MS-1-O-1802 3D elemental mapping of the atoms in bimetallic nanocrystals
A three dimensional (3D) characterization of complex heterostructures is required for an optimized understanding of their properties. For example, it is known that the optical properties of bimetallic Au@Ag nanostructures are largely determined by the presence of certain surface facets and interfaces. Furthermore, effects such as alloying or intermixing of the atoms at the interfaces results in a shift of the plasmon resonances, urging the need for a thorough 3D study at the atomic scale. [1] Electron tomography is a technique to obtain 3D reconstructions based on a series of 2D projection images and recently, different approaches enable a 3D investigation at the atomic scale as well. [2-4]
Here, we apply a tomography approach which is based on compressive sensing to reconstruct the atomic lattice of Au@Ag bimetallic nanoparticles having important applications in the field plasmonics. In order to obtain a reliable reconstruction, 5 high resolution HAADF-STEM projection images are acquired along different orientations of the nanorod and used as an input for a tomographic reconstruction. More detailed information about the experimental set-up can be found in [5]. 3D visualizations of the results are illustrated in figure 1, where a visual distinction can be made between the Au core (yellow) and the surrounding Ag shell (blue). Figures 1a-c correspond to visualizations where the sample was tilted by 0º, 45º and 90º around the [010] axis, respectively. The resulting Fourier transforms correspond to the expected symmetry for a fcc crystal structure and the atomic lattice can clearly be recognized from the visualizations themselves.
Since the reconstruction is based on HAADF-STEM projection images, the intensity of the reconstructed atoms scales with their atomic weight. Therefore, Ag and Au atoms can be identified by analysing intensity profiles through the reconstruction. An example of such an intensity profile is presented in figure 2 which enables the labelling of each atom in the cross sections presented in figures 2b and 2c to be either Ag or Au. The result is shown in figures 2d and 2e yielding a correct indexing of the facets composing the interfaces. We conclude that the interface between the Au core and the Ag shell is sharp, without intermixing and mainly composed of {520} facets where bevels are observed at the <100> and the <110> directions. This type of detailed information is crucial to understand and optimize the physical properties of these materials. [6]
[1] M.B. Cortie and A.M. McDonagh, Chemical Reviews 11 (2011) 3713-3735
[2] S. Van Aert et al., Nature 470 (2011) 374-377
[3] M.C. Scott et al., Nature 483 (2012) 444-447
[4] B. Goris et al., Nature Materials 11 (2012) 930-935
[5] B. Goris et al., Nano Letters 13 (2013) 4236-4241
The authors acknowledge support from the European Research Council (ERC Grants # 24691-COUNTATOMS and #335078-COLOURATOMS) and from the Flemish Fund for Scientific Research.
Fig. 1: (a-c) 3D renderings of the reconstruction viewed along different directions where the sample was tilted around the [010] axis for 0º, 45º and 90º. The atoms in the Au core are rendered yellow whereas the surrounding Ag shell is shown in blue. The Fourier transforms of these projected views correspond to a fcc crystal lattice. |
Fig. 2: (a) Three orthogonal slices through the reconstruction show the structure of the nanorod. (b,c) Detailed view of the slices through the reconstruction. An intensity profile is acquired along the direction indicated by the white rectangle in (b). (d,e) Slices corresponding to (b) and (c), in which each Au atom is indicated by a yellow circle. |