Researchers at the U.S. Department of Energy’s Ames Laboratory have discovered a new type of Weyl semimetal, a material that opens the way for further study of Weyl fermions, a type of massless elementary particle hypothesized by high-energy particle theory and potentially useful for creating high-speed electronic circuits and quantum computers (Nature Materials, “Spectroscopic evidence for a type II Weyl semimetallic state in MoTe2”).

Researchers created a crystal of molybdenum and tellurium, one of only a few compounds that had been predicted to host a new and recently postulated type of Weyl state, where the hole and electron bands normally separated by an indirect gap touch at a few Weyl points. Those points are equivalent to magnetic monopoles in the momentum space and are connected by Fermi arcs.

A combination of angle resolved photoemission spectroscopy (ARPES), modelling, density functional theory and careful calculations were used to confirm the existence of this new type of Weyl semimetal.

This material provides an exciting new platform to study the properties of Weyl fermions, and may lead the way to more new materials with unusual transport properties.

“This an important, interdisciplinary discovery because it allows us to study many aspects of these exotic particles predicted by high energy physics theory in solid state, without need for extremely expensive particle accelerators,” said Adam Kaminsky, Ames Laboratory scientist and professor in the Department of Physics and Astronomy at Iowa State University. “From my perspective as solid state physicist it is absolutely extraordinary to observe two bands touching each other at certain points and being connected by Fermi arcs – objects that are prohibited to exist in “ordinary” materials.”

(Source:  www.nanowerk.com; Aug 15, 2016; http://tinyurl.com/z34tl6q)

A powerful new material developed by Northwestern University chemist William Dichtel and his research team could one day speed up the charging process of electric cars and help increase their driving range. An electric car currently relies on a complex interplay of both batteries and supercapacitors to provide the energy it needs to go places, but that could change.

“Our material combines the best of both worlds – the ability to store large amounts of electrical energy or charge, like a battery, and the ability to charge and discharge rapidly, like a supercapacitor,” said Dichtel, a pioneer in the young research field of covalent organic frameworks (COFs).

Dichtel and his research team have combined a COF – a strong, stiff polymer with an abundance of tiny pores suitable for storing energy – with a very conductive material to create the first modified redox-active COF that closes the gap with other older porous carbon-based electrodes.

“COFs are beautiful structures with a lot of promise, but their conductivity is limited,” Dichtel said. “That’s the problem we are addressing here. By modifying them – by adding the attribute they lack – we can start to use COFs in a practical way.”

And modified COFs are commercially attractive: COFs are made of inexpensive, readily available materials, while carbon-based materials are expensive to process and mass-produce.

Dichtel, the Robert L. Letsinger Professor of Chemistry at the Weinberg College of Arts and Sciences, is presenting his team’s findings today (Aug. 24) at the American Chemical Society (ACS) National Meeting in Philadelphia. Also today, a paper by Dichtel and co-authors from Northwestern and Cornell University was published by the journal ACS Central Science.

To demonstrate the new material’s capabilities, the researchers built a coin-cell battery prototype device capable of powering a light-emitting diode for 30 seconds.

The material has outstanding stability, capable of 10,000 charge/discharge cycles, the researchers report. They also performed extensive additional experiments to understand how the COF and the conducting polymer, called poly(3,4-ethylenedioxythiophene) or PEDOT, work together to store electrical energy.

Dichtel and his team made the material on an electrode surface. Two organic molecules self-assembled and condensed into a honeycomb-like grid, one 2-D layer stacked on top of the other. Into the grid’s holes, or pores, the researchers deposited the conducting polymer.

Each pore is only 2.3 nanometers wide, but the COF is full of these useful pores, creating a lot of surface area in a very small space. A small amount of the fluffy COF powder, just enough to fill a shot glass and weighing the same as a dollar bill, has the surface area of an Olympic swimming pool.

The modified COF showed a dramatic improvement in its ability to both store energy and to rapidly charge and discharge the device. The material can store roughly 10 times more electrical energy than the unmodified COF, and it can get the electrical charge in and out of the device 10 to 15 times faster.

“It was pretty amazing to see this performance gain,” Dichtel said. “This research will guide us as we investigate other modified COFs and work to find the best materials for creating new electrical energy storage devices.”

Journal reference:

“Superior Charge Storage and Power Density of a Conducting Polymer-Modified Covalent Organic Framework.” In addition to Dichtel, other authors are Ryan P. Bisbey, of Northwestern; Catherine R. Mulzer (nee DeBlase, first author), currently at Dow Electronic Materials; and Luxi Shen, James R. McKone, Na Zhang and Hector D. Abruna, of Cornell.

(Source:  www.energy-daily.com; Aug 26, 2016; http://tinyurl.com/guk3v52)