  Two-dimensional (2D) layered materials, transition metal dichalcogenides and
black phosphorus, have attracted much interest from the viewpoints of
fundamental physics and device applications. The establishment of new
functionalities in anisotropic layered 2D materials is a challenging but
rewarding frontier, owing to their remarkable optical properties and prospects
for new devices. Here, we report the anisotropic optical properties of layered
2D monochalcogenide of germanium sulfide (GeS). Three Raman scattering peaks
corresponding to the B3g, A1g, and A2g modes with strong polarization
dependence are demonstrated in the GeS flakes, which validates polarized Raman
spectroscopy as an effective method for identifying the crystal orientation of
anisotropic layered GeS. Photoluminescence (PL) is observed with a peak at
around 1.66 eV that originates from the direct optical transition in GeS at
room temperature. Moreover, determination of the polarization dependent
characteristics of the PL and absorption reveals an anisotropic optical
transition near the band edge of GeS, which is also supported by the density
functional theory calculations. This anisotropic layered GeS presents the
opportunities for the discovery of new physical phenomena and will find
applications that exploit its anisotropic properties.
