In this project funded by the Macromolecular, Supramolecular and Nanochemistry Program of the Chemistry Division, Steven George of the University of Colorado at Boulder will develop new surface chemistries for depositing hybrid organic-inorganic films using molecular layer deposition (MLD) techniques. The goal is to fabricate functional films with tunable mechanical, electrical, chemical, dielectric, optical and thermal properties to enable the construction of thin film devices. Targeted properties include: high cross-linking between polymer chains in the film for high toughness and high critical strain and compressibility for flexibility and volume expansion tolerance. The broader impacts involve student training and development, collaborations with companies interested in using hybrid organic-inorganic MLD films in future products, and communicating the excitement of science to elementary school students via the University of Colorado Wizard Program.
This work will develop new way to make technologically important thin films with customizable mechanical properties. These films could impact technologies that use protective and barrier layers, flexible and conductive films, and compliant and functional coatings.
" focused primarily on the mechanical and electrical properties of hybrid organic-inorganic polymer films. These hybrid organic-inorganic polymer films were grown using a procedure involving stepwise, self-limiting surface reactions known as molecular layer deposition (MLD) techniques. This MLD procedure allows for atomic layer control of thin film growth. The resulting thin films are also conformal to the initial substrate. Consequently, these MLD films can be deposited on high surface area substrates with high aspect ratios. MLD is similar to atomic layer deposition (ALD) which is also based on stepwise, sequential self-limiting surface reactions. For ALD, each surface reaction incorporates an element into the growing film. For MLD, a molecular fragment is incorporated into the growing film. This molecular fragment is usually an organic entity. Consequently, MLD allows for the growth of organic and hybrid organic-inorganic films. ALD and MLD can also be combined to control the composition of the hybrid organic-inorganic film. By combining a certain number of ALD surface reactions with a certain number of MLD surface reactions, the film composition can be varied from pure organic to pure inorganic. Composition control was important for this project because the mechanical and electrical properties of hybrid organic-inorganic films are dependent on composition. One important goal was to demonstrate that the mechanical and electrical properties could be tuned by varying the composition of the MLD films. A practical motivation for this research was that hybrid organic-inorganic films with tunable mechanical and electrical properties will be important for the fabrication of devices on flexible substrates. The organic component of the hybrid organic-inorganic film provides flexibility compared with the inorganic component that is very brittle. A secondary goal was to demonstrate additional new surface chemistries for the fabrication of hybrid organic-inorganic films. The various hybrid organic-inorganic films have different functionalities and will serve, for example, as insulating, conducting and protecting layers in devices on flexible substrates. We first demonstrated the tunable mechanical properties of ALD:MLD alloys using Al2O3 ALD/Alucone MLD alloys. Alucone MLD is a hybrid organic-inorganic polymer deposited using trimethylalumininum (TMA) and organic alcohols. We showed that we could tune the density, refractive index, elastic modulus and hardness of the ALD:MLD alloys over a wide range. The measured results were within the bounds predicted by the "rule of mixtures". Similar results were also observed for ZrO2 ALD/Zircone MLD alloys. Zircone MLD is a hybrid organic-inorganic polymer deposited using zirconium tert-butoxide and organic alcohols. We also explored the tunable electrical conductivity of ZnO ALD/Zincone MLD alloys. ZnO ALD itself is a reasonable electrical conductor. Zincone MLD is a hybrid organic-inorganic polymer deposited using zirconium tert-butoxide and organic alcohols. For conducting zincone MLD films, we used hydroquinone (HQ) as the organic alcohol. HQ is an aromatic molecule and more electrical conductivity was expected because of the aromaticity. We discovered that the pure zincone MLD film had negligible conductivity. However, the ALD:MLD alloys grown by repeating 1 cycle of ZnO ALD with 1 cycle of zincone MLD (1:1 alloy) and 2 cycles of ZnO ALD with 2 cycles of zincone MLD (2:2 alloy) displayed significant electrical conductivity. The electrical conductivity was more than x10 times higher than the electrical conductivity of ZnO ALD by itself. These results are important because they may lead to a new class of conducting, transparent and flexible films that could be used in place of indium tin oxide (ITO). We also examined new chemistry for titanicone MLD films using TiCl4 together with the organic alcohols ethylene glycol and glycerol. These titanicone studies were significant because this work led us to examine the post-processing of MLD films using heating and UV light exposure. Both annealing and UV exposure in O2 removed the organic constituents in the titanicone MLD films and produced TiO2 films. In the case of UV exposure, the TiO2 films were porous. This post-processing theme has been developed further in our new NSF work on the pyrolysis of MLD films. In addition, we explored the growth of alucone MLD films using glycerol instead of ethylene glycol. Glycerol has three hydroxyl groups and should lead to higher cross-linking and tougher MLD films. Hoping to fabricate extremely flexible films, we also examined the growth of polysiloxane MLD films using various silane precursors such as dimethylmethoxychlorosilane or diisopropylisopropoxysilanol and water. We also worked on incorporating unreacted TMA into MLD films to fabricate H2O chemical getter films. We demonstrated the growth of Lewis acid-base films using TMA as the Lewis acid precursor and varoius diamines, such as triethylenediamine, as the Lewis base precursor. In addition, we explored the growth of alucone MLD films using TMA and glycidol. Glycidol is a heterobifunctional reactant and minimizes the number of double reactions compared with homobifunctional precursors like ethylene glycol.
