Chirality-specific carbon nanotubes (CNTs) remain a critical target for advanced applications in electronics, photonics, and quantum technologies due to their diameter- and twist-dependent electronic properties. Achieving precise control over CNT chirality requires targeted synthesis approaches, with catalyst engineering and templated growth emerging as key strategies. Additionally, metrology techniques such as optical spectroscopy play an indispensable role in verifying chirality and assessing synthesis outcomes.

Catalyst engineering is a primary method for chirality-specific CNT growth, where the structure and composition of catalytic nanoparticles dictate the nucleation and extension of specific (n,m) species. Transition metals such as Fe, Co, and Ni are commonly used, but their size, crystallographic orientation, and alloying state must be carefully controlled. Studies have demonstrated that catalysts with well-defined facets can template the formation of CNTs with particular chiral angles. For example, Co-W and Co-Mo bimetallic catalysts have shown preferential growth of semiconducting CNTs with chiral indices such as (6,5) and (7,6), attributed to the lattice matching between the catalyst surface and the growing CNT edge. The diameter of the catalyst nanoparticle is another critical parameter, as it directly influences the CNT diameter. Nanoparticles in the 1–3 nm range tend to produce single-walled CNTs with diameters between 0.7–2 nm, where specific chiralities are more energetically favorable. Pre-treatment of catalysts with ammonia or hydrogen can further refine their activity and selectivity by removing amorphous carbon and stabilizing active sites.

Templated growth offers another pathway to chirality control by providing a physical or chemical scaffold that guides CNT formation. One approach involves using crystalline substrates such as quartz or sapphire, where epitaxial interactions between the CNT and the substrate lattice promote alignment and chirality selection. For instance, CNTs grown on ST-cut quartz substrates exhibit a high proportion of (6,5) and (7,5) tubes due to the lattice matching between the CNT and the substrate. Another templating strategy employs molecular seeds or carbon precursors with predefined structures. Short CNT segments or organic templates with aromatic structures can serve as nucleation points, biasing the growth toward specific chiralities. DNA-wrapped CNT templates have also been explored, where the helical structure of DNA interacts with the growing CNT to influence its chirality. While templated growth can yield high selectivity, challenges remain in scalability and maintaining uniformity across large substrates.

Metrology is essential for validating chirality-specific synthesis, with optical spectroscopy serving as a non-destructive and high-throughput tool. Raman spectroscopy is widely used to probe CNT vibrational modes, where the radial breathing mode (RBM) provides direct information on the tube diameter. By correlating RBM frequencies with diameter-dependent Kataura plots, researchers can identify specific (n,m) species. For example, RBMs in the 250–350 cm⁻¹ range typically correspond to diameters of 0.7–1.0 nm, where chiralities like (6,5) and (7,5) are prominent. Photoluminescence excitation (PLE) spectroscopy is another powerful technique, particularly for semiconducting CNTs. PLE maps plot excitation versus emission wavelengths, revealing distinct peaks corresponding to specific chiralities. The (6,5) CNT, for instance, exhibits characteristic excitation and emission at around 570 nm and 980 nm, respectively. Near-infrared (NIR) absorption spectroscopy complements these methods by detecting electronic transitions between van Hove singularities, which are unique to each chirality. The E₁₁ and E₂₂ transitions for metallic and semiconducting CNTs provide additional fingerprints for identification.

Despite progress, challenges persist in achieving absolute chirality purity. Catalyst engineering often yields mixtures of chiralities due to slight variations in nanoparticle size and structure. Templated growth can suffer from defects or incomplete template-CNT interactions, leading to heterogeneous outputs. Advanced characterization techniques, including single-nanotube spectroscopy and high-resolution transmission electron microscopy (HRTEM), are necessary to resolve these issues at the individual tube level. Statistical analysis across large sample areas is also critical to assess the overall selectivity of a synthesis method.

Future directions may involve combining catalyst design with templating strategies to enhance selectivity further. For example, pre-patterning catalyst nanoparticles on lattice-matched substrates could leverage both approaches synergistically. Computational modeling of CNT-catalyst interactions could also provide predictive insights into optimal growth conditions for target chiralities. As metrology techniques continue to improve, real-time monitoring of CNT growth may enable dynamic adjustments to synthesis parameters, pushing the boundaries of chirality-specific production.

In summary, chirality-specific CNT synthesis relies on precise catalyst engineering and templated growth strategies, supported by robust optical spectroscopy techniques for validation. While challenges remain in achieving perfect selectivity, ongoing advances in synthesis and characterization hold promise for unlocking the full potential of CNTs in tailored applications.