Sorting Transition-Metal Dichalcogenide Nanotubes by Centrifugation

Tungsten disulfide (WS2) nanotubes are cylindrical, multiwall nanotubes with various diameters and wall numbers. They can exhibit various unique properties depending on their structures and thus preparing samples with uniform structures is important for understanding their basic properties and applications. However, most synthesis methods have difficulty to prepare uniform samples, and thus, a purification method to extract nanotubes with a selected diameter and wall number must be developed. Here, we demonstrate a solution-based purification of WS2 nanotubes using a surfactant solution. Stable dispersions of nanotubes were prepared using nonionic surfactants, which enabled us to sort the diameters and wall numbers of the nanotubes through a centrifugation process. By optimizing the conditions, we successfully obtained thin nanotubes with a mean diameter of 32 nm and mean wall number of 13 with relatively small distributions. Finally, we clarified the relationships between the structure and optical properties of the nanotubes

Separations of Metallic and Semiconducting Carbon Nanotubes by Using Sucrose as a Gradient Medium

The separation of metallic and semiconducting single-wall carbon nanotubes (SWCNTs) was achieved using sucrose as a gradient medium in density-gradient ultracentrifugations (sucrose-DGU). By lowering the temperature during sucrose-DGU and tuning the concentrations of the surfactants, metallic and semiconducting SWCNT samples were obtained in high purity. The purity of the metallic and the semiconducting SWCNTs obtained by the sucrose-DGU was estimated to be 69% and 95%, respectively, from their optical absorption spectra. It is well-known that the amounts and types of surfactants significantly influence the separations. However, the authors found that the temperature during centrifugation was also an important parameter that improved the metal−semiconductor separation capability

Influence of Aromatic Environments on the Physical Properties of β-Carotene

The influence of the surrounding aromatic environment on the properties of β-carotene (Car) was investigated for a simple system comprising single-walled carbon nanotubes (SWCNTs) encapsulating Car molecules. Both metallic- and semiconducting-type SWCNTs encapsulating Car were prepared, and their physical properties were investigated using optical measurements and first-principles calculations. The optical absorption peaks of encapsulated Car in metallic and semiconducting SWCNTs were slightly different, which is thought to be caused by the difference in polarizability of the two types of SWCNTs. The Raman frequency of the CC stretching mode of Car in the metallic SWCNTs was 3 cm−1 down-shifted from that in the semiconducting SWCNTs. This down-shift could not be explained by the difference of dielectric environments of the metallic and the semiconducting SWCNTs. One possible origin for the shift is a difference in the amount of charge on the encapsulated Car, which was supported by theoretical calculations. From the results of this study, it can be concluded that the electronic structure of the nanotube walls influences the properties of encapsulated molecules

Thermoelectric Power of a Single van der Waals Interface between Carbon Nanotubes

Control of van der Waals interfaces is crucial for fabrication of nanomaterial-based high-performance thermoelectric devices because such interfaces significantly affect the overall thermoelectric performances of the device due to their relatively high thermal resistance. Such interfaces could induce different thermoelectric power from the bulk, i.e., interfacial thermoelectric power. However, from a macroscopic point of view, a correct evaluation of the interfacial thermoelectric power is difficult owing to various interface configurations. Therefore, the study of the thermoelectric properties at a single interface is crucial to address this problem. Herein, we used in situ transmission electron microscopy and nanomanipulation to investigate the thermoelectric properties of carbon nanotubes and their interfaces. The thermoelectric power of the bridged carbon nanotubes was individually measured. The existence of the interfacial thermoelectric power was determined by systematically changing the contact size between the two parallel nanotubes. The effect of interfacial thermoelectric power was qualitatively supported by Green’s function calculations. When the contact length between two parallel nanotubes was less than approximately 100 nm, the experimental results and theoretical calculations indicated that the interface significantly contributed to the total thermoelectric power. However, when the contact length was longer than approximately 200 nm, the total thermoelectric power converged to the value of a single nanotube. The findings herein provide a basis for investigating thermoelectric devices with controlled van der Waals interfaces and contribute to thermal management in nanoscale devices and electronics

