The use of templates in the synthesis of nanostructured materials allows precise control of shape and size, scalability, and stabilization for long-term storage. In recent experimental work [1-3], the synthesis of high-quality ZnSe quantum dots, nanorods, and nanoplates in novel microemulsion and liquid crystal templates has been demonstrated.

To understand the underlying physical and chemical mechanisms of the growth process, a multi-scale modeling approach has been developed that couples mesoscopic lattice Monte Carlo simulations of particle synthesis in the dispersed spherical nanodomains of a microemulsion with macroscopic finite element simulations of transport phenomena in the two-phase system.

The growth of ZnSe quantum dots in microemulsion templates is used as an example. In a typical case, the Zn precursor is dissolved in the spherical droplets of the microemulsion and is converted to ZnSe by an irreversible reaction with a Se precursor that diffuses through the surfactant layer. The stochastic model describes diffusion of the Zn precursor inside a droplet, formation of ZnSe nuclei at the interface by a reaction with a Se precursor that diffuses through the surfactant layer, diffusion and coalescence of ZnSe nuclei into clusters inside the droplet and formation of a single nanocrystal per nanodroplet by coalescence of the clusters. The effects of varying the Se precursor flux through the surfactant layer on the evolution of the particle population inside the droplet and the morphology of the final particle are being studied using computer simulations.

The particle-particle collision data from the lattice Monte Carlo simulations are incorporated into a dynamic thermal analysis model of cluster-cluster coalescence 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). Binary cluster-cluster coalescence simulations have revealed the possibility of melting and recrystallization of nanoparticles, thus explaining the formation of single crystals in a medium that is at room temperature. In this work, the effects of multiple collisions are studied to determine the final morphology of the particles for different initial concentrations of the Zn reactant in the droplet. The model predictions are compared to experimental data.

A microfluidic system for ZnSe quantum dot synthesis in microemulsions is being designed using a multi-scale modeling approach. A macroscopic model that describes transport phenomena in the two phases of the microemulsion by treating them as interpenetrating continua is coupled to the lattice Monte Carlo model describing quantum dot formation in a single nanodroplet being transported along a streamline in the microfluidic system. The models are coupled through the interfacial flux of the Se precursor into the droplet. The effects of operating conditions, such as inlet flow rate, initial concentration of Zn precursor in the droplets and rate of supply of Se precursor to the microemulsion, on the growth rate and morphology of nanocrystals are being studied. The objective is to identify optimal designs and operating conditions that yield single crystalline particles.

References

1. G. N. Karanikolos, P. Alexandridis, G. Itskos, A. Petrou, and T. J. Mountziaris, Langmuir 20, 550 (2004).

2. G.N. Karanikolos, P. Alexandridis, R. Mallory, A. Petrou, and T. J. Mountziaris, Nanotechnology, 16, 2372-2380 (2005)

3. G.N. Karanikolos, N.-L. Law, R. Mallory, A. Petrou, P. Alexandridis, and T. J. Mountziaris, Nanotechnology, 17, 3121-3128 (2006).

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