Molecule Derived Nanomaterials: Chemical Concepts for Composition, Morphology and Particle Size Control
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Molecule Derived Nanomaterials: Chemical Concepts for Composition, Morphology and Particle Size Control Sanjay Mathur Institute of New Materials D-66041 Saarbruecken, Germany ABSTRACT Conventional material synthesis procedures rely on the intrinsic chemical behaviours (e.g., different hydrolysis rates of the reactants in solution phase reactions or different vapour pressure or thermal stability in the gas phase reactions) of the different components, which make the material properties susceptible to inaccuracies due to an increased number of process variables. As a consequence, phase separation and element segregation are present at the nanometer scale, although the global stoichiometry of the product may correspond to the desired composition. In this context, the use of well-defined molecular precursors is a promising approach to grow extended solid-state structures from atomic constituents. INTRODUCTION Nanomaterials research is seeking methods to achieve a precise control over composition, particle size, size-distribution and morphology [1]. The conventional syntheses (e.g., solid-state reactions, co-precipitation, polymeric precursor methods and combustion reactions) involving solid-state or liquid phase reactions of two or more components fail to meet the challenge of a designed synthesis. The chemical syntheses based on the controlled reactions (interactions) of atoms or molecules are promising alternatives for a rational synthesis of nanostructured materials. However, a straight forward synthesis is not guaranteed by all (chemical) approaches and the processing of complex systems, for instance, those containing two or more different metals or metastable phases, are difficult to achieve. As a result, component segregation, mixed phases or non-ideal stoichiometry are observed in the final ceramic material, the main reason being the intrinsic differences in the chemical properties of different precursor species used or generated in the processing. In this context, metal-organic precursors synthesized to meet the composition of the target material the so-called single molecular precursors are of substantial interest, because they augment the advantages of chemical processing and simultaneously reduce the process parameters [2]. The purpose-built ensemble of the phase-forming elements serves as an efficient one-step strategy for growing nanomaterials from a molecular level [3]. THE PRECURSOR CONCEPT AND ITS CHEMICAL INFLUENCE The molecular precursors containing predetermined metal-ligand linkages (e.g., metalcarbon for carbides, metal-nitrogen for nitrides, metal-oxygen for oxides, etc.) are an attractive choice to achieve homogeneous cation distribution at the atomic level and a defined stoichiometry in the final material. The superior potential of molecular level synthesis
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demonstrated in several cases generally leads to the simplistic assumption that a ‘homogeneous chemical cocktail’ is sufficient for synthesizing pure nanomaterials. On the contrary, a chemically controlled synthesis demands p
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