  Here, we have identified the monolayer phase of Bi2O2Se as a promising
two-dimensional semiconductor with ultra-high carrier mobility and giant
electric polarization. Due to the strong reconstruction originated from the
interlayer electrostatic force, we have applied structure prediction algorithms
to explore the crystalline geometry of Bi2O2Se monolayer with the lowest total
energy. Considering Se and Te belong to the same group, Bi2O2Te monolayer is
also investigated based on a similar scheme. Further calculations suggest that
the high carrier mobility is maintained in the monolayer phase and the moderate
band gap will lead to the strong optical absorption in the visible light
region. In particular, the electron mobility in Bi2O2Te can reach as high as
3610 cm2V-1s-1 at room temperature, which is almost ten times of conventional
transition metal dichalcogenides (TMD) family. Because of the strong structural
anisotropy, a remarkable spontaneous in-plane and out-of-plane electric
polarization is additionally revealed along with significant piezoelectric
properties, endowing them as promising candidates in the area of photovoltaic
solar cells, optoelectronic materials and field effect transistors.
