Abstract
This thesis focuses on the novel methods using plasma processes for synthesizing fluorescent and phosphorescent polymers for light emitting diodes and for efficient conversion of carbon dioxide into fuels and polymers. The process that converts monomers into plasma polymers and a series of products via the formation of gas-phase radicals and their recombination at radical sites during film growth and particle formation is known as plasma polymerization. The plasma chemical reactions of conventional monomers and of low-molecular weight substances can be initiated by plasma at atmospheric- or low-pressure. Plasma-produced radicals, radical fragments, radical-sites at solid surface or gaseous monomers, etc., can initiate a classical chain-growth polymerization to form polymer and/or produce a series of gaseous/liquid/solid products. In this sense, it is a special form of plasma-enhanced chemical vapor deposition (PECVD), since the radical processes dominate the macromolecule formation yielding mainly amorphous, more or less cross-linked structures. Some monomers might not undergo polymerization by conventional activation, indicating a main difference between plasma and conventional polymerization. The underlying growth mechanism is known as Rapid Step-Growth Polymerization (RSGP). It is assumed that the gas molecules and particles (monomer and reactive gas) travel through the active zone (bulk plasma and plasma/sheath boundary region), where radical formation is taking place within the gas phase, and then enter a passive zone (plasma sheath and surface growth region) yielding recombination and stable products. The recombination of reactive species and reactivation of reaction products determine the plasma chemical reactions. Furthermore, the surface takes part in plasma polymerization by radical sites, third-body reactions, and etching processes that lead to ablation and re-deposition. Hence, a plasma polymer typically results from the competition between etching and deposition processes and it has a more or less cross-linked structure, depending on the plasma species present during plasma chemical reactions. Generally, one of the intentions of plasma polymerization is to produce ultra-thin, pinhole-free polymer-like layers with a defined, regular structure, but with variable composition In part I, We describe a facile one-step fabrication process of thin film conjugated polymers for efficient optoelectronic devices from naphthalenes by plasma polymerization through a new monomer feeding method. The resulting 3-D aromatic chain networks of the polymerized films were in good agreement with the predictions based on monomer bond rupture routes analysis. The photoluminescence (PL) spectra of plasma-polymerized films featured short-lived inter-molecular emissions arising from self-assembly of the naphthalene units in the cross-linked architectures. When functioning between the electrodes for light emitting, the plasma films manifested electroluminescence (EL) spectra that showed a steep rise at the threshold wavelength red-shifted relative to the PL spectra, indicating prevailing charge tunneling to lower excited states by the accumulated interfacial charges. The wide distribution of conjugation lengths in the plasma films provided the subsequent energy transfer routes and gave rise to the broad EL emission band that produced stable pure white light of CIE coordinates of (0.34, 0.33) with excellent efficiencies. This dry deposition process allows the use of multiple monomers for property tailoring and may be used for large-area depositions. In part II, we unveil a novel approach of fabricating white phosphorescent polymers for light-emitting diodes (LEDs) was successfully developed via processes of plasma polymerization and complex chelation. Direct injection of high concentrations of aromatic monomer, 1,10-phenanthroline (Ph), into plasma reaction chamber allowed the polymerization to take place in high yields with excellent retention of desired monomer functionalities. Constructed by conjugated segments of various lengths in their backbones, the synthesized polymer emitted broad-band photoluminescence spectra from 380 to 600 nm. Subsequent refluxing with the transition metal iridium (Ir) resulted Ir-polymer complexes that manifested broad-band fluorescence from 400 to 600 nm and phosphorescence from 450 to 700 nm. The phosphorescent emissions were assigned to a combination of triplet ligand-centered π-π* and metal-to-ligand (ML) charge transfer emissions. Between electrodes, the polymer-Ir complexes emanated full-spectral white-light emissions that are composed of ligand-centered π-π* emission with maxima at 435 nm and ML charge transfer emission with maxima at 605 nm. The devices exhibited voltage-stable white luminescence with the Commission Internationale de l’Enclairage (CIE) coordinates of (0.25,0.26), a maximum luminance efficiency of 7.44 cd/A, and a maximum luminance of 4486 cd/m2. This simple approach offers great potential for the synthesis and engineering of white organic LED phosphorescent materials. In part III, the present work reports on the conversion of carbon dioxide (CO2) and selected hydrocarbons (hexane, decane, decadiene and toluene) into oxygenates in gaseous, liquid and solid compounds by low-temperature non-catalytic plasma activation. Under plasma activation, the CO2 molecule dissociated into CO and O radicals and reacted with hydrocarbon fragments to form oxygenated compounds (alcohols, esters, aldehydes, ethers, carboxylic acids) in gaseous, liquid and solid products. The distribution of oxygenated compounds in C-O types (alcohol, ether) is higher than that in C=O types (ketone, aldehyde), it is because the reaction rate of O radicals is higher than that of CO radicals. The hydrogen atoms would terminate chain propagation into polymers and cap with O radicals to form alcohol compounds. The reaction routes of CO2 with selected hydrocarbon molecules were also proposed. With the proper selection of counter-molecules, a high yield of liquid fuel can be achieved. This study clarifies the reaction routes for CO2 and hydrocarbon molecules under plasma activation and affords proper selections of molecules for optimal syntheses with CO2 without catalysts. In Part IV, we report on the direct conversion of carbon dioxide (CO2) and water vapor (H2O) into liquid fuels by low-temperature non-catalytic atmospheric pressure plasma jet. The basic principle of this approach is to utilize plasma electrons in which the major fraction of the energy transferred to the CO2 and H2O molecules by vibrational excitation. Under plasma reactions, where the gas-phase collisions dominate processes, the CO2 and H2O molecules are dissociated into CO, O, OH, and H radicals and recombine to form oxygenated compounds (alcohols, ketones, carboxylic acids, aldehydes), light hydrocarbons (methane and ethane), H2, CO and O2. From GC/MS results, the major compounds in liquid products are methanol, ethanol, acetone and acetic acid. With the increasing water vapor concentration in the feed, the concentration of the liquid and gaseous oxygenated products is also increased. The results manifest even continuous and low-cost methodology that produces stable atmospheric-pressure plasma to convert CO2 into fuels.