Research Projects

Research Projects



Nano (Metal) Clusters

 

Noble metal molecular nanoparticles, MMNPs, -mainly Au and Ag- are revolutionary new materials that consist of a distinct number of metal atoms protected by a certain number of stabilizing ligands. They have received keen interest due to their unique optical and electronic properties featured by discrete molecular-like energy levels, size specific HOMO-LUMO gap, and, in multiple of cases, enhanced photoluminescence.

In our lab, we investigate several aspects of MMNPs. These involve the establishment of new methods to enhance their functionality either by ligand exchange or incorporating mixed ligands, the development of high-resolution separation techniques such as gel electrophoresis and ultracentrifugation, in addition to their utilization as active sites in solar, catalytic, electrochemical and photocatalytic materials. We also engaged in crystallization of these nanoclusters for better understanding their fascinating properties. 







Selected Reference List: 


  • AbdulHalim, L. G., Ashraf, S., Katsiev, K., Kirmani, A. R., Kothalawala, N., Anjum, D. H., ... & Bakr, O. M. (2013). A scalable synthesis of highly stable and water dispersible Ag 44 (SR) 30 nanoclusters. Journal of Materials Chemistry A, 1(35), 10148-10154.
  • AbdulHalim, L. G., Kothalawala, N., Sinatra, L., Dass, A., & Bakr, O. M. (2014). Neat and Complete: Thiolate-Ligand Exchange on a Silver Molecular Nanoparticle. Journal of the American Chemical Society, 136(45), 15865-15868.

 

 

Continuous flow synthesis

 

The solution syntheses of mono- and bi-metallic nanoparticles have attracted the interest of researchers due to their fascinating properties and technological applications in as an example catalysis and photovoltaic. However, these nanoparticle systems are inherently difficult to scale-up because of the sensitivity of their growth kinetics towards changes in the experimental parameters including the way of introduction of reagents, temperature variation and reaction volume. Moreover, the reproducibility of the batch synthesis is challenging and difficult. Hence, the continuous-flow synthesis has made this challenge to be doable. Additionally, the flow reactor synthesis offers many advantages over the batch one including uniform heat, low utilization of chemicals during the optimization process and the ability to quickly test all reaction parameters and to safely use unpleasant odors chemicals. In top of that, the continuous-flow system can pave the way to conduct reactions at elevated temperature and pressure that could not be trivial on batch.

Our group has illustrated the usefulness and the advantages of the continuous-flow system by publishing two articles and filing patents on this method and more to come J. For example, Pan et al. reported that it can be possible to separate the nucleation and the growth of colloidal nanocrystals by continuous-flow system as shown in figure 1. This automated synthesis was greatly beneficial for producing the highest photovoltaic-quality colloidal quantum dots. Also, Alec et al. described the benefit of this method in synthesising shape- and size- controlled bimetallic nanoparticles as shown in figure 2.

 


Selected Reference List:

 

  • Pan, J.; El-Ballouli, A. O; Rollny, L.; Voznyy, O.; Burlakov, V.; Goriely, A.; Sargent, E. H; Bakr, M. O. “Automated Synthesis of Photovoltaic-Quality Colloidal Quantum Dots using Separate Nucleation and Growth Stages”, ACS Nano, 2013, 7, 10158–1016
  •  Mehenni, H.; Sinatra, L.; Mahfouz, R.; Katsiev, K.; Bakr, O. M., Rapid continuous flow synthesis of high-quality silver nanocubes and nanospheres. RSC Advances 2013, 3 (44), 22397-22403.

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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 system 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

 


Perovskite Solar Cells: Fundamental Investigation & Device Engineering


In the past few years, a well-known material called methylammoinum lead triiodide has suddenly become exceptionally popular in photovoltaic community.  Specially, the power conversion efficiency of perovskite solar cells (PSCs) has dramatically increased from reported 3.8% to certified 20.1% in just five years through low-cost manufacturing procedures. To date, the popularity of the hybrid perovskite shows no sign of abating. Perovskite has worked wonders shown unprecedented steep performance curve in photovoltaics, and we need to know why.

Aimed for better understanding on the ultimate superiority of perovskite materials, we successfully made centimeter-scale organolead trihalide perovskite single crystals with exceptionally low trap-state density for accurate determination of the material’s intrinsic optical and electrical properties. Both CH3NH3PbBr3 and CH3NH3PbI3 single crystals demonstrated exceptionally long diffusion length exceeding 10 micrometers as calculated from the measured PL lifetime and carrier mobility. This study, as published in Science, demonstrates that PSCs stand to see further breakthroughs through substantial improvement in the crystallinity in perovskite layer.

We are currently interested in deep investigation on the fundamental aspects of perovskite materials and the working mechanism of PSCs, specifically from photo- and electrical-physical perspective angles. We also seek to develop cost-effective device engineering protocols and other application of perovskites.

We are all enthusiastic. Come and join us, and enjoy the perovskite fever.


Selected Reference List: