Master of Science
Dr. Heng-Yong Nie
Amongst various surface modification techniques, hyperthermal hydrogen induced cross-linking (HHIC) has been used to modify the surface of polymeric samples. In this novel and innovative technique neutral hydrogen projectiles with appropriate kinetic energy are produced to generate carbon radicals on the impacted surface through the collision-induced C-H bond breaking. Subsequently, this phenomenon results in cross-linking hydrocarbon chains in the treated polymeric samples.
Verifying the validity of cross-linking process through experiments is the target of first part of presented dissertation. Spin-coated poly(methyl methacrylate) (PMMA) films on silicon wafer were exposed to hydrogen projectiles for different durations, while the other conditions related to HHIC reactor were maintained the same. We propose the usage of time-of-fight secondary ion mass spectrometry (TOF-SIMS) negative ion spectra in order to gauge the degree of cross-linking in the impacted surfaces. TOF-SIMS depth profiles of PMMA characteristic ion fragments have also been used to study the cross-linking penetration into polymeric surface. It has been demonstrated that cross-linking depth into polymeric surface by the HHIC treatment does not increase linearly with the exposure time.
In the second part of the experiments, application of cross-linked polymeric surfaces was studied for organic thin film transistors (OTFTs) efficiency improvement. However, before using a cross-linked dielectric layer in the configuration of OTFTs a set of experiments was designed to figure out the best conditions for the HHIC treatment. Experiments included spin-coated PMMA films on silicon wafers which were treated with HHIC technique for different duration up to 30 min. Afterwards, cross-linked polymeric surfaces were subjected to contact with an organic solvent; the morphology of residual films were investigated with atomic force microscopy (AFM). We clarified that at least 5 min of treatment time is necessary to make efficient cross-linked surface on the polymeric sample for device fabrication purposes. Subsequently, cross-linking technique was applied in a simple model of multilayer device consist of PMMA as the dielectric layer and poly(3-hexylthiophene) (P3HT) as the organic semiconductor layer. By means of TOF-SIMS depth profile data the multi-layered structure of the mentioned device was studied and a significant improvement in the interfacial configuration of polymeric films owing to the cross-linking process was observed.
Consequently, as the main goal of this research, HHIC technique was applied in the solution-processable OTFTs configurations by treating the polymeric gate dielectric layer and making it resistant against the attack from the solvent contained in the solution used for building the subsequent layer (semiconductor). Use of cross-linked polymer dielectric layer resulted in enhancement of OTFTs performance.
Naderi Gohar, Soheila, "Enhancing the Performance of Organic Thin Film Transistors By Cross-Linking the Organic Gate Dielectric" (2013). Electronic Thesis and Dissertation Repository. 1754.