Charge Transfer from n-Doped Nanocrystals: Mimicking Intermediate Events in Multielectron Photocatalysis

In Situ Studies of Multi-Carrier Dynamics in Electrochemically-Charged Colloidal CdSe/CdS Core/Shell Quantum Dots

The application of semiconductor nanocrystals (NCs) in optoelectronic devices and photocatalysis inevitably involves them in charged states. The carrier and exciton dynamics of electrochemically charged NCs in solutions have yet to be reported. Herein, the electrochemical charging effects in colloidal CdSe/CdS core/shell quantum dots (QDs) are systematically investigated using static spectroelectrochemistry (SEC) and in situ transient absorption (TA) spectroscopy. Static SEC reveals the presence of in-gap trap states from 0.9 eV below the conduction band (CB) edge. Negligible changes in TA spectra and kinetics were observed from open circuit potential (OCP) to more anodic potentials within the QD band gap. At cathodic potentials, the negatively charged QDs show band edge trion decay with a lifetime of 690 ± 31 ps and slower 1P to 1S electron relaxation with time constants of 12.4 ± 0.8 ps assigned to the spin blockade effect and 316 ± 35 ps assigned to the phonon bottleneck effect. Our study reveals rich effects of charging on QD excited state under nearly native conditions.

Revealing the Phonon Bottleneck Limit in Negatively Charged CdS Quantum Dots

The capture of photoexcited hot electrons in semiconductors before they lose their excess energy to cooling is a long-standing goal in photon energy conversion. Semiconductor nanocrystals have large electron energy spacings that are expected to slow down electron relaxation by phonon emission, but hot electrons in photoexcited nanocrystals, nevertheless, cool rapidly by energy transfer to holes. This makes the intrinsic phonon-bottleneck-limited hot electron lifetime in nanocrystals elusive. We used a combination of theory and experiments to probe the hot-electron dynamics of negatively charged cadmium sulfide (CdS) colloidal quantum dots (QDs) in the absence of holes. Experiments found that these hot electrons cooled on a 100 ps time scale. Theoretical simulations predicted that pure phonon-bottleneck-limited hot electron cooling occurs on a similar time scale. This similarity suggests that the experimental measurements reflect the upper limit on the hot-electron lifetimes in these CdS QDs and the lower limit on the rates of processes that can harvest those hot electrons.

Coherent manipulation of photochemical spin-triplet formation in quantum dot-molecule hybrids

The interconversion between singlet and triplet spin states of photogenerated radical pairs is a genuine quantum process, which can be harnessed to coherently manipulate the recombination products through a magnetic field. This control is central to such diverse fields as molecular optoelectronics, quantum sensing, quantum biology and spin chemistry, but its effect is typically fairly weak in pure molecular systems. Here we introduce hybrid radical pairs constructed from semiconductor quantum dots and organic molecules. The large g-factor difference enables us to directly observe the radical-pair spin quantum beats usually hidden in previous studies, which are further accelerated by the strong exchange coupling of radical pairs enabled by the quantum confinement of quantum dots. The rapid quantum beats enable the efficient and coherent control of charge recombination dynamics at room temperature, with the modulation level of the yield of spin-triplet products reaching 400%.

Hot Electron Cooling in n-Doped Colloidal Nanoplatelets Following Near-Infrared Intersubband Excitation

Intersubband transition was recently discovered in colloidal nanoplatelets, but the associated intersubband carrier relaxation dynamics remains poorly understood. In particular, it is crucial to selectively excite the intersubband transition and to follow the hot electron dynamics in the absence of valence-band holes. This is achieved herein by exciting the predoped electrons in CdSe/ZnS nanoplatelets using near-infrared femtosecond pulses and monitoring nonequilibrium electron dynamics using broad-band visible pulses. We find that the n = 2 electrons relax to the n = 1 subband and establish a Fermi-Dirac distribution within 200 fs, and finally reach an equilibrium with the lattice within a few ps. The cooling dynamics depend mainly on the excitation fluence but weakly on the doping density and the lattice temperature. These characteristics are well captured by our numerical simulation that explicitly accounts for the state occupation effect and optical phonon scattering.

Meter-scale chemiluminescent carbon nanodot films for temperature imaging

Chemiluminescence (CL), as one class of luminescence driven by chemical reaction, exhibits obvious temperature-dependence in its light emission process. Herein, temperature-dependent CL emission of carbon nanodots (CDs) in the chemical reaction of peroxalate and hydrogen peroxide is demonstrated and temperature imaging based on the temperature-dependent CL has been established for the first time. In detail, the temperature-dependent CL emission of CDs in the chemical reaction of peroxalate and hydrogen peroxide is observed, and the linear relationship between the CL intensity and temperature is demonstrated in both the CL solution and film, enabling their applications in temperature sensing and imaging capabilities. The increase of the CL emission with temperature can be attributed to the accelerated electron exchange between the CDs and intermediate generated in the peroxalate system. Meter-scale chemiluminescent CD films have been constructed. The CL sensor based on the films presents a high spatial resolution of 0.4 mm and an outstanding sensitivity of 0.08 °C^-1, which is amongst the best values for the thermographic luminophores. With the unique temperature response and flexible properties, non-planar, meter-scale and sensitive palm temperature imaging has been achieved. These findings present new opportunities for designing CL-based temperature probes and thermography.

