Epitaxial growth of shells on III–V semiconductor quantum dot (QD) cores yields improved fluorescence quantum efficiency and stability toward their implementation in light emission technologies. Here, we control the shell morphology and crystal structure and investigate their effects on the emission properties of heterovalent III–V/II–VI core/shell QDs. This is achieved by tuning the ZnSe shell growth mode from kinetic to thermodynamic regimes via adjusting the precursor reactivity. When combined with high-temperature Ostwald ripening, this approach enables controlled tuning of shell morphology between tetrahedral and spherical-like, accompanied by a transformation of the shell crystal structure from zinc-blende to wurtzite. The position of the III–V cores within the III–V/ZnSe core/shell QDs varies under the different growth modes, being closer to the edge in the former. Moreover, the spherical architecture exhibits a higher photoluminescence quantum yield (PLQY) and improved stability. Such morphological and crystal-type differences directly affect the band alignment and exciton confinement, leading to tunable emission spectra and exciton dynamics, as confirmed by quantum mechanical simulations of the band gap exciton energies. This study deepens the understanding of heteroepitaxial growth and emission control in III–V/II–VI core/shell QDs, enabling advanced QD design toward optimization for diverse light emission applications.

Quantum dots (QDs) coated with π-conjugated ligands display triplet energy transfer (TET), which opens the path for photon upconversion via QD photosensitization. Herein we study the effect of the ligand binding and its orientation on the triplet energy transfer efficiency through analysing the quenching of the QD photoluminescence. Comparing anthracene ligands with different anchoring groups, we find that replacing carboxylate with thiol or dithiol groups enhances quenching rates by factors of 3 and 4.5, respectively. To obtain this quantitative information, we devise a modified Stern–Volmer model taking into account the Poisson distribution of the ligand binding on the QDs. To this end, we show that bound anthracene-based ligands exhibit distinct spectral changes in their absorption spectra, including a ligand-dependent bathochromic shift with a modified vibronic progression and broadened spectral width. These changes, related to the deprotonation of the anchoring groups upon binding and the confined environment on the QD surface, enable the distinction of the crossover from bound to free ligands upon ligand addition. This allows us to incorporate accurate ligand binding stoichiometry to extract reliable quenching rates. Consistent with DFT calculations, the improved quenching for the thiolated anthracenes is ascribed to the parallel orientation of the π-system relative to the QD surface enabling larger orbital overlap that leads to faster TET rates via the Dexter mechanism. This work contributes to the design principles for efficient QD–organic hybrid systems towards improved triplet energy transfer.

The 2023 Nobel Prize awarded to Moungi G. Bawendi, Louis E. Brus, and Alexei Ekimov “for the discovery and synthesis of quantum dots” (QDs) marks a milestone in the field to which I devoted the past 30-years of my career. In this perspective, I reflect on key concepts and directions in my research journey. I began by exploring the “artificial atom” nature of QDs while advancing the development of III-V QDs. Shape control, particularly in rods, captured my attention due to its impact on dimensionality related properties. I also discovered semiconductor-metal hybrid nanocrystals and uncovered synergetic effects, highlighting their transformative role in photocatalysis and heavy doping. My work extended to QD applications in displays and, more recently, to forming coupled QD molecules, continuing the artificial atom theme. I conclude by outlining future directions and challenges, envisioning a bright future for this vibrant field at the intersection of materials and physical chemistry.

Quantum dots (QDs) are prominent nanometric light emitters, featuring quantum behavior due to the quantum confinement effect. This effect is responsible for the size-dependent optoelectronic properties of QDs, which makes them a versatile building block for various applications. Moreover, the bright emission of QDs and narrow spectral lines make them ideal for display applications. However, QDs also undergo nonradiative processes that can impair their functionality in display devices. This chapter delves into the physics and photophysics of QDs. It discusses quantum confinement and size and shape effects, heterostructures and surface effects, the absorption and emission spectra, charge dynamics, stability, and collective effects, aiming to shed light on the intricate nature of the QDs' photophysics.

