Smallest Carbon Nanotube Is $3\mathrm{\AA }$ in Diameter

Previous energetic considerations have led to the belief that carbon nanotubes (CNTs) of $4\AA$ in diameter are the smallest stable CNTs. Using high-resolution transmission electron microscopy, we find that a stable $3\AA$ CNT can be grown inside a multiwalled carbon nanotube. Density functional calculations indicate that the $3\AA$ CNT is the armchair $\mathrm{CNT}(2,2)$ with a radial breathing mode at $787{\mathrm{cm}}^{-1}$. Each end can be capped by half of a ${\mathrm{C}}_{12}$ cage (hexagonal prism) containing tetragons.

Figure 1

HRTEM image of $3\AA$ CNT formed inside MWNT. The diameter of $3\mathrm{\AA }$ CNT and the interlayer space of MWNT, $3.4\mathrm{\AA }$, are indicated by a pair of lines marked by arrows. Each end is marked by two horizontal arrows, $A$ and $A^{\prime }$, and is capped by half of a ${\mathrm{C}}_{12}$ cage containing two tetragons (see the density functional calculations in text).

Figure 2

Diameter of $3\AA$ CNT, and the diameters of CNTs in MWNT plotted as a function of the number of CNTs in MWNT. Diameters of $3\AA$ CNT, the innermost tube in MWNT, and the second CNT in MWNT are shown in the upper left inset. The lower right inset is an image contrast profile across the MWNT at the position marked by two vertical arrows, $B$ and $B^{\prime }$, in Fig. 1.

Figure 3

Supercell models of isolated $\mathrm{CNT}(2,2)$ and graphene strip. (a) “Open-ended” model with total energy of $0.29\mathrm{eV}/\mathrm{\text{atom}}$. (b) “Closed” model, $0.24\mathrm{eV}/\mathrm{\text{atom}}$. (c) “Triangle cap-ended” model, $0.23\mathrm{eV}/\mathrm{\text{atom}}$. (d) “Square cap-ended” model, $0.17\mathrm{eV}/\mathrm{\text{atom}}$. (e) Graphene strip obtained by unrolling $\mathrm{CNT}(2,2)$, $0.0\mathrm{eV}/\mathrm{\text{atom}}$ as reference.