Borophene, a monoatomic layer of boron atoms, stands out among two-dimensional (2D) materials, with its versatile properties of polymorphism, metallicity, plasmonics, superconductivity, tantalizing for physics exploration and next-generation devices. Yet its phases are all synthesized on and stay bound to metal substrates, hampering both characterization and use. The growth on the inert insulator would allow post-synthesis exfoliation of borophene, but its weak adhesion to such substrate results in a very high 2D-nucleation barrier preventing clean borophene growth. This challenge can be circumvented in a devised and demonstrated here, with ab initio calculations, strategy. Naturally present 1D-defects, the step-edges on h-BN substrate surface, enable boron epitaxial assembly, reduce the nucleation dimensionality and lower the barrier by an order of magnitude (to 1.1 eV or less), yielding v1/9 phase. Weak borophene adhesion to the insulator makes it readily accessible for comprehensive property tests or transfer into the device setting.
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