Directed Assembly of High Aspect Ratio Nanoparticles for Hierarchical Materials

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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.