The emerging materials are called metamaterials (MMs). The properties thereof are dominated by the scattering properties of the individual unit cells. Consequently, if the spatial density of unit cells is sufficiently high, the propagating light will not sense every individual unit cell but rather an effective medium. The entire stream of research gained momentum upon appreciating that these unit cells made from a small number of plasmonic NPs are still much smaller than the wavelengths of interest. These first demonstrations of the fabrication of a unit cell by self-assembly initiated an enormous effort to design more complex nanostructures by bottom-up techniques. demonstrated the possibility of arranging two strongly coupled nanospheres (sometimes termed a dimer) with nanometer precision using DNA and molecular linkers, respectively. Therefore, the ultimate problem that has to be solved is the sufficiently dense arrangement of the NPs in a desired geometry. If strongly coupled, the emergence of new resonances will be observed and changing the geometrical parameters of the entire arrangement allows for the tuning of all properties such as resonance position, resonance strength and lifetime. Moreover, to observe a response from the unit cell that does not just correspond to that of an electric dipole of an isolated NP, the coupling of two or more NPs in a unit cell has to be achieved. The localized surface plasmon polariton resonances (LSPRs) of the NPs in the visible and near IR can be exploited to achieve an optical response that differs dramatically from that of a diluted metal. Plasmonic nanoparticles (NPs), such as nanospheres or nanorods, serve as the tiny building blocks that the unit cells are made from. The primary goal of these bottom-up techniques is the fabrication of unit cells tailored at the nanoscale to exhibit a desired optical response in the visible and near infrared (IR) regime. In this field, chemical self-assembly techniques are applied to fabricate nanostructures required for applications in optics and photonics. The fusion of the fields of colloidal nanochemistry and nanooptics has fostered the emergence of an entirely new scientific area that deals with self-assembled plasmonic nanostructures. We document the state-of-the-art but also critically assess the problems that have to be overcome. Emphasis is put on bottom-up amorphous metamaterials. This review shall document recent progress in this field of research. Eventually, novel applications have to be perceived that are adapted to the specifics of the self-assembled nanostructures. Such self-assembled nanostructures require novel analytical means to describe their properties, innovative designs of functional elements that possess a desired near- and far-field response, and entail genuine nanofabrication and characterization techniques. ![]() The precise spatial arrangement across larger dimensions is not possible in most cases leading essentially to amorphous structures. Achievable structures are characterized by a high degree of nominal order only on a short-range scale. ![]() There, self-assembly methods and techniques from the field of colloidal nanochemistry are used to build complex functional unit cells in solution from an ensemble of simple building blocks, i.e., in most cases plasmonic nanoparticles. These limitations can be mitigated by relying on bottom-up nanofabrication technologies. Therefore, bulk plasmonic structures are difficult to fabricate and the periodic arrangement causes undesired effects, e.g., strong spatial dispersion is observed in metamaterials. The structures available are usually planar and periodically arranged. However, it often causes disadvantages as well. This offers great degrees of freedom to tailor the geometry with unprecedented precision. Nowadays for the sake of convenience most plasmonic nanostructures are fabricated by top-down nanofabrication technologies.
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