Tailoring the optical gap of silicon quantum dots without changing their size.

The absorption of photons through the direct generation of spatially separated excitons at dot-ligand interfaces is proposed as a promising strategy for tailoring the optical gap of small silicon quantum dots independent of their size. This removes a primary drawback for the use of very small dots in broad range of applications. For instance, the strategy can be applied to solar energy technologies to align the absorption of such dots with the peak of the solar spectrum. The key is to establish both a Type-II energy level alignment and a strong electronic coupling between the dot and ligand. Our first principles analysis indicates that connecting conjugated organic ligands to silicon quantum dots using vinyl connectivity can satisfy both requirements. For a prototype assembly of 2.6 nm dots, we predict that triphenylamine termination will result in a 0.47 eV redshift along with an enhanced near-edge absorption character. Robustness analyses of the influence of oxidation on absorption and of extra alkyl ligands reveal that the control of both factors is important in practical applications.