Masato Ohnishi

Modulation of thermal and thermoelectric transport in individual carbon nanotubes by fullerene encapsulation

The potential impact of encapsulated molecules on the thermal properties of individual carbon nanotubes (CNTs) has been an important open question since the first reports of the strong modulation of electrical properties in 2002. However, thermal property modulation has not been demonstrated experimentally because of the difficulty of realizing CNT-encapsulated molecules as part of thermal transport microstructures. Here we develop a nanofabrication strategy that enables measurement of the impact of encapsulation on the thermal conductivity (κ) and thermopower (S) of single CNT bundles that encapsulate C 60, Gd@C 82 and Er 2@C 82. Encapsulation causes 35-55% suppression in κ and approximately 40% enhancement in S compared with the properties of hollow CNTs at room temperature. Measurements of temperature dependence from 40 to 320 K demonstrate a shift of the peak in the κ to lower temperature. The data are consistent with simulations accounting for the interaction between CNTs and encapsulated fullerenes.

Effects of defects on thermoelectric properties of carbon nanotubes

Carbon nanotubes (CNTs) have recently attracted attention as materials for flexible thermoelectric devices. To provide theoretical guideline of how defects influence the thermoelectric performance of CNTs, we theoretically studied the effects of defects (vacancies and Stone-Wales defects) on its thermoelectric properties; thermal conductance, electrical conductance, and Seebeck coefficient. The results revealed that the defects mostly strongly suppresses the electron conductance, and deteriorates the thermoelectric performance of a CNT. By plugging in the results and the intertube-junction properties into the network model, we further show that the defects with realistic concentrations can significantly degrade the thermoelectric performance of CNT-based networks. Our findings indicate the importance of the purification of CNTs for improving CNT-based thermoelectrics.

Effects of uniaxial compressive strain on the electronic-transport properties of zigzag carbon nanotubes

Recently, substantial attention has been paid to the strain sensitivity of the carbon nanotubes’ (CNTs’) electronic properties. In this study, the relationships between the geometric structures and electronic states of zigzag CNTs under uniaxial compressive strain were investigated. We found that different factors dominate the electronic states of zigzag CNTs depending on the strain regions: the initial stage of the strain loading, which lasts until column-buckling deformation begins, and the strain regions corresponding to column- and shell-buckling deformations. Because shell-buckling deformation significantly increases the π-orbital angle, the angle between the ρ orbital axis vectors of adjacent atoms, strong localization of the density of states (LDOS) occurs in the buckled area. We also analyzed the current able to pass through deformed CNTs using a tight-binding-based Green’s function approach and determined that the current can be significantly suppressed by applying uniaxial compressive strain. Our method of predicting the electronic state of a deformed CNT based on the π-orbital angle is expected to be useful for predicting the electronic properties of CNT-based electronic devices and sensors.

Electronic properties and strain sensitivity of CVD-grown graphene with acetylene

Although many studies have shown that large-area monolayer graphene can be formed by chemical vapor deposition (CVD) using methane gas, the growth of monolayer graphene using highly reactive acetylene gas remains a big challenge. In this study, we synthesized a uniform monolayer graphene film by low-pressure CVD (LPCVD) with acetylene gas. On the base of Raman spectroscopy measurements, it was found that up to 95% of the as-grown graphene is monolayer. The electronic properties and strain sensitivity of the LPCVD-grown graphene with acetylene were also evaluated by testing the fabricated field-effect transistors (FETs) and strain sensors. The derived carrier mobility and gauge factor are 862–1150 cm2/(Vs) and 3.4, respectively, revealing the potential for high-speed FETs and strain sensor applications. We also investigated the relationship between the electronic properties and the graphene domain size.

Two-Photon Absorption in Tin Dioxide Single Crystal at 532 nm

Single crystals of tin dioxide synthesized by a hydrolysis reaction of stannic chloride are submitted to open-aperture Z-scan measurements at 532 nm. The two-photon absorption coefficient of the crystal is determined-to be 7.6±1.3 cm/GW. The value is larger by about three times than that predicted theoretically. One of the reasons for the discrepancy is given as the applicable limit of the theory in which a two-band model is assumed in the analysis.

Quantitative evaluation of orbital hybridization in carbon nanotubes under radial deformation using π-orbital axis vector

When a radial strain is applied to a carbon nanotube (CNT), the increase in local curvature induces orbital hybridization. The effect of the curvature-induced orbital hybridization on the electronic properties of CNTs, however, has not been evaluated quantitatively. In this study, the strength of orbital hybridization in CNTs under homogeneous radial strain was evaluated quantitatively. Our analyses revealed the detailed procedure of the change in electronic structure of CNTs. In addition, the dihedral angle, the angle between π-orbital axis vectors of adjacent atoms, was found to effectively predict the strength of local orbital hybridization in deformed CNTs.

