Ball milling induced borophene flakes fabrication.

To fill the knowledge gap for borophene, as the youngest member of the two-dimensional (2D) nanomaterials family, a facile, cost effective, scalable and reproducible fabrication route is still strongly required. Among so far studied techniques the potential of pure mechanical processes such as ball milling is not explored yet. Therefore, in this contribution, we explore the efficiency to exfoliate bulk boron into a few-layered borophene induced by mechanical energy in the planetary ball mill. It was revealed that the resulting flakes thickness and distribution can be controlled by (i) rotation speed (250-650 rpm), (ii) time of ball-milling (1-12 hours), and mass loading of bulk boron (1-3 g). Furthermore, the optimal conditions for the ball-milling process to induce efficient mechanical exfoliation of boron were determined to be 450 rpm, 6 hours, and 1 g (450 rpm_6 h_1 g), which resulted in the fabrication of regular and thin few-layered borophene flakes (∼5.5 nm). What is more, the mechanical energy induced during ball-milling, and the heat generated inside, affected the structure of borophene resulting in different crystalline phases. Besides being an additional and interesting discovery, it will also open up opportunities to investigate the relevance between the properties and the emerging phase. Structures labeled as β-rhombohedral, γ-orthorhombic, τ-B and the conditions under which they appear, have been described. Therefore, in our study, we open a new door to obtain a bulk quantity of few-layered borophene for further fundamental studies and practical potential assessment.