In both double-walled carbon nanotube field-effect transistors (DWCNT-FETs) and single-walled carbon nanotube field-effect transistors (SWCNT-FETs), “clockwise” hysteretic transfer characteristics are observed. These characteristics can be attributed to mobile ions or charged clusters in surfactant layers around the tube channels. Compared with SWCNT-FETs, more significant surfactant influences on the transfer characteristics are observed in DWCNT-FETs due to superior surfactant adsorption properties of double-walled carbon nanotubes. Our findings suggest that not only water molecules but also the surfactants play critical roles in dominating the hystereses, especially when the carbon nanotubes are double walled.

In situ Raman scattering is performed on an individual semiconducting double-walled carbon nanotube (DWNT) in a field-effect transistor (FET) geometry, while the transfer characteristics of the DWNT-FET are measured. Through studying the Raman spectra with response to forward and backward gate voltage (V(gs)) sweeping, respectively, we observe hysteresis loops in the curves of G(-) peak frequency and the intensity ratio of G(-) to G(+) (I(G(-))/I(G(+))) as a function of V(gs). These loops correlate very well with the hysteretic transfer characteristics of the device. The clear correlations suggest that G(-) peak line width and I(G(-))/I(G(+)) increase with the carrier concentration in the DWNT induced by V(gs). In addition, unique G(-) peak line width variations with V(gs) can be attributed to interband electron transitions between the energy bands of two concentric shells of the DWNT excited by G phonons.

The current instability of carbon nanotube field effect transistors (CNTFETs) is systematically studied under the influence of applied voltages, surfactants and temperatures. The devices were fabricated from carbon nanotubes and sodium dodecyl benzene sulfonate (SDBS) suspension using an ac dielectrophoresis (DEP) technique. The source and drain current for as-prepared p-type CNTFETs show an increase with time for the on-state, but a decrease for the off-state. Comparisons between constant and intermittent biasing conditions reveal that mobile ions could be the origin of the current instability. After removal of adsorbed SDBS, opposite transient behaviors of the current were observed, which can be attributed to the charge trapping induced screening effect.

Abnormal Raman scattering from a large-diameter ultralong single-walled carbon nanotube (SWCNT) is studied in detail. Along the SWCNT, the Raman spectra show the frequencies of 1553, 1563, and 2597 cm(-1) for G(+), G(-), and G' peaks, respectively, much lower than the corresponding frequencies well-reported both experimentally and theoretically. The significant downshifts in the peaks frequencies can be attributed to self-built tensile strain, which is likely caused by carbon nanodots decorated on the tube. After infrared laser heating is performed to one point of it, all of the Raman modes are found to shift to higher frequencies and approach their conventional values. We suggest that the SWCNTs with larger diameters easily possess such self-built strain compared to small-diameter SWCNTs because of the weaker curvature effect for the larger ones.

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