Influence of ligand shape and steric hindrance on the composition of the nanocrystal ligand shell

Organic ligands play a key role in the synthesis of colloidal semiconductor nanocrystals or quantum dots. Generally they consist of a functional group and an aliphatic chain, with carboxylic acids, thiols and phosphonic acids as typical examples. The functional group ensures the binding to the nanocrystal surface, while the stability of the dispersion strongly depends on the interactions between the organic chains of the adjacent ligands. A number of studies already addressed the binding strength and the type of binding between the nanocrystal surface and the ligand yet none discuss the effect of the organic chain on the ligand exchange. By means of NMR spectroscopy, we examine the ligand shell composition of CdSe nanocrystals originally capped with oleic acid (OA), when exposed to a linear carboxylic acid. Regardless of chain length, we see a one-to-one exchange between the carboxylic acids. The composition of the ligand shell closely matches that of the ligand mixture in solution, indicating that the ligand shell can be seen as an ideal mixture of both ligands. As a consequence, a mixed ligand shell can easily be prepared by adding a ligand mixture with desired composition to the nanocrystal dispersion. On the other hand, when the CdSe nanocrystals are exposed to a branched carboxylic acid with two long aliphatic chains, like 2-hexyldecanoic acid, the ligand shell mainly consists of OA moieties. We interpret these results using an exchange process where the incoming ligand not only displaces oleic acid but also occupies additional space in the ligand shell to accommodate both aliphatic chains. Hence, given a one-for-one exchange reaction, steric hindrance in a fully packed ligand shell will prevent complete ligand exchange. These results can be very useful in view of producing nanocrystals with lower ligand densities by means of synthesis with these branched carboxylic acids

Aminophosphines : a double role in the synthesis of colloidal indium phosphide quantum dots

Aminophosphines have recently emerged as economical, easy-to-implement precursors for making InP nanocrystals, which stand out as alternative Cd-free quantum dots for optoelectronic applications. Here, we present a complete investigation of the chemical reactions leading to InP formation starting from InCl3 and tris(dialkylamino)phosphines. Using nuclear magnetic resonance (NMR) spectroscopy and single crystal X-ray diffraction, we demonstrate that injection of the aminophosphine in the reaction mixture is followed by a transamination with oleylamine, the solvent of the reaction. In addition, mass spectrometry and NMR indicate that the formation of InP concurs with that of tetra(oleylamino)phosphonium chloride. The chemical yield of the InP formation agrees with this 4 P(+III) -> P(-III) + 3 P(+V) disproportionation reaction occurring, since full conversion of the In precursor was only attained for a 4:1 P/In ratio. Hence it underlines the double role, of the aminophosphine as both precursor and reducing agent. These new insights will guide further optimization of high quality InP quantum dots and might lead to the extension of synthetic protocols toward other pnictide nanocrystals

Studies of organic ligands at the nanoparticle surface with solution NMR

When studying colloidal nanoparticles (NPs) with NMR, we focus on the ligands that surround them. Used during synthesis to control nucleation and growth, they end up as a monolayer covering the NP surface and stabilizing the NP colloidal suspension. In the last few years we have develop the application of NMR techniques to characterize these systems [1]. It turns out that plenty of information on the characteristics of NP systems is already present in the 1D proton spectrum. For instance, when studying NPs stabilized with oleic acid or oleylamine ligands (OL), we focus especially on the alkene resonance at around 5.5 ppm. Looking at different OL-NP systems with a different core composition, a different ligand density or in a different solvent, we noticed that the peak shape of the alkene resonance varies considerably

