We applied a noncontacting surface acoustic wave technique to measure the conductivity of aligned multiwall carbon nanotube films. By changing intensities of the parallel and perpendicular electric field components at the surface of the film sample, we observed different temperature dependencies of the conductivity. It was found that the temperature dependence of the conductivity parallel to the tube axis is comparable to that of a single-wall carbon nanotube, namely, there is a metallic–nonmetallic transition at temperature of 280 K. On the other hand, the temperature dependence of the conductivity perpendicular to the tube axis is similar to that of a graphene sheet.
The graphene/LiNbO3 structure exists in an interfacial stress-free state at the temperature at which the graphene was transferred onto the LiNbO3 substrate surface. Coupling of a surface acoustic wave with this structure revealed drastic changes in the properties of the propagating elastic wave around the critical temperature of the stress-free state. Three states, namely, tensile stress, stress-free, and compressive stress, were successively observed at the surface of the LiNbO3 substrate as the temperature was increased through the critical point. The interfacial stress increased prior to the occurrence of sliding friction and approached a constant value when the frictional force exceeded the van der Waals interaction between the graphene and LiNbO3. Consequently, the interfacial stress exhibited a step-like temperature dependence around the critical temperature of the stress-free state. The results obtained in this study indicate that the temperature used to prepare graphene layers on a substrate is a crucial parameter owing to the instability of the electrical and mechanical properties of the graphene/substrate in the vicinity of this temperature. Therefore, in the fabrication of graphene-based electronic devices, room temperature should be avoided during the preparation of the graphene layers on the substrate.
The dielectric loss in C60 films was studied by a noncontacting technique utilizing the external electric fields associated with surface acoustic waves (SAW) on a piezoelectric crystal. A sharp increase in loss was observed at temperatures below 220 K together with other structure not found with standard SAW measurements. We believe that these features are due to induced current in C60, causing joule loss, and to the formation of localized dipole moments by charge transfer between adsorbed O2 and C60 molecules, giving rise to thermally activated relaxation.
Electrical transport properties of the nanocrystalline Er 3 N@C 80 with fcc crystal structure were characterized by measuring both temperature-dependent d.c. conductance and a.c. impedance. The results showed that the Er 3 N@C 80 sample has characteristics of n-type semiconductor and an electron affinity larger than work function of gold metal. The Er 3 N@C 80 /Au interface has an ohmic contact behavior and the contact resistance was very small as compared with bulk resistance of the Er 3 N@C 80 sample. The charge carriers in the sample were thermally excited from various trapped levels and both acoustic phonon and ionic scatterings become a dominant process in different temperature regions, respectively. At temperatures below 250 K, the activation energy of the trapped carrier was estimated to be 35.5 meV, and the ionic scattering was a dominant mechanism. On the other hand, at temperatures above 350 K, the activation energy was reduced to 15.9 meV, and the acoustic phonon scattering was a dominant mechanism. In addition, a polarization effect from the charge carrier was observed at low frequencies below 2.0 MHz, and the relative intrinsic permittivity of the Er 3 N@C 80 nanocrystalline lattice was estimated to be 4.6 at frequency of 5.0 MHz.
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