Superposition Principle in Auger Recombination of Charged and Neutral Multicarrier States in Semiconductor Quantum Dots

Application of colloidal semiconductor quantum dots (QDs) in optical and optoelectronic devices is often complicated by unintentional generation of extra charges, which opens fast nonradiative Auger recombination pathways whereby the recombination energy of an exciton is quickly transferred to the extra carrier(s) and ultimately dissipated as heat. Previous studies of Auger recombination have primarily focused on neutral and, more recently, negatively charged multicarrier states. Auger dynamics of positively charged species remains more poorly explored due to difficulties in creating, stabilizing, and detecting excess holes in the QDs. Here we apply photochemical doping to prepare both negatively and positively charged CdSe/CdS QDs with two distinct core/shell interfacial profiles (“sharp” <i>versus</i> “smooth”). Using neutral and charged QD samples we evaluate Auger lifetimes of biexcitons, negative and positive trions (an exciton with an extra electron or a hole, respectively), and multiply negatively charged excitons. Using these measurements, we demonstrate that Auger decay of both neutral and charged multicarrier states can be presented as a superposition of independent <i>elementary</i> three-particle Auger events. As one of the manifestations of the <i>superposition principle</i>, we observe that the biexciton Auger decay rate can be presented as a sum of the Auger rates for independent negative and positive trion pathways. By comparing the measurements on the QDs with the “sharp” <i>versus</i> “smooth” interfaces, we also find that while affecting the absolute values of Auger lifetimes, manipulation of the shape of the confinement potential does not lead to violation of the superposition principle, which still allows us to accurately predict the biexciton Auger lifetimes based on the measured negative and positive trion dynamics. These findings indicate considerable robustness of the superposition principle as applied to Auger decay of charged and neutral multicarrier states, suggesting its generality to quantum-confined nanocrystals of arbitrary compositions and complexities

Charge Transfer from <i>n</i>‑Doped Nanocrystals: Mimicking Intermediate Events in Multielectron Photocatalysis

In multielectron photocatalytic reactions, an absorbed photon triggers charge transfer from the light-harvester to the attached catalyst, leaving behind a charge of the opposite sign in the light-harvester. If this charge is not scavenged before the absorption of the following photons, photoexcitation generates not neutral but charged excitons from which the extraction of charges should become more difficult. This is potentially an efficiency-limiting intermediate event in multielectron photocatalysis. To study the charge dynamics in this event, we doped CdS nanocrystal quantum dots (QDs) with an extra electron and measured hole transfer from <i>n</i>-doped QDs to attached acceptors. We find that the Auger decay of charged excitons lowers the charge separation yield to 68.6% from 98.4% for neutral excitons. In addition, the hole transfer rate in the presence of two electrons (1290 ps) is slower than that in the presence one electron (776 ps), and the recombination rate of charge separated states is about 2 times faster in the former case. This model study provides important insights into possible efficiency-limiting intermediate events involved in photocatalysis

Electron Transfer into Electron-Accumulated Nanocrystals: Mimicking Intermediate Events in Multielectron Photocatalysis II

The overall efficiency of multielectron photocatalytic reactions is often much lower than the charge-separation yield reported for the first charge-transfer (CT) event. Our recent study has partially linked this gap to CT from charge-accumulated light harvesters. Another possible intermediate event lowering the efficiency is CT into charge-accumulated nanocatalysts. To study this event, we built a “toy” system using nanocrystal quantum dots (QDs) doped with extra electrons to mimick charge-accumulated nanocatalysts. We measured electron transfer (ET) from photoexcited molecular light harvesters into doped QDs using transient absorption spectroscopy. The measurements reveal that the pre-existing electron slows down ET from 37.8 ± 2.2 ps in the neutral sample to 93.4 ± 8.6 ps in the singly doped sample, accelerates charge recombination (CR) from 7.02 ± 0.84 to 3.69 ± 0.25 ns, and lowers the electron-injection yield by ∼14%. This study uncovers yet another possible intermediate event lowering the efficiency of multielectron photocatalysis

Exciton Localization and Dissociation Dynamics in CdS and CdS–Pt Quantum Confined Nanorods: Effect of Nonuniform Rod Diameters

