Chiral carbon nanotubes (CNTs) exhibit unique electronic properties determined by their chiral indices (n,m), which dictate whether they behave as metals or semiconductors. Post-synthesis separation of CNTs by chirality is critical for applications requiring uniform electronic characteristics, such as high-performance transistors, sensors, and optoelectronic devices. Two prominent techniques for achieving this separation are density gradient ultracentrifugation (DGU) and chromatography, each leveraging distinct physicochemical interactions to isolate specific chiralities.
Density gradient ultracentrifugation exploits differences in buoyant densities of CNTs wrapped with surfactant molecules. When dispersed in a medium such as sodium cholate or sodium deoxycholate, CNTs form complexes whose densities vary with diameter and electronic type. A density gradient is prepared using iodixanol or sucrose solutions, and the CNT dispersion is layered on top. During ultracentrifugation, CNTs migrate to equilibrium positions where their buoyant densities match the local gradient density. Metallic and semiconducting CNTs separate due to subtle differences in surfactant packing, with semiconducting species typically occupying lower density regions. Further refinement using iterative centrifugation or tailored surfactant mixtures can isolate specific (n,m) species. For instance, single-chirality enrichment of (6,5) CNTs has been achieved with purity exceeding 90%.
Chromatographic methods rely on differential adsorption of CNTs onto stationary phases. In size-exclusion chromatography, larger-diameter CNTs elute faster due to weaker interactions with the porous matrix. More selective separations are achieved via affinity chromatography, where functionalized stationary phases preferentially bind certain chiralities. DNA-wrapped CNTs exhibit sequence-dependent binding affinities, enabling high-resolution separation. For example, poly(T) sequences selectively isolate (6,5) CNTs, while (GT)n repeats target (9,1) species. Gel chromatography using agarose or hydroxypropyl cellulose also resolves CNTs by chirality, with elution order correlating with diameter and electronic structure.
The electronic properties of CNTs are intrinsically linked to chirality. A CNT is metallic if (n-m) is divisible by 3; otherwise, it is semiconducting with a bandgap inversely proportional to diameter. For instance, a (6,5) CNT (diameter ~0.75 nm) exhibits a bandgap of ~1.2 eV, while a (10,5) CNT (~1.0 nm) has a ~0.8 eV gap. These properties enable tailored applications:
- **High-performance transistors**: Semiconducting CNTs with large bandgaps (e.g., (7,5)) provide high on/off ratios (>10^4) and carrier mobilities (>10,000 cm²/V·s), outperforming silicon in ballistic transport regimes.
- **Optoelectronics**: Narrow-bandgap species like (9,1) are ideal for near-infrared photodetectors, while direct bandgaps enable efficient electroluminescence in LEDs.
- **Quantum devices**: Armchair CNTs (e.g., (6,6)) exhibit Luttinger liquid behavior, useful for studying correlated electron phenomena.
Chiral separation remains a bottleneck for scalable CNT electronics, but advances in DGU and chromatography continue to improve yield and purity. Combined with deterministic placement techniques, these methods pave the way for chirality-specific CNT circuits, enabling next-generation nanoelectronics with unmatched performance metrics.