Size-dependent valence and conduction band-edge energies of semiconductor nanocrystals

Composition-dependent band structure parameters and band-gap bowing effect in a caesium lead mixed halide system: a cyclic voltammetry investigation

Cyclic voltammetry techniques have been employed to study the effect of halide substitution on the band edge parameters and band gap bowing effect in the case of CsPbX3 [X = I, Br, Cl] perovskite nanocrystals (PNCs). A series of compositions, viz. CsPbI3, CsPb(I-Br)3, CsPbBr3, CsPb(Br-Cl)3 and CsPbCl3, have been prepared by a hot injection method. From powder XRD and HR-TEM analysis, the formation of a highly crystalline, cubic phase of the perovskite having size in the range from 7-20 nm has been confirmed. Sharp peaks in the photoluminescence spectra suggest the formation of quantum dots with narrow-size distribution. The composition-dependent optical band gap (εopgap) for CsPbX3 displays a systematic shift towards shorter wavelengths from I to Br to Cl substitutions. The cyclic voltammetry investigation on the dispersion of PNCs in nonaqueous solvents yielded prominent cathodic and anodic peaks. These are correlated to conduction (e1) and valence band edge (h1) positions, respectively. The h1 has been decreased substantially with I to Br to Cl in CsPbX3. Meanwhile, e1 shows a marginal increase. The values derived from CV data demonstrated an excellent match with UVPS results, reported for a similar system. From these results, the quasi-particle gap (εqpgap) and exciton binding energy have been estimated for all the compositions. The negative band gap bowing effect noted in these PNCs is attributed to the size quantization effect. The band-edge parameters reported in this work will be valuable in matching these heterojunctions with suitable electron/hole transport materials for optimum device-performance.

Probing surface interactions in CdSe quantum dots with thiocyanate ligands

Surface chemistry dictates the optoelectronic properties of semiconductor quantum dots (QDs). Tailoring these properties relies on the meticulous selection of surface ligands for efficient passivation. While long-chain organic ligands boast a well-understood passivation mechanism, the intricacies of short inorganic ionic ligands remain largely unexplored. This study sheds light on the surface-passivation mechanism of short inorganic ligands, particularly focusing on SCN- ions on CdSe QDs. Employing steady-state and time-resolved infrared spectroscopic techniques, we elucidated the surface-ligand interactions and coordination modes of SCN--capped CdSe QDs. Comparative analysis with studies on CdS QDs unveils intriguing insights into the coordination behavior and passivation efficacy of SCN- ions on Cd2+ rich QD surfaces. Our results reveal the requirement of both surface-bound (strong binding) and weakly-interacting interfacial SCN- ions for effective CdSe QD passivation. Beyond fostering a deeper understanding of surface-ligand interactions and highlighting the importance of a comprehensive exploration of ligand chemistries, this study holds implications for optimizing QD performance across diverse applications.

Heavy-Metal-Free Colloidal Quantum Dots: Progress and Opportunities in Solar Technologies

Colloidal quantum dots (QDs) hold great promise as building blocks in solar technologies owing to their remarkable photostability and adjustable properties through the rationale involving size, atomic composition of core and shell, shapes, and surface states. However, most high-performing QDs in solar conversion contain hazardous metal elements, including Cd and Pb, posing significant environmental risks. Here, a comprehensive review of heavy-metal-free colloidal QDs for solar technologies, including photovoltaic (PV) devices, solar-to-chemical fuel conversion, and luminescent solar concentrators (LSCs), is presented. Emerging synthetic strategies to optimize the optical properties by tuning the energy band structure and manipulating charge dynamics within the QDs and at the QDs/charge acceptors interfaces, are analyzed. A comparative analysis of different synthetic methods is provided, structure-property relationships in these materials are discussed, and they are correlated with the performance of solar devices. This work is concluded with an outlook on challenges and opportunities for future work, including machine learning-based design, sustainable synthesis, and new surface/interface engineering.

Narrowband Organic/Inorganic Hybrid Afterglow Materials

Narrowband afterglow materials display interesting functions in high-quality anti-counterfeiting and multiplexed bioimaging. However, there is still a limited exploration of these afterglow materials, especially for those with a full width at half maxima (FWHM) around 30 nm. Here, we report the fabrication of narrowband organic/inorganic hybrid afterglow materials via energy transfer technology. Coronene (Cor) with a long phosphorescence feature and broad phosphorescence band is selected as the donor for energy transfer, and inorganic quantum dots (QDs) of CdSe/ZnS with a narrowband emission are used as acceptors. Upon doping into the organic matrix, the resultant three-component materials exhibit a narrowband afterglow with an afterglow lifetime of approximately 3.4 s and an FWHM of 31 nm. The afterglow wavelength of the afterglow materials can be controlled by the QDs. This work based on organic/inorganic hybrids provides a facile approach for developing multicolor and narrowband afterglow materials, as well as opens a new way for expanding the features of organic afterglow for multifunctional applications. It is expected to rely on narrowband afterglow emitters to solve the "spectrum congestion" problem of high-density information storage in optical anti-counterfeiting and information encryption.

Charge Transfer from Quantum-Confined 0D, 1D, and 2D Nanocrystals

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D-2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.

The size-dependent valence and conduction band-edge energies of Cu quantum dots

Ultra-small metal particles having band gaps are regarded as a new class of functional materials. We investigated the size dependencies of the band-edge energies on Cu quantum-dots in the size range of 0.7-2.1 nm. The extremely high conduction band-edge energies owing to the strong quantum-size effects were observed for sizes below 1 nm.

Solvation Shifts the Band-Edge Position of Colloidal Quantum Dots by Nearly 1 eV

The optoelectronic properties of colloidal quantum dots (cQDs) depend critically on the absolute energy of the conduction and valence band edges. It is well known these band-edge energies are sensitive to the ligands on the cQD surface, but it is much less clear how they depend on other experimental conditions, like solvation. Here, we experimentally determine the band-edge positions of thin films of PbS and ZnO cQDs via spectroelectrochemical measurements. To achieve this, we first carefully evaluate and optimize the electrochemical injection of electrons and holes into PbS cQDs. This results in electrochemically fully reversible electron injection with >8 electrons per PbS cQDs, allowing the quantitative determination of the conduction band energy for PbS cQDs with various diameters and surface compositions. Surprisingly, we find that the band-edge energies shift by nearly 1 eV in the presence of different solvents, a result that also holds true for ZnO cQDs. We argue that complexation and partial charge transfer between solvent and surface ions are responsible for this large effect of the solvent on the band-edge energy. The trend in the energy shift matches the results of density functional theory (DFT) calculations in explicit solvents and scales with the energy of complexation between surface cations and solvents. As a first approximation, the solvent Lewis basicity can be used as a good descriptor to predict the shift of the conduction and valence band edges of solvated cQDs.

