Mmm

To act as a suitable chemical source for material synthesis, the precursor compound should fulfill a set of requirements. The basic criteria for designing and synthesizing a metal-organic precursor are

• Simple and scalable synthesis

• High purity and purifiability by sublimation or distillation

• Easy handling and long shelf life

• Clean decomposition behavior

Figure 6. Molecular structure of different Ba-Zr(Ti) alkoxide clusters: (a) [BaZr2(OBu')10] (Ba:Zr = 1:2), (b) [Ba2Zr(OBu')8(HOBu')2(THF)] (Ba:Zr = 2:1), (c) [BaTi3(OPr')14] (Ba:Ti = 1:3) and (d) [BaZr(OPr')r (OH)(HOPr')3]2 (Ba:Zr = 1:1). Reprinted with permission from [23], M. Veith, J. Chem. Soc., Dalton Trans. 2405 (2002). © 2002, Royal Society of Chemistry.

Figure 6. Molecular structure of different Ba-Zr(Ti) alkoxide clusters: (a) [BaZr2(OBu')10] (Ba:Zr = 1:2), (b) [Ba2Zr(OBu')8(HOBu')2(THF)] (Ba:Zr = 2:1), (c) [BaTi3(OPr')14] (Ba:Ti = 1:3) and (d) [BaZr(OPr')r (OH)(HOPr')3]2 (Ba:Zr = 1:1). Reprinted with permission from [23], M. Veith, J. Chem. Soc., Dalton Trans. 2405 (2002). © 2002, Royal Society of Chemistry.

• Must not form stable residues

• Decomposition at low temperatures

• No(?) chemical hazards (nontoxic and noncorrosive byproducts).

In addition to the above attributes, metal-organics should meet the following requirements to be SSPs:

• Appropriate element ratio compatible with the target material

• Sufficient stability of the molecular framework to survive different experimental conditions (e.g., thermolysis and hydrolysis)

• Low molecular weight and low nuclearity to achieve acceptable vapor pressure

• Predefined reaction/decomposition chemistry

• Minimum heteroatom content in the ligands used to build or stabilize the molecule.

Further properties of a "tailor-made" precursor are subject to the nature and condition of the processing methods. These specific criteria, for example, can be high vapor pressure, a large temperature window between evaporation and decomposition, and thermal lability for the methods involving gas-phase decomposition, whereas high solubility, stability in solution (no dissociations or rearrangements), and high reactivity are of interest for solution-based transformation. It must be pointed out that all of these conditions are seldom met in a single molecular species, and the quest for an "ideal" precursor is responsible for the rapidly increasing chemistry-material science interface.

Although a wealth of information on the synthesis and characterization of inorganic molecular derivatives is available, a continuous input based on new synthetic principles and reaction chemistry is desired to discover a provisional and plausible sequence of transformation that will convert a starting configuration (molecular precursor) into the desired final state (nanomaterial). The current approach of using molecular building blocks to grow nanomaterials is backward chaining, where the role of precursor chemistry in determining the structure and purity of the synthesized material is analyzed in a retrosynthetic manner. The concept of designing molecular precursors to mimic and act as preforms for known solid-state structures is still in its infancy and needs a systematic exploration to elucidate the potential of this methodology in terms of obtaining device-quality materials with improved performance when compared with materials obtained by conventional routes. Furthermore, the diversity of molecular structures and structural motifs available to synthetic chemists acts as a pointer for future research dealing with the synthesis of inorganic materials with unusual structures, new compositions, and possibly unknown properties.

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