Carbon nanohorns are a unique class of carbon-based nanomaterials characterized by their horn-like tubular structures terminating in conical tips. These structures typically aggregate into spherical assemblies, forming dahlia-like or bud-like morphologies. The electronic and thermal properties of carbon nanohorns are influenced by their structural features, including horn length, diameter, cone angle, and aggregation state. These properties make them distinct from other carbon nanomaterials like graphene and carbon nanotubes while sharing some similarities due to their sp²-hybridized carbon networks.

The electronic properties of carbon nanohorns are primarily determined by their conical geometry and the presence of pentagonal defects at the tips. Unlike carbon nanotubes, which exhibit metallic or semiconducting behavior based on chirality, carbon nanohorns generally display semiconducting characteristics due to the curvature-induced disruption of the π-electron system. The bandgap of single-walled carbon nanohorns typically ranges between 0.2 to 0.5 eV, depending on the cone angle and tip structure. Larger cone angles introduce greater curvature, leading to increased bandgap values. Aggregation further modifies electronic behavior by introducing inter-horn interactions, which can create localized electronic states and alter charge transport mechanisms.

Electrical conductivity in carbon nanohorns is anisotropic, with higher conductivity along the tubular axis compared to radial directions. The presence of defects, such as pentagons at the tips and possible heptagonal defects along the walls, scatters charge carriers, reducing overall conductivity compared to defect-free graphene or carbon nanotubes. However, post-synthesis treatments like oxidation or doping can enhance conductivity by introducing additional charge carriers or creating conductive pathways between aggregated nanohorns. For instance, nitrogen doping has been shown to increase the conductivity of carbon nanohorns by shifting the Fermi level into the conduction band.

Thermal properties of carbon nanohorns are equally noteworthy. The thermal conductivity of individual nanohorns is high along the tubular axis, similar to carbon nanotubes, due to the strong sp² carbon bonds and efficient phonon transport. However, thermal conductivity values are generally lower than those of graphene or single-walled carbon nanotubes due to increased phonon scattering at the conical tips and defect sites. Measurements indicate that the thermal conductivity of carbon nanohorn assemblies ranges between 500 to 1000 W/m·K, significantly lower than the 3000 to 5000 W/m·K observed in graphene. Aggregation further reduces effective thermal conductivity due to increased interfacial thermal resistance between individual nanohorns.

Heat dissipation in carbon nanohorns is influenced by their high surface area and porous structure. The aggregated assemblies provide numerous pathways for heat transfer through phonon propagation and radiation, but the interfaces between nanohorns act as scattering centers, limiting overall heat dissipation efficiency. This makes them less effective than graphene for thermal management applications requiring isotropic heat spreading but suitable for applications where localized heat dissipation or thermal insulation is desired. The presence of interstitial spaces in nanohorn aggregates also contributes to reduced thermal conductivity by introducing air gaps that impede phonon transport.

Structural variations play a critical role in modulating these properties. Horn length directly affects electronic transport, with longer nanohorns exhibiting reduced resistance due to fewer interfacial scattering events per unit length. Conversely, shorter nanohorns display higher resistance because charge carriers encounter more tip-related defects. The cone angle, typically between 20° to 40°, influences both electronic and thermal properties. Smaller cone angles result in less curvature-induced strain, leading to bandgap reduction and improved thermal conductivity. Larger cone angles increase strain, widening the bandgap and enhancing phonon scattering.

Aggregation state is another key factor. Isolated nanohorns exhibit superior electronic and thermal transport compared to aggregated forms. Dahlia-like aggregates, where nanohorns radiate outward from a central core, show anisotropic thermal and electrical conductivity, with higher values along the radial direction. Bud-like aggregates, with more disordered packing, exhibit isotropic but lower overall conductivity due to increased inter-horn contact resistance. The density of aggregates also matters; loosely packed assemblies have lower thermal conductivity than densely packed ones due to greater interfacial resistance.

Comparisons with other carbon nanomaterials highlight the unique position of carbon nanohorns. Graphene exhibits superior electronic mobility and thermal conductivity due to its extended planar structure and minimal defects. Carbon nanotubes, depending on chirality, can outperform nanohorns in conductivity but lack the high surface area and porous aggregation of nanohorn assemblies. The electronic properties of carbon nanohorns are more tunable than those of graphene due to the ability to modify tip structure and aggregation, but they generally cannot match the extreme conductivity values of defect-free graphene or metallic carbon nanotubes.

In thermal applications, carbon nanohorns are less efficient than graphene or aligned carbon nanotube arrays but offer advantages in specific scenarios. Their porous aggregates make them suitable for thermoelectric applications where low thermal conductivity is desirable to maintain a high temperature gradient. The combination of moderate thermal conductivity and high surface area also makes them useful in heat storage or insulation applications where graphene would be overly conductive.

The electronic and thermal properties of carbon nanohorns can be further tailored through chemical functionalization. Covalent attachment of functional groups to the nanohorn tips or sidewalls can modify bandgap and conductivity. Oxygen-containing groups, for example, introduce localized states near the Fermi level, reducing conductivity but enabling tunable electronic behavior for sensor applications. Non-covalent functionalization with polymers or surfactants preserves the intrinsic properties while improving dispersion and interfacial contact in composite materials.

In summary, carbon nanohorns exhibit a unique combination of electronic and thermal properties shaped by their conical geometry and aggregation behavior. Their semiconducting nature, moderate conductivity, and tunable bandgap distinguish them from other carbon nanomaterials, while their thermal properties strike a balance between high conductivity and porous insulation. Structural variations such as horn length, cone angle, and aggregation state provide additional levers to optimize these properties for specific applications, from electronics to thermal management. While they may not surpass graphene or carbon nanotubes in individual performance metrics, their structural versatility and aggregation-dependent behavior make them a compelling material for specialized uses where their unique properties can be fully exploited.