  We carried out first-principles calculations to investigate the electronic
properties of the monolayer blue phosphorene (BlueP) decorated by the group-IVB
transition-metal adatoms (Cr, Mo and W), and found that the Cr-decorated BlueP
is a magnetic half metal, while the Mo- and W-decorated BlueP are
semiconductors with band gaps smaller than 0.2 eV. Compressive strains make the
band gaps close and reopen and band inversions occur during this process, which
induces topological transitions in the Mo-decorated BlueP (with strain of
-5.75%) and W-decorated BlueP (with strain of -4.25%) from normal insulators to
topological insulators (TIs). The TI gap is 94 meV for the Mo-decorated BlueP
and 150~220 meV for the W-decorated BlueP. Our findings pave the way to
engineer topological phases in the monolayer BlueP with transition-metal
adatoms.
