Excitation dependent photoluminescence from quantum confined ultrasmall SnS sheets

Black phosphorus analogous tin(II) sulfide (SnS) has recently emerged as an attractive building block for photonic and optoelectronic devices due to its intrinsic anisotropic response. Two-dimensional SnS has shown to exhibit in-plane anisotropy in optical and electrical properties. However, the limitations in growing ultrasmall structures of SnS hinder the experimental exploration of anisotropic behavior in low dimension. Here, we present an elegant approach of synthesizing highly crystalline nanometer-sized SnS sheets. Ultrasmall SnS exhibits two distinct valleys along armchair and zig-zag directions due to in-plane structural anisotropy like bulk SnS. We find that in SnS nanosheets, the bandgaps corresponding to two valleys are increased due to the quantum confinement effect. Moreover, the photoluminescence (PL) from SnS quantum dots (QDs) is excitation energy dependent. Our spectroscopic studies infer that PL of SnS QDs originates from the two non-degenerate valleys.