Nanocrystal Arrays: Self-Assembly and Physical Properties
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Over the last few years, research in the area of nanoscience has blossomed into an independent and highly interdisciplinary area. Materials in the nanometer scale (size range 10 Å to 1 µm) are typically referred to as nanoparticles, nanocrystals, nanorods, or nanowires, depending on their crystallinity and shape. Here we refer to them simply all as nanocrystals, without distinguishing the differences. The unique material properties in this size range come from several sources: 1) quantum size effects, where confinement of charge carriers in a small space leads to discrete energy levels; 2) classical charging effects, which originate from the discrete nature of the electrical charge; and 3) surface/interface effects, where the properties of surface or interface atoms become much more significant, as the surface-to-volume ratio increases with decreasing of particle size. Many novel properties of single, isolated nanocrystal have been investigated during the past two decades, such as size-dependent optical absorption and luminescence in semiconductors, coulomb blockade phenomena in charge transfer, or enhanced surface magnetic moments in magnetic nanocrystals.
By comparison, assemblies of nanocrystals have only begun to be studied in a systematic fashion in recent years. The exciting aspect of nanocrystal arrays is that they form a truly new class of materials, where the basic building blocks are nanocrystals instead of atoms. The properties of these materials not only depend on which chemical elements are used to form the building blocks, but also depend on how many atoms are in each building block and how strongly coupled these building blocks are. Traditional materials can be either crystalline or amorphous, depending on the arrangement of the constituent atoms. Similarly, nanocrystal arrays can also be ordered or disordered. In the former case, they are referred to as nanocrystal superlattices (NCSs).
The purpose of this article is to introduce several recent developments in the field, focusing on the experimental point of view. In “Formation of Nanocrystal Arrays,” experimental issues regarding the formation of nanocrystal arrays will be discussed and, in particular, the conditions for controlled formation by self-assembly. In “Electronic Transport” and “Optical Properties,” we consider electronic and optical transport properties of nanocrystal arrays, with the main theme being the collective phenomena in these systems, rather than behavior related to the individual nanocrystal. “Conclusion” contains a brief discussion of outstanding issues and concluding remarks. Because of the limited space, we will focus our article on arrays formed by chemically synthesized nanocrystals only. Therefore lithographically patterned quantum dot arrays will not be discussed. For aspect not covered here and for additional, in depth information on nanocrystal arrays, we refer the reader to Refs.