(4ep) In Situ Grafting and Growing of Organic Functional Layers Via Initiated Chemical Vapor Deposition
Initiated chemical vapor deposition (iCVD) is a very benign solventless process that produces conformal and thin (<100nm) coatings for a variety of technological applications. It enables the room temperature modification with functional polymers on virtually any substrate: organic or inorganic, rigid or flexible, planar or three-dimensional, dense or porous. During iCVD polymerization, monomers are delivered to the surface in vapor phase and undergo polymerization and thin film formation simultaneously. Free radical polymerization is initiated by filament heating with a typical current value of 1-2 Amp for a total power of 0.0086-0.0343 Watts cm-2. This energy-efficient process thus enables the cost-effective passivation of silicon for semiconductor, photovoltaic and biosensor applications. By eliminating the need to dissolve monomers or macromolecules, iCVD prevents solvent damage to delicate substrates such as reverse osmosis membranes. CVD film growth proceeds from the substrate up, allowing for interfacial engineering, real-time monitoring, and thickness control. Ultra-thin coatings (~30nm) have been fabricated for membrane-based separations to obtain high permeate flux. iCVD has shown successful results in the rational design of micro- and nano-engineered materials to control molecular interactions at material surfaces.
However, during the iCVD process, monomers physisorb to the substrate surface and polymerize; the lack of covalent bonding between coating and substrate results in delamination and loss of function when the coated device is subject to stress or heat. Here, we report on the new chemistries we developed for iCVD to control interactions at material surface down to the molecular level. Both traditional iCVD polymerization and novel interface reactions have been utilized. We report for the first time a novel antifouling coating with excellent chlorine-resistance to mitigate fouling on membranes used in sea water desalination. In situ interface grafting of the coating directly to the surface of commercial membranes has been developed to enhance stability and durability. A second example involves a novel interface reaction on silicon, which enables the organic vapor passivation of silicon at room temperature. Passivation is essential for the fabrication of high-quality thin film photovoltaic devices. Excellent passivation quality with surface recombination velocities less than 10cm s-1 has been obtained with the iCVD passivation scheme and the passivation is stable for >200 hours in air, corresponding to the highest passivation quality reported to date for air-stable organic passivation. Through these two examples, we demonstrated the ability of iCVD to graft and grow polymer thin films on both organic and inorganic substrates for better control of interface interactions at the molecular level and greater robustness and durability of as-fabricated devices.