Two dimensional (2D) colloidal PbS nanoplatelets (NPLs) with a thickness of 1.8–2.8 nm have been synthesized using a single-molecule precursor approach with lead octadecylxanthate. The lateral dimensions were tuned by varying the reaction temperature, growth time, and capping ligands. Transmission electron microscopy and X-ray diffraction reveal that the NPLs have an orthorhombic crystal structure rather than the rocksalt phase usually reported for bulk and nanostructured PbS. The 1.8 nm thickness, in combination with the tunable lateral dimensions, results in a blue-shifted absorption peak at 715–730 nm and a 48–68 nm narrow emission spectrum with a surprisingly small, 18 nm Stokes shift at room temperature. The fluorescence lifetime of these PbS NPLs is 2 orders of magnitude shorter than the typical lifetime in 0D colloidal PbS quantum dots, highlighting the advantageous properties of colloidal 2D nanostructures that combine strong transversal with weak lateral confinement.

Colloidal nanocrystals (NCs) are attractive materials for light-emitting applications thanks to their flexible synthesis, size-dependent properties, and bright emission. Yet, colloidal NCs still present a narrow gain band (full-width halfmaximum around 10 nm), which limits their application to single-color lasers. Widening of the gain band by specifically engineered NCs can further improve the prospect of this class of materials toward the fabrication of solution-processed white-emitting or color-tunable lasers. Here, we report broadband amplified spontaneous emission (ASE) from wurtzite CdSe/CdS "giant-shell" nanocrystals (g-NCs) with an unprecedented large core up to 7.5 nm in diameter that were synthesized through a continuous injection route. The combination of large core and shell enables ASE from different CdSe optical transitions as well as from the CdS. Importantly, thin films of g-NCs with a large CdSe core (7.5 and 5.1 nm in diameter) show ASE at different colors with a similar threshold, indicating that light emission amplification can be achieved from different optical transitions simultaneously. Tuning of the core diameter allows obtaining ASE in a wide spectral range, and blending of g-NCs with different core sizes gives rise to a continuous amplified spontaneous emission band from green to red (510 to 650 nm). Drop-cast films of CdSe/CdS g-NCs demonstrate simultaneous dual-color random lasing under nanosecond-pulsed excitation.

We calculated the absorption coefficient of colloidal wurtzite CdSe/CdS nanocrystals (NCs), ranging from spherical to strongly elongated shapes (aspect ratio up to 21) by correlating the NC absorbance spectrum to the NC elemental composition and overall dimensions. We compared experimental data with numerical finite-element calculations of the NC absorption coefficient, which can account for the nonspheroidal NC shape and the influence of the organic ligand shell. The results unveiled that quantum confinement effects influence the NC absorption properties, even at relatively high photon energies (∼4.2 eV). From a practical point of view, we provide a general expression for the absorption coefficient that only requires the knowledge of the NC aspect ratio and CdSe/CdS volume ratio, giving access to a fast and nondestructive estimate of NC concentrations in solution and single-particle absorption cross sections, relevant for a wide range of photonic applications. More fundamentally, the unique aspect ratio-dependence of the absorption coefficient allowed us to derive the real (εr) and imaginary (εi) parts of the dielectric constant, demonstrating that even at energies far above the band gap the NC absorption coefficient differs from bulk due to a reduction of both εr and εi. As the methods presented are general, our analysis can be applied to a wide range of materials of varying composition and yields comprehensive insight into the optical constants of colloidal nanocrystals. (Graph Presented)

We present the synthesis of colloidally stable ultrasmall (diameter of 1.5 ± 0.6 nm) and fluorescent copper clusters (Cu-clusters) exhibiting outstanding quantum efficiencies (up to 67% in THF and approximately 30% in water). For this purpose, an amphiphilic block copolymer poly(ethylene glycol)-block-poly(propylene sulfide) (MPEG-b-PPS) was synthesized by living anionic ring-opening polymerization. When CuBr is mixed with the living polymer chains in THF, the formation of Cu-clusters is detected by the appearance of the fluorescence. The cluster growth is quenched by the addition of water, followed by THF removal. The structural features of the MPEG-b-PPS copolymer control the cluster formation and the stabilization: the poly(propylene sulfide) segment acts as coordinating and reducing agent for the copper ions in THF, and imparts a hydrophobic character. This hydrophobic block protects the Cu-clusters from water exposure, thus allowing to obtain a stable emission in water. The PEG segment instead provides the hydrophilicity, rendering the Cu-clusters water-soluble. To obtain fluorescent and stable Cu-clusters exhibiting outstanding quantum efficiencies, the removal of the excess of free polymer and copper salt was crucial. The Cu-clusters are also colloidally and optically stable in physiological media and showed bright fluorescence even when taken up by HeLa cells, being noncytotoxic when administered at a Cu dose between 10 nM and 1.6 μM. Given the very small size of the Cu-clusters, localization and fluorescent staining of cell nucleus is achieved, as demonstrated by confocal cell imaging performed at different Cu-cluster doses and at different incubation temperatures.