(735c) Radial Elemental Distribution Analysis of Spherical Core/Shell Nanocrystals with STEM/EDX

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 (σ_Ge/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, 154–168 (2009).

3.        Strasser,
P. et al. Lattice-strain control of the activity in dealloyed core–shell
fuel cell catalysts. Nat. Chem. 2, 454–460 (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,
3411–3419 (2013).

5.        Tschirner,
N. et al. Interfacial alloying in CdSe/CdS heteronanocrystals: A Raman
spectroscopy analysis. Chem. Mater. 24, 311–318 (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, 8263–8270 (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.