Defect control during CVD-growth for high performance MoS₂-based self-powered photodetector

Abstract

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) materials have unique band structure as well as excellent electrical and optical properties, which exhibit great advantages in optoelectronic devices. Chemical vapor deposition (CVD), a method to realize the synthesis of large-scale 2D TMDs materials, will inevitably introduce defects in the growth process, thus decreasing the performance of 2D TMDs-based optoelectronic devices. In order to fundamentally address this issue, we proposed a method to gradually regulate the reaction concentration of precursor during growth. As a result, the suitable concentration of precursor can effectively enhance the probability of covalent binding of X−M (X: S, Se, etc.; M: Mo, W, etc.), thus suppressing the generation of vacancy defects. Furthermore, we explored sulfur vacancy (V_S) on the performance of 2D molybdenum disulfide-based (MoS₂-based) self-powered devices through constructing p-type silicon/MoS₂ (p-Si/MoS₂) based p–n heterojunction. The photodetector composed of optimized MoS₂ nanosheets exhibited high responsivity (330.14 A·W⁻¹), fast response speed (40 μs/133 μs), and excellent photovoltage stability. This method of regulating the low temperature region during CVD growth can realize the preparation of high-quality TMDs films and be applied in high-performance optoelectronic devices.

Graphical abstract

Keywords

defect control / sulfur vacancy / chemical vapor deposition (CVD) / photodetector / transition-metal dichalcogenides (TMDs)

1 Introduction

Due to the advantages of tunable band gap (1–2 eV) [1, 2], strong light-matter interaction [3, 4] and ultrafast carrier transport [5], the 2D layered TMDs materials have been widely used in the field of the optoelectronic devices [6], such as phototransistors [7−10], solar cells [11, 12], and especially photodetection [13−16]. At present, CVD is generally considered as the most promising method for the controlled large-scale synthesis of TMDs [17−19]. However, according to the second law of thermodynamics, the defects are more likely to generate in TMDs synthesized by CVD method because of the unstable and low formation energy of defects during the reaction. The presence of defects in TMDs-based heterojunctions can decrease the performance of optoelectronic devices, such as responsivity, response speed, and photovoltage stability. Therefore, it is of great significance to study how to suppress the generation of defects to some extent and decrease the negative impact of defects on optoelectronic devices.

The defects in TMDs include vacancies, impurities, attached atoms, etc. [20], among which vacancy defects are the most stable structural defects. In order to efficiently repair the vacancy defects, most studies have focused on post-treatment. Post-treatment refers to processes of treating TMDs film grown on the substrate. The main method is surface passivation treatment [21−26], which can fill vacancy defects with oxygen(O)/sulfur(S)/other atoms to repair the surface overhanging bonds of materials. It is considered as an effective method to improve the performance of devices. For example, Lee et al. [21] used O plasma to heal V_S, improving the channel quality and mobility of FETs. Gao et al. [22] efficiently repaired V_S by annealing under S vapor after CVD growth. Schwarz et al. [23] healed V_S using chemical thiol reagents adsorption at active vacancy sites. Above methods can only repair the defects on the surface of TMDs, while the defects inside the material are not treated. In fact, most post-treatment methods not only require complex processes, but also increase the preparation cost and are extremely easy to damage the surface. Therefore, it is crucial to study how to suppress the generation of vacancy defects during the preparation stage of growth. We took the growth of MoS₂ as an example. Zhou et al. [27] discussed the relationship between formation energy of point defects and the S chemical potential, and found that the formation energy of V_S is the lowest regardless of the environment rich in S or Mo. Therefore, the V_S is the most easily formed vacancy defect during CVD growth. Many reports have altered the growth environment [28−31] during CVD growth to suppress the production of V_S, such as regulating growth temperature, turning pressure, and using single-phase precursors. For example, Tomar et al. [28] explored the effect of growth temperature in high temperature region on the morphology and vacancy defects of MoS₂ film. Abidi et al. [29] altered the pressure to suppress the generation of V_S during growth. Mawlong et al. [30] introduced additional DMS in the liquid-phase precursor to create a S-rich environment that suppressed V_S formation, but additional chemical solution is very likely to introduce impurities. However, it is unknown whether the higher concentration of S source will synthesize MoS₂ nanosheets with larger size and lower V_S density. Therefore, it is necessary to systematically discuss the dependence of precursor concentration on the growth of 2D-MoS₂ nanosheets and its application in optoelectronic devices.

In this work, we proposed a method to gradually regulate the reaction concentration of precursor during CVD growth. Furthermore, the MoS₂ nanosheets were transferred to p-Si substrates to form p-Si/MoS₂ heterojunction photodetectors, which realized self-powered characteristics. Interestingly, too low temperature of S source cannot drive the reaction completely, and too high temperature of S source will lead to undesired nucleation and growth due to ultra-high S chemical potential. The appropriate S source temperature can effectively enhance the probability of Mo–S covalent binding and suppress the generation of V_S, which facilitates reaction to synthesize high-quality MoS₂ nanosheets. This contributes to light absorption and efficient photocarriers separation of interface for the application in the high-performance p-Si/MoS₂ self-powered photodetector.

2 Experimental

2.1 Preparation of MoS₂ nanosheets

The double temperature zone tube furnace (KTL1700-1400, Nanjing University instrument factory) was used to grow MoS₂ nanosheets. 200 mg S powder (99.98%, Aldrich) and 50 mg MoO₃ powder (99.5%, Aldrich) were used as precursors. The temperature in the low temperature zone was set at 150–180 °C to create a different concentration of S vapor environment, and the temperature in the high temperature zone was 700 °C, and the reaction was maintained for 60 minutes. The reaction was carried out in an Ar gas environment with a flow rate of 15 sccm.

2.2 Photodetector fabrication

The p-Si/MoS₂ self-powered photodetector was constructed by transferring MoS₂ nanosheets onto p-Si by wet transfer technology. A small amount of PMMA solution (about 20 µL) was dripped onto MoS₂/SiO₂/Si substrate surface and placed into a Spin Coater (4000 r/min for 30 s). Place it on the heating platform to dry at 80 °C for 5 min, and then keep it at 150 °C for 15 min (so that PMMA and MoS₂ were fully integrated). The PMMA/MoS₂ layer was separated from the substrate by placing the substrate in a NaOH aqueous solution (30%). Then the PMMA/MoS₂ layer was washed in ultra-pure water to remove the remaining NaOH, and fished out with p-Si substrate. Place it again on the Heating Platform (80 °C for 5 min, and then 150 °C for 15 min). Then, PMMA/MoS₂/p-Si substrate was placed in an acetone solution to remove PMMA and it was cleaned by immersion in absolute ethanol and dry the sample.

2.3 Characterizations

The size of MoS₂ nanosheets grown at different temperatures of S source was evaluated under an optical microscope (DM4000, Germany). The number of growing MoS₂ layers was confirmed by Raman spectrometer (labRAM HR Evolution) with a 532 nm linearly polarized light. The photoluminescence test was used to evaluate the defect content. X-ray diffractometry (D8 Focus, Germany) was used to judge the phase constituent of MoS₂ nanosheets. The current–voltage (I–V) test system included a Keithley 2400 digital source meter, white light laser (GTEBL-01), and probe station (EPS-300). The laser with a wavelength of 532 nm was focused through the lens and irradiated on the surface of the device. The pulsed photovoltaic response test system mainly included an oscilloscope (Tektronix MDO3032), white light laser (GTEBL-01), optical chopper (SR540), and probe station (EPS-300). The wavelength of light was 532 nm, and the frequency and power density of the chopper were 2000 Hz and 180 mW·cm⁻², respectively. The system was used to evaluate the response speed of the device.

3 Results and discussion

Fig.1(a) shows an experimental process of CVD method to grow MoS₂ materials. We used double temperature zone tube furnace to precisely control the temperature of MoO₃ powder and S powder precursors, respectively. The redox reaction equation for MoS₂ synthesized by CVD method is as follows:

(1)7{\mathrm{S}}_2+4{\mathrm{M}\mathrm{o}\mathrm{O}}_3\to 4{\mathrm{M}\mathrm{o}\mathrm{S}}_2+6{\mathrm{S}\mathrm{O}}_2.

We controlled the temperature of S precursor by regulating low temperature zone, creating different S vapor concentrations in the MoS₂ growth process. Specifically, MoO₃ powder is first reduced by S vapor, forming volatile MoO_3−x. Further, the partially reduced MoO_3−x is transported downstream by high-purity Ar carrier gas, which adsorbed and diffused on the surface of the Silicon dioxide (SiO₂) substrate. Finally, the MoO_3−x will be further reduced by the S vapor near the substrate, thus obtaining the MoS₂ nanosheets. During the chemical synthesis of MoS₂, the stability of crystal growth will be also affected by too high or too low concentration of S vapor. The following experimental part will be discussed in depth. Fig.1(b) illustrates the formation process of V_S in MoS₂ nanosheets by CVD growth. In order to ensure sufficient supply of S source, we used 200 mg S and 50 mg MoO₃ for the condition that the ratio of Mo:S is less than 1:2. We start from a hexagonal nucleation to discuss the growth process of MoS₂ nanosheets. The three sides of the hexagonal nucleus are the zigzag ends of Mo atom, and the other three sides are the zigzag ends of S atom. In an abundant S supply environment, the energy of S atomic edge is more stable than the Mo atomic edge. The unsaturated Mo atoms with two hanging bonds are easier to meet and bond with free S atoms, which promotes the hexagonal nucleus to grow into triangular MoS₂ nanosheets with S edge. In this process, the concentration of S vapor directly determines the driving speed of the redox reaction, which becomes the key to the growth of MoS₂ as to the reaction equation (1). Ultra-low concentration of S vapor cannot drive the forward reaction completely, while ultra-high concentration of S vapor leads to too fast reaction speed, causing the energetic instability and irregular crystal growth. In order to create the optimal local chemical environment rich in S vapor and grow high-quality MoS₂ nanosheets, we deliberately controlled the temperature of S source in the low temperature zone from 150 to 180 °C, including six temperature states, as shown in Fig.1(c).

Fig.2 shows the optical images of MoS₂ nanosheets grown on the SiO₂/Si substrates at different temperatures of S source. The growth process of MoS₂ depends on the Gibbs free energy of the reaction associated with the S chemical potential, such as nucleation, aggregation, and self-assembly. As elaborated by the following observation: (i) When the temperature of S source was 150 °C, black particles can be seen on the substrate surface, which is identified as not fully sulfurized MoO_3−x by Raman [See Fig. S1 in the Electronic Supplementary Materials (ESM)]. Due to the ultra-low concentration of S vapor will lead to the low S chemical potential, there is insufficient energy to overcome the barrier to initiate nucleation and growth of MoS₂. (ii) The high S chemical potential (i.e., the S-rich chemical environment) is more favorable to drive the reaction of MoS₂ by CVD method. It can promote the Mo atoms easier bonding with the surrounding free S atoms, increasing the probability of Mo−S covalent binding, thus speeding up the reaction rate. According to the principle of crystal growth, the growth rate of the crystal surface determines the shape and size of crystal [32]. When the temperature of S source was increased from 160 to 170 °C, the majority size of MoS₂ nanosheets was doubly enlarged. This phenomenon reflects that MoS₂ is easier to overcome the barrier of nucleation and agglomeration with the increase of concentration of S vapor. Besides, high S chemical potential will facilitate the growth of high-quality MoS₂. The following Raman, X-ray diffraction (XRD) and photoluminescence (PL) will further verify the defect density regulated by S vapor concentration. (iii) When the growth temperature reaches 175 °C and 180 °C, the energy in the reaction process is unstable due to ultra-high S chemical potential, leading to the undesired nucleation and growth of MoS₂ nanosheets. The introduction of the defect state can be attributed to the fact that the growth rate of Mo edge is much faster than that of S edge, making the growth energy of the whole MoS₂ crystal unstable. In summary, when the temperature of S source ranges from 160 to 170 °C, it is more conducive to the growth of large-scale and ultra-thin MoS₂ nanosheets. The application of MoS₂ nanosheets with different defect densities to optoelectronic devices will be discussed in the following sections.

In order to investigate the influence of temperature on sample crystal quality, we used Raman and XRD to characterize MoS₂ nanosheets grown at the temperature of S source ranging from 160 to 170 °C. Fig.3(a) shows the Raman spectra of MoS₂ nanosheets. Two sharp and strong peaks were detected, corresponding to the in-plane vibration mode {\mathrm{E}}_{2\mathrm{g}}^1 and the out-of-plane vibration mode A_1g, respectively. The wave difference of A_1g−{\mathrm{E}}_{2\mathrm{g}}^1 Raman peak at 160 °C, 165 °C, and 170 °C is 21.91 cm⁻¹, 21.14 cm⁻¹, and 20.36 cm⁻¹, respectively (See Fig. S2 in the ESM), suggesting that the MoS₂ is bilayer [33]. Fig.3(b) shows the full width at half maxima (FWHM) for the three sets of MoS₂ nanosheets. As the temperature of S source increases, the FWHM of the {\mathrm{E}}_{2\mathrm{g}}^1 and A_1g mode become narrower and the fluctuation of the statistical average is smaller, indicating that the crystallization quality of the MoS₂ nanosheets was significantly improved [34]. Fig.3(c)–(e) show the XRD patterns for MoS₂ nanosheets grown at different temperatures of S source. Compared with the Powder Diffraction File (PDF) card [Fig.3(f)], the XRD pattern has six diffraction peaks when the temperature of S source is 170 °C, corresponding to the (002), (004), (100), (104), (110), and (112) lattice planes, respectively. At the same time, the peaks are significantly sharper, which verifies the high crystal quality of MoS₂ nanosheets. As the temperature of S source decreases, some diffraction peaks of MoS₂ nanosheets are difficult to detect. It further confirmed that the suitable S vapor concertation contributed to the growth of MoS₂ nanosheets with high crystal quality.

To evaluate the effect of V_S on the PL of MoS₂ nanosheets, PL spectra of three samples were compared, as shown in Fig.4(a). As the temperature of S source increases, the intensity of PL peak enhances and blueshift occurs. In the process of photoexcitation, MoS₂ nanosheets absorb photon energy and generate excitons inside. Excitons have three main channels in the process of recombination luminescence. Intrinsic luminescence of A and B excitons is produced by splitting of the spin orbit of the high price band. The V_S defect trapping excitons cause the V_S defect state luminescence. Therefore, the Gaussian fitting was performed for the PL signal to verify the mechanism behind it, as shown in Fig.4(b). Among them, dark blue, light blue and green regions correspond to V_S defect state PL peak (I_Vs), A exciton PL peak (I_A), and B exciton PL peak (I_B), respectively. As the temperature of S source increases, the area of A excitons PL peak gradually occupies whole spectrum. In order to further quantify the change of PL intensity of three components, the variation trend of PL intensity is shown in Fig.4(c), and the inset shows the percentage of V_S defect state in the total spectrum. It can be clearly seen that the ratio of the area occupied by the V_S PL peak to the total area of the whole spectrum gradually decreases. This result confirms that the V_S density associated with defect state decreases as the temperature of S source increases. In essence, the formation energy of V_S is directly related to the supply of S source or Mo source. The higher S source temperature leads to higher S chemical potential, resulting in higher V_S formation energy [27]. Therefore, V_S is not easy to form when the temperature of S source is 170 °C. For the MoS₂ nanosheets with optimized V_S density, the PL intensity associated with A and B excitons is enhanced by 10 times and 2 times, respectively. The reason for the enhancement of intrinsic luminescence is that the reduction of V_S density will reduce the number of excitons trapped by the defect state, thus enhancing the efficiency of radiative recombination luminescence for intrinsic excitons [35]. Meanwhile, PL blueshift is ascribed to the upward shift of the valence band maximum caused by V_S defect state, leading to the band gap reduction of MoS₂ [36].

Fig.5(a) shows the schematic diagram of the current–voltage (I–V) response test system. Fig.5(b) shows the energy band diagram of p-Si/MoS₂ based p–n junction under light irradiation. The built-in field is induced by the electric potential difference between MoS₂ and p-Si. The built-in field makes the device generate photogenerated electromotive force to realize the self-powered characteristics [37]. Fig.5(c) shows I–V characteristic curves of optimized p-Si/MoS₂ self-powered photodetector, which exhibits excellent rectification characteristics in dark state. As the light power density increases, the forward current of the p-Si/MoS₂ self-powered photodetector remains essentially unchanged, while the reverse current is significantly enhanced. This situation is consistent with the typical properties of photovoltaic photodetectors [38]. Fig.5(d) shows the dependence of photocurrent on the light power density. The qualitative relationship between light power density and photocurrent is

(2)I_{\mathrm{p}\mathrm{h}}={CP}^{\beta },

where I_ph, C, and P represent the photocurrent, the coeffcient of proportionality, and the light power density, respectively. With the increase of light power density, the photocurrent (I_ph = I_λ – I_dark) of three p-Si/MoS₂ self-powered photodetectors increases. The value of β reflects the defect density of film, and the closer to 1, the lower the defect content. The optimized p-Si/MoS₂ photodetector has the highest β value (0.98), indicating an ultra-low defect density. This means that more photogenerated electron–hole pairs can be converted into photocurrent, rather than being trapped by the defect states of interface. Therefore, the optimized photodetector exhibits excellent photoresponse characteristics. