Electron Stability and Negative-Tetron Luminescence in Free-Standing Colloidal n-Type CdSe/CdS Quantum Dots

Abstract

We examine the effects of CdS shell growth on photochemical reduction of colloidal CdSe quantum dots (QDs) and describe the spectroscopic properties of the resulting n-type CdSe/CdS QDs. CdS shell growth greatly slows electron trapping. Because of this improvement, complete two-electron occupancy of the 1S conduction-band orbital is achieved in CdSe/CdS QDs and found to be much more stable than in past experiments. Simultaneous photoluminescence at two different energies is now observed from QDs possessing two excess conduction-band electrons, reflecting competing recombination of discretized 1S and 1P conduction-band electrons within photogenerated four-carrier negative tetrons (three electrons and one hole). Stable occupancy of the 1P level is not achievable under these conditions, and possible reasons are discussed. The stability and accessibility of these multielectron configurations, and the facile spectroscopic detection of negative tetrons, both make photodoped core/shell QDs attractive for exploring the physical properties of free-standing heavily n-doped colloidal CdSe-based QDs.

Electrochemical Modulation of the Photophysics of Surface-Localized Trap States in Core/Shell/(Shell) Quantum Dot Films

“…When approximately two electrons are injected into the 1S e levels, a second PL band is observed, which is typically assigned to radiative recombination involving electrons in the higher 1P-state in the CB and is also referred to in literature as negative tetron PL (Figures e,g and d). ,− We find that for thin CdS shells, there is negligible to no negative tetron luminescence (Figure c), even if the 1S e level is almost completely filled (and the 1S 3/2 1S e transition almost entirely bleached, ⟨ N 1S e ⟩ = 1.7, Figure S7) according to differential absorbance measurements (Figure d). This means that a part of the ensemble of the QD film has two electrons in the CB edge, and therefore that optical excitation should give rise to population of the 1P e state in those QDs.…”

Engineering Auger recombination in colloidal quantum dots via dielectric screening

“…For Auger recombination, theoretical 4,47 and experimental 51 studies reported that Auger recombination rate of a negative tetron should be 3–5 times faster than that for the corresponding negative trion, which is inconsistent with the results for multiple dots for either sample (Fig. 4f).…”

Photoexcitation Dynamics in Electrochemically Charged CdSe Quantum Dots: From Hot Carrier Cooling to Auger Recombination of Negative Trions

“…At −2 V, since QDs become double charged, the corresponding excited states are called tetrons, with three electrons and a hole . The hot electron relaxation (380 fs) has about the same time scale as in the case of the negative trions.…”