Applications of Atmospheric Pressure Plasma in Surface Engineering

Abstract

Plasma processing of materials has grown to be a key technology for various industrial applications. Low pressure plasmas have found wide applications; they require expensive vacuum systems and need orderly maintenance. Atmospheric pressure plasma jets (APPJs) on the other hand are less technically demanding. APPJs can generate a high flux of active species and are a promising alternative to low pressure plasmas for surface treatment. For an APPJ the plasma is not confined within the dimensions of the electrodes and can be directed towards the desired region. This dissertation is aimed at three novel applications of atmospheric pressure plasmas: printing nanomaterials, functionalization of nanomaterials and deactivation of airborne microbes. For all the contributions presented in this thesis emphasis have be given on studying the effects of plasma on surfaces. A novel APPJ based printing technique is proposed and developed to address issues of material degradation in conventional printing techniques. The process involves printing using nano-colloidal ink. The novelty of this printing technique is that it can tune the electronic properties of the nanomaterials in-situ while printing. Near edge X-ray fine structure (NEXAFS) spectroscopy of the deposited copper nanoparticles confirmed that the oxidation state of copper can be reduced equally at the surface and in the bulk. Also, by varying the gas mixture in the plasma the morphology of the films can be varied from uniform to porous. The film formed from copper nanoparticles tend to be insulating but can be transformed to conductive films through use of APPJ processes. Graphene oxide (GO) has found applications in multi-junction devices as charge transport layers or transparent electrodes. This is because, the work function of GO can be tuned to the device specifications. A lower power APPJ has been used to dope GO films with nitrogen. High resolution X-ray photoelectron (XPS) and NEXAFS spectroscopy revealed that the plasma induces finite changes in the surface chemistry and also influences the electronic properties of GO. Kelvin probe microscopy on the functionalized films has shown that the work function of GO can be tuned by 120 mV. This variation has been linked to the specific nitrogen configuration in the graphitic lattice. An APPJ device has also been used for depositing graphene oxide (GO) films. Hummer’s method is widely used for the oxidative exfoliation of graphite. Due to the use of strong acids, the resultant GO suspension is highly acidic and need extensive dilution to neutralize pH. It has been demonstrated for the first time that an APPJ can in-situ reduce highly acidic graphene oxide while deposition. XPS and NEXAFS spectroscopy revealed marked differences in the oxygen containing functional groups after deposition. Both NEXAFS and Raman spectroscopy revealed the healing of sp2 graphitic structure. Subsequent increase in conductivity was also observed from the electrostatic force microscopy measurements. The decontamination of airborne microbes using an atmospheric pressure dielectric barrier discharge has been demonstrated. Here, air has been used as the process gas and is found to be highly efficient in the inactivation of microbes. After interaction with the plasma, the physical structure of the microbes was found to be severely distorted and changes in surface chemical composition were also observed from NEXAFS studies. These physiochemical changes lead to the annihilation of microbes.