  The two-dimensional boron monolayers were reported to be metallic both in
previous theoretical predictions and experimental observations, however, we
have firstly found a family of boron monolayers with the novel semiconducting
property as confirmed by the first-principles calculations with the
quasi-particle G0W0 approach. We demonstrate that the vanished metallicity
characterized by the pz-derived bands cross the Fermi level is attributed to
the motif of a triple-hexagonal-vacancy, with which various semiconducting
boron monolayers are designed to realize the band-gap engineering for the
potential applications in electronic devices. The semiconducting boron
monolayers in our predictions are expected to be synthesized on the proper
substrates, due to the similar stabilities to the ones observed experimentally.
