Influence of Precursor Design on the Growth of Nanomaterials
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Influence of Precursor Design on the Growth of Nanomaterials Sanjay Mathur*, Hao Shen, Eva Hemmer, Thomas Ruegamer and Christian Holzapfela CVD Division, Leibniz Institute of New Materials a Institute of Functional Materials, Saarland University D-66041 Saarbruecken, Germany ABSTRACT Chemical processing of inorganic materials demands an understanding of the precursor chemistry at the molecular level. Although chemical compounds imitating atomic composition or bonding features of solid phases are efficient templates for a controlled evolution of nanomatter, the intrinsic advantages of this approach, such as atomic level mixing of the constituents and phase-selective synthesis, rely on the chemistry involved in the transformation of molecules to materials. Therefore, a better understanding of principles underlying chemical processing is necessary to enable a rational synthesis of materials. We have deposited MgAl2O4 thin films by the chemical vapor deposition of two Mg-Al alkoxide precursors, [MgAl2(OPri)8] and [MgAl2(OBut)8], which reveal that precursor attributes such as vapor pressure and ligand elimination mechanisms influence the microstructure and material properties of the spinel films. Under similar growth conditions, [MgAl2(OPri)8] produces rough and poorly crystalline spinel films, whereas crystalline deposits and dense microstructure were obtained in the case of [MgAl2(OBut)8]. The films were characterized by XRD, SEM, TEM, XPS and nano-indentation studies. INTRODUCTION Chemical processing using molecular precursors is one of the favored routes for the controlled synthesis of nanomaterials [1]. The near molecular size of nanoscopic structures opens up the possibility of creating extended frameworks from molecular building blocks [2]. The soft chemistry methods are indeed desirable and enable better control over size-dispersion, structure, chemical composition and phase purity. In addition, mixed-metal precursors can be used for a selective synthesis of metastable compounds by severely limiting diffusion through low crystallization temperature (kinetically controlled). The segregation of elements, in the synthesis of multi-component phase, is thermodynamically favored especially if the system could lower the free energy by redistributing its constituents into two or more phases. This phenomenon is crucial for 'dry' processing such as gas phase reactions where the precursor molecules are present in an energetic state [3]. We have recently reported on the synthesis of YMO3 phases (M = Al, Fe) by employing single molecular precursors ([YM(OPri)6(PriOH)]2) with pre-existent Y-O(R)M units that efficiently prevent the separation of Y and M during processing [4]. We discuss here the chemical vapor deposition (CVD) of MgAl2O4 spinel films from heterometal alkoxide precursors [MgAl2(OPri)8] and [MgAl2(OBut)8], which possess similar Mg:Al ratio but differ in the constitution of ligands. This work demonstrates the influence of initial molecular composition on the final material properties of the targeted soli
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