Molecular Junctions by Joining Single-Walled Carbon Nanotubes

Crossing single-walled carbon nanotubes can be joined by electron beam welding to form molecular junctions. Stable junctions of various geometries are created in situ in a transmission electron microscope. Electron beam exposure at high temperatures induces structural defects which promote the joining of tubes via cross-linking of dangling bonds. The observations are supported by molecular dynamics simulations which show that the creation of vacancies and interstitials induces the formation of junctions involving seven- or eight-membered carbon rings at the surface between the tubes.

Figure 1

(a) A SWNT of ca. 2.0 nm diam (running from bottom-left diagonally towards top right) crossing with an individual SWNT of ca. 0.9 nm diam. (b) 60 sec of electron irradiation promotes a molecular connection between the thin and the wide tube, forming an “X” junction. Schematics show that this junction is twisted out of the plane. Molecular models of each image are provided; heptagonal rings are indicated in red.

Figure 2

High-resolution transmission electron microscopy image and molecular model of a Y junction created following electron irradiation of an “X” structure. One of the arms of the “X” junction vanished due to continuous sputtering under the electron beam, and a three-terminal junction remained. The junction exhibits tubes of different diameters which are molecularly joint.

Figure 3

(a)–(c) HRTEM images of a “T”-like” junction formed after irradiating a preformed Y junction. This junction may not be a perfect “T” (90° angle between the crossing tubes) because we observe the projection of the object. The sequence shows the motion of this junction and the rotation by 180° under the electron beam; note the circular cross section in one of the tubes (b). Atomic models of the junctions are depicted below.

Figure 4

Sequences of merging between two crossing (8,8) carbon nanotubes (diameter ca. 1.1 nm) into a unique X-like junction. (a) The TBMD simulation starts with the random creation of 20 vacancies in the lattices of two tubes in the localized neighboring region (top view). (b) After 10 ps, two links between the two defective carbon structures are formed via carbon chains (side view). (c) After 100 ps, the connection between the two tubes is established, although some $s{p}^3$-carbon atoms (red) and dangling bonds (green) are still remaining. (d) After 220 ps, surface reconstruction occurs and the carbon system approaches an X junction. The reconstructed surface contains six heptagons, one octagon, one pentagon, and two dangling bonds.