Carbon nanotubes (CNTs) synthesized via chemical vapor deposition (CVD) represent a critical advancement in nanomaterials due to their exceptional electrical, thermal, and mechanical properties. CVD-based growth offers scalability and control, making it suitable for industrial applications. The process involves catalytic decomposition of carbon-containing gases on substrates, with parameters such as catalyst design, gas precursors, and substrate choice significantly influencing CNT quality, yield, and chirality.

Catalyst design is central to CVD-grown CNTs. Transition metals like iron (Fe) and cobalt (Co) are widely used due to their high carbon solubility and catalytic activity. Nanoparticle size and composition dictate CNT diameter and wall number. For single-walled CNTs (SWCNTs), catalysts are typically in the 1–5 nm range, while larger particles favor multi-walled CNTs (MWCNTs). Bimetallic catalysts, such as Fe-Co alloys, enhance yield and uniformity by optimizing carbon diffusion rates. Catalyst support materials, including alumina (Al₂O₃) and magnesium oxide (MgO), prevent nanoparticle agglomeration at high temperatures. Pretreatment steps like reduction in hydrogen further refine catalyst activity.

Gas precursors supply the carbon source. Methane (CH₄) and acetylene (C₂H₂) are common, with decomposition kinetics influencing growth. CH₄ requires higher temperatures (900–1000°C) but produces cleaner CNTs due to fewer side reactions. C₂H₂ decomposes at lower temperatures (600–800°C) but may introduce amorphous carbon without precise flow control. Ethylene (C₂H₄) and carbon monoxide (CO) are alternatives, with CO enabling selective SWCNT growth via the HiPCO process. Gas ratios (e.g., H₂:CH₄) affect nucleation density, with hydrogen etching amorphous carbon and promoting aligned growth.

Substrates play a dual role as catalyst supports and growth templates. Flat substrates like silicon wafers with oxide layers enable horizontal CNT arrays, while porous substrates like quartz facilitate vertical alignment. Substrate roughness and surface energy influence catalyst adhesion and CNT orientation. For instance, patterned catalyst deposition on silicon dioxide (SiO₂) enables site-specific growth for device integration. Temperature gradients across substrates can further direct CNT alignment via thermophoretic forces.

Chirality control remains a major challenge in CVD synthesis. CNT electronic properties depend on their chiral indices (n,m), with metallic (n-m divisible by 3) and semiconducting (n-m not divisible by 3) types arising from different atomic arrangements. Most CVD methods produce a mix of chiralities, necessitating post-synthesis sorting. Strategies like catalyst engineering (e.g., Co-W alloys) and growth parameter tuning (e.g., low-pressure CVD) have improved selectivity, but achieving >90% semiconducting or metallic CNTs at scale remains elusive. Epitaxial seeding on crystalline substrates shows promise for chirality-specific growth.

CNT growth mechanisms are categorized as tip-growth or base-growth, dictated by catalyst-substrate interactions. In tip-growth, the catalyst particle detaches from the substrate, remaining at the CNT tip as carbon precipitates from its base. This occurs with weak catalyst-substrate adhesion (e.g., Fe on silica). In base-growth, the catalyst stays anchored, with CNT extrusion from its top. Strong adhesion (e.g., Co on alumina) favors this mode. The growth mechanism affects CNT alignment and defect density, with base-growth typically yielding straighter tubes.

Applications of CVD-grown CNTs in electronics leverage their high carrier mobility and current density. In transistors, semiconducting SWCNTs serve as channels, with ballistic transport enabling high-speed operation. Challenges include achieving uniform doping and minimizing contact resistance. For interconnects, metallic CNTs outperform copper in electromigration resistance and conductivity at nanoscale widths. Integration requires dense, aligned bundles and low-resistance vias. Advances in transfer techniques (e.g., dry spinning) have enabled CNT incorporation into silicon CMOS platforms.

Thermal management is another key application, with CNT films dissipating heat efficiently in high-power devices. Their mechanical flexibility also suits wearable electronics and flexible displays. However, scalable synthesis of defect-free, aligned CNTs remains critical for commercial adoption.

In summary, CVD-based CNT synthesis offers a versatile route to high-performance nanomaterials, with catalyst design, gas chemistry, and substrate engineering enabling controlled growth. Chirality control and growth mechanisms are active research areas, while electronic applications benefit from CNTs' unique properties. Continued optimization of CVD parameters will drive advancements in nanoelectronics and beyond.