For the first time, Northwestern University engineers have created a double layer of atomically flat borophene, a feat that defies the natural tendency of boron to form non-planar clusters beyond the single-atomic-layer limit.
Although known for its promising electronic properties, borophene—a single-atom-layer-thick sheet of boron—is challenging to synthesize. Unlike its analog two-dimensional material graphene, which can be peeled away from innately layered graphite using something as simple as scotch tape, borophene cannot merely be peeled away from bulk boron. Instead, borophene must be grown directly onto a substrate.
And if growing one layer was difficult, growing multiple layers of atomically flat borophene seemed impossible. Because bulk boron is not layered like graphite, growing boron beyond single atomic layers leads to clustering rather than planar films.
"When you try to grow a thicker layer, the boron wants to adopt its bulk structure," said Northwestern's Mark C. Hersam, co-senior author of the study. "Rather than remaining atomically flat, thicker boron films form particles and clusters. The key was to find growth conditions that prevented the clusters from forming. Until now, we didn't think you could go beyond one layer. Now we have moved into unexplored territory between the single atomic layer and the bulk, resulting in a new playground for discovery."
The research will be published Aug. 26 in the journal Nature Materials.
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