Enormous Temperature Coefficient of Resistance in Carbon Nanotube/Oxide Ceramic Composites

Bolometer-type infrared sensors, which can detect and visualize infrared without cooling, are utilized in various fields such as human thermography, inspection equipment for structures and food, night vision to support car driving at night, and security cameras. Our research group is engaged in the research and development of an infrared imaging sensor using semiconducting single-walled carbon nanotubes (SWCNTs) as the bolometer material [1-2]. Using the semiconducting SWCNT extracted by the “electric-field inducing layer formation (ELF)” method [3], and by controlling the manufacturing condition of the SWCNT network, we achieved a temperature coefficient of resistance (TCR) of about -6%/K [1], which was three times larger than the conventional bolometer material, VO_x[4].<br/>In this study, we developed SWCNT/oxide ceramics composites and evaluated the magnitude of TCR for potential confirmation as a bolometer material. As a result, in an elementary evaluation, a TCR of about -15%/K was obtained.<br/>The manufacturing method of the composite is as follows. After producing an oxide ceramic particle aqueous suspension, we drop-casted the suspension onto a p-type Si substrate with a thermal oxide film with Au electrodes, and dried and solidified it to form an oxide ceramic porous film. Then, we formed a silane coupling self-organizing monomolecular film (SAM) on the surface of the oxide ceramic particles of the porous film, and dripped a SWCNT dispersion liquid onto the film, and formed a SWCNT network on the surface of the oxide particles and the substrate. Then, we dropped a polymethylmethacrylate (PMMA) solution, baked it to fix the SWCNT/ceramic oxide composite, and removed the ceramic oxide film and SWCNTs where PMMA was not formed. For comparison, simple SWCNT network elements were also fabricated in a similar process.<br/>From the microstructure observation by scanning electron microscope (SEM) and transmission electron microscope (TEM), we confirmed that the developed composite material forms a three-dimensional network of SWCNTs based on a 2-3 μm thick oxide ceramic porous film. The oxide ceramic particles used had an average particle size of about 1μm, and the SWCNTs were present on the particles and the substrate surface and also existed in a form bridging the voids between the particles.<br/>The substrate used for the TCR evaluation had electrodes on the backside as gate electrode, and the drain and source electrodes on the top surface. Thin-film transistor (TFT) devices were fabricated by forming materials such as the SWCNT/oxide ceramic composites and simple SWCNT networks on the surface as channels between the drain and source electrodes. We evaluated the output and transfer characteristics of the TFTs. These measurements were performed at 25°C and 30°C for the TCR calculation. As a result, it was confirmed that the best performance was achieved when Zn_1-xMg_xP₂O₇ was used as the oxide ceramic. In the composite, a TCR of about 15%/K was achieved, although the issue of hysteresis presented in the transport characteristics of the evaluated TFT.<br/>In conclusion, by compositing SWCNT and oxide ceramic, we realized enormous TCR, and demonstrated the potential for further improving the performance of SWCNT bolometers.<br/> <br/>Acknowledgments: Part of this study was supported by Innovative Science and Technology Initiative for Security Grant No. JPJ004596, ATLA, Japan.<br/> <br/>[1] T. Tanaka. et. al., 2022 MRS Fall Meeting, NM02.09.08.<br/>[2] T. Tanaka. et. al., 2023 MRS Fall Meeting, EL07.06.10<br/>[3] K. Ihara, et. al., J. Phys. Chem. C, 115, 2011, 22827.<br/>[4] C. Chen, et. al., Sen. Act. A. Phys. 90, 2001, 212.