  By using first-principles calculations, we investigated the monolayer $ReS_2$
with vacancies under strain engineering, specifically focusing on its energy of
formation, band gap, electron density of states, effective mass and optical
properties. The calculated results disclose that S4 defect is more likely to
form than other kinds of vacancies. Asymmetric deformation induced by strain
makes its band structure transformation from direct band gap to indirect band
gap. The analysis of the partial density of states indicates that the Re-d,
Re-p and S-d orbitals are the major components of the defect states, being
different from $MoS_2$, the defect states locate both above and below the Fermi
level. Moreover, the effective mass was sensitive and anisotropic under the
external strain. The reflection spectrum can be greatly tuned by the external
strains, which indicates that the ReS2 monolayer has promising applications in
nanoscale strain sensor and conductance-switch FETs.
