Gas-modulated optoelectronic properties of monolayer MoS₂ for photodetection applications

The integration of next-generation multifunctional semiconducting technology is based on the ability to tune the characteristics of 2D semiconductors effortlessly.¹ Defect engineering is an established approach for tuning the electrical and optical characteristics of semiconductors.² Although several studies have recently been conducted on 2D materials and their fascinating qualities, controlling the creation of defects and, consequently, the inherent characteristics of these materials is a critical step toward making them realistically feasible.² Among the various 2D materials, transition metal dichalcogenides (TMDs), such as monolayer MoS₂ (M-MoS₂), are considered promising 2D materials for next-generation optoelectronic devices.³ It has an atomically thin structure with a high surface-to-volume ratio, high absorption, stability, flexibility, and an appropriate bandgap of ∼1.8 eV.^4,5 These unique characteristics of M-MoS₂ make it a substitute for semiconducting materials in fabricating photovoltaic cells, LEDs, and photodetectors (PDs) because they have different properties that suit them well with existing semiconductor technologies.^6,7 The built-in defects in M-MoS₂, specifically sulfur vacancies (V_s), significantly influence the performance of devices by altering carrier concentration, mobility, and interaction with external stimuli.^8,9 Therefore, an investigation is essential to fully understand the impact of inherent defects, such as V_s, on the optical behavior of M-MoS₂ under different gas circumstances.

There are several obstacles to achieving scalable and controlled defect engineering in M-MoS₂. Various approaches have been reported to investigate the defects, including UV photon irradiation, chemical functionalization, electron/ion beam irradiation, laser processing, thermal annealing, and plasma treatment.^2,10–12 Despite their effectiveness in inducing defects, they often lead to structural degradation and unpredictable properties in MoS₂-based devices, increase production costs, and limit practical application. This study presents a novel method for defect engineering in M-MoS₂ caused by exposure to gaseous environments. In particular, V_s can be controlled by the exposure of M-MoS₂ to gaseous environments of H₂S and NO₂ without compromising its structural integrity. We also investigated the effect of external perturbation on dynamic optoelectronic properties and emphasized the significant role played by V_s in determining their photoresponse behavior. These gases introduce charge states while modifying the photoconductivity of the materials, thus influencing carrier generation and recombination rates. The findings could significantly advance the area of sensing based on 2D materials and enable the development of highly selective and sensitive sensors for next-generation sensing applications. It also offers a reliable and reproducible defect engineering solution for 2D materials.

Using a two-zone CVD technique, the M-MoS₂ growth was optimized on a SiO₂/Si substrate. The precursors such as molybdenum oxide (MoO₃) and sulfur (S) powders were loaded in ceramic boats in different zones after flushing the tube at 1050 °C. Zone-1 was maintained at 160 °C (S), zone-2 was maintained at 780 °C (MoO₃), and argon gas continuously flowed at 40 SCCM in the precisely controlled environment. For the optimization of M-MoS₂, Raman spectroscopy (Jobin Yvon instrument) and atomic force microscopy (AFM; Multimode V Veeco system) were used to analyze the vibrational modes and high-resolution topographic images to confirm its formation. Furthermore, the surface composition of the thin film was examined using x-ray photoelectron spectroscopy (Scienta Omicron with an Al Kα source) by calibrating the C 1s position at 284.6 eV. The optical properties of the film are analyzed using the AvaSoft-8 UV–visible spectroscopy software (Avantes spectrometer) and the Edinburgh photoluminescence (PL) (FLS-980) D2D2 spectrometer, respectively. The device was fabricated on a single MoS₂ flake using the lithography technique. The size of the triangular flake is 26 μm, and the spacing between the electrodes is 1 μm. We obtained the photoresponse of the PD device using a Keithley 2450 source meter as the data collection unit and a probe station equipped with an S-10 Triax cable. The current detection limit of the PD device is in the femtoampere range and has a precision of 100 ns in response time evaluation.

The schematic presentation and the temperature profile of the experimental setup used to synthesize M-MoS₂ are shown in Figs. 1(a) and 1(b), respectively. The optical microscopic image of grown M-MoS₂ is shown in Fig. 1(c), indicating well-defined, isolated triangular MoS₂-flakes with sharp edges of high crystallinity. The triangular shape of the crystallites mirrors the threefold symmetry of MoS₂, suggesting that they are single-crystalline.¹³ The top view of the Field Emission Scanning Electron Microscopy (FESEM) image of one flake and the optical image of the monolayer grown sample is shown in Figs. S1(a) and S1(b) of the supplementary material. The high-resolution topographic images obtained by AFM [Fig. 1(d)] reveal a uniform flake and thickness. The determined thickness is ∼0.7 nm. Moreover, the Raman spectra show peaks at 385.24 and 403.24 cm⁻¹, revealing the presence of two distinct peaks with a Raman shift of 18.0 cm⁻¹, corresponding to the E¹_2g vibrational mode for the I-plane atom and the A_1g mode for out-of-plane vibration [illustrated in Fig. 1(e)]. A satellite peak around 380 cm⁻¹ can be observed, attributed to a localized vibrational mode of a longitudinal optical branch [LO(M)] caused by V_s. In addition, to support the claim of uniform growth, Raman mapping was observed, revealing the uniform distribution of MoS₂ in the flake shown in Fig. 1(f).¹⁴ 

Furthermore, to analyze the chemical composition of M-MoS₂, Mo 3d core level spectra are shown in Fig. 1(g). [The survey scan illustrated in Fig. S1(c) confirms the purity of M-MoS2.] The spectra exhibited doublets for Mo 3d_5/2 (229.31 eV) and Mo 3d_3/2 (232.42 eV), with a spin–orbit parting of 3.11 eV and distinctly separated Mo–S (Mo⁺⁴) bonding in MoS₂. The evidence peak of the Mo–S bond can be observed at 226.39 eV. The S 2p core level [Fig. 1(d) of the supplementary material] confirms the oxidation state. In particular, the doublet 2p_1/2 and 2p_3/2 appear at 163.54 and 162.40 eV, respectively, with a peak splitting energy of 1.14 eV.¹⁴ The stoichiometry was calculated using the Mo 3d core level of spectra of XPS.¹⁵ [Equation (S1)] The estimated ratio of Mo and S, around 1:1.8, suggests a sulfur deficiency in the grown MoS₂ flakes. Further analysis was conducted on the optical characteristic UV–Vis absorption spectra; distinct excitonic peaks at ∼695 and 610 nm were observed. These peaks represent the excitons A and B, confirming that M-MoS₂ exhibits a semiconducting nature and a direct bandgap, which is essential for its application in PDs.¹⁶ 

Figure 1 confirms the growth of M-MoS₂ with a sulfur deficiency that consists of a Mo atom sandwiched between two S atoms, as shown in Fig. 1S(e) of the supplementary material. To play with the defect, the device was fabricated, and the engineering of the device was observed in different stimuli. We have used the concentration of gases (H₂S: 10 ppm, NO₂: 10 ppm) and the power of the light source (10 mW/cm² for 636 nm wavelength). Figure 2(a) depicts the schematic image [optical image of the device displayed in Fig. 1(f) of the supplementary material] of the device used to study the impact of various gases on the performance of the device. The I–V characteristics of gases without light illumination are shown in Fig. 2(b). The dark current (I_d) at 0 V in the air was found to be 3.02 pA, while I_d for the gases H₂S and NO₂ was 2.27 and 5.07 pA, respectively.

