Debunking top-down borophene: a multimodal reassessment of boron sonication and milling

The rapid growth of reports claiming the top-down synthesis of free-standing borophene from boron has generated substantial ambiguity regarding the true nature of the resulting materials. Here, representative sonication- and milling-based processing routes were investigated through multimodal structural, chemical, and optical characterization. Boron powders were treated by bath sonication in water, hydrogen peroxide, dimethylformamide, and isopropanol, as well as by ball milling under air and argon atmospheres. In all cases, no evidence of known borophene polymorphs was observed. Instead, the treatments produced fragmented β‑boron grains coated by defect-rich B_xO_y/B₂O₃ and boric-acid-derived surface species. Sonication promotes oxidation and hydrolysis, while ball milling induces partial amorphization and ultrathin oxide-layer formation. The observed photoluminescence originates from defect states in oxidized boron networks rather than from quantum confinement in putative borophene nanosheets. Processing in dimethylformamide can also induce solvent carbonization, potentially leading to misassignment of carbonaceous fluorescent species as boron nanostructures. These findings challenge current interpretations of top-down borophene synthesis and establish experimentally grounded criteria for distinguishing borophene from defect-mediated β‑boron-derived nanostructures.