Metastable and Nanostructured Materials
Transmission Electron Microscopy - Surface Diffusion
Aberration-corrected high-resolution transmission electron microscopy (HRTEM) is used to investigate atomic diffusion at the surfaces of nanoparticles in particular during their coalescence. As the transport of material during inter-particle coalescences occurs predominantly via surface diffusion, a thorough and quantitative understanding of the latter is mandatory. Since the incident electron beam may effectively promote the sintering, possibilities of measuring the surface diffusion in situ in an electron microscope provides the general opportunity to shed some light on the interaction between the high-energy electron beam and the sample.
The motion of atoms (or atom columns) at the surfaces of Au nanoparticles can be directly followed in the microscope. Both, Au icosahedra on amorphous carbon and Au and Pt octahedra whose surfaces extend into the vacuum and which can thus be imaged quasi substrate-free (see figure) were studied. For a quantitative estimate of the diffusion coefficient the temporal fluctuations of the occupation of individual atom columns were measured from time series of HRTEM images. Although (i) HRTEM only provides a 2D projection of the 3D situation and (ii) the atoms are indistinguishable, a characteristic atomic jump frequency can be measured from which the surface self-diffusion coefficient is derived.
Two HRTEM images (of a series of 22) showing one half of a Au octahedron. The time delay between the two images is 1.2 s. Two columns which experience changes in their occupation are highlighted by red circles.
Below, you find three movies of surface diffusion on Au nanoparticles:
- Coalescence of two Au Icosahedra:
Electron beam-induced coalescence (sintering) of two adjacent Au nanoparticles on a thin amorphous carbon substrate. Once the particles are in contact, the Au atoms are diffusing across the surface towards the sintering neck to grow the neck thereby minimizing the surface free energy.
- Coalescence of two Au Icosahedra (Close-up):
Atomic surface diffusion on an icosahedral Au nanoparticle under the influence of the imaging electron beam.
- Surface diffusion on a Au octahedron (cf. Fig.):
Atomic surface diffusion on an octahedral Au nanoparticle imaged without the substrate (the particle extends into a hole and is thus seen substrate-free). At the lower left facet a unstable surface reconstruction of the (100) surface into a quasi-hexagonal (111) surface layer is observable.
Further reading: A. Surrey et al., Nano Lett. 12, p. 6071 (2012) URL