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

Strong reducing agents (<-2.0 V vs saturated calomel electrode (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 light-emitting diodes (LEDs) and sacrificial amine reductants, aryl chlorides and phosphate esters with reduction potentials up to -3.4 V vs SCE are photoreductively cleaved to afford hydrodefunctionalized or functionalized products. In contrast to small-molecule catalysts, QDs are stable under these conditions and turnover numbers up to 47 500 have been achieved. These conditions can also effect other challenging reductions, such as tosylate protecting group removal from amines, debenzylation of benzyl-protected alcohols, and reductive ring opening of cyclopropane carboxylic acid derivatives.

Strengthened Optical Nonlinearity of V2C Hybrids Inlaid with Silver Nanoparticles

The investigation of nonlinear optical characteristics resulting from the light–matter interactions of two-dimensional (2D) nano materials has contributed to the extensive use of photonics. In this study, we synthesize a 2D MXene (V2C) monolayer nanosheet by the selective etching of Al from V2AlC at room temperature and use the nanosecond Z-scan technique with 532 nm to determine the nonlinear optical characters of the Ag@V2C hybrid. The z-scan experiment reveals that Ag@V2C hybrids usually exhibits saturable absorption owing to the bleaching of the ground state plasma, and the switch from saturable absorption to reverse saturable absorption takes place. The findings demonstrate that Ag@V2C has optical nonlinear characters. The quantitative data of the nonlinear absorption of Ag@V2C varies with the wavelength and the reverse saturable absorption results from the two-photon absorption, which proves that Ag@V2C hybrids have great potential for future ultrathin optoelectronic devices.

Effect of Ultrafast Broadband Nonlinear Optical Responses by Doping Silver into Ti3C2 Nanosheets at Visible Spectra

Ti3C2 nanosheet is a newly discovered two-dimensional (2D) clan. It turns out to have encouraging applications for electromagnetic shielding and energy storage. Here, Ag@ Ti3C2 hybrids are precisely synthesized by using the one-step solution processing method. Also, their ultrafast broadband nonlinear optical responses in the visible region are studied systematically through nanosecond open-aperture Z-scan and transient absorption techniques. The mechanism of two-photon absorption (TPA) is disclosed in the visible region (409–532 nm). When the laser energy is low and the wavelength is longer than 400 nm, nonlinear absorption cannot happen. Meanwhile, as the laser energy increases, two photons will be absorbed by the electrons in the valence band and the electrons will jump to the conduction band. The process is named as two-photon absorption which will make the specimen show reverse saturable absorption (RSA) properties. What is more, the ultrafast carrier dynamics of the specimen are studied by using the transient absorption. The result shows that the decay contains two phases: the fast and then the slow one. The two phases first come from electron–phonon and then from phonon–phonon interactions, respectively. The electron transfer and charge carrier trapping processes are further verified by the outcomes of similar measurements on Ag@ Ti3C2 hybrids. Besides, the two decay processes increase together with the pump fluence. These results show that Ti3C2 nanosheet has potential applications in broadband optical limiter.

Slow Auger Recombination of Trapped Excitons Enables Efficient Multiple Electron Transfer in CdS-Pt Nanorod Heterostructures

Solar-to-fuel conversion reaction often requires multiple proton-coupled electron transfer (PCET) processes powered by the energetic electrons and/or holes generated by the absorption of multiple photons. The effective coupling of multiple electron transfer from the light absorber with the multiple PCET reactions at the catalytic center is one of the key challenges in efficient and selective conversion of solar energy to chemical fuels. In this paper, we examine the dynamics of multiple electron transfer in quantum confined CdS nanorods with a Pt tip, in which the CdS rod functions as the light absorber and the Pt tip the catalytic center. By excitation-fluence-dependent transient absorption spectroscopic measurements, we show that the multiexciton Auger recombination rate in CdS rods follows a carrier-collision model, k_n^A = n²(n - 1)/4k₂^A, with a biexciton lifetime (1/k₂^A) of 2.0 ± 0.2 ns. In CdS-Pt nanorods, electron transfer kinetics from the CdS conduction band edge to the Pt show negligible dependence on the excitation fluence, occurring with a half-life time of 5.6 ± 0.6 ps. The efficiency of multiple exciton dissociation by multiple electron transfer to Pt decreases from 100% in biexciton states to ∼41% at 22 exciton state due to the competition with Auger recombination. The half-lifetime of the n-charge separated state recombination (with n electrons in the Pt and n holes in the CdS) decreases from 10 μs in the single charge separated state to 42 ns in nine charge separated states. Our findings suggest the possibility of driving multielectron photocatalytic reactions under intense illumination and controlling product selectivity through multielectron transfer.

