Mixed-phase amorphous-crystalline (a-c) systems are both of technological and theoretical relevance.[1] In particular, this is the case of  nanocrystalline (nc) materials, where a distribution of nanosized crystal grains is embedded into an amorphous  matrix. nc-systems are useful for advanced optoelectronics,  structural engineering, and many other technological applications.

nc-systems are in most cases thermodynamically metastable.  Since the free energy is larger in the amorphous phase than in the corresponding crystalline one, a-c systems tend to recrystallize.[2] It is possible to take advantage of this property in order to synthesize new materials by solid phase crystallization.[3] A comprehensive physical understanding and theoretical modeling of the recrystallization phenomena is mostly needed and useful for technological impact.

We investigate at the atomic scale the microstructure evolution of a two-phase amorphous-crystalline system. We focus on the case of textured nanocrystalline silicon here described as a distribution of cylindrical grains embedded into an amorphous matrix.

We prove that the growth of an isolated grain is nonuniform and it can be described by a power law model.[2] Furthermore, we study the case of a distribution of grains in absence of nucleation. The atomistic results are used to work out a comparison with Kolmogorov-Johnson-Mehl-Avrami (KJMA) mesoscopic model describing the kinetics of a first-order phase transformation. Deviations from the KJMA are observed that are mainly due to atomic-scale features. We include such effects by using an improved version of the KJMA theory. Finally, the optoelectronic properties of the nc-systems are studied during the recrystallization process by means of large scale empirical tight binding calculations. 

[1] A. Mattoni and L. Colombo, submitted for publication (2008)

[2] A. Mattoni and L. Colombo, Phys. Rev. Lett. 99 205501 (2007)

[3] A. Mattoni and L. Colombo,  Phys. Rev. B 69 045204 (2004)

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1. Molecular dynamics results showing continuum theory failure in describing the elastic behavior of nanoparticles embedded in Si-based systems
2. Defects in silicon nanocrystals embedded in amorphous silicon oxide