Optical Properties of Strongly Coupled Quantum Dot-Ligand Systems

Semiconductor Band Structure, Symmetry, and Molecular Interface Hybridization for the Chemist

Merging molecular bonding concepts with semiconductor- and materials-based concepts of band structure is challenging due to the mutually exclusive historical development and notations used in those respective fields: symmetry adapted linear combinations (SALCs) and Mulliken terms for molecules, versus k space and Bloch sums for materials. This lack of commonality brings the issue of hybridization (aka electronic coupling) between molecules and materials to the forefront in many aspects of modern chemical research─including nanocrystal properties, solar energy conversion, and molecular computing. It is thus critical to establish a holistic approach to hybridizing orbital (molecule) and plane-wave (semiconductor/material) systems to better describe symmetry-based molecule|material bonding and the corresponding symmetry-adapted molecular orbital (MO) diagrams. Such a unified approach would enable the construction of testable hypotheses about the role of symmetry and electronic structure in determining the extent of electronic coupling between molecular orbitals and semiconductor band structure. This Perspective provides an analysis and compendium of "translations" between the physics and chemistry language of group theory. In this vein, this approach describes the symmetries─and corresponding point groups─that occur in k space along the available descent in symmetry pathways (k space vectors). As a result, chemists may arrive at a more intuitive understanding of the band symmetries of semiconductors, as well as insights into the corresponding algebraic formulations. This analysis can ultimately generate MO diagrams for hybrid molecule|material systems. Lastly, an Outlook provides some context to the application of this analysis to modern problems at the interface of molecular and materials chemistry.

Controlling Electronic Coupling of Acene Chromophores on Quantum Dot Surfaces through Variable-Concentration Ligand Exchange

Controlling the binding of functional organic molecules on quantum dot (QD) surfaces and the resulting ligand/QD interfacial structure determines the resulting organic-inorganic hybrid behavior. In this study, we vary the binding of tetracenedicarboxylate ligands bound to PbS QDs cast in thin films by performing solid-state ligand exchange of as-produced bound oleate ligands. We employ comprehensive Fourier-transform infrared (FTIR) analysis coupled with ultraviolet-visible (UV-vis) spectrophotometric measurements, transient absorption, and Density Functional Theory (DFT) simulations to study the QD/ligand surface structure and resulting optoelectronic properties. We find that there are three primary QD/diacid structures, each with a distinct binding mode dictated by the QD-ligand and ligand-ligand intermolecular and steric interactions. They can be accessed nearly independently of one another via different input ligand concentrations. Low concentrations produce mixed oleate/tetracene ligand structures where the tetracene carboxylates tilt toward QD surfaces. Intermediate concentrations produce mixed oleate/tetracene ligand structures with ligand-ligand interactions through intramolecular hydrogen bonding with the ligands perpendicular to the QD surface and weaker QD/ligand electronic interactions. High concentrations result in full ligand exchange, and the ligands tilt toward the surface while the QD film compacts. When the tetracene ligands tilt or lie flat on the QD surface, the benzene ring π-system interacts strongly with the p-orbitals at the PbS surface and produces strong QD-ligand interactions evidenced through QD/ligand state mixing, with a coupling energy of ≈700 meV.

Threonine functionalized colloidal cadmium sulfide (CdS) quantum dots: The role of solvent and counterion in ligand induced chiroptical properties

The functionalization of semiconductor nanocrystals, quantum dots (QDs), with small organic molecules has been studied extensively to gain better knowledge on how to tune the electronic, optical and chiroptical properties of QDs. Chiral QDs have progressively emerged as key materials in a vast range of applications including biosensing and biorecognition, imaging, asymmetric catalysis, optoelectronic devices, and spintronics. To engage the full potential of the unique properties of chiral nanomaterials and be able to prepare them with tailorable chiroptical characteristics, it is essential to understand how chirality is rendered from chiral molecular ligands at the surface of nanocrystals to the electronic states of QDs. Using a series of polar protic and aprotic solvents together with ammonium (NH4+), tetramethylammonium (TMA+), and tetrabutylammonium (TBA+) countercations in the preparation of threonine-functionalized cadmium sulfide (Thr-CdS) QDs by phase transfer ligand exchange approach, we demonstrated the significance of the role both the solvent and the countercations play in the transfer of chirality from chiral molecular ligand to achiral semiconductor QDs as apparent by the modulations of the signatures and anisotropy of the circular dichroism (CD) spectra. Moreover, we have utilized tetrabutylammonium countercation to successfully synthesize chiral QDs in nonpolar cyclohexane solvent for the first time. This study provides further insights into the origin of the ligand induced chirality of colloidal nanomaterials and facilitates the synthesis of tailormade chiral QDs.

Efficient photon upconversion enabled by strong coupling between silicon quantum dots and anthracene

Hybrid structures formed between organic molecules and inorganic quantum dots can accomplish unique photophysical transformations by taking advantage of their disparate properties. The electronic coupling between these materials is typically weak, leading photoexcited charge carriers to spatially localize to the dot or to a molecule at its surface. However, we show that by converting a chemical linker that covalently binds anthracene molecules to silicon quantum dots from a carbon-carbon single bond to a double bond, we access a strong coupling regime where excited carriers spatially delocalize across both anthracene and silicon. By pushing the system to delocalize, we design a photon upconversion system with a higher efficiency (17.2%) and lower threshold intensity (0.5 W cm^-2) than that of a corresponding weakly coupled system. Our results show that strong coupling between molecules and nanostructures achieved through targeted linking chemistry provides a complementary route for tailoring properties in materials for light-driven applications.