Colloidal n-Doped CdSe and CdSe/ZnS Nanoplatelets

Colloidal semiconductor nanoplatelets (NPLs) are chemical versions of well-studied quantum wells (QWs). For QWs, gating and carrier doping are standard tools to manipulate their optical, electric, or magnetic properties. It would be highly desirable to use pure chemical methods to dope extra charge carriers into free-standing colloidal NPLs to achieve a similar level of manipulation. Here we report colloidal n-doped CdSe and CdSe/ZnS NPLs achieved through a photochemical doping method. The extra electrons doped into the conduction band edges are evidenced by exciton absorption bleaches recoverable through dedoping and the appearance of new intersub-band transitions in the near-infrared. A high surface ligand coverage is the key to successful doping; otherwise, the doped electrons can be depleted likely by unpassivated surface cations. Large trion binding energies of 20-30 meV are found for the n-doped CdSe NPLs, which, in contrast, are reduced by 1 order of magnitude in CdSe/ZnS core/shell NPLs due to dielectric screening. Furthermore, we identify a long-lived negative trion with a lifetime of 1.5-1.6 ns that is likely dominated by radiative recombination.

Marcus inverted region of charge transfer from low-dimensional semiconductor materials

A key process underlying the application of low-dimensional, quantum-confined semiconductors in energy conversion is charge transfer from these materials, which, however, has not been fully understood yet. Extensive studies of charge transfer from colloidal quantum dots reported rates increasing monotonically with driving forces, never displaying an inverted region predicted by the Marcus theory. The inverted region is likely bypassed by an Auger-like process whereby the excessive driving force is used to excite another Coulomb-coupled charge. Herein, instead of measuring charge transfer from excitonic states (coupled electron-hole pairs), we build a unique model system using zero-dimensional quantum dots or two-dimensional nanoplatelets and surface-adsorbed molecules that allows for measuring charge transfer from transiently-populated, single-charge states. The Marcus inverted region is clearly revealed in these systems. Thus, charge transfer from excitonic and single-charge states follows the Auger-assisted and conventional Marcus charge transfer models, respectively. This knowledge should enable rational design of energetics for efficient charge extraction from low-dimensional semiconductor materials as well as suppression of the associated energy-wasting charge recombination.

Marcus inverted region for charge transfer from low-dimensional semiconductor materials has been long sought after. Here, the authors reveal this region by directly measuring charge transfer from single-charge states rather than excitonic states.

Ultrafast photophysical process of bi-exciton Auger recombination in CuInS2 quantum dots studied by transient-absorption spectroscopy

Auger recombination is an ultrafast and unnegligible photophysical process in colloidal semiconductor quantum dots (QDs) due to competition with charge separation or radiative recombination processes, pivotal for their applications ranging from bio-labeling, light-emitting diodes, QD lasing to solar energy conversion. Among diverse QDs, ternary chalcopyrite is recently receiving significant attention for its heavy-metal free property and remarkable optical performance. Given deficient understanding of the Auger process for ternary chalcopyrite QDs, CuInS₂ QDs with various sizes are synthesized as a representative and the bi-exciton lifetime (τ_BX) is derived by virtue of ultrafast time resolved absorption spectrum. The trend of τ_BX varying with size is consistent with the universal scaling of τ_BX versus QD volume (V): τ_BX = γV. The scaling factor γ is 6.6 ± 0.5 ps·nm^-3 for CuInS₂ QDs, and the bi-exciton Auger lifetime is 4-5 times slower than typical CdSe QDs with the same volume, suggesting reduced Auger recombination rate in ternary chalcopyrite. This work facilitates clearer understanding of Auger process and provides further insight for rational design of light-harvesting and emitting devices based on ternary chalcopyrite QDs.

