Tuning of band-edges in type-I core-shell nanocrystals through band-offset engineering: selective quantum confinement effect

The addition of a shell layer onto semiconducting quantum dots is an approach to control the optical bandgap of core-shell systems. In this direction, we have grown several core-shell nanostructures having a type-I heterostructure configuration. The nature of the energy-offset has been varied by a suitable shell-material with the aim to control the conduction and the valence band-offsets separately. Confined holes or electrons could hence be relaxed selectively leading to an increase in the valence band-edge or a decrease in the conduction band-edge. In this work, after forming such core-shell systems with a control over the shell thickness, we characterized the nanostructures with scanning tunneling spectroscopy in order to determine the density of states and finally the conduction and the valence band-edges of the core-shell systems. We found that while a large band-offset strictly localizes the carriers in the core, a small perturbation indeed delocalizes the carriers up to the shell-layer shifting the relevant band-edge towards the Fermi energy and thereby decreasing the transport gap. The decrease in the transport gap was in agreement with the optical absorption spectra. The results provide a novel route to delocalize a selective type of carrier up to the shell layer of core-shell nanostructured systems.