(518d) Multi-Scale Modeling of Quantum Dot Synthesis in Microemulsions and Liquid Crystals

A lattice-Monte Carlo simulation technique has been developed that describes the formation of ZnSe nanocrystals (quantum dots) inside the spherical nanodomains formed by self-assembly of a ternary system containing an amphiphilic block copolymer, a polar continuous phase (formamide) and a non-polar dispersed phase (heptane) [1,2,3]. The stochastic model describes diffusion of diethylzinc molecules inside the droplets of the dispersed phase, nucleation of ZnSe at the interface by a spontaneous and irreversible reaction between dielthylzinc and hydrogen selenide diffusing through the surfactant layer, and diffusion and coalescence of ZnSe clusters inside the droplets of the dispersed phase leading to the formation of a single nanocrystal per nanodroplet. The stochastic simulation was calibrated by using a deterministic diffusion-reaction model describing diethylzinc depletion from a nanodroplet due to a fast interfacial reaction. The motion of molecules and clusters was programmed according to their diffusivity, which was estimated by using the Stokes-Einstein equation. The formation of stable "magic" clusters with a close-caged structure was monitored during each simulation run and the predicted size variation of the final nanoparticle population was recorded.

A thermal analysis of cluster-cluster coalescence was performed using a macroscopic model describing: (1) Energy released due to surface area reduction. (2) Energy accumulation in the coalescing particles that can lead to melting. (3) Energy dissipation to the surrounding medium that can lead to evaporation of the heptane and formation of a thin insulating layer of vapor surrounding each particle (in analogy to the Leidenfrost effect in boiling). The simulations reveal the possibility of melting and recrystallization of nanoparticles, thus explaining the formation of single crystals in a medium that is at room temperature.

A microfluidic system for quantum dot synthesis in microemulsions has been designed by coupling a model that describes the two phases of the microemulsion as interpenetrating continua with the mesoscopic model of quantum dot formation in a single nanodroplet. The models were coupled through the interfacial flux of hydrogen selenide. The effects of operating conditions, such as inlet flow rate and supply of hydrogen selenide to the microemulsion, on the growth rate of clusters and nanocrystals have been studied using this multi-scale modeling approach.

References

1. G.N. Karanikolos, et al., Langmuir, 20(3), 550-553 (2004)

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

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