Breakdown of the electron delocalization in hexagonal borophene toward tunable energy gap

The absence of energy gap (E-g) in borophene polymorphs, where multi-center bonds favor electron delocalization and form metallic phases, restricts their potential applications in microelectronic and optoelectronic industry. Herein, we propose to incorporate chalcogenide atoms to breakdown the electron delocalization in hexagonal borophene and introduce an E-g. Based on first principle calculation, we report a group of semi-conducting boron chalcogenide monolayers, denoted as B2X (X = O, Se and Te), with an indirect E-g of 2.84, 0.97 and 1.04 eV under HSE06 levels, respectively. Moreover, the introduced E-g can be modulated by external strains (+/- 6%) affordable by substrates, with electronic effective mass being comparable or even lower than those of MoS2 along certain directions. Furthermore, B2X monolayers exhibit desirable light absorption properties under visible light, which can be improved by stacking bilayers. The current study presents an alternative route to introduce the E-g in borophene, endows them with new vitality and highlights their promise in flexible electronics, optoelectronics and photovoltaic devices.