Highly Crystalized Cl-Doped SnO2 Nanocrystals for Stable Aqueous Dispersion Toward High-Performance Perovskite Photovoltaics

Tin dioxide (SnO2 ) with high conductivity and low photocatalytic activity has been reported as one of the best candidates for highly efficient electron transport layer (ETL) in perovskite solar cell (PSC). The state-of-the-art SnO2 layer is achieved by chemical bath deposition with tunable properties, while the commercial SnO2 nanocrystals (NCs) with low tunability still face the necessity of further improvement. Here, a kind of highly crystallized Cl-doped SnO2 NCs is reported that can form very stable aqueous dispersion with shelf life up to one year without any stabilizer, which can facilitate the fabrication of PSCs with satisfactory performance. Compared to the commercial SnO2 NCs regardless of the extrinsic Cl-doping conditions, the intrinsic Cl-doped SnO2 NCs effectively suppress the energy barrier and reduces the trap state density at the buried interface between perovskite and ETL. Consequently, stable PSCs based on such Cl-doped SnO2 NCs achieve a champion efficiency up to ≈25% for small cell (0.085 cm2 ) and ≈20% for mini-module (12.125 cm2 ), indicating its potential as a promising candidate for ETL in high-performance perovskite photovoltaics.

Hybrid CdSe/ZnS Quantum Dot-Gold Nanoparticle Composites Assembled by Click Chemistry: Toward Affordable and Efficient Redox Photocatalysts Working with Visible Light

A new modular, easy-to-synthesize photocatalyst was prepared by assembling colloidal CdSe/ZnS quantum dots (QD) and gold nanoparticles (AuNP) via their ligands thanks to copper-catalyzed azide to alkyne cycloaddition (CuAAC) click chemistry. The resulting composite (QD-AuNP) photocatalyst was tested with a benchmark photoredox system previously reported by our group, for which QD alone acted as a photocatalyst but with a modest quantum yield (QY = 0.06%) and turnover number (TON = 350 in 3 h) due to poor charge separation. After optimization, the QD-AuNP composites exhibited much improved photocatalytic performances: up to five times higher TON (2600 in 3 h) and up to 24 times faster reaction in the first 10 min of visible irradiation. Such an improvement is attributed to an efficient electron transfer from QD to AuNP in the photoexcited QD-AuNP composites, which ensures a much better charge separation than that in QD alone. This was confirmed by studying both (i) the quenching of the QD photoluminescence during the synthesis of the QD-AuNP composites and (ii) the blue shift of the AuNP plasmon absorption band due to the accumulation of up to 7400 electrons per AuNP in QD-AuNP composites under visible light irradiation in the presence of electron donors.

Semiconductor Quantum Dots Act as Photocatalysts for Carbon-Carbon Bond Formation: Selective Functionalization of Xanthene's 9H Position

The potential of CdSe, CdS, MoS2, and WS2 QDs as semiconductor photocatalysts for selective functionalization of the xanthene 9H position through carbon-carbon bond formation has been investigated. Our study reveals valuable insights into the energy-transfer and electron-transfer pathways involved in these reactions, as well as the radical polar crossover (RPC) and triplet-to-triplet energy transfer (TTEnT) processes. Notably, this approach offers a range of intriguing features, including visible-light-mediated processes, inexpensive catalytic systems, mild reaction conditions, broad substrate scope, unfunctionalized starting materials, and suitability for gram-scale synthesis. This study makes a significant contribution to the newly emerging field of QD-catalyzed reactions, paving the way for future explorations.

Synthesis of biologically important tetrahydroisoquinoline (THIQ) motifs using quantum dot photocatalyst and evaluation of their anti-bacterial activity

Our study introduces an efficient photocatalytic approach for synthesizing biologically significant C1-substituted tetrahydroisoquinoline (THIQ) motifs, employing WS2 quantum dots (QDs) as catalysts. This method enables the formation of C-C and C-P bonds at the C1 position of the THIQ motif. The resulting compounds exhibit substantial antimicrobial activity against methicillin-resistant Staphylococcus aureus (MRSA) bacteria, with low minimum inhibitory concentration (MIC) values. Notably, the WS2 QD catalyst demonstrates recyclability and suitability for gram-scale reactions, underscoring the sustainability and scalability of our approach. Overall, our research presents a versatile and cost-effective strategy for synthesizing C1-substituted THIQ derivatives, highlighting their potential as novel therapeutic agents in biology and medicinal chemistry.

Efficient Hole Transfer from CdSe Quantum Dots Enabled by Oxygen-Deficient Polyoxovanadate-Alkoxide Clusters

A limitation of the implementation of cadmium chalcogenide quantum dots (QDs) in charge transfer systems is the efficient removal of photogenerated holes. Rapid hole transfer has typically required the ex situ functionalization of hole acceptors with groups that can coordinate to the surface of the QD. In addition to being synthetically limiting, this strategy also necessitates a competitive binding equilibrium between the hole acceptor and native, solubilizing ligands on the nanocrystal. Here we show that the incorporation of oxygen vacancies into polyoxovanadate-alkoxide clusters improves hole transfer kinetics by promoting surface interactions between the metal oxide assembly and the QD. Investigating the reactivity of oxygen-deficient clusters with phosphonate-capped QDs reveals reversible complexation of the POV-alkoxide with a phosphonate ligand at the nanocrystal surface. These findings reveal a new method of facilitating QD-hole acceptor association that bypasses the restrictions of exchange interactions.

