Single atoms and small atomic clusters offer a range of novel, tunable properties for a number of applications such as selective catalysis [1]. Achieving precise control of the desired properties of these systems first requires an understanding of the interaction between the cluster and its support. Advances in aberration-corrected scanning transmission electron microscopy (STEM) mean that atomic resolution imaging and characterisation is now achievable for many materials. Observing individual atoms and small clusters remains difficult, however, due to low signal-to-noise ratio and beam-induced motions causing blurring during image acquisition. One route around these problems is to acquire rapid image sequences in an effort to reduce the electron dose and also to capture any dynamic behaviour of the atoms. Making use of the spatial and temporal correlations between frames, and using a novel processing method based on singular value thresholding [2], we have developed robust approaches to recover individual atomic positions, including STEM acquisition rates of 10 frames per second or better [3].

We have applied the approach to the study of catalytically-important copper atoms on few-layer graphene oxide (GO), where the presence of functional groups on GO may aid the control of deposited clusters by acting as preferential pinning sites. Processing and analysis of an annular dark-field STEM image sequence reveals a range of behaviours, with some strongly-pinned atoms and other more mobile atoms undertaking random walks on the surface (Figure 1b). Further investigation of the jump distances (Figure 1c) and mean-squared displacements (Figure 1d) reveals that the diffusion of adatoms on GO is anomalous. We combine this information with ab-initio DFT calculations to provide new insight into the formation and behaviour of small atom clusters under an electron beam, and the interactions between few-atom catalysts and high surface area supports.

References

[1] Tyo, EC, Vajda, S. (2015). Nat. Nanotechnol. 10, 577-588.

[2] Candes EJ, Sing-Long CA, Trzasko JD. (2013). IEEE Trans. Signal Process. 61, 4643-4657.

[3] Furnival T, Leary R, Midgley PA. (2016). Manuscript submitted.

The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement 291522-3DIMAGE.

Figures:

Figure 1. (a) Processed frame from an annular dark-field STEM image sequence of copper atoms on graphene oxide. (b) Extracted atom trajectories from the sequence. (c) Distribution of the x components of the steps in the trajectories, showing non-Gaussian behaviour. (d) Mean-squared displacement of the atoms, showing sub-linear scaling with time.

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EMC Abstracts - https://emc-proceedings.com/abstract/time-resolved-imaging-and-analysis-of-single-atom-diffusion-on-graphene-oxide/