Spectroelectrochemistry of Colloidal CdSe Quantum Dots

Abstract

Solution-phase spectroelectrochemistry was used to study electron injection into colloidal CdSe quantum dots (QDs) with sizes ranging from 3.4 to 11.1 nm in tetrahydrofuran (THF). The absorbance and photoluminescence of the QDs were monitored in response to both charging and discharging cycles, and the optical changes were reversible on a timescale of minutes. Bleaching of the QD 1S 3/2h 1S e exciton state was used to determine the conduction band energy levels. We found that the negative trion state was stable in THF for hours under an applied cathodic potential. Both the degree of bleaching and the recovery of the exciton state depended on the applied potential. Based on the current and charge measurements, we found that between 10 and 150 electrons were injected into the QDs, depending on the electrode potential and QD size. Most of the electron injection occurred below the band edge and led to quenching of the QD photoluminescence. The potential at which injection into QDs occurred depended on the nature of the QD ligands.

Permanent Electrochemical Doping of Quantum Dot Films through Photopolymerization of Electrolyte Ions

“…This is not trivial because in experiments that investigate doping of QDs, it is often found that more electrons are injected than that are present in the conduction band. 32 , 64 …”

CdS Quantum Dots as Potent Photoreductants for Organic Chemistry Enabled by Auger Recombination

“…115,118 Light scattering due to QD aggregation may contribute to the broad feature, however both observed spectral changes were fully reversible after exposure to oxygen (Figure 4B, green trace), strongly suggesting that they arise from the presence of injected electrons residing in the CB (the exciton bleaching) as well as newly formed surface states (the additional broad features). 103,118 To confirm the origin of these spectral changes within the catalytic reaction, we undertook spectroelectrochemistry studies, as previously employed to study CdSe nanocrystals 67,119,120 and deeply reducing photocatalytic systems. 12,19 Consistent with reports of CdS band positions, we found that cathodic reduction of 5.9 nm CdS QDs at -2.2 V vs. SCE was sufficient to electrochemically dope the QDs with ~0.5 electrons per QD within 1 h (based on the magnitude of absorbance bleaching of the 1S e feature at 464 nm), 70 mirroring exactly the spectral changes observed in photodoping experiments (Figure 4C, , green to dark blue traces and Supporting…”

“…To address the need for continued development of photoredox catalysts, our group and others have been interested in new applications of QDs in organic chemistry. 26, In addition to their high photostability, QDs exhibit tunable, size-dependent optical and redox properties; are made in single-step syntheses with no chromatography from abundant precursors; 62 reversibly bind to many molecules at once (typically 1 -5 ligands/nm 2 of QD surface are found for closely related CdSe QDs [63][64][65] ) through common organic functional groups (-CO 2 H, -PO 3 H, -SH, -NH 2 ); can become charged with many electrons at once without decomposing; 66,67 and undergo many electronic processes with no direct analogue in smallmolecules. 68 Inspired by reports of conPET-type photoreduction mechanisms operative within commonly used photocatalyst systems, 12,24,69 we envisioned that QDs could achieve a similar mode of reactivity, while also addressing the catalyst stability and availability challenges of organocatalyst-mediated photoreductions.…”

Core-Size-Dependent Trapping and Detrapping Dynamics in CdSe/CdS/ZnS Quantum Dots

“…The mechanism of carrier detrapping has been explained by involving (i) an activation barrier, supported through temperature-dependent lifetime investigations, and (ii) quantum mechanical tunneling . Several approaches have been reported for the design of nearly defect free QDs; for example, overcoating CdSe with a graded as well distinct CdS/ZnS shell. , Despite of all these efforts, trap-states persist in core–shell QDs. , It is difficult to probe trap-states using ensemble measurements since these provide average properties. One of the convenient ways to investigate trap-states in core–shell QD is by using time-resolved single-particle PL studies …”

“…17,29 Despite of all these efforts, trap-states persist in core−shell QDs. 30,31 It is difficult to probe trap-states using ensemble measurements since these provide average properties. One of the convenient ways to investigate trap-states in core−shell QD is by using time-resolved single-particle PL studies.…”