Compelling Evidence for the epsilon-Phase InSe Crystal by Oblique Incident Second Harmonic Generation

Indium selenide (InSe), a layered semiconductor with direct band gap and high carrier mobility, holds promising applications in bendable electronics and ultrafast optoelectronics. Yet its crystal structure exhibits polytypism with four different stacking orders (gamma-, epsilon-, beta- and delta-phases), arising from the weak van der Waals interlayer coupling. These phases are nearly-degenerate in energy but are predicted to have contrasting electronic structures for versatile applications. It remains highly challenging to distinguish between these polytypes due to the lack of noninvasive tools that are sensitive enough to the interlayer structural variations. Here, the unambiguous discrimination of different InSe polytypes using symmetry-sensitive oblique incident optical second harmonic generation (SHG) is demonstrated. Surprisingly, the epsilon-phase is found to be dominant, although the samples from two popular commercial vendors (2D Semiconductors and Six Carbon Technology) are claimed to be the gamma-phase. These results would help promoting the precise application of InSe crystal for piezoelectric transducer and strain sensing, as well as showing the oblique incident SHG to be a powerful structural analytical tool for van der Waals layered materials.