Raman spectroscopy study of CdS nanorods and strain induced by the adsorption of 4-mercaptobenzoic acid

In this study, near- and off-resonance Raman spectra of cadmium sulfide (CdS) quantum rods (NRs) and 4-mercaptobenzoic acid (4-MBA) adsorbed CdS NRs are reported. The envelopes of characteristic optical phonon modes in the near-resonance Raman spectrum of CdS NRs are deconvoluted by following the phonon confinement model. As compared with off-resonant Raman spectra, optical phonon modes scattering cross section is amplified significantly in near-resonance Raman spectra through the Fröhlich interaction. The Huang–Rhys factor defining the strength of the Fröhlich interaction is estimated (∼0.468). Moreover, the adsorption of different concentrations of 4-mercaptobenzoic acid (4-MBA) onto CdS NRs produces surface strain in CdS NRs originating due to surface reconstruction and consequently blue and red shifts in off-resonance (514.5 nm) Raman spectra depending on the concentration of 4-MBA. These consequences are attributed to compressive and tensile strains, respectively. Relative to bulk CdS powder as the reference, strain in CdS NRs increases with decreasing 4-MBA concentrations. In off-resonance Raman spectra of 4-MBA adsorbed CdS NRs, the full width at half maxima of phonon modes (1-LO and 2-LO) and intensity ratio I2-LO/I1-LO increase with decreasing 4-MBA concentration.

Using intrinsic properties of quantum dots to provide additional security when uniquely identifying devices

Unique identification of optical devices is important for anti-counterfeiting. Physical unclonable functions (PUFs), which use random physical characteristics for authentication, are advantageous over existing optical solutions, such as holograms, due to the inherent asymmetry in their fabrication and reproduction complexity. However, whilst unique, PUFs are potentially vulnerable to replication and simulation. Here we introduce an additional benefit of a small modification to an established model of nanoparticle PUFs by using a second measurement parameter to verify their authenticity. A randomly deposited array of quantum dots is encapsulated in a transparent polymer, forming a tag. Photoluminescence is measured as a function of excitation power to assess uniqueness as well as the intrinsic nonlinear response of the quantum material. This captures a fingerprint, which is non-trivial to clone or simulate. To demonstrate this concept practically, we show that these tags can be read using an unmodified smartphone, with its built-in flash for excitation. This development over constellation-style optical PUFs paves the way for more secure, facile authentication of devices without requiring complex fabrication or characterisation techniques.

Electron Transfer at Quantum Dot–Metal Oxide Interfaces for Solar Energy Conversion

Electron transfer at a donor–acceptor quantum dot–metal oxide interface is a process fundamentally relevant to solar energy conversion architectures as, e.g., sensitized solar cells and solar fuels schemes. As kinetic competition at these technologically relevant interfaces largely determines device performance, this Review surveys several aspects linking electron transfer dynamics and device efficiency; this correlation is done for systems aiming for efficiencies up to and above the ∼33% efficiency limit set by Shockley and Queisser for single gap devices. Furthermore, we critically comment on common pitfalls associated with the interpretation of kinetic data obtained from current methodologies and experimental approaches, and finally, we highlight works that, to our judgment, have contributed to a better understanding of the fundamentals governing electron transfer at quantum dot–metal oxide interfaces.

Bound-Ion Pair X-Type Ligation of Cadmium and Zinc Dithiocarbamates on Cadmium Selenide Quantum Belts

Wurtzite CdSe quantum belts with L-type n-octylamine, L-type ammonia, or Z-type Cd(oleate)₂ ligands are exchanged for several metal-dithiocarbamate ligands [M(S₂CNR₁R₂)₂]: Cd(S₂CNPhMe)₂, Cd(S₂CNEt₂)₂, Zn(S₂CNPhMe)₂, and Zn(S₂CNEt₂)₂. Successful ligand exchange with all M(S₂CNR₁R₂)₂ compounds occurs from {CdSe[Cd(oleate)₂]_0.19} quantum belts (QBs), which induce similar spectral shifts in the absorption spectra of the ligand-exchanged QBs. Spectroscopic data, experimentally determined lattice strains, and ligand exchanges with [Na][Et₂NCS₂] and [NH₄][MePhNCS₂] establish that the [M(S₂CNR₁R₂)₂] ligands bind as bound-ion-paired X-type ligands with (S₂CNR₁R₂)^- groups ligated directly to the QB surfaces and [M(S₂CNR₁R₂)]⁺ groups serving as the charge-balancing ion-paired countercations. The X-type dithiocarbamate ligands do not impart any special electronic effects to the CdSe QBs.

All-inorganic halide-perovskite polymer-fiber-photodetector for high-speed optical wireless communication

The use of optical carrier frequencies will enable seamless data connection for future terrestrial and underwater internet uses and will resolve the technological gap faced by other communication modalities. However, several issues must be solved to propel this technological shift, which include the limitations in designing optical receivers with large detection areas, omnidirectionality, and high modulation bandwidth, mimicking antennas operating in the radio-frequency spectrum. To address this technological gap, herein, we demonstrate halide-perovskite-polymer-based scintillating fibers as a near-omnidirectional detection platform for several tens-to-hundreds of Mbit/s optical communication in both free space and underwater links. The incorporation of all-inorganic CsPbBr₃ nanocrystals by engineering the nanocrystal concentration in an ultraviolet-curable polymer matrix ensures a high photoluminescence quantum yield, Mega-Hertz modulation bandwidth and Mbit/s data rate suitable to be used as a high-speed fibers-based receiver. The resultant perovskite polymer-based scintillating fibers offer flexibility in terms of shape and near-omnidirectional detection features. Such fiber properties also introduce a scalable detection area which can resolve the resistance-capacitance and angle-of-acceptance limits in planar-based detectors, which conventionally impose a trade-off between the modulation bandwidth, detection area, and angle of view. A high bit rate of 23 Mbit/s and 152.5 Mbit/s was achieved using an intensity-modulated laser for non-return-to-zero on-off-keying (NRZ-OOK) modulation scheme in free-space and quadrature amplitude modulation orthogonal frequency-division multiplexing (QAM-OFDM) modulation scheme in an underwater environment, respectively. Our near-omnidirectional optical-based antenna based on perovskite-polymer-based scintillating fibers sheds light on the immense possibilities of incorporating functional nanomaterials for empowering light-based terrestrial- and underwater-internet systems.

