Bright Cool White Emission from Ultrasmall CdSe Quantum Dots

In this work, we employ a post-synthesis ligand treatment of ultrasmall CdSe quantum dots (QDs) with citric acid (CA) to increase the quantum yield (QY) of the as-synthesized CdSe from an average of 5.7% to a maximum of 61.3%. This is the highest reported value for a single-component white-light emitting QD. The white-light emission of the ultrasmall CdSe QDs is derived from three surface states: pinned emission at a trap state associated with the phosphonic acid surface ligand, a surface Se state, and conventional deep trap states. The CA-treated CdSe QDs were found to recover band gap emission from the phosphonic acid ligand trap-state emission, exhibiting classic quantum confinement behavior. This band gap emission peak also becomes more prominent, resulting in a cooler white emission, as indicated by the shift in the Commission Internationale de l'Eclairage coordinates from an average of (0.30, 0.36) to (0.26, 0.32). Compared to a previous method of increasing the QY by a post-synthesis treatment with formic acid, the CA-treated CdSe QDs are brighter and more stable in solution over time, which opens the possibility for use in multiple applications.