Millisecond Kinetics of PbS Quantum Dots Using Droplet-based Microfluidics with On-line Absorption and Fluorescence Spec
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Millisecond Kinetics of PbS Quantum Dots Using Droplet-based Microfluidics with On-line Absorption and Fluorescence Spectroscopy Ioannis Lignos1, Stavros Stavrakis1, Ardita Kilaj1 and Andrew deMello1 1 Institute for Chemical & Bioengineering, Department of Chemistry & Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1, 8093 ABSTRACT We report a novel approach for the on-line characterization of nucleation and growth kinetics of lead sulfide (PbS) quantum dots using droplet-based microfluidics. Monodisperse NIR-emitting PbS with optical bandgap between 680 to 1200 nm can be formed rapidly using two reaction schemes at different operating temperatures between 70 and 130oC and the temporal evolution of the absorption and fluorescence spectra are monitored in real-time using a microfluidic platform with an on-line absorption and fluorescence optical system. Therefore, this microfluidic platform is able to provide quantitative information on a millisecond (ms) time frame regarding the size, size distribution, concentration and emission characteristics of the generated nuclei and particles. To our knowledge, this represents the first microfluidic approach for the study of the nucleation and growth in high-temperature colloidal crystallization using in-situ absorption and photoluminescence spectroscopy. INTRODUCTION Semiconductor nanocrystals (NCs) - quantum dots - have been of great interest due to their tunable optical and electronic properties with particle size1,2. IV-VI nanocrystalline materials such as PbS have a small band gap and they show superior electronic and optoelectronic properties for near-infrared (NIR) applications3. Since their luminescence can cover the broad wavelength region of NIR, they have found significant application in Schottkybased solar cells, photodiodes and in vitro/in vivo biological imaging1,2,4. However, a thorough understanding of the synthetic processes of IV-VI quantum dots remains a significant challenge. A variety of flask-based methods have been shown to be successful in producing high-quality IV-VI quantum dots, applying ‘hot injection’ or ‘heat-up’ synthetic approaches5. However, macroscale techniques are usually ineffective in providing information regarding nucleation and growth kinetics of nanocrystalline materials because the extraction of absorption and PL spectra is performed off-line (the reaction is quenched and the measurements are carried out at room temperature)6. Since the time-scale of the nucleation stage is in the range of milliseconds to seconds, it is essential to perform real time measurements for the extraction of kinetic data6. In recent years, the use of microfluidic systems for performing complex chemical experiments for nanoparticle synthesis has become increasingly popular7,8,9. Herein, we describe the real-time characterization of nucleation and growth of PbS quantum dots using droplet-based microfluidics. The microfluidic platform is integrated with both visible and near-infrared (VIS/NIR) absorption and fluorescence system, which can be performed whi
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