Nucleation dynamics and pattern formation
in nanoclustering
far from the equilibrium

• I. Background

The synthesis of nanoparticles has both fundamental scientific interest and many technological applications because electrical, optical, and magnetic properties depend strongly on their size. The technique is how to achieve monodisperse nanoparticles (methods: seed-mediated growth, digestive ripening process or by surfactant exchange reactions. Park et.al. implemented a new procedures for the synthesis of monodisperse nanocrystals of metals without going through a laborious size-sorting process.) The key issue is separate nucleation and growth.
1. Size: we will adopt Schmid's definitions in which metal nanoclusters are unambiguously defined as those metal aggregates smaller than 100 A (Le., 10 nm) in diameter and metal colloids are defined as those aggregates bigger than 100 A.
2. Historically, colloids and nanoclusters have been prepared in aqueous solutions in the presence of stabilizing agents, resulting in either so-called "charge stabilization" (e.g., by the adsorption of ions such as C1-) or "steric stabilization" (e.g., by the surface adsorption of polymers).
3. "magic number" clusters: Idealized representation of hexagonal close-packed (hcp), full-shell clusters. Surface atoms are (10n2+2). M13=1+(10+2), M55=M13+(10*2^2+2), M147, M309, M561...
4. "Evidence supporting a homogeneous nucleation, then autocatalytic growth mechanism for the formation of the nearly monodispersed Ir nanoclusters"[Lin, Y. and R.G. Finke,94]
5. High-resolution transmission electron microscopy (TEM) to monitore nucleation and growth in situ (原位监测！) for quantum dot materials.
6. A transmission electron microscope capable of identifying individual atoms or defects in a crystal lattice has much to offer materials scientists. It has now been used to study the early stages of nanocluster nucleation and growth in semiconductors. Although there are numerous theories and models of nucleation, there is little or no experimental data available on the atomic configurations near the critical nucleus size, where clusters are more likely to grow than to shrink. Most transmission electron microscopy (TEM) studies are confined to the growth of large stable clusters (exceeding about 2 nm in diameter) -- well beyond the nucleation stage (whereas the critical nucleus might consist of only two or three atoms.) We have had no direct information about what is going on during the nucleation period.
7. Because the entire process of cluster nucleation has not been accessible to microscopy in the past,Monte Carlo models have been devised with parameters adjusted to fit the available information.
8. Ganz[PRL68, 1567 (1992)] DIRECT MEASUREMENT OF DIFFUSION BY HOT TUNNELING MICROSCOPY, observing individual atomic interchanges. The scanning process does not affect the diffusion. For temperatures from 24 to 79-degrees-C, the diffusion obeys an Arrhenius law with an activation energy of 0.54 +/- 0.03 eV. The STM was typically operated at 100pA, tuning current +2V bias on the sample, scanned region between 100A to 390A."
9. Close systems v.s. open systems: Decay kinetics in ‘closed’ systems where no exchange of a solvent component (monomers, impurity atoms) between the solution andthe environment is taken into account. the nonequlibrium decay kinetic of such systems can considerably differ from those expected from the thermodynamical point of view. The kinetics becomes more complicated in ‘open’ systems, where the exchange of solvent components with the environment is allowed. Investigation of such systems is not only interesting from the fundamental point of view, but is important as an analytical backgroundof advancedmethod s for the nanostructure manufacturing. Subsequent nucleation kinetics is governed by stochastic processes of absorption andd esorption of monomers at the precipitate interfaces, andd i,usional supply of monomers from the interior of the matrix to the precipitates.

• II. The classic LaMer mechanism

In the classical LaMer mechanism, a short burst of nucleation from a supersaturated solution is followed by the slow growth of particles without any significant additional nucleation, thereby achieving a complete separation of nucleation and growth.

• III. Watzky and Finke mechanism:

  Slow, continuous nucleation (A->B) and fast, autocatalytic surface growth (A+B->2B). To achieve monodisperse nanoclusters requires a separation of nucleation and growth. The consumption A in the autocatalytic step is accounted to play the role of shutting off the nucleation.

References:

1. V. K. LaMer, R. H. Dinegar, J. Am. Chem. Soc. 72, 4847 (1950).
2. M. A. Watzky, R. G. Finke, J. Am. Chem. Soc. 119, 10382 (1997); K. R. Brown, M. J. Natan, Langmuir 14, 726 (1998).