Graphene nanoribbons (GNR) are one of the most promising candidates for the fabrication of graphene-based nanoelectronic devices such as high mobility field effect transistors (FET). In this presentation, we report a novel high-yield fabrication method of high quality another type of GNR, fully flattened carbon nanotubes (flattened CNTs), using solution-phase extraction of inner tubes from large-diameter multi-wall CNTs (MWNTs). In addition, we also report that the flattened CNTs can be transformed into multi-layer GNR by etching edge of flattened CNTs.
Our previous study shows that inner CNTs can be efficiently extracted from double-wall CNTs by vigorous sonication with water containing surfactants. We have applied this technique to extract 1-3 layers CNTs of large diameter from arc-grown MWNTs to fabricate of fully flattened CNTs; the extracted large-diameter CNTs spontaneously collapse to form fully flattened CNTs. The extraction process efficiently works, and we found that approximately 80 % of MWCNTs provide flattened CNTs of high quality and purity. Low-magnification transmission electron microscopy (TEM) observations of the flattened CNTs show that flattened CNTs have with of typically 30 nm. The observed structure of the flattened CNTs is very similar to GNR. Detailed TEM observations using aberration corrected TEM and electron diffraction measurements shows that flattened CNTs have ribbon like structure with barbell-like cross section. In addition, edge of the flattened CNTs can be etched to produce multilayer GNR with real edges by hydrogen plasma treatment. Formation of real edge was also characterized by aberration corrected TEM, which shows that smooth straight edge of flattened CNTs was converted to disordered edge.
Measurements of the low-bias conductance of isolated flattened CNTs as a function of gate voltage shows that the flattened CNTs display ambipolar conduction. Estimated band gap based on temperature dependence of conductivity measurements of isolated flattened CNTs was 10 ~ 15 meV, which is consistent with the observed FET characteristics. We think that interlayer interaction plays an important role in opening band gap of flattened CNTs.
In conclusion, we have successfully fabricated novel nanocarbon materials, flattened CNTs. The flattened CNTs show ambipolar FET characteristics, which indicates that the flattened CNTs have band gap. In addition, flattened CNTs can be converted to GNR, which can be novel production method of GNR.