Tunable intertube spacing in single-walled carbon nanotube bundles

The structure of ternary compounds involving alkali, tetrahydrofuran (THF) and single-walled carbon nanotubes have been investigated using neutron diffraction (ND). Hydrogen-deuterium substitution in THF, as well as the study of different alkali-based compounds, allow a layered structure around the nanotubes to be determined. ND results indicate that the alkali cations form a monolayer surrounding each tube of the bundle, while THF molecules intercalate between the decorated tubes and at the surface of the bundles. In spite of this insertion, the triangular bundle structure is preserved, albeit with a much larger lattice parameter, which depends on the size of the inserted cation.

Figure 1

Neutron diffraction patterns of pristine SWNT (bottom), ${\mathrm{LiC}}_{5.88}$-TDF (middle), and ${\mathrm{LiC}}_{5.88}$-THF (top). The upper line of each pair of plain lines is the experimental ND pattern, while the lower line is the result of a calculation based on the structural parameters described in the text. The dotted line is the background (power law) added to the calculation of the ND pattern of the pristine SWNT sample in order to reproduce the small angle scattering coming from impurities. The dashed line is a calculation of the ND pattern of the pristine sample without the background.

Figure 2

(Color online) Schematic representations of the structure of a ${\mathrm{LiC}}_6$-TDF bundle of seven tubes [atomically resolved (a)] and approximated by homogeneous densities of scatterers (b). (c) Decomposition of a ${\mathrm{LiC}}_6$-TDF bundle of 19 tubes into two contributions: from carbon nanotubes and lithium shells, and from the TDF structure.

Figure 3

Calculated ND patterns of the constitutive objects of a compound of stochiometry ${\mathrm{LiC}}_{5.88}$-THF (left part), and ${\mathrm{LiC}}_{5.88}$-TDF (right part): total intensity (black bold line), intensity of the carbon nanotubes and the lithium shells (gray thin line), THF or TDF (dotted line), and the crossed interference term (dashed line).

Figure 4

(Color online) Left part: Structure of the ${\mathrm{KC}}_{5.88}$-THF (top) and the ${\mathrm{LiC}}_{5.88}$-THF (bottom) compound after energy minimization. Right part: Experimental ND patterns of the ${\mathrm{KC}}_{5.88}$-THF sample (upper line) and the ${\mathrm{LiC}}_{5.88}$-THF (lower dotted line, shown for comparison). The bold line is a calculated ND pattern of the ${\mathrm{KC}}_{5.88}$ sample involving the structural parameters described in the text.