  Atomically thin binary 2-dimensional (2D) semiconductors exhibit diverse
physical properties depending on their composition, structure and thickness. By
adding another element in those materials, which will lead to formation of
ternary 2D materials, the property and structure would greatly change and
significantly expanded applications could be explored. In this work, we report
structural and optical properties of atomically thin chromium thiophosphate
(CrPS4), a ternary antiferromagnetic semiconductor. Its structural details were
revealed by X-ray and electron diffractions. Transmission electron microscopy
showed that preferentially-cleaved edges are parallel to diagonal Cr atom rows,
which readily identified their crystallographic orientations. Strong in-plane
optical anisotropy induced birefringence that also enabled efficient
determination of crystallographic orientation using polarized microscopy. The
lattice vibrations were probed by Raman spectroscopy for the first time and
exhibited significant dependence on thickness of crystals exfoliated down to
single layer. Optical absorption determined by reflectance contrast was
dominated by d-d type transitions localized at Cr3+ ions, which was also
responsible for the major photoluminescence peak at 1.31 eV. The spectral
features in the absorption and emission spectra exhibited noticeable
thickness-dependence and hinted a high photochemical activity for single layer
CrPS4. The current structural and optical investigation will provide a firm
basis for future study and application of this novel magnetic semiconductor.
