Phase Identification and Strong Second Harmonic Generation in Pure ε-InSe and Its Alloys

Two-dimensional material indium selenide (InSe) has offered a new platform for fundamental research in virtue of its emerging fascinating properties. Unlike 2H-phase transition-metal dichalcogenides (TMDs), epsilon phase InSe with a hexagonal unit cell possesses broken inversion symmetry in all the layer numbers, and predicted to have a strong second harmonic generation (SHG) effect. In this work, we find that the as-prepared pure InSe, alloyed InSe1-xTex and InSe1-xSx (x = 0.1 and 0.2) are epsilon phase structures and exhibit excellent SHG performance from few-layer to bulk-like dimension. This high SHG efficiency is attributed to the non-centrosymmetric crystal structure of the epsilon-InSe system, which has been clearly verified by aberration-corrected scanning transmission electron microscopy (STEM) images. The experimental results show that the SHG intensities from multilayer pure epsilon-InSe and alloyed InSe0.9Te0.1 and InSe1-xSx (x = 0.1 and 0.2) are around 1-2 orders of magnitude higher than of the monolayer TMD systems and even superior to that of GaSe with the same thickness. The estimated nonlinear susceptibility chi((2)) of epsilon-InSe is larger than that of epsilon-GaSe and monolayer TMDs. Our study provides first-hand information about the phase identification of epsilon-InSe and indicates an excellent candidate for nonlinear optical (NLO) applications as well as the possibility of engineering SHG response by alloying.