Spectroscopic Evidence for the Contribution of Holes to the Bleach of Cd-Chalcogenide Quantum Dots

Abstract

In transient absorption (TA) measurements on Cd-chalcogenide quantum dots (QDs), the presence of a band-edge (BE) bleach signal is commonly attributed entirely to conduction-band electrons in the 1S(e) state, neglecting contributions from BE holes. While this has been the accepted view for more than 20 years, and has often been used to distinguish electron and hole kinetics, the reason for the absence of a hole contribution to the BE-bleach has remained unclear. Here, we show with three independent experiments that holes do in fact have a significant impact on the BE-bleach of well-passivated Cd-chalcogenide QD samples. Transient absorption experiments on high photoluminescence quantum yield CdSe/CdS/ZnS core–shell–shell QDs clearly show an increase of the band-edge bleach as holes cool down to the band edge. The relative contribution of electron-to-hole bleach is 2:1, as predicted by theory. The same measurements on core-only CdSe QDs with a lower quantum yield do not show a contribution of holes to the band-edge bleach. We assign the lack of hole bleach to the presence of ultrafast hole trapping in samples with insufficient passivation of the QD surface. In addition, we show measurements of optical gain in core–shell–shell QD solutions, providing clear evidence of a significant hole contribution to the BE transient absorption signal. Finally, we present spectroelectrochemical measurements on CdTe QDs films, showing the presence of a BE-bleach for both electron and hole injections. The presence of a contribution of holes to the bleach in passivated Cd-chalcogenides QDs bears important implications for quantitative studies on optical gain as well as for TA determinations of carrier dynamics.

Atomic fluctuations in electronic materials revealed by dephasing

“…Dephasing arises from two contributions: incoherent relaxation processes and decorrelation of participating states. The X 2 → X 1 hole relaxation occurs with a timescale of 200 fs for CdSe QDs of this size, which is much slower than other dephasing mechanisms and is thus safely neglected (71)(72)(73)(74)(75). The exact timescale for relaxation depends on a number of sample parameters, but fast population decay would be visible in the kinetics shown in Fig.…”

Surface passivation extends single and biexciton lifetimes of InP quantum dots

“…In QDs, the XB results from the state-filling effect of photogenerated electrons and/or holes, which block the ground-state excitonic transition of the QDs and leads to an absorbance decrease (or bleach) feature in TA spectra. 2 , 41 , 42 Due to the large difference in the effective mass of electrons ( m e = 0.08) and holes ( m h = 0.64) in InP QDs, 15 the density of hole states is much larger than that of electrons, and XB signal is expected to originate mostly from the state-filling effect of electrons. The nature of PAs in QDs vary between different systems and have been previously shown to originate from the intraband transition of photogenerated electrons 15 or free/trapped holes.…”

Overcoming the exciton binding energy in two-dimensional perovskite nanoplatelets by attachment of conjugated organic chromophores

“…The occurrence of electron transfer also explain the fast initial decay of the bleach of the NPLs mentioned above. It is well-documented that excitons in NPLs and nanocrystals exhibit a larger ground state bleach than charged particles, but at the same wavelength 45 .…”