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. 2021 Sep 9;6(37):24075–24081. doi: 10.1021/acsomega.1c03522

Rapid Degradation of the Electrical Properties of 2D MoS2 Thin Films under Long-Term Ambient Exposure

Bhim Chamlagain †,*, Saiful I Khondaker †,‡,*
PMCID: PMC8459407  PMID: 34568686

Abstract

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The MoS2 thin film has attracted a lot of attention due to its potential applications in flexible electronics, sensors, catalysis, and heterostructures. Understanding the effect of long-term ambient exposure on the electrical properties of the thin film is important for achieving many overreaching goals of this material. Here, we report for the first time a systematic study of electrical property variation and stability of MoS2 thin films under ambient exposure of up to a year. The MoS2 thin films were grown via the sulfurization of 6 nm thick molybdenum films. We found that the resistance of the samples increases by 114% just in 4 weeks and 430% in 4 months and they become fully insulated in a year of ambient exposure. The dual-sweep current–voltage (I–V) characteristic shows hysteretic behavior for a 4-month-old sample which further exhibits pronounced nonlinear I–V curves and hysteretic behavior after 8 months. The X-ray photoelectron spectroscopy measurements show that the MoS2 thin film gradually oxidizes and 13.1% of MoO3 and 11.8% oxide of sulfur were formed in 4 months, which further increased to 23.1 and 12.7% in a year, respectively. The oxide of the sulfur peak was not reported in any previous stability studies of exfoliated and chemical vapor deposition-grown MoS2, suggesting that the origin of this peak is related to the distinct crystallinity of the MoS2 thin film due to its smaller grain sizes, abundant grain boundaries, and exposed edges. Raman studies show the broadening of E2g1 and A1g peaks with increasing exposure time, suggesting an increase in the disorder in MoS2. It is also found that coating the MoS2 thin film with polymethylmethacrylate can effectively prevent the electrical property degradation, showing only a 6% increase in resistance in 4 months and 40% over a year of ambient exposure.

Introduction

Thin films of two-dimensional (2D) transition-metal dichalcogenides (TMDs) prepared via the chalcogenization of metal and metal oxide films have attracted significant research interest due to their uniform large-area coverage, controlled synthesis in a wide range of thicknesses, and cost-effective relatively easier synthesis process.1−8 This growth method not only provides control over the thickness of the TMD films but also transitions from horizontal to vertical orientation of the TMD sheet with the initial thickness of the metal film used.5,9 The large-area MoS2 thin film prepared via the sulfurization of molybdenum (Mo) films has been demonstrated as a potential candidate for catalysis applications due to its abundant exposed edges in vertically oriented sheets.6 Vertical and lateral heterojunctions along with their application in photodetection were also demonstrated.10,11 Recently, the film has been investigated for possible applications in flexible electronics and gas sensing.12,13 However, TMD films have much smaller grain sizes, typically in the range of 10–100 nm, which give rise to significantly more grain boundaries than their exfoliated and coevaporation chemical vapor deposition (CVD)-based counterparts.8 Furthermore, the vertical sheet of TMDs has abundant exposed edges which could be prone to ambient gases and moisture. The interaction of ambient gases and moisture with the thin film could pose major challenges in not only the practical realization of the thin-film devices but also the reproducibility of the results.

