System Bits: April 16

Characterizing 2D borophene
Researchers at Rice and Northwestern universities collaborated on a method to view the polymorphs of 2D borophene crystals, providing insights into the lattice configurations of the two-dimensional material.

Boris Yakobson, a materials physicist at Rice’s Brown School of Engineering, and materials scientist Mark Hersam of Northwestern led a team that not only discovered how to see the nanoscale structures of borophene lattices but also built theoretical models that helped characterize the crystalline forms. Their results are published in Nature Communications.

Borophene differs from graphene in producing the material and takes a different form – an array of hexagons for graphene and a grid of triangles for borophene. Borophene’s polymorphs can have more than one crystal structure, making it harder to characterize than graphene.

Yakobson said there could theoretically be more than 1,000 forms of borophene, each with unique characteristics.

“It has many possible patterns and networks of atoms being connected in the lattice,” he said.

The project started at Hersam’s Northwestern lab, where researchers modified the blunt tip of an atomic force microscope with a sharp tip of carbon and oxygen atoms. That gave them the ability to scan a flake of borophene to sense electrons that correspond to covalent bonds between boron atoms. They used a similarly modified scanning tunneling microscope to find hollow hexagons where a boron atom had gone missing.

Scanning flakes grown on silver substrates under various temperatures via molecular-beam epitaxy showed them a range of crystal structures, as the changing growth conditions altered the lattice.

“Modern microscopy is very sophisticated, but the result is, unfortunately, that the image you get is generally difficult to interpret,” Yakobson said. “That is, it’s hard to say an image corresponds to a particular atomic lattice. It’s far from obvious, but that’s where theory and simulations come in.”

Yakobson’s team used first-principle simulations to determine why borophene took on particular structures based on calculating the interacting energies of both boron and substrate atoms. Their models matched many of the borophene images produced at Northwestern.

“We learned from the simulations that the degree of charge transfer from the metal substrate into borophene is important,” he said. “How much of this is happening, from nothing to a lot, can make a difference.”

The researchers confirmed through their analysis that borophene is also not an epitaxial film. In other words, the atomic arrangement of the substrate doesn’t dictate the arrangement or rotational angle of borophene.

Image credit: Xiaolong Liu/Northwestern University

Looking to the future, Hersam said, “The development of methods to characterize and control the atomic structure of borophene is an important step toward realizing the many proposed applications of this material, which range from flexible electronics to emerging topics in quantum information sciences.”

The Office of Naval Research, the National Science Foundation, the Department of Energy’s Office of Science, and the Northwestern University International Institute for Nanotechnology supported the research.

Preventing lithium-ion battery fires
The University of Illinois Chicago’s College of Engineering looked into ways of keeping lithium-ion batteries from catching fire. Coating the cathode of a lithium-ion battery with graphene sheets turned out to be a useful method of keeping oxygen confined to the cathode, limiting its mixing with other flammable materials. The UIC researchers reported their findings in the Advanced Functional Materials journal.

“We thought that if there was a way to prevent the oxygen from leaving the cathode and mixing with other flammable products in the battery, we could reduce the chances of a fire occurring,” said Reza Shahbazian-Yassar, associate professor of mechanical and industrial engineering in the UIC College of Engineering and corresponding author of the paper.

It turns out that a material Shahbazian-Yassar is very familiar with provided a perfect solution to this problem. That material is graphene — a super-thin layer of carbon atoms with unique properties. Shahbazian-Yassar and his colleagues previously had used graphene to help modulate lithium buildup on electrodes in lithium metal batteries.

Shahbazian-Yassar and his colleagues knew that graphene sheets are impermeable to oxygen atoms. Graphene is also strong, flexible and can be made to be electrically conductive. Shahbazian-Yassar and Soroosh Sharifi-Asl, a graduate student in mechanical and industrial engineering at UIC and lead author of the paper, thought that if they wrapped very small particles of the lithium cobalt oxide cathode of a lithium battery in graphene, it might prevent oxygen from escaping.

First, the researchers chemically altered the graphene to make it electrically conductive. Next, they wrapped the tiny particles of lithium cobalt oxide cathode electrode in the conductive graphene.

