Controllable Synthesis of Atomically Thin 1T‐SnSe2 Flakes and Its Linear Second Harmonic Generation with Layer Thickness

As an important member of the IVA–VIA group compounds, 2D SnSe2 has emerged as a perfect platform for developing diverse applications, especially in high‐performance optoelectronic devices and data storage, etc. However, the bottom‐up synthesis of large‐area uniform, atomically thin SnSe2 crystals with controlled thicknesses has not yet been realized. Herein, we report the large‐area uniform growth of monolayer (1L), bilayer (2L), and few‐layer (FL) 1T‐SnSe2 single‐crystal flakes on mica substrates via a facile chemical vapor deposition (CVD) route. The feeding amount of Sn precursor and flow rate of hydrogen carrier are found to be the key parameters for the thickness‐controlled growth of uniform SnSe2 flakes. More intriguingly, obvious second harmonic generation (SHG) is revealed in the retained inversion symmetry structure of 1T‐SnSe2, with its intensity showing linear dependence with the thickness from monolayer to multilayers. The new findings reported herein should pave the ways for the thickness‐tunable growth of atomically thin SnSe2 crystals, and their unique optical property explorations and applications in nonlinear optics. The large‐area uniform growth of monolayer, bilayer and few‐layer 1T‐SnSe2 single‐crystal flakes is presented via a chemical vapor deposition route. The thickness‐tunable 1T‐SnSe2 flakes allow to establish a good criterion to identify the thickness and its unique optical properties. Intriguingly, we find obvious second harmonic generation in the inversion symmetry structure, with its intensity showing linear dependence.