Introduction

Typically, a nanostructure is a material structure assembled from a layer or cluster of atoms with size on the order of nanometers [1-5]. A number of methods exist for the synthesis of nanostructured materials, including synthesis from atomic or molecular precursors (chemical or physical vapor deposition, gas condensation, chemical precipitation, aerosol reactions, biological templating, etc.), processing of bulk precursors (mechanical attrition, crystallization from the amorphous state, phase separation, etc.), and processes in nature (e.g., biological systems) [6-12]. Although it is general, this definition underlines the fact that a positional control over constituting atomic or molecular units is inherently associated with the synthesis of nanostructured materials. When evaluated dimensionally, the domain of nanoscale structures (<100 nm) lies between that of ordinary, macroscopic or mesoscale products and microdevices on the one hand, and single atoms or molecules on the other. For this reason, nanostructured materials comprising a countable number of polyatomic or molecular units represent a scale of matter where entirely new properties, not known in the bulk counterparts, are manifested and dominate their behaviors. The most well-known example is the change in color of CdS nanocrystals when their size is reduced to a few nanometers [13]. Such mechanisms include quantum effects, statistical time variations of properties and their scaling with structure size, dominant surface interactions, and the absence of defects in the volume of nanocrystals. These effects endow nanoscale particles and structures thereof with unique mechanical, electronic, magnetic, optical, chemical, etc. properties [6, 8, 14]. Bulk materials constructed from nanoparticles also exhibit new properties such as enhanced plasticity and surface reactivity, and more uniform structures [15].

The unique properties of assembled nanostructures largely attributed to their small size are responsible for the great strides in the development of new synthetic methodologies that would allow the prescribed synthesis of any desired nanostructured material. Methods for controlling the shape and size of inorganic nanocrystals are evolving rapidly with the increasing understanding of basic size-dependent scaling laws, which are important for a wide range of applications. Furthermore, nanoparticulate systems display some special properties like dispersability of an immiscible phase, light weight, extremely high surface area, control over the scattering of light and electronic states (useful in optoelectronic features), and the relatively higher energetic state of atoms and molecules present at the surface of a nanoparticle when compared with those in the bulk. The higher energy associated with the surface also drives

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Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 4: Pages (131-191)

the growth of phases and poses the greatest challenge to the controlled synthesis of nanosized particles [7, 8].

The various possibilities for fabricating nanomaterials through the control of the sizes of their constituent clusters or particles can be broadly divided in the so-called top-down (transformative) and bottom-up (synthetic) approaches. The top-down processes are based on the transformation of bulk materials into nanoscaled components, largely by mechanical methods, whereas the bottom-up approach relies on the synthesis of large architectures from smaller, well-defined nanosized units. Conventional materials with a grain size anywhere from hundreds of microns (^m) to millimeters (mm) are also made of polyatomic or molecular assemblies of matter, where the distribution (ordered or statistical) and multiplicity of such building units are responsible for the familiar averaged properties of bulk materials. Because these macroscale properties effectively extend down to the microscale, traditional manufacturing techniques are experiencing a miniaturization trend for the fabrication of microstructures (e.g., in microelectronics) in a top-down approach. On the other hand, well-known monoatomic or molecular units offer the ultimate building blocks for an atom-by-atom or molecule-by-molecule synthesis of nano-structures in a bottom-up fashion. However, the possibility of building extended solid-state structures from molecular building blocks demands a comprehensive and thorough analysis of synthesis and the composition-structure-property relationship. This review is intended to provide an overview of the efforts made in the chemical synthesis of nanomateri-als with the use of molecular metal-organic precursors, with special emphasis given to single-source precursors (SSPs) [16-31]. The term "single source" in a strict sense applies to compounds containing all of the phase-forming elements in a single molecular source (e.g., (Me2AlNH2)3 for AlN) that can be used to perform a one-step synthesis of the desired (nano)material. In addition to the inherent advantages (low-temperature synthesis, phase purity, better composition, and morphology control, etc.) of chemical methods, this concept allows the "preformation" of a solid-state material on the molecular scale.

The research work reviewed here is focused on molecular routes to inorganic nanomaterials and will not refer to the progress of molecular nanotechnology, which is largely related to the understanding of molecular phenomena involved in the self-assembly or self-organization of molecular building blocks to form supramolecular structures that interact to form a higher ordered material (aggregate or crystalline) [32-34]. The emphasis in this contribution will be on the state of the art of molecular concepts developed to obtain inorganic nanoparticles or nanostructured films by chemical routes.

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