Synthesis and optical properties of WS2 nanotubes with relatively small diameters.

Tungsten disulfide (WS2) nanotubes exhibit various unique properties depending on their structures, such as their diameter and wall number. The development of techniques to prepare WS2 nanotubes with the desired structure is crucial for understanding their basic properties. Notably, the synthesis and characterization of multi-walled WS2 nanotubes with small diameters are challenging. This study reports the synthesis and characterization of small-diameter WS2 nanotubes with an average inner diameter of 6 nm. The optical absorption and photoluminescence (PL) spectra of the as-prepared nanotubes indicate that a decrease in the nanotube diameter induces a red-shift in the PL, suggesting that the band gap narrowed due to a curvature effect, as suggested by theoretical calculations.

Semiconducting Transition Metal Dichalcogenide Heteronanotubes with Controlled Outer-Wall Structures.

Transition metal dichalcogenide (TMDC) nanotubes exhibit unique physical properties due to their nanotube structures. The development of techniques for synthesizing TMDC nanotubes with controlled structures is very important for their science and applications. However, structural control efforts have been made only for the homostructures of TMDC nanotubes and not for their heterostructures that provide an important platform for their two-dimensional counterparts. In this study, we synthesized heterostructures of TMDC nanotubes, MoS2/WS2 heteronanotubes, and demonstrated a technique for controlling features of their structures, such as diameters, layer numbers, and crystallinity. The diameter of the heteronanotubes could be tuned with inner nanotube templates and was reduced by using small-diameter WS2 nanotubes. The layer number and crystallinity of the MoS2 outer wall could be controlled by controlling their precursors and synthesis temperatures, resulting in the formation of high-crystallinity TMDC heteronanotubes with specific chirality. This study can expand the research of van der Waals heterostructures.

Hall effect in gated single-wall carbon nanotube films.

The presence of hopping carriers and grain boundaries can sometimes lead to anomalous carrier types and density overestimation in Hall-effect measurements. Previous Hall-effect studies on carbon nanotube films reported unreasonably large carrier densities without independent assessments of the carrier types and densities. Here, we have systematically investigated the validity of Hall-effect results for a series of metallic, semiconducting, and metal-semiconductor-mixed single-wall carbon nanotube films. With carrier densities controlled through applied gate voltages, we were able to observe the Hall effect both in the n- and p-type regions, detecting opposite signs in the Hall coefficient. By comparing the obtained carrier types and densities against values derived from simultaneous field-effect-transistor measurements, we found that, while the Hall carrier types were always correct, the Hall carrier densities were overestimated by up to four orders of magnitude. This significant overestimation indicates that thin films of one-dimensional SWCNTs are quite different from conventional hopping transport systems.

Control of Thermal Conductance across Vertically Stacked Two-Dimensional van der Waals Materials via Interfacial Engineering.

A comprehensive understanding of the roles of various nanointerfaces in thermal transport is of critical significance but remains challenging. A two-dimensional van der Waals (vdW) heterostructure with tunable interface lattice mismatch provides an ideal platform to explore the correlation between thermal properties and nanointerfaces and achieve controllable tuning of heat flow. Here, we demonstrate that interfacial engineering is an efficient strategy to tune thermal transport via systematic investigation of the thermal conductance (G) across a series of large-area four-layer stacked vdW materials using an improved polyethylene glycol-assisted time-domain thermoreflectance method. Owing to its rich interfacial mismatch and weak interfacial coupling, the vertically stacked MoSe2-MoS2-MoSe2-MoS2 heterostructure demonstrates the lowest G of 1.5 MW m-2 K-1 among all vdW structures. A roadmap to tune G via homointerfacial mismatch, interfacial coupling, and heterointerfacial mismatch is further demonstrated for thermal tuning. Our work reveals the roles of various interfacial effects on heat flow and highlights the importance of the interfacial mismatch and coupling effects in thermal transport. The design principle is also promising for application in other areas, such as the electrical tuning of energy storage and conversion and the thermoelectricity tuning of thermoelectronics.

Thermally activated polymorphic transition from a 1D ribbon to a 2D carpet: squaric acid on Au(111).

Polymorphic transition from the 1D ribbon to the 2D carpet superstructure of squaric acid molecules on Au(111) was achieved through a thermally activated process. Our combined STM and DFT study revealed that the molecular arrangements in 1D and 2D superstructures are determined by the stability of their conformational isomers and assembled structures, respectively.