Auger decay of multiple excitons represents a significant obstacle to photonic applications of semiconductor quantum dots (QDs). This nonradiative process is particularly detrimental to the performance of QD-based electroluminescent and lasing devices. Here, we demonstrate that semiconductor quantum shells with an “inverted” QD geometry inhibit Auger recombination, allowing substantial improvements to their multiexciton characteristics. By promoting a spatial separation between multiple excitons, the quantum shell geometry leads to ultralong biexciton lifetimes (>10 ns) and a large biexciton quantum yield. Furthermore, the architecture of quantum shells induces an exciton–exciton repulsion, which splits exciton and biexciton optical transitions, giving rise to an Auger-inactive single-exciton gain mode. In this regime, quantum shells exhibit the longest optical gain lifetime reported for colloidal QDs to date (>6 ns), which makes this geometry an attractive candidate for the development of optically and electrically pumped gain media.
Digestive ripening (DR) represents a powerful strategy for improving the size homogeneity of colloidal nanostructures. It relies on the ligand-mediated dissolution of larger nanoparticles in favor of smaller ones and is often considered to be the opposite of Ostwald ripening. Despite its successful application to size-focusing of metal colloids, digestive ripening of semiconductor nanocrystals has received little attention to date. Here, we explore this synthetic niche and demonstrate that ligand-induced ripening of semiconductor nanocrystals exhibits an unusual reaction path. The unique aspect of the DR process in semiconductors lies in the thermally activated particle coalescence, which leads to a significant increase in the nanocrystal size for temperatures above the threshold value (T th = 200–220 °C). Below this temperature, nanoparticle sizes focus to an ensemble average diameter just like in the case of metal colloids. The existence of the thermal threshold for coalescence offers an expedient strategy for controlling both the particle size and the size dispersion. Such advanced shape control was demonstrated using colloids of CdS, CdSe, CsPbBr3, and CuZnSnS4, where monodisperse samples were obtained across broad diameter ranges. We expect the demonstrated approach to be extended to other semiconductors as a simple strategy for tuning the nanoparticle morphology.
Two-dimensional (2D) semiconductor nanocrystals processed from solution have become an attractive material platform for the development of optoelectronic devices. The synergy of the 2D charge carrier confinement and soft fabrication methods has enabled new paradigms in solid-state lighting, biosensing, and energy harvesting. In this Perspective, we will summarize recent research trends in the synthesis and applications of 2D semiconductor materials, focusing on three morphological classes: metal chalcogenides, halide perovskites, and transition metal dichalcogenides. Our aim is to describe the role that 2D geometry plays in the optical and electronic properties of these semiconductor nanostructures and to discuss emerging opportunities in their synthesis and applications.
Assemblies of metal nanostructures and fluorescent molecules represent a promising platform for the development of biosensing and near-field imaging applications. Typically, the interaction of molecular fluorophores with surface plasmons in metals results in either quenching or enhancement of the dye excitation energy. Here, we demonstrate that fluorescent molecules can also engage in a reversible energy transfer (ET) with proximal metal surfaces, during which quenching of the dye emission via the energy transfer to localized surface plasmons can trigger delayed ET from metal back to the fluorescent molecule. The resulting two-step process leads to the sustained delayed photoluminescence (PL) in metal-conjugated fluorophores, as was demonstrated here through the observation of increased PL lifetime in assemblies of Au nanoparticles and organic dyes (Alexa 488, Cy3.5, and Cy5). The observed enhancement of the PL lifetime in metal-conjugated fluorophores was corroborated by theoretical calculations based on the reverse ET model, suggesting that these processes could be ubiquitous in many other dye− metal assemblies.
Artificial solids of CsPbX3 perovskite nanocrystals (NC) are well known for their promising charge transport characteristics. The long-range diffusion of photoinduced charges in these materials is attributed to the unique electronic structure of CsPbX3 NCs associated with high defect tolerance and a low disorder of excited-state energies. Here, we show that the same set of electronic properties allows CsPbBr3 NC solids to act as superior energy transport materials, which support a long-range diffusion of electrically neutral excitons. By performing time-resolved bulk quenching measurements on halide-treated CsPbBr3 NC films, we observed average exciton diffusion lengths of 52 and 71 nm for I–- and Cl–-treated solids, respectively. Steady-state fluorescence quenching studies have been employed to explain such a large diffusion length as due to a high defect tolerance and a low disorder of exciton energies in CsPbBr3 NC solids. We expect that the demonstrated ability of halide-treated CsPbBr3 NC solids to support a long-range exciton transport could be beneficial for applications in light energy concentration, as was demonstrated in this work through energy transfer measurements in assemblies of perovskite NC donors and CdSe quantum dot acceptors.
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