Ultra-low detection limit chemoresistive NO₂ gas sensor using single transferred MoS₂ flake: an advanced nanofabrication

Abstract

In this work, a method of fabricating a NO₂ nano-sensor working at room temperature with a low detectable concentration limit is proposed. A 2D-MoS₂ flake is isolated by transferring a single MoS₂ flake to SiO₂/Si substrate, followed by applying an advanced e-beam lithography (EBL) to form a metal contact with Au/Cr electrodes. The resulting chemoresistive nano-sensor using a single MoS₂ flake was applied to detect a very low concentration of NO₂ at the part-per-billion (ppb) level. This result is obtained due to the ability to create microscopic nano-sized MoS₂ gaps using e-beam lithography (300 nm–400 nm). Experimental results also show that the sensor can capture changes in concentration and send the information out extremely quickly. The response and recovery time of the sensor also reached the lowest point of 50 and 75 ms, outperforming other sensors with a similar concentration working range.

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1. Introduction

The amount of poisonous gases emitted into the environment has significantly increased due to industrial development as well as improvements in quality of life and transit accessibility, causing substantial harm to the ecosystem and the general public, making it necessary to monitor the air quality in metropolitan areas.^1,2 Among the aforementioned air contaminants, NO₂ is one of the most deadly as at concentrations over 1 ppm, it causes or aggravates respiratory disorders including emphysema and bronchitis as well as severe damage to lung tissues in humans.³ Therefore, research on gas sensors is helpful for the NO₂ emission monitoring and controlling process and is a potential research direction where a large number of scientists are currently interested.

Two-dimensional (2D) materials have been applied in various research applications and industries because of their potential in chemical gas sensing, electronics, optoelectronics, optics, and energy generation.^4–7 In previous works, the resistive NO₂ gas sensor was fabricated using 2D materials (molybdenum disulfide MoS₂/graphene hybrid), which showed high sensitivity, good selectivity, and a low detection limit.⁸ However, the concentration limit of that type of sensor is still relatively high and is not suitable for monitoring NO₂ in the surrounding environment. The nano-sensor deployed by the combination of two-dimensional (2D) materials of atomic-layer thickness and the advantages of nanofabrication is an urgent research direction because of excellent outcomes in the chemical gas sensing field. However, the biggest challenge in this research direction is how we can make sensor devices at the nanoscale to quickly trap these changes in 2D materials, i.e., what is the optimal design and what are the proper materials. This has been the motivation for us to conduct experiments to fabricate a highly sensitive nanoscale NO₂ gas sensor using advanced materials.

MoS₂ has recently been suggested as a promising material for NO₂ gas sensing^9–12 because, when combined with graphene to create composite or hybrid materials, it increases sensitivity due to synergistic effects and eliminates the issue of high recovery time of graphene-only NO₂ sensor.¹³ Moreover, the challenge in developing chemoresistive sensors is the distance between electrodes, which is limited by e-beam lithography technology. The short distance between electrodes and the sensing area at truly nanoscale give such advantages as fast response and low detection limit.¹⁴ In this work, we transferred a single MoS₂ flake to SiO₂/Si and applied advanced e-beam lithography to make chemoresistive nanosensors to detect a very low concentration of NO₂ gas.

2. Experimental section

2.1 Transferred MoS₂ and fabrication of chemoresistive nanosensor

As previously mentioned,⁸ MoS₂ was deposited together with the CVD-based synthesis of graphene utilizing copper foil and transferred to SiO₂/Si. To fabricate a chemoresistive NO₂ gas sensor, two steps of e-beam lithography were applied. In the first step, the array of Au/Cr marker was fabricated on SiO₂/Si using 100 nm thick PMMA as e-beam resist. Then, the single MoS₂ flake was transferred to SiO₂/Si (with Au/Cr marker on top) using 3M Scotch tape. The intermediate SEM observation step is to find the MoS₂ flake on SiO₂/Si after the transferring process and identify the MoS₂ flake position using Au/Cr marker. After identifying the MoS₂ flake location on the array Au/Cr marker with high precision, the second e-beam step was performed on PMMA (100 nm)/MoS₂/SiO₂/Si to make metal contact by the lift-off process. Then, two Au/Cr electrodes were deposited on MoS₂/SiO₂/Si by e-beam evaporation to make the final chemoresistive nano-sensor device. The illustration of chemoresistive NO₂ nanosensor fabrication process is shown in Fig. 1.

