Tuning the Postfocused Size of Colloidal Nanocrystals by the Reaction Rate: From Theory to Application

Abstract

We show that adjusting the reaction rate in a hot injection synthesis is a viable strategy to tune the diameter of colloidal nanocrystals at the end of the size distribution focusing, i.e., the postfocused diameter. The approach is introduced by synthesis simulations, which describe nucleation and growth of colloidal nanocrystals from a solute or monomer that is formed in situ out of the injected precursors. These simulations indicate that the postfocused diameter is reached at almost full yield and that it can be adjusted by the rate of monomer formation. We implement this size-tuning strategy using a particular CdSe quantum dot synthesis that shows excellent agreement with the model synthesis. After demonstrating that the reaction rate depends in first order on the Cd and Se precursor concentration, the proposed strategy of size control is explored by varying the precursor concentration. This enables the synthesis of colloidal nanocrystals with a predefined size at almost full yield and sharp size distributions. In addition, we demonstrate that the same tuning strategy applies to the synthesis of CdS quantum dots. This result is highly relevant especially in the context of reaction upscaling and automation. Moreover, the results obtained challenge the traditional interpretation of the hot injection synthesis, in particular the link between hot injection, burst nucleation, and sharp size distributions.

Real-Time in Situ Probing of High-Temperature Quantum Dots Solution Synthesis

“…Our experimental results can be compared to recent work by the Hens group 24,36,41 who used both UV/vis ex-situ experiments and numerical resolution of nucleation and growth equation to unravel the formation mechanism of quantum dots in similar conditions. Remarkably, the evolution of the nanocrystal concentration in their simulations reproduce very well the trends observed in our experiments, where a brief nucleation phase is followed by a decrease in nanocrystal concentration.…”

“…From the absorbance at 350 nm, we can extract the concentration of quantum dots in solution 35,36 (Supporting Information Figure S5). Though the 10 s time resolution in these spectroscopy experiments does not permit a detailed characterization of the nucleation phase, we observe a decrease in the nanocrystal concentration during the first 100 s of the synthesis.…”

In situ measurement of exciton energy in hybrid singlet-fission solar cells

“…Previously, our group fabricated photovoltaic devices with pentacene/lead sulphide (PbS) nanocrystal heterojunctions 29 . However, PbS nanocrystal syntheses often yield only moderately monodisperse samples, presumably owing to the rapid reaction of their precursors [41][42][43] . In this study, we fabricate bilayer photovoltaic cells with pentacene, using lead selenide (PbSe) nanocrystals as the infrared absorbing material.…”

Size-controlled growth of spherical nanoparticles of Y-doped BaZrO3 perovskite

“…As the molar concentration of cations increases, the number of nuclei formed also increases, and thus, a higher nucleation rate results in finer particles [34,35]. Kwon et al [36] and later Abe et al [37,38] developed a model for nucleation and growth of colloidal particles from solutions following the classical nucleation and growth theory, in which they proposed a precursor to monomer (P ? M) transition model for nucleation of nanocrystals.…”

“…Moreover, as super saturation (S) increases, nucleation rate (J N ) also increases according to Eq. (2) [37,38]. The monomer in the present case can be taken as the hydrolyzed complexes that form in the base solution as described above.…”