With the increasing consumption of nanomaterials in a variety of applications, our environment becomes more and more exposed to different kinds of (possibly toxic) nanomaterials having variable sizes and shapes, raising up the requirement to sense and monitor the presence of nanomaterials. Here, we propose and demonstrate a porous-silicon based optical sensing platform, capable of sensing nanoparticles of a given distribution of sizes and shapes, but independent of their chemical, mechanical, or electrical properties. A white light optical interference technique has been utilized to transduce nanoparticles trapped in the porous matrix into an optical signal. We have found an unusual optical sensing response that substantially increases the sensing bandwidth of the porous-silicon based optical sensor, which follows a Hill-equation type behavior that is characterized by a logarithmic response at low nanoparticle's concentration and saturation at high concentrations. These universal characteristics of the sensors are explained by the anomalous and limited diffusion of the nanoparticles via a quasi-1D geometry of the pore's matrix. Very low concentration of nanoparticles, of the order of few μg/ml, has been measured by this sensing technique.

Semiconductor–metal hybrid nanoparticles (HNPs) are promising materials for photocatalytic applications, such as water splitting for green hydrogen generation. While most studies have focused on Cd containing HNPs, the realization of actual applications will require environmentally compatible systems. Using heavy-metal free ZnSe-Au HNPs as a model, we investigate the dependence of their functionality and efficiency on the cocatalyst metal domain characteristics ranging from the single-atom catalyst (SAC) regime to metal-tipped systems. The SAC regime was achieved via the deposition of individual atomic cocatalysts on the semiconductor nanocrystals in solution. Utilizing a combination of electron microscopy, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy, we established the presence of single Au atoms on the ZnSe nanorod surface. Upon increased Au concentration, this transitions to metal tip growth. Photocatalytic hydrogen generation measurements reveal a strong dependence on the cocatalyst loading with a sharp response maximum in the SAC regime. Ultrafast dynamics studies show similar electron decay kinetics for the pristine ZnSe nanorods and the ZnSe-Au HNPs in either SAC or tipped systems. This indicates that electron transfer is not the rate-limiting step for the photocatalytic process. Combined with the structural-chemical characterization, we conclude that the enhanced photocatalytic activity is due to the higher reactivity of the single-atom sites. This holistic view establishes the significance of SAC-HNPs, setting the stage for designing efficient and sustainable heavy-metal-free photocatalyst nanoparticles for numerous applications.

Photocatalytic hydrogen generation offers a sustainable method for generating solar fuels. Indium phosphide (InP) nanocrystal quantum dots (QDs), with their adjustable band gaps and versatile surface properties, present an eco-friendly alternative to heavy-metal-based semiconductor NCs as photocatalysts. We report the synthesis of wurtzite InP (w-InP) QDs and their performance as photocatalysts for hydrogen generation from water by using the red part of the solar spectrum. Size-controlled w-InP QDs with absorption edges extending to 750 nm were synthesized via a cation exchange route. Stabilized in water with sulfides, these QDs demonstrated higher hydrogen generation efficiencies compared with other narrow-band-gap QDs. The overall hydrogen generation efficiency sharply decreases with the size. A mixed-size approach combining the high efficiency of small QDs with the broad absorption range of large QDs enhances the solar-to-hydrogen conversion by a more effective utilization of the solar spectrum. Such an approach shows promise for effective solar fuel generation.

The utilization of photoelectrochemical cells (PEC) for converting solar energy into fuels (e.g., hydrogen) is a promising method for sustainable energy generation. We demonstrate a strategy to enhance the performance of PEC devices by integrating surface-functionalized zinc selenide (ZnSe) semiconductor nanocrystals (NCs) into porous polymeric carbon nitride (CN) matrices to form a uniformly distributed blend of NCs within the CN layer via electrophoretic deposition (EPD). The achieved type II heterojunction at the CN/NC interface exhibits intimate contact between the NCs and the CN backbone since it does not contain insulating binders. This configuration promotes efficient charge separation and suppresses carrier recombination. The reported CN/NC composite structure serves as a photoanode, demonstrating a photocurrent density of 160 ± 8 μA cm–2 at 1.23 V vs a reversible hydrogen electrode (RHE), 75% higher compared with a CN-based photoelectrode, for approximately 12 h. Spectral and photoelectrochemical analyses reveal extended photoresponse, reduced charge recombination, and successful charge transfer at the formed heterojunction; these properties result in enhanced PEC oxygen production activity with a Faradaic efficiency of 87%. The methodology allows the integration of high-quality colloidal NCs within porous CN-based photoelectrodes and provides numerous knobs for tuning the functionality of the composite systems, thus showing promise for achieving enhanced solar fuel production using PEC.

