Control of Surface State Emission via Phosphonic Acid Modulation in Ultrasmall CdSe Nanocrystals: The Role of Ligand Electronegativity

Abstract

Pinned emission from ultrasmall (<1.7 nm) CdSe nanocrystals is dominated by surface trap state emission. In sharp contrast to bulk defect emission, optical properties of the nanocrystal can be tuned through surface chemistry. We demonstrate the ability to alter this pinned emission from 2.79 to 2.91 eV by varying the phosphonic acid ligand. The blue shifting emission is attributed to the increased electronegativity of shorter chain length ligands. Further, the quantum yield of these nanocrystals can be enhanced by increasing the chain length of the ligand. These results indicate that for ultrasmall nanocrystals, the surface ligands play the dominant role in the emission properties.