|Nanostructural Engineering via Sol-Gel Processing and ALD: From Thin Films to Bio-Inorganic Composites
Ceramics and Materials Engineering
|Dr. John Bartlett of the Australian Nuclear Science and Technology Organisation (ANSTO) Government Laboratory
10:30 a.m. in Fiber Optics Auditorium, Busch Campus
Sol gel chemistry provides the “nanostructural engineer” with a range of well defined nanosized precursors for the production of thin films with tailored properties, thus providing the basis of a “bottom up” approach to the construction of materials with controlled nanostructures. This talk will present two novel approaches to the control of nanostructural evolution in oxide and nanohybrid films using sol gel processing:
• In the first approach, ordered oxo organo titanium (IV) clusters (including [Ti6O4](OR)8(OOR’)8 and [Ti6O6](OR)6(OOR’)6 prepared by controlled hydrolysis of carboxylic acid modified alkoxides are used to deposit titania coatings via spin coating and subsequent annealing. The corresponding microstructures were characterised using spectroscopic ellipsometry, atomic force microscopy, X-ray diffraction and cross-sectional TEM. The structure of the oxo organo titanium (IV) clusters was characterised using single crystal X-ray diffraction and vibrational spectroscopy (FTIR and Raman). The role of precursor chemistry, size and internal structure (particularly oxygen and titanium coordination in the ordered clusters) in mediating the structural evolution and crystallisation behaviour of the films will be discussed and compared to that of more conventional sol gel precursors.
• The second approach involves the synthesis of organic-inorganic hybrids, which offer a versatile new approach for fabricating thin films for a wide range of applications, including sensors, waveguides, integrated optics, and corrosion/scratch resistant coatings. A critical issue for industrial applications of these coatings is their mechanical behaviour and adhesion to the substrate. By controlling key process variables such as the nature and volume fraction of the organic group, the mechanical properties of the hybrid coatings can be readily modulated for specific functions. In particular, increasing the size of the organic substituent led to a corresponding increase in the ductility of the film, together with improved adhesion. The relationship between the connectivity of the inorganic network, the volume fraction and size of the organic modifier, and the mechanical response will be discussed. The design of nanohybrid precursors, in which the organic moiety is capable of self assembling, and thus playing an active “structure directing” role will also be discussed, and the properties of coatings and nanopowders produced from such precursors will be described.
The use of related bulk sol gel matrices for the immobilisation of functioning biological species will also be briefly addressed.
Atomic layer deposition (ALD) methods provide an alternative approach to the deposition of nanostructured coatings, which is complementary to sol gel processing. ALD involves the use of sequential surface chemical reactions to saturate a surface with a (sub)monolayer of reactive compounds such as metal alkoxides or covalent halides, followed by reaction with a second compound such as water to “grow” coatings layer by layer. The self-limiting nature of the reactions ensures excellent conformality, even on very convoluted substrates, and sequential processing results in exquisite control over film thickness, albeit at rather slow deposition rates (<100nm/hr). The use of such methods to produce multilayered coatings with controlled nanostructures, and the role of substrate surface chemistry in mediating structural evolution, will be discussed.