Although a few studies of the stability of mechanically exfoliated and coevaporation CVD-based 2D MoS2 under ambient exposure have been reported mainly focusing on structural and optical property variation,14−19 the study of the stability of the MoS2 thin film has not been reported to date. In addition, a systematic study of the electrical property variation of MoS2 grown by any technique has not been investigated. For CVD-grown monolayer MoS2, oxidation at grain boundaries and sulfur (S) vacancies has been reported with the progressive oxidation of the film over time.14 In this film, grain boundaries have primarily exposed Mo sites, which promote the formation of MoO3. Theoretical calculations also showed the pronounced oxidation and absorption of the atmospheric gases at the defect centers and grain boundaries.20−22 These findings suggest that TMDs with abundant grain boundaries are more susceptible to atmospheric oxygen and moisture. In another study, it has been reported that decreasing the S vacancy density by adjusting the growth conditions of CVD MoS2 monolayers could slow the degradation,19 suggesting that defects and grain boundaries in the film are crucial for the film degradation. In contrast to exfoliated and CVD-based MoS2, the thin film has distinct crystallinity, which has abundant grain boundaries and edges in the film.8 Therefore, it is critical to investigate the long-term stability of the MoS2 thin film under ambient exposure. In addition, since MoS2 is an electronic material with many potential applications in electronics and optoelectronics, it is important to systematically study the effect of long-term ambient exposure on the electronic properties of MoS2.

Here, we present a systematic study on the variation of electrical properties of the large-area MoS2 thin film prepared via the sulfurization of Mo films by keeping it under ambient conditions. We observed that the resistance of MoS2 devices monotonically increases with time and the average resistance of the devices increases by 114% in just 4 weeks, which further increases by 430% in 4 months and the samples become completely insulated in a year. In addition, the dual-sweep current–voltage (I–V) characteristic becomes nonlinear and shows hysteretic behavior after 4 months, which becomes pronounced with increasing exposure time. X-ray photoelectron spectroscopy (XPS) measurements show that the formation of the oxides of Mo and S due to the oxidation of the MoS2 film gradually increases with time. The oxide of the sulfur peak was not reported in any previous stability studies of exfoliated or CVD-grown MoS2, suggesting that the origin of this peak is related to the distinct crystallinity of the film due to its smaller grain sizes and orientation of the sheet. We observed a shift of Mo 3d and S 2p XPS doublet peaks to lower binding energies (BEs) for aged MoS2 films, suggesting a relative shift of the Fermi level toward the valence band edge, consistent with electrical transport measurement data. In addition, XPS data also showed that organic contaminants and moisture are absorbed on the aged MoS2 films, which suggest that the localized electrons at the MoO3 sites promote the absorption of polar molecules and moisture from the atmosphere, resulting in the observed hysteresis effect in the I–V curves. Raman studies showed an increase of full width at half-maximum (fwhm) of E2g1 and A1g peaks with increasing exposure time, suggesting an increase in the disorder in MoS2 films. Our study suggests that the MoS2 film actively interacts with atmospheric oxygen to gradually increase MoO3 and oxide of the sulfur amount in the film, which increases the resistance of the film and eventually becomes electrically insulated in a year. We also found that coating the MoS2 thin film with polymethylmethacrylate (PMMA) can effectively prevent the electrical property degradation, showing only a 6% increase in resistance in 4 months and 40% over a year of ambient exposure. Our study reported here on the long-term stability, especially the variation of electrical properties of the MoS2 thin film is an important step forward in achieving the overreaching goals of MoS2 thin films in practical applications.

Results and Discussion

Figure 1a shows a digital image of a 6 nm thick Mo film, while Figure 1b shows a digital image of the same film after sulfurization. The change of color indicates that the Mo film was successfully sulfurized to form the MoS2 thin film. Raman spectra of the sulfurized film presented in Figure 1c show two prominent peaks at 386.2 and 411.0 cm–1, corresponding to the in-plane E2g1 and out-of-plane A1g vibrational modes of MoS2, respectively, with a position difference of 24.8 cm–1, indicating the formation of the multilayer MoS2 film.23−27 For electrical transport measurements, MoS2 devices of 100 μm channel length and 300 μm channel width were fabricated by depositing 5 nm/35 nm of Cr/Au using a shadow mask. Figure 1d shows the current–voltage (I–V) characteristics of a representative pristine MoS2 device. The resistance calculated from the linear region of the I–V curve was found to be 168 MΩ. We have measured a total of 27 MoS2 devices that were fabricated on the same chip, and the resistance of the devices varies from 165 to 318 MΩ with an average value of 266 MΩ (presented in Figure 1e and Supporting Information, Figure S1). The device-to-device variation of resistance is not due to the variation of the thickness of the films as we have measured the thickness of the Mo film at different locations and found the thickness to be uniform. In addition, all the devices have the same dimensions. Therefore, the device-to-device resistance variation could be due to the local inhomogeneity of the MoS2 film resulting from the growth process. These values of resistance are within the range of reported resistance values of the MoS2 film prepared by similar methods.9,10,12,28−30

