MS-1-P-2194 Nucleation texture of metal nanoparticles on amorphous substrates

Crystals nucleating homogeneously tend to adopt their
equilibrium shapes to minimise the barrier for
nucleation. For heterogeneous nucleation on a substrate,
the Wulff shape of the crystal itself is translated,
rotated and truncated by the substrate. The orientation
and level of truncation determimes the volume of the
so-called Winterbottom shape. We have calculated the
preferred orientation of heterogeneous nucleation on an
amorphous (isotropic) substrate by assuming that the
wetting of the solid nucleus on the substrate is
constant for the different orientations of nucleation.
Under the given conditions, the preferred orientation of
nucleation is the one for which the exposed volume of
the crystal on the substrate is minimum, as for such an
orientation the nucleation barrier is the minimum.

Theoretical calculations for obtaining minimum energy
Winterbottom shapes of nuclei of FCC metal at their
preferred orientations for a range of wetting conditions
have been done and the results are shown in Figure 1.
Experimentally, we have attempted to estimate the
orientation of nucleation of few hundreds of FCC metal
nuclei in order to statistically conclude the preferred
direction of orientation of heterogeneous nucleation on
an amorphous carbon substrate. Precession Electron
Diffraction (PED) technique is being used to scan over
regions containing a good number of nuclei and obtain an
orientation map from which the nucleation orientation of
the metal nuclei is to be determined.

Very fine nuclei of Au or Pt nanoparticles have been
nucleated on functionalized amorphous Carbon coated
Copper grid by microwave reduction of the precursor
salts in ethylene glycol medium. Electron diffraction
pattern obtained from such fine nuclei do not contain
enough number of spots for a reliable indexing of the
pattern using standard diffraction patterns for the
particular metal. So, the orientation map obtained from
the sample has a very low reliability index. For
optimization of conditions to obtain reliable
orientation mapping, PED scan on homogeneously nucleated
Au particles of around 8 nm diameter [inset of Figure 2
(a)] have been carried out and the resulting orientation
map has been shown in Figure 2(b). Here the 8 nm
particles could be resolved properly, as can be seen in
the virtual bright field image of the scanned area in
Figure 2(a) but the reliability index is poor because of
the polycrystalline nature of the Au nanoparticles.
Results on the nucleation texture of different
nanoparticles will be presented with detailed analysis
of the suitable microscopy conditions required for the
same.