K.I. Hunter, J.T. Held, K.A. Mkhoyan, U.R. Kortshagen
University of Minnesota,
Keywords: nonthermal plasma, nanoparticle, quantum dot, silicon, germanium, core/shell
Summary:In this work, we present an enabling technology for the dry synthesis of core/shell heterostructured nanoparticles using a low-pressure nonthermal plasma reactor. The free-standing nanoparticles produced by this plasma approach are on the size scale of quantum confinement (2-10 nm) such that their optoelectronic properties are highly dependent on their physical dimensions, specifically core and shell radii. Through in-flight control of epitaxial shell thickness in the gas-phase, we demonstrate the ability to tune nanocrystal optoelectronic properties beyond size variation alone. This approach is tailored towards the incorporation of materials requiring high synthesis temperatures (e.g. group IV semiconductors – Si and Ge) in heterostructured quantum dots. The technology presented here builds on previous work using low-pressure nonthermal plasmas to produce highly-luminescent, quantum-confined Si nanocrystals with a narrow size distribution , which exhibit significant potential for a variety of optoelectronic applications including in solid-state lighting  and solar energy applications . In pursing this technology, we were motivated by the success of solution-phase core/shell growth, which has proven to be indispensable in the colloidal nanocrystal community as a means to improve optoelectronic properties of II-VI crystallites . However, the dry synthesis approach demonstrated here has numerous, significant advantages to wet chemical methods. For one, our plasma process does not require the use of ligands at any step in the process, facilitating the incorporation of as-produced nanocrystals into electrically conductive thin films. In place of ligands, the unipolar negative charging of the nanoparticles by electrons within the plasma environment effectively suppresses particles agglomeration during core and shell growth. Additionally, the highly non-equilibrium synthesis conditions present in the nonthermal plasma leads to more effective doping of nanoparticles than is often demonstrated by solution-phase approaches . Finally, our plasma processes focus on the minimization of chemical waste while utilizing sustainable, non-toxic materials. Ultimately, this all gas phase approach allows for the generation of heterostructured nanocrystal materials currently inaccessible through traditional solution-based processes.  Mangolini, L.; Thimsen, E.; Kortshagen, U. Nano Letters. 2005, 5, 655–659.  Cheng, K. Y.; Anthony, R.; Kortshagen, U. R.; Holmes, R. J. 2011. “High-Efficiency Silicon Nanocrystal Light-Emitting Devices.” Nano Letters 11 (5): 1952–56. doi:10.1021/nl2001692.  Liu, C.Y.; Holman, Z.; Kortshagen, U.R. 2009. “Hybrid Solar Cells from P3HT and Silicon Nanocrystals.” Nano Lett. 9 (1): 449–52. doi:10.1021/nl8034338  Li, J. J.; Wang, Y. A.; Guo, W.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X. Journal of the American Chemical Society. 2003, 125, 12567–12575.  Pi, X. D.; Gresback, R.; Liptak, R. W.; Campbell, S. A.; Kortshagen, U. R. Applied Physics Letters. 2008, 92, 2–5.