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