Directed Assembly of High Aspect Ratio Nanoparticles for Hierarchical Materials

高纵横比纳米颗粒（HARN），如纳米棒和纳米管，跨越微米和纳米域，并为分层材料特别关键。与方向相关的额外自由度创造了诱人的机会，但也带来了额外的挑战。HARN的隐式各向异性反映在它们的固有特性和包含它们的组件的丰富结构多样性中。例如，虽然横向量子限制可以沿着单个纳米线的长轴沿着产生独特的传输特性，但是也可以引入内部结构或组成变化，从而在单个HARN内产生复杂的功能。另一方面，为了使用其固有的功能，HARN必须集成到更广泛的结构中。从这个角度来看，高纵横比允许形成具有可调节密度、取向、连接性和长度尺度的开放网络或支架。如果组装过程可以控制，那么复杂的特定架构就有了空间，在脚手架中有有序的分支或结点，针对给定的应用进行了优化。设想这一机会的一种方法是想象金属有机框架（MOF）的丰富性乘以结构和功能的多样性，这些多样性可以通过使用HARN作为一系列长度尺度的连接支柱来引入。请注意，尽管术语“HARNS”可以被理解为包括扁球形颗粒，如纳米粘土片晶，但这里的重点是扁长杆、管和纤维。在概念层面上，分层材料的组装代表了材料科学的下一个挑战;掌握在整个长度尺度上完全控制物质的方法，将开辟科学和应用的新前景。另一方面，最简单的HARN网络已经与广泛的应用非常相关;现有的例子包括用于电容器电极的对齐CNT阵列，具有超低电渗透阈值的航空航天环氧树脂纳米复合材料，以及二氧化钛纳米棒光致发光中的有效电子收集。目前的这些网络几乎都是随机设计的，或者充其量是单轴定向的。不同长度尺度的HARN架构的合理设计将在结构和功能性能方面产生根本性的改进。

High aspect ratio nanoparticles (HARNs), such as nanorods and nanotubes, span both the micron and the nano domains, and are especially critical for hierarchical materials. The extra degrees of freedom associated with orientation create appealing opportunities but additional challenges. The implicit anisotropy of HARNs is reflected both in their intrinsic properties and in the rich structural variety of assemblies containing them. For example, while transverse quantum confinement can generate unique transport properties along the long axis of an individual nanowire, it is also possible to introduce internal structural or compositional variation, creating complex functionality within a single HARN . On the other hand, in order to use its inherent functionalities, the HARN must be integrated into a wider structure. From this perspective, high aspect ratio allows the formation of open networks or scaffolding with adjustable density, orientation, connectivity, and length scale. If the assembly process can be controlled, there is scope for complicated specific architectures, with ordered branches or junctions in the scaffolding, optimised for given applications. One way to envision the opportunity is to imagine the richness of metal organic frameworks (MOFs) multiplied by the structural and functional diversity that could be introduced by using HARNs as linking struts at a range of lengthscales. Note that although the term 'HARNS' could be taken to include oblate particles, such as nanoclay platelets, the focus here is on prolate rods, tubes, and fibres.At a conceptual level, the assembly of hierarchical materials represents the next challenge in materials science; mastering methods to control matter fully, across the lengthscales, will open up new vistas of science and application. On the other hand, the simplest networks of HARNs are already extremely relevant to a wide range of applications; existing examples include aligned CNT arrays for capacitor electrodes, aerospace epoxy nanocomposites with ultra-low electrical percolation thresholds, and efficient electron collection in titania nanorod photovoltaics. These current networks are almost all randomly designed, or at best, uniaxially oriented. The rational design of HARN architectures across different lengthscales will yield radical improvements in structural and functional performance.