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

Honarfar, Alireza; Mourad, Hassan; Lin, Weihua; Polukeev, Alexey V.; Rahaman, Ahibur; Abdellah, Mohamed; Chábera, Pavel; Pankratova, Galina; Gorton, Lo; Zheng, Kaibo; Pullerits, Tõnu

doi:10.1021/acsaem.0c02478

Cited by 9 publications

(15 citation statements)

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“…For estimation of the band structure of the CdSe QDs, we used cyclic voltammetry as a common tool for measuring the energy level positions in semiconductors. ,, Measurement of CV of the QDs was done in a three-electrode electrochemical cell. The QD assembly onto TiO 2 -FTO serves as the working electrode, and a platinum plate is used as the counter electrode (see the Methods section).…”

Section: Resultsmentioning

confidence: 99%

“…However, it is not a priori known whether a positive or negative trion has been formed by the photocharging. For a controlled charging of QDs, photochemical doping − or electrochemistry methods ,− can be used. While the negative trion properties have been extensively studied, ,,, only a few articles address the photophysics of the positively charged QDs. ,, Partially, this is related to the fact that in QDs, the spectroscopic signatures of valence band (VB) holes are much weaker than the state-filling signal from the conduction band electrons. , Earlier, the hole-related trions have been studied using photochemical doping to prepare positively charged CdSe/CdS core shell QDs. ,, …”

Section: Introductionmentioning

confidence: 99%

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Ultrafast Spectroelectrochemistry Reveals Photoinduced Carrier Dynamics in Positively Charged CdSe Nanocrystals

et al. 2021

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Extra charges in semiconductor nanocrystals are of paramount importance for their electrically driven optoelectronic and photovoltaic applications. Optical excitations of such charged nanocrystals lead to rapid recombination via an Auger process, which can deteriorate the performance of the corresponding devices. While numerous articles report trion Auger processes in negatively charged nanocrystals, optical studies of well-controlled positive charging of nanocrystals and detailed studies of positive trions remain rare. In this work, we used electrochemistry to achieve positive charging of CdSe nanocrystals, so-called quantum dots (QDs), in a controlled way. Femtosecond transient absorption spectroscopy was applied for in situ investigation of the charge carrier dynamics after optical excitation of the electrochemically charged QD assembly on TiO 2 . We observe that without bias (i.e., neutral QDs), subpicosecond hot carrier cooling is followed by multiple phases of the dynamics corresponding to electron injection and transfer to the TiO 2 . Positive charging first leads to activation of the hole traps close to the valence band maximum, which opens a rapid recombination channel of the optical excitation. A further increase in the positive bias interrupts the electron injection to TiO 2 , and if nanocrystals are positively charged, it leads to Auger relaxation in a few hundred picosecond timescale. This study represents a step toward the understanding of the effect of positive charging on the performance of semiconductor nanocrystals under conditions which closely mimic their potential applications.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Ultrafast Spectroelectrochemistry Reveals Photoinduced Carrier Dynamics in Positively Charged CdSe Nanocrystals

et al. 2021

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“… 6 Throughout the years, numerous studies have addressed a broad set of fundamental questions regarding excited states and their dynamics in QDs. 7 − 12 Several recent studies have addressed issues like high-intensity effects 13 − 16 and the influence of charging on excited-state dynamics 17 , 14 —all important from the point of view of possible optoelectronic applications of QDs.…”

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confidence: 99%

Excited States and Their Dynamics in CdSe Quantum Dots Studied by Two-Color 2D Spectroscopy

Wang

Lenngren

Amarotti

et al. 2022

J. Phys. Chem. Lett.

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Quantum dots (QDs) form a promising family of nanomaterials for various applications in optoelectronics. Understanding the details of the excited-state dynamics in QDs is vital for optimizing their function. We apply two-color 2D electronic spectroscopy to investigate CdSe QDs at 77 K within a broad spectral range. Analysis of the electronic dynamics during the population time allows us to identify the details of the excitation pathways. The initially excited high-energy electrons relax with the time constant of 100 fs. Simultaneously, the states at the band edge rise within 700 fs. Remarkably, the excited-state absorption is rising with a very similar time constant of 700 fs. This makes us reconsider the earlier interpretation of the excited-state absorption as the signature of a long-lived trap state. Instead, we propose that this signal originates from the excitation of the electrons that have arrived in the conduction-band edge.

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“…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…”

Section: Resultsmentioning

confidence: 99%

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

Widness

Enny

McFarlane-Connelly

et al. 2022

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Strong reducing agents (< -2.0 V vs SCE) enable a wide array of useful organic chemistry, but suffer from a variety of limitations. Stoichiometric metallic reductants such as alkali metals and SmI2 are commonly employed for these reactions, however considerations including expense, ease of use, safety, and waste generation limit the practicality of these methods. Recent approaches utilizing energy from multiple photons or electron-primed photoredox catalysis have accessed reduction potentials equivalent to Li0 and shown how this enables selective transformations of aryl chlorides via aryl radicals. However, in some cases low stability of catalytic intermediates can limit turnover numbers. Herein we report the ability of CdS nanocrystal quantum dots (QDs) to function as strong photoreductants and present evidence that a highly reducing electron is generated from two consecutive photoexcitations of CdS QDs with intermediate reductive quenching. Mechanistic experiments suggest that Auger recombination, a photophysical phenomenon known to occur in photoexcited anionic QDs, generates transient thermally excited electrons to enable the observed reductions. Using blue LEDs and sacrificial amine reductants, aryl chlorides and phosphate esters with reduction potentials up to -3.4 V vs. SCE are photo-reductively cleaved to afford hydrodefunctionalized or functionalized products. In contrast to small molecule catalysts, the QDs are stable under these conditions and turnover numbers up to 47500 have been achieved. These conditions can also effect other challenging reductions, such as tosylate protecting group removal from amines, debenzylation of alcohols, and reductive ring-opening of cyclopropanecarboxylic acid derivatives.

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

Cited by 9 publications

References 49 publications

Ultrafast Spectroelectrochemistry Reveals Photoinduced Carrier Dynamics in Positively Charged CdSe Nanocrystals

Ultrafast Spectroelectrochemistry Reveals Photoinduced Carrier Dynamics in Positively Charged CdSe Nanocrystals

Excited States and Their Dynamics in CdSe Quantum Dots Studied by Two-Color 2D Spectroscopy

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

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