Modelling of nanoscale phononic crystals

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The tremendous technological progress of the last century has been achieved to a large extent by a precise control of electrons and photons. If a similar level of control over phonons could be achieved, this might enable new technological breakthroughs. Possible applications include improved harvesting of waste energy, novel cooling techniques, new characterization methods for soft materials and signal processing devices. This research program focuses on the properties of two-dimensional nanostructures as candidates for phononic metamaterials. Periodic lattices built from nanoparticles and nanowires have unique vibrational properties that are different from isolated nanoparticles or nanowires and their corresponding bulk materials. The periodic structure of the material leads to the effect of Bragg scattering which can create forbidden frequency ranges called band gaps. Wave whose frequencies are in such a band gap cannot propagate through the material. Band gaps in a material can be used to guide signals through a strucutre and to build active communication devices. In addition to this, two-dimensional nanomaterials with specific, fine-tuned vibrational or thermal properties can be created through variations of the dimensions of the nanowires and nanoparticles.Periodic two-dimensional nanostructures have not been studied extensively yet. The first goal of this research program is therefore to obtain a basic understanding of the structure of the vibrational spectrum and the thermal conductivity of such assemblies and the factors that influence these properties. Specifically, the impact of imperfections, such as the grain boundaries at the interface between nanoparticles and nanowires, are studied. The main computational method utilized for the studying of the vibrational and thermal properties of model nanostructures is molecular dynamics simulation. Molecular dynamics simulations make it possible to study the vibrational spectrum as well as the thermal conductivity of a material on the same footing. In addition to this, they account for the effect of the atomic structure (including defects) of the nanoparticles and nanowires. Large-scale simulations of system with more than 10 million atoms are carried out on Compute Canada high-performance computers. These systems will use model potentials for silicon as well as simplified force modes. The molecular dynamics simulations are complemented by finite-element computations to obtain a better insight into the structure of the vibrational modes and to allow simulations of complete devices such as a waveguide or a hypersonic diode.