Nanotechnologies / Materials
Medical / Healthcare, Environment monitoring, Smart devices

Development of photoactivated gas sensor using
two-dimensional semiconducting materials
Division of Electrical, Electronic and Infocommunications Engineering, Graduate School of Engineering
Assistant Professor Hiroshi Tabata
Researchmap https://researchmap.jp/read0156510

Nanotechnologies / Materials

Abstract
Two-dimensional semiconducting materials, such as transition
metal dichalcogenides (TMDCs) are promising sensing materials
for gas sensors, because their electrical properties are sensitive tothe adsorption of gas molecules due to their high surface-to-volume ratio. On the other hand, the TMDCs are also known to havea strong interaction with light, having a high optical absorption
coefficient in the visible region, about 10 times higher than that ofbulk semiconductors such as silicon and germanium. In this study,
we focused on molybdenum disulfide (MoS2), a kind of the TMDC,
and investigated the effect of light illumination on the response ofa FET type sensor based on the MoS2 monolayers to NO2 gas. We
revealed that visible light illumination with a photon energy higher
than the bandgap (1.8 eV) of the MoS2 monolayer significantly improves the sensitivity and response/recovery characteristics.

▍ Significance of the research and Future perspective
The significance of this research lies in the fact that the response
characteristics of the sensor can be tuned by adding a third element, light, to the interaction between the two-dimensional semiconducting material and gas molecules. Since light illumination
provides excellent sensor performance even at room temperature,
it is expected to be developed into ultra-low power consumption
gas sensors required for IoT sensors by combining it with energy-efficient micro light sources such as micro-LEDs. Furthermore,by selecting wavelength and modulating intensity of the illumination light, it may be possible to achieve the advanced molecular
recognition required for the detection of biological gas species.

▍ Background & Results
With the recent development of Internet of Things (IoT) technology, the importance of gas sensors is increasing. Gas sensors for
IoT application need to be small, low cost, and have low power
consumption, in addition to high sensitivity, high reliability, and highselectivity. Semiconductor gas sensors are promising candidates
for IoT sensors because of their small size, low cost, and relatively
high sensitivity, but they have the drawback of high-power consumption due to the need for high temperature operation. Therefore, sensor activation by light illumination is attracting attention as
an alternative to activation by heat. Conventionally, ultraviolet light,
which is harmful to the human body, has been used for the photoactivation. In this study, we found that the sensor response canbe efficiently activated at room temperature even by visible light insensors based on the MoS2 monolayers as the sensing material.
Figure 1 shows the schematic and microscopic image of themonolayer MoS 2-FET sensor, and the photograph of the setup
for sensor measurement. Figures 2a and 2b show the dynamic
response of the sensor to 50 ppb NO2 measured under light illumination with an irradiance of 86 mW/cm2 and dark conditions. NO2 isdifficult to desorb at room temperature because of its high adsorption energy, therefore the current hardly recovers by gas purging in
the dark condition. In contrast, under light illumination, the current
fully recovered in a short time by gas purging. Both the response
and recovery speeds of the sensor tended to increase as the irradiance increased, which implies that the photo-generated electrons
and holes in MoS2 promote the adsorption and desorption of gas
molecules, respectively (see Figure 2c).
The sensitivity and the limit of detection of the sensor were estimated to be 8.6%/ppb and 0.15 ppb, respectively. These sensor
performance metrics meet the requirements of the environmental
quality standard in Japan（0.04−0.06 ppm as the daily average forhourly values）, indicating that this sensor can be used for monitoring air pollution gas.

Figure 1

Figure 2

P a t e n t
Tabata, Hiroshi; Matsuyama, Hiroaki; Goto, Taishi et al. Visible-Light-Activated Response Originating from Carrier-Mobility Modulation of NO2 Gas Sensors
Based on MoS2 Monolayers. ACS Nano. 2021; 15(2): 2542-2553. doi: 10.1021/acsnano.0c06996
URL http://nmc.eei.eng.osaka-u.ac.jp/index_j.html
K e y w o r d MoS2, nitrogen dioxide sensor, photoconductance, photoactivated gas response, photostimulated desorption

Tr e a t i s e