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.