Colloidal Quantum Dots (QDs), the building blocks of modern displays and optoelectronic devices, have reached the highest level of size and shape control, and stability during the last 30 years. However, full utilization of their potential requires integration or assembly of more than one nanocrystal as in the case of Coupled Quantum Dots Molecules (CQDM), where two core-shell QDs are fused to form two emission centers in close proximity. These CQDMs were recently shown to switch color under an applied electric field at room temperature. Here we use cryogenic single particle spectroscopy of single CQDMs under an electric field to show that various mechanisms can contribute to the spectrum change under an applied electric field at cryogenic temperatures.  The first mechanism is the control of the delocalized electron wave function when the electric field is applied along the dimer axis. The electric field bends the conduction band and forces the electron wave function to localize in one of the QDs yielding preferential emission of that particular center. In addition, we found that QDs and CQDMs could become sensitive to surface traps under an electric field. In the case of CQDMs, that can result in decreasing the intensity of one of the QDs while increasing the other QD's intensity. Moreover, we show that there are surface charges which screen the applied electric field in some of the QDs. This as well can result in electric field-induced color-tuning of CQDMs. Understanding the underlying mechanisms responsible for spectral shifts under applied electric fields is critical for the development of color-tunable devices utilizing CQDMs, including efficient displays and single photon sources.

We explore the potential of nanocrystals (a term used equivalently to nanoparticles) as building blocks for nanomaterials, and the current advances and open challenges for fundamental science developments and applications. Nanocrystal assemblies are inherently multiscale, and the generation of revolutionary material properties requires a precise understanding of the relationship between structure and function, the former being determined by classical effects and the latter often by quantum effects. With an emphasis on theory and computation, we discuss challenges that hamper current assembly strategies and to what extent nanocrystal assemblies represent thermodynamic equilibrium or kinetically trapped metastable states. We also examine dynamic effects and optimization of assembly protocols. Finally, we discuss promising material functions and examples of their realization with nanocrystal assemblies.

A unique on-chip method for the direct correlation of optical properties, with atomic-scale chemical–structural characteristics for a single quantum dot (QD), is developed and utilized in various examples. This is based on performing single QD optical characterization on a modified glass substrate, followed by the extraction of the relevant region of interest by focused-ion-beam–scanning electron microscope processing into a lamella for high resolution scanning transmission electron microscopy (STEM) characterization with atomic scale resolution. The direct correlation of the optical response under an electric field with STEM analysis of the same particle allows addressing several single particle phenomena: first, the direct correlation of single QD photoluminescence (PL) polarization and its response to the external field with the QD crystal lattice alignment, so far inferred indirectly; second, the identification of unique yet rare few-QD assemblies, correlated directly with their special spectroscopic optical characteristics, serving as a guide for future designed assemblies; and third, the study on the effect of metal island growth on the PL behavior of hybrid semiconductor–metal nanoparticles, with relevance for their possible functionality in photocatalysis. This work, therefore, establishes the use of the direct on-chip optical–structural correlation method for numerous scenarios and timely questions in the field of QD research.

Phosphide-based nanocrystals (NCs), including InP and Cu3–xP, are relevant for applications in light-emitting devices and catalysis, yet their synthetic design is limited in terms of size range and homogeneity. We report the synthesis of uniform and size-controlled emissive wurtzite-phase InP NCs formed via cation exchange from Cu3–xP. First, size-controlled Cu3–xP NCs are synthesized by the formation of metallic Cu0 NCs and their phosphidation to Cu3–xP. By changing the ligands and precursor concentrations, the NC size is varied between 5 and 13 nm. Using cation exchange, InP NCs are then generated. As the surface of InP NCs is prone to oxidation and defects that decrease their emission, we performed a reaction with NOBF4. This yields InP NCs with resolved absorption features and efficient band-gap emission as a result of impurity removal and surface passivation. The effect of water, acid, and halides on the balance of NC etching and surface passivation is studied. With this approach, high-quality wurtzite InP NCs are obtained while the emission is tuned between 810 and 600 nm. The obtained NCs are potential building blocks for catalytic and optoelectronic applications.

