Celso de Mello Donegá

Indium phosphide-based quantum dots (QDs) are leading Cd- and Pb-free alternatives for photonic applications spanning 400-920 nm. However, their long-term photostability under prolonged illumination remains poorly understood. In this study, we systematically investigate the optical response of colloidal InP/ZnSe/ZnS core/shell/shell QDs exposed to white light under a dynamic inert atmosphere (glovebox filled with circulating N₂ with O₂ < 0.1 ppm, H₂O < 1.0 ppm). Upon illumination, the QDs exhibit irreversible photodarkening, defined as a reduction in photoluminescence quantum yield with a nearly unchanged absorption spectrum. Photodarkening shows a linear dependence on absorbed photon dose, with photoexcitation deep into the conduction band inducing it more rapidly than resonant photoexcitation. Additionally, prolonged photoexposure leads to an emission redshift and an enhanced delayed-emission component, consistent with the selective photodarkening of individual QDs. Experiments in air and controlled humidity (10% and 90% relative humidity) indicate that photodarkening primarily arises from photooxidation by oxygen, whereas water promotes QD photoetching. This is further corroborated by control experiments in which the QDs are illuminated under dry oxygen (2O) and in a cuvette sealed by fusion under a N₂ atmosphere with 2 and 2O. These experiments show that photodarkening does not occur in QDs contained in the fusion-sealed cuvette, while under dry oxygen the photodarkening rates of the QDs are even faster than in dry air. These findings advance understanding of InP-based QD photodegradation and provide a framework for quantifying their stability under light stress.

Quantum dot (QD)‐based luminescent solar concentrators (LSCs) promise to revolutionize solar energy technology by replacing building materials with energy‐harvesting devices. However, QDs degrade under air, limiting the long‐term performance of QD‐LSCs. This study introduces an innovative approach to prevent QDs degradation by utilizing a photoactive polymer matrix (maleic anhydride‐grafted poly(styrene‐b‐ethylene‐co‐butylene‐b‐styrene, SEBS‐g‐MA). This strategy has been tested outdoors over a 2‐year period on five LSCs, followed by characterization of the weathered devices. The tested LSCs consist of three QD‐LSCs (CuInS ₂ /ZnS, InP/ZnSe/ZnS, CdSe/CdS/ZnS core/shell QDs), alongside a Lumogen dye‐based LSC and a luminophore‐free LSC. The study yields several findings: 1) SEBS‐g‐MA undergoes photochemistry outdoors, 2) SEBS‐g‐MA accelerates the photodegradation of Lumogen, 3) the power conversion efficiency of CdSe‐based QD‐LSC drops by 80% due to reduction of the photoluminescence quantum yield, and 4) under illumination SEBS‐g‐MA protects CuInS ₂ and InP‐based QDs from degradation, ensuring a stable performance during the entire study. This work thus demonstrates for the first time that the interaction between the luminophores and the matrix is a critical determinant of the long‐term success of LSCs. Leveraging on the fact that this is the longest outdoor study to date, we propose design rules for highly efficient and stable QD‐LSCs.

Incorporating magnetic ions into semiconductor nanocrystals has emerged as a prominent research field for manipulating spin-related properties. The magnetic ions within the host semiconductor experience spin-exchange interactions with photogenerated carriers and are often involved in the recombination routes, stimulating special magneto-optical effects. The current account presents a comparative study, emphasizing the impact of engineering nanostructures and selecting magnetic ions in shaping carrier–magnetic ion interactions. Various host materials, including the II–VI group, halide perovskites, and I–III–VI2 in diverse structural configurations such as core/shell quantum dots, seeded nanorods, and nanoplatelets, incorporated with magnetic ions such as Mn2+, Ni2+, and Cu1+/2+ are highlighted. These materials have recently been investigated by us using state-of-the-art steady-state and transient optically detected magnetic resonance (ODMR) spectroscopy to explore individual spin-dynamics between the photogenerated carriers and magnetic ions and their dependence on morphology, location, crystal composition, and type of the magnetic ion. The information extracted from the analyses of the ODMR spectra in those studies exposes fundamental physical parameters, such as g-factors, exchange coupling constants, and hyperfine interactions, together providing insights into the nature of the carrier (electron, hole, dopant), its local surroundings (isotropic/anisotropic), and spin dynamics. The findings illuminate the importance of ODMR spectroscopy in advancing our understanding of the role of magnetic ions in semiconductor nanocrystals and offer valuable knowledge for designing magnetic materials intended for various spin-related technologies.

