(735c) Radial Elemental Distribution Analysis of Spherical Core/Shell Nanocrystals with STEM/EDX Conference: AIChE Annual MeetingYear: 2017Proceeding: 2017 AIChE Annual MeetingGroup: Materials Engineering and Sciences DivisionSession: Semiconducting Quantum Dots I: Surface Chemistry and Assemblies Time: Thursday, November 2, 2017 - 1:16pm-1:32pm Authors: Held, J., University of Minnesota Hunter, K. I., University of Minnesota Mkhoyan, K. A., University of Minnesota Kortshagen, U. R., University of Minnesota Radial Elemental Distribution Analysis of Spherical Core/Shell Nanocrystals with STEM/EDX Jacob T. Held1, Katharine I. Hunter2, Uwe R. Kortshagen2, K. Andre Mkhoyan1 1.Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, MN 55455. 2.Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455. Semiconductor quantum dots exhibit many useful size-dependent optoelectronic properties.1 Shells can be used to protect such nanocrystals from oxidation and surface trap states, and by managing strain and band alignment between the core and shell, they can be used to control optoelectronic and catalytic properties.2,3 However, many of these properties are sensitive to the interface between the core and shell.4 Despite its importance, characterizing the radial composition of core/shell nanocrystals remains a significant challenge.5 In this study, we demonstrate a method for quantifying the radial distributions of elements in spherical core/shell nanocrystals by fitting simulated distributions to radially-averaged scanning transmission electron microscopy/energy dispersive X-ray (STEM/EDX) spectrum images of the nanocrystals. Plasma-grown spherical Ge/Si core/shell nanocrystals with well-controlled core and shell dimensions6 were used as an ideal test case for demonstration of this analysis. These crystals were directly deposited onto holey/thin double carbon grids and transferred under Ar into an FEI-Titan G2 60-300 equipped with a Super-X EDX system. The microscope was operated at at 60 kV and 125 pA beam current with a 25 mrad convergence angle. STEM/EDX spectrum images were acquired with frame-by-frame drift correction with a dwell time of 3 μs/pixel and a pixel resolution of 0.03 nm/pixel. The resulting spectrum images (Figure 1) were radially-averaged around the centroid of a fit ellipse (aspect ratio <1.05) to produce 1D datasets for further analysis. An error function-based model of the radial composition of spherical core/shell nanocrystals was developed, accounting for interface broadening, surface roughness, and residual core material in the shell. The modelled spherical radial distributions were numerically projected down the cylindrical Z axis and convoluted with a Gaussian probe function to account for STEM electron beam spread, producing a fit to the experimental EDX data. The simulation parameters (core and shell radii, interface broadening, surface roughness, and shell composition) were then optimized by minimizing the sum-squared error of the fit, defining the uncertainty as the maximum change in each parameter necessary to increase the fitting error by 5%. Figure 2 shows the results of this analysis for the crystal in Figure 1. Because only a single parameter is used to define the alloying and roughness at each interface, it is impossible to decouple these features through the analysis demonstrated here. However, the interface broadening values, expressed as standard deviations, are upper bounds on the total broadening of each interface (including non-sphericity, roughness, and alloying). For example, the analysis in Figure 2 quantitatively shows that the alloyed region between the Ge core and Si shell shown here is at most only 1-2 unit cells (σGe/Si ≈ 0.5 nm). This technique can be readily applied to other spherical core/shell systems with a wide variety of chemistries, and could be expanded for use in other well-defined geometries. 1. Mangolini, L. Synthesis, properties, and applications of silicon nanocrystals. J. Vac. Sci. Technol. B Microelectron. Nanom. Struct. 31, 20801 (2013). 2. Reiss, P., Protière, M. & Li, L. Core/shell semiconductor nanocrystals. Small 5, 154168 (2009). 3. Strasser, P. et al. Lattice-strain control of the activity in dealloyed coreshell fuel cell catalysts. Nat. Chem. 2, 454460 (2010). 4. Bae, W. K. et al. Controlled alloying of the core-shell interface in CdSe/CdS quantum dots for suppression of auger recombination. ACS Nano 7, 34113419 (2013). 5. Tschirner, N. et al. Interfacial alloying in CdSe/CdS heteronanocrystals: A Raman spectroscopy analysis. Chem. Mater. 24, 311318 (2012). 6. Hunter, K. I., Held, J. T., Mkhoyan, K. A. & Kortshagen, U. R. Nonthermal Plasma Synthesis of Core/Shell Quantum Dots: Strained Ge/Si Nanocrystals. ACS Appl. Mater. Interfaces 9, 82638270 (2017). Figure 1. Spectrum image data of a single Ge/Si core/shell nanocrystal. (a) High-angle annular dark field (HAADF) image of the crystal obtained after spectrum image acquisition. (b) Composite Ge (green) and Si (red) EDX map. (c,d) EDX maps of the Ge core and Si shell. Figure 2. Analysis of the radial distribution of elements in a Ge/Si core/shell nanocrystal. (a) Radially-averaged EDX data from the crystal shown in Figure 1 and the corresponding fit profiles. (b) Spherical radial concentrations of Ge and Si from the best-fit model. (c) Optimized values for each fitting parameter. Topics: Nanomaterials Particle Characterization