Synthesis of Conjugated Polymers for Selective Sorting of Semiconducting Single Walled Carbon Nanotubes and Electron Beam Radiation Effect on Field Effect Transistors Performance

Abstract

Single-walled carbon nanotubes have aroused many interest as one of the nanomaterials for future electronic devices due to their outstanding characters such as mechanical and electrical properties. Researches have explored to implementation of their electronic applications such as field-effect transistors. Therefore, various separation techniques to isolate the electronically-pure SWNTs have been developed for the utilization in electronic devices. In particular, conjugated polymer wrapping of SWNT has specially been great attracted as a method for isolation of SWNTs due to their advantages of high selectivity toward semiconducting (sc-) SWNT and simple polymer sorting process. Many wrapping polymers such as polyflourene and polythiophene derivatives have been studied for the selective dispersion of sc-SWNT, but the selection rule between the main chain of polymer and specific sc-SWNT still remain question. Chapter 1 introduces the conjugated polymers, selective sorting for semiconducting single walled carbon nanotubes and developments for the enhancement of Field effect transistors. In Chapter 2, To enable the application of semiconducting single-walled carbon nanotubes (sc-SWNTs) in advanced technologies, the SWNT chiralities must be sorted from the mixture of as-synthesized SWNTs. Interestingly, conjugated polymers have the ability to sort sc-SWNTs with different chiralities selectively, and this sorting effect is dependent on the polymer backbone structure. However, design principles for polymer backbones that allow the selective separation of desired SWNT chiralities are still unknown. To investigate the relationship between the polymer backbone structure and the sorting effect systematically, we used three pyrene moieties to prepare different structural topologies of diketopyrrolopyrrole (DPP)-based polymers. As expected, the backbone of three synthesized polymers is gradually linearized from P(1,8-pyDPP) to P(1,6-pyDPP) and P(2,7-pyDPP). Further, these polymers successfully enrich sc-SWNTs of the high-pressure carbon monoxide (HiPco) SWNTs by using an efficient washing process. We obtained high-quality sc-SWNT solutions without impurities or excess polymer by using P(1,6-pyDPP) because of the good dispersibility arising from the slightly kinked backbone. It was also found that the very kinked backbone of P(1,8-pyDPP) increase the preference for specific diameters of SWNTs, whereas the linear backbone of P(2,7-pyDPP) was favorable for the sorting of sc-SWTNs with large diameters. We believe that our findings regarding the polymer-backbone-topology-dependent sorting ability for sc-SWNTs offers important fundamental information for the structural design of conjugated polymers. In Chapter 3, Quinoidal polymers of PQEDOT-2T and PQEDOT-2fBT are introduced for the investigation of structure-property relationship on selective sorting of sc-SWNTs. By using bithiophene (2T) and difluorobenzothiadiazole (2fBT) as counter part, the PQEDOT-2T show lower distortion angle of backbone than PQEDOT-2fBT. The property influence the UV-Vis absorption spectra, chain packing structure and performance of organic field effect transsitor. The quinoidal polymers are firstly utilized for the selective sorting of sc-SWNT with simple wrapping process. From the UV-Vis-Nir absorption specra, they show the sharp and intense peaks of sc-SWNTs which mean the effective polymer wrapping of surface on sc-SWNTs. Also, the PQEDOT-2T exhibit more efficient selective sorting of (8,6) and (9,5) than PQEDOT-2fBT. Finally, the presence of m-SWNTs are investigated with Raman spectra and CNT-FETs that the results indicate the purity of sc-SWNTs above 99%. The structure-property relationship would be the background for the development of conjugated polymer and SWNTs hybrids materials for electronic devices. In Chapter 4, electron beam radiation are introduced to modulate the SWNT network for high performance CNT-FET. P(1,6-pyDPP) polymers are utilized for selective sorting of sc-SWNTs and the (1,6-pyDPP)/HiPco hybrids solution are successfully produced with simple wrapping process. The electron beam radiation directely irradiates in the hybrid films with 10 MeV. The morphology of the irradiated films have no change up to 70 kGy. However, the absorption spectra in irradiated films are drastically decreased in radiation dose from 10 and 20 kGy. The ratio of ID/IG in raman spectra increase up to 1.08 which indicate the large defect sites in SWNTs. Also, the surface composition exhibits the increase of portion about sp3 amorphous carbon. Interestingly, the mobility of CNT-FETs increase about twice at radiation dose of 10 kGy, compared with non-irradiated CNT-FET. Despite the degradation of polymers and SWNTs, the enhancement of mobility may mean the decrease of contact resistance because the electron beam radiation can form the new bonds beween polymer-SWNTs and SWNT-SWNTs juntions. Although the mechanism about construting new bonds by using electron beam is still unknown, the electron beam radiation is demonstrated as powerful tools for the tuning of nano-material films.