  Monolayer group-III monochalcogenides (MX, M = Ga, In; X = S, Se, Te), an
emerging category of two-dimensional (2D) semiconductors, hold great promise
for electronics, optoelectronics and catalysts. By first-principles
calculations, we show that the phonon dispersion and Raman spectra as well as
the electronic and topological properties of monolayer MX can be tuned by
oxygen functionalization. Chemisorption of oxygen atoms on one side or both
sides of the MX sheet narrows or even closes the band gap, enlarges work
function, and significantly reduces the carrier effective mass. More
excitingly, InS, InSe and InTe monolayers with double-side oxygen
functionalization are 2D topological insulators with sizeable bulk gap up to
0.21 eV. Their low-energy bands near the Fermi level are dominated by the px
and py orbitals of atoms, allowing band engineering via in-plane strains. Our
studies provide viable strategy for realizing quantum spin Hall effect in
monolayer group-III monochalcogenides at room temperature, and utilizing these
novel 2D materials for high-speed and dissipationless transport devices.