Colloidal Inorganic Ligand-Capped Nanocrystals: Fundamentals, Status, and Insights into Advanced Functional Nanodevices

Colloidal nanocrystals (NCs) are intriguing building blocks for assembling various functional thin films and devices. The electronic, optoelectronic, and thermoelectric applications of solution-processed, inorganic ligand (IL)-capped colloidal NCs are especially promising as the performance of related devices can substantially outperform their organic ligand-capped counterparts. This in turn highlights the significance of preparing IL-capped NC dispersions. The replacement of initial bulky and insulating ligands capped on NCs with short and conductive inorganic ones is a critical step in solution-phase ligand exchange for preparing IL-capped NCs. Solution-phase ligand exchange is extremely appealing due to the highly concentrated NC inks with completed ligand exchange and homogeneous ligand coverage on the NC surface. In this review, the state-of-the-art of IL-capped NCs derived from solution-phase inorganic ligand exchange (SPILE) reactions are comprehensively reviewed. First, a general overview of the development and recent advancements of the synthesis of IL-capped colloidal NCs, mechanisms of SPILE, elementary reaction principles, surface chemistry, and advanced characterizations is provided. Second, a series of important factors in the SPILE process are offered, followed by an illustration of how properties of NC dispersions evolve after ILE. Third, surface modifications of perovskite NCs with use of inorganic reagents are overviewed. They are necessary because perovskite NCs cannot withstand polar solvents or undergo SPILE due to their soft ionic nature. Fourth, an overview of the research progresses in utilizing IL-capped NCs for a wide range of applications is presented, including NC synthesis, NC solid and film fabrication techniques, field effect transistors, photodetectors, photovoltaic devices, thermoelectric, and photoelectrocatalytic materials. Finally, the review concludes by outlining the remaining challenges in this field and proposing promising directions to further promote the development of IL-capped NCs in practical application in the future.

Directing Energy Transfer in Halide Perovskite-Chromophore Hybrid Assemblies

Directing the flow of energy and the nature of the excited states that are produced in nanocrystal-chromophore hybrid assemblies is crucial for realizing their photocatalytic and optoelectronic applications. Using a combination of steady-state and time-resolved absorption and photoluminescence (PL) experiments, we have probed the excited-state interactions in the CsPbBr₃-Rhodamine B (RhB) hybrid assembly. PL studies reveal quenching of the CsPbBr₃ emission with a concomitant enhancement of the fluorescence of RhB, indicating a singlet-energy-transfer mechanism. Transient absorption spectroscopy shows that this energy transfer occurs on the ∼200 ps time scale. To understand whether the energy transfer occurs through a Förster or Dexter mechanism, we leveraged facile halide-exchange reactions to tune the optical properties of the donor CsPbBr₃ by alloying with chloride. This allowed us to tune the spectral overlap between the donor CsPb(Br_1-xCl_x)₃ emission and acceptor RhB absorption. For CsPbBr₃-RhB, the rate constant for energy transfer (k_ET) agrees well with Förster theory, whereas alloying with chloride to produce chloride-rich CsPb(Br_1-xCl_x)₃ favors a Dexter mechanism. These results highlight the importance of optimizing both the donor and acceptor properties to design light-harvesting assemblies that employ energy transfer. The ease of tuning optical properties through halide exchange of the nanocrystal donor provides a unique platform for studying and tailoring excited-state interactions in perovskite-chromophore assemblies.

Vibronic Excitons and Conical Intersections in Semiconductor Quantum Dots

Surface defects and organic surface-capping ligands affect the photoluminescence properties of semiconductor quantum dots (QDs) by altering the rates of competing nonradiative relaxation processes. In this study, broadband two-dimensional electronic spectroscopy reveals that absorption of light by QDs prepares vibronic excitons, excited states derived from quantum coherent mixing of the core electronic and ligand vibrational states. Rapidly damped coherent wavepacket motions of the ligands are observed during hot-carrier cooling, with vibronic coherence transferred to the photoluminescent state. These findings suggest a many-electron, molecular theory for the electronic structure of QDs, which is supported by calculations of the structures of conical intersections between the exciton potential surfaces of a small ammonia-passivated model CdSe nanoparticle.

Influence of Ligand Structure on Excited State Surface Chemistry of Lead Sulfide Quantum Dots

The ligand-nanocrystal boundaries of colloidal quantum dots (QDs) mediate the primary energy and electron transfer processes that underpin photochemical and photocatalytic transformations at their surfaces. We use mid-infrared transient absorption spectroscopy to reveal the influence that ligand structure and bonding to nanocrystal surfaces have on the changes of the excited state surface chemistry of this boundary in PbS QDs and the corresponding impact on charge transfer processes between nanocrystals. We demonstrate that oleate ligands undergo marked changes in their bonding to surfaces in the excitonic excited states of the nanocrystals, indicating that oleate passivated PbS surfaces undergo significant structural changes following photoexcitation. These changes can impact the surface mobility of the ligands and the ability of redox shuttles to approach the nanocrystal surfaces to undergo charge transfer in photocatalytic reactions. In contrast, markedly different transient vibrational features are observed in iodide/mercaptoproprionic acid passivated PbS QD films that result from charge transfer between neighboring nanocrystals and localization of holes at the nanocrystal surfaces near MPA ligands. This ability to distinguish the influence that excitonic excited states vs charge transfer processes have on the surface chemistry of the ligand-nanocrystal boundary lays the groundwork for exploration of how this boundary can be understood and controlled for the design of nanocrystalline materials tailored for specific applications in solar energy harvesting and photocatalytic reactions.

