Abstract
We report on the calibration of gas sensors based on two transition metal dichalcogenides, molybdenum disulfide or tungsten diselenide, grown by the thermally assisted conversion of patterned Mo or W. The sensors showed non-stationary behavior when exposed to ammonia (NH3) in the range of 10–100 parts per million at room temperature. This drawback hampered the calibration of the sensors. Applying the time-differential signal output (TDSO) enabled us to overcome the issue since the maxima of TDSO were uniquely and linearly correlated to the NH3 concentration. The outcomes show that TDSO is a powerful, reliable, and valid approach when gas sensors are exposed to both oxidizing and reducing atmospheres.
1. Introduction
The absence of a detected steady-state signal represents a longstanding hindrance, which affects solid-state gas sensors. Gas sensors based on two-dimensional materials (2DMs) and transition metal dichalcogenides (TMDCs) are not immune from such a drawback. Due to the great potential of 2DMs and TMDCs in the sensing field, especially in terms of extremely low sensitivity, there is an urgent need to treat the lack of a steady-state signal. So far, the approach named time-differential signal output (TDSO) has been a powerful means of overcoming this hindrance, although TDSO has been applied to sensors exposed upon oxidizing analyte, i.e., nitrogen dioxide (NO2) [1]. Here, we present TDSO while exposing the chemiresistors (CRs) based on MoS2 or WSe2 to a reducing analyte, i.e., ammonia (NH3).
2. Materials and Methods
We deposited molybdenum disulfide (MoS2) or tungsten diselenide (WSe2) by thermally assisted conversion (TAC) of patterned Mo or W. The chalcogens’ vapors were produced from the solid source at ∼115 °C and ∼220 °C, for S and Se, respectively. The vapors were then diffused into the metal layers deposited on Si/SiO2 substrate to form MoS2 and WSe2 at 750 and 600 °C, respectively [2]. On the pre-deposited TMDCs, we realized the resistors depositing the metal contacts via hard marks, i.e., Ti/Au for MoS2 and Ni/Au for WSe2 [2].
3. Discussion
Figure 1a illustrates the typical behavior of the signal when the sensors detect NH3 at room temperature (RT). The current keeps decreasing along the entire exposure without ever reaching the steady state. However, reporting the maxima of TDSO as a function of the NH3 concentration (Figure 1b,c), we observe linear correlations for both MoS2- and WSe2-based chemiresistors, as described in Ref [1]. We reported the absolute values of TDSO peaks because of the decreasing current (black curves) reported in the Insets. The red curves in the Insets show TDSO.
Figure 1.
(a) Real-time percentage current variation (black curve) of MoS2-sensor exposed to pulses of NH3 at increasing concentrations (red rectangles). Each pulse lasts 150 s while dry N2 is used as a buffer gas. Pressure and temperature were kept constant at 150 Torr and RT, respectively. Maxima of the differential curves as a function of the NH3 concentration for (b) MoS2- and (c) WSe2-based sensors. The WSe2-based sensor is calibrated in a narrower range because of the scarce response at concentrations lower than 40 ppm and TDSO peaks not being well defined.
The values of coefficient R2 show a close agreement between the experimental data and the linear fit, indicating that TDSO is a powerful, reliable, and valid approach when gas sensors are exposed to both oxidizing and reducing atmospheres [3,4]. These outcomes successfully mark a step forward in the treatment of gas sensors showing non-stationary behavior, independently from the investigated analyte.
Author Contributions
Conceptualization, data curation, formal analysis, writing—original draft preparation, writing—review and editing, F.R.; methodology, K.L., N.M. and M.M.; project administration, funding acquisition, G.S.D. All authors have read and agreed to the published version of the manuscript.
Funding
This project has received funding from the European Union’s Horizon 2020 under grant agreement no. 785219 (Graphene Flagship). This project was supported with funds from the German Federal Ministry for Education and Research (BMBF) in the context of the national security research program.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Dataset is available on request from the corresponding author.
Acknowledgments
The authors would like to thank Riley Gatensby (AMBER and School of Chemistry, Trinity College Dublin) for the fabrication of the devices.
Conflicts of Interest
The authors declare no conflicts of interest.
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