Density of states in a single PbSe/PbS core-shell quantum dot measured by scanning tunneling spectroscopy

The electronic configuration of a set of PbSe/PbS core-shell colloidal quantum dots (CQDs), with a common core radius of 1.5 nm having a PbS shell of a variable width from 0.75 to 2.5 nm, was investigated by scanning tunneling spectroscopy. The conductance resonance spectra were correlated with a band tunneling process, monitoring the individual electronic levels of conduction and valence bands. The energy band gap of the various samples, derived from the conductance spectra, were compared with the values measured by the absorption spectra, as well as with a theoretical evaluation, using the extended four-band envelope function theory across the PbSe/PbS material boundary. The experimental results showed a decrease in the energy band gap, as well as a decrease in a few higher energy interband transitions, with the increase in the shell thickness, in close agreement with the theoretical prediction, which correlated it with a delocalization of the carriers' wave functions over the entire core-shell structure. However, the increase in the outer radius of the core-shell CQDs has a slightly smaller influence than a simple growth in size of PbSe cores, due to the existence of Fermi-energy offset, discontinuity in the effective mass and the dielectric constant at the core-shell interface.