Optical properties of five-layer and ten-layer CrI₃ films under high pressure: Insights from in situ Raman and UV–visible spectroscopy

The two-dimensional van der Waals ferromagnetic semiconductor CrI₃ provides an ideal platform for exploring the interplay among structural, electronic and magnetic degrees of freedom. In this work, we systematically investigate the thickness-dependent optical properties of five-layer and ten-layer CrI₃ under hydrostatic pressure up to 27.9 GPa by in situ Raman and UV–visible absorption spectroscopy. All A_g Raman modes exhibit a continuous blueshift with increasing pressure. The low-frequency modes (${\mathrm{A}}_{\mathrm{g}}^1$ – ${\mathrm{A}}_{\mathrm{g}}^3$) are mainly associated with enhanced interlayer coupling, whereas the high-frequency modes (${\mathrm{A}}_{\mathrm{g}}^4$ – ${\mathrm{A}}_{\mathrm{g}}^6$) reflect the suppression of surface vibrations. The Raman modes disappear at approximately 4.9 GPa for the five-layer sample and 11.2 GPa for the ten-layer sample, indicating a stronger strain sensitivity in thinner CrI₃. Optical absorption measurements show a pronounced redshift of the absorption edge, accompanied by bandgap narrowing from 2.26 eV to 1.26 eV in five-layer CrI₃. At comparable pressures, the five-layer sample consistently exhibits a wider bandgap than the ten-layer one, which is attributed to quantum confinement effects and reduced interlayer hybridization. Above 12.7 GPa, the bandgap reduction becomes less pronounced, probably due to enhanced Cr 3d/I 5p orbital overlap and strengthened superexchange interactions. These results reveal a clear layer-dependent structure–electronic coupling in CrI₃ under compression and provide useful insights into pressure modulation of van der Waals magnetic semiconductors.