Optical Characteristics of the Surface Defects in InP Colloidal Quantum Dots for Highly Efficient Light-Emitting Applications

The colloidal quantum dots (QDs) have inherent multiple dangling bonds (DBs) on the surface atoms due to the intrinsic weak bonding nature and steric hindrance of organic ligands. Such DBs can be the trap sites for charge carriers, leading to the reduction of luminescence efficiency, but their detailed characteristics are still unclear. In this study, we disclose the electronic and optical features of the surface DBs of InP QDs via density functional calculations combined with experimental evidence. For InP core, both In-DB and P-DB create invariant DB energy levels with respect to the core size, and their optical transition intensities exhibit an order of magnitude smaller than the band-edge transition. The In-DB and P-DB generate a deep trap level at -3.947 eV and a shallow trap level at -5.717 eV, and the deep trap level corresponds to the origin to induce the trap emissions. The passivation with ZnS shell on InP core significantly modifies the optical properties of both DBs to the radiative transition even when the passivating shell partially covers the InP surface. The ZnS shell growth pushes the energy levels of the In-DB and P-DB to near the band edges and makes the orbitals more delocalized. Such modified roles of the DBs significantly improve the optical intensities comparable to those of the band-edge transition, which is validated by the absorption calculations and luminescence measurements.