multinary [11,12] chalcogenides of transition metals as well as P-block elements (e.g., groups 13-15). [13][14][15][16][17][18][19][20][21] These chalcogenides form the basis for the explosion of experimental and theoretical research in the recent decade into the layer-number dependent optical, electrical properties, and potential applications of these 2D materials. [22][23][24][25] While a 2D structure for P-block elements besides graphite carbon is uncommon, black phosphorous (BP) has been an outstanding example, which adopts a GeS-type structure. [16][17][18][19][20][21] The connection between BP and the monochalcogenide GeS is quite obvious: they are isoelectronic with ten valence electrons per pair of atoms. [26][27][28][29] The monochalcogenide of group IV elements (i.e., Ge, Sn) has also been known as four-six-enes. [30] The most important difference from phosphorene is that they do not possess inversion symmetry for monolayer (or stacks with odd layer numbers), thus making them important candidates for exploiting piezo/ferro-electronic response as well as valleypolarized optical properties. [29,[31][32][33][34] Recently, their strong relevance with phosphorene and the chalcogenide 2D materials has stimulated growing interest in exploration of the potential properties from valleytronics to optical nonlinearity. [28,[35][36][37][38] Similar to many transition metal dichalcogenides, they have been predicted to show indirect to direct bandgap crossover at the monolayer limit, [28,[39][40][41] which however still await experimental confirmation. On the other hand, their bulk counterparts have been known for unconventional superconductivity, [42] ultrahigh thermoelectric figure of merit, [43][44][45] and photovoltaic properties, [41,[46][47][48] to name a few. The interplay between structural two-dimensionality and these features deserves a thorough examination, which is hampered by the limited access of high quality samples. Up to present, sheets of GeSe (or SnSe) with thickness of around 100 nm as well as Ge(Sn)Se nanowires and nanoplates have been reported by vapor transport process and liquid phase growth, [49][50][51][52][53][54][55][56][57][58][59] while access to few-layer and monolayer samples with high crystallinity by a top-down approach remain quite limited.In the present work, SnSe nanosheets were synthesized by sonication assisted exfoliation in different organic solvents. High quality FL SnSe nanosheets and clear layer number dependent optical bandgap is found, which is corroborated by our density functional theory (DFT) calculations. In addition, we also characterized the nonlinear optical absorption of FL SnSe Monochalcogenides of group IV elements have been considered as phosphorene analogs due to their similar crystal and electronic structure. Here, few-layer SnSe nanosheets are synthesized by a sonication-assisted liquid phase exfoliation process and their linear and nonlinear optical properties are examined. The as-exfoliated few-layer (FL) SnSe demonstrates layer thickness from 2 to 10 nm a...