The fusion step in the formation of colloidal quantum dot molecules, constructed from two core/shell quantum dots, dictates the coupling strength and hence their properties and enriched functionalities compared to monomers. Herein, studying the monomer size effect on fusion and coupling, we observe a linear relation of the fusion temperature with the inverse nanocrystal radius. This trend, similar to that in nanocrystal melting, emphasizes the role of the surface energy. The suggested fusion mechanism involves intraparticle ripening where atoms diffuse to the reactive connecting neck region. Moreover, the effect of monomer size and neck filling on the degree of electronic coupling is studied by combined atomistic-pseudopotential calculations and optical measurements, uncovering strong coupling effects in small QD dimers, leading to significant optical changes. Understanding and controlling the fusion and hence coupling effect allows tailoring the optical properties of these nanoscale structures, with potential applications in photonic and quantum technologies.

Nanocomposites are constructed from a matrix material combined with dispersed nanosized filler particles. Such a combination yields a powerful ability to tailor the desired mechanical, optical, electrical, thermodynamic, and antimicrobial material properties. Colloidal semiconductor nanocrystals (SCNCs) are exciting potential fillers, as they display size-, shape-, and composition-controlled properties and are easily embedded in diverse matrices. Here we present their role as quantum photoinitiators (QPIs) in acrylate-based polymer, where they act as a catalytic radical initiator and endow the system with mechanical, photocatalytic, and antimicrobial properties. By utilizing ZnO nanorods (NRs) as QPIs, we were able to increase the tensile strength and elongation at break of poly(ethylene glycol) diacrylate (PEGDA) hydrogels by up to 85%, unlike the use of the same ZnO NRs acting merely as fillers. Simultaneously, we endowed the PEGDA hydrogels with post-polymerization photocatalytic and antimicrobial activities and showed their ability to decompose methylene blue and significantly eradicate antibiotic-resistant bacteria and viral pathogens. Moreover, we demonstrate two fabrication showcase methods, traditional molding and digital light processing printing, that can yield hydrogels with complex architectures. These results position SCNC-based systems as promising candidates to act as all-in-one photoinitiators and fillers in nanocomposites for diverse biomedical applications, where specific and purpose-oriented characteristics are required.

Colloidal semiconductor quantum dots are robust emitters implemented in numerous prototype and commercial optoelectronic devices. However, active fluorescence colour tuning, achieved so far by electric-field-induced Stark effect, has been limited to a small spectral range, and accompanied by intensity reduction due to the electron–hole charge separation effect. Utilizing quantum dot molecules that manifest two coupled emission centres, we present a unique electric-field-induced instantaneous colour-switching effect. Reversible emission colour switching without intensity loss is achieved on a single-particle level, as corroborated by correlated electron microscopy imaging. Simulations establish that this is due to the electron wavefunction toggling between the two centres, induced by the electric field, and affected by the coupling strength. Quantum dot molecules manifesting two coupled emission centres may be tailored to emit distinct colours, opening the path for sensitive field sensing and colour-switchable devices such as a novel pixel design for displays or an electric-field-induced colour-tunable single-photon source.

Coupled colloidal quantum dot molecules (CQDMs) are an emerging class of nanomaterials, manifesting two coupled emission centers and thus introducing additional degrees of freedom for designing quantum-dot-based technologies. The properties of multiply excited states in these CQDMs are crucial to their performance as quantum light emitters, but they cannot be fully resolved by existing spectroscopic techniques. Here we study the characteristics of biexcitonic species, which represent a rich landscape of different configurations essentially categorized as either segregated or localized biexciton states. To this end, we introduce an extension of Heralded Spectroscopy to resolve the different biexciton species in the prototypical CdSe/CdS CQDM system. By comparing CQDMs with single quantum dots and with nonfused quantum dot pairs, we uncover the coexistence and interplay of two distinct biexciton species: A fast-decaying, strongly interacting biexciton species, analogous to biexcitons in single quantum dots, and a long-lived, weakly interacting species corresponding to two nearly independent excitons. The two biexciton types are consistent with numerical simulations, assigning the strongly interacting species to two excitons localized at one side of the quantum dot molecule and the weakly interacting species to excitons segregated to the two quantum dot molecule sides. This deeper understanding of multiply excited states in coupled quantum dot molecules can support the rational design of tunable single- or multiple-photon quantum emitters.