P- and N-type InAs nanocrystals with innately controlled semiconductor polarity

InAs semiconductor nanocrystals (NCs) exhibit intriguing electrical/optoelectronic properties suitable for next-generation electronic devices. Although there is a need for both n- and p-type semiconductors in such devices, InAs NCs typically exhibit only n-type characteristics. Here, we report InAs NCs with controlled semiconductor polarity. Both p- and n-type InAs NCs can be achieved from the same indium chloride and aminoarsine precursors but by using two different reducing agents, diethylzinc for p-type and diisobutylaluminum hydride for n-type NCs, respectively. This is the first instance of semiconductor polarity control achieved at the synthesis level for InAs NCs and the entire semiconductor nanocrystal systems. Comparable field-effective mobilities for holes (3.3 × 10-3 cm2/V·s) and electrons (3.9 × 10-3 cm2/V·s) are achieved from the respective NC films. The mobility values allow the successful fabrication of complementary logic circuits, including NOT, NOR, and NAND comprising photopatterned p- and n-channels based on InAs NCs.

Rhombohedral boron monosulfide as a metal-free photocatalyst

Most of previous photocatalysts contain metal species, thus exploring a metal-free photocatalyst is still challenging. A metal-free photocatalyst has an advantage for the development of economical and non-toxic artificial photosynthesis system and/or environmental purification applications. In this study, rhombohedral boron monosulfide (r-BS) was synthesized by a high-pressure solid-state reaction, and its photocatalytic properties were investigated. r-BS absorbed visible light, and its photocurrent action spectrum also exhibited visible light responsivity. The r-BS evolved hydrogen (H2) from water under ultraviolet (UV) as well as under visible light irradiation, and its internal quantum efficiency reached 1.8% under UV light irradiation. In addition to the H2 evolution reaction, the r-BS photocatalyst drove carbon dioxide (CO2) reduction and dye oxidation reactions under UV irradiation. Although bare r-BS was not so stable under strong light irradiation in water, cocatalyst modification improved its stability. These results indicate that r-BS is a new class of non-metal photocatalyst applicable for H2 production, CO2 reduction, and environmental purification reactions.

Ultrasmall water-stable CsPbBr3 quantum dots with high intensity blue emission enabled by zeolite confinement engineering

Ultrasmall CsPbBr3 perovskite quantum dots (PQDs) as promising blue-emitting materials are highly desired for full-color display and lighting applications, but their inferior efficiency and poor ambient stability hinder extensive applications. Herein, a "break-and-repair" strategy has been developed to tightly confine monodispersed ultrasmall CsPbBr3 PQDs in a zeolite. In this strategy, the CsPbBr3 PQDs are introduced into the zeolite via a high temperature evaporation method, wherein the perovskite precursors break the zeolite framework, and amino acids and silane are then used to fix the damaged framework and lock the perovskite QDs within the matrix. By modulating the synthetic conditions to control the growth of CsPbBr3, PQDs with ultrasmall size of 2 nm have been obtained in the zeolite, giving emission centered at 460 nm with a high quantum yield of 76.93%. Strikingly, the PQDs@zeolite composite exhibits water-induced reversible photoluminescence promoted by the coordination between the amino acids and PQDs in a dynamic manner, achieving enhanced water stability (14 days in aqueous solution). This work provides a new perspective for the synthesis of water-stable blue-emitting perovskite composites for potential applications in lighting fields.

Molecular imprinting-based indirect fluorescence detection strategy implemented on paper chip for non-fluorescent microcystin

Fluorescence analysis is a fast and sensitive method, and has great potential application in trace detection of environmental toxins. However, many important environmental toxins are non-fluorescent substances, and it is still a challenge to construct a fluorescence detection method for non-fluorescent substances. Here, by means of charge transfer effect and smart molecular imprinting technology, we report a sensitive indirect fluorescent sensing mechanism (IFSM) and microcystin (MC-RR) is selected as a model target. A molecular imprinted thin film is immobilized on the surface of zinc ferrite nanoparticles (ZnFe2O4 NPs) by using arginine, a dummy fragment of MC-RR. By implementation of IFSM on the paper-based microfluidic chip, a versatile platform for the quantitative assay of MC-RR is developed at trace level (the limit of detection of 0.43 μg/L and time of 20 min) in real water samples without any pretreatment. Importantly, the proposed IFSM can be easily modified and extended for the wide variety of species which lack direct interaction with the fluorescent substrate. This work offers the potential possibility to meet the requirements for the on-site analysis and may explore potential applications of molecularly imprinted fluorescent sensors.

Molecular surface programming of rectifying junctions between InAs colloidal quantum dot solids

Heavy-metal-free III-V colloidal quantum dots (CQDs) show promise in optoelectronics: Recent advancements in the synthesis of large-diameter indium arsenide (InAs) CQDs provide access to short-wave infrared (IR) wavelengths for three-dimensional ranging and imaging. In early studies, however, we were unable to achieve a rectifying photodiode using CQDs and molybdenum oxide/polymer hole transport layers, as the shallow valence bandedge (5.0 eV) was misaligned with the ionization potentials of the widely used transport layers. This occurred when increasing CQD diameter to decrease the bandgap below 1.1 eV. Here, we develop a rectifying junction among InAs CQD layers, where we use molecular surface modifiers to tune the energy levels of InAs CQDs electrostatically. Previously developed bifunctional dithiol ligands, established for II-VI and IV-VI CQDs, exhibit slow reaction kinetics with III-V surfaces, causing the exchange to fail. We study carboxylate and thiolate binding groups, united with electron-donating free end groups, that shift upward the valence bandedge of InAs CQDs, producing valence band energies as shallow as 4.8 eV. Photophysical studies combined with density functional theory show that carboxylate-based passivants participate in strong bidentate bridging with both In and As on the CQD surface. The tuned CQD layer incorporated into a photodiode structure achieves improved performance with EQE (external quantum efficiency) of 35% (>1 μm) and dark current density < 400 nA cm-2, a >25% increase in EQE and >90% reduced dark current density compared to the reference device. This work represents an advance over previous III-V CQD short-wavelength IR photodetectors (EQE < 5%, dark current > 10,000 nA cm-2).

