(422e) Simulation of Quantum Dot Synthesis in Microemulsion Templates

Semiconductor nanocrystals (quantum dots) have attracted significant attention because of their size-tunable optical and electronic properties that make them attractive for applications in biosensors, high-efficiency solar cells, and high-density optoelectronics. The use of templates in the synthesis of semiconductor nanocrystals allows precise control of their shape and size [1-3]. It also enables easy scale-up for commercial production.

We have developed a Lattice Kinetic Monte Carlo (LKMC) technique to simulate the synthesis of quantum qots in the dispersed phase of microemulsion templates formed by self assembly of a ternary system that includes a polar solvent, a non-polar solvent, and an amphiphilic block copolymer [1-3]. The components of the microemulsion have been selected to eliminate droplet-droplet coalescence that leads to the formation of particle aggregates. Nucleation of a binary semiconductor, such as ZnSe or CdSe, occurs by an irreversible reaction between a precursor dissolved in the solvent forming the dispersed phase and a second precursor that diffuses into the droplets from the continuous phase. Experimental observations indicate that a single nanocrystal is obtained in each droplet of the microemulsion, whose size can be controlled by the concentration of the precursor dissolved in the dispersed phase.

The LKMC simulations track the diffusion of precursor molecules inside a spherical droplet, reaction with a second precursor at the interface to form nuclei, cluster formation by coalescence of nuclei, and formation of a single particle by cluster-cluster coalescence. In the LKMC simulations the precursor molecules, nuclei, and clusters are modeled as hard spheres, whose diffusivity depends on their size. The LKMC technique has been validated by comparing its predictions for diffusion out of a sphere with a fast reaction at the interface with those of a PDE-based model. An optimal lattice size was selected that makes the LKMC predictions to be lattice-independent.

Parametric studies have been performed to elucidate the effects of cluster coalescence efficiency and nucleation rate on the evolution of cluster sizes and the time required for formation of a single nanocrystal inside the droplet. By introducing appropriate scaling quantities, a generalized dimensionless diagram was constructed that allows a priori estimation of particle formation times for a variety of growth scenaria.

The LKMC simulations elucidate the underlying cluster-cluster coalescence mechanism and the evolution of intermediate cluster size distributions. The simulations predict that the time required for formation of a single nanocrystal initially increases with final particle size, but quickly passes through a maximum and subsequently decreases. In the region of interest for forming II-VI quantum dots with final particle diameters between 1nm and 7nm, the overall formation time decreases with particle size. This is due to the formation of larger ?sweeper? clusters at higher precursor concentrations that are efficient collision partners for smaller clusters, despite their slow diffusivity, and lead to faster formation of a single final particle.

References

1. G. N. Karanikolos, et al., Langmuir 20, 550 (2004).

2. G.N. Karanikolos, et al., Nanotechnology, 16, 2372-2380 (2005).

3. G.N. Karanikolos, et al., 17, 3121-3128 (2006).