Unveiling Layer-Dependent Phase Transition and Lattice Dynamics in Two-Dimensional InSe.

Two-dimensional (2D) indium selenide (InSe) has attracted considerable interest due to its superior ballistic transport properties, superplasticity, and thermoelectric properties. Ferroelectricity and a variety of other intriguing physical characteristics. These arise from its van der Waals (vdW) layered structure, interlayer coupling, and intralayer interactions. The vibrational modes of 2D InSe are highly sensitive to thickness. The phase transitions in 2D materials, which are critical to their properties and applications, are closely related to interlayer and intralayer vibrations. However, the effect of the thickness on these vibrational behaviors during phase transitions remains insufficiently understood. In this study, we investigate the Raman spectra of β-InSe with layer numbers (LN) ranging from 4 to 33 under high pressure and construct a pressure LN phase diagram. Unexpectedly, due to the quantum confinement and defect effects, InSe flakes with fewer layers require more energy to undergo phase transitions which is confirmed by PL experiments and DFT calculations, irrespective of whether pressure is being increased or decreased. This research establishes a solid foundation for exploring and characterizing interlayer and intralayer lattice dynamics through pressure engineering in vdW materials.