Figure 1.

Figure 1

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Digital image of the (a) molybdenum film, (b) MoS2 film after the sulfurization of the Mo film, (c) Raman spectrum of the pristine MoS2 thin film, (d) current–voltage (I–V) characteristics of a representative pristine MoS2 device, and (e) histogram of resistance for the pristine MoS2 devices.

The devices were then left exposed under atmospheric conditions (lab temperature, pressure, and humidity) for a period of a year during which their electrical characterizations were periodically measured. Figure 2a shows the I–V characteristics of the MoS2 device presented in Figure 1d measured every week for up to 6 weeks and then at 9, 12, and 16 weeks. The calculated values of resistance of this device with time are presented in Figure 2b. The resistance of the device increases monotonically with time. After 1 week, the resistance increased to 193 MΩ, an increase of 14%. The resistance continued to increase by 61% in 2 weeks, 92% in 3 weeks, 114% in 4 weeks, 164% in 5 weeks, 175% in 6 weeks, 316% in 9 weeks, 415% in 12 weeks, and 508% in 16 weeks. We have measured all the 27 MoS2 devices after 4 months, and an increase of the resistance was observed for every sample (Figure 2c and Supporting Information, Figure S1). The resistance of the 4-month-old samples varies from 976 to 1990 MΩ with an average value of 1405 MΩ, which is a ∼430% increase compared to the average resistance of the pristine sample. While there are no reports on the electrical property variation of MoS2 thin films due to long-term ambient exposure, there is only one report of the electrical property variation of the CVD-grown MoS2 monolayer, which showed that the drain current was decreased by up to 2 orders of magnitude after a month of ambient exposure.14 Assuming that the reported current in the CVD-grown monolayer is Ohmic, this would mean that the resistance variation in the CVD-grown MoS2 monolayer is faster than that in the thin-film sample. This is quite surprising given that thin-film MoS2 has smaller grain sizes with significantly more grain boundaries compared to the CVD-grown monolayer. However, we note that the reported CVD-grown MoS2 device was fabricated after 3 months by keeping the sample in marginal vacuum which makes the comparison complicated. Since the thickness of the MoS2 thin film is ∼14 nm, which is ∼20 times higher than that of the CVD-grown MoS2, one could argue that in the thin film, upper layers could protect the lower layers from ambient effects, which might explain the less dramatic electrical property variation for a 1-month-old sample; however, we note that a 1-year-old sample is completely insulated, suggesting that every layer is affected by ambient exposure and top layers do not protect the bottom layers.

Figure 2.

Figure 2

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(a) I–V characteristics of the MoS2 device (presented in Figure 1d) measured every week for up to 6 weeks and then at 9, 12, and 16 weeks. (b) Calculated values of the resistance of the device with time and (c) histogram of resistance for 27 devices on a chip measured after 4 months.

