Physical properties of materials at the nanoscale

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This research program aims at understanding how the atomic-scale structure of materials affects their properties on the larger scale, focusing on nanoscale systems, viz. nanoparticles, nanojunctions, etc. For this purpose, we resort to a variety of computational schemes that provide the relevant physics of the systems, in particular molecular dynamics simulations, and at the same time allow us to examine the effect of different parameters individually. Our research program will focus on three different themes: (i) Laser-matter interactions, in particular ablation of solid materials by ultrashort (several fs to hundreds of ps), intense laser pulses, a problem to which we have largely contributed in the past. We will in particular examine the problem of ablation and the subsequent production of nanoparticles in targets immersed in a liquid (typically water), a method routinely used for the production of nanoparticles, which are themselves used e.g. in medical applications. We will investigate the “manipulation” of the laser pulse so as to optimize the structure of the ejected matter and achieve tailor-made properties – a method referred to as pulse-shaping. Finally, we will examine the warm-dense matter and “plasma” regimes, where the physics is dominated by non-thermal processes, and in particular Coulomb effects (repulsion, explosion, charge effects). (ii) Nanothermics: We are interested in determining how heat is transported and dissipated in nanoscale materials, in particular semiconductors and carbon-based materials. For this purpose, we examine the phonon contributions to heat transport, i.e. how the various modes contribute individual heat currents and how they cross-correlate. We have found that these correlations act against heat current decay, which explains why one-dimensional materials are such good heat conductors. We will extend this work to larger scale structures, more realistic models, and higher dimensionalities. (iii) Disordered materials: Our aim here is to understand at the fundamental level the structure of the prototypical amorphous semiconductor a-Si, in particular the nature of defects and their influence on local order and relaxation. We expect that answers will come from a detailed comparison of structural data for a-Si with those for a-Ge which have recently become available from high energy x-ray diffraction experiments. Such a comparison provides “contrast” in the data and will give some understanding of the various contributions to order. We will first develop/optimize a model for a-Ge, which will be compared to our model for a-Si. Using these two models, we will elaborate a method for simulating more closely the difference between as-made and annealed samples, a problem that has not yet been properly addressed.