Two-Dimensional Group Ⅳ-Ⅵ Monochalcogenides:Synthesis And Optical Properties

Monochalcogenides of group IV elements are considered as isoelectronic and isostructural analogue of black phosphorous.They have attracted considerable attention owing to their unique two-dimensional(2D)structure and size-dependent physical and chemical properties,which can be used in optoelectronics,energy and sensing,such as solar cells,lithium-ion batteries,gas sensors and photocatalysis.Here,we demonstrate the synthesis and optical properties of atomically thin GeSe and SnSe by direct sonication-assisted liquid phase exfoliation(LPE)of bulk microcrystalline powders in organic solvents.First,it is found that bulk GeSe powders can be exfoiliated in many solvents to form high-quality GeSe nanosheets.The thickness of the nanosheets are dependent on the exfoliation conditions,and highly crystalline few-layer GeSe sheets of 4-10 layer stacks with lateral sizes over 200 nm can be obtained.In ambient atmosphere,the LPE sheets deposited on the substrate demonstrate strong resistance against degradation,while decomposition into elemental Ge and Se nanostructures occurs at a moderate rate for ethanol dispersions.Density functional theory calculation together with optical characterizations confirms the blue-shifted bandgap for the GeSe sheets as a result of strong quantum confinement effect.In addition,we show that the few-layer GeSe sheets with optical bandgaps of 1.18 eV-1.41 eV allow for efficient solar light harvesting for photocurrent generation based on a photoelectrochemical cell.Our joint theoretical and experimental results suggest that GeSe sheets of atomic thickness could be a new 2D semiconductor that can be exploited for potential applications in optoelectronics and photonics.Second,few-layer SnSe nanosheets were also synthesized by a sonication-assisted liquid phase exfoliation process and their linear and nonlinear optical properties were examined.The as-exfoliated few-layer(FL)SnSe demonstrate layer thickness from 2 nm to 10 nm and lateral size of 150 nm with high crystallinity.Optical measurement confirms the layer-number dependent bandgap,which is also corroborated by first-principle calculation.In addition,the FL-SnSe shows strong reverse saturable absorption,which is characterized by an ultrafast response time of picosecond scale.The nonlinear response in FL SnSe is explained by defect-involved recombination of photo-generated carried as well as the Auger scattering process,which is futher enhanced by structural two-dimensionality.