Spectral widths and Stokes shifts in InP-based quantum dots

InP-based quantum dots (QDs) have Stokes shifts and photoluminescence (PL) line widths that are larger than in II-VI semiconductor QDs with comparable exciton energies. The mechanisms responsible for these spectral characteristics are investigated in this paper. Upon comparing different semiconductors, we find the Stokes shift decreases in the following order: InP > CdTe > CdSe. We also find that the Stokes shift decreases with core size and decreases upon deposition of a ZnSe shell. We suggest that the Stokes shift is largely due to different absorption and luminescent states in the angular momentum fine structure. The energy difference between the fine structure levels, and hence the Stokes shifts, are controlled by the electron-hole exchange interaction. Luminescence polarization results are reported and are consistent with this assignment. Spectral widths are controlled by the extent of homogeneous and inhomogeneous broadening. We report PL and PL excitation (PLE) spectra that facilitate assessing the roles of homogeneous and different inhomogeneous broadening mechanisms in the spectra of zinc-treated InP and InP/ZnSe/ZnS particles. There are two distinct types of inhomogeneous broadening: size inhomogeneity and core-shell interface inhomogeneity. The latter results in a distribution of core-shell band offsets and is caused by interfacial dipoles associated with In-Se or P-Zn bonding. Quantitative modeling of the spectra shows that the offset inhomogeneity is comparable to but somewhat smaller than the size inhomogeneity. The combination of these two types of inhomogeneity also explains several aspects of reversible hole trapping dynamics involving localized In3+/VZn2- impurity states in the ZnSe shells.

Synthesis of Layered Lead-Free Perovskite Nanocrystals with Precise Size and Shape Control and Their Photocatalytic Activity

Halide perovskites have attracted enormous attention due to their potential applications in optoelectronics and photocatalysis. However, concerns over their instability, toxicity, and unsatisfactory efficiency have necessitated the development of lead-free all-inorganic halide perovskites. A major challenge in designing efficient halide perovskites for practical applications is the lack of effective methods for producing nanocrystals with precise size and shape control. In this work, a layered perovskite, Cs4ZnSb2Cl12 (CZS), is found from calculations to exhibit size- and facet-dependent optoelectronic properties in the nanoscale, and thus, a colloidal method is used to synthesize the CZS nanoparticles with size-tunable morphologies: zero- (nanodots), one- (nanowires and nanorods), two- (nanoplates), and three-dimensional (nanopolyhedra). The growth kinetics of the CZS nanostructures, along with the effects of surface ligands, reaction temperature, and time were investigated. The optoelectronic properties of the nanocrystals varied with size due to quantum confinement effects and with shape due to anisotropy within the crystals and the exposure of specific facets. These properties could be modulated to enhance the visible-light photocatalytic performance for toluene oxidation. In particular, the 9.7 nm CZS nanoplates displayed a toluene to benzaldehyde conversion rate of 1893 μmol g-1 h-1 (95% selectivity), 500 times higher than the bulk synthesized CZS, and comparable with the reported photocatalysts. This study demonstrates the integration of theoretical calculations and synthesis, revealing an approach to the design and fabrication of novel, high-performance colloidal perovskite nanocrystals for optoelectronic and photocatalytic applications.

Serum Lipidomic Fingerprints Encode Early Diagnosis and Staging of Lung Cancer on a Novel PbS/Au-Layered Substrate

Lung cancer (LC) is a major cause of mortality among malignant tumors. Early diagnosis through lipidomic profiling can improve prognostic outcomes. In this study, a uniform PbS/Au-layered substrate that enhances the laser desorption/ionization process, an interfacial process triggered on the substrate surface upon laser excitation, was designed to efficiently characterize the lipidomic profiles of LC patient serum. By controlling the stacking arrangement and particle sizes of PbS QDs and AuNPs, the optimized substrate promotes the generation of excited electrons and creates an enhanced electric field that polarizes analyte molecules, facilitating ion adduction formation ([M + Na]⁺ and [M + K]⁺) and enhancing detection sensitivity down to the femtomole level. Combining multivariate statistics and machine learning, a distinct lipidomic biomarker panel is successfully identified for the early diagnosis and staging of LC, with an accurate prediction validated by an area under the curve of 0.9479 and 0.9034, respectively. We also found that 18 biomarkers were significantly correlated with six metabolic pathways associated with LC. These results demonstrate the potential of this innovative PbS/Au-layered substrate as a sensitive platform for accurate diagnosis of LC and facilitate the development of lipidomic-based diagnostic tools for other cancers.

CdS Quantum Dots for Metallaphotoredox-Enabled Cross-Electrophile Coupling of Aryl Halides with Alkyl Halides

Semiconductor quantum dots (QDs) offer many advantages as photocatalysts for synthetic photoredox catalysis, but no reports have explored the use of QDs with nickel catalysts for C-C bond formation. We show here that 5.7 nm CdS QDs are robust photocatalysts for photoredox-promoted cross-electrophile coupling (40 000 TON). These conditions can be utilized on small scale (96-well plate) or adapted to flow. NMR studies show that triethanolamine (TEOA) capped QDs are the active catalyst and that TEOA can displace native phosphonate and carboxylate ligands, demonstrating the importance of QD surface chemistry.

Characterizing the Semiconductor Nanocrystal Surface through Chemical Reactivity

ConspectusMany desirable and undesirable properties of semiconductor nanocrystals (NCs) can be traced to the NC surface due to the large surface-to-volume ratio. Therefore, precise control of the NC surface is imperative to achieve NCs with the desired qualities. Ligand-specific reactivity and surface heterogeneity make it difficult to accurately control and tune the NC surface. Without a molecular-level appreciation of the NC surface chemistry, modulating the NC surface is impossible and the risk of introducing deleterious surface defects is imminent. To gain a more comprehensive understanding of the surface reactivity, we have utilized a variety of spectroscopic techniques and analytical methods in concert.This Account describes our use of robust characterization techniques and ligand exchange reactions in effort to establish a molecular-level understanding of NC surface reactivity. The utility of NCs in target applications such as catalysis and charge transfer hangs on precise tunability of NC ligands. Modulating the NC surface requires the necessary tools to monitor chemical reactions. One commonly utilized analytical method to achieve targeted surface compositions is ¹H nuclear magnetic resonance (NMR) spectroscopy. Here we describe our use of ¹H NMR spectroscopy to monitor chemical reactions at CdSe and PbS NC surfaces to identify ligand specific reactivity. However, seemingly straightforward ligand exchange reactions can vary widely depending on the NC materials and anchoring group. Some non-native X-type ligands will irreversibly displace native ligands. Other ligands exist in equilibrium with native ligands. Depending on the application, it is important to understand the nature of exchange reactions. This level of understanding can be obtained by extracting exchange ratios, exchange equilibrium, and reaction mechanism information from ¹H NMR spectroscopy to establish precise NC reactivity.Reactivity that occurs through multiple, parallel ligand exchange mechanisms can involve both the liberation of metal-based Z-type ligands in addition to reactivity of X-type ligands. In these reactions, ¹H NMR spectroscopy fails to discern between an X-type oleate or a Z-type Pb(oleate)₂ because only the alkene resonance of the organic constituent is probed by this method. Multiple, parallel reaction pathways occur when thiol ligands are introduced to oleate-capped PbS NCs. This necessitated the use of synergistic characterization methods including ¹H NMR spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, and inductively coupled plasma mass spectrometry (ICP-MS) to characterize both surface-bound and liberated ligands.Similar analytical methods have been employed to probe the NC topology, which is an important, but often overlooked, component to NC reactivity given the facet-specific reactivity of PbS NCs. Through the tandem use of NMR spectroscopy and ICP-MS, we have monitored the liberation of Pb(oleate)₂ as an L-type ligand is titrated to the NC to determine the quantity and equilibrium of Z-type ligands. By studying a variety of NC sizes, we correlated the number of liberated ligands with the size-dependent topology of PbS NCs.Lastly, we incorporate redox-active chemical probes into our toolbox to study NC surface defects. We describe how the site-specific reactivity and relative energetics of redox-active surface-based defects are elucidated using redox probes and show that this reactivity is highly dependent on the surface composition. This Account is designed to encourage readers to consider the necessary characterization techniques needed establish a molecular-level understanding of NC surfaces in their own work.

