AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Gas-Phase Production of Aluminum-Doped Zinc Oxide Nanocrystalline Thin Films
BENJAMIN GREENBERG, Shreyashi Ganguly, Eray Aydil, Uwe R. Kortshagen, University of Minnesota
Abstract Number: 139 Working Group: Nanoparticles and Materials Synthesis
Abstract Doped semiconductor nanocrystals (NCs) can serve as low-cost building blocks for a myriad of optoelectronic devices, including solar cells, light-emitting diodes, and electrochromic windows. Although many of these NCs can be synthesized in the liquid phase, a number of challenges arise: as-synthesized NC surfaces are covered with electrically insulating ligands, dopants are often nonuniformly distributed within an NC, doping efficiencies are often low, and NC size and dopant concentration are difficult to control simultaneously. All of these issues can be attributed to inherent limitations of colloidal synthesis: the ligands are necessary for preventing agglomeration, and difficulties in doping result from solvation forces which render impurity incorporation energetically unfavorable.
We demonstrate that gas-phase synthesis is a promising route to circumventing and overcoming these challenges by exploring nonthermal plasma synthesis of aluminum-doped zinc oxide (AZO) NCs, a prototypical and industrially relevant material. Using diethylzinc (DEZ) and trimethylaluminum (TMA) as precursors, we have developed a scalable high-throughput process for synthesizing transparent conductive AZO NCs with diameters between 4 and 12 nm. By varying the TMA-to-DEZ ratio in the feed gas, we control the doping fraction and thus the free electron concentration in the 10$^(19)-to-10$^(20)-cm$^(-3) range, as indicated by the localized surface plasmon resonance (LSPR) absorption features in the Fourier transform infrared (FTIR) spectra. We calculate doping efficiencies by comparing electron concentrations determined from the LSPRs to Al concentrations measured by inductively coupled plasma mass spectroscopy; we find that, over a wide range of dopant concentrations, our doping efficiencies are significantly higher than those typically achieved in colloidal synthesis. Importantly, doping efficiency and dopant concentration do not decrease as NC size is reduced from 12 to 4 nm. Finally, combining plasma synthesis with supersonic impaction deposition, we produce nanocrystalline thin films and achieve doping-tunable resistivities on the order of 10$^(-3) ohm-cm.
This work was supported primarily by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013. Part of this work was carried out in the College of Science and Engineering Characterization Facility, University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC program.