Visible-Light-Activated Response Originating from Carrier-Mobility Modulation of NO₂ Gas Sensors Based on MoS₂ Monolayers.

Abstract

Some gas sensors exhibit significant increases in their sensitivity and response/recovery rates under light illumination. This photoactivation of the gas response is considered a promising alternative to conventional thermal activation, which requires high power consumption. Thin layers of molybdenum disulfide (MoS₂) are known to exhibit an effective photoactivated gas response under visible light. However, the mechanism of the photoactivated response has not yet been studied in detail. In this study, we fabricated field-effect-transistor (FET) gas sensors based on MoS₂ monolayers and investigated their photoactivated gas responses to NO₂ gas under illumination at various irradiances of visible light. A photocurrent was generated mainly due to the photovoltaic effect, which decreased upon exposure to NO₂. The conductance-based sensor response showed a dependence on NO₂ concentration according to the Langmuir adsorption isotherm, thereby suggesting that the response is proportional to the surface coverage of NO₂ molecules on the MoS₂ layer. The response and recovery rates showed a linear increase with increasing irradiance. Analysis based on the Langmuir adsorption model revealed that both photostimulated adsorption and desorption are involved in the photoactivated response. In contrast, despite the strong dependence of the photocurrent on the irradiance, the magnitude of the sensor response was independent of the irradiance. Based on this result and the change in transfer characteristics of the FET during NO₂ exposure, we concluded that the fast response/recovery of the photoactivated response is due to the carrier mobility modulation of MoS₂, which is caused by the dipole scattering of adsorbed NO₂ molecules.