Observable Hole-State Kinetics and Its Implications for Optical Gain in Hole-Engineered Quantum Dots

Abstract

Disentangling carrier kinetics is of both fundamental and technological importance for optoelectronics, while the hole-state dynamics in Cd chalcogenide quantum dots (QDs) remains far from understood. Herein, we take the CdZnS-based alloyed QDs as the model system to examine the hole-state dynamics by transient absorption (TA) spectroscopy. The spectroscopic signature of the hole states could be directly manifested from the TA spectra, even in low-quality CdZnS QDs, which arises from the distinctive features of alloyed QDs. By completely passivating the hole traps, the resulting CdZnS/ZnS QDs feature a significant hole contribution of 17% to the TA bleach and associated bleach buildup kinetics. These merits endow the CdZnS-based QDs with excellent optical gain properties, where an ultralong gain lifetime (1.1 ns) and a broad gain bandwidth (>300 meV) have been observed thanks to the positive trion-suppressed Auger recombination and the retarded hot hole cooling under high excitation intensities. Such a slow hot carrier cooling (200 ps) in the alloyed QDs further indicates their potential in other optoelectronic devices, such as the hot carrier solar concentrators. These findings clarify the characters of the valence band hole and offer insights into the manipulation of hole states for optimizing device performances.