New modelling capability for nano-confined phase change materials

全球对更小、更节能的设备的需求一直在持续，因为硅微电子产品的生产规模在稳步下降，从2000年代中期的65纳米工艺到最新英特尔处理器的14纳米工艺。为了在本世纪20年代中期以后继续保持这一趋势，将需要尺寸仅为1-2nm的器件，可能使用硅的替代品。在这种情况下，导线的横截面可能不超过2x2或3x3个原子，其中相关的材料物理由表面和约束效应主导，导致相应的大块材料的结构和电子特性截然不同。这种金属线可以通过“巴基管”的碳纳米管（CNTs）中的熔盐结晶形成，从而产生尽可能小的横截面纳米晶体，有时被称为费曼晶体。对这些碳纳米管封装结构的基本材料物理学的研究仍处于起步阶段，英国的实验学家在这方面处于领先地位。其中一位申请者（斯隆）最近的工作特别令人兴奋，他证明了这些金属丝在碳纳米管中的弯曲应变下，有可能在具有显著不同性质的纳米晶体结构之间发生转变。这些“相变”特性为纳米级机电开关和非易失性存储器开辟了道路，同时也为材料中最小长度尺度的相变的基础研究提供了一个平台。受这些结果的启发，我们当前项目的目标是开发一种计算建模能力，以帮助解释实验，了解相变行为的起源，并指导我们的实验同事研究具有潜在优势特性的化合物。与直觉相反的是，由于对称性的降低，模拟纳米线的计算成本可能比体晶体更高。我们将解决当前可用的建模工具在应用于这些系统时的局限性。这将涉及到对现有软件的重大修改和对各种近似的严格研究，以增加模拟的可追溯性。我们将在这些系统中应用最先进的结构预测方法，这是一项非常重要的工作，因为电线可能具有非晶体（例如螺旋）对称性，并通过计算与封装电线的电子和振动特性相关的光谱直接连接到相关实验。最后，我们将研究纳米晶体相变的热力学和动力学，了解何时以及如何快速地影响结构变化，以评估该机制在器件应用中的效用。

The global demand for smaller and more energy efficient devices has been sustained by a steady decrease in the scale on which silicon microelectronics can be manufactured, from 65nm processes in the mid 2000s to 14nm in the very latest Intel processors. To continue this trend beyond the mid 2020s devices with dimensions of just 1-2nm will be required, likely using alternatives to silicon.In this regime, the cross section of a wire might be no more than 2x2 or 3x3 atoms across, where the relevant materials physics is dominated by surface and confinement effects leading to dramatically different structural and electronic properties to the corresponding bulk material. Such wires can be formed by crystallisation of a molten salt within carbon nanotubes (CNTs) of "Buckytubes", leading to the smallest cross section nano crystals possible, sometimes referred to as Feynman crystals. Research into the fundamental materials physics of these CNT-encapsulated structures is still in its infancy, with UK experimentalists leading the way. Particularly exciting recent work by one of the applicants (Sloan) has demonstrated the possibility of these wires undergoing transitions between nano-crystalline structures with markedly different properties, in response to bending strain in the CNT. These "phase change" properties open the way for nanoscale electromechanical switches and non-volatile memory, as well as providing a playground for fundamental studies of phase changes at the smallest length scale possible in a material.Our aim with the current project, inspired by these results, is to develop a computational modelling capability to aid in interpretation of experiments, understand the origin of the phase change behaviour, and guide our experimental colleagues toward compounds with potentially advantageous properties. Counterintuitively, due to a reduction in symmetry, the computational expense of simulating nanowires can be more demanding when compared to bulk crystals. We will address the limitations of currently available modelling tools when applied to these systems. This will involve significant modifications to existing software and a rigorous study of the various approximations one might employ to increase the tractability of simulations.We will apply cutting-edge methods in structure prediction to these systems, a non-trivial exercise due to the possibility wires with non-crystalline (e.g. helical) symmetry, and connect directly to relevant experiments by computing spectra related to the encapsulated wire's electronic and vibrational properties. Finally, we will study the thermodynamics and kinetics of nano-crystalline phase change, developing an understanding of when and how rapidly structural changes are affected to assess the utility of this mechanism for device applications.