  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.