The changes in I_d are caused by the interactions of H₂S and NO₂ gases with the surface of MoS₂. Interactions with H₂S reduce I_d by the passivation of defect states, while NO₂ increases the defects, which act as trap states, leading to higher I_d.¹⁷ To understand the optoelectronic properties, the I–V in 626 nm light illumination under different gases is explained in Fig. S2 of the supplementary material. Furthermore, the transit response of the device using a 626 nm light source under different gas environments (H₂S, air, and NO₂) has been recorded. The power of the light was adjustable, and the applied bias was maintained at 0.5 V. Figure 2(c) shows I_ph with time under various gas environments, and the source was periodically switched on/off at 10-s intervals. We have observed significant changes in I_d at 0.5 V under air, H₂S, and NO₂; the values were 0.79, 0.58 and 1.4 nA, respectively. The current was increased by light illumination with air, H₂S, and NO₂ exposure. The values were 7, 9, and 4.8 nA, respectively, indicating responsiveness to different gases. Using I_ph, the performance of the device was evaluated in terms of R, detectivity (D), and noise equivalent power (NEP).^18,19 In the air environment, the device exhibited R, D, and NEP of 10 296 A/W, 1.2 × 10¹² Jones, and 1.56 × 10⁻¹⁸ W Hz^−1/2, respectively, as illustrated in Fig. 2(d) and Fig. S3 of the supplementary material (Table S1 of the supplementary material). When we exposed the device to H₂S gas, the R was increased by 136%, and another parameter was also enhanced. In contrast, the device performance in the NO₂ gas environment was decreased, as discussed in the supplementary material. In addition, we also investigated the influence of different gases, as shown in Fig. 2(e). Under H₂S, air, and NO₂ gases, the rise time was calculated as 615, 625, and 687 ms and the decay times were 846 ms, 1.5 s, and 1.58 s, respectively. The response time (τ) under air was taken as a baseline. When exposed to H₂S, the device displayed significantly faster rise and decay times, indicating enhanced photocarrier generation and recombination due to decreased Schottky barrier height and increased electron concentration. However, in the case of the NO₂ gas environment, slower rise and decay times were exhibited, suggesting slower photocarrier dynamics due to increased Schottky barrier height and reduced electron concentration. The fitted curve closely matched the experimental data, confirming the positive impact of H₂S on enhancing the photodetection performance of the device.

Inducing defects in the device due to exposure to environmental gas happens in three stages. (1) H₂S gas molecules come into contact with the surface of MoS₂, where they can become physically or chemically adsorbed. At defect sites or edges with a higher binding energy, H₂S molecules typically split into HS⁻ and H⁺ ions. (2) The adsorbed H₂S molecules act as electron donors, transferring electrons to the MoS₂, which changes the electron concentration and impacts the sensitivity and response time of the PD. (3) Passivation or modification of trap states occurs through an interaction with H₂S.^17,20 In addition, H₂S modifies the height of the Schottky barrier, influencing the carrier injection efficiency and affecting the performance of PD. On the contrary, exposure to NO₂ induces p-type doping in MoS₂ by accepting electrons from the surface, as shown in Fig. S4 of the supplementary material. This reduces the number of electrons and changes the performance of PD. The interaction of NO₂ and H₂S with M-MoS₂ affects the carrier and changes the Schottky barrier height inversely, as shown in Figs. 3(e) and 3(f). Understanding the interactions of M-MoS₂ with gas-specific changes and PD performance is important for future research.

In summary, we have explored the growth of M-MoS₂ using the CVD technique and investigated the effect of defect engineering on the performance of PD. The fabricated device shows an excellent responsivity of 1.02 × 10⁴ A/W and an ultralow NEP on the order of 10⁻¹⁸ W Hz^−1/2 in the air environment. The impact of V_s and its interactions with H₂S, NO₂, and ambient conditions have been studied. It was observed that the device performance improved (136% in responsivity) in the H₂S and decreased (68%) in the NO₂ environment compared to the air. Our findings highlight the critical role of V_s in determining the sensitivity and selectivity of MoS₂-based optical sensors. Exposure to H₂S led to the passivation of V_s, resulting in enhanced PL intensity, faster carrier recombination times, and decreased I_d due to reduced non-radiative recombination pathways. Meanwhile, exposure to NO₂ caused PL quenching, slower recombination times, and increased dark current, which indicates enhanced non-radiative recombination and charge trapping. These changes underscore the sensitivity of M-MoS₂-based devices to gas interactions and highlight their potential for sensing applications in various environments. The photocurrent and response time measurements also demonstrated the sensitivity of M-MoS₂ to gas exposure, with faster response times and increased photocurrent upon H₂S exposure, while NO₂ exposure resulted in prolonged response times and decreased photocurrent. This research significantly advances the understanding of defect engineering in 2D materials for gas-sensing applications, providing detailed mechanistic insights into how V_s influences gas interactions. This work establishes a robust foundation for future research in optimizing MoS₂-based devices for environmental monitoring and industrial applications.