Modelling of nanoscale phononic crystals

上个世纪的巨大技术进步在很大程度上是由于对电子和光子的精确控制而实现的。如果能够实现对声子的类似控制，这可能会带来新的技术突破。可能的应用包括改进废弃能源的收集，新型冷却技术，软材料和信号处理设备的新表征方法。******这个研究项目的重点是二维纳米结构作为声子超材料的候选材料的特性。由纳米粒子和纳米线构成的周期性晶格具有独特的振动特性，与孤立的纳米粒子或纳米线及其相应的块状材料不同。材料的周期性结构导致了布拉格散射效应，这种效应会产生被称为带隙的禁止频率范围。频率在这种带隙内的波不能通过材料传播。材料中的带隙可用于引导信号通过结构并构建有源通信设备。除此之外，通过改变纳米线和纳米颗粒的尺寸，可以制造出具有特定的、微调的振动或热性能的二维纳米材料。******周期二维纳米结构尚未得到广泛的研究。因此，该研究计划的第一个目标是获得对振动谱结构和此类组件的导热性以及影响这些特性的因素的基本了解。具体而言，研究了纳米颗粒和纳米线之间界面晶界等缺陷的影响。******用于研究模型纳米结构的振动和热性能的主要计算方法是分子动力学模拟。分子动力学模拟使得在相同的基础上研究材料的振动谱和导热性成为可能。除此之外，他们还解释了纳米粒子和纳米线的原子结构（包括缺陷）的影响。在加拿大的高性能计算机上进行了超过1000万个原子的系统的大规模模拟。这些系统将使用硅的模型电位以及简化的力模式。分子动力学模拟辅以有限元计算，以更好地了解振动模式的结构，并允许模拟完整的器件，如波导或高超音速二极管。

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.