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Few-Layer to Multilayer Germanium (II) Sulfide: Synthesis, Structure, Stability, and Optoelectronics.

Among 2D/layered semiconductors, group IV monochalcogenides such as SnS(e) and GeS(e) have attracted attention as phosphorene/black phosphorus analogues with anisotropic structure and predicted unusual properties. In contrast to SnS, for which bottom-up synthesis has been reported, few-layer GeS has been realized primarily via exfoliation from bulk crystals. Here we report the synthesis of large (up to >20 μm), faceted single crystalline GeS flakes with anisotropic properties using a vapor transport process. In-situ electron microscopy is used to identify the thermal stability and sublimation pathways, and demonstrates that the GeS flakes are self-encapsulated in a thin, sulfur-rich amorphous GeSx shell during growth. The shell provides exceptional chemical stability to the layered GeS core. In contrast to exfoliated GeS, which rapidly degrades during exposure to air, the synthesized GeS-GeSx core-shell structures show no signs of chemical attack and remain unchanged in air for extended time periods. Measurements of the optoelectronic properties by photoluminescence spectroscopy show a tunable bandgap due to out-of-plane quantum confinement in flakes with thickness below 100 nm. Cathodoluminescence (CL) spectroscopy with nanoscale excitation provides evidence for interfacial charge transfer due to a type II heterojunction between the crystalline core and amorphous shell. Measurements by locally excited CL yield a minority carrier (electron) diffusion length in the p-type GeS core Ldiff = 0.27 μm, on par with diffusion lengths in the highest-quality layered chalcogenide semiconductors.