  Symmetry breaking generally induce exotic physical properties, particularly
for low-dimensional materials. Herein we demonstrate that symmetry breaking
induces a giant Stark effect in 2D Janus materials using group IV-V monolayers
with a four-atom-layer structure as a model system, which are constructed by Ge
and As element substitution of symmetrical SnSb monolayer. A linear giant Stark
effect is found in Janus semiconductor monolayers, as verified by the band gap
variation up to 134 meV of Sn2SbAs monolayer, which is 30 times larger than
that of SnSb monolayer (4 meV) when the applied electric field is increased
from -0.30 to 0.30 V/{\AA}. By considering the induced electronic field, we
propose a generalized and effective formula that efficiently determines the
band gap variation owing to Stark effect. The calculated results from proposed
formula are well agreement with those from DFT-HSE06 functional. The giant
Stark effect is originated from the large spatial separation of centers of the
conduction band minimum and valence band maximum states of Janus structure due
to its intrinsic potential gradient. The wide-range tuning of band gap under
electronic field shows potential applications of 2D Janus materials in
optoelectronic devices.