One-dimensional colloidal multicomponent semiconductor nanorods, such as CdSe–CdS dot-in-rod, have been extensively studied as a promising class of new materials for solar energy conversion because of the possibilities of using the band alignment of component materials and the rod-diameter-dependent quantum confinement effect to control the location of electrons and holes and to incorporate catalysts through the growth of Pt tips. Here we used CdS nanorods as an example to study the effect of nonuniform diameters along the rod on the exciton localization and dissociation dynamics in CdS and (platinum tipped) CdS–Pt nanorods. We showed that, in CdS nanorods prepared by seeded growth, the presence of a bulb with a larger diameter around the CdS seed resulted in an additional absorption band lower in energy than the exciton in the CdS rod. As a result, excitons generated in the CdS rod could undergo ultrafast localization to the bulb region in addition to trapping on the CdS rod. We observed that the Pt tip led to fast exciton dissociation by electron transfer. However, excitons localized on the CdS bulb showed slower average ET rates than those localized in the rod region. Our findings suggested that the effect of rod morphology should be carefully considered in designing multicomponent nanorods for solar energy conversion applications

Observation of Internal Photoinduced Electron and Hole Separation in Hybrid Two-Dimentional Perovskite Films

Two-dimensional (2D) organolead halide perovskites are promising for various optoelectronic applications. Here we report a unique spontaneous charge (electron/hole) separation property in multilayered (BA)₂(MA)_<i>n</i>−1Pb_<i>n</i>I_3<i>n</i>+1 (BA = CH₃(CH₂)₃NH₃⁺, MA = CH₃NH₃⁺) 2D perovskite films by studying the charge carrier dynamics using ultrafast transient absorption and photoluminescence spectroscopy. Surprisingly, the 2D perovskite films, although nominally prepared as “<i>n</i> = 4”, are found to be mixture of multiple perovskite phases, with <i>n</i> = 2, 3, 4 and ≈ ∞, that naturally align in the order of <i>n</i> along the direction perpendicular to the substrate. Driven by the band alignment between 2D perovskites phases, we observe consecutive photoinduced electron transfer from small-<i>n</i> to large-<i>n</i> phases and hole transfer in the opposite direction on hundreds of picoseconds inside the 2D film of ∼358 nm thickness. This internal charge transfer efficiently separates electrons and holes to the upper and bottom surfaces of the films, which is a unique property beneficial for applications in photovoltaics and other optoelectronics devices

“Intact” Carrier Doping by Pump–Pump–Probe Spectroscopy in Combination with Interfacial Charge Transfer: A Case Study of CsPbBr₃ Nanocrystals

Carrier doping is important for semiconductor nanocrystals (NCs) as it offers a new knob to tune NCs’ functionalities, in addition to size and shape control. Also, extensive studies on NC devices have revealed that under operating conditions NCs are often unintentionally doped with electrons or holes. Thus, it is essential to be able to control the doping of NCs and study the carrier dynamics of doped NCs. The extension of previously reported redox-doping methods to chemically sensitive materials, such as recently introduced perovskite NCs, has remained challenging. We introduce an “intact” carrier-doping method by performing pump–pump–probe transient absorption spectroscopy on NC–acceptor complexes. The first pump pulse is used to trigger charge transfer from the NC to the acceptor, leading to NCs doped with a band edge carrier; the following pump–probe pulses measure the dynamics of carrier-doped NCs. We performed this measurement on CsPbBr₃ NCs and deduced positive and negative trion lifetimes of 220 ± 50 and 150 ± 40 ps, respectively, for 10 nm diameter NCs, both dominated by Auger recombination. It also allowed us to identify randomly photocharged excitons in CsPbBr₃ NCs as positive trions

Plasmon-Induced Hot Electron Transfer from the Au Tip to CdS Rod in CdS-Au Nanoheterostructures

The plasmon-exciton interaction mechanisms in CdS–Au colloidal quantum-confined plexcitonic nanorod heterostructures have been studied by transient absorption spectroscopy. Optical excitation of plasmons in the Au tip leads to hot electron injection into the CdS rod with a quantum yield of ∼2.75%. This finding suggests the possibility of further optimization of plasmon-induced hot electron injection efficiency through controlling the size and shape of the plasmonic and excitonic domains for potential light harvesting applications

