Band-edge oscillator strength of colloidal CdSe/CdS dot-in-rods: comparison of absorption and time-resolved fluorescence spectroscopy

Abstract

We studied the oscillator strength f of the band gap transition in heteronanocrystals (hNCs) with a spherical CdSe core embedded in an elongated CdS shell. A comparison with f of core-only CdSe NCs confirmed a reduction of the electron-hole overlap in hNCs with a band gap larger than 2.05 eV or smaller than 1.98 eV. However, the decrease in f is limited to about 50% when compared to CdSe NCs, suggesting that residual confinement still localizes the electron near the core. We correlated f with the radiative lifetime obtained from multiexponential photoluminescence (PL) decay traces. The different components were attributed to radiative decay, or deep and shallow carrier trapping, respectively, using the PL quantum efficiency (QE) as a guideline. Our data highlight the challenges associated when extracting the radiative decay, and demonstrate the added value of absorption spectroscopy to obtain the band-edge oscillator strength and the associated radiative recombination rate in colloidal hNCs.

Electronic coupling in colloidal quantum dot molecules; the case of CdSe/CdS core/shell homodimers

“…spectroscopy) measurements (See supplementary materials for more details) [53][54][55] . The absorption cross section of non-fused dimers (green Fig.…”

“…However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS DOI:10.1063/1.5128086 spectroscopy) measurements (See supplementary materials for more details) [53][54][55] . The absorption cross section of non-fused dimers (green Fig.…”

Switchable dissociation of excitons bound at strained CdTe/CdS interfaces

“…The transition from delocalised (quasi type-II regime) to localised electrons (type-I regime) occurs between CdSe seed sizes of 3 to 4 nm. 6,9,10 A similar system in which electrons and holes separate spatially at a heterointerface with staggered band alignment (type-II structure) has been fabricated using ZnSe nanocrystals as seeds in ZnSe/CdS nanorods. 5,11,12 Different interpretations exist in the literature concerning the delocalisation and spatial separation of electrons and holes in these systems.…”

“…15 However, all reported lifetimes for CdSe/CdS support coulomb-bound excitons and holes localised in the potential well of the seed particles. 10 Charge carrier dynamics are further complicated by surface states. It was shown that in pure CdS nanorods over 99% of the photoexcited holes get trapped in surface states within the first picoseconds after excitation, which is observed using TA spectroscopy as a weak, broad photoinduced absorption below the band gap.…”

“…Smith et al have shown that compression of small nanocrystals by a mismatched shell distorts the electronic structure strongly enough to shift it between the type-I and type-II regime. 28 A deviation from the type-I behaviour of CdSe/CdS seeded nanorods with large (>4.5 nm) seeds was attributed to strain by Angeloni et al 10 Christodoulou et al have shown for much larger CdSe/CdS rod-in-rod nanocrystals that lattice mismatch between core and shell leads to piezoelectric potentials at the opposite ends of the elongated seed. These also cause band distortions that lead to formation of a type-II system.…”