Fig. 1 The illustration of the chemoresistive NO₂ nanosensor fabrication process.

2.2 Evaluation methods

The morphology and nanofabrication of the MoS₂/SiO₂/Si sample were determined using high-resolution scanning electron microscopy (HR-SEM; SU 8010, Hitachi) and high-resolution e-beam lithography (e-Raith 150). High-resolution transmission electron microscopy (HR-TEM; JEM-2100F, JEOL) was used to gauge the atomic structure and thickness of transferred MoS₂. Raman spectroscopic measurements were conducted in WITec Raman imaging with a 532 nm laser source to identify MoS₂ quality before/after the transferring process. The NO₂ gas sensor was installed inside a sealed chamber that was attached to NO₂ gas cylinders with various concentrations. The I–V characteristics and resistance of the sensor were recorded using a semiconductor probe station (Keithley SCS-4200). The detailed measurement setup for the NO₂ gas sensor was described in previous work.⁸

3. Results and discussion

As-grown MoS₂–graphene hybrid and a single MoS₂ flake on SiO₂/Si after the transfer process are depicted in SEM images in Fig. 2(a). It is obvious that the MoS₂ with 2D crystallite was grown on graphene and was triangle-shaped. The triangle-like MoS₂ flakes are with average sizes of around 1–2 μm. The MoS₂ flakes were grown in a large-scale area on graphene with uniform thickness as shown in AFM images in Fig. 2(b). In Fig. 2(c), the transferred MoS₂ seems to induce defects during the transfer process by breaking into smaller flakes compared with the original MoS₂ flake as-deposited on graphene (see AFM image in Fig. 2(b)). In order to measure the thickness of MoS₂ flakes on graphene, the AFM with tapping mode was carried out. Results in MoS₂ flake average thickness is around 5 nm, according to 10 layers of MoS₂, consistent with the report in ref. 8. Fig. 2(d) shows the cross-sectional view of HR-TEM of transferred MoS₂ on SiO₂/Si substrate, which indicates 8 layers of MoS₂.

Fig. 2 MoS₂ flakes deposited on graphene: (a) SEM image, (b) AFM analysis, (c) SEM images of isolated MoS₂ flake on SiO₂/Si after the transferring process and (d) cross-sectional view of HR-TEM analysis of transferred MoS₂ on SiO₂/Si (with the help of focused-ion-beam (FIB) to prepare sample specimen).

