Studies on the Formation of CdS Nanoparticles from Solutions of (NMe4)4[Cd10S4(SPh)16]

Nucleation control of quantum dot synthesis in a microfluidic continuous flow reactor

The use of microfluidics in chemical synthesis is topical due to the potential to improve reproducibility and the ability promptly interrogate a wide range of reaction parameters, the latter of which is necessary for the training of artificial intelligence (AI) algorithms. Applying microfluidic techniques to semiconductor nanocrystals, or quantum dots (QDs), is challenging due to the need for a high-temperature nucleation event followed by particle growth at lower temperatures. Such a high-temperature gradient can be realized using complex, segmented microfluidic reactor designs, which represents an engineering approach. Here, an alternative chemical approach is demonstrated using the cluster seed method of nanoparticle synthesis in a simple microfluidic reactor system. This enables quantum dot nucleation at lower temperatures due to the presence of molecular organometallic compounds (NMe4)4[Cd10Se4(SPh)16] and (NMe4)4[Zn10Se4(SPh)16]. This integration of cluster seeding with microfluidics affords a new mechanism to tailor the reaction conditions for optimizing yields and tuning product properties.

Core-Doped [(Cd1-xCox)10S4(SPh)16]4- Clusters from a Self-Assembly Route

The incorporation of substitutional Co²⁺ impurities in [Cd₁₀S₄(SPh)₁₆]^4- (Cd₁₀) molecular clusters prepared by the self-assembly method where Na₂S is the sulfur precursor and a redox method where elemental S is the sulfur precursor is studied. The Co²⁺ ions provide unique spectroscopic and chemical handles to monitor dopant speciation during cluster formation and determine what role, if any, other cluster species play during Cd₁₀ cluster formation. In contrast to the redox method that produces exclusively surface-exchanged Co²⁺-doped Cd₁₀ (Co:Cd₁₀), the preparation of Cd₁₀ by the self-assembly method in the presence of Co²⁺ ions results in Co²⁺ incorporation at both the surface and core sites of the Cd₁₀ cluster. Electrospray ionization mass spectrometry (ESI-MS) analysis of the dopant distribution for the self-assembly synthesis of Co:Cd₁₀ is consistent with a near-Poissonian distribution for all nominal dopant concentrations albeit with reduced actual Co²⁺ incorporation. At a nominal Co²⁺ concentration of 50%, we observe incorporation of up to seven Co²⁺ ions within the Cd₁₀ self-assembled cluster compared to a maximum of only four Co²⁺ dopants in the Cd₁₀ redox clusters. The observation of up to seven Co²⁺ dopants must involve substitution of at least three core sites within the Cd₁₀ cluster. Electronic absorption spectra of the Co²⁺ ligand field transition in the heavily doped Co:Cd₁₀ clusters display clear deviation with the surface-doped Co²⁺-doped Cd₁₀ clusters prepared by the redox method. We hypothesize that the coordination of Co²⁺ and S^2- ions in solution prior to cluster formation, which is possible only with the self-assembly method, is critical to the doping of Co²⁺ ions within the Cd₁₀ cores.

Theoretical analysis of structures and electronic spectra in molecular cadmium chalcogenide clusters

We present calculated structural and optical properties of molecular cadmium chalcogenide nonstoichiometric clusters with a size range of less than 1 nm to more than 2 nm with well-defined chemical compositions and structures in comparison to experimental characterization and previous theoretical work. A unified treatment of these clusters to obtain a fundamental understanding of the size, ligand, and solvation effects on their optical properties has not been heretofore presented. The clusters belong to three topological classes, specifically supertetrahedral (Tn), penta-supertetrahedral (Pn), and capped supertetrahedral (Cn), where n is the number of metal layers in each cluster. The tetrahedrally shaped Tn clusters examined in this work are Cd(ER)4(2-) (T1), Cd4(ER)10(2-) (T2), and Cd10E4 (')(ER)16(4-) (T3), where R is an organic group, E and E' are chalcogen atoms (sulfur or selenium). The first member of the Pn series considered is M8E'(ER)16(2-). For the Cn series, we consider the first three members, M17E4 (')(ER)28(2-), M32E14 (')(ER)36L4, and M54E32 (')(ER)48L4(4-) (L = neutral ligand). Mixed ligand clusters with capping ER groups replaced by halogen or neutral ligands were also considered. Ligands and solvent were found to have a large influence on the color and intensity of the electronic absorption spectra of small clusters. Their effects are generally reduced with increasing cluster sizes. Blueshifts were observed for the first electronic transition with reduced size for both cadmium sulfide and cadmium selenide series. Due to weakly absorbing and forbidden transitions underlying the one-photon spectra, more care is needed in interpreting the quantum confinement from the clusters' lowest-energy absorption bands.

Surface-functionalized CdS clusters with recognition sites near the interface: selective luminescence response to lipophilic phenols

A series of water-soluble cadmium sulfide clusters bearing an alkyl-chain layer between the inorganic core and the outer PEG layer were synthesized by the ligand-exchange reaction of Cd(10)S(4)(SPh)(12) with thiols functionalized by an N-(ω-PEGylated alkyl) amide moiety. The photoluminescence titration experiments in aqueous media revealed that clusters with a sufficiently hydrophobic inner environment exhibit definite emission enhancements upon the addition of bisphenol A or 4-nonylphenol. The dramatic effect of the alkyl chain length on the emission responses demonstrated that the hydrophobic layer around the inorganic surface serves as guest binding sites to facilitate the access of the lipophilic phenols near the organic-inorganic interface. A marked preference for the lipophilic phenols over related compounds, such as methylated bisphenol A, long-chain n-alkanol, and nonlipophilic phenols, was observed in the emission responses of the "hydrophobic" cluster, suggesting that not only the hydrophobic interaction but also the attractive force involving the phenolic OH group contributes to the positive responses. The results of control experiments and IR studies indicated that the hydrogen bonding interaction between the phenolic OH group and the amide group in the surface organic units is responsible for the positive emission responses. The present work shows that the precise tuning of the molecular recognition environments near the organic-inorganic interface is useful for developing guest-specific functions.