Calibration of Nonstationary Gas Sensors Based on Two-Dimensional Materials.

Two-dimensional materials (2DMs) have high potential in gas sensing, due to their large surface-to-volume ratio. However, most sensors based on 2DMs suffer from the lack of a steady state during gas exposure, hampering sensor calibration. Here, we demonstrate that analysis of the time differential of the signal output enables the calibration of chemiresistors based on platinum or tungsten diselenide (PtSe2, WSe2) and molybdenum disulfide (MoS2), which present nonstationary behavior. 2DMs are synthesized by thermally assisted conversion of predeposited metals on a silicon/silicon dioxide substrate and therefore are integrable with standard complementary metal-oxide semiconductor (CMOS) technology. We analyze the behavior of the sensors at room temperature toward nitrogen dioxide (NO2) in a narrow range from 0.1 to 1 ppm. This study overcomes the problem of the absence of steady-state signals in 2DM gas sensors and thus facilitates their usage in this highly important application.

Introduction

The monitoring of toxic and flammable gases at low concentration is challenging, especially in domestic and industrial environments.[1] Among the waste species produced nowadays, nitrogen dioxide (NO2) represents one of the most common exhaust gases as it is usually obtained from numerous industrial applications. This oxidizing gas can have a severe effect on human health, particularly in terms of permanent damage to the respiratory system, even at concentration levels as low as 1 ppm (parts per million).[2,3] As such, gas sensors capable of detecting extremely low concentrations are highly sought after.[4] Over the previous few decades, metal oxides (MO) have shown their potential as sensing materials due to their high sensitivity to pollutant gases, small size of devices, and low cost.[1,5] However, it is known that MO-based sensors only perform well when working at temperatures typically higher than 400 °C, with consequent huge consumption of energy.[5−8]

Sensors based on low-dimensional materials, including carbon nanotubes, graphene, and black phosphorus, have been widely investigated as promising alternatives to MO.[2,9−15][2,9−15] Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been widely investigated as promising gas-sensing materials due to their high surface-to-volume ratio, favorable surface energy levels for gas adsorption, high mobilities, and high current on/off ratios.[16−19] A number of TMDCs have shown low detection limits for NO2 operating at room temperature (RT).[19−23] Recently, platinum diselenide (PtSe2) has gained huge interest for selective NO2 sensing with a low limit of detection (LOD) in the range of few ppb and an extremely fast response time (few seconds) when exposed to 0.1 ppm.[24,25]

However, most gas sensors based both on TMDCs and other low-dimensional materials suffer significantly from drawbacks such as the lack of a steady state during the gas exposure and slow recovery kinetics. These two hindrances can hamper the use of sensors since two characteristic parameters, the response time and the signal variation, cannot be properly determined.[26,27] In previous reports,[28−30] we have demonstrated that the limitations can be overcome simply by analyzing the time differential of the signal output (TDSO) instead of the output itself. In those papers, the sensing material consisted of multilayered graphene (MLG), synthesized through different routes.

Here, we demonstrate the reliability of the TDSO approach applying it to chemiresistors (CRs) based on three different TMDCs: platinum diselenide (PtSe2), tungsten diselenide (WSe2), and molybdenum disulfide (MoS2). The sensors show continuously rising current during a gas exposure longer than 2 min, similar to those based on MLG.[28−31] By means of TDSO, we are able to properly calibrate and compare the sensors, highlighting the promising potentialities of these materials in the field of gas sensors working at room temperature (RT).

Results and Discussion

Raman and X-ray photoelectron spectroscopy (XPS) results reported in our previous publications show the stochiometric transformation of Pt, Mo, and W into PtSe2, MoS2, and WSe2, respectively, by thermally assisted conversion (TAC).[20−22,25,32−34] Here, we present the application of TDSO to the devices based on TMDCs reported in those papers, in which a detailed characterization of TMDCs is presented.

Figure shows the optical images and the current–voltage (I–V) characteristics of the devices based on the aforementioned TMDCs. The linearity of the I–V characteristics (Figure b,c) recorded for the investigated devices suggests that the possibility that Schottky barriers at the contacts to the PtSe2, MoS2, and WSe2 films can be excluded. For the PtSe2-based devices, the values of sheet resistance (RS) are in the range of a few hundred kΩ/sq (Table ). The variance of the resistance values can likely be attributed to thickness variation and roughness of the sputtered Pt film at the nanoscale level.

