Effect of Surfactants on the Thermoelectric Performance of Double-Walled Carbon Nanotubes

Thermoelectrics are a promising solution to the recovery of some of the 60% of the worldwide energy wasted as heat. However, their conversion efficiency is low and the best performing materials are brittle, toxic, and made of expensive ceramics. The challenge in developing better performing materials is in disrupting the electrical vs thermal conductivity correlation, to achieve low thermal conductivity simultaneously with a high electrical conductivity. Carbon nanotubes allow for the decoupling of the electronic density of states from the phonon density of states and this paper shows that flexible, thin films of double-walled carbon nanotube (DWCNT) can form effective n- and p-doped semiconductors that can achieve a combined Seebeck coefficient of 157.6 mu V K-1, the highest reported for a single DWCNT device to date. This is achieved through selected surfactant doping, whose role is correlated with the length of the hydrocarbon chain of the hydrophobic tail group of the surfactant's molecules. CNTs functionalized with Triton X-405 show the highest output power consisting of a single junction of p- and n-type thermoelectric elements, reaching as high as 67 nW for a 45 K temperature gradient. Thus enabling flexible, cheaper, and more efficient thermoelectric generators through the use of functionalized CNTs.