We also studied the hysteresis effect on the I–V curves to gain more insights into the electrical property variation by measuring dual-sweep (forward and reverse) I–V characteristics of the devices. This is presented in Figure 3, where we show I–V curves of a representative device measured immediately after fabrication, 4, 8, and 10 months of ambient exposure. It is noted that the hysteresis behavior on the I–V curve of the MoS2 sample due to aging has never been reported before. Figure 3a shows the I–V curve for the pristine MoS2 sample measured from −1 to 1 V (forward) and then from 1 to −1 V (reverse), while Figure 3b–d represents I–V curves of the same sample measured after 4, 8, and 10 months, respectively. The I–V curve of the pristine device does not show any hysteretic behavior, while very little hysteresis was observed for the 4-month-old sample. However, significant hysteresis was observed for 8- and 10-month-old samples. The observed hysteretic behavior suggests the formation of trap charge states on the film for aged samples, which is originated from the ambient effect.31 Due to the nonlinear hysteretic nature of the I–V curve, we did not calculate the resistance of samples that were more than 4-month-old. After a year, the sample became completely electrically insulated. Interestingly, the film color also changed for a 1-year-old sample (Supporting Information, Figure S2) in comparison to the pristine film. We have attempted to measure gate dependence electrical properties by using highly doped Si as a back gate; however, no gate dependence was observed similar to what has been reported in a few other reports.9,12,28,32

Figure 3.

Figure 3

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Dual-sweep I–V characteristics of the MoS2 device measured (a) immediately after fabrication, (b) in 4 months, (c) in 8 months, and (d) in 10 months.

To identify the chemical states of the film, which are responsible for the observed changes in electronic transport properties, we performed XPS measurements of the pristine, 4-month-old (when a hysteretic I–V curve appears), and 1-year-old (when the film becomes completely electrically insulated) MoS2 films (Figure 4). The symbols represent experimental data and the solid lines represent convoluted spectra. Figure 4a shows the Mo 3d core-level XPS spectra of the pristine MoS2 with three prominent peaks expected at 226.0, 229.1, and 232.2 eV corresponding to the BEs of S 2s, Mo4+ 3d5/2, and Mo4+ 3d3/2 electrons, respectively.32,33Figure 4d shows the S 2p core-level XPS spectra of the same sample with peak positions at 161.7 and 162.8 eV corresponding to S2+ 2p3/2 and S2+ 2p1/2 spin–orbit split components of MoS2, respectively.30,33,34 For the 4-month-old MoS2 film, the Mo 3d XPS spectra (Figure 4b) show a slight shift to lower BEs. In addition, two new peaks were observed at 232.8 and 235.2 eV, which correspond to Mo6+ peaks due to the formation of MoO3.32,33,35 The S 2p XPS spectra (Figure 4e) of the same sample also show a slight shift to lower BEs, and a new peak appeared at 168.0 eV, which belongs to S6+ due to the oxidation of S.33,35 The downshift of BEs indicates a relative shift of the Fermi level toward the valence band edge, suggesting the p-doping of MoS2 films due to oxidation,33 which is consistent with electrical transport measurements. Similarly, the XPS spectra of Mo and S for the 1-year-old MoS2 film are presented in Figure 4c,f, respectively. For this sample, all the peaks corresponding to Mo 3d, S 2s, and S 2p were observed along with additional peaks at 232.4 and 235.2 corresponding to Mo6+ and at 168.0 eV corresponding to S6+. Interestingly, Mo6+ peaks of a 1-year-old sample became more prominent than the 4 month-old-sample, while the S6+ peak remained almost the same, which suggests that more MoS2 is converted into MoO3, while the oxide of sulfur remains almost constant.

Figure 4.

Figure 4

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XPS spectra of the MoS2 thin film. Mo 3d core-level XPS spectra of the (a) pristine MoS2 film, (b) 4-month-old film, and (c) 1-year-old film. S 2p core-level XPS spectra of the (d) pristine MoS2 film, (e) 4-month-old film, and (f) 1-year-old film. The spectra were deconvoluted using Gaussian–Lorentzian curves. The symbols are the experimental points, and the solid lines are the deconvolution of the data. (g) Percentage of MoS2 transformed into MoO3 with time, (h) percentage of MoS2 transformed into the oxide of sulfur with time, and (i) variation of the S/Mo ratio with time.

