FEB 19, 2014 12:00 AM PST

Thin Boron-Based Films: Potential Competitors For Graphene

WRITTEN BY: Jen Ellis
Graphene has been studied extensively for its potential use in microelectronic applications. Among other things, it's incredibly conductive in a very thin film, and thus could make an excellent material for transparent conducting applications such as touch screen displays. The structure of graphene is a single atomic layer of carbon in a classic six-sided honeycomb pattern - essentially the graphite used in ordinary pencil lead shaved down to a single layer of atoms.

Chemists and engineers have wondered if the same sort of structure could be developed using boron, which occupies the space one step below carbon on the periodic table. A boron-based lattice is predicted to be fully metallic in nature and could possibly be an even better conductor than graphene, which has semi-conducting characteristics. However, since boron has one less electron in its outer shell compared to carbon, the six-atom honeycomb configuration is not possible. To form an atomic layer, a significantly different structure would have to be formed.

Researchers at Brown University have recently shown that such a configuration is possible, and that, at least theoretically, a "borophene" molecule can be synthesized. Their work was summarized in the recent issue of the journal Nature Communications.

Scientists have theorized that a borophene structure would likely be made up of 36 boron atoms in a series of triangular lattices (a B36 cluster), collectively forming a hexagon with a smaller hexagonal hole in the middle. These patterns can in turn fit together in a larger-scale honeycomb-shaped 2-dimensional sheet similar to graphene.

The next step was to produce and test the properties of boron clusters. By exposing bulk boron to a laser, the team created a vapor of boron atoms, then used a helium jet to effectively freeze the vapor into small atomic clusters. With a second laser, an electron was dislodged from the structure and then measured through photoelectron spectroscopy to determine its electron binding energy - a measure of how tightly the cluster holds on to electrons. Spectroscopy results revealed the B36 cluster showed an extremely low binding energy compared to other cluster formations of boron, and also gave evidence that the structure was symmetrical.

The leader of the research team at Brown, Lai-Sheng Wang, enlisted the help of a former collaborator, Professor Jun Li from Tsinghua University in Beijing, to perform modeling for confirmation of the findings. Using a supercomputer at the Pacific Northwest National Lab, they were able to model the electron binding spectra of approximately 3000 different boron cluster arrangements. The modeling verified that the planar hexagonal form with a smaller hexagonal hole was a very close match to the spectra obtained in the laboratory testing, suggesting that the proposed B36 cluster is a feasible structure to synthesize.

The next challenge is to come up with a synthesis process to make the material and verify its properties. To compete with graphene, the synthesis will need to be relatively economical, since graphene has a huge head start in development, applications and patents, and prices are likely to fall as demand increases. Even so, it is worth watching the future development of borophene to see if it can overcome these hurdles and find a place in the microelectronics and nanotechnology fields.
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