Gate-bias tunable humidity sensors based on rhenium disulfide field-effect transistors Amir Zulkefli 1,2 , Bablu Mukherjee 1 , Takuya Iwasaki 1,3 , Ryoma Hayakawa 1 , Shu Nakaharai 1* , and Yutaka Wakayama 1,2* 1 International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan 2 Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 1-1 Namiki, Tsukuba 305-0044, Japan 3 International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan * E-mail: WAKAYAMA.Yutaka@nims.go.jp; NAKAHARAI.Shu@nims.go.jp Received September 29, 2020; revised November 27, 2020; accepted December 10, 2020; published online December 29, 2020 We investigate the humidity sensing performance and mechanism of few-layer-thick rhenium disulfide (ReS 2 ) field-effect transistors (FETs) under gate bias operation. Consequently, a negative gate bias exhibits the sensor response, exceeding 90% mainly in the low relative humidity (RH) range. Meanwhile, the threshold voltage change was discovered to be a superior sensing parameter to achieve a broad monitoring of RH range with high response and sensitivity. The approach obtained a practical sensitivity of 0.4 V per 1% RH, which exceed a majority of previous studies with the pristine 2D materials. Besides, our devices display reversible adsorption–desorption and long-term stability operations even after a one- month period. This suggests the sensor capacity to function in real-time applications with a short response and recovery times. These outcomes offer support in the development of adaptable tunable humidity sensors based on ReS 2 FETs. © 2020 The Japan Society of Applied Physics Supplementary material for this article is available online 1. Introduction The identification of humidity level is crucial in different contexts which encompass production control in agriculture, industrial and environmental monitoring, and medical diagnostic. 1–5) The increased demand of humidity sensing requires high response and sensitivity, and a wide detection range. 6) Field-effect transistor (FET) is becoming a promising approach for such humidity sensors because of the simple device structure, and well-developed device fabrication process and measurements. 7,8) In addition, the current mod- ulation by gate biasing offers an advantage in a variation of sensor performance. 9,10) Two-dimensional (2D) transition metal dichalcogenides (TMDCs) materials have attracted attention in future elec- tronic/optoelectronic device-based sensors. 11–13) The atten- tion is contributed by the unique physical, optical, and electrical characteristics such as naturally high surface-area- to-volume ratio that is vital for gas sensing. In particular, rhenium disulfide (ReS 2 ) has grown significant research interest due to its promising application of transistors, detectors, and sensors. 14–17) Unlike other conventional TMDCs like molybdenum disulfide (MoS 2 ) or tungsten disulfide (WS 2 ), ReS 2 has 1 T crystal structure, showing weak interlayer coupling property. 18–21) Inspired by these characteristics, many noteworthy researches with ReS 2 have been investigated, extending their research stages from material synthesis to transistors, photodetectors, non-volatile memory, and gas sensors. 22–27) These studies imply that ReS 2 is a promising contender for future ultrathin nanoelectronics applications. Considerable work to enhance the ReS 2 -based gas sensor has been reported ranging from heterostructure, intentional plasma-induced defected material, surface functionalization, and photo-assisted effect. 27–29) However, the investigation of humidity sensing performance and mechanism based on gate- bias effect, especially gas–solid interactions have not yet been explored. To this extent, it would be fascinating to examine the ReS 2 FET-based humidity sensor under gate- bias effect for further progress of high-performance sensor. In this study, we investigated ReS 2 FET-based humidity sensors. The sensing performance including response, sensi- tivity, reversibility, and stability were examined in an orderly approach. This study focuses on the gate voltage operation to explain their functions in the sensing mechanism. In con- sequence, high sensing response was achieved especially in low relative humidity (RH) range by utilizing the negative gate bias effect. Meanwhile, sensing response based on the threshold voltage change was discovered to be a superior sensing parameter for a broad RH range monitoring, together with high response and sensitivity. A practical sensitivity of 0.4 V per 1% RH was attained. This research aims to provide a thorough understanding of the functions of gate biasing, contributing to the development of a high-quality humidity sensor based on ReS 2 FET. In comparison with the corre- sponding SSDM 2020 extended abstract, 30) more in-depth discussions and additional experimental results are given in this paper. These include electrical, sensing, response-re- covery time, reversibility, and stability properties of ReS 2 FET-based humidity sensor. 2. Experimental methods 2.1. ReS 2 nanoflakes preparation and characterization The gold (Au) assisted exfoliation technique was used to prepare ReS 2 nanoflakes on a SiO 2 /Si substrate. 29,31) This technique is efficient to prepare large-scale uniform ReS 2 nanoflakes. Initially, ReS 2 crystal (HQ Graphene supplier) was mechanically exfoliated on a Nitto Denko thermal tape (Model NO319Y-4LSC). Next, a 100 nm thick Au film was directly deposited on an exfoliated ReS 2 /thermal tape. Additional thermal tape was utilized to exfoliate the Au/ReS 2 layers from the previous ReS 2 /Au coated thermal tape. Subsequently, these exfoliated Au/ReS 2 layers were pasted onto the SiO 2 /Si substrate before detaching the thermal tape at 100 °C. To completely remove the residual © 2020 The Japan Society of Applied Physics SBBH01-1 Japanese Journal of Applied Physics 60, SBBH01 (2021) REGULAR PAPER https://doi.org/10.35848/1347-4065/abd2a0