When they looked at the graphene-wrapped lithium cobalt oxide particles using electron microscopy, they saw that the release of oxygen under high heat was reduced significantly compared with unwrapped particles.
Next, they bound together the wrapped particles with a binding material to form a usable cathode, and incorporated it into a lithium metal battery. When they measured released oxygen during battery cycling, they saw almost no oxygen escaping from cathodes even at very high voltages. The lithium metal battery continued to perform well even after 200 cycles.

“The wrapped cathode battery lost only about 14% of its capacity after rapid cycling compared to a conventional lithium metal battery where performance was down about 45% under the same conditions,” Sharifi-Asl said.

“Graphene is the ideal material for blocking the release of oxygen into the electrolyte,” Shahbazian-Yassar said. “It is impermeable to oxygen, electrically conductive, flexible, and is strong enough to withstand conditions within the battery. It is only a few nanometers thick so there would be no extra mass added to the battery. Our research shows that its use in the cathode can reliably reduce the release of oxygen and could be one way that the risk for fire in these batteries — which power everything from our phones to our cars — could be significantly reduced.”

This research was supported in part by the National Science Foundation.

More sensitive measurement of semiconductor materials
Microscale, nanoscale, and 2D semiconductor materials must have their quality measured in order to determine their suitability for electronic devices. The sensitivity of those measurements must be enhanced to identify the properties of such materials.

Daniel Wasserman, an associate professor in the Department of Electrical and Computer Engineering in the Cockrell School of Engineering, led the team that built the physical system, developed the measurement technique capable of achieving this level of sensitivity and successfully demonstrated its improved performance. Their work was reported in Nature Communications.

The team’s design approach was focused on developing the capability to provide quantitative feedback on material quality, with particular applications for the development and manufacturing of optoelectronic devices. The method demonstrated is capable of measuring many of the materials that engineers believe will one day be ubiquitous to next-generation optoelectronic devices.

Optoelectronics is the study and application of electronic devices that can source, detect and control light. Optoelectronic devices that detect light, known as photodetectors, use materials that generate electrical signals from light. Photodetectors are found in smartphone cameras, solar cells, and in the fiber-optic communication systems that make up our broadband networks. In an optoelectronic material, the amount of time that the electrons remain “photoexcited,” or capable of producing an electrical signal, is a reliable indicator of the potential quality of that material for photodetection applications.

The current method used for measuring the carrier dynamics, or lifetimes, of photoexcited electrons is costly and complex and only measures large-scale material samples with limited accuracy. The University of Texas at Austin team decided to try using a different method for quantifying these lifetimes by placing small volumes of the materials in specially designed microwave resonator circuits. Samples are exposed to concentrated microwave fields while inside the resonator. When the sample is hit with light, the microwave circuit signal changes, and the change in the circuit can be read out on a standard oscilloscope. The decay of the microwave signal indicates the lifetimes of photoexcited charge carriers in small volumes of the material placed in the circuit.

“Measuring the decay of the electrical (microwave) signal allows us to measure the materials’ carrier lifetime with far greater accuracy,” Wasserman said. “We have discovered it to be a simpler, cheaper and more effective method than current approaches.”

Carrier lifetime is a critical material parameter that provides insight into the overall optical quality of a material while also determining the range of applications for which a material could be used when it’s integrated into a photodetector device structure. For example, materials that have a very long carrier lifetime may be of high optical quality and therefore very sensitive, but may not be useful for applications that require high-speed.

“Despite the importance of carrier lifetime, there are not many, if any, contact-free options for characterizing small-area materials such as infrared pixels or 2D materials, which have gained popularity and technological importance in recent years,” Wasserman said.

One area certain to benefit from the real-world applications of this technology is infrared detection, a vital component in molecular sensing, thermal imaging and certain defense and security systems.

“A better understanding of infrared materials could lead to innovations in night-vision goggles or infrared spectroscopy and sensing systems,” Wasserman said.

High-speed detectors operating at these frequencies could even enable the development of free-space communication in the long wavelength infrared – a technology allowing for wireless communication in difficult conditions, in space or between buildings in urban environments.

The research was funded by Air Force Research Laboratories and is part of an ongoing collaboration between Wasserman and his Mid-IR Photonics Group at UT, close collaborators at Eglin Air Force Base, and researchers from The Ohio State University, the University of Wisconsin, and Sandia National Laboratories.