  Defect engineering is promising to tailor the physical properties of
two-dimensional (2D) semiconductors for function-oriented electronics and
optoelectronics. Compared with the extensively studied 2D binary materials, the
origin of defects and their influence on physical properties of 2D ternary
semiconductors have not been clarified. In this work, we thoroughly studied the
effect of defects on the electronic structure and optical properties of
few-layer hexagonal Znln2S4 via versatile spectroscopic tools in combination
with theoretical calculations. It has been demonstrated that the Zn-In
anti-structural defects induce the formation of a series of donor and acceptor
levels inside the bandgap, leading to rich recombination paths for defect
emission and extrinsic absorption. Impressively, the emission of donor-acceptor
pair (DAP) in Znln2S4 can be significantly tailored by electrostatic gating due
to efficient tunability of Fermi level (Ef). Furthermore, the layer-dependent
dipole orientation of defect emission in Znln2S4 was directly revealed by back
focal plane (BFP) imagining, where it presents obviously in-plane dipole
orientation within a dozen layers thickness of Znln2S4. These unique features
of defects in Znln2S4 including extrinsic absorption, rich recombination paths,
gate tunability and in-plane dipole orientation will definitely benefit to the
advanced orientation-functional optoelectronic applications.