Shewanella oneidensis MR-1 respires CdSe quantum dots for photocatalytic hydrogen evolution

Living bio-nano systems for artificial photosynthesis are of growing interest. Typically, these systems use photoinduced charge transfer to provide electrons for microbial metabolic processes, yielding a biosynthetic solar fuel. Here, we demonstrate an entirely different approach to constructing a living bio-nano system, in which electrogenic bacteria respire semiconductor nanoparticles to support nanoparticle photocatalysis. Semiconductor nanocrystals are highly active and robust photocatalysts for hydrogen (H2) evolution, but their use is hindered by the oxidative side of the reaction. In this system, Shewanella oneidensis MR-1 provides electrons to a CdSe nanocrystalline photocatalyst, enabling visible light-driven H2 production. Unlike microbial electrolysis cells, this system requires no external potential. Illuminating this system at 530 nm yields continuous H2 generation for 168 h, which can be lengthened further by replenishing bacterial nutrients.

Influence of Capping Ligands, Solvent, and Thermal Effects on CdSe Quantum Dot Optical Properties by DFT Calculations

Cadmium selenide nanomaterials are very important materials in photonics, catalysis, and biomedical applications due to their optical properties that can be tuned through size, shape, and surface passivation. In this report, static and ab initio molecular dynamics density functional theory (DFT) simulations are used to characterize the effect of ligand adsorption on the electronic properties of the (110) surface of zinc blende and wurtzite CdSe and a (CdSe)₃₃ nanoparticle. Adsorption energies depend on ligand surface coverage and result from a balance between chemical affinity and ligand-surface and ligand-ligand dispersive interactions. In addition, while little structural reorganization occurs upon slab formation, Cd···Cd distances become shorter and the Se-Cd-Se angles become smaller in the bare nanoparticle model. This originates mid-gap states that strongly influence the absorption optical spectra of nonpassivated (CdSe)₃₃. Ligand passivation on both zinc blende and wurtzite surfaces does not induce a surface reorganization, and thus, the band gap remains nonaffected with respect to bare surfaces. In contrast, structural reconstruction is more apparent for the nanoparticle, which significantly increases its highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap upon passivation. Solvent effects decrease the band gap difference between the passivated and nonpassivated nanoparticles, the maximum of the absorption spectra being blue-shifted around 20 nm by the effect of the ligands. Overall, calculations show that flexible surface cadmium sites are responsible for the appearance of mid-gap states that are partially localized on the most reconstructed regions of the nanoparticle that can be controlled through appropriate ligand adsorption.

π-Electron-Extended Triazine-Based Covalent Organic Framework as Photocatalyst for Organic Pollution Degradation and H2 Production from Water

Herein, we report the efficient preparation of π-electron-extended triazine-based covalent organic framework (TFP-TPTPh COF) for photocatalysis and adsorption of the rhodamine B (RhB) dye molecule, as well as for photocatalytic hydrogen generation from water. The resultant TFP-TPTPh COF exhibited remarkable porosity, excellent crystallinity, high surface area of 724 m2 g−1, and massive thermal stability with a char yield of 63.41%. The TFP-TPTPh COF demonstrated an excellent removal efficiency of RhB from water in 60 min when used as an adsorbent, and its maximum adsorption capacity (Qm) of 480 mg g−1 is among the highest Qm values for porous polymers ever to be recorded. In addition, the TFP-TPTPh COF showed a remarkable photocatalytic degradation of RhB dye molecules with a reaction rate constant of 4.1 × 10−2 min−1 and an efficiency of 97.02% under ultraviolet–visible light irradiation. Furthermore, without additional co-catalysts, the TFP-TPTPh COF displayed an excellent photocatalytic capacity for reducing water to generate H2 with a hydrogen evolution rate (HER) of 2712 μmol g−1 h−1. This highly active COF-based photocatalyst appears to be a useful material for dye removal from water, as well as solar energy processing and conversion.

Synthesis and Characterization of the Orthorhombic Sn3 O4 Polymorph

An unexplored tin oxide crystal phase (Sn₃ O₄ ) was experimentally synthesized via a facile hydrothermal method. After tuning the often-neglected parameters for the hydrothermal synthesis, namely the degree of filling of the precursor solution and the gas composition in the reactor head space, an unreported X-ray diffraction pattern was discovered. Through various characterization studies, such as Rietveld analysis, energy dispersive X-ray spectroscopy, and first-principles calculations, this novel material was characterized as orthorhombic mixed-valence tin oxide with the composition Sn^II ₂ Sn^IV O₄ . This orthorhombic tin oxide is a new polymorph of Sn₃ O₄ , which differs from the reported conventional monoclinic structure. Computational and experimental analyses showed that orthorhombic Sn₃ O₄ has a smaller band gap (2.0 eV), enabling greater absorption of visible light. This study is expected to improve the accuracy of hydrothermal synthesis and aid the discovery of new oxide materials.

