Double-walled carbon nanotubes: scalable synthesis, patterning, and multifunctional applications

Double-walled carbon nanotubes (DWCNTs), renowned for their exceptional electrical, mechanical, and optical properties, are esteemed as ideal candidate materials for next-generation flexible electronics and optoelectronic devices. However, achieving controlled synthesis and effective into functional devices remain significant challenges. This thesis addresses these challenges, by presenting a growth method for DWCNTs using a sulfur-assisted floating catalyst chemical vapor deposition (FC-CVD) process, developing a novel technique for fabricating patterned CNT electrodes, and further demonstrating numerous applications of the synthesized DWCNTs. With the aim of promoting synthesis of DWCNTs, elemental sulfur was incorporated into a CH4-ferrocene (FeCp2) FC-CVD system. Systematic studies demonstrated that optimizing the sulfur concentration significantly enhances the quality of carbon nanotube (CNT) films, achieving a sheet resistance (𝑅𝑅𝑠𝑠) of 61.4 Ω·sq-1 at 90% transmittance (90%T) following AuCl3 doping. Detailed analysis further indicated that sulfur has a minimal effect on the size distribution of catalyst particles, which solely depends on FeCp2 concentration. Interestingly, by regulating the sulfur concentration, a growth transition from single-walled to double-walled CNTs has been achieved, resulting in the synthesis of DWCNTs with an exceptional yield up to 87%. Additionally, electron diffraction analysis showed no evidence of preferred chirality in DWCNT growth. This study highlights sulfur’s critical role in optimizing DWCNT synthesis while maintaining precise control over film properties and structural characteristics. Owing to the achievements made in scalable synthesis of DWCNTs, a widely applicable and contamination-free patterning technique was developed. This technique integrates seamlessly with the FC-CVD reactor, enabling the direct deposition of CNT aerosols onto pre-patterned filters, followed by effortless dry transfer onto diverse substrates. The fabricated grid-patterned CNT electrodes exhibited ultra-low sheet resistance (𝑅𝑅𝑠𝑠) about 1.3 Ω∙sq−1 at 90%T, along with excellent mechanical flexibility, highlighting their potential for high-performance, multifunctional applications in flexible electronics. Finally, the extensive applications based on DWCNTs were explored. Customized patterned CNT electrodes were employed to fabricate color-tunable alternating current electroluminescence (ACEL) devices, showcasing notable resilience under mechanical deformation. In addition, DWCNTs were implemented into the in-situ synthesis of MoS2/DWCNT heterostructures, which showed promise for high-performance field-effect transistors by enhancing charge transport within MoS2. The CNT patterning technique was further extended to fabricating wafer-scale, all-CNT photodetectors, by combining highly conductive DWCNTs with high-purity semiconducting SWCNTs. These photodetectors achieved an extraordinary photoresponsivity of 44 AW-1, highlighting the versatility and technological promise of DWCNT-based systems for a wide range of applications. In conclusion, these achievements presented in this thesis advance the controlled synthesis of DWCNTs, establish a scalable and innovative fabrication route for streamlined device integration, and highlight their potential across various applications, including flexible electronics, advanced optoelectronics, and heterostructure devices.