The percentages of the MoO3 formation and oxide of S are shown in Figure 4g,h, respectively. Remarkably, 13.1% MoS2 converted to MoO3 in 4 months, which increased to 23.1% in a year. On the other hand, 11.8% of oxide of S was calculated for the 4-month-old film, which slightly increased to 12.7% for a 1-year-old film (Figure 4h). The S/Mo ratio for the pristine MoS2 film was 1.99:1, which decreased to 1.94:1 for the 4-month-old sample and further decreased to 1.81:1 for a 1-year-old sample (Figure 4i). Although the observed 13.1% of MoO3 in the 4-month-old thin-film sample is comparable to reported MoS2 conversion into MoO3 (14.4%) for the CVD-grown monolayer in 6 months,14 the formation of the oxide of S was not observed for any previous stability studies of exfoliated or CVD-grown MoS2. However, the formation of the oxide of S was observed in the oxygen plasma-treated MoS2 thin film prepared by the same method,33 suggesting that the origin of this peak is related to the distinct crystallinity of the film originated from its smaller grain sizes, abundant grain boundaries, and exposed edges. These results suggest that the MoS2 thin film leads to the incorporation of atmospheric oxygen with Mo and S for the simultaneous formation of their oxides. Since the formation of MoO3 and oxide of S in the film is due to the absorption of oxygen from the atmosphere, the overall oxidation of the MoS2 film is faster than the oxidation of CVD-grown monolayer MoS2. In addition, organic contaminants and water were also detected on the MoS2 film (Supporting Information, Figures S3 and S4). This result suggests that the localized electrons at MoO3 domains attract the polar molecules and moisture from the atmosphere, which is the origin of the observed hysteresis in I–V curves in electrical transport measurements. We rule out the degradation of contact as a possible mechanism for the observed increase of resistance in the aged samples. This is due to the fact that the MoS2 film underneath the metal contacts is expected to protect the film from interaction with the atmospheric gases (similar to how PMMA protects the MoS2 film discussed in the next section). As a result, the contact resistance is expected to remain the same with time. Previous morphological studies on the ambient-exposed CVD-grown MoS2 monolayer and exfoliated flakes showed cracks and pits.14−16,19 In our morphological studies using atomic force microscopy (AFM) and scanning electron microscopy (SEM) (Supporting Information, Figure S5), we did not observe any visible cracks in the MoS2 thin film after long-term ambient exposure. However, we observed an increase in surface roughness with time along with dot- and dendrite-like structures, which can be explained due to the oxidation of the film, the absorbance of organic contaminants, and moisture.16 These observations are consistent with the observed increase of resistance and hysteresis for the aged devices.

To further investigate the ambient effects on the MoS2 film, Raman spectra of the MoS2 samples were recorded after ambient exposure. The measured Raman spectra of 4-month- and 1-year-old samples are shown in Figure 5a. Raman spectra of the pristine sample are also presented in the same plot for comparison (red curve of Figure 5a). As in the pristine film, two MoS2 characteristic peaks were observed for the 4-month-old sample at 385.8 and 412.3 cm–1 corresponding to the in-plane E2g1 and out-of-plane A1g vibrational modes. For the 4-month-old sample, the A1g peak was blue-shifted by 1.3 cm–1 compared with that of the pristine sample, suggesting that the p-doping of the MoS2 film further supports the results of XPS and electrical transport measurements.36 Similarly, E2g1 and A1g peaks of a 1-year-old sample were observed at 385.6 and 412.8 cm–1 with an additional blue shift of the A1g peak by 0.5 cm–1. In addition, we observed the broadening of Raman peaks of the films after ambient exposure. The fwhm of the pristine MoS2 sample was measured to be 7.3 and 8.7 cm–1 for A1g and E2g peaks, respectively, which increased to 8.5 and 10.3 cm–1 for the 4-month-old sample (Figure 5b). The fwhm of A1g and E2g1 peaks further increased to 11.0 and 11.5 cm–1 for a 1-year-old sample, respectively. The observed broadening of Raman peaks of older MoS2 samples has been attributed to the disorder introduced in the MoS2 film due to the incorporation of oxygen.33 The inset of Figure 5a shows Raman spectra at the low-wavenumber region. For the 4-month-old sample, we observed prominent peak intensity at 224 and 285 cm–1, which was very weak for the pristine sample. For a 1-year-old sample, the same peaks were observed along with an additional peak at 201 cm–1. All these peak positions coincide with the MoO3 Raman peaks.37

