Numerical modeling of melting of nano-clusters. Microscopic mechanism
Résumé
The fundamental physical properties of nanocrystals, such as their electronic band structure and optical activity, can be drastically different from those of the corresponding bulk materials, mainly due to their large surface-to-volume ratio. Among variety of properties, variation (lowering) of melting temperature of nanoparticles is a very important issue as it decreases the functional range of the solid phase. In this talk we discuss mechanism of melting of nanoclusters from a perspective of numerical modeling and relate the computational results to experimental observations. Conventionally, when studying melting of nanoparticles, the particles and the host medium are both in direct contact with a temperature source. In such conditions, melting of the nanoparticles starts at their surfaces, at reduced temperatures [1,2]. The typical dependence of the melting temperature on the size of nanoparticles is presented in the Figure 1. It shows that generally for particles built from smaller number of atoms melting is observed at lower temperature. However, the microscopic mechanism of melting may be affected by many factors. In particular, the irradiation with laser light can selectively and homogeneously excite the metallic nanoparticles without direct heating of the particle environment. The time of the laser pulse can be very short (fs) and conse-quently the melting may happen in non-equilibrium conditions. Also, the initial structural changes may lead to intermediate non-homogenous structures. Therefore, the microscopic mechanism of melting may be complex, and depending not only on the size of particles but also on theirs initial structures, rate of heating and the local temperature (kinetic energy of particles) distribution. We will show how the above factors affect the mechanism of melting of nanoparticles. The numerical simulation results will be confronted with the experimental observations which show that, although the as-prepared nanorods are defect-free, point and planar defects are present in gold nanorods after laser irradiation. The defects are mainly (multiple) twins and stacking faults (planar defects). They are the precursors that drive the convertion of nanorods (110) facets into the more stable (100) and (111) facets and hence minimize their surface energy. These observations suggest that short-laser pulsed photothermal melting begins with the creation of defects inside the nanorods followed by surface reconstruction and diffusion [3].