Multiple exciton generation in isolated and interacting silicon nanocrystals

An important challenge in the field of renewable energy is the development of novel nanostructured solar cell devices which implement low-dimensional materials to overcome the limits of traditional photovoltaic systems. For optimal energy conversion in photovoltaic devices, one important requirement is that the full energy of the solar spectrum is effectively used. In this context, the possibility of exploiting features and functionalities induced by the reduced dimensionality of the nanocrystalline phase, in particular by the quantum confinement of the electronic density, can lead to a better use of the carrier excess energy and thus to an increment of the thermodynamic conversion efficiency of the system. Carrier multiplication, i.e. the generation of multiple electron-hole pairs after absorption of one single high-energy photon (with energy at least twice the energy gap of the system), can be exploited to maximize cell performance, promoting a net reduction of loss mechanisms. Over the past fifteen years, carrier multiplication has been recorded in a large variety of semiconductor nanocrystals and other nanostructures. Owing to the role of silicon in solar cell applications, the mission of this review is to summarize the progress in this fascinating research field considering carrier multiplication in Si-based low-dimensional systems, in particular Si nanocrystals, both from the experimental and theoretical point of view, with special attention given to the results obtained by ab initio calculations.

Ligand-dependent nano-mechanical properties of CdSe nanoplatelets: calibrating nanobalances for ligand affinity monitoring

The influence of ligands on the low frequency vibration of cadmium selenide colloidal nanoplatelets of different thicknesses is investigated using resonant low frequency Raman scattering. The strong vibration frequency shifts induced by ligand modifications as well as sharp spectral linewidths make low frequency Raman scattering a tool of choice to follow ligand exchange as well as the nano-mechanical properties of the NPLs, as evidenced by a carboxylate to thiolate exchange study. Apart from their molecular weight, the nature of the ligands, such as the sulfur to metal bond of thiols, induces a modification of the NPLs as a whole, increasing the thickness by one monolayer. Moreover, as the weight of the ligands increases, the discrepancy between the mass-load model and the experimental measurements increase. These effects are all the more important when the number of layers is small and can only be explained by a modification of the longitudinal sound velocity. This modification takes its origin in a change of the lattice structure of the NPLs, that reflects on their elastic properties. These nanobalances are finally used to characterize ligand affinity with the surface using binary thiol mixtures, illustrating the potential of low frequency Raman scattering to finely characterize nanocrystal surfaces.

Dithiocarbamate Complexes as Single Source Precursors to Nanoscale Binary, Ternary and Quaternary Metal Sulfides

Nanodimensional metal sulfides are a developing class of low-cost materials with potential applications in areas as wide-ranging as energy storage, electrocatalysis, and imaging. An attractive synthetic strategy, which allows careful control over stoichiometry, is the single source precursor (SSP) approach in which well-defined molecular species containing preformed metal-sulfur bonds are heated to decomposition, either in the vapor or solution phase, resulting in facile loss of organics and formation of nanodimensional metal sulfides. By careful control of the precursor, the decomposition environment and addition of surfactants, this approach affords a range of nanocrystalline materials from a library of precursors. Dithiocarbamates (DTCs) are monoanionic chelating ligands that have been known for over a century and find applications in agriculture, medicine, and materials science. They are easily prepared from nontoxic secondary and primary amines and form stable complexes with all elements. Since pioneering work in the late 1980s, the use of DTC complexes as SSPs to a wide range of binary, ternary, and multinary sulfides has been extensively documented. This review maps these developments, from the formation of thin films, often comprised of embedded nanocrystals, to quantum dots coated with organic ligands or shelled by other metal sulfides that show high photoluminescence quantum yields, and a range of other nanomaterials in which both the phase and morphology of the nanocrystals can be engineered, allowing fine-tuning of technologically important physical properties, thus opening up a myriad of potential applications.

Quenched or alive quantum dots: The leading roles of ligand adsorption and photoinduced protonation

HYPOTHESIS

The fluorescence emission of water-soluble CdTe quantum dots (QDs) capped with mercaptocarboxylic acids (MCAs) is known to be pH-dependent. However, this behaviour is quite different from a study to another, so that literature suffers from a lack of coherence. Here we assume that the QD fluorescence efficiency is actually driven by the acid-base equilibrium of MCA thiol groups, and that light-excited QDs open a non-radiative relaxation path through photoinduced protonation.

EXPERIMENTS

We address this issue by examining colloidal CdTe QDs with (time-resolved) fluorescence spectroscopy under various conditions of acidity and light excitation.

FINDINGS

It appears that the emission of QDs is quenched below a critical pH value of 6.87, and that light excitation power strengthens this quenching. We thus demonstrate the existence of an additional photochemical process and developed a mathematical modeling accounting for all our experimental results. With only three parameters, it is possible to accurately predict the fluorescence decay of QDs over time, at any pH. Further, we also related the critical pH value of 6.87 to QD surface properties, explaining why observations may differ from a study to another and making the literature much more coherent.

Spatial Dependence of the Dipolar Interaction between Quantum Dots and Organic Molecules Probed by Two-Color Sum-Frequency Generation Spectroscopy

Given the tunability of their optical properties over the UV–Visible–Near IR spectral range, ligand-capped quantum dots (QDs) are employed for the design of optical biosensors with low detection threshold. Thanks to non-linear optical spectroscopies, the absorption properties of QDs are indeed used to selectively enhance the local vibrational response of molecules located in their vicinity. Previous studies led to assume the existence of a vibroelectronic QD–molecule coupling based on dipolar interaction. However, no systematic study on the strength of this coupling has been performed to date. In order to address this issue, we use non-linear optical Two-Color Sum-Frequency Generation (2C-SFG) spectroscopy to probe thick QD layers deposited on calcium fluoride (CaF2) prisms previously functionalized by a self-assembled monolayer of phenyltriethoxysilane (PhTES) molecules. Here, 2C-SFG is performed in Attenuated Total Reflection (ATR) configuration. By comparing the molecular vibrational enhancement measured for QD–ligand coupling and QD–PhTES coupling, we show that the spatial dependence of the QD–molecule interactions (∼1/r3, with r the QD–molecule distance) is in agreement with the hypothesis of a dipole–dipole interaction.

