Exploring Carbon Dots: Green Nanomaterials for Unconventional Lasing.

In recent years, the progress toward lighting miniaturization is focused on luminescent nanomaterials. Among them, fluorescent carbon dots (CDs) are receiving increasing attention thanks to their astonishing optical properties complemented by their intrinsic biocompatibility and low toxicity. The CDs can be easily dispersed in water, organic solvents or incorporated in polymeric matrices, preserving their emission properties. However, the relationship between their structural and optical properties is still not fully elucidated, motivating a consistent research effort for the comprehension of their features. Nevertheless, CDs demonstrate to be efficient gain materials for lasing, thanks to their high quantum yield (QY), emission tunability in the visible and near infrared (NIR) range, short lifetimes, and high absorption cross section, even if the synthetic reproducibility, the low reaction yield and the spectral width of the emission may limit their effective exploitation. This review summarizes the latest advancements in the investigation of the characteristic properties of CDs that make laser action possible, illustrating optical geometries for lasing and random lasing, both in solution and solid state, and the few currently demonstrated breakthroughs. While the journey toward their effective application is still long, the potential of CD-based laser sources is promising in various technological fields and futuristic perspectives will be discussed.

High-Efficiency FRET Processes in BODIPY-Functionalized Quantum Dot Architectures.

Efficient FRET systems are developed combining colloidal CdSe quantum dots (QDs) donors and BODIPY acceptors. To promote effective energy transfer in FRET architectures, the distance between the organic fluorophore and the QDs needs to be optimized by a careful system engineering. In this context, BODIPY dyes bearing amino-terminated functionalities are used in virtue of the high affinity of amine groups in coordinating the QD surface. A preliminary QD surface treatment with a short amine ligand is performed to favor the interaction with the organic fluorophores in solution. The successful coordination of the dye to the QD surface, accomplishing a short donor-acceptor distance, provides effective energy transfer already in solution, with efficiency of 76 %. The efficiency further increases in the solid state where the QDs and the dye are deposited as single coordinated units from solution, with a distance between the fluorophores down to 2.2 nm, demonstrating the effectiveness of the coupling strategy.

PbS Quantum Dots Decorating TiO2 Nanocrystals: Synthesis, Topology, and Optical Properties of the Colloidal Hybrid Architecture.

Fabrication of heterostructures by merging two or more materials in a single object. The domains at the nanoscale represent a viable strategy to purposely address materials' properties for applications in several fields such as catalysis, biomedicine, and energy conversion. In this case, solution-phase seeded growth and the hot-injection method are ingeniously combined to fabricate TiO2/PbS heterostructures. The interest in such hybrid nanostructures arises from their absorption properties that make them advantageous candidates as solar cell materials for more efficient solar light harvesting and improved light conversion. Due to the strong lattice mismatch between TiO2 and PbS, the yield of the hybrid structure and the control over its properties are challenging. In this study, a systematic investigation of the heterostructure synthesis as a function of the experimental conditions (such as seeds' surface chemistry, reaction temperature, and precursor concentration), its topology, structural properties, and optical properties are carried out. The morphological and chemical characterizations confirm the formation of small dots of PbS by decorating the oleylamine surface capped TiO2 nanocrystals under temperature control. Remarkably, structural characterization points out that the formation of heterostructures is accompanied by modification of the crystallinity of the TiO2 domain, which is mainly ascribed to lattice distortion. This result is also confirmed by photoluminescence spectroscopy, which shows intense emission in the visible range. This originated from self-trapped excitons, defects, and trap emissive states.

Surface-Engineered Cesium Lead Bromide Perovskite Nanocrystals for Enabling Photoreduction Activity.

In recent years, colloidal lead halide perovskite (LHP) nanocrystals (NCs) have exhibited such intriguing light absorption properties to be contemplated as promising candidates for photocatalytic conversions. However, for effective photocatalysis, the light harvesting system needs to be stable under the reaction conditions propaedeutic to a specific transformation. Unlike photoinduced oxidative reaction pathways, photoreductions with LHP NCs are challenging due to their scarce compatibility with common hole scavengers like amines and alcohols. In this contribution, it is investigated the potential of CsPbBr3 NCs protected by a suitably engineered bidentate ligand for the photoreduction of quinone species. Using an in situ approach for the construction of the passivating agent and a halide excess environment, quantum-confined nanocubes (average edge length = 6.0 ± 0.8 nm) are obtained with a low ligand density (1.73 ligand/nm2) at the NC surface. The bifunctional adhesion of the engineered ligand boosts the colloidal stability of the corresponding NCs, preserving their optical properties also in the presence of an amine excess. Despite their relatively short exciton lifetime (τAV = 3.7 ± 0.2 ns), these NCs show an efficient fluorescence quenching in the presence of the selected electron accepting quinones (1,4-naphthoquinone, 9,10-phenanthrenequinone, and 9,10-anthraquinone). All of these aspects demonstrate the suitability of the NCs for an efficient photoreduction of 1,4-naphthoquinone to 1,4-dihydroxynaphthalene in the presence of triethylamine as a hole scavenger. This chemical transformation is impracticable with conventionally passivated LHP NCs, thereby highlighting the potential of the surface functionalization in this class of nanomaterials for exploring new photoinduced reactivities.