Methods of growth and transfer of carbon nanotubes onto substrates for advanced electrical and electronic applications

• Main Author: Ng, Zhi Kai
• Other Authors: Teo Hang Tong, Edwin
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/170734
• Institution: Nanyang Technological University

Carbon Nanotubes (CNT) are one-dimensional nanomaterials made entirely out of carbon atoms. The theoretical electric current density of metallic CNT is estimated to be ~ 4×10^9 A/cm^2, which is 1000 times higher than that of copper. CNT possess exceptionally high Young’s modulus of over 1.2 TPa and a tensile strength of ~ 200 GPa due to the unique arrangement and bonding of carbon atoms. Chemical vapor deposition (CVD) method is one of the established techniques to synthesize CNT due to its ability to precisely control the process parameters such as growth substrates, flow rates of precursors and temperature. Typically, substrates such as Si/SiO2 wafers deposited with growth catalyst are utilized for the CVD synthesis of CNT. Subsequently, the CNT cannot be easily removed without mechanical damage or deformation due to their strong adherence to the underlying substrate. From the applications perspective, vertically aligned CNT (VACNT) are required in a free-standing form for the development of high-performance electronics. Potential applications of free-standing VACNT include thermal interfacing, electronics packaging, thermal management, etc. In this thesis, a water assisted CVD method for “self” delamination of VACNT from the growth substrate is proposed. It is hypothesized that the water vapour introduced post-synthesis of CVD creates an oxidative environment that weakens the CNT-catalyst bonds, causing self-delamination of VACNT from the growth substrate. Subsequently, the influence of various growth parameters such substrate oxide thickness, amount of water vapor and etching time on efficiency of delamination was evaluated and optimized to yield wafer-scale free-standing VACNT, without requiring any form of mechanical handling. A significant key finding of the delamination study is that the SiO2 surface plays a crucial role for large scale self-delamination of VACNT. Another potential research direction is to synthesize VACNT on non-planar or flexibles substrates, for applications such as electromagnetic interference shielding, lightning strike protection and CNT based flexible electronics. However, these substrates may not withstand the CVD chemical environment and high growth temperature. Therefore, to accomplish deposition of VACNT on flexible substrates, a water-assisted approach is proposed in this thesis, to transfer the delaminated CNT synthesized on Si/SiO2 substrates. The proposed methodology has been validated by the successful transfer of large area (10 cm x 10 cm), CNT membrane on flexible substrates such as PET, copper foil, glass fabric and carbon cloth. Mechanical bending tests (100 cycles at 90°) ascertained stable adhesion of CNT to the transferred substrates. The electrical properties of the VACNT remained unchanged after the transfer process and maintained ohmic contact with the conductive substrate. Therefore, the proposed water assisted delamination and transfer processes are beneficial for upscaling the synthesis of CNT on various substrates for a wide range of applications. Subsequently, this thesis also explores another viable method to directly synthesize CNT on all surfaces of non-planar substates such as glass fabric (GF) with an intention to develop lightweight, mechanically robust, and electrically conducting composite materials. However, the main concern of CNT growth on non-planar substrates is the uneven­­ deposition of catalysts due to the direction dependence of the E-beam deposition method. In this context, a dip-coating method for catalyst deposition is developed and optimised to synthesize CNT on GF. Parameters such as number of dip-coating cycles, rate of fabric withdrawal from dip-coating solution, fabric annealing temperature, growth time, growth temperature and catalyst ratio have been optimized to synthesize the GF/CNT composite. It was observed that the sheet resistance of GF/CNT could be tuned over a wide range from 40 Ω/square to 10^9 Ω/square by modulating the catalyst ratio and growth temperature. A large area GF/CNT composite of 10 cm x 10 cm have also been successfully synthesized as a showcase of its scalability. Recently, 3D interconnected porous graphene foams (3DGF) have shown a good potential in many applications such as thermal fillers, composite materials, batteries, etc., due to its high surface area and electrically conductive properties. The surface area of the 3DGF can be further improved by incorporating high surface to volume ratio materials such as CNT. In this thesis, a method with optimized growth parameters was developed to synthesize CNT on 3DGF, which yielded a covalently bonded 3DGF/CNT composite. This composite was utilized as a battery electrode and its electrical performance was evaluated by incorporating nickel cobalt oxide as the active material. This novel composite electrode yielded 60 times the charging current density (0.25 mA/cm^2 vs 15 mA/cm^2) of the previously reported electrode without compromising the specific capacity. This allows the fabricated battery to charge and discharge at a much faster rate. These observations indicate the potential to achieve a lightweight, conductive, high surface area composite electrodes. Lastly, ongoing research, insights, research directions and potential applications are elaborated. Applications such as VACNT/copper foil for fabrication of a field emission neutraliser, growth of CNT onto 3D-printed ceramics for electromagnetic shielding applications, and the utilisation of 3DGF/CNT as a high surface area electrode for other electrocatalytic materials are discussed. Although CNT research has made tremendous amounts of progress since its discovery, there is still a significant scope and requirement for large-scale synthesis of CNT on various substrates for deployment in high performance electronics.