The paper was published in Science 374, 1616-1620 (2021)
This work was done in collaboration with a number of foreign institutes, with the main experiment carried out at NIMS (Tsukuba, Japan) by Prof. D.M.Tang. Using in situ transmission electron microscopy, we applied heating and mechanical strain to alter the local chirality and thereby control the electronic properties of individual single-wall carbon nanotubes. A transition trend toward a larger chiral angle region was clearly observed (Fig. (c)).
(a) Schematics of a CNT intramolecular transistor with local chirality altered by mechanical strain and Joule heating. (b) TEM image of a SWCNT intramolecular transistor with a channel length of ~2.8 nm. (c) Changes of the chiral angles revealing a converging trend toward large angles. (d) Atomic structure of a (10,7) SWCNT with the basic vectors b1 and b2, the chiral angle χ and misorientation angle α for a 5|8|5 defect. (e) Schematics and formation energies of a pair of dislocations, including the sublimation of a carbon dimer and bond rotation steps. (f) Predicted changes of nanotube chiral angles along chirality transitions.
Generally, plastic deformation and chirality transformations of CNTs have been attributed to the dislocation activities. In theory, two mechanisms have been discussed for the nucleation of dislocations in CNTs, i.e. bond flip driven by stress and thermally activated carbon dimer evaporation. The chirality changes due to gliding of dislocations nucleated from bond flip and associated Stone–Thrower–Wales (STW) defects have been studied in detail to predict a decreasing trend for the chiral angles to near zigzag type chirality due to the energetically favorable (0,1) dislocations, which is opposite to the experimentally observed increasing trend for the chiral angles in this work. As mentioned, a “near critical condition” was used in our experiments so that the CNT was exposed at a high temperature from Joule heating and accompanied with a slow elongation in a quasi-static process. Therefore, we considered the orientation dependent formation energy of the dislocations generated from evaporation of carbon dimers (C2) and associated formation of 5|8|5 defects. The 5|8|5 defects can yield 5|7 dislocation cores responsible for the chirality transitions (Fig. (e)). Calculated formation energy of 5|8|5 defects indicated that at a low chiral angle, the (1,0) dislocation was energetically favored. Therefore, nanotubes with originally small chiral angles tend to increase the chiral angle during the transitions. In a high angle region, the formation energies of 5|8|5 defects associated with (1,0) and (0,1) dislocations become close. This explains the fluctuation behaviors of the chiral angles in the high angle region. The ratio of probabilities of forming (1,0) and (0,1) dislocations was calculated to predict the chirality dynamics (Fig. (f)), showing a converging trend toward larger angles, consistent with the experimental observations. Thus, we have studied a method of local chirality modification that allows to realize metal-semiconductor-metal contact in single-walled carbon nanotubes, i.e., to create an intramolecular nanotube-based transistor.
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