Deposition of a stable and high concentration of carboxylic acid functional groups onto a silicon surface via a tailored remote atmospheric pressure plasma process

Processes to introduce carboxylic acid functional groups onto surfaces have been widely applied in various applications, such as molecular grafting for biosensors, biocompatibility improvement, gas filter/adsorption in environmental engineering and the enhancement of interfacial adhesion between fillers (such as carbon or glass fibre) and matrices (such as epoxy resin) in composite materials. To develop an environmental-friendly process to introduce a stable and high concentration of carboxylic acid functional groups, Acrylic Acid (AAc) was deposited onto silicon wafers using remote argon atmospheric plasma processing (APP) and vapour phase grafting in a bespoke Pyrex cylindrical chamber. The chamber stands vertically with an APP nozzle installed at the top, a sample platform downstream and an AAc vapour inlet on the chamber wall. In this way, both the effect of atmospheric gas on the process and AAc monomer fragmentation during APP can be limited. Silicon specimens were exposed to a combination of plasma gas and AAc vapour. Surface wettability was evaluated by measurement of water contact angles and chemical composition was analyzed using X-ray Photoelectron Spectroscopy (XPS). The peak at the binding energy of 289.2 eV, which fits with COOH components in the C1s spectrum of poly-AAc, was used to determine the contribution of the carboxylic acid groups depositing on the surface because it was absent in the C1s peak of untreated silicon wafers. A more stable and hydrophilic wettability and the binding energy peak at 289.2 eV from XPS show the presence of carboxylic acid groups on the surface of samples treated with AAc vapour with the remote argon-APP. A higher intensity peak at 289.2 eV was detected, compared with the surfaces activated by a conventional argon-APP in which the AAc vapour is exposed to the plasma glow, suggesting a potential, rapid and novel remote APP process for carboxylic acid functional group deposition.