Improving the performance of carbon nanotube sensors by selective noise removal

Today, in addition to traditional visualisation techniques, such as X-rays, for crack and defect detection, researchers in the field of nondestructive testing are actively using electromagnetic waves for detailed material identification. In particular, ultra-wideband and multi-wavelength measurements of millimetre waves and infrared rays, which are intermediate between visible light and radio waves, contribute to the sophistication of material identification. From the perspective of image measurement, which is the primary inspection method, carbon nanotubes (CNTs), which show high absorption in the same band, are considered promising as image sensor materials. CNT film photothermoelectric (PTE) sensors enable nondestructive material composition identification, considering their application in inspection equipment. However, in the design of CNT film materials (such as size and composition), the challenge is to balance the light detection response strength and the noise during PTE sensor operation. The response of CNT films is minimal, in the μV range, so there is a problem that they are easily affected by noise. Based on this understanding, this study demonstrates a method to selectively reduce the resistance of CNT films by focusing on additional liquid surfactants and the intrinsic properties of the pure material. The ionic surfactant used in this study dissolves when immersed in water. When immersed in pure water, the resistance of the CNT film decreases by about 25%. In previous studies, researchers controlled the electrical resistance value by increasing the Fermi level or the volume of the CNTs; however, instead of reducing the electrical resistance, the Seebeck coefficient and optical detection response intensity were reduced. However, in this study, we successfully reduced only the electrical resistance. The Seebeck coefficient of the CNT film remained almost unchanged even after exposure to pure water. In addition, no significant change was observed in the absorbance before and after water immersion. Various challenges remain for the operation of the CNT film PTE sensor, especially its resistance to high-speed operation and the engineering application of CNT film sensors with reduced noise. In this study, we will also report on these additional concerns at the conference.

Hayato Hamashima is conducting research into the properties of carbon nanotubes and graduated with a Bachelor of Engineering in 2025.