Giant Thermopower Effects from Molecular Physisorption on Carbon Nanotubes

Results are presented of in situ studies of the thermoelectric power and four-probe resistance of single-walled carbon nanotube films during the adsorption of cyclic hydrocarbons ${\mathrm{C}}_6{\mathrm{H}}_{2n}$ ($n=3–6$). The size of the change in these transport parameters is found to be related to the $\pi$ electron population of the molecule, suggesting the coupling between these $\pi$ electrons and those in the nanotube wall may be responsible for the observed effects. A transport model for the SWNT film behavior is presented, incorporating the effects of a new scattering channel associated with the adsorbed molecules.

Figure 1

In situ 40 °C thermoelectric power ($S$) vs time ($t$) during successive exposure of a degassed SWNT film to vapors of six-membered ring molecules ${\mathrm{C}}_6{\mathrm{H}}_{2n}$; $n=3–6$. The dashed lines are guides to the eye.

Figure 2

In situ 40 °C four-probe resistance ($R$) vs time ($t$) during successive exposure of a degassed SWNT film to vapors of six-membered ring molecules ${\mathrm{C}}_6{\mathrm{H}}_{2n}$; $n=3–6$.

Figure 3

$S$ vs $\Delta R/R_0$ during exposure to ${\mathrm{C}}_6{\mathrm{H}}_{2n}$ ($n=3–6$). The dashed curve is a fit of a quadratic function $S=a+b(\Delta R/R_0)+c(\Delta R/R_0{)}^2$ to the data, where $a=-6.2\mu \mathrm{V}/\mathrm{K}$, $b=-8.6\mu \mathrm{V}/\mathrm{K}$, and, $c=301\mu \mathrm{V}/\mathrm{K}$.

Figure 4

Temperature dependence of the thermopower of the degassed SWNT after saturation coverage of the various ${\mathrm{C}}_6{\mathrm{H}}_{2n}$ molecules. The dashed lines are guides to the eye.