Rubidium localization in single-walled carbon nanotube bundles: Structural study

X-ray and neutron-diffraction investigations of rubidium-intercalated single-walled carbon nanotubes are reported. Ab initio calculations conducted in combination with our experiments show that for a single Rb ion the most energetically favorable intercalation site is the interstitial channel between three tubes in a bundle. At higher doping levels, as the Rb content increases, this site becomes however unfavored with respect to the interior of the tubes or the external surface of the bundle. Model simulations of the diffraction patterns, capable of well reproducing both the x-ray and neutron-diffraction patterns, indicate that only the latter insertion sites are compatible with the experimental data. Finally we show that the bundle surface site is the most probable one in the case of saturation at an estimated stoichiometry close to ${\text{RbC}}_8$.

Figure 1

(Color online) Comparison of saturated Raman spectra (in blue) and present inserted sample Raman spectra.

Figure 2

(Color online) X-ray and neutron-diffraction patterns before and after intercalation. X ray: the horizontal line indicates the zero level for the inserted sample.

Figure 3

Schematic representation of the possible intercalation sites in a nanotube bundle. The different sites and distance are labeled according to the text. The gray-shaded areas indicate the region that can actually be occupied by the Rb intercalants at the (undoped) equilibrium lattice parameter (bottom panel) or in the case of lattice expansion (top panel).

Figure 4

(Color online) The collection of bundles used for model calculations. Bundles of 3, 6, 7, 12, 13, 18, and 19 tubes. The sixth bundle is shown in Fig. 5. The different sites occupied by ${\text{Rb}}^{+}$ ions are shown in the figure. From left to right: raw, grooves (GS), triangular channels (t), interior (i) random, interior (i) surface, and grooves (GS).

Figure 5

(Color online) The sixth bundle. The occupied site is the exterior surface of the bundle with two layers (S). Right: view of the ${\text{Rb}}^{+}$ ions along the tube axis (projection).

Figure 6

(Color online) Diffraction patterns simulated for seven bundles and stoichiometry ${\text{RbC}}_8$. Right is for neutrons and left for x rays.

Figure 7

(Color online) Ring effect. Left: patterns for only one bundle. Right: patterns for the different sites and for the same bundle (Rb only). All curves (left and right) are simulated for x rays.

Figure 8

(Color online) Left: simulated $I\left(Q\right)$ for the (S) site with different layers on the bundle surface (x-rays). Right: simulated $I\left(Q\right)$ for the interior site (I) with different stoichiometries (x-rays).

Figure 9

(Color online) Rb intercalation in graphite stoichiometry ${\text{RbC}}_8$.