  In this work, we investigate the mechanical and electronic properties of
monolayer antimonene in its most stable beta-phase using first-principles
calculations. The upper region of its valence band is found to solely consist
of lone pair p-orbital states, which are by nature more delocalized than the
d-orbital states in transition metal dichalcogenides, implying superior
transport performance of antimonene. The Young's and shear moduli of
beta-antimonene are observed to be ~25% higher than those of bulk antimony,
while the hexagonal lattice constant of the monolayer reduces significantly
(~5%) from that in bulk, indicative of strong inter-layer coupling. The ideal
tensile test of beta-antimonene under applied uniaxial strain highlights ideal
strengths of 6 GPa and 8 GPa, corresponding to critical strains of 15% and 17%
in the zigzag and armchair directions, respectively. During the deformation
process, the structural integrity of the material is shown to be better
preserved, albeit moderately, in the armchair direction. Interestingly, the
application of uniaxial strain in the zigzag and armchair directions unveil
direction-dependent trends in the electronic band structure. We find that the
nature of the band gap remains insensitive to strain in the zigzag direction,
while strain in the armchair direction activates an indirect-direct band gap
transition at a critical strain of 4%, owing to a band switching mechanism. The
curvature of the conduction band minimum increases during the transition, which
suggests a lighter effective mass of electrons in the direct-gap configuration
than in the free-standing state of equilibrium. The work function of
free-standing beta-antimonene is 4.59 eV and it attains a maximum value of 5.07
eV under an applied biaxial strain of 4%.
