We have developed a process for chemical purification of carbon nanotubes for solution-processable thin-film transistors (TFTs) having high mobility. Films of the purified carbon nanotubes fabricated by simple drop coating showed carrier mobilities as high as 164 cm²V⁻¹s⁻¹, normalized transconductances of 0.78 Sm⁻¹, and on/off current ratios of 10⁶. Such high performance requires the preparation of a suspension of micrometer-long and highly purified semiconducting single-walled carbon nanotubes (SWCNTs). Our purification process includes length and electronic-type selective trapping of SWCNTs using recycling gel filtration with a mixture of surfactants. The results provide an important milestone toward printed high-speed and large-area electronics with roll-to-roll and ink-jet device fabrication. Open image in new window

Progress on researches of two-dimensional (2D) metals strongly relies on development of the growth technique. Studies on preparation of 2D metals have so far been limited, and this is in stark contrast to the situation of 2D semiconductors, where various layered semiconductors, including MoS ₂ , WS ₂ , MoSe ₂ , WSe ₂ , have been isolated in its monolayer form. In this work, we have developed a facile method to prepare 2D metallic transition metal dichalcogenides (TMDCs) by chemical vapor deposition (CVD) method, where direct growth of few-layered NbS ₂ (3R phase) on atomically flat hexagonal boron nitride (hBN) has been demonstrated. Structural characterization of the so-grown NbS ₂ was performed with atomic force microscopy, optical microscopy, electron microscopy and optical spectroscopy, revealing that the utilization of hBN as growth substrates is a key factor for the first successful CVD growth of 2D metallic TMDCs with large single-domain size (several μ m). Electrical transport measurements have clearly shown that NbS ₂ atomic layers down to few-layer-thickness are metal. The current study opens up a new synthetic route for controllable growth of 2D layered metallic materials, which is of great importance in study of rich physics in 2D metals, as well as in search for novel 2D superconductors.

Carbon nanotubes have long been described as rolled-up graphene sheets. It is only fairly recently observed that longitudinal cleavage of carbon nanotubes, using chemical, catalytical and electrical approaches, unzips them into thin graphene strips of various widths, the so-called graphene nanoribbons. In contrast, rolling up these flimsy ribbons into tubes in a real experiment has not been possible. Theoretical studies conducted by Kit et al. recently demonstrated the tube formation through twisting of graphene nanoribbon, an idea very different from the rolling-up postulation. Here we report the first experimental evidence of a thermally induced self-intertwining of graphene nanoribbons for the preferential synthesis of (7, 2) and (8, 1) tubes within parent-tube templates. Through the tailoring of ribbon’s width and edge, the present finding adds a radically new aspect to the understanding of carbon nanotube formation, shedding much light on not only the future chirality tuning, but also contemporary nanomaterials engineering.