Ultrafast Charge Carrier Delocalization in CdSe/CdS Quasi-Type II and CdS/CdSe Inverted Type I Core–Shell: A Structural Analysis through Carrier-Quenching Study

Abstract

We have employed femtosecond transient absorption spectrocopy to monitor charge carrier delocalization in CdSe/CdS quasi-type II and CdS/CdSe inverted type I core–shell nanocrystals (NCs). Interestingly, CdSe and CdS QD pairs can make both type I and quasi-type II core–shell structures, depending on their band alignment and charge carrier localization. Steady-state optical absorption and luminescence studies show a gradual red-shift in both optical absorption and emission spectra in CdSe/CdS core–shell with increasing CdS shell thickness. The luminescence quantum yield in CdSe/CdS core–shell drastically increases with shell thickness. Notably, CdS/CdSe inverted core–shell shows a huge red-shift both in absorption and luminescence which closely matches with the band edge photoluminescence (PL) of pure CdSe QDs (shell). However, the luminescence quantum yield does not change much with shell thickness. Depending on their band energy level alignment, the charge carrier (electron and hole) delocalization in both the core–shells have been demonstrated using electron (benzoquinone, BQ) and hole (pyridine, Py) quencher. The bleach recovery kinetics of CdS/CdSe core–shell recovers faster in the presence of both BQ and Py. However, for CdSe/CdS core–shell, the bleach recovers faster only in the presence of BQ while the bleach dynamics remain unaffected in the presence of Py. Our experimental observations suggest that in CdSe/CdS quasi-type II core–shell, photoexcited electrons are localized in CdS shell and holes are localized in CdSe core; however, in CdS/CdSe inverted core–shell both electrons and holes are localized in the CdSe shell.

An Insight into the Interface through Excited‐State Carrier Dynamics for Promising Enhancement of Power Conversion Efficiency in a Mn‐Doped CdZnSSe Gradient Alloy

“…This slow electron cooling due to the spatially decoupled electron–hole wavefunction in the quasi‐type II structure has already been reported by us. , , Previously, we reported an extremely slow electron cooling of 8 ps in CdSe 0.3 S 0.7 alloy NCs, which was attributed to spatially decoupled electrons and holes in the CdSe/CdS quasi‐type II structure. Similarly, the gradient structure of CdSe/CdS/ZnSe/ZnS in the CdZnSeS alloy NC in the present investigation allows the electrons and holes to decouple due to the quasi‐type II nature of the inner CdSe/CdS NCs.…”

“…We have already mentioned that the synthesized NCs have a core–shell structure with a gradient composition of CdSe/CdS/ZnSe/ZnS, for which both absorption and PL spectra are dominated by the inner CdSe/CdS NC (Scheme ). As the energy level suggests, the band structure of CdSe/CdS forms a quasi‐type II structure in the core of the alloyed structure and, therefore, the appearance of CT state absorption can be attributed to direct transfer of charge from the CdSe state to the CdS state. This gradient band structure also allows electrons to be delocalized throughout the conduction band of the NCs, which ensures a very high emission QY (≈50 %).…”

“… We have already mentioned that a CdSe/CdS quasi‐type II NC structure exists in the core of the gradient alloy, in which electrons can delocalize throughout the CB, whereas holes can be localized to the valence band of the CdSe core. , Therefore, the additional long cooling component of 4 ps for the undoped alloy NCs and 5 ps for the Mn‐doped alloy NCs can be attributed to the electron cooling time in the delocalized conduction‐band state before relaxation to the conduction‐band edge. This slow electron cooling due to the spatially decoupled electron–hole wavefunction in the quasi‐type II structure has already been reported by us.…”

Tuning Hole and Electron Transfer from Photoexcited CdSe Quantum Dots to Phenol Derivatives: Effect of Electron‐Donating and ‐Withdrawing Moieties

“…El-Sayed et al verified the effect of electron (benzoquinone (BQ)) and hole (4-aminothiophenol( TP)) quenchers on the bleaching recovery dynamics of CdSe QDs. [62,63] The additional fast component (3 ps)i nt he multiexponential recovery dynamics of CdSe/4NP can be considered as the lifetimeo f as hort-lived CT complex, similart oap revious report [67] ( Table 1). On the other hand, in the presence of TP (hole quencher), they observed extremely slow bleaching recovery (> 1ns).…”

Compositional and structural engineering of inorganic nanowires toward advanced properties and applications

“…The CdS‐CdSe system can form a quasi‐type‐II band alignment through a Fermi level alignment owing to the small Δ E c and the low effective mass of electrons . Photogenerated electrons are commonly assumed to extend along the entire structure, whereas holes are predicted to be strongly confined in the valence band of the CdSe shell, which results in a very good charge‐separation efficiency.…”