Core/Shell Magic-Sized CdSe Nanocrystals

Silver-doped CdSe magic-sized nanocrystals

Magic-sized nanocrystals (MSNCs) grow via jumps between very specific sizes. This discrete growth is a possible avenue toward monodisperse nanomaterials that are completely identical in size and shape. In spite of this potential, MSNCs have seen limited study and application due to their poor optical properties. Specifically, MSNCs are limited in their range of emission wavelengths and commonly exhibit poor photoluminescence quantum yields (PLQYs). Here, we report silver doping of CdSe MSNCs as a strategy to improve the optical properties of MSNCs. Silver doping leads to controllable shifts in emission wavelength and significant increases in MSNC PLQYs. These results suggest that doped MSNCs are interesting candidates for displays or luminescent solar concentrators. Finally, we demonstrate that the doping process does not affect the magic size of our MSNCs, allowing further photophysical study of this class of nanomaterial.

Selenium reduction pathways in the colloidal synthesis of CdSe nanoplatelets

Several established procedures are now available to prepare zinc blende CdSe nanoplatelets. While these protocols allow for detailed control over both thickness and lateral dimensions, the chemistry behind their formation is yet to be unraveled. In this work, we discuss the influence of the solvent on the synthesis of nanoplatelets. We confirmed that the presence of double bonds, as is the case for 1-octadecene, plays a key role in the evolution of nanoplatelets, through the isomerization of the alkene, as confirmed by nuclear magnetic resonance spectroscopy and mass spectrometry. Consequently, 1-octadecene can be replaced as a solvent (or solvent mixture), however, only by one that also contains α protons to CC double bonds. We confirm this via synthesis of nanoplatelets in hexadecane spiked with a small amount of 1-octadecene, and in the aromatic solvent 1,2,3,4-tetrahydronaphthalene (tetralin). At the same time, the chemical reaction leading to the formation of nanoplatelets occurs to some extent in saturated solvents. A closer examination revealed that an alternative formation pathway is possible, through interaction of carboxylic acids, such as octanoic acid, with selenium. Next to shedding more light on the synthesis of CdSe nanoplatelets, fundamental understanding of the precursor chemistry paves the way to use optimized solvent admixtures as an additional handle to control the nanoplatelet synthesis, as well as to reduce potential self-polymerization hurdles observed with 1-octadecene.

Room temperature synthesis of CdSe/CdS triangular nanoemitters and their stabilization in colloidal state and sol-gel glass

Heterostructured cadmium-based core-shell nanoparticles (NPs) are the subject of research because of not only fundamental scientific advances but also a range of technological applications. To increase the range of applications of nanoparticles, it is possible to immobilise them in sol-gel glass that can be easily manufactured and shaped, keeping the properties of the dispersed particles. This allows the creation of new bulk optical materials with tailored properties, opening up opportunities for various technological applications such as lighting or sensing. Herein we report the synthesis of core-shell CdSe/CdS triangular-shaped nanoparticles under an atmosphere of oxygen and at room temperature. A detailed characterisation of the obtained NPs was carried out. The interesting effect of the gelling agent (tetra-n-butylammonium fluoride) on the triangular nanoparticles in solution and the stability of the emission properties over time was investigated. Sol-gel glasses with entrapped triangular NPs were prepared, and their photoluminescence properties were compared with those obtained in colloidal solutions.

Highly efficient near-infrared light-emitting diodes based on Zn:CuInSe2/ZnS//ZnS quantum dots with double shell engineering