Electronic Structure and Excited State Dynamics of Cadmium Chalcogenide Nanorods

The cylindrical quasi-one-dimensional shape of colloidal semiconductor nanorods (NRs) gives them unique electronic structure and optical properties. In addition to the band gap tunability common to nanocrystals, NRs have polarized light absorption and emission and high molar absorptivities. NR-shaped heterostructures feature control of electron and hole locations as well as light emission energy and efficiency. We comprehensively review the electronic structure and optical properties of Cd-chalcogenide NRs and NR heterostructures (e.g., CdSe/CdS dot-in-rods, CdSe/ZnS rod-in-rods), which have been widely investigated over the last two decades due in part to promising optoelectronic applications. We start by describing methods for synthesizing these colloidal NRs. We then detail the electronic structure of single-component and heterostructure NRs and follow with a discussion of light absorption and emission in these materials. Next, we describe the excited state dynamics of these NRs, including carrier cooling, carrier and exciton migration, radiative and nonradiative recombination, multiexciton generation and dynamics, and processes that involve trapped carriers. Finally, we describe charge transfer from photoexcited NRs and connect the dynamics of these processes with light-driven chemistry. We end with an outlook that highlights some of the outstanding questions about the excited state properties of Cd-chalcogenide NRs.

Sequential Carrier Transfer Can Accelerate Triplet Energy Transfer from Functionalized CdSe Nanocrystals

Nanocrystal (NC)-sensitized triplet-fusion upconversion is a rising strategy to convert long-wavelength, incoherent light into higher-energy output photons. Here, we chart the photophysics of tailor-functionalized CdSe NCs to understand energy transfer to surface-anchored transmitter ligands, which can proceed via correlated exciton transfer or sequential carrier hops. Varying NC size, we observe a pronounced acceleration of energy transfer (from k_quench = 0.0096 ns^-1 ligand^-1 to 0.064 ns^-1 ligand^-1) when the barrier to hole-first sequential transfer is lowered from 100 ± 25 meV to 50 ± 25 meV. This acceleration is 5.1× the expected effect of increased carrier wave function leakage, so we conclude that sequential transfer becomes kinetically dominant under the latter conditions. Last, transient photoluminescence shows that NC band-edge and trap states are comparably quenched by functionalization (up to ∼98% for sequential transfer) and exhibit matched dynamics for t > 300 ns, consistent with a dynamic quasi-equilibrium where photoexcitations can ultimately be extracted even when a carrier is initially trapped.

Review: Quantum Dot Light-Emitting Diodes

Quantum dot light-emitting diodes (QD-LEDs) are one of the most promising self-emissive displays in terms of light-emitting efficiency, wavelength tunability, and cost. Future applications using QD-LEDs can cover a range from a wide color gamut and large panel displays to augmented/virtual reality displays, wearable/flexible displays, automotive displays, and transparent displays, which demand extreme performance in terms of contrast ratio, viewing angle, response time, and power consumption. The efficiency and lifetime have been improved by tailoring the QD structures and optimizing the charge balance in charge transport layers, resulting in theoretical efficiency for unit devices. Currently, longevity and inkjet-printing fabrication of QD-LEDs are being tested for future commercialization. In this Review, we summarize significant progress in the development of QD-LEDs and describe their potential compared to other displays. Furthermore, the critical elements to determine the performance of QD-LEDs, such as emitters, hole/electron transport layers, and device structures, are discussed comprehensively, and the degradation mechanisms of the devices and the issues of the inkjet-printing process were also investigated.

Amine Hole Scavengers Facilitate Both Electron and Hole Transfer in a Nanocrystal/Molecular Hybrid Photocatalyst

A well-known catalyst, fac-Re(4,4'-R₂-bpy)(CO)₃Cl (bpy = bipyridine; R = COOH) (ReC0A), has been widely studied for CO₂ reduction; however, its photocatalytic performance is limited due to its narrow absorption range. Quantum dots (QDs) are efficient light harvesters that offer several advantages, including size tunability and broad absorption in the solar spectrum. Therefore, photoinduced CO₂ reduction over a broad range of the solar spectrum could be enabled by ReC0A catalysts heterogenized on QDs. Here, we investigate interfacial electron transfer from Cd₃P₂ QDs to ReC0A complexes covalently bound on the QD surface, induced by photoexcitation of the QD. We explore the effect of triethylamine, a sacrificial hole scavenger incorporated to replenish the QD with electrons. Through combined transient absorption spectroscopic and computational studies, we demonstrate that electron transfer from Cd₃P₂ to ReC0A can be enhanced by a factor of ∼4 upon addition of triethylamine. We hypothesize that the rate enhancement is a result of triethylamine possibly altering the energetics of the Cd₃P₂-ReC0A system by interacting with the quantum dot surface, deprotonation of the quantum dot, and preferential solvation, resulting in a shift of the conduction band edge to more negative potentials. We also observe the rate enhancement in other QD-electron acceptor systems. Our findings provide mechanistic insights into hole scavenger-quantum dot interactions and how they may influence photoinduced interfacial electron transfer processes.

Visible-light induced three-component reaction for α-aminobutyronitrile synthesis by C-C bond formation using quantum dots as photocatalysts

We describe a three-component reaction of malononitrile, benzaldehyde and N,N-dimethylaniline using aluminium doped CdSeS/CdZnSeS(Al)/ZnS quantum dots (QDs) as visible light catalysts to synthesize α-aminobutyrilitriles at room temperature and under mild conditions. The reactions exhibit high functional group tolerance, and the well dispersed quantum dot catalysts are highly efficient with a turnover number (TON) greater than 1.1 × 10³ and can be recycled at least three times without significant loss of catalytic activity.

CdSe-Decorated Flowerlike CaMoO4 Microspheres with Enhanced Hydrogen Production Activity

Photocatalytic hydrogen production technology from water is a more effective and promising method to solve energy and environmental crises. In this work, flowerlike CaMoO₄ microspheres were successfully synthesized by an ultrasonic precipitation method and modified with variable concentrations of CdSe NCs. CdSe/CaMoO₄ microspheres showed increased light absorption ability, larger relative surface area, lower electrochemical impedance, and longer fluorescence lifetime. The photocatalytic hydrogen production rate of CdSe/CaMoO₄ microspheres could reach up to 10 162.33 μmol g^-1 h^-1. The constructed type-I heterostructure improved the separation of photogenerated electrons and inhibited the rapid recombination of photogenerated electrons and holes, thus enhancing the photocatalytic hydrogen production performance. CdSe/CaMoO₄ with high hydrogen production activity would be an efficient photocatalyst for hydrogen production applications.