Controllable chiral behavior of type-II core/shell quantum dots adjusted by shell thickness and coordinated ligands

Chiral semiconductor nanomaterials induced by capped chiral ligands are of great interest for both theoretical studies and advanced applications. In this study, CdTe/CdSe quantum dots (QDs), defined as type-II core/shell nanostructure, with the advantage of a good separation of holes and electrons are imparted chirality with L/D-cysteine and L/D-penicillamine molecules. Circular dichroism (CD) at exciton transitions from cysteine- and penicillamine-capped QDs is different in shape and intensity. CD intensities decrease with increasing shell thickness from three monolayers to six monolayers, indicating a decreased hybridization degree between the holes in CdTe core and the electrons in chiral ligands. Elevated cysteine concentration leads to decreased g-factor, probably due to an altered binding mode from tridentate to bidentate. Our observations provide further insights into the understanding of chiral phenomenon as well as optimized design and applications of chiral nanostructures.

Prospects of Coupled Organic-Inorganic Nanostructures for Charge and Energy Transfer Applications

We review the field of organic-inorganic nanocomposites with a focus on materials that exhibit a significant degree of electronic coupling across the hybrid interface. These nanocomposites undergo a variety of charge and energy transfer processes, enabling optoelectronic applications in devices which exploit singlet fission, triplet energy harvesting, photon upconversion or hot charge carrier transfer. We discuss the physical chemistry of the most common organic and inorganic components. Based on those we derive synthesis and assembly strategies and design criteria on material and device level with a focus on photovoltaics, spin memories or optical upconverters. We conclude that future research in the field should be directed towards an improved understanding of the binding motif and molecular orientation at the hybrid interface.

Controlling the dimension of the quantum resonance in CdTe quantum dot superlattices fabricated via layer-by-layer assembly

In quantum dot superlattices, wherein quantum dots are periodically arranged, electronic states between adjacent quantum dots are coupled by quantum resonance, which arises from the short-range electronic coupling of wave functions, and thus the formation of minibands is expected. Quantum dot superlattices have the potential to be key materials for new optoelectronic devices, such as highly efficient solar cells and photodetectors. Herein, we report the fabrication of CdTe quantum dot superlattices via the layer-by-layer assembly of positively charged polyelectrolytes and negatively charged CdTe quantum dots. We can thus control the dimension of the quantum resonance by independently changing the distances between quantum dots in the stacking (out-of-plane) and in-plane directions. Furthermore, we experimentally verify the miniband formation by measuring the excitation energy dependence of the photoluminescence spectra and detection energy dependence of the photoluminescence excitation spectra.

Designing quantum dot superlattices remains a challenge. Here, the authors present CdTe quantum dot superlattices via the layer-by-layer assembly and verify the miniband formation by measuring the excitation energy the dependence of the photoluminescence spectra and the detection energy dependence of the excitation spectra.

Spectral shift, electronic coupling and exciton delocalization in nanocrystal dimers: insights from all-atom electronic structure computations

Delocalization of excitons promoted by electronic coupling between clusters or quantum dots (QD) changes the dynamical processes in nanostructured aggregates enhancing energy transport. A spectroscopic shift of the absorption spectrum upon QD aggregation is commonly observed and ascribed to quantum mechanical coupling between neighbouring dots but also to exciton delocalization over the sulphur-based ligand shell or to other mechanisms as a change in the dielectric constant of the surrounding medium. We address the question of electronic coupling and exciton delocalization in nanocrystal aggregates by performing all-atom electronic structure calculations in models of colloidal QD dimers. The relation between spectral shift, interdot coupling and exciton delocalization is investigated in atomistic detail in models of dimers formed by CdSe clusters kept together by bridging organic ligands. Our results support the possibility of obtaining exciton delocalization over the dimer and point out the crucial role of the bridging ligand in enhancing interdot electronic coupling.

Solid-state nuclear magnetic resonance studies of nanoparticles

In this trends article, we review seminal and recent studies using static and magic-angle spinning solid-state NMR to study the structure of nanoparticles and ligands attached to nanoparticles. Solid-state NMR techniques including one-dimensional multinuclear NMR, cross-polarization, techniques for measuring dipolar coupling and internuclear distances, and multidimensional NMR have provided insight into the core-shell structure of nanoparticles as well as the structure of ligands on the nanoparticle surface. Hyperpolarization techniques, in particular solid-state dynamic nuclear polarization (DNP), have enabled detailed studies of nanoparticle core-shell structure and surface chemistry, by allowing unprecedented levels of sensitivity to be achieved. The high signal-to-noise afforded by DNP has allowed homonuclear and heteronuclear correlation experiments involving nuclei with low natural abundance to be performed in reasonable experimental times, which previously would not have been possible. The use of DNP to study nanoparticles and their applications will be a fruitful area of study in the coming years as well.

Dynamic Ligand Surface Chemistry of Excited PbS Quantum Dots

The ligand shell around colloidal quantum dots mediates the electron and energy transfer processes that underpin their use in optoelectronic and photocatalytic applications. Here, we show that the surface chemistry of carboxylate anchoring groups of oleate ligands passivating PbS quantum dots undergoes significant changes when the quantum dots are excited to their excitonic states. We directly probe the changes of surface chemistry using time-resolved mid-infrared spectroscopy that records the evolution of the vibrational frequencies of carboxylate groups following excitation of the electronic states. The data reveal a reduction of the Pb-O coordination of carboxylate anchoring groups to lead atoms at the quantum dot surfaces. The dynamic surface chemistry of the ligands may increase their surface mobility in the excited state and enhance the ability of molecular species to penetrate the ligand shell to undergo energy and charge transfer processes that depend sensitively on distance.

