# Following quantum dot growth with McSAS

Thanks to Benjamin Abécassis (via Twitter), I got involved in an interesting project. Dr. Abécassis and coworkers had been doing in-situ measurements during synthesis of quantum dots, and were using McSAS to analyse their data. That allowed them to figure out what mechanistic models are unlikely to be valid for this synthesis and highlight one which is consistent with the measurements. The very nice Nano Letters publication that came out of this has just been published last Friday and is available here.

Firstly (before discussing the work) credit where credit is due: my contribution consisted mostly of advice and verification calculations, the bulk of the data analysis had already been done. However, Dr. Abécassis and co-workers considered the McSAS method to be a crucial part of their research and therefore wished to add me to the author list. I am very thankful for this gesture, and hope that my somewhat minor contributions had a positive effect on the manuscript.

Quantum dots are useful little fluorescent particles which emit one particular wavelength of photons when struck by shorter-wavelength photons. They can therefore be used to replace inefficiently transmitting colour filters. Their use in televisions is forthcoming, and they are also promising for use in solar cells, lasers, medical imaging and transistors.

With their optical properties defined by their size, understanding of the growth processes (which can lead to size control) is a quite important task. The techniques used by Dr. Abécassis to study quantum dots include synchrotron SAXS and WAXS, and laboratory UV-vis. These techniques allowed for monitoring of CdSe quantum dot formation under high-temperature synthesis conditions with a time resolution of $1/\mathrm{s}$.

SAXS was used to determine the size of the nanoparticles, and peak broadening analysis of WAXS was used to estimate the crystallite size. As expected, the WAXS size is always smaller than the SAXS size, indicating a certain degree of polycrystallinity. As measurements were done on absolute scale, the absolute volume fraction of nanoparticles could be extracted, and cross-checked using UV-vis.

However, what matters is not that you collect data, but that you use it constructively. In this case, the data could be used to rule out some mechanistic models for nanoparticle formation and growth. Using this data, diffusion-limited and reaction-limited growth could be ruled out, leaving only monomer generation as the rate-limiting step. There are strong indications that this monomer generation is limited by the availability of the reactive Se necessary to form the CdSe monomers.

All in all, this is a very nice investigation resulting in a clear answer. My congratulations to everyone for working it out and putting it all together in this nice, concise form!

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