Nanoscopic Characterization Reveals that Bulk Amorphous Elementary Boron Is Composed of a Ladder-like Polymer with B₄ as the Structural Unit

As the initially discovered allotrope of boron, amorphous elementary boron (AE-B) has been reported for more than two centuries. Several possible structures of AE-B have been proposed during the past decades. Due to its noncrystalline nature, however, the structure of AE-B has not yet been determined. We notice that AE-B can be dissolved in organic solvents, although the solubility is very low. After surface adsorption from solution, the individual or the self-assembled structure of AE-B molecules can be characterized at the single-molecule or nanoscopic level, which may be helpful to reveal the molecular structure of AE-B. Atomic force microscopy (AFM) imaging shows that AE-B is a chain-like molecule with a thickness (or height) of 0.17 ± 0.01 nm, which agrees well with the diameter of a B atom, demonstrating that the structure of an AE-B molecule contains only one layer of B atoms. Results from high-resolution transmission electron microscopy (HRTEM) indicate that AE-B molecules can be self-assembled into a nanosheet with parallel lines. The width of each line is 0.27 nm, and the periodical length along the chain axial direction is 0.32 ± 0.01 nm. These results indicate that AE-B is composed of a ladder-like inorganic polymer with B 4 as the structural unit. This conclusion is supported by the single-chain elasticity obtained by single-molecule AFM and quantum mechanical calculations. We expect that this fundamental study is not only an ending of the two-century-old scientific mystery but also the beginning of the research and applications of AE-B (ladder B) as a polymeric material. The research strategy may be also used to study other amorphous inorganic materials.