Origin of n type properties in single wall carbon nanotube films with anionic surfactants investigated by experimental and theoretical analyses.

We investigated the origin of n-type thermoelectric properties in single-wall carbon nanotube (SWCNT) films with anionic surfactants via experimental analyses and first-principles calculations. Several types of anionic surfactants were employed to fabricate SWCNT films via drop-casting, followed by heat treatment at various temperatures. In particular, SWCNT films with sodium dodecylbenzene sulfonate (SDBS) surfactant heated to 350 °C exhibited a longer retention period, wherein the n-type Seebeck coefficient lasted for a maximum of 35 days. In x-ray photoelectron spectroscopy, SWCNT films with SDBS surfactant exhibited a larger amount of sodium than oxygen on the SWCNT surface. The electronic band structure and density of states of SWCNTs with oxygen atoms, oxygen molecules, water molecules, sulfur atoms, and sodium atoms were analyzed using first-principles calculations. The calculations showed that sodium atoms and oxygen molecules moved the Fermi level closer to the conduction and valence bands, respectively. The water molecules, oxygen, and sulfur atoms did not affect the Fermi level. Therefore, SWCNT films exhibited n-type thermoelectric properties when the interaction between the sodium atoms and the SWCNTs was larger than that between the oxygen molecules and the SWCNTs.