  Monochalcogenides of germanium (or tin) are considered as stable
isoelectronic and isostructural analogue of black phosphorous. Their
two-dimensional (2D) forms have been just predicted to shown strong
thickness-dependent physical properties, and even indirect to direct band gap
crossover at the monolayer limit. Here, we demonstrate the synthesis of
atomically thin GeSe by direct sonication-assisted exfoliation of bulk
microcrystalline powders in solvents. The exfoliated few-layer GeSe sheets
characterize high crystallinity with lateral sizes over 100 nm and,
importantly, strong resistance against oxidation and degradation in ambient
conditions. Density functional theory calculation combined with optical
characterizations confirm the layer-number dependent optical bandgap, which,
for few-layer sheets, is optimal for solar light harvesting, and promising for
relevant applications, such as optoelectronics and photonics.