Impurity doping of low-dimensional semiconductors is an interesting route towards achieving control over carrier dynamics and energetics, e.g., to improve hot carrier extraction, or to obtain strongly Stokes shifted luminescence. Such studies remain, however, underexplored for the emerging family of III-V colloidal quantum dots (QDs). Here, we show through a detailed global analysis of multiresonant pump-probe spectroscopy that electron cooling in copper-doped InP quantum dot (QDs) proceeds on subpicosecond time scales. Conversely, hole localization on Cu dopants is remarkably slow (1.8 ps), yet still leads to very efficient subgap emission. Due to this slow hole localization, common Auger assisted pathways in electron cooling cannot be blocked by Cu doping III-V systems, in contrast with the case of II-VI QDs. Finally, we argue that the structural relaxation around the Cu dopants, estimated to impart a reorganization energy of 220 meV, most likely proceeds simultaneously with the localization itself leading to efficient luminescence.

The formation mechanisms of colloidal magic-size semiconductor nanostructures have remained obscure. Herein, we report the room temperature synthesis of three species of ultrathin CdTe magic-size nanowires (MSNWs) with diameters of 0.7 +/- 0.1 nm, 0.9 +/- 0.2 nm, and 1.1 +/- 0.2 nm, and lowest energy exciton transitions at 373, 418, and 450 nm, respectively. The MSNWs are obtained from Cd(oleate)2 and TOP-Te, provided diphenylphosphine and a primary alkylamine (RNH2) are present at sufficiently high concentrations, and exhibit sequential, discontinuous growth. The population of each MSNW species is entirely determined by the RNH2 concentration [RNH2] so that single species are only obtained at specific concentrations, while mixtures are obtained at concentrations intermediate between the specific ones. Moreover, the MSNWs remain responsive to [RNH2], interconverting from thinner to thicker upon [RNH2] decrease and from thicker to thinner upon [RNH2] increase. Our results allow us to propose a mechanism for the formation and interconversion of CdTe MSNWs and demonstrate that primary alkylamines play crucial roles in all four elementary kinetic steps (viz., monomer formation, nucleation, growth in length, and interconversion between species), thus being the decisive element in the creation of a reaction pathway that leads exclusively to CdTe MSNWs. The insights provided by our work thus contribute toward unravelling the mechanisms behind the formation of shape-controlled and atomically precise magic-size semiconductor nanostructures.

Copper-doped II-VI and copper-based I-III-VI₂ colloidal quantum dots (CQDs) have been at the forefront of interest in nanocrystals over the past decade, attributable to their optically activated copper states. However, the related recombination mechanisms are still unclear. The current work elaborates on recombination processes in such materials by following the spin properties of copper-doped CdSe/CdS (Cu@CdSe/CdS) and of CuInS₂ and CuInS₂/(CdS, ZnS) core/shell CQDs using continuous-wave and time-resolved optically detected magnetic resonance (ODMR) spectroscopy. The Cu@CdSe/CdS ODMR showed two distinct resonances with different g factors and spin relaxation times. The best fit by a spin Hamiltonian simulation suggests that emission comes from recombination of a delocalized electron at the conduction band edge with a hole trapped in a Cu²⁺ site with a weak exchange coupling between the two spins. The ODMR spectra of CuInS₂ CQDs (with and without shells) differ significantly from those of the copper-doped II-VI CQDs. They are comprised of a primary resonance accompanied by another resonance at half-field, with a strong correlation between the two, indicating the involvement of a triplet exciton and hence stronger electron-hole exchange coupling than in the doped core/shell CQDs. The spin Hamiltonian simulation shows that the hole is again associated with a photogenerated Cu²⁺ site. The electron resides near this Cu²⁺ site, and its ODMR spectrum shows contributions from superhyperfine coupling to neighboring indium atoms. These observations are consistent with the occurrence of a self-trapped exciton associated with the copper site. The results presented here support models under debate for over a decade and help define the magneto-optical properties of these important materials.