Aluminum nanocrystals are emerging as a promising alternative to silver and gold for various applications ranging from plasmonic functionalities to photocatalysis and as energetic materials. Such nanocrystals often exhibit an inherent surface oxidation layer, as aluminum is highly reactive. Its controlled removal is challenging but required, as it can hinder the properties of the encaged metal. Herein, two wet-chemical colloidal approaches toward the surface coating of Al nanocrystals, which afford control over the surface chemistry of the nanocrystals and the oxide thickness, are presented. The first approach utilizes oleic acid as a surface ligand by its addition toward the end of the Al nanocrystals synthesis, and the second approach is the post-synthesis treatment of Al nanocrystals with NOBF₄, in a "wet" colloidal-based approach, which is found to etch and fluorinate the surface oxides. As surface chemistry is an important handle for controlling materials' properties, this research paves a path for manipulating Al nanocrystals while promoting their utilization in diverse applications.

Electron transfer is a fundamental process in chemistry, biology, and physics. One of the most intriguing questions concerns the realization of the transitions between nonadiabatic and adiabatic regimes of electron transfer. Using colloidal quantum dot molecules, we computationally demonstrate how the hybridization energy (electronic coupling) can be tuned by changing the neck dimensions and/or the quantum dot sizes. This provides a handle to tune the electron transfer from the incoherent nonadiabatic regime to the coherent adiabatic regime in a single system. We develop an atomistic model to account for several states and couplings to the lattice vibrations and utilize the mean-field mixed quantum-classical method to describe the charge transfer dynamics. Here, we show that charge transfer rates increase by several orders of magnitude as the system is driven to the coherent, adiabatic limit, even at elevated temperatures, and delineate the inter-dot and torsional acoustic modes that couple most strongly to the charge transfer dynamics.

Binary compositions of surface ligands are known to improve the colloidal stability and fluorescence quantum yield of nanocrystals (NCs), due to ligand–ligand interactions and surface organization. Herein, we follow the thermodynamics of a ligand exchange reaction of CdSe NCs with alkylthiol mixtures. The effects of ligand polarity and length difference on ligand packing were investigated using isothermal titration calorimetry (ITC). The thermodynamic signature of the formation of mixed ligand shells was observed. Correlating the experimental results with thermodynamic mixing models has allowed us to calculate the interchain interactions and to infer the final ligand shell configuration. Our findings demonstrate that, in contrast to macroscopic surfaces, the small dimensions of the NCs and the subsequent increased interfacial region between dissimilar ligands allow the formation of a myriad of clustering patterns, controlled by the interligand interactions. This work provides a fundamental understanding of the parameters determining the ligand shell structure and should help guide smart surface design toward NC-based applications.

Nanochemistry provides powerful synthetic tools allowing one to combine different materials on a single nanostructure, thus unfolding numerous possibilities to tailor their properties toward diverse functionalities. Herein, we review the progress in the field of semiconductor–metal hybrid nanoparticles (HNPs) focusing on metal–chalcogenides–metal combined systems. The fundamental principles of their synthesis are discussed, leading to a myriad of possible hybrid architectures including Janus zero-dimensional quantum dot-based systems and anisotropic quasi 1D nanorods and quasi-2D platelets. The properties of HNPs are described with particular focus on emergent synergetic characteristics. Of these, the light-induced charge-separation effect across the semiconductor–metal nanojunction is of particular interest as a basis for the utilization of HNPs in photocatalytic applications. The extensive studies on the charge-separation behavior and its dependence on the HNPs structural characteristics, environmental and chemical conditions, and light excitation regime are surveyed. Combining the advanced synthetic control with the charge-separation effect has led to demonstration of various applications of HNPs in different fields. A particular promise lies in their functionality as photocatalysts for a variety of uses, including solar-to-fuel conversion, as a new type of photoinitiator for photopolymerization and 3D printing, and in novel chemical and biomedical uses.