Figure 5.

Figure 5

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(a) Raman spectra of pristine, 4-month-old, and 1-year-old MoS2 films. The inset shows zoom-in Raman spectra of MoS2 E2g1 and A1g peaks and spectra at the low-wavenumber range. (b) fwhm of E2g and A1g peaks with an ambient exposure time.

To further confirm that the observed electrical property degradation is due to the ambient exposure, we encapsulated MoS2 thin-film devices with PMMA and measured electronic transport properties after 4 months and a year. We note that PMMA has been used to prevent the ambient degradation of extremely air-sensitive 2D materials.38,39Figure 6a shows the representative I–V characteristics of pristine- and PMMA-covered 4-month- and 1-year-old MoS2 devices. The resistance for the pristine sample was 295 MΩ, which slightly increased to 312 MΩ in 4 months and 355 MΩ in a year. Figure 6b shows a histogram of resistance for all 27 MoS2 samples in the chips. The average resistance of the PMMA-coated samples increased by only 6% compared to that of the pristine samples, whereas the average resistance of the unencapsulated samples increased by 430% in 4 months. In addition, the average resistance of the PMMA-coated samples increased by 40% in a year, whereas unencapsulated samples became completely insulated. The small increase of the resistance of the PMMA-coated device with time might be the effect of the moisture absorbed by PMMA because it can absorb as high as 2% of water.40 XPS measurements of the PMMA-coated 1-year-old sample show all the Mo 3d and S 2p doublet peaks at the same BEs as the pristine sample within an error margin of 0.1 eV, and only less than 1% MoO3 was detected (Figure 6c,d). These results confirm that the observed electrical property degradation of the film is due to the effect of ambient exposure and PMMA coating on the MoS2 film can effectively prevent the degradation of its electrical properties.

Figure 6.

Figure 6

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(a) Representative I–V curve of a pristine MoS2, 4-month-old, and a 1-year-old PMMA-coated device. (b) Histogram of resistance for pristine- and PMMA-coated 4-month- and 1-year-old MoS2 devices. (c) Mo 3d and (d) S 2p core-level XPS spectra of the MoS2 film measured in a year, which was stored with PMMA coating. The symbols are the experimental points, and the solid lines are the deconvolution of the data using the Gaussian–Lorentzian fit.

Conclusions

In conclusion, we report for the first time a systematic study of the electrical property variation of the MoS2 thin film under long-term ambient exposure. We demonstrated that the resistance of the MoS2 thin-film samples, prepared via the low-pressure sulfurization of the Mo film, monotonically increases with the ambient exposure time and becomes completely insulated in a year. The dual-sweep I–V curve of the pristine device did not show any hysteretic behavior, while significant hysteresis in I–V curves was observed for 8- and 10-month-old samples. XPS measurements reveal the gradual increase of the oxidation of the MoS2 film to form oxides of Mo and S simultaneously along with a shift of the Mo and S peaks to lower BEs. This causes a decrease of free electron carrier in the MoS2 film, which explains the observed increase of resistance in electrical transport measurements. The localization of electrons at MoO3 domains attracts the polar molecules and moisture from atmosphere, which is the origin of the observed hysteresis in I–V curves for aged samples. We also showed that the PMMA encapsulation of the MoS2 thin film can effectively prevent electrical property degradation, which shows only a 6% increase in resistance in 4 months and a 40% increase in a year.

