Band offsets in InP/ZnSe nanocrystals evaluated using two-photon transitions analysis

Abstract

We present a semi-analytical theoretical kp-study of the energy structure and optical transitions in spherical core–shell InP/ZnSe nanocrystals. We use the eight-band Kane model and the six-band Luttinger Hamiltonian in the spherical approximation to calculate the electron and hole energy spectra, respectively. The influence of the Coulomb interaction is considered perturbatively. The one- and two-photon absorption spectra are calculated as functions of the band offsets between the InP core and ZnSe shell. Exciton states responsible for the main features in the two-photon absorption spectra of InP/ZnSe nanocrystals are identified and the spectral dependence of the linear-circular dichroism signal is predicted. We show that in the presence of inhomogeneous broadening, the transition to the ground two-photon-active exciton state can be hidden behind intense transitions to higher-lying states. A comparison of the calculated one- and two-photon absorption spectra with the available experimental data shows that, depending on the lattice strain in the InP core, the range of possible valence band offsets is 0.85–1 eV. The determined range exceeds the natural valence band offset of 0.57 eV expected from first-principles calculations, indicating the presence of electric dipoles formed by preferential Zn–P bonds at the InP/ZnSe heterointerface.