Figure 1

(a) Optical image of the chip containing the eight devices based on PtSe2 with labeled pads. The dashed red rectangle surrounds the PtSe2 film. (b) I–V characteristics of the PtSe2-based resistors reported in Table S1. (c) I–V characteristics of the resistors based on MoS2 (blue line) and WSe2 (red line). Inset: sketch of the MoS2 (WSe2) sensor.

Table 1

Differential Sensitivity (DS) of PtSe2-Based Sensors

device	differential sensitivity [nA/(s·ppm)]
#1	0.91 ± 0.07
#2	0.93 ± 0.07
#3	0.82 ± 0.11
#4	0.83 ± 0.06
#5	0.93 ± 0.06
#6	0.99 ± 0.08
#7	0.88 ± 0.04
#8	0.80 ± 0.04

Figure a illustrates the real-time current behavior (black line) of PtSe2-CR #1 upon sequential exposures of NO2 (red rectangles). The signals recorded for the other tested PtSe2-based devices are shown in the Supporting Information (Figure S1).

Figure 2

(a) Real-time current behavior (black curve) of PtSe2-CR #1 during exposure to increasing concentrations of NO2 (red dashed rectangles). Dry N2 is used as a buffer gas. (b) Signals recorded upon exposure to different concentrations of NO2. The current is normalized at the value reached when each gas exposure starts.

In each exposure window, the current shows an increasing signal, never reaching the stationary state, especially for NO2 concentrations higher than 0.5 ppm. Similar results are reported elsewhere.[28−31] The lack of a steady state is more evident in Figure b, where the signals recorded upon exposure to different concentrations are grouped. It can be seen that the variation of the current toward 100 ppb of NO2 (black curve) is almost negligible compared to the others at higher concentrations.

Figure b also shows that the higher the concentration of the injected gas, the smaller the return of the current to the initial state. Noteworthily, based on the definition of the term “recovery” as the time required to reduce the stationary signal value by 90%,[26] we need to carefully adopt this term to describe the restoration of the signal after the gas pulse. At concentrations lower than 0.5 ppm, the current value is almost restored to the initial value in 150 s. When the sensor is exposed to the highest concentration (1 ppm), for instance, the current is lowered by less than 30% from the value reached when the gas is removed. These outcomes straightforwardly point out two features: first, the continuous integrating capability of the sensors and second, the difficulty in removing the adsorbed molecules in a timescale feasible for applications in environmental conditions.[28] These two limitations, which commonly affect the sensors based on 2D materials, are mathematically overcome by the TDSO approach.[28] In Figure a, the method is applied to the signal recorded on device #1. The TDSO (red curve) is overlapped upon the transients of the sensor (black line), where I0 and I represent the values of the current when the gas is injected and stopped, respectively, during each exposure. The TDSO peaks are well distinguishable for each gas pulse except at 0.1 ppm (Figure a).

Figure 3

(a) Real-time percentage current variation (black curve) of CR #1 during the exposure to increasing concentrations of NO2 and corresponding TDSO (red curve). Analogous results are obtained for the other seven devices (see the Supporting Information). (b) Maxima of the differential curves determined for the eight devices (see the Supporting Information) and plotted as a function of the NO2 concentration.

At that concentration, the peak is not well visible because of the small variation of the signal (Figure b). As the rise time of the sensor is in the range of tens of seconds, the TDSO peaks are located quite close to the position of the gas inlet (t0), as mathematically modeled in ref (28) (Figure S2).

The maxima of TDSO plotted as a function of the NO2 concentration in the range 0.2–1.0 ppm (Figure b) display comparable behavior for the complete set of sensors, especially in terms of linearity of the fitting curves. To properly compare the performance of the sensors, the most powerful tool is differential sensitivity (DS), defined as the slope of the calibration curve.[28] Further studies are ongoing to address the definition of the limit of detection through the TDSO approach. For the eight devices investigated in this paper, the values of the DS are comparable within a maximum variation of about 20% (Table ).