Change in Spatial Distribution of State Densities of Carbon Nanotubes Under Anisotropic Strain Field

In any electronic devices and sensors, unexpected changes in their function occur due to internal strain caused by contact of different materials which leads to thermal deformation or lattice mismatch. Thus, understanding of the effect of strain on electronic properties of carbon nanotubes (CNTs) is indispensable for assuring the reliability of CNTs-based electronic devices and for developing new electronic devices and sensors. In this study, the change in spatial distribution of state densities of zigzag CNTs under radial strain is analyzed by using first-principles calculation. The analysis shows that when a radial strain is applied to a CNT, its state densities are localized at high curvature regions. Such localization of state densities decrease their energies, and then decrease the band gap. In addition, since the behavior of the state energy under the radial strain is dominated by its spatial distribution, the strain sensitivity of CNTs depends on their chirality. The founding gives a guideline on how to fabricate high-performance novel CNTs devices, sensors, for example, biaxial strain sensor.© 2014 ASME

Change in electronic properties of carbon nanotubes caused by local distortion under axial compressive strain

In this study, the change in electronic properties of carbon nanotubes (CNTs) under axial compressive strain was analyzed by using Greens function method based on tight-binding approach. A single (9, 0) CNT structure was used for the calculation. The column buckling of the CNT was occurred and a kink was formed in the buckled CNT with increasing compressive strain. After the formation of a kink, the local density of state (LDOS) at the energy higher than 0.3 eV was strongly localized in the buckled region. The conductance of the CNT with a kink was suppressed by the scattering of electrons due to the localization of LDOS. Therefore, electronic properties of the CNT changes drastically by buckling deformation when CNTs are subject to combined bending and axial compression.

Effect of Three Dimensional Strain on the Electronic Properties of Graphene Nanoribbons

Graphene has great potential for ultra-sensitive strain sensors applications due to its high mechanical strength and good compatibility with the traditional semiconductor process. In the current study, we investigated the effect of tensile and bending deformations on the electronic states of graphene nanoribbons (GNRs) using density functional theory (DFT) to clarify the underlying mechanism of the piezoresistive properties of graphene. It is found that the electronic structure of armchair graphene nanoribbons (AGNRs) is very sensitive to the tensile deformation. When a uniaxial tensile stress is applied to AGNRs with width Na = 10, the band structure is modified, leading to the change in band gap approximately from 0 eV to 1.0 eV. The band gap values of bent AGNRs decrease significantly when the maximum local dihedral angle exceeds a critical value due to the orbital hybridization. Based on these knowledge, we fabricated a strain sensor using the graphene film grown by thermal chemical vapor deposition (CVD) method on Cu foil. The strain sensor is fabricated directly on the graphene-coated Cu foils by using the standard photolithography process and reactive ion etching (RIE) and then transferred onto a stretchable and flexible polydimethysiloxane (PDMS) substrate. The one-dimensional tensile test and three-dimensional bending test are performed to investigate the piezoresistive properties. A gauge factor 3.4 was achieved under the tensile deformation. The fabricated strain sensor also exhibits good performance to detect bending deformation.Copyright

Characterization of the Electronic Properties and Strain Sensitivity of Graphene Formed by C2H2Chemical Vapor Deposition

We succeed in synthesizing large-area single-layer graphene sheets with different grain size using C2H2 chemical vapor deposition process. Our graphene shows high uniformity and low sheet resistance to 1080Ω/□. By fabricating graphene-based field effect transistors (FETs), the relation between the nucleation density and the electronic properties of CVD grpahene are investigated. We found that the nucleation density can severely affect the defects formation in graphene, leading to the change in the electronic properties of graphene. We also check the strain sensitivity of CVD graphene. The as-grown graphene/Cu film was fixed onto the SiO2/Si substrate with a double-sided tape. The strain device is fabricated directly on the graphene-coated Cu foils by using the standard photolithography and reactive ion etching (RIE) process. Then the device is transferred onto a stretchable and flexible polydimethysiloxane (PDMS) substrate. By using a motorized stage, the tensile test is performed to investigate the piezoresistive properties of graphene-based strain sensors. The one-dimensional tensile test is performed to investigate the piezoresistive properties. A gauge factor 3.4 was achieved under the tensile deformation.Copyright