N-Heterocyclic Carbenes as Reversible Exciton-Delocalizing Ligands for Photoluminescent Quantum Dots

Delocalization of excitons within semiconductor quantum dots (QDs) into states at the interface of the inorganic core and organic ligand shell by so-called "exciton-delocalizing ligands (EDLs)" is a promising strategy to enhance coupling of QD excitons with proximate molecules, ions, or other QDs. EDLs thereby enable enhanced rates of charge carrier extraction from, and transport among, QDs and dynamic colorimetric sensing. The application of reported EDLs-which bind to the QDs through thiolates or dithiocarbamates-is however limited by the irreversibility of their binding and their low oxidation potentials, which lead to a high yield of photoluminescence-quenching hole trapping on the EDL. This article describes a new class of EDLs for QDs, 1,3-dimethyl-4,5-disubstituted imidazolylidene N-heterocyclic carbenes (NHCs), where the 4,5-substituents are Me, H, or Cl. Postsynthetic ligand exchange of native oleate capping ligands for NHCs results in a bathochromic shift of the optical band gap of CdSe QDs (R = 1.17 nm) of up to 111 meV while the colloidal stability of the QDs is maintained. This shift is reversible for the MeNHC-capped and HNHC-capped QDs upon protonation of the NHC. The magnitude of exciton delocalization induced by the NHC (after scaling for surface coverage) increases with the increasing acidity of its π system, which depends on the substituent in the 4,5-positions of the imidazolylidene. The NHC-capped QDs maintain photoluminescence quantum yields of up to 4.2 ± 1.8% for shifts of the optical band gap as large as 106 meV.

Chemical Reactions Involving the Surface of Metal Chalcogenide Quantum Dots

This invited feature article focuses on the chemical reactions involving the surface ions of colloidal quantum dots (Qdots). Emphasis is placed on ion-exchange, redox, and complexation reactions. The pursuit of reactions involving primarily the cations on the surface results in changes in the optical properties of the Qdots and also may confer new properties owing to the newly formed surface species. For example, the cation-exchange reaction, leading to systematic removal of the cations present on the as-synthesized Qdots, enhances the photoluminescence quantum yield. On the other hand, redox reactions, involving the dopant cations in the Qdots, could not only modulate the photoluminescence quantum yield but also give rise to new emission not present in the as-synthesized Qdots. Importantly, the cations present on the surface could be made to react with external organic ligands to form inorganic complexes, thus providing a new species defined as the quantum dot complex (QDC). In the QDC, the properties of Qdots and the inorganic complex are not only present but also enhanced. Furthermore, by varying reaction conditions such as the concentrations of the species and using a mixture of ligands, the properties could be further tuned and multifunctionalization of the Qdot could be achieved. Thus, chemical, magnetic, and optical properties could be simultaneously conferred on the same Qdot. This has helped in externally controlled bioimaging, white light generation involving individual quantum dots, and highly sensitive molecular sensing. Understanding the species (i.e., the newly formed inorganic complex) on the surface of the Qdot and its chemical reactivity provide unique options for futuristic technological applications involving a combination of an inorganic complex and a Qdot.

PbS Nanocrystals Made with Excess PbCl2 Have an Intrinsic Shell that Reduces Their Stokes Shift

The use of excess PbCl₂ in the synthesis of PbS nanocrystals has become a convenient route to produce narrow-line-width infrared emitters. However, these materials have found limited adoption in optoelectronic devices-even compared to PbS nanocrystals prepared with lead oleate. Here, using both transmission electron microscopy and small-angle X-ray scattering, we show that excess PbCl₂ results in larger-diameter PbS nanocrystals for the same excitonic features, which is consistent with the formation of an intrinsic insulating shell. We observe further differences in excess-lead-chloride nanocrystals consistent with a shell, including lattice strain and smaller Stokes shifts for intermediate sizes (⌀: 4.8-6.8 nm) that match the passivation/rigidification predicted for a chloride-terminate surface. Our results clarify and rationalize the divergent properties of PbS nanocrystals prepared using different synthetic methodologies, give guidance for device implementation, and offer a new target for synthetic control.

Ligand-Enhanced Energy Transport in Nanocrystal Solids Viewed with Two-Dimensional Electronic Spectroscopy

We examine CdSe NCs functionalized with the exciton-delocalizing ligand phenyldithiocarbamate (PDTC) using two-dimensional electronic spectroscopy (2DES). PDTC forms hybrid molecular orbitals with CdSe's valence band that relax hole spatial confinement and create potential for enhanced exciton migration in NC solids. We find PDTC broadens the intrinsic line width of individual NCs in solution by ∼30 meV, which we ascribe to modulation of NC band edge states by ligand motion. In PDTC-exchanged solids, photoexcited excitons are mobile and rapidly move to low-energy NC sites over ∼30 ps. We also find placing excitons into high-energy states can accelerate their rate of migration by over an order of magnitude, which we attribute to enhanced spatial delocalization of these states that improves inter-NC wave function overlap. Our work demonstrates that NC surface ligands can actively facilitate inter-NC energy transfer and highlights principles to consider when designing ligands for this application.

Carboxylate Anchors Act as Exciton Reporters in 1.3 nm Indium Phosphide Nanoclusters

Developing interfacial probes of ligand-nanocluster interactions is crucial for understanding and tailoring the optoelectronic properties of these emerging nanomaterials. Using transient IR spectroscopy, we demonstrate that ligand vibrational modes of oleate-capped 1.3 nm InP nanoclusters report on the photogenerated exciton. The exciton induces an intensity change in the asymmetric carboxylate stretching mode by 57% while generating no appreciable shift in frequency. Thus, the observed difference signal is attributed to an exciton-induced change in the dipole magnitude of the asymmetric carboxylate stretching mode. Additionally, the transient IR data reveal that the infrared dipole change is dependent on the geometry of the ligand bound to the nanocluster. The experimental results are interpreted using TDDFT calculations, which identify how the spatial dependence of an exciton-induced electron density shift affects the vibrational motion of the carboxylate anchors. More broadly, this work demonstrates transient IR spectroscopy as a useful method for characterizing ligand-nanocluster coupling interactions.

