Layer-number dependent and structural defect related optical properties of InSe†
Two-dimensional (2D) InSe is an attractive semiconductor because of its bandgap in the near infrared region, high carrier mobility and chemical stability. Here, we present systematic investigations on the layer-dependent optical properties of few-layer InSe (2–6 layers). We develop a quantitative calibration map using optical images and Raman and photoluminescence (PL) spectroscopy to directly identify the layer numbers of the InSe flakes. This is facilitated by the significant difference in the optical contrast or Raman/PL spectra of InSe with different thicknesses. Moreover, excitonic states in few-layer InSe and in the bulk are probed by temperature-dependent PL spectroscopy. An emerging peak arising from the recombination of excitons bound to localized states is observed at low temperature. These states arise from structural defects which can also be induced via electron beam irradiation. The PL emission from bound excitons could be modified to be even stronger than the emission from near band edge recombination. This provides a new strategy to improve the PL emission efficiency of 2D InSe.