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.

/pdf/nonaqueous-chemistry-of-group-4-oxo-clusters-and-colloidal-225f8ww7.pdf

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

We take here the perspective of a nanochemist on the field of colloidal nanocrystals, focusing specifically on nanocrystal synthesis. Considering the three components of a colloidal synthesis, we discuss the chemistry that occurs with precursors, ligands, and solvents. Insight into the coordination chemistry and the organic chemistry of nanocrystal syntheses brings us one step closer to a retro-synthetic analysis of a nanocrystal reaction. We also reflect on different crystallization mechanisms (either under thermodynamic or kinetic control). We consider the possibility that the different models are simply describing different experimental conditions and are not fundamentally at odds with one another. However, we do critically evaluate the use of the terms monomer and burst nucleation. Finally, we discuss good chemical practices for a nanochemist, and we try to define nanocrystal purity. This perspective will hopefully inspire researchers in colloidal nanoscience to think more about chemical equations, consider reaction by-products, and come together as a field to agree on standard reporting practices for colloidal nanocrystals. 

Chemical Considerations for Colloidal Nanocrystal Synthesis

http://www.cell.com/article/S2451929422003308/pdf

Structure of a subnanometer-sized semiconductor Cd14Se13 cluster

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. 

https://biblio.ugent.be/publication/8762297/file/8762298

Monoalkyl Phosphinic Acids as Ligands in Nanocrystal Synthesis

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.

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

Magic-sized semiconductor nanocrystals (MSNCs) grow via discrete jumps between specific sizes. Despite their potential to offer atomically precise structures, their use has been limited by poor stability and trap-dominated photoluminescence. Recently, CdSe MSNCs have been grown to larger sizes. We exploit such particles and demonstrate a method to grow shells on CdSe MSNC cores via high-temperature synthesis. Thin CdS shells lead to dramatic improvements in the emissive properties of the MSNCs, narrowing their fluorescence line widths, enhancing photoluminescence quantum yields, and eliminating trap emission. Although thicker CdS shells lead to decreased performance, CdxZn1-xS alloyed shells maintain efficient and narrow emission lines. These alloyed core/shell crystallites exhibit a tetrahedral shape, in agreement with a recent model for MSNC growth. Our results indicate that MSNCs can compete with other state-of-the-art semiconductor nanocrystals. Furthermore, these core/shell structures will allow further study of MSNCs and their potential for atomically precise growth.

Core/Shell Magic-Sized CdSe Nanocrystals.

Magic-sized semiconductor nanocrystals (MSNCs) grow via discrete jumps between specific sizes. Despite their potential to offer atomically precise structures, their use has been limited by poor stability and trap-dominated photoluminescence. Recently, syntheses have been reported that produce CdSe MSNCs over a larger size series. We exploit such particles and demonstrate a method to grow shells on CdSe MSNC cores. Thin CdS shells lead to dramatic improvements in the emissive properties of the MSNCs, narrowing their fluorescence linewidths, enhancing photoluminescence quantum yields, and eliminating trap emission. While thicker CdS shells lead to decreased performance, CdxZn1-xS alloyed shells maintain efficient and narrow fluorescence lines. These alloyed core/shell crystallites also exhibit a tetrahedral shape, in agreement with a recent model for MSNC growth. Our results indicate that MSNCs can compete with other state-of-the-art semiconductor nanocrystals. Furthermore, these core/shell structures will allow further study of MSNCs and their potential for atomically precise growth.

/pdf/core-shell-magic-sized-semiconductor-nanocrystals-b573ovmava.pdf

Jacob T. Held

Papers

Core/Shell Magic-Sized CdSe Nanocrystals.

Core/Shell Magic-Sized Semiconductor Nanocrystals