Effect of the Core/Shell Interface on Auger Recombination Evaluated by Single-Quantum-Dot Spectroscopy

Abstract

Previous single-particle spectroscopic studies of colloidal quantum dots have indicated a significant spread in biexciton lifetimes across an ensemble of nominally identical nanocrystals. It has been speculated that in addition to dot-to-dot variation in physical dimensions, this spread is contributed to by variations in the structure of the quantum dot interface, which controls the shape of the confinement potential. Here, we directly evaluate the effect of the composition of the core-shell interface on single- and multiexciton dynamics via side-by-side measurements of individual core-shell CdSe/CdS nanocrystals with a sharp versus smooth (graded) interface. To realize the latter type of structures we incorporate a CdSexS1-x alloy layer of controlled composition and thickness between the CdSe core and the CdS shell. We observe that while having essentially no effect on single-exciton decay, the interfacial alloy layer leads to a systematic increase in biexciton lifetimes, which correlates with the increase in the biexciton emission efficiency, as inferred from two-photon correlation measurements. These observations provide direct experimental evidence that in addition to the size of the quantum dot, its interfacial properties also significantly affect the rate of Auger recombination, which governs biexciton decay. These findings help rationalize previous observations of a significant heterogeneity in the biexciton lifetimes across similarly sized quantum dots and should facilitate the development of "Auger-recombination-free" colloidal nanostructures for a range of applications from lasers and light-emitting diodes to photodetectors and solar cells.

Ultrafast optical microscopy of single monolayer molybdenum disulfide flakes

“…By tuning the probe photon energy through the MoS 2 band gap (both indirect and direct), our UOM measurements show that conduction and valence band states are rapidly populated on a sub-picosecond (ps) time scale in a MoS 2 monolayer after photoexcitation at 3.1 eV, consistent with previous work [14][15][16][17][18] . Pump fluence-dependent measurements reveal that subsequent carrier relaxation in our samples is primarily due to surface-related defects and trap states, not the Auger processes observed in previous measurements on MoS 2 and other semiconductor nanosystems 12,[18][19][20][21][22][23] . We also observed an increase in the carrier relaxation time with an increase in the number of MoS 2 layers, likely due to the well known layer-dependent changes in the electronic structure 16,24 .…”

Active photonic devices based on colloidal semiconductor nanocrystals and organometallic halide perovskites

“…3f) [29]. Strategies to minimize this mechanism include the incorporation of a thick CdS shell around a CdSe has [30], or by growing an intermediate CdSe x S 1−x layer CdSe and the CdS [31].…”

Interfacial engineering of core/shell heterostructured nanocrystal quantum dots for light-emitting applications

“…As a result of the persistent efforts to tailor the formulations at the interfaces, quantum-dot light-emitting diodes with external quantum efficiencies of over 10% with green, blue, and red NQDs have been successfully realized ( Figure 5(b)) [5]. In addition, composition (energy) gradients help suppress AR, which limits the realization of NQD lasers [29,48,50]. Recently, Park et al [18] reported a comprehensive theoretical and experimental study on the relationship between the structural formulation of core/shell NQDs and their optical performance, specifically the biexciton (two electron-hole pairs) quantum yields ( Figure 6(a)).…”