ε-InSe single crystals grown by a horizontal gradient freeze method†
Indium selenide (InSe) single crystals have been considered as promising candidates for future optical, electrical, and optoelectronic device applications. In particular, due to its 2H stacking and non-centrosymmetric feature, ε-InSe is highly desirable for second harmonic generation. However, due to its complex phase diagram, the high chemical activity of the elements, and the large formation energy, single-phase crystal growth of ε-InSe still remains a challenge. In this work, we present our efforts using a horizontal gradient freeze (HGF) method to fabricate InSe single crystals for the first time, whose high-quality crystallinity has been confirmed by both X-ray diffraction (XRD) and transmission electron microscopy (TEM). Moreover, we further combine the photoluminescence, Raman spectroscopy and infrared absorption data to identify this single crystal to be ε-InSe. Finally, thickness-dependent second harmonic generation (SHG) measurements were carried out on our few-layer InSe samples, and the results demonstrate its promising application for non-linear optics.