InAs colloidal quantum dots synthesis via aminopnictogen precursor chemistry

Despite their various potential applications, InAs colloidal quantum dots have attracted considerably less attention than more classical II-VI materials because of their complex syntheses that require hazardous precursors. Recently, aminophosphine has been introduced as a cheap, easy-to-use and efficient phosphorus precursor to synthesize InP quantum dots. Here, we use aminopnictogen precursors to implement a similar approach for synthesizing InAs quantum dots. We develop a two-step method based on the combination of aminoarsine as the arsenic precursor and aminophosphine as the reducing agent. This results in state-of-the-art InAs quantum dots with respect to the size dispersion and band gap range. Moreover, we present shell coating procedures that lead to InAs/ZnS(e) core/shell quantum dots that emit in the infrared region. This innovative synthesis approach can greatly facilitate the research on InAs quantum dots and may lead to synthesis protocols for a wide range of III-V quantum dots

Influence of ligand shape and steric hindrance on the composition of the nanocrystal ligand shell

Organic ligands play a key role in the synthesis of colloidal semiconductor nanocrystals or quantum dots. Generally they consist of a functional group and an aliphatic chain, with carboxylic acids, thiols and phosphonic acids as typical examples. The functional group ensures the binding to the nanocrystal surface, while the stability of the dispersion strongly depends on the interactions between the organic chains of the adjacent ligands. A number of studies already addressed the binding strength and the type of binding between the nanocrystal surface and the ligand yet none discuss the effect of the organic chain on the ligand exchange. By means of NMR spectroscopy, we examine the ligand shell composition of CdSe nanocrystals originally capped with oleic acid (OA), when exposed to a linear carboxylic acid. Regardless of chain length, we see a one-to-one exchange between the carboxylic acids. The composition of the ligand shell closely matches that of the ligand mixture in solution, indicating that the ligand shell can be seen as an ideal mixture of both ligands. As a consequence, a mixed ligand shell can easily be prepared by adding a ligand mixture with desired composition to the nanocrystal dispersion. On the other hand, when the CdSe nanocrystals are exposed to a branched carboxylic acid with two long aliphatic chains, like 2-hexyldecanoic acid, the ligand shell mainly consists of OA moieties. We interpret these results using an exchange process where the incoming ligand not only displaces oleic acid but also occupies additional space in the ligand shell to accommodate both aliphatic chains. Hence, given a one-for-one exchange reaction, steric hindrance in a fully packed ligand shell will prevent complete ligand exchange. These results can be very useful in view of producing nanocrystals with lower ligand densities by means of synthesis with these branched carboxylic acids

Economic and size-tunable synthesis of InP/ZnE (E = S, Se) colloidal quantum dots

We present synthesis protocols, based on indium halide and aminophosphine precusors, that allow for the economic, up-scaled production of InP quantum dots (QDs). The reactions attain a close to full yield conversion with respect to the indium precursor and we demonstrate that size tuning at full chemical yield is possible by changing the nature of the indium halide salt. In addition, we present ZnS and ZnSe shell growth procedures that lead to InP/ZnS and InP/ZnSe core/shell QDs that emit from 510 to 630 nm with an emission line width between 46 and 63 nm. This synthetic method is an important step toward performing Cd-free QDs, and it could help the transfer of colloidal QDs from the academic field to product applications

Reaction chemistry/nanocrystal property relations in the hot injection synthesis of quantum dots

Quantum dots (QDs) or colloidal semiconductor nanocrystals attract extensive interest in both industry and science nowadays due to their opto-electronic properties that strongly depend on their size, structure, shape and composition. To further develop applications, providing a rational basis to explore and understand how hot injection synthesis parameters affect these properties is key. Various literature studies indicate that the composition of the reaction mixture in which nanocrystals are formed is related to the size the nanocrystals attain at the end of the reaction. We analyze several of these ‘reaction chemistry/nanocrystal property relations’ by combining reaction simulations with an experimental investigation on CdSe quantum dot syntheses. We find that increasing the free acid concentration in the reaction mixture has the same effect on a real synthesis as raising the solute solubility in the simulations and explain the increase of the QD size as result ing from an enhanced consumption of the solute by nanocrystal growth. Similarly, we address the effect of increasing precursor concentrations to an increasing monomer formation rate, which results in a higher nanocrystal concentration and therefore a smaller nanocrystal size. Finally, we relate size tuning by the ligand chain length to the coordination of the solute by these ligands