Isolated rigid rod behavior of functionalized single-wall carbon nanotubes in solution determined via small-angle neutron scattering

The measurement of small-angle neutron scattering on functionalized single-wall carbon nanotubes in solution is reported. Without the use of surfactants, isolated rigid rod behavior of functionalized single-wall carbon nanotubes has been observed. The onset of pinning between the nanotubes at different pinning distances depending on the concentration is also detected, as well as some aggregation for the more concentrated samples. We present evidence of a multiscale organization of these samples: the molecular covalent structure of single-wall nanotubes, the supramolecular structure of bundles, and a loose three-dimensional network formed by pinning of the bundles. Since most organic electronic devices are fabricated from blends of small molecules or nanocrystals with a polymeric matrix by spin coating from solution, the understanding of the solution properties of such nanocrystals is important in order to improve the performance of these devices.

Figure 1

Schematic draft of the molecular structure of both functionalized single-wall carbon nanotubes: $f_1$ SWNT is the pentyl-ester SWNT and $f_2$ SWNT is the conjugated hybrid of $C_{60}$-fullerene and SWNT.

Figure 2

HRTEM image of a functionalized carbon nanotube and a bundle of nanotubes. The diameters of nanotubes are of 1.5 nm, and those of bundles are of around 10 nm (bar: 10 nm).

Figure 3

SANS intensity profiles for pentyl-ester single-wall carbon nanotubes dissolved in deuterated toluene at 0.4, 0.1, 0.05, and 0.025% wt. The plot includes the three instrumental configurations used: small (circles), medium (triangles), and large (squares) angles. No solvent subtraction was performed. We can see a clear $Q^{-1}$ behavior over a wide range of $Q$ values. In addition the signal from deuterated toluene is plotted for comparison.

Figure 4

(Color online) Detail of the small-angle scattering measurement for all the samples. The lower $Q$ limit for every sample is pointed by an arrow. The corresponding number indicates the pinning distance deduced from this limit. The horizontal segment indicates the onset of the solvent signal in the plot, which terminates the $Q^{-1}$ behavior without reaching its theoretical limit deduced from the diameter of the SWNTs. Filled symbols are medium-angle configuration measurements.

Figure 5

Detail of the medium-angle scattering measurement for three samples: 0.4 and 0.1% wt of $f_1$ SWNT (squares and triangles) and 0.05% wt $f_2$ SWNT (circles) corrected by solvent subtraction. The observed peaks have been fitted with a Gaussian function (lines).