Unscrambling Mixed Elements using High Angle Annular Dark Field Scanning Transmission Electron Microscopy

Karel H. W. van den Bos, Annick De Backer, Gerardo T. Martinez, Naomi Winckelmans, Sara Bals, Peter D. Nellist, and Sandra Van Aert

Phys. Rev. Lett. 116, 246101 – Published 17 June 2016

Abstract

The development of new nanocrystals with outstanding physicochemical properties requires a full three-dimensional (3D) characterization at the atomic scale. For homogeneous nanocrystals, counting the number of atoms in each atomic column from high angle annular dark field scanning transmission electron microscopy images has been shown to be a successful technique to get access to this 3D information. However, technologically important nanostructures often consist of more than one chemical element. In order to extend atom counting to heterogeneous materials, a new atomic lensing model is presented. This model takes dynamical electron diffraction into account and opens up new possibilities for unraveling the 3D composition at the atomic scale. Here, the method is applied to determine the 3D structure of Au@Ag core-shell nanorods, but it is applicable to a wide range of heterogeneous complex nanostructures.

Received 13 March 2016

DOI:https://doi.org/10.1103/PhysRevLett.116.246101

Physics Subject Headings (PhySH)

Nanostructures

Dark-field microscopy Electron diffraction Scanning transmission electron microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Karel H. W. van den Bos¹, Annick De Backer¹, Gerardo T. Martinez¹, Naomi Winckelmans¹, Sara Bals¹, Peter D. Nellist², and Sandra Van Aert^1,*

¹EMAT, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium
²Department of Materials, University of Oxford, 16 Parks Road, Oxford OX1 3PH, United Kingdom

^*Sandra.VanAert@uantwerpen.be

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Vol. 116, Iss. 24 — 17 June 2016

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Images

Figure 1
The atomic lensing model for different column configurations compared to image simulations and a linear model neglecting channeling. (a) An eight-atom-thick column of Co atoms containing 1 Pt atom at different depth locations. (b) A fcc Au-Ag crystal in [110] zone-axis orientation, with Au and Ag atoms alternating. (c) A column composed of Al, Ag, and Au in a random ordering. The multislice approach with emission potential is used with parameters $U_{a} = 300 kV$ , $C_{1} = - 8.3 nm$ , $C_{3} = 0.035 mm$ , $C_{5} = 0 mm$ , and $α = 22 mrad$ , and a detector collection range of 90–180 mrad [28].
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Figure 2
The atomic lensing model visualized. (a) The lensing effect in an atomic column modeled as the superposition of individual atoms focusing electrons on the column. (b) The normalization of the first derivative of the scattering cross section with thickness by the scattering cross section of a single freestanding atom gives the lensing effect of a column of atoms on an absolute scale. (c) Subtracting the lensing effect of different atomic columns gives the lensing effect of the individual atoms.
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Figure 3
Experimental HAADF STEM images of a Ag-coated Au nanorod. (a),(b) The number of (a) Ag and (b) Au atoms. (c),(d) The thickness profiles along the [ $1 \bar{1} 0$ ] direction of the single layer indicated in (a) and (b). (e),(f) The number of (e) Ag and (f) Au atoms from a second viewing direction. (g),(h) The thickness profiles along the [ $0 \bar{1} 0$ ] direction of the single layer indicated in (e) and (f). (c),(g) Circular profiles are fitted to the Ag shell, indicated in blue, to estimate the total thickness. (d) The cylindrical shape of the Au core is determined by fitting a second circular profile to atom-counting results along the [ $1 \bar{1} 0$ ] direction. (h) This fitted profile matches with the atom counting results along the [ $0 \bar{1} 0$ ] direction. (i) The reconstructed 3D model of the nanorod. (j) A cross section of the nanorod. The single layer is indicated in green (i),(j).
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Figure 4
(a) A 3D cross section of the Au@Ag nanorod used as input structure. Au and Ag atoms are represented by, respectively, red and blue spheres. The green area indicates the estimated range of possible depth locations of the gold atoms per atomic column. (b) An image simulation of the input structure viewed from the top, including Poisson noise and Gaussian source size broadening (with a FWHM of 0.7 Å). The incoming electron dose is $5 \times 10^{5} e l e c t r o n s / Å^{2}$ (c) The histogram of the scattering cross sections of the different columns configurations. (d) The number of Ag and Au atoms per column. Image simulations are carried out by using the multislice approach with emission potential with parameters $U_{a} = 300 kV$ , $C_{1} = - 8.3 nm$ , $C_{3} = 0.035 mm$ , $C_{5} = 0 mm$ , and $α = 22 mrad$ , and a detector collection range of 90–170 mrad [28].
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