Optical Transitions of CuInS2 Nanoparticles: Two Types of Absorption and Two Types of Emission

CuInS2 (CIS) nanoparticles would be attractive alternatives to replace toxic elements in widely investigated nanoparticles, when the optical properties such as the broad absorption and the luminescence with a large Stokes shift are clarified. Hence, the optical transitions of CIS nanoparticles are investigated. The absorption spectrum shows two types of electronic transitions due to the splitting of the valence band, whose energy difference is influenced by the size of the nanoparticles. The asymmetric shape of the photoluminescence spectrum indicates the coexistence of two emissive states such as surface and band-gap states with distinctive energies and lifetimes. The low-energy absorption and band-gap emission exhibit the weakly allowed nature of optical transitions aided by the electron–phonon coupling, which explains the broad band and large Stokes shift, although the ground and lowest excited states are involved in both the low-energy absorption and the band-gap emission. Consequently, an understanding of the optical properties of CIS increases its potential for appropriate applications as an alternative to quantum dots containing toxic elements.