Final Activity Report Summary - MULTIFERRO-SOL-GEL (Multiferroic nanostructures: a non-aqueous sol-gel approach.)
At present there is a large gain of interest in multiferroic materials. Multiferroics are compounds which present simultaneously two or more long range ferroic-like ordering, such as ferromagnetic, ferromagnetic, antiferromagnetic, ferroelectric or ferroelastic. Since they usually exhibit coupling between the magnetic and electrical order parameters, they are of vital interest both academically and technologically. These materials are the best candidates for obtaining magnetoelectric coupling, which consists of a spontaneous magnetisation switched by an applied electric field and a spontaneous polarisation switched by an applied magnetic field. These compounds offer the possibility of multiple state memory elements, where data could be stored both as electrical and magnetic polarisations.
Since these compounds and nanocomposites are mostly synthesised via the pulsed laser deposition technique (PLD) or molecular beam epitaxy (MBE), we planned to use a deposition process which could be adapted to an industrial process by coupling both non-aqueous nanoparticles synthesis and atomic layer deposition using non-aqueous routes. The first challenge was to find a chemical way to deposit complex oxides at low temperatures with a high conformality. Indeed, the coating of a film comprised of nanoparticles of 8 to 10 nm diameter needed a deposition technique which allowed for the deposition of nano-sized complex structures.
A new chemical process applied to ALD for oxide thin films deposition was first developed to reach these goals. This process was based on the reaction of a metal alkoxide precursor (Hf(OtBu)4, Ti(OiPr)4, Ta(OEt)5,..) with a carboxylic acid and enabled the oxide thin film deposition on many supports, such as silicon substrate, carbon nanotubes, natural wool, polymers, cellulose, etc. at temperatures ranging from 50 °C to 350 °C with high conformality. The main benefits of employing carboxylic acids as oxygen sources were their less oxidative nature as compared to traditional oxygen sources and the possibility to grow films at relatively low temperatures on various substrates.
In the aim to experiment on and optimise this ALD process, simple oxide thin films deposition, e.g. HfO2, TiO2, Ta2O5, V2O4, were deposited on many kinds of support, from monocristalline substrates to carbon nanotubes, natural fibers, wool, etc. HfO2 and TiO2 thin films presented good electrical properties, such as high permittivity and low leakage current; therefore they were suitable for microelectronic applications. These results were encouraging in view of the deposition of multiferroic structures in the next step of this study. This process was also adapted to carbon nanotubes coating, using once again HfO2, TiO2 and V2O4, and, more particularly, with V2O4 for gas sensors applications.
As the deposition of more complex oxides was more delicate, since there were no volatile alkaline earth alkoxide precursors, the method was adapted for the use of amide precursors. The employment of a new barium precursor (Ba(N(SiMe3)2(THF)2) coupled with a titanium precursor (Ti(NEt2)4), i.e. amide precursors, allowed for the deposition of an oxide thin film composed of both Ti and Ba. The same was not achieved using alkoxide precursors.
In the mean time monodisperse ferromagnetic nanoparticles of 10 nm in diameter, such as CoFe2O4, were synthesised using non-aqueous sol-gel approaches. Monolayers of these nanoparticles were deposited onto different substrates using the Langmuir-Blodgett technique. By using this method almost perfect monolayers were obtained. They were in the process to be coated by ALD with different oxide materials by the time of the project completion, in order to measure transport and multiferroic properties.
All in all, we demonstrated that the ALD process that was developed during this project appeared to be a promising route for future studies and applications in various fields, such as gas sensing or (photo)catalysis and for the fabrication of advanced multifunctional, i.e. comprising multiferroic, nanomaterials for microelectronics. This process would certainly allow for applications adapted to high quality oxide thin film deposition at low temperatures on supports sensitive to oxygen or water.
Since these compounds and nanocomposites are mostly synthesised via the pulsed laser deposition technique (PLD) or molecular beam epitaxy (MBE), we planned to use a deposition process which could be adapted to an industrial process by coupling both non-aqueous nanoparticles synthesis and atomic layer deposition using non-aqueous routes. The first challenge was to find a chemical way to deposit complex oxides at low temperatures with a high conformality. Indeed, the coating of a film comprised of nanoparticles of 8 to 10 nm diameter needed a deposition technique which allowed for the deposition of nano-sized complex structures.
A new chemical process applied to ALD for oxide thin films deposition was first developed to reach these goals. This process was based on the reaction of a metal alkoxide precursor (Hf(OtBu)4, Ti(OiPr)4, Ta(OEt)5,..) with a carboxylic acid and enabled the oxide thin film deposition on many supports, such as silicon substrate, carbon nanotubes, natural wool, polymers, cellulose, etc. at temperatures ranging from 50 °C to 350 °C with high conformality. The main benefits of employing carboxylic acids as oxygen sources were their less oxidative nature as compared to traditional oxygen sources and the possibility to grow films at relatively low temperatures on various substrates.
In the aim to experiment on and optimise this ALD process, simple oxide thin films deposition, e.g. HfO2, TiO2, Ta2O5, V2O4, were deposited on many kinds of support, from monocristalline substrates to carbon nanotubes, natural fibers, wool, etc. HfO2 and TiO2 thin films presented good electrical properties, such as high permittivity and low leakage current; therefore they were suitable for microelectronic applications. These results were encouraging in view of the deposition of multiferroic structures in the next step of this study. This process was also adapted to carbon nanotubes coating, using once again HfO2, TiO2 and V2O4, and, more particularly, with V2O4 for gas sensors applications.
As the deposition of more complex oxides was more delicate, since there were no volatile alkaline earth alkoxide precursors, the method was adapted for the use of amide precursors. The employment of a new barium precursor (Ba(N(SiMe3)2(THF)2) coupled with a titanium precursor (Ti(NEt2)4), i.e. amide precursors, allowed for the deposition of an oxide thin film composed of both Ti and Ba. The same was not achieved using alkoxide precursors.
In the mean time monodisperse ferromagnetic nanoparticles of 10 nm in diameter, such as CoFe2O4, were synthesised using non-aqueous sol-gel approaches. Monolayers of these nanoparticles were deposited onto different substrates using the Langmuir-Blodgett technique. By using this method almost perfect monolayers were obtained. They were in the process to be coated by ALD with different oxide materials by the time of the project completion, in order to measure transport and multiferroic properties.
All in all, we demonstrated that the ALD process that was developed during this project appeared to be a promising route for future studies and applications in various fields, such as gas sensing or (photo)catalysis and for the fabrication of advanced multifunctional, i.e. comprising multiferroic, nanomaterials for microelectronics. This process would certainly allow for applications adapted to high quality oxide thin film deposition at low temperatures on supports sensitive to oxygen or water.