Ring Structures from Nanoparticles and Other Nanoscale Building Blocks
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Over the last two decades nanomaterials have attracted growing interest because of their unique, potentially useful electronic, magnetic, and optical properties. For example, carbon nanotubes possess tunable electronic structure from metals to insulators. Nanoparticles can act as “artificial atoms” which, when assembled into meso- or macroscopic structures may lead to novel functional materials. Thus self-assembled particle arrays not only preserve the properties of individual particles but can also exhibit new behaviors due to interparticle correlations and coupling. The ability to organize nanoscale components into specific geometrical arrangements, with defined interconnections, underpins much of their promise for future applications as well as current fundamental studies. Here we focus on the organization of nanoparticles and, to a lesser extent, other nanoscopic building blocks into ringlike arrangements ranging over five decades of length scale, from macroscopic (∼ 1 mm) to nanoscopic (∼ 10 nm). In addition to the simple geometry or a ring, a variety of other arrangements have been realized. For example, following earlier reports of close-packed crystalline nanoparticle superstructures, Korgel et al. analyzed nanoparticle self-assembly into superlattices in detail highlighting the effects of size-selection, ligand coverage, and interparticle attraction. Detailed reviews on the fabrication and properties of nanoparticle superlattices are now available. Other efforts have focused on organizing nanoparticles according to highly specific biological interactions or using self-organized polymeric media such as block copolymers to spatially template nanoparticle synthesis and distribution. Albeit the focus of this review restricts it from broadly considering the general theme of nanomaterial self-assembly, the above and related efforts continue to make critical advances toward functional materials and devices incorporating nanoscale components.
The formation of ringlike assemblies, whether over macroscopic or nanoscale dimensions, is intriguing for reasons other than simple curiosity about their physical or chemical origin. Thus continuous rings, for example, will develop circulating currents when magnetic flux is applied across the ring. This “persistent” current has a periodicity as a function of the enclosed magnetic flux. Moreover, the electronic states of the ring are likewise functions of the applied field, leading to magnetically tunable optical and other properties that, in addition to being fundamentally interesting, may provide unique capabilities in applications. While a variety of routes to microscopic rings have been reported, including lithography, direct chemical synthesis, and diffusive coalescence on free liquid surfaces, self-assembly of ring structures using wetting and capillary phenomena has perhaps been the most common. By its nature, self-assembly often provides a facile fabrication route, and generally similar methods can be effectively used to organize different particle types, making these methods fairly general. In what follows, various mechanisms that have led to the formation of ring structures will be described.