Speaker
Description
Semiconductor CuInS$_2$ (CIS) quantum dots (QDs) are highly promising for optoelectronic applications for their low toxicity, excellent charge-transport properties, and remarkable photoluminescence features. To develop our fundamental understanding of nanoscale photo-physics and optimize the design of devices, several mechanisms of recombination in CIS QDs have been proposed, modelling the population dynamics of correlated electron-hole pairs (bound excitons). While most suggest that radiative recombination results from the interaction between an electron in the conduction band and a hole in the so-called confined hole state (CHS), no research has provided direct experimental evidence for the existence of this intra-gap state. The existence of such a state, arising from a defect of Cu$^+$ that can get oxidized to Cu$^{2+}$ by the hole, would explain the large Stokes shift and broad photoluminescence exhibited by the CIS QDs and has been theoretically predicted and postulated for a long time. In this study, we aim to observe and understand the formation process of the CHS, followed by the localization of a hole in the CHS. The novelty of this project stems from the experimental methodologies employed to tackle this problem: we use a combination of ultrafast optical and X-ray free electron laser (XFEL) pump-probe techniques on tunned CIS QDs. This approach allows us to observe the spectral fingerprints of photogenerated charge carriers directly on the fundamental timescales of their formation and relaxation within a QD but also, with atomic spatial resolution provided by hard X-rays delivered by an XFEL. Moreover, the element and oxidation state specificity of X-ray, serves as a direct probe to track recombination and localization dynamics of photogenerated holes. Furthermore, the femtosecond-resolved optical studies complement our results by monitoring of the photoinduced dynamics of photoelectrons. This extended study grants new insights into this class of materials and correlates the observed charge carrier dynamics, influenced by the laser fluence.
| Scientific Topics | Solid State Physics |
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