Strong Quantum Confinement Effect in the Optical Properties of Ultrathin α‐In2Se3

The effect of quantum confinement in the optical absorption spectra of atomically thin {\alpha}-In2Se3 crystals is studied, observing a huge thickness-dependent shift in the optical band gap of exfoliated {\alpha}-In2Se3 flakes. The band gap variation reported here is among the largest found in semiconductor crystals and spans a region of the near-UV spectrum uncovered by other 2D semiconductors.