Tunable Near-Infrared Emission from CdSe/CdTe/CdSe Core/Shell/Shell Quantum Dots

Core/shell nanostructures of type I and type II band-offset materials have been pivotal in improving photoluminescence and imparting charge separation in well-established colloidal quantum dots. In addition to these features, the multishell motif provides a simple means to expand the emission spectral range and spatially fine-tune electron and hole wave functions. Here, we explore the CdSe/CdTe/CdSe core/shell/shell structure with a total diameter of ≲7 nm, within the practical limits of readily accessible synthesis capabilities. Systematic variations of the dimensions of the core and the shells are carried out to examine how each of these structural parameters affects optical properties. Emission in the red to near-infrared range from 677 to 1057 nm (1.83 to 1.17 eV), photoluminescence quantum yields of up to 88%, and line widths as narrow as 109 meV (41.8 nm for the peak position of 677 nm) are achieved. Even with a total size limit of ∼7 nm with an ∼1 nm thick CdTe shell, the reddest emission energy can approach the expected energy separation between the bulk CdSe conduction band edge and CdTe valence band edge to within tens of millielectronvolts. Comparison to simple effective mass approximation provides insights and guidelines on achieving independent control over the effective band gap and the electron-hole overlap.