Quantum Dots

Quantum Dots (QDs) are semiconductor nanocrystals; whose size, shape, chemical composition, and inter-particle connection play key roles in determining their physical and chemical properties. Monodispersed QDs with narrow size distribution are crucial for various QD-based applications, including bio-imaging, photo-catalysis, and optoelectronics. Nevertheless, scaling up the material synthesis via the traditional batch method lessens crystal quality and monodispersity, due to reduced control of the reaction conditions; and thus the nucleation and growth processes. In our lab, we adopt the continuous-flow reactor synthesis approach to synthesize a wide range of nanocrystallites and QDs, in large-scale, while maintaining precise control of the reaction conditions (temperature, reactions time, etc..). Our aim is to develop automated synthesis procedures of high-quality materials in order to support QDs in fulfilling their tremendous industrial promise in various applications.

Fig. 1 shows a schematic example of the flow reactor sy

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stem employed in our lab to synthesize PbS QDs. Our flow reactor synthesis of PbS QDs leads to solar cells having performance similar to that of comparable batch-synthesized QDs. Specifically, we find that only when using a dual-temperature-stage flow reactor synthesis are the QDs of sufficient quality to achieve high performance. The dual-temperature stage serves in separating the nucleation and growth processes, and we have used a kinetic model to explain and optimize these processes within the reactor. Compared to conventional single-stage flow-synthesized QDs, we achieve superior quality nanocrystals via the optimized dual-stage reactor, with high photoluminescence quantum yield (50%) and narrow full width-half-maximum. This process allows continuous and large-scale synthesis of QDs without scarifying crystal quality and monodispersity.

Moreover, we also investigate the charge-carrier dynamics of QDs, with transient spectroscopic techniques, to better understand the multiple exciton generation (MEG), and charge transfers (CT) at QD interfaces. Such information is crucial for increasing the conversion efficiency of QD based-solar cell devices.

 

Selected Reference List:

• El-Ballouli, A. O; Alarousu, E.; Bernardi, M.; Aly, S. M.; Lagrow, A. P.; Bakr, O. M.; Mohammed, O. F. “Quantum Confinement-Tunable Ultrafast Charge Transfer at the PbS Quantum Dot and PCBM Fullerene Interface”,  JACS, 2014, 136, 6952–6959

 

• El-Ballouli, A. O; Alarousu, E.; Usman, A.; Pan, J; Bakr, O. M.; Mohammed, O. F. “Real-Time Observation of Ultrafast Intraband Relaxation and Exciton Multiplication in PbS Quantum Dots”, ACS Photonics, 2014, 3, 285-292