Modelling of nanoscale phononic crystals

上个世纪的巨大技术进步在很大程度上是通过对电子和光子的精确控制实现的。如果能够实现对声子的类似水平的控制，这可能会使新的技术突破成为可能。可能的应用包括改进废能的收集，新的冷却技术，软材料和信号处理设备的新表征方法。*本研究计划侧重于作为声子超材料候选者的二维纳米结构的性质。由纳米颗粒和纳米线构建的周期晶格具有不同于孤立的纳米颗粒或纳米线及其相应的块体材料的独特的振动特性。这种材料的周期性结构导致了布拉格散射的影响，这可能会产生被称为带隙的禁止频率范围。频率在这样的带隙内的波不能通过材料传播。材料中的带隙可以用来引导信号穿过结构，并用来建造有源通信设备。此外，还可以通过改变纳米线和纳米颗粒的尺寸来制备具有特定的、微调的振动或热学性质的二维纳米材料。因此，这项研究计划的第一个目标是基本了解振动光谱的结构和此类组件的导热系数以及影响这些性能的因素。具体地说，研究了缺陷的影响，例如纳米颗粒和纳米线之间的界面处的晶界。*研究模型纳米结构振动和热学性质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光谱和热导率成为可能。除此之外，它们还考虑了纳米颗粒和纳米线的原子结构(包括缺陷)的影响。在加拿大Compute高性能计算机上对原子数超过1000万的系统进行了大规模模拟。这些系统将使用硅的模型电势以及简化的力模式。分子动力学模拟得到有限元计算的补充，以更好地了解振动模式的结构，并允许模拟完整的设备，如波导或高超声速二极管。

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.