Directed Assembly of High Aspect Ratio Nanoparticles for Hierarchical Materials

纳米棒和纳米管等高深宽比纳米颗粒(HARN)跨越微米级和纳米级，对层次化材料尤为重要。与定位相关的额外自由度创造了吸引人的机会，但也带来了额外的挑战。哈恩斯的隐含各向异性既反映在它们的固有性质上，也反映在含有它们的丰富的结构多样性上。例如，虽然横向量子限制可以沿着单个纳米线的长轴产生独特的传输特性，但也可能引入内部结构或成分变化，在单个Harn中创建复杂的功能。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，高纵横比允许形成密度、方向、连通性和长度比例可调的开放网络或脚手架。如果组装过程可以控制，就有空间建造复杂的特定体系结构，脚手架中有有序的分支或连接，针对特定的应用进行优化。一种预见机会的方法是想象金属有机骨架(MOF)的丰富性乘以结构和功能的多样性，这可以通过使用Harns作为在一系列长度尺度上的连接支柱来引入。请注意，虽然“哈恩斯”一词可以被理解为包括扁平颗粒，如纳米粘土小片，但这里的重点是长形棒、管和纤维。在概念层面上，分层材料的组装代表着材料科学的下一个挑战；掌握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密切相关；现有的例子包括电容器电极的定向碳纳米管阵列，具有超低电渗流阈值的航空航天环氧纳米复合材料，以及二氧化钛纳米棒光伏中的高效电子收集。这些目前的网络几乎都是随机设计的，或者充其量是单轴定向的。对不同长度尺度的HARN建筑进行合理的设计，将在结构和功能性能方面产生根本性的改善。

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.