Broadband Visible Nonlinear Absorption and Ultrafast Dynamics of the Ti3C2 Nanosheet

The Ti3C2 nanosheet, as a new two-dimensional (2D) group, has been found to have attractive characteristics as material for electromagnetic shielding and energy storage. In this study, the nonlinear broadband absorption and ultrafast dynamics of the Ti3C2 nanosheet were investigated using nanosecond open-aperture Z-scan and transient absorption techniques. The mechanism of two-photon absorption (TPA) was revealed in the visible region (475-700 nm). At lower incident energies, nonlinear absorption could not happen. When the laser energy increased to 0.64 GW/cm2, electrons in the valence band could absorb two photons and jump to the conduction band, with TPA occurring, which meant that the sample exhibited reverse saturable absorption (RSA). In addition, when transient absorption was used to investigate the ultrafast carrier dynamics of the sample, it demonstrated that the relaxation contains a fast decay component and a slow one, which are obtained from electron-phonon and phonon-phonon interactions, respectively. Moreover, with the increasing pump fluence, the fast decay lifetime τ1 increased from 3.9 to 4.5 ps, and the slow one τ2 increased from 11.1 to 13.2 ps. These results show that the Ti3C2 nanosheet has potential applications in broadband optical limiters.

Coulomb Barrier for Sequential Two-Electron Transfer in a Nanoengineered Photocatalyst

Multielectron photocatalysis requires sequential, multiple charge transfer from the light absorber to the catalytic site. As a result, many-body effects induced by charge accumulation play a fundamental role in these reactions, especially when photocatalysts are miniaturized to the nanoscale. Here, we study sequential two-electron transfer in a state-of-the-art nanophotocatalyst, CdSe@CdS dot-in-rod (DIR) decorated with Pt tips, using pump-pump-probe transient absorption spectroscopy. Following the first electron transfer (ET) from DIR to the Pt tip, the second ET needs to not only compete with Auger recombination of a positively charged exciton but also experience a large Coulomb barrier exerted by two holes. As a result, both the ET rate and efficiency decrease by an order of magnitude. Analysis using a dissociation-limited long-range charge transfer model reveals that the Coulomb barrier of the second ET is ∼60 meV higher than that of the first one. This study not only uncovers the mechanism and efficiency bottleneck of a real multielectron photocatalyst but also provides general guidelines for the design of multielectron photocatalytic systems.

Physical mechanism of special type photoinduced charge transfer in one-photon and two-photon absorption of Mobius rings

In this work, the monomers and dimers of the Mobius ring were studied using density functional theory (DFT), symmetric matching perturbation theory (SAPT) and various excited state wave function analysis methods. The one-photon and two-photon absorption and their charge transfer excitation characteristics of the Mobius ring were qualitatively and quantitatively analyzed. The exchange-related induced super-exchange charge transfer, charge sequence transfer and local excitation-enhanced charge transfer were analyzed and discussed. The relationship between the super-exchange action and the transition distance and the degree of electron hole density separation is obtained. A mechanism to enhance charge transfer excitation was discovered during the two-photon transition.

White Photoluminescent Ti3C2 MXene Quantum Dots with Two-Photon Fluorescence

A recently created class of inorganic 2D materials, MXenes, has become a subject of intensive research. Reducing their dimensionality from 2D to 0D quantum dots (QDs) could result in extremely useful properties and functions. However, this type of research is scarce, and the reported Ti3C2 MXene QDs (MQDs) have only shown blue fluorescence emission. This work demonstrates a facile, high-output method for preparing bright white emitting Ti3C2 MQDs. The resulting product is two layers thick with a lateral dimension of 13.1 nm. Importantly, the as prepared Ti3C2 MQDs present strong two-photon white fluorescence. Their fluorescence under high pressure is also investigated and it is found that the white emission is very stable and the pressure makes it possible to change from cool white emission to warm white emission. Hybrid nanocomposites are then fabricated by polymerizing Ti3C2 MQDs in polydimethylsiloxane (PDMS) solution, and the bright white emitting hybrid materials in white light-emitting diodes are used. This work provides a facile and general approach to modulate various nanoscale MXene materials, and could further aid the wide development of applications for MXene materials in various optical-related fields.

Picosecond multi-hole transfer and microsecond charge-separated states at the perovskite nanocrystal/tetracene interface

Hole transfer (HT) is often kinetically sluggish compared to electron transfer (ET), which increases recombination losses and thus limits the efficiency of many energy conversion devices such as light-emitting diodes, solar cells and solar-fuel production systems. Recently introduced lead halide perovskites and their nanocrystals (NCs) have emerged as an important class of energy conversion materials. Here we report that tetracene molecules can enable ultrafast and efficient HT from perovskite NCs. Transient absorption measurements reveal that HT occurs in 7.6 ± 0.2 ps, and the charge-separated states are extremely long-lived (5.1 ± 0.3 μs). Such exceptional charge separation properties are leveraged to demonstrate the dissociation of up to 5.6 excitons per NC by multi-hole transfer for the first time. These results not only suggest that tetracenes are an effective hole-extracting material for perovskite devices, but also have important implications for using perovskite NCs as sensitizers and tetracenes as redox mediators to drive single and even multi-electron photochemical reactions.

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.