Efficient Extraction of Trapped Holes from Colloidal CdS Nanorods

Cadmium Sulfide (CdS) nanostructures have been widely applied for solar driven H₂ generations due to its suitable band gap and band edge energetics. For an efficient photoreduction reaction, hole scavenging from CdS needs to compete favorably with many recombination processes. Extensive spectroscopic studies show evidence for hole trapping in CdS nanostructures, which naturally leads the concern of extracting trapped holes from CdS in photocatalytic reactions. Here, we report a study of hole transfer dynamics from colloidal CdS nanorods (NRs) to adsorbed hole acceptor, phenothiazine (PTZ), using transient absorption spectroscopy. We show that >99% of the holes were trapped (with a time constant of 0.73 ps) in free CdS NRs to form a photoinduced transient absorption (PA) feature. In the presence of PTZ, we observed the decay of the PA feature and corresponding formation of oxidized PTZ⁺ radicals, providing direct spectroscopic evidence for trapped hole transfer from CdS. The trapped holes were extracted by PTZ in 3.8 ± 1.7 ns (half-life) to form long-lived charge separated states (CdS^–-PTZ⁺) with a half lifetime of 310 ± 50 ns. This hole transfer time is significantly faster than the slow conduction band electron–trapped hole recombination (half lifetime of 67 ± 1 ns) in free CdS NRs, leading to an extraction efficiency of 94.7 ± 9.0%. Our results show that despite rapid hole trapping in CdS NRs, efficient extraction of trapped holes by electron donors and slow recombination of the resulting charge-separated states can still be achieved to enable efficient photoreduction using CdS nanocrystals

Beyond Band Alignment: Hole Localization Driven Formation of Three Spatially Separated Long-Lived Exciton States in CdSe/CdS Nanorods

Colloidal one-dimensional semiconductor nanoheterostructures have emerged as an important family of functional materials for solar energy conversion, although the nature of the long-lived exciton state and their formation and dissociation dynamics remain poorly understood. In this paper we study these dynamics in CdSe/CdS dot-in-rod (DIR) NRs, a representative of 1D heterostructures, and DIR-electron-acceptor complexes by transient absorption spectroscopy. Because of a quasi-type II band alignment of CdSe and CdS, it is often assumed that there exists one long-lived exciton state with holes localized in the CdSe seed and electrons delocalized among CdSe and CdS. We show that excitation into the CdS rod forms three distinct types of long-lived excitons that are spatially localized in the CdS rod, in and near the CdSe seed and in the CdS shell surrounding the seed. The branching ratio of forming these exciton states is controlled by the competition between the band offset driven hole localization to the CdSe seed and hole trapping to the CdS surface. Because of dielectric contrast induced strong electron–hole interaction in 1D materials, the competing hole localization pathways lead to spatially separated long-lived excitons. Their distinct spatial locations affect their dissociation rates in the presence of electron acceptors, which has important implications for the application of 1D heterostructures as light-harvesting materials

Ultrafast Charge Separation and Long-Lived Charge Separated State in Photocatalytic CdS–Pt Nanorod Heterostructures

Colloidal semiconductor–metal nanoheterostructures that combine the light-harvesting ability of semiconductor nanocrystals with the catalytic activity of small metal nanoparticles show promising applications for photocatalysis, including light-driven H₂ production. The exciton in the semiconductor domain can be quenched by electron-, hole-, and energy transfer to the metal particle, and the competition between these processes determines the photocatalytic efficiency of these materials. Using ultrafast transient absorption spectroscopy, we show that, in CdS–Pt heterostructures consisting of a CdS nanorod with a Pt nanoparticle at one end, the excitons in the CdS domain dissociate by ultrafast electron transfer (with a half-life of ∼3.4 ps) to the Pt. The charge separated state is surprisingly long-lived (with a half-life of ∼1.2 ± 0.6 μs) due to the trapping of holes in CdS. The asymmetry in the charge separation and recombination times is believed to be the key feature that enables the accumulation of the transferred electrons in the Pt tip and photocatalysis in the presence of sacrificial hole acceptors