Publication date: 1st April 2013
Semiconductor nanocrystals show unique energetic, electronic and optical properties which depend tremendously upon their size. Latest reports have focused on lead sulfide - quantum dots (PbS-QDs).[1][2] While macroscopic lead sulfide has a bandgap of 0.37 eV (which corresponds to a wavelength of 3300 nm), synthesized nanocrystals have diameters between 3 to 8 nm and appropriate excitonic peaks from 1000 to 1600 nm. Due to small bandgap energies (0.8 to 1.2 eV) the optoelectronical transitions of PbS nanocrystals cover the near-infrared region. In addition, they absorb the whole wavelength range below 1000 nm and are highly resistant against heating and bleaching. These properties predestine PbS-QD’s to be used for photovoltaic applications and IR-emitters in LED’s.[3][4]
The classical synthetic routes enable the production of particles with proper homogeneous size dispersions, therewith narrow emission bandwidths and quantum yields above 50%.[1][2] However, common hot-injection syntheses show various issues which can be eliminated by passing on to continuous flow systems. These methods minimize concentration- and temperature-gradients during the reaction and offer perfect mixing of the fluids. Due to these points, producing reasonable amounts of oleic acid stabilized PbS (up to kg/year) with narrow size distribution and a high degree of reproducibility is only possible and economically realizable by a continuous flow reactor. In addition these methods increase the laboratory safety as the expose to chemicals can be reduced to a minimum.
Here we will present our progress in synthesizing spherical, monodisperse, highly luminescent PbS-quantum dots by continuous flow methods. We are able to produce particles with an average FWHM (emission peak) of 115 nm (≙142 meV), compared to single particle line widths averages of 100 meV.[5] The regulation of particle growth and properties by varying temperature, flow speed and precursor parameters will be shown. Working with a continuous flow system allows us to synthesize quantum dots from 3 to 8 nmwith an enormous high degree of reproducibility.
Absorption- and Emissionspectra for three PbS-nanoparticles reactor-samples (synthezised on different days with different stock solutions).
[1] Hines, M.A.; Scholes, G.D. Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared Emission: Observation of Post-Synthesis Self-Narrowing of Particle Size Distribution. Adv. Mater., 2003, 15, 1844-1849. [2] Moreels, I.; Justo, Y.; De Geyter, B.; Haustraete, K.; Martins, J.C.; Hens, Z. Size-Tunable, Bright and Stable PbS Quantum Dots: A Surface Chemistry Study. ASC Nano, 2011, 5, 2004-2012. [3] Tang, J.; Kemp, K. W.; Hoogland, S.; Joeng, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang. X.; Debnath, R.; Cha, D.; Chou, K. W.; Fischer, A.; Amassian, A.; Asbury, J. B.; Sargent, E.H. Quantum Dot Photovoltaics in the Extreme Quantum Confinement Regime: The Surface-Chemical Origins of Exceptional Air- and Light-Stability. ACS Nano, 2010, 4, 869-878. [4] Rauch, T.; Böberl, M.; Tedde, S. F.; Fürst, J.; Kovalenko, M. V.; Hesser, G.; Lemmer, U.; Heiss, W.; Hayden, O. Near-infrared imaging with quantum-dot-sensitized organic photodiondes. Nature Photonics, 2009, 3, 332-336. [5] Peterson, J. J.; Krauss, T. D. Fluorescence Spectroscopy of Single Lead Sulfide Quantum Dots. Nano Letters, 2006, 6, 3, 510-514.
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