Chiral-Angle Distribution for Separated Single-Walled Carbon Nanotubes

Chiral indices (n,m) of metallic and semiconducting single-walled carbon nanotubes (SWNTs) selectively separated via the density-gradient ultracentrifugation process were individually assigned by using an aberration-corrected transmission electron microscope (TEM) operated at 80 kV. Our statistical analysis revealed that armchair (n,n) and chiral (n,n-3) SWNTs with large chiral angles (>20°) are dominant metallic nanotubes in the separated samples, whereas such a noticeable preference of particular indices was not observed for semiconducting nanotubes. Some significant discrepancies were found between the TEM and spectroscopic results on the major chiral indices and the metal/semiconductor ratios in these SWNTs

Extraction of Linear Carbon Chains Unravels the Role of the Carbon Nanotube Host

Linear carbon chains (LCCs) have been shown to grow inside double-walled carbon nanotubes (DWCNTs), but isolating them from this hosting material represents one of the most challenging tasks toward applications. Herein we report the extraction and separation of LCCs inside single-walled carbon nanotubes (LCCs@SWCNTs) extracted from a double-walled host LCCs@DWCNTs by applying a combined tip-ultrasonic and density gradient ultracentrifugation (DGU) process. High-resolution transmission electron microscopy, optical absorption, and Raman spectroscopy show that not only short LCCs but clearly long LCCs (LLCCs) can be extracted and separated from the host. Moreover, the LLCCs can even be condensed by DGU. The Raman spectral frequency of LCCs remains almost unchanged regardless of the presence of the outer tube of the DWCNTs. This suggests that the major importance of the outer tubes is making the whole synthesis viable. We have also been able to observe the interaction between the LCCs and the inner tubes of DWCNTs, playing a major role in modifying the optical properties of LCCs. Our extraction method suggests the possibility toward the complete isolation of LCCs from CNTs

Chirality-Dependent Combustion of Single-Walled Carbon Nanotubes

The chirality-dependent combustion of single-walled carbon nanotubes (SWCNTs) during oxidation in both air and hydrogen peroxide was investigated using photoluminescence and Raman spectroscopy. Under air oxidation conditions, SWCNTs with a higher chiral angle and smaller diameter were observed to decompose more rapidly. The decomposition rate of each chiral index was determined from the reaction rate analysis, and it was found that the reaction is governed by a “local curvature radius” along the C−C bond. The reaction barriers for breaking the C−C bonds after cycloaddition with oxygen molecules were obtained for 10 types of SWCNTs using first-principles calculations. The barrier heights were found to depend on the local curvature radius, which showed good agreement with the experimental results. On the other hand, for the oxidation reaction in hydrogen peroxide, oxygen radicals decomposed the smaller-radius SWCNTs more rapidly without any chirality selection. As a result, two different chirality distributions for SWCNTs with similar diameters were obtained by these oxidation processes

Control of high-harmonic generation by tuning the electronic structure and carrier injection

High-harmonic generation (HHG), which is generation of multiple optical harmonic light, is an unconventional nonlinear optical phenomenon beyond perturbation regime. HHG, which was initially observed in gaseous media, has recently been demonstrated in solid state materials. How to control the extreme nonlinear optical phenomena is a challenging subject. Compared to atomic gases, solid state materials have advantages in controlling electronic structures and carrier injection. Here, we demonstrate control of HHG by tuning electronic structure and carrier injection using single-walled carbon nanotubes (SWCNTs). We reveal systematic changes in the high-harmonic spectra of SWCNTs with a series of electronic structures from a metal to a semiconductor. We demonstrate enhancement or reduction of harmonic generation by more than one order of magnitude by tuning electron and hole injection into the semiconductor SWCNTs through electrolyte gating. These results open a way to control HHG within the concept of field effect transistor devices

Heat and Charge Carrier Flow through Single-Walled Carbon Nanotube Films in Vertical Electrolyte-Gated Transistors: Implications for Thermoelectric Energy Conversion

Understanding the relationship between electrical and thermal characteristics of thin films is crucial for their thermal management. However, there are hardly any reliable experimental data on this relationship because measurement methods that can evaluate both characteristics in the same direction are very limited. Here, we report a measurement technique that can simultaneously evaluate electrical and thermal characteristics by combining time-domain thermoreflectance and vertical electrolyte-gated transistors. We demonstrate that the thermal conductivity of single-walled carbon nanotube films is independent of the vertical current density, which is modulated over 4 orders of magnitude, indicating a route to improve thermoelectric conversion efficiencies

Control of Thermal Conductance across Vertically Stacked Two-Dimensional van der Waals Materials <i>via</i> 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