The research group of Professor Yutaka Majima and Assistant Professor Trong Tue Phan (at the time of research) at the Laboratory for Materials and Structures, Institute for Innovative Research, Tokyo Institute of Technology, investigated the electrode spacing (gap length) of variable-resistance gas sensors, and developed a gas sensor whose response speed was 300 times higher than those of conventional gas sensors, using a 20 nm-gap electrode.
A gas sensor is a device that detects the presence and concentration of gas in the atmosphere or environment. Among these, the variable-resistance gas sensor is a conventional type sensor that detects the type and concentration of gas through a built in variable resistor that changes it’s resistance value in response to gas concentration. According to Professor Majima, "We have been developing technology to readily fabricate platinum nanostructures by electron beam lithography (EBL). For this research, we attempted to improve performance by decreasing the gap length, which corresponds to the distance between the electrodes of the gas sensor, to as small a distance as possible." As platinum is a harder material than precious metals such as gold and palladium, it is difficult to fabricate a structure with a gap ≤20 nm using EBL. To overcome this, the research group developed a technique to achieve a 20 nm gap length by optimizing the EBL process, and prepared a cerium oxide film as a gas detection material via the solution process. Specifically, using a bottom-contact nanogap gas sensor as a base, the gap length was realized by forming a cerium oxide film, followed by manufacturing a top-contact nanogap gas sensor to fabricate a platinum nanogap electrode. Introducing nanogap electrodes into the variable-resistance gas sensor results in a faster response and improved sensitivity than those of existing gas sensors.
The gap lengths of commercially used variable-resistance gas sensors are at the micrometer scale. The response speeds of the newly developed gas sensor with a 20 nm gap length and the conventional gas sensor with a 12 μm gap length are 10 s and 3200 s, respectively, implying that the response speed of the new device is approximately 300 times higher. Professor Majima concluded, "As we can achieve such a high speed simply by decreasing the gap length, we hope that these new nanogap electrodes will be widely employed in variable resistance gas sensors in the future".
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