Size Control at Maximum Yield and Growth Kinetics of Colloidal II–VI Semiconductor Nanocrystals

Abstract

We investigated the growth kinetics of CdSe nanocrystals for a hot-injection colloidal synthesis, as a function of selected key process parameters. In this investigation, the synthesis consisted in the injection of trioctylphosphine-selenium into cadmium oleate, followed by nucleation and growth stages. The evolution of size and size distribution of the nanocrystals was monitored during the synthesis via UV−visible absorption and photoluminescence spectroscopy. Three growth parameters have been extracted from the experimental data through a general kinetic growth model, and discussed: the initial particle size, the growth rate, and the particle size at the end of the growth. A modification of the classical nucleation theory has been developed to explain and predict the equilibrium (final) size of the nanoparticles, by taking into consideration the consumption of monomers during crystal formation and growth. The model accurately predicts the trends of the crystal size as a function of the oleic acid concentration. The model represents a valuable tool for the study of colloidal crystal growth providing new insight into the physical and chemical processes behind the nucleation and growth. Moreover, it enables exploration of new limits in terms of typical synthetic conditions, aiming at the optimization of the synthesis yield for every single case at handa very important goal in view of the ever-growing need for large-scale fabrication of colloidal nanostructures.

Identifying Clusters and/or Small-Size Quantum Dots in Colloidal CdSe Ensembles with Optical Spectroscopy

“…Regarding the red shifts of the absorption and emission peaks, as well as the values of the fwhm, the samples from Batches a and b are similar, and these features are comparable to those of QD samples that were taken from a single CdSe reaction batch that underwent a growth in size. − ,,− ,− The presence and near-absence of trap emission for the samples from Batches a and b, respectively, are worthy of notice. The nature of the surface ligand for the two batch samples should be identical, , while the Cd and Se reactivity in Batch a is much higher than that in Batch b due to the presence of HPPh 2 . − See Figure S1-3 for results pertaining to Batch a but without the use of HPPh 2 .…”

“…Figure shows the optical absorption and PL spectra of one Batch a sample and of one Batch b sample. The two CdSe samples display essentially similar bandwidths in optical absorption and PL, which are comparable to that of a single conventional CdSe QD ensemble. − Figure suggests that Batch a produced mixtures of clusters including Clusters-429, -437, -453, and -467 (denoted in reference to their absorption peak positions in wavelength nanometers (nm)). Figure indicates that Batch b produced small-size QDs mainly, together with a relatively small amount of clusters such as Cluster-467.…”

“…The two CdSe samples present essentially similar bandwidths of optical absorption and PL, the feature of which is comparable to that shown by single conventional CdSe QD ensembles. − This similarity is not sufficient to support the conclusion that the two CdSe samples contain small-size QDs free of clusters. It is well-known that to characterize the size and size distribution of one ensemble of clusters or QDs the approach should be quite different.…”

“…Optical absorption and photoluminescence (PL) have been shown to be practical tools for estimating the size and size distribution in colloidal semiconductor cadmium chalcogenide quantum dot (CdE QD) samples. − The peak positions of the bandgap absorption and emission have been correlated with the size of QDs, while the full width at half-maximum (fwhm) has been related directly to the size distribution in a QD sample. For a single CdSe QD species (with a bandgap of ∼545 nm), a value of fwhm of ∼17 nm was reported, together with that of ∼33 nm for the corresponding ensemble .…”

“…For QD samples extracted sequentially from a reaction batch, the peak positions of their bandgap absorption and emission normally red shift in a continuous fashion. − ,− ,− It is noteworthy that there is another type of evolution of the peak positions, which is discontinuous and attributed to the presence of clusters instead of QDs. − The key difference between clusters and QDs should be their structures. Meanwhile, the PL spectra may contain features from both band-edge and trap emission.…”