Narrow-gap physical vapour deposition synthesis of ultrathin SnS1−xSex (0 ≤ x ≤ 1) two-dimensional alloys with unique polarized Raman spectra and high (opto)electronic propertiesElectronic supplementary information (ESI) available: Optical images of the SnS1−xSex alloyed nanosheets by different wafer methods, schematic diagrams for the traditional growth and NGPVD growth of 2D SnS1−xSex alloyed nanosheets, the SEM-EDS data of SnS1−xSex nanosheets, SEM images of 2D SnS1−xSex nanosheets via a two

Here we report ultrathin SnS 1− x Se x alloyed nanosheets synthesized via a narrow-gap physical vapour deposition approach. The SnS 1− x Se x alloy presents a uniform quadrangle shape with a lateral size of 5-80 μm and a thickness of several nanometers. Clear orthorhombic symmetries and unique in-plane anisotropic properties of the 2D alloyed nanosheets were found with the help of X-ray diffraction, high resolution transmission electron microscopy and polarized Raman spectroscopy. Moreover, 2D alloyed field-effect transistors were fabricated, exhibiting a unipolar p-type semiconductor behavior. This study also provided a lesson that the thickness of the alloyed channels played the major role in the current on/off ratio, and the high ratio of 2.10 × 10 2 measured from a large ultrathin SnS 1− x Se x device was two orders of magnitude larger than that of previously reported SnS, SnSe nanosheet based transistors because of the capacitance shielding effect. Obviously enhanced Raman peaks were also found in the thinner nanosheets. Furthermore, the ultrathin SnS 0.5 Se 0.5 based photodetector showed a highest responsivity of 1.69 A W −1 and a short response time of 40 ms under illumination of a 532 nm laser from 405 to 808 nm. Simultaneously, the corresponding highest external quantum efficiency of 392% and detectivity of 3.96 × 10 4 Jones were also achieved. Hopefully, the narrow-gap synthesis technique provides us with an improved strategy to obtain large ultrathin 2D nanosheets which may tend to grow into thicker ones for stronger interlayer van der Waals forces, and the enhanced physical and (opto)electrical performances in the obtained ultrathin SnS 1− x Se x alloyed nanosheets prove their great potential in the future applications for versatile devices. An improved physical vapour deposition for achieving ultrathin SnS 1− x Se x alloyed nanosheets with unique anisotropic Raman characteristics and good (opto)electrical performance.