Methods

MoS2 Film Growth

The MoS2 thin film was grown via the low-pressure sulfurization of the molybdenum (Mo) film in a furnace equipped with a 1 in. quartz tube. A silicon substrate with a 250 nm thick thermal oxide capping layer was rinsed with acetone, isopropyl alcohol, and deionized water, followed by rinsing with oxygen plasma for 10 min to ensure the cleanliness of the substrate. The Mo film of 6 nm thickness was deposited on the clean Si/SiO2 substrate by thermal evaporation and placed in the center of the furnace. Sulfur (S) powder (400 mg, 99.9%, Sigma-Aldrich) was kept in a separate quartz crucible in the upstream side at a distance of 16.5 cm from the center of the furnace. The chamber was purged with argon (Ar) gas (99.995% purity) for the removal of any residual oxygen and water vapor in the chamber and pumped down to a base pressure of ∼30 mTorr. The flow rate of Ar gas was adjusted to 130 standard cubic centimeter per minute (sccm) to transport S vapor to the center of the furnace. The furnace was then heated to the growth temperature of 800 °C at a ramping rate of 15°/min and was maintained at that temperature for 55 min. After the growth, the chamber was allowed to cool to room temperature naturally.

AFM and Raman Characterization

A tapping mode atomic force microscope (Veeco instruments, Dimension 3100) was used to determine the topography of the film. Raman characterization was performed using a WITec alpha 300 RA confocal Raman microscope with a laser source of an excitation wavelength of 532 nm and a power of <1 mW under ambient conditions at room temperature. A 100× objective was used to focus the laser beam at a spot. Raman emission was collected and dispersed by a grating of 1800 lines-per-mm with a data accumulation duration of 3 s.

XPS Characterization

XPS measurements of the MoS2 samples were performed using a Thermo Scientific (ESCALAB Xi) XPS system with a monochromatic Al Kα radiation source. A pass energy of 20 eV with a 0.1 eV scanning step was used for photoelectron detection. XPS spectra were taken onto the sample surface with a scan area of 300 × 300 μm2, and a carbon (C) 1s reference line at a BE of 284.8 eV was used to calibrate the charging effect.

Device Fabrication and Transport Characterization

For the electrical transport characterization of the MoS2 film, 5 nm/40 nm Cr/Au electrodes were deposited on top of the film using a shadow mask. The drain–source electrodes were deposited with a deposition rate of 0.05 Å/s at a base pressure of 5 × 10–7 mBar. The electrical transport measurements of the devices were performed in a probe station using a Keithley 2400 source meter and a current preamplifier (DL instruments 1211) interfaced with the Lab View program (National Instrument). All electrical measurements were carried out at room temperature in a two-probe configuration. The devices are kept under ambient conditions (room temperature, atmospheric pressure, and humidity 51%) and measured again for stability comparison. For PMMA encapsulation, the MoS2 devices were coated with ∼120 nm thick PMMA (molecular weight of 950k). After 4 months, PMMA was removed by dipping into acetone and electronic transport properties were measured. This was repeated for the 1-year-old samples.

Acknowledgments

This work was supported by the U.S. National Science Foundation (NSF) under grant no. 1728309. We acknowledge Sajeevi Withanage and Prof. Laurene Tetard for helping with Raman measurements.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c03522.

  • Box plot of the resistance variation of the pristine MoS2 thin film and after left under ambient conditions with and without PMMA coating, optical microscopic images of the large-area pristine MoS2 thin film and after keeping under ambient conditions, XPS O 1s spectra of pristine and ambient-exposed MoS2 films, XPS C 1s spectra of pristine and ambient-exposed MoS2 films, and AFM and SEM surface topography images of pristine and ambient-exposed MoS2 thin films (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c03522_si_001.pdf (609.4KB, pdf)

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