Fig. 3 shows the Raman spectra of a few layers of MoS₂ growth on graphene and their Raman mapping. In Fig. 3(a), two Raman peaks were assigned to MoS₂ material (E_g at ∼378.5 cm⁻¹ and A_1g at ∼405.8 cm⁻¹).⁸ The E_g modes are doubly degenerate in-plane vibrational modes and A_1g is an out-of-plane vibration mode, which agrees well with previous observations in β-phase bulk MoS₂.¹⁵ In comparison with the Raman spectrum of transferred pure MoS₂ flakes (transferring from MoS₂/graphene/SiO₂/Si sample) on SiO₂/Si substrate, both peak frequencies of β-phase (E_g and A_1g) of MoS₂–graphene shift to the higher wavenumber region. The Raman peaks of transferred MoS₂ flakes on SiO₂/Si, as shown in Fig. S1,† are at 370.9 and 401.2 cm⁻¹ for E_g and A_1g mode, respectively. The shift in Raman peaks of MoS₂ material is due to either a strain/stress free state after transferring from graphene or the influence of the thickness of a few-layers MoS₂ on Raman shift.¹⁵ The G and 2D bands of graphene were assigned to two minor peaks at 1584.3 cm⁻¹ and 2677.2 cm⁻¹, respectively. The in-plane vibration of sp²-carbon atoms was attributed to the G band, while defects in graphene were associated with the D band's breathing modes of six-atom rings. The D band, which is at around 1350 cm⁻¹ indicating defects in graphene, was not present in the sample after depositing MoS₂ as in Fig. 3(a). However, the 2D peaks indicating the number of layers of graphene or graphene quality appeared at 2677.2 cm⁻¹. The intensities of graphene-related peaks are lower than MoS₂ peaks because of either the thickness of the few-layer MoS₂ or the focus point of the laser from Raman. Fig. 3(b) shows an optical image of MoS₂–graphene and the selected area for Raman mapping. Fig. 3(c and d) are images for Raman mapping for average peak intensity centered at 405.8 cm⁻¹ and 378.5 cm⁻¹ for A_1g and E_g mode of MoS₂ material, respectively. The A_1g peak has clear contrast on the area of MoS₂ flakes in comparison with the E_g peak, which indicated the higher intensity of A_1g mode in MoS₂ flakes. Fig. 3(e and f) are images of Raman mapping of G and 2D peaks related to graphene, respectively. In conversion, the mapping of peaks related to graphene has more uniformity in color, except at the graphene grain boundary, which gives higher intensity in Fig. 3(e and f) due to the thicker thickness of folded graphene at the boundary. These Raman mapping images confirm the MoS₂ as isolated flakes on the base planar graphene layer.

Fig. 3 (a) Raman spectra of MoS₂–graphene on SiO₂/Si and (b) optical image with selected area for Raman mapping, (c) Raman mapping for A_1g mode, (d) Raman mapping for E_g mode of MoS₂, Raman mapping for (e) G peak and (f) 2D peak of graphene.

Fig. 4(a) shows the fabricated chemoresistive sensor using a single transferred MoS₂ flake. The Au/Cr metal contact on a single MoS₂ flake is with a distance of 300 nm between two electrodes. The fabricated sensor was tested several times during the day and on different days. Results showed a consistent response with the same testing conditions (NO₂ gas concentrations and bias voltage). Fig. 4(b–d) shows the sensor responses with a low concentration of NO₂ gas. The I–V curves were recorded with synthetic air and different NO₂ gas concentrations of 1, 10, and 100 ppb as shown in Fig. 4(b). The fabricated sensor showed a clearly different response with various NO₂ concentrations. Fig. 4(b) indicates the maximum response of the fabricated sensor with 100 ppb NO₂. I–V curves in Fig. 4(b) show that the sensor response increased dramatically in the forwarding region when exposed to NO₂ gas. The non-linear behavior in the I–V characteristic of Au/MoS₂/Au sample is due to either space-charge limited current in 2D materials¹⁶ or oxidized MoS₂ surface giving the nano-junction with Au electrode. With the same sample structure without MoS₂ flakes, pure graphene sample (with two Au/Cr electrodes and a distance of 300 nm) shows linear ohmic contact and negligible change when exposed to 100 ppb NO₂ gas (see Fig. S2 in ESI†). Fig. 4(c) shows the transient response of the fabricated sensor with 5 V bias exposed under 3 repeat cycles of 1, 10, and 100 ppb. The sensor response shows good repeatability and the gas-induced-current increased by two orders of its value when exposed to NO₂ gas. The response/recovery time of the fabricated sensor is estimated at around 50/75 ms. Fig. 4(d) shows the transient response to 3 cycles of 100 ppb NO₂ at a lower bias voltage of 1 V. The current increased only 4-fold with 100 ppb NO₂ at 1 V bias, which is much smaller than with a 5 V bias. The NO₂ gas generated more current in Au–MoS₂–Au at a higher bias voltage in comparison with a low bias voltage due to the influence of the external electrical field on electron–hole separation and charge collection at the electrode.^14,16 The performance of the fabricated NO₂ gas sensor can be compared with previously developed sensors (Table 1).^17–21 All of them are sensors that work at NO₂ concentrations at ppb, so all comparisons are fair. It is evident that the proposed sensor exhibited a low limit of detection and high sensitivity, among others. In addition, the response/recovery time of the proposed sensor also outperforms others by being more than 60 times faster. In Table 1, the sensor response (S) at fixed bias voltage is defined as the ratio of current change when the sensor is exposed to NO₂:

S = I_g/I_a (1)

where I_a is the current of the sensor in the presence of synthesized air only, and I_g is the current in the presence of NO₂ at given concentrations. The response time is defined as the time required for the sensor to reach 90% of the current change (ΔI) when the sensor is exposed to a given concentration of NO₂. The recovery time is defined as the time needed to recover 90% of the initial baseline after NO₂ is turned off.

Fig. 4 (a) SEM image of the fabricated nanosensor of Au–MoS₂–Au, (b) I–V characteristics of sensors with various NO₂ concentrations, (c) repeatability response of sensor with different NO₂ concentrations of 1, 10, 100 ppb at 5 V bias (d) transient response to 3 cycles of 100 ppb at 1 V bias.

Table 1 Comparison of representative NO₂ gas sensors

Materials	Experimental detected concentration (ppb)	Sensor response (S = Ig/Ia)	Response/recovery time	Condition	Ref.
ZnO	25	0.16	—	100 °C UV light	17
ZnO	25	1.65	—	RT UV light	18
Cu2O–CuO microflowers-2	10	1.6	75 s/100 s	187 °C 30% RH	19
SnS2 nanopetals	5	19.7	179 s/782 s	RT	20
CuO nanoflakes/rGO	50	20.6	31.8 s/60.6 s	RT	21
MoS2/SiO2	1	5.1	50 ms/75 ms	RT	This work
MoS2/SiO2	10	12.9	50 ms/75 ms	RT	This work

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The sensor response of pure graphene with NO₂ is shown in Fig. S2(b and c).† Obviously, the Au–MoS₂–Au showed significantly higher responsivity in comparison with pure graphene (for responsivity of pure graphene, see Fig. S1 in ESI†). The sensor made of Au–MoS₂–Au has increased responsivity from 4 to 100 with 100 ppb NO₂ at 1 V and 5 V bias, respectively. Meanwhile, the sensor response of pure graphene is not significant, which is less than 1% in Fig. S2(b and c),† at 1 V and 5 V bias. The selectivity of the fabricated sensor towards NO₂, in comparison with common toxic gases/moisture such as H₂, H₂O, CO, and CO₂, is shown in Fig. S2(d).† In Fig. S2(d),† examined gases/moisture were measured at 1000 ppb, and the negative sensor response indicates that the current was reduced with H₂, CO, and CO₂ gas. Results showed good selectivity of the fabricated sensor towards NO₂ gas, which is promising for practical applications.

4. Conclusions

In this work, MoS₂ was successfully deposited on graphene by thermal CVD. The single MoS₂ flake was then transferred successfully to SiO₂/Si substrate as isolated flakes. Advanced e-beam lithography was applied to fabricate the nanosensor with a single MoS₂ flake only. The created nanosensor could identify low NO₂ concentrations between 1 and 100 ppb at room temperature, showing a fast response/recovery of 50/75 ms. Thanks to the recent advantages of nanofabrication in combination with the development of 2D materials, the chemical gas sensor at truly nanoscale showed its advantages with low detection limit and fast response and is promising for detecting NO₂ at the atomic level in practical applications. In future works, the performance of the fabricated sensor will be considered over a long-term period.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research is funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2018.33.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ra06228c

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