It is well known that semiconductor nanocrystals (NCs) present outstanding properties that make them ideal candidates to be used in different nanotechnology fields, like health, mobility, energy storage or communications. In fact, several scientific works describing well-defined methods to synthesize and characterize highly emissive and stable NCs have been already published. Despite this, usually these advances are the result of trial-mistake approaches, since there is still a lack of knowledge in the events occurring during nucleation and growth of NCs. For example, it is unknown how are the structures of the first stable nuclei that are able to grow and form the final NCs, or if there is a more stable crystal structure that is always formed and then it can develop in different crystal structures due to different growth pathways. Reaction parameters like precursor ratio, temperature and reaction time or capping ligands influence nucleation and growth events and therefore should be deeply studied.

To gain knowledge on the nucleation and growth events it is necessary to study these processes “in-situ”, which implies the use of devices able to reach the millisecond time scale, where nucleation events take place. In this way, and with the aim of in-situ study nucleation and growth events on NCs, we have design and assemble a state-of-the-art continuous-flow device for the synthesis of NCs. This device is highly versatile and allows the synthesis of NCs at temperatures as high as 380 C with reaction times in the millisecond time scale. Besides, a UV-VIS-NIR spectrometer is attached, enabling the in-situ study of the absorption properties of the resulting NCs. A home-build flow cell has been incorporated to perform also SAXS/WAXS experiments with synchrotron radiation. CdSe has been chosen as model system.

The experiments have been done considering the effect of temperature and reaction time on nucleation and growth. Nucleation studies have been performed with the minimum reaction time (ms), while growth experiments require longer reaction times (from 2 to 50 s). Our results show that NPs are the result of the co-operative growth of CdSe clusters of different sizes: first, very small clusters are formed (absorption at ≈ 368 nm) and then, under certain temperatures (150 ≤ T ≤ 180 C) grow to form intermediate clusters (absorption at ≈ 404 nm). At temperatures between 180 C and 210 C these clusters grow to form clusters absorbing at 437 nm. Higher temperatures (T ≥ 210 C) produce the growth to form NPs of different sizes.