Elementary Charge Characterization of Single Quantum Dots in Solution

Measuring quantum dot properties at the single-particle level in their native liquid environment provides a powerful means of deepening our understanding of quantum dot systems and advancing their applications. In this work, we successfully measure the electrical charge of individual CdSe/CdS core/shell quantum dots, with diameters of 15 and 25 nm, in a nonpolar liquid, with precision at the elementary charge level. This is accomplished by combining laser scanning microscopy with high-field electrophoresis, where the observed electrophoretic mobilities show clear clustering around values corresponding to discrete charge states. A thermodynamic charging model captures the dominant features of the probability distribution and size dependence of the observed charges. This research opens the possibility to study charge-related optical and electronic phenomena at the single-quantum-dot level in solution.