stages to a temperature of slightly more than 700 degrees Celsius — hotter than had been attempted before -– the team produced pure layers of graphene on a rhodium catalyst substrate. The stepwise heating and higher temperature overcame challenges seen in earlier efforts to produce graphene directly from hydrocarbon precursors. 

And due to its lower cost and simplicity, the technique could open new potential applications for graphene, which has attractive physical and electronic properties. The work also provides a novel mechanism for the self-evolution of carbon cluster precursors whose diffusional coalescence results in the formation of the graphene layers.