By Josh Perry, Editor
[email protected]

Researchers at Rice University (Houston, Texas) and Northwestern University (Evanston, Ill.) have discovered that different lattice arrangements of borophene, the 2-D form of boron, can be combined into new and interesting crystal-like formations with properties that could benefit electronics manufacturers.

Scanning tunneling electron microscope images of line defects in 1-to-6 and 1-to-5 borophene, indicated by blue and red arrowheads, respectively, show how the defects align in a way that preserves the synthetic material’s metallic nature. (Hersam Research Group)

According to a report from Rice, borophene is a synthetic 2-D material that forms from atoms linked in triangles, although occasionally the atoms go missing and leave hexagonal vacancies. This study focused on borophene, formed on a silver substrate through atomic boron deposition in a vacuum, with vacancies in one of every five triangles and one per every six in the lattice.

“The lab found that at temperatures between 440 and 470 degrees Celsius (824-878 degrees Fahrenheit), both 1-to-5 and 1-to-6 phases grew simultaneously on the silver substrate, which acts as a template that guides the deposition of atoms into aligned phases,” the article explained. “The lab’s interest was heightened by what happened where these domains met. Unlike what they had observed in graphene, the atoms easily accommodated each other at the boundaries and adopted the structures of their neighbors.”

This meant more exotic combinations of borophene, including ratios of 4-to-21 and 7-to-36. Unlike graphene, these combinations did not produce disorganized materials but rather formed perfect structures and seamless alignment.

“This implied that unlike insulating defects in otherwise metallic graphene, they have minimal impact on the material’s electronic properties at room temperature,” the article added. “At low temperature, the material shows evidence of a charge density wave, a highly ordered flow of electrons.”

Calculations also suggest that there are slight difference in stiffness, thermal conductivity, and electrochemical properties, which imply that borophene can be tuned for specific applications.

The research was recently published in Nature Materials. The abstract stated:

“Two-dimensional (2D) boron (that is, borophene) was recently synthesized following theoretical predictions. Its metallic nature and high in-plane anisotropy combine many of the desirable attributes of graphene and monolayer black phosphorus.

“As a synthetic 2D material, its structural properties cannot be deduced from bulk boron, which implies that the intrinsic defects of borophene remain unexplored. Here we investigate borophene line defects at the atomic scale with ultrahigh vacuum (UHV) scanning tunnelling microscopy/spectroscopy (STM/STS) and density functional theory (DFT).

“Under suitable growth conditions, borophene phases that correspond to the v_1/6 and v_1/5 models are found to intermix and accommodate line defects in each other with structures that match the constituent units of the other phase. These line defects energetically favour spatially periodic self-assembly that gives rise to new borophene phases, which ultimately blurs the distinction between borophene crystals and defects.

“This phenomenon is unique to borophene as a result of its high in-plane anisotropy and energetically and structurally similar polymorphs. Low-temperature measurements further reveal subtle electronic features that are consistent with a charge density wave (CDW), which are modulated by line defects.

“This atomic-level understanding is likely to inform ongoing efforts to devise and realize applications based on borophene.”