To further prove the reliability of the TDSO approach, we tested CRs based on MoS2 and WSe2. Figure reports the calibration curves obtained by applying TDSO on both sensors. For MoS2-CR (Figure a), the peaks were determined from the same test protocol (inset of Figure a) used for PtSe2-CRs. WSe2-CR, instead, was revealed to be scarcely sensitive to NO2 concentrations lower than 0.4 ppm, showing no intense TDSO peaks (inset of Figure b). As such, we slightly modified the protocol, exposing the WSe2-CR up to 5 ppm of NO2.

Figure 4

Calibration curves of (a) MoS2- and (b) WSe2-based CR. The insets show the transients upon sequential NO2 exposures and TDSO, respectively, in black and red.

Because of the decreasing current shown by both sensors, we reported the absolute values of the TDSO peaks as a function of the NO2 concentration. The n- or p-type behavior of MoS2 is already shown in other reports,[22,35,36] while we are further investigating the behavior of WSe2. Nevertheless, in the present paper, we mostly focus on TDSO to overcome the calibration issue. The linearity of the calibration curve first confirms the reliability of the TDSO approach. The DS of MoS2-CR (0.6 ± 0.1 nA/(s·ppm)) indicates that TAC-grown MoS2 is slightly more sensitive than TAC-grown PtSe2 toward NO2 in the investigated range.

A fairer comparison between the results presented here and those reported in the literature can be based on the minimum detectable concentration of gas. Except for the WSe2-based CR, the sensors presented here are able to distinguish down to a hundred ppb of NO2 working at RT. These findings are in close agreement with up to date results.[16,25,37] By further refinement of the materials synthesis route, it is very likely that the detectable gas concentration will be lowered significantly.

Conclusions

We have analyzed the sensing properties of chemiresistors based on PtSe2, MoS2, and WSe2 toward NO2. The devices were able to detect concentrations of NO2 in N2 down to few hundred ppb at RT. We addressed the issue of a nonstationary state shown during the gas exposure by applying the TDSO approach. We definitively proved that the maxima of TDSO are uniquely and linearly correlated with the NO2 concentration in the range 0.1–1 ppm. The TDSO approach allowed us to properly calibrate the sensors based on three different TMDCs, showing the potential of these materials in the field of gas sensors operating at RT.

Experimental Section

Films of PtSe2, MoS2, and WSe2 were synthesized by thermally assisted conversion (TAC) of Pt, Mo, and W, respectively. Thin layers of Pt (0.5 nm), Mo (10 nm), and W (20 nm) were sputtered onto SiO2 (300 nm)/Si substrates. The samples were then loaded into a quartz tube furnace and heated at 400, 750, and 600 °C, respectively. The vapors of the chalcogens were produced at ∼115 °C (S) and ∼220 °C (Se) and diffused into the metal layers to form PtSe2, MoS2, and WSe2, respectively. The TAC process is described in detail in previous reports.[20,21,25]

To fabricate the gas sensors, we used shadow masks to sputter Pt, Mo, and W layers only on selected areas. After the synthesis of patterned PtSe2, MoS2, and WSe2 by the TAC process, metal contacts (Ni/Au for PtSe2 or WSe2, Ti/Au for MoS2) were deposited using hard masks to define the sensing areas.[21,25] For instance, Figure illustrates the PtSe2-based CRs. Each device is about 1 × 0.2 mm2. The electrical measurements were performed using a Keithley 2636 Source Meter Unit and a Keithley 3706 System Switch/Multimeter. We tested the gas sensors based on TMDC in a custom-made chamber with a volume of about 10 cL and remote-controllable mass-flow controllers (MFCs). The pressure and temperature were kept constant at 150 Torr and room temperature (RT). A constant flow of the gas mixture (100 sccm) was flushed in the chamber. We injected 10 ppm of NO2 into the test chamber, diluting it with dry nitrogen (N2) to achieve concentrations ranging from 0.1 to 1 ppm. Each of the ten gas pulses and subsequent recovery steps lasted 150 s. The sensors were biased at 1 V. The resistance of the devices upon the periodic gas exposure was simultaneously monitored.