|I. GENERAL FEATURES
I.1. What are the properties of the
precursors relevant for solution routes?
Basic requirements are purity, high yield and
thus selective synthetic routes, easy handling and storage, non-toxicity.
Suitable physical properties are solubility - generally in non-aqueous
media- for sol-gel applications or "wet MOCVD", volatility and thus liquids
or solids with low melting points for conventional MOCVD. In terms of
transformation into the materials, high ceramic yields and controlled
conversion are desired. This implies to control the hydrolysis rates, having
access to a suitable rheology (stable sols, homogeneous gels, viscosity...)
especially for coatings and, for the materials required in their crystalline
form, low temperatures of crystallization. Special chemical functionalities
such as donor-acceptor or polymerisable sites may be necessary for special
properties such as NLO (non linear optics), hybrids or photopatterning.
Multicomponent oxides such as electroceramics can be obtained from mixtures
of precursors or from molecules in which two metals M and M’ are chemically
associated in a "single source" precursor. The formulation of the latter
should match the stoichiometry between the metals required by the material.3,4.
Control of hydrolysis, of surface properties such as hydrophobicity, of
microstructure (porosity, homogeneity) of the final material, formation of
gels for coatings or embedding, possibility to accede to hybrid materials
and thus to organic-inorganic arrays with covalent bonding1, 4, 5
are other desired properties .
I.2. Comparison of the different types of
A rough comparison of the basic physical and
chemical properties of various oxide sources is given below (for classical
alkoxide ligands OR such as ethoxides (OEt), propoxides (OPr), butoxides (OBu),…):
Solubility (in organic solvents):
> M(b-dik)n > M(O2CR)n
> M(OR)n >> M(O2CR)n
| M(OR)n >> M(b-dik)n
Facility to form stable heterometalic species
M(OR)n > M(O2CR)n > M(b-dik)n
Hydrolysis (and handling):
>> M(O2CR)> > M(b-dik)n
Metal alkoxides can meet solubility as well
as volatility requirements and thus be applied in MOCVD processes,
especially if they are liquids. Their high ability to cross-link in the
presence of water requires storage under inert atmosphere. The high
reactivity (lability) of the metal-alkoxide bond makes them useful starting
compounds for a variety of heteroleptic species (ie species with different
types of ligands) such as M(OR)n-xZx (Z = b-dik or
O2CR) (see eq 1).
Acetates M(O2CMe)n are
commercially available often as hydrates (Li, Pb, La, …). They can be
obtained anhydrous by heating with acetic anhydride or with 2-methoxyethanol
(in situ for the latter). These carboxylates are generally insoluble in
organic solvents due to the trend of those ligands to act, as shown by Fig
1, as bridging or bridging-chelating ones, thus giving oligomers or polymers
[M(O2CR)n]m where m stands for the degree of association or
molecular complexity or nuclearity).
|Fig. 1: Most common
coordination modes of carboxylate ligands
They might be dissolved in the presence of
metal alkoxides (see III). 2-Ethylhexanoates M(O2CCHEtnBu)n
are the carboxylates with the smallest number of carbon atoms which are
soluble in organic media for most elements. A large number of carboxylate
derivatives are available for aluminium. Formate Al(O2CH)3(H2O) and carboxylate-alumoxanes [Al(O)x(OH)y(O2CR)z]m
could be prepared from inexpensive feedstocks namely gibsite or boehmite.7
Carboxylic acid exchange reactions applied to the formate gave derivatives
with tailored solubility properties, spinnable and displaying polymerisable
sites. Liquid carboxylates8 were also prepared for a number of
metals (Al, Mg, Ba, Ca, Y) by reacting 2-[2-methoxyethoxy]ethoxyacetic acid
(MEEA) with acetates, hydroxides or carbonates. Carboxylates are generally
non volatile (although volatility was observed with bulky R groups such as
tertiobutyl as for instance for bismuth).
1.2.3 Metal b-diketonates6b
are mostly used in material science for their volatility properties. Most of
b-diketonates are monomeric (m = 1 due to achelating behaviour of the
ligand) but association (m>1) is the rule for divalent, large elements such
as alkaline earth metals (Ca, Sr, Ba). Their volatility makes them
attractive for CVD techniques and they act as source of oxide or of metals (especially
for late transition metals). The volatility is governed by the bulk of the R
and R' groups as well as the nature of the metal which will determine the
degree of association m.
b-diketonates form easily adducts M(b-dik)nLx
with neutral molecules L mostly with nitrogen or oxygen donor sites such as
water, alcohols, ethers, amines. Formation of adducts reduces association
but stability problems in the vapor phase must be considered.9
Acetylacetonates (R = R' = CH3 = Me) which provide the best
ceramic yield are generally used for solution routes.
Tetramethylheptanedionates (R = R' = tBu) are precursors of choice for CVD