Near-infrared (NIR) quantum dot-based light-emitting diodes (QLEDs) developed rapidly in the fields of biomedical applications, telecommunications, sensing and diagnostics. However, it remains an enormous challenge for the synthesis of high-quality NIR QD materials with low toxicity or non-toxicity, high photoluminescence (PL) quantum yields (QYs) and high stability. Herein, we used a facile method to synthesize large-sized (8 nm) and thick-shell NIR Zn:CuInSe₂/ZnS//ZnS QDs by engineering a double ZnS shell. The resulting NIR QDs exhibited high PL QYs of 80%, and excellent photochemical stability, which could be ascribed to the decreased lattice mismatch of the core/shell interface by the introduced Zn element into CuInSe₂ cores and the energetic defect passivation of the double ZnS shell engineering. Furthermore, the high-quality Zn:CuInSe₂/ZnS//ZnS QDs based LEDs exhibited the maximum external quantum efficiency (EQE) of 3.0%, 4.0% and 2.5% for PL peaks located at 705, 719 and 728 nm, respectively. This efficiency is comparable to that of the outstanding PbS- and InAs-based NIR QLEDs, as well as the avoidance of toxic heavymetal and/or hazardous reagents in this work. The synthesized high-quality Zn:CuInSe₂/ZnS//ZnS QDs could be expected to promote the potential applications of heavy-metal-free QDs in the NIR fields.

Exciton Dynamics in Colloidal CdS Quantum Dots with Intense and Stokes Shifted Photoluminescence in a Single Decay Channel

CdS quantum dots (QDs), synthesized by a sol-gel method, exhibit significantly Stokes shifted bright photoluminescence (PL), predominantly from the trap states. Surprisingly, the PL decay at the emission maximum is single-exponential. This is an unusual observation for as-prepared QDs and indicates a narrow distribution in the nature of trap states. A closer look reveals an additional fast component for the decays at shorter emission wavelengths, presumably due to the band edge emission, which remains elusive in the steady-state spectra. Indeed, a significantly narrower and blue-shifted emission band is observed in the decay-associated spectra. The contribution of this component to the steady-state PL intensity is shown to be overwhelmed by that of the significantly stronger trap emission. Exciton dynamics in the quantum dots is elucidated using transient absorption spectra, in which the stimulated emission is observed even at low pump power.

Monoalkyl Phosphinic Acids as Ligands in Nanocrystal Synthesis

Ligands play a crucial role in the synthesis of colloidal nanocrystals. Nevertheless, only a handful molecules are currently used, oleic acid being the most typical example. Here, we show that monoalkyl phosphinic acids are another interesting ligand class, forming metal complexes with a reactivity that is intermediate between the traditional carboxylates and phosphonates. We first present the synthesis of n-hexyl, 2-ethylhexyl, n-tetradecyl, n-octadecyl, and oleylphosphinic acid. These compounds are suitable ligands for high-temperature nanocrystal synthesis (240-300 °C) since, in contrast to phosphonic acids, they do not form anhydride oligomers. Consequently, CdSe quantum dots synthesized with octadecylphosphinic acid are conveniently purified, and their UV-vis spectrum is free from background scattering. The CdSe nanocrystals have a low polydispersity and a photoluminescence quantum yield up to 18% (without shell). Furthermore, we could synthesize CdSe and CdS nanorods using phosphinic acid ligands with high shape purity. We conclude that the reactivity toward TOP-S and TOP-Se precursors decreases in the following series: cadmium carboxylate > cadmium phosphinate > cadmium phosphonate. By introducing a third and intermediate class of surfactants, we enhance the versatility of surfactant-assisted syntheses.

Nonaqueous Chemistry of Group 4 Oxo Clusters and Colloidal Metal Oxide Nanocrystals

We review the nonaqueous precursor chemistry of the group 4 metals to gain insight into the formation of their oxo clusters and colloidal oxide nanocrystals. We first describe the properties and structures of titanium, zirconium, and hafnium oxides. Second, we introduce the different precursors that are used in the synthesis of oxo clusters and oxide nanocrystals. We review the structures of group 4 metal halides and alkoxides and their reactivity toward alcohols, carboxylic acids, etc. Third, we discuss fully condensed and atomically precise metal oxo clusters that could serve as nanocrystal models. By comparing the reaction conditions and reagents, we provide insight into the relationship between the cluster structure and the nature of the carboxylate capping ligands. We also briefly discuss the use of oxo clusters. Finally, we review the nonaqueous synthesis of group 4 oxide nanocrystals, including both surfactant-free and surfactant-assisted syntheses. We focus on their precursor chemistry and surface chemistry. By putting these results together, we connect the dots and obtain more insight into the fascinating chemistry of the group 4 metals. At the same time, we also identify gaps in our knowledge and thus areas for future research.