(154f) Oscillatory Microprocessor for High-Throughput in-Situ Characterization of Semiconductor Nanocrystals

Emergence of semiconductor nanocrystals, known as quantum dots (QDs), with unique physicochemical properties have enabled breakthrough applications at cellular and organism scales in biological imaging, and at device scales in light emitting diodes, solar cells and displays.^1-3 Owing to the quantized energy levels associated with nanometre-sized QDs, their corresponding absorption and photoluminescence emission spectra are directly correlated and tuned with the size of QDs.^{4, 5} Although the batch scale processes for the solution-phase preparation of QDs was introduced ~ 20 years ago,⁴ but still remains as the main competitor for screening, exploration and investigation of QDs. Despite the diversity of the demonstrated applications of QDs with batch scale techniques, these strategies present deficiencies for consistent preparation of QDs due to their poor heat and mass transfer and inaccurate control over the reaction conditions. Moreover, the manual nature of batch scale techniques makes it challenging and time-consuming for high-throughput screening and fundamental studies of colloidal QDs. Shrinking the reaction vessel size from litres to microliters and diffusion length scale from centimetres to hundreds of microns would significantly enhance the heat and mass transfer rates during the synthesis process of QDs, while enabling precise control over the reaction parameters. Over the past decade, different single/multi-phase microscale strategies have demonstrated promising alternatives to the batch scale synthesis for robust and high throughput screening of a large parameter space associated with QD synthesis.^{2, 6, 7} However, the intrinsic dependency of the degree of mixing and residence time (via average flow velocity) associated with continuous microscale platforms makes it impossible to reproduce the same mixing characteristics for different QD synthesis times. In addition, most of microscale strategies would still require ~mL volume of QD precursors for each experimental condition (e.g., concentration and reaction time), due to the dead volumes associated with tubings and waiting timesrequired to reach steady state conditions. 

In this work, building on our previously developed oscillatory flow reactor,³ we designed and developed an integrated and modular flow-based strategy for in-situ characterization and fundamental studies of synthesis of colloidal semiconductor nanocrystals. The automated oscillatory microprocessor consists of a 12 cm long Teflon tubular reactor (0.0625 inch inner diameter, Fluorinated ethylene propylene, FEP) embedded within an aluminium chuck, two fiber-coupled blue LEDs (405 nm) and photodetectors as well as a fiber-coupled UV-Vis light source and a miniature spectrometer. The integration of the two-phase oscillatory platform with spectral characterization tools (i.e., absorption and fluorescence spectroscopy) enables real-time in-situ monitoring of the in-flow prepared QDs which is challenging, and in some cases even impossible to accomplish in batch scale synthesis. Utilizing the developed oscillatory microprocessor, we screened the nucleation and growth stages of II-VI (CdSe and CdTe) and III-V (InP) QDs via monitoring the absorption spectra evolution of the formed QDs, in-situ, over a wide range of temperatures (160 °C-220 °C ) and reaction times (3 s-900 s), while using only 10 μL of the QD precursors. A time-series of absorption spectra with a time delay of 3-15 s was obtained at each reaction temperature from a 10 μL droplet containing the QDs while moving back and forth within a 12 cm long reactor with an average velocity of 1-2 cm/s. To the best of our knowledge, our developed two-phase experimental platform is the first small-scale strategy which enables in-situ spectral characterization of II-VI and III-V QDs, providing further insights on the kinetics and mechanisms of nucleation and growth stages of colloidal nanocrystals. The oscillatory microprocessor could further be applied towards high-throughput in-situstudies of core/shell QDs.

References

1.         W. C. W. Chan and S. Nie, Science 281(5385), 2016-2018 (1998).

2.         A. M. Nightingale, T. W. Phillips, J. H. Bannock and J. C. de Mello, Nat. Commun 5(2014).

3.         M. Abolhasani, N. C. Bruno and K. F. Jensen, Chem. Commun.(2015).

4.         C. B. Murray, D. J. Norris and M. G. Bawendi, Journal of the American Chemical Society 115(19), 8706-8715 (1993).

5.         D. J. Norris and M. G. Bawendi, Physical Review B 53(24), 16338-16346 (1996).

6.         A. M. Nightingale and J. C. deMello, Adv. Mater. 25(13), 1813-1821 (2013).

7.         B. K. H. Yen, A. Günther, M. A. Schmidt, K. F. Jensen and M. G. Bawendi, Angew. Chem. 117 (34), 5583-5587 (2005).