Role of Superlattice Phonons in Charge Localization Across Quantum Dot Arrays.

Understanding charge transport in semiconductor quantum dot (QD) assemblies is important for developing the next generation of solar cells and light-harvesting devices based on QD technology. One of the key factors that governs the transport in such systems is related to the hybridization between the QDs. Recent experiments have successfully synthesized QD molecules, arrays, and assemblies by directly fusing the QDs, with enhanced hybridization leading to high carrier mobilities and coherent band-like electronic transport.

Photocatalytic Semiconductor-Metal Hybrid Nanoparticles: Single-Atom Catalyst Regime Surpasses Metal Tips.

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.

Collective Interactions of Quantum-Confined Excitons in Halide Perovskite Nanocrystal Superlattices.

Collective optical properties can emerge from an ordered ensemble of emitters due to interactions between the individual units. Superlattices of halide perovskite nanocrystals exhibit collective light emission, influenced by dipole-dipole interactions between simultaneously excited nanocrystals. This coupling changes both the emission energy and rate compared to the emission of uncoupled nanocrystals.

NC Meets CN: Porous Photoanodes with Polymeric Carbon Nitride/ZnSe Nanocrystal Heterojunctions for Photoelectrochemical Applications.

The utilization of photoelectrochemical cells (PEC) for converting solar energy into fuels (e.g., hydrogen) is a promising method for sustainable energy generation.

Nanocrystal Assemblies: Current Advances and Open Problems.

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.

Seeing is believing: Correlating optoelectronic functionality with atomic scale imaging of single semiconductor nanocrystals.

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.

The Effect of Monomer Size on Fusion and Coupling in Colloidal Quantum Dot Molecules.

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.

ZnO Quantum Photoinitiators as an All-in-One Solution for Multifunctional Photopolymer Nanocomposites.

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.

Electric-field-induced colour switching in colloidal quantum dot molecules at room temperature.

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.

Two Biexciton Types Coexisting in Coupled Quantum Dot Molecules.

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.

Controlling the Surface of Aluminum Nanocrystals: From Aluminum Oxide to Aluminum Fluoride.

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.

Incoherent nonadiabatic to coherent adiabatic transition of electron transfer in colloidal quantum dot molecules.

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.

Spontaneous Patterning of Binary Ligand Mixtures on CdSe Nanocrystals: From Random to Janus Packing.

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).

Rich Landscape of Colloidal Semiconductor-Metal Hybrid Nanostructures: Synthesis, Synergetic Characteristics, and Emerging 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.

Zn-Doped P-Type InAs Nanocrystal Quantum Dots.

Doped heavy metal-free III-V semiconductor nanocrystal quantum dots (QDs) are of great interest both from the fundamental aspects of doping in highly confined structures, and from the applicative side of utilizing such building blocks in the fabrication of p-n homojunction devices. InAs nanocrystals (NCs), that are of particular relevance for short-wave IR detection and emission applications, manifest heavy n-type character poising a challenge for their transition to p-type behavior. The p-type doping of InAs NCs is presented with Zn - enabling control over the charge carrier type in InAs QDs field effect transistors.

Hybridization and deconfinement in colloidal quantum dot molecules.

The structural, electronic, and optical properties of CdSe/CdS core-shell colloidal quantum dot molecules, a new class of coupled quantum dot dimers, are explored using atomistic approaches. Unlike the case of dimers grown by molecular beam epitaxy, simulated strain profile maps of free-standing colloidal dimers show negligible additional strain resulting from the attachment. The electronic properties of the relaxed dimers are described within a semiempirical pseudopotential model combined with the Bethe-Salpeter equation within the static screening approximation to account for electron-hole correlations.

Composite Indium Tin Oxide Nanofibers with Embedded Hematite Nanoparticles for Photoelectrochemical Water Splitting.

Hematite is a classical photoanode material for photoelectrochemical water splitting due to its stability, performance, and low cost. However, the effect of particle size is still a question due to the charge transfer to the electrodes. In this work, we addressed this subject by the fabrication of a photoelectrode with hematite nanoparticles embedded in close contact with the electrode substrate.

Ligands Mediate Anion Exchange between Colloidal Lead-Halide Perovskite Nanocrystals.

The soft lattice of lead-halide perovskite nanocrystals (NCs) allows tuning their optoelectronic characteristics via anion exchange by introducing halide salts to a solution of perovskite NCs. Similarly, cross-anion exchange can occur upon mixing NCs of different perovskite halides. This process, though, is detrimental for applications requiring perovskite NCs with different halides in close proximity.

Complete Mapping of Interacting Charging States in Single Coupled Colloidal Quantum Dot Molecules.

Colloidal quantum dots (CQDs), major building blocks in modern optoelectronic devices, have so far been synthesized with only one emission center where the exciton resides. Recent development of coupled colloidal quantum dots molecules (CQDM), where two core-shell CQDs are fused to form two emission centers in close proximity, allows exploration of how charge carriers in one CQD affect the charge carriers in the other CQD. Cryogenic single particle spectroscopy reveals that while CQD monomers manifest a simple emission spectrum comprising a main emission peak with well-defined phonon sidebands, CQDMs exhibit a complex spectrum with multiple peaks that are not all spaced according to the known phonon frequencies.

Flow synthesis of photocatalytic semiconductor-metal hybrid nanocrystals.

Semiconductor-metal hybrid nanostructures are promising materials for photocatalytic applications, providing high efficiencies compared to their composing counterparts. So far, the synthesis of such hybrid nanoparticles was limited to batch reactors, achieving tunability while demonstrating how each of the nanocrystals' characteristics affects photocatalytic performances. Yet, new methodologies should be established to increase the synthetic yield while maintaining high control over the resulting structures.

Sulfide Ligands in Hybrid Semiconductor-Metal Nanocrystal Photocatalysts: Improved Hole Extraction and Altered Catalysis.

Hybrid semiconductor-metal nanocrystals manifest efficient photocatalytic activity related to the metal domain promoting charge carrier separation and providing an active catalytic site. The surface properties of such nanoparticles are also of paramount importance in determining their photocatalytic activity. Addressing the combination of surface effects in catalysis on metals, with the electronic properties of hybrid nanoparticles, we examined the effect of coating CdS-Au hybrid nanoparticles with sulfide, an anion that is expected to bind strongly to both domains, on the photocatalytic functionality.

Luminescent Anisotropic Wurtzite InP Nanocrystals.

Indium phosphide (InP) nanocrystals are emerging as an alternative to heavy metal containing nanocrystals for optoelectronic applications but lag behind in terms of synthetic control. Herein, luminescent wurtzite InP nanocrystals with narrow size distribution were synthesized via a cation exchange reaction from hexagonal CuP nanocrystals. A comprehensive surface treatment with NOBF was performed, which removes excess copper while generating stoichiometric In/P nanocrystals with fluoride surface passivation.

Neck Barrier Engineering in Quantum Dot Dimer Molecules via Intraparticle Ripening.

Coupled colloidal quantum dot (CQD) dimers represent a new class of artificial molecules composed of fused core/shell semiconductor nanocrystals. The electronic coupling and wave function hybridization are enabled by the formation of an epitaxial connection with a coherent lattice between the shells of the two neighboring quantum dots where the shell material and its dimensions dictate the quantum barrier characteristics for the charge carriers. Herein we introduce a colloidal approach to control the neck formation at the interface between the two CQDs in such artificial molecular constructs.