The interparticle distance limit for multiple exciton dissociation in PbS quantum dot solid films

Understanding the behaviour of multiple exciton dissociation in quantum dot (QD) solid films is of fundamental interest and paramount importance for improving the performance of quantum dot solar cells (QDSCs). Unfortunately, the charge transfer behaviour of photogenerated multiple exciton in QD solid films is not clear to date. Herein, we systematically investigate the multiple exciton charge transfer behaviour in PbS QD solid films by using ultrafast transient absorption spectroscopy. We observe that the multiple exciton charge transfer rate within QD ensembles is exponentially enhanced as the interparticle distance between the QDs decreases. Biexciton and triexciton dissociation between adjacent QDs occurs via a charge transfer tunneling effect just like single exciton, and the charge tunneling constants of the single exciton (β₁: 0.67 ± 0.02 nm^-1), biexciton (β₂: 0.68 ± 0.05 nm^-1) and triexciton (β₃: 0.71 ± 0.01 nm^-1) are obtained. More importantly, for the first time, the interparticle distance limit (≤4.3 nm) for multiple exciton charge transfer between adjacent QDs is found for the extraction of multiple excitons rapidly before the occurrence of Auger recombination. This result points out a vital and necessary condition for the use of multiple excitons produced in PbS QD films, especially for their applications in QDSCs.

Fundamental Properties in Colloidal Quantum Dots

A multidisciplinary approach for the production and characterization of colloidal quantum dots, which show great promise for implementation in modern optoelectronic applications, is described. The approach includes the design and formation of unique core/shell structures with alloy-composed layers between the core and the shell. Such structures eliminate interfacial defects and suppress the Auger process, thus reducing the known fluorescence blinking and endowing the quantum dots with robust chemical and spectral stability. The unique design enables the generation and sustained existence of single and multiple excitons with a defined spin-polarized emission recombination. The studies described herein implement the use of single-dot magneto-optical measurements and optically detected magnetic resonance spectroscopy, for direct identification of interfacial defects and for resolving exciton fine structure. The results are of paramount importance for a fundamental understanding of optical transitions in colloidal quantum dots, with an impact on appropriate materials design for practical applications.

Coordination chemistry in the design of heterogeneous photocatalysts

Heterogeneous catalysts have been widely used for photocatalysis, which is a highly important process for energy conversion, owing to their merits such as easy separation of catalysts from the reaction products and applicability to continuous chemical industry and recyclability. Yet, homogenous photocatalysis receives tremendous attention as it can offer a higher activity and selectivity with atomically dispersed catalytic sites and tunable light absorption. For this reason, there is a major trend to combine the advantages of both homogeneous and heterogeneous photocatalysts, in which coordination chemistry plays a role as the bridge. In this article, we aim to provide the first systematic review to give a clear picture of the recent progress from taking advantage of coordination chemistry. We specifically summarize the role of coordination chemistry as a versatile tool to engineer catalytically active sites, tune light harvesting and maneuver charge kinetics in heterogeneous photocatalysis. We then elaborate on the common fundamentals behind various materials systems, together with key spectroscopic characterization techniques and remaining challenges in this field. The typical applications of coordination chemistry in heterogeneous photocatalysis, including proton reduction, water oxidation, carbon dioxide reduction and organic reactions, are highlighted.

Effects of Direct Solvent-Quantum Dot Interaction on the Optical Properties of Colloidal Monolayer WS2 Quantum Dots

Because of the absence of native dangling bonds on the surface of the layered transition metal dichalcogenides (TMDCs), the surface of colloidal quantum dots (QDs) of TMDCs is exposed directly to the solvent environment. Therefore, the optical and electronic properties of TMDCS QDs are expected to have stronger influence from the solvent than usual surface-passivated QDs due to more direct solvent-QD interaction. Study of such solvent effect has been difficult in colloidal QDs of TMDC due to the large spectroscopic heterogeneity resulting from the heterogeneity of the lateral size or (and) thickness in ensemble. Here, we developed a new synthesis procedure producing the highly uniform colloidal monolayer WS₂ QDs exhibiting well-defined photoluminescence (PL) spectrum free from ensemble heterogeneity. Using these newly synthesized monolayer WS₂ QDs, we observed the strong influence of the aromatic solvents on the PL energy and intensity of monolayer WS₂ QD beyond the simple dielectric screening effect, which is considered to result from the direct electronic interaction between the valence band of the QDs and molecular orbital of the solvent. We also observed the large effect of stacking/separation equilibrium on the PL spectrum dictated by the balance between inter QD and QD-solvent interactions. The new capability to probe the effect of the solvent molecules on the optical properties of colloidal TMDC QDs will be valuable for their applications in various liquid surrounding environments.

Alkyl-Thiol Ligand-Induced Shape- and Crystalline Phase-Controlled Synthesis of Stable Perovskite-Related CsPb2Br5 Nanocrystals at Room Temperature

Controlled synthesis of colloidal all-inorganic lead halide perovskite semiconductor nanocrystals, such as CsPbBr₃, with tunable size, shape, composition, and crystalline phase have recently attracted wide interest for photonic and optoelectronic applications. Herein, we report a new strategy for using alkyl-thiols to induce the transformation of CsPbBr₃ to perovskite-related cesium lead halide (CsPb₂Br₅) with controlled morphology and a crystalline phase at room temperature. By rational tuning the ratios of the alkyl-thiol ligands to alkyl-amines or to alkyl-acids, the as-synthesized colloidal nanocrystals can be rationally controlled from orthorhombic crystalline-phase CsPbBr₃ to tetragonal-phase CsPb₂Br₅ nanosheets and nanowires with high yield. Significantly, the tetragonal CsPb₂Br₅ nanowires and nanosheets have high stability in high-temperature and high-humidity environments. These findings may open new directions for large-scale synthesis of shape- and crystalline phase-controlled perovskite nanocrystals for high-performance, low-cost optical electronic and optoelectronic devices.

