Trions Stimulate Electronic Coupling in Colloidal Quantum Dot Molecules

Recent synthetic progress has enabled the controlled fusion of colloidal CdSe/CdS quantum dots in order to form dimers, manifesting electronic coupling in their optical response. While this "artificial H₂ molecule" constitutes a milestone toward the development of nanocrystal chemistry, the strength of the coupling has proven to be smaller than intended. The reason is that, when an exciton is photoinduced in the system, the hole localizes inside the CdSe cores and captures the electron, often preventing substantial delocalization across the dimer. Here, we predict, by means of k·p theory and configuration interaction calculations, that using trions instead of neutral excitons or biexcitons restores the electron delocalization. Positive trions are particularly apt because the strong hole-hole repulsion makes electron delocalization robust against moderate asymmetries in the cores, thus maintaining a homodimer-like behavior. The hybridization energies are sufficiently large to preserve the molecular character beyond cryogenic temperatures.