Gas-phase Synthesis of Semiconducting Single-walled Carbon Nanotubes for Advanced Electronics

Abstract

The advancement of electronics towards portable, high-performance, and flexible technologies faces significant challenges as device sizes continue to shrink. This has driven the exploration of advanced nanomaterials, with single-walled carbon nanotubes (SWCNTs), standing out due to their unique one-dimensional structure and exceptional properties, including superior conductivity, transparency, flexibility, and stability. However, scalable and controllable synthesis of SWCNT remains a critical challenge for their large-scale integration into nanoelectronics.

This dissertation focuses on the synthesis of semiconducting SWCNTs (s-SWCNTs) and their application in electronics, with the prime goal of achieving high-purity s-SWCNTs and effectively integrating them into electronic devices. The study employs a floating catalyst chemical vapor deposition (FCCVD) method to enable continuous large-scale synthesis. By optimizing growth parameters, such as temperature, gas composition, feeding rate, and carbon sources, we achieved a controllable synthesis of s-SWCNTs with a purity as high as 94%, one of the highest purities attained through direct synthesis. Water and carbon dioxide are believed to act as oxidizing agents, selectively etching metallic SWCNTs and enhancing the yield of s-SWCNTs. The resulting nanotubes have a mean diameter of approximately 1 nm and a mean length of 6.38 μm, significantly longer than those produced via solution-sorting methods. Furthermore, these synthesized s-SWCNTs demonstrated a mean mobility of 376 cm2V-1s-1 and an on-off ratio of up to 8.33×106 in individual-SWCNT field effect transistors (FETs). The carrier mobility in the optimal FETs approaches the theoretical limit for 1 nm SWCNT on a SiO2 substrate at room temperature.

Importantly, this dissertation introduces a novel, lithography-free fabrication technique for creating flexible, wafer-scale, all-CNT device arrays, addressing common issues of contamination and damage that typically arise during traditional wet processing. The resulting wafer-scale all-CNT photodetector arrays exhibit excellent uniformity, wearability, environmental stability, and notable broadband photoresponse. These photodetectors achieve a high responsivity of 44 A/W, significantly outperforming similar CNT photodetectors fabricated using photolithography and solution sorting, even with purities of up to 99% s-SWCNT. Furthermore, the dry transfer manufacturing process was employed to fabricate transparent flexible electrodes, successfully transferring SWCNT film onto MoS₂, thereby optimizing the photodetectors' light absorption and carrier separation, leading to improved responsivity.