![]() ![]() The last decade has witnessed the successful synthesis of metal nanocrystals in a variety of shapes. These and many other examples clearly illustrate the importance of shape control to the efficient utilization of metal nanocrystals. Of course, the facets exposed on a nanocrystal have a strong correlation with the shape. For example, Pt can selectively catalyze different types of chemical reactions, with the facets being most active for reactions involving H 2 and CO, respectively. The selectivity, however, is most sensitive to the packing of atoms on the surface or the exposed facets of a nanocrystal. In the case of catalysis, it is well-established that the activity of a metal nanocrystal can be enhanced by reducing its size. For example, in the case of localized surface plasmon resonance (LSPR) and surface-enhanced Raman scattering (SERS), both computational and experimental studies have demonstrated that the shape and structure of a Au or Ag nanocrystal play the most important role in determining the number, position, and intensity of LSPR modes, as well as the spectral region or polarization dependence for effective molecular detection via SERS. In principle, one can tailor and fine-tune the properties of a metal nanocrystal by controlling any one of these parameters, but the flexibility and scope of change are highly sensitive to the specific parameter. The properties of a metal nanocrystal are determined by a set of physical parameters that may include its size, shape, composition, and structure (e.g., solid versus hollow). ![]() Significantly, most of these applications require the use of metals in a finely divided state, preferably in the form of nanocrystals with precisely controlled properties. New applications for metals in areas such as photonics, sensing, imaging, and medicine are also being developed. Metals also possess a range of wonderful properties, and many metals have found extensive use in applications that include catalysis, electronics, photography, and information storage, among others. Most metals crystallize in the same cubic close-packed ( ccp) structure, a face-centered cubic ( fcc) lattice that allows for easy characterization. Among all kinds of inorganic solids, metals deserve our special attention because they represent more than two thirds of the elements in the period table. This notion explains why nanocrystals have been the primary source for discovering and studying quantum size effects, with examples of quantized excitation, Coulomb blockade, metal-insulator transition, and superparamagnetism. In principle, the electron confinement by a nanocrystal provides the most powerful means to manipulate the electronic, optical, and magnetic properties of a solid material. The ability to generate such minuscule crystals is central to advances in many areas of modern science and technology. Interest in nanocrystals has been growing steadily due to their unique position as a bridge between atoms and bulk solids as well as their fascinating properties and potential applications. They also are characterized by a single-domain crystalline lattice, without the complicating presence of grain boundaries. Nanocrystals are crystals with at least one dimension between 1 and 100 nm. We conclude this article with personal perspectives on the directions toward which future research in this field might take. Toward the end of this article, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. ![]() The aim of this article is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Nanocrystals are fundamental to modern science and technology. ![]()
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