Design of a Hole Trapping Ligand

A new ligand that covalently attaches to the surface of colloidal CdSe/CdS nanorods and can simultaneously chelate a molecular metal center is described. The dithiocarbamate-bipyridine ligand system facilitates hole transfer through energetic overlap at the inorganic-organic interface and conjugation through the organic ligand to a chelated metal center. Density functional theory calculations show that the coordination of the free ligand to a CdS surface causes the formation of two hybridized molecular states that lie in the band gap of CdS. The further chelation of Fe(II) to the bipyridine moiety causes the presence of seven midgap states. Hole transfer from the CdS valence band to the midgap states is dipole allowed and occurs at a faster rate than what is experimentally known for the CdSe/CdS band-edge radiative recombination. In the case of the ligand bound with iron, a two-step process emerges that places the hole on the iron, again at rates much faster than band gap recombination. The system was experimentally assembled and characterized via UV-vis absorbance spectroscopy, fluorescence spectroscopy, time-resolved photoluminescence spectroscopy, and energy dispersive X-ray spectroscopy. Theoretically predicted red shifts in absorbance were observed experimentally, as well as the expected quench in photoluminescence and lifetimes in time-resolved photoluminescence.

Effects of the capping ligands, linkers and oxide surface on the electron injection mechanism of copper sulfide quantum dot-sensitized solar cells

QDSCs are an effective alternative to fossil fuels. However, it is difficult to differentiate the effect of each component in optimization. DFT calculations are combined with a bottom-up approach to differentiate the effect of each component on the electronic structure and absorption spectra.

Probing the Interaction of Quantum Dots with Chiral Capping Molecules Using Circular Dichroism Spectroscopy

Circular dichroism (CD) induced at exciton transitions by chiral ligands attached to single component and core/shell colloidal quantum dots (QDs) was used to study the interactions between QDs and their capping ligands. Analysis of the CD line shapes of CdSe and CdS QDs capped with l-cysteine reveals that all of the features in the complex spectra can be assigned to the different excitonic transitions. It is shown that each transition is accompanied by a derivative line shape in the CD response, indicating that the chiral ligand can split the exciton level into two new sublevels, with opposite angular momentum, even in the absence of an external magnetic field. The role of electrons and holes in this effect could be separated by experiments on various types of core/shell QDs, and it was concluded that the induced CD is likely related to interactions of the highest occupied molecular orbitals of the ligands with the holes. Hence, CD was useful for the analysis of hole level-ligand interactions in quantum semiconductor heterostructures, with promising outlook toward better general understanding the properties of the surface of such systems.

Electronic Processes within Quantum Dot-Molecule Complexes

The subject of this review is the colloidal quantum dot (QD) and specifically the interaction of the QD with proximate molecules. It covers various functions of these molecules, including (i) ligands for the QDs, coupled electronically or vibrationally to localized surface states or to the delocalized states of the QD core, (ii) energy or electron donors or acceptors for the QDs, and (iii) structural components of QD assemblies that dictate QD-QD or QD-molecule interactions. Research on interactions of ligands with colloidal QDs has revealed that ligands determine not only the excited state dynamics of the QD but also, in some cases, its ground state electronic structure. Specifically, the article discusses (i) measurement of the electronic structure of colloidal QDs and the influence of their surface chemistry, in particular, dipolar ligands and exciton-delocalizing ligands, on their electronic energies; (ii) the role of molecules in interfacial electron and energy transfer processes involving QDs, including electron-to-vibrational energy transfer and the use of the ligand shell of a QD as a semipermeable membrane that gates its redox activity; and (iii) a particular application of colloidal QDs, photoredox catalysis, which exploits the combination of the electronic structure of the QD core and the chemistry at its surface to use the energy of the QD excited state to drive chemical reactions.

Subpicosecond Photoinduced Hole Transfer from a CdS Quantum Dot to a Molecular Acceptor Bound Through an Exciton-Delocalizing Ligand

This paper describes the enhancement of the rate of hole transfer from a photoexcited CdS quantum dot (QD), with radius R = 2.0 nm, to a molecular acceptor, phenothiazine (PTZ), by linking the donor and acceptor through a phenyldithiocarbamate (PTC) linker, which is known to lower the confinement energy of the excitonic hole. Upon adsorption of PTC, the bandgap of the QD decreases due to delocalization of the exciton, primarily the excitonic hole, into interfacial states of mixed QD/PTC character. This delocalization enables hole transfer from the QD to PTZ in <300 fs (within the instrument response of the laser system) when linked by PTC, but not when linked by a benzoate group, which has a similar length and conjugation as PTC but does not delocalize the excitonic hole. Comparison of the two systems was aided by quantification of the surface coverage of benzoate and PTC-linked PTZ by (1)H NMR. This work provides direct spectroscopic evidence of the enhancement of the rate of hole extraction from a colloidal QD through covalent linkage of a hole acceptor through an exciton-delocalizing ligand.

Roles of adsorption sites in electron transfer from CdS quantum dots to molecular catalyst cobaloxime studied by time-resolved spectroscopy

Electron transfer from CdS quantum dots (QDs) to cobaloxime (Co(dmgH)₂pyCl) is demonstrated by transient absorption spectroscopy (TAS), and further confirmed using photoluminescence (PL) techniques.

Synergism at the Nanoscale

Photoactive nanoparticles are smart systems that exhibit unique optical properties. In general, their intrinsic properties are size dependent. The degree and type of response to size are both related to their composition. Nanoparticles usually require to be capped with organic ligands in order to be dispersible in an aqueous or organic media, thus leading to nanoparticle colloidal dispersions and enhancing the processability of the material. The organic ligand also plays a key role in their preparation. In addition, the high surface-to-volume ratio of the nanoparticles combined with the affinity of the ligands for the nanoparticle surface can be used to place a large number of functional molecules at their periphery. The purpose of this chapter is to understand the synergism between nanoparticles and organic ligands with regard to their preparation, performance, and applicability.