Beyond exfoliation: a critical assessment of borophene claims from β-boron and a framework for evidence in 2D boron research

The discovery of borophene, a two-dimensional boron polymorph stabilized on metal substrates, has promoted intense efforts to obtain free-standing 2D boron sheets. Since its first epitaxial synthesis on Ag(111), several reports have claimed the formation of borophene sheets through mechanical or liquid-phase “exfoliation” of bulk β-rhombohedral boron. These claims, if correct, would represent a breakthrough in fabricating non-van der Waals bidimensional materials. In this review, we have critically evaluated the available experimental evidence in the published data. In the first part of the article, the structure of β-Boron (β-B) and borophene has been described; the presence and potential role of defects in producing 2D materials via top-down synthesis have been then analyzed. In the second part, we have discussed how the absence of van der Waals gaps in β-B precludes true “exfoliation” and is inconsistent with the theoretical framework. The formation of nanostructures upon “exfoliation” is better explained as defect-mediated cleavage formation of nanoscale intrinsic van der Waals gaps boron fragments, often oxidized, rather than true borophene polymorphs (β12, χ3). Based on this analysis, we propose diagnostic criteria for identifying the phases obtained through top-down synthesis processes starting from β-B. This critical review highlights how to avoid structural assignment errors and outlines best practices for phase identification in the search for 2D materials derived from non-layered solids.