Recent progress and future prospects on halide perovskite nanocrystals for optoelectronics and beyond

As an emerging new class of semiconductor nanomaterials, halide perovskite (ABX3, X = Cl, Br, or I) nanocrystals (NCs) are attracting increasing attention owing to their great potential in optoelectronics and beyond. This field has experienced rapid breakthroughs over the past few years. In this comprehensive review, halide perovskite NCs that are either freestanding or embedded in a matrix (e.g., perovskites, metal-organic frameworks, glass) will be discussed. We will summarize recent progress on the synthesis and post-synthesis methods of halide perovskite NCs. Characterizations of halide perovskite NCs by using a variety of techniques will be present. Tremendous efforts to tailor the optical and electronic properties of halide perovskite NCs in terms of manipulating their size, surface, and component will be highlighted. Physical insights gained on the unique optical and charge-carrier transport properties will be provided. Importantly, the growing potential of halide perovskite NCs for advancing optoelectronic applications and beyond including light-emitting devices (LEDs), solar cells, scintillators and X-ray imaging, lasers, thin-film transistors (TFTs), artificial synapses, and light communication will be extensively discussed, along with prospecting their development in the future.

Direct detection of spin polarization in photoinduced charge transfer through a chiral bridge

It is well assessed that the charge transport through a chiral potential barrier can result in spin-polarized charges. The possibility of driving this process through visible photons holds tremendous potential for several aspects of quantum information science, e.g., the optical control and readout of qubits. In this context, the direct observation of this phenomenon via spin-sensitive spectroscopies is of utmost importance to establish future guidelines to control photo-driven spin selectivity in chiral structures. Here, we provide direct proof that time-resolved electron paramagnetic resonance (EPR) can be used to detect long-lived spin polarization generated by photoinduced charge transfer through a chiral bridge. We propose a system comprising CdSe quantum dots (QDs), as a donor, and C60, as an acceptor, covalently linked through a saturated oligopeptide helical bridge (χ) with a rigid structure of ∼10 Å. Time-resolved EPR spectroscopy shows that the charge transfer in our system results in a C60 radical anion, whose spin polarization maximum is observed at longer times with respect to that of the photogenerated C60 triplet state. Notably, the theoretical modelling of the EPR spectra reveals that the observed features may be compatible with chirality-induced spin selectivity, but the electronic features of the QD do not allow the unambiguous identification of the CISS effect. Nevertheless, we identify which parameters need optimization for unambiguous detection and quantification of the phenomenon. This work lays the basis for the optical generation and direct manipulation of spin polarization induced by chirality.

Direct Detection of Long-Range Interdomain Auger Recombination in Dumbbell-Shaped Quasi-Type-II Nanoparticle

Multicarrier dynamics in heterostructured ZnS-AgInS₂ (ZAIS) dumbbell-like nanoparticle (nanodumbell), which consists of two visible-light absorptive domains (ellipsoidal tip domains) directly linked to each end of a 22 nm length rod domain of the ZAIS nanodumbell with a quasi-type-II heterostructure, was investigated by femtosecond transient absorption spectroscopy under variable excitation intensities. Quantitative analysis together with the numerical simulations for the excitation intensity dependence of the dynamics revealed that only one electron-hole pair survived in the overall dumbbell as a consequence of Auger recombination, even though multiple carriers were formed on both terminal tip domains. This result strongly suggested carrier-carrier interaction between the tip domains, leading to the long-range Auger recombination via tunneling across a rod potential barrier.

Biocatalytic conversion of sunlight and carbon dioxide to solar fuels and chemicals

This review discusses the progress in the assembly of photosynthetic biohybrid systems using enzymes and microbes as the biocatalysts which are capable of utilising light to reduce carbon dioxide to solar fuels. We begin by outlining natural photosynthesis, an inspired biomachinery to develop artificial photosystems, and the rationale and motivation to advance and introduce biological substrates to create more novel, and efficient, photosystems. The case studies of various approaches to the development of CO2-reducing microbial semi-artificial photosystems are also summarised, showcasing a variety of methods for hybrid microbial photosystems and their potential. Finally, approaches to investigate the relatively ambiguous electron transfer mechanisms in such photosystems are discussed through the presentation of spectroscopic techniques, eventually leading to what this will mean for the future of microbial hybrid photosystems.

Synthesis, properties and catalysis of quantum dots in C–C and C-heteroatom bond formations

Luminescent quantum dots (QDs) represent a new form of carbon nanomaterials which have gained widespread attention in recent years, especially in the area of chemical sensing, bioimaging, nanomedicine, solar cells, light-emitting diode (LED), and electrocatalysis. Their extremely small size renders some unusual properties such as quantum confinement effects, good surface binding properties, high surface‐to‐volume ratios, broad and intense absorption spectra in the visible region, optical and electronic properties different from those of bulk materials. Apart from, during the past few years, QDs offer new and versatile ways to serve as photocatalysts in organic synthesis. Quantum dots (QD) have band gaps that could be nicely controlled by a number of factors in a complicated way, mentioned in the article. Processing, structure, properties and applications are also reviewed for semiconducting quantum dots. Overall, this review aims to summarize the recent innovative applications of QD or its modified nanohybrid as efficient, robust, photoassisted redox catalysts in C–C and C-heteroatom bond forming reactions. The recent structural modifications of QD or its core structure in the development of new synthetic methodologies are also highlighted. Following a primer on the structure, properties, and bio-functionalization of QDs, herein selected examples of QD as a recoverable sustainable nanocatalyst in various green media are embodied for future reference.

Direct Laser Patterning of CdTe QDs and Their Optical Properties Control through Laser Parameters

Direct laser patterning is a potential and powerful technique to localize nanomaterials within a host matrix. The main goal of this study is to demonstrate that by tuning some parameters of a laser source, like power and laser pulse frequency, it is possible to modify and tune the optical properties of the generated quantum dots (QDs) within a host matrix of a specific chemical composition. The study is realized by using cadmium telluride (CdTe) QD precursors, embedded in polymethylmethacrylate (PMMA) host matrix, as starting materials. The patterning of the CdTe QDs is carried out by using a UV nanosecond laser source at 355. Fluorescence microscopy and photoluminescence spectroscopy, associated with transmission electron microscopy, indicate that it is possible to obtain desired patterns of QDs emitting from green to red of the visible spectrum, due to the formed CdTe QDs. Preliminary highlights of the CdTe QDs' formation mechanism are given in terms of laser power and laser pulse frequency (repetition rate).

A cobalt mimochrome for photochemical hydrogen evolution from neutral water

A system for visible light-driven hydrogen production from water is reported. This system makes use of a synthetic mini-enzyme known as a mimochrome (CoMC6*a) consisting of a cobalt deuteroporphyrin and two attached peptides as a catalyst, [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) as a photosensitizer, and ascorbic acid as a sacrificial electron donor. The system achieves turnover numbers (TONs) up to 10,000 with respect to catalyst and optimal activity at pH 7. Comparison with related systems shows that CoMC6*a maintains the advantages of biomolecular catalysts, while exceeding other cobalt porphyrins in terms of total TON and longevity of catalysis. Herein, we lay groundwork for future study, where the synthetic nature of CoMC6*a will provide a unique opportunity to tailor proton reduction chemistry and expand to new reactivity.

Advances and Challenges in Heavy-Metal-Free InP Quantum Dot Light-Emitting Diodes

Light-emitting diodes based on colloidal quantum dots (QLEDs) show a good prospect in commercial application due to their narrow spectral linewidths, wide color range, excellent luminance efficiency, and long operating lifetime. However, the toxicity of heavy-metal elements, such as Cd-based QLEDs or Pb-based perovskite QLEDs, with excellent performance, will inevitably pose a serious threat to people's health and the environment. Among heavy-metal-free materials, InP quantum dots (QDs) have been paid special attention, because of their wide emission, which can, in principle, be tuned throughout the whole visible and near-infrared range by changing their size, and InP QDs are generally regarded as one of the most promising materials for heavy-metal-free QLEDs for the next generation displays and solid-state lighting. In this review, the great progress of QLEDs, based on the fundamental structure and photophysical properties of InP QDs, is illustrated systematically. In addition, the remarkable achievements of QLEDs, based on their modification of materials, such as ligands exchange of InP QDs, and the optimization of the charge transport layer, are summarized. Finally, an outlook is shown about the challenge faced by QLED, as well as possible pathway to enhancing the device performance. This review provides an overview of the recent developments of InP QLED applications and outlines the challenges for achieving the high-performance devices.

Insight into morphology dependent charge carrier dynamics in ZnSe-CdS nanoheterostructures

Semiconductor nanoheterostructures (NHSs) are being increasingly used for the photocatalytic conversion of solar energy in which photo-induced charge separation is an essential step and hence it is necessary to understand the effect of various factors such as size, shape, and composition on the charge transfer dynamics. Ultrafast transient absorption spectroscopy is used to investigate the nature and dynamics of photo-induced charge transfer processes in ZnSe-CdS NHSs of different morphologies such as nanospheres (NSs), nanorods (NRs), and nanoplates (NPs). It demonstrates the fast separation of charge carriers and localization of both charges in adjacent semiconductors, resulting in the formation of a charge-separated (CS) state. The lifetime of the charge-separated state follows the order of NSs < NPs < NRs, emphasizing the effect of morphology on the enhancement of photo-induced charge separation and suppression of backward recombination. The separated charge carriers have been utilized in visible light driven hydrogen production and the hydrogen generation activity follows the same order as that for the lifetime of the CS state, underlining the role of charge separation efficiency. Therefore, the variation of the morphology of NHSs plays a significant role in their charge carrier dynamics and hence the photocatalytic hydrogen production activity.

Bioelectronic Platform to Investigate Charge Transfer between Photoexcited Quantum Dots and Microbial Outer Membranes

Photosynthetic semiconductor biohybrids (PSBs) convert light energy to chemical energy through photo-driven charge transfer from nanocrystals to microorganisms that perform bioreactions of interest. Initial proof-of-concept PSB studies with an emphasis on enhanced CO₂ conversion have been encouraging; however, bringing the broad prospects of PSBs to fruition is contingent on establishing a firm fundamental understanding of underlying interfacial charge transfer processes. We introduce a bioelectronic platform that reduces the complexity of PSBs by focusing explicitly on interactions between colloidal quantum dots (QDs), microbial outer membranes, and native, small-molecule redox mediators. Our model platform employs a standard three-electrode electrochemical cell with supported outer membranes of Pseudomonas aeruginosa, pyocyanin redox mediators, and semiconducting CdSe QDs dispersed in an aqueous electrolyte. We present a comprehensive electrochemical analysis of this platform via electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and chronoamperometry (CA). EIS reveals the formation and electronic properties of supported outer membrane films. CV reveals the electrochemically active surface area of P. aeruginosa outer membranes and that pyocyanin is the sole species that performs redox with these outer membranes under sweeping applied potential. CA demonstrates that photoexcited charge transfer in this system is driven by the reduction of pyocyanin at the QD surface followed by diffusion of reduced pyocyanin through the outer membrane. The broad applicability of this platform across many bacterial species, QD architectures, and controlled environmental conditions affords the possibility to define design principles for future PSB systems to synergistically integrate concurrent advances in genetically engineered organisms and inorganic nanomaterials.

Using Spectator Ligands to Enhance Nanocrystal-to-Molecule Electron Transfer

Semiconductor nanocrystals (NCs) have emerged as promising photocatalysts. However, NCs are often functionalized with complex ligand shells that contain not only charge acceptors but also other "spectator ligands" that control NC solubility and affinity for target reactants. Here, we show that spectator ligands are not passive observers of photoinduced charge transfer but rather play an active role in this process. We find the rate of electron transfer from quantum-confined PbS NCs to perylenediimide acceptors can be varied by over a factor of 4 simply by coordinating cinnamate ligands with distinct dipole moments to NC surfaces. Theoretical calculations indicate this rate variation stems from both ligand-induced changes in the free energy for charge transfer and electrostatic interactions that alter perylenediimide electron acceptor orientation on NC surfaces. Our work shows NC-to-molecule charge transfer can be fine-tuned through ligand shell design, giving researchers an additional handle for enhancing NC photocatalysis.