[en] Highlights: • Self-contraction of silk fibroin induces localized densification of carbon nanotubes. • Carbon nanotubes can be densified at and below the micrometer scale. • The tensile strength of CNT fibers and narrow ribbons are increased remarkably. High densification of carbon nanotubes (CNTs) is important for high utilization efficiency of their superior properties in macroscopic assemblies. However, the conventional “top-down” compressing strategies have met problems to modify CNT assemblies at and below the micrometer scale. Here we report a molecular way to strap CNTs together via the self-contraction of silk fibroin (SF) during its drying process, resulting in a localized densification below the micrometer scale. Importantly, after the thermal removal of SF molecules, the densified assembly was well maintained. The SF-induced densification increased the average strength from 355 MPa to 960 MPa for CNT fibers, and from 1.45 GPa to 1.82 GPa for CNT ribbons, which contain much more CNTs on the surface.

[en] Highlights: • The surface of CNT fiber acts as a shield and hinders the densification of interior. • The untwisted CNT strip is fully densified and shows size-invariant tensile strength. • The untwisted CNT strip is stiffer and stronger than the twisted CNT fiber. The way how carbon nanotubes (CNTs) are assembled together determines the utilization efficiency of mechanical property in their macroscopic assembly materials. For one-dimensional (1D) assemblies, CNTs are often assembled under drawing and twisting into a fiber structure with a twist angle. In this study, an untwisted 1D assembly, CNT strip, is introduced, inside which the CNTs are not only aligned but also overall parallel to strip axis. Due to the shielding effect of the twisted fiber surface, the interior of CNT fiber is loosely packed, and the fiber is more stretchable and hard to become stiff. On the contrary, CNT strip with high straightness or undirectionality can utilize the CNT's mechanical property much more efficiently, as reflected by its higher strength and modulus. These insights can guide different applications of CNT fibers and strips in textile.

[en] Highlights: • Bromine infiltration increases the electrical conductivity of carbon nanotube fiber. • Interfacial electron transfer from carbon to bromine makes carbon nanotubes p-doped. • Bromine-infiltrated carbon nanotube fiber can be used as a functional wire. Highly conducting and lightweight wires can be created based on carbon nanotube (CNT) assembly materials, where the inter-tube electron transport plays a key role. Here we report a gas infiltration of bromine to improve the electrical properties of CNT fibers, owing to the enhanced inter-tube electron hopping. Although Br infiltration is mainly a physical absorption process, with just a little fraction to form covalent bonding with the defect of CNTs, it can induce electron transfer from CNT to Br. Together with the densification effect, there are more pathways for interfacial electron transport between the CNTs. As a result, the infiltration efficiently increases the electrical conductivity from 2.66 × 10⁵ to 1.63 × 10⁶ S/m, by 6-fold. Based on the high performance stability, the composite fiber can be used as lightweight and functional conductor for wearable smart devices.