Toward
a Quantitative Understanding of Symmetry Reduction
Involved in the Seed-Mediated Growth of Pd Nanocrystals
- Publication date
- Publisher
Abstract
We
report a quantitative analysis of the symmetry reduction phenomenon
involved in the seed-mediated growth of Pd nanocrystals under dropwise
addition of a precursor solution. In addition to the elimination of
self-nucleation, the dropwise approach allows for the formation of
a steady state for the number of precursor ions in the growth solution,
which only fluctuates in a narrow range defined by experimental parameters
such as the initial concentration of precursor solution and the injection
rate. We can deterministically control the growth mode (symmetric
vs asymmetric) of a seed by tuning these parameters to quantitatively
manipulate the reaction kinetics and thus the lower and upper limits
that define the steady state. We demonstrate that there exists a correlation
between the growth mode and the lower limit of precursor ions in the
steady state of a seed-mediated growth process. For the first few
drops of precursor solution, the resultant atoms will only be deposited
on a limited number of available sites on the seed if the lower limit
of the steady state is below a critical value. Afterward, the deposition
of atoms will be largely confined to these initially activated sites
to induce symmetry reduction if atom deposition is kept at a faster
rate than surface diffusion by controlling the lower limit of precursor
ions in the steady state. Otherwise, the migration of atoms to other
regions through surface diffusion can access other sites on the surface
of a seed and thus lead to the switch of growth mode from asymmetric
to symmetric. Our study suggests that symmetry reduction can only
be initiated and retained by keeping the atom deposition at a rate
slow enough to limit the number of initial nucleation sites on a seed
but fast enough to beat the surface diffusion process