Understanding the Local Seebeck Coefficient of Carbon Nanotube Fibers Using the Photothermoelectric Effect

The applications of carbon nanotube fibers (CNTF) are broad because of their flexibility, high specific strength, and outstanding thermal and electrical properties. Although CNTFs have a hierarchical structure, their macroscopic properties, are usually discussed and investigated at the scale of the whole fiber, with a lack of understanding of the local properties, such as the Seebeck coefficient and the Fermi energy. Here, we study the variation of the Seebeck coefficient along the fibers by using the photothermoelectric (PTE) effect. The photovoltage is measured as a function of position, and the laser-induced temperature profile is obtained by a robust steady-state thermal model. The Seebeck coefficient as a function of position along the fiber can be obtained from the measured, spatially mapped photovoltage and temperature profile. We observe a correlation between the variation of the Seebeck coefficient and the shift of Raman modes, both related to the doping level and the Fermi energy. We find the Seebeck coefficient fluctuation in the pristine fiber is due to the nonuniformity of the doping level and the Fermi energy. With an established model to correlate the thermoelectric response and the Fermi energy, our PTE-based method can probe the Fermi energy fluctuation along the fiber with a resolution better than 1 meV, which is far beyond the capability of commercial Raman spectroscopy. This study shows a nondestructive method to quantify the uniformity of CNTF at the micrometer scale, key for fabricating more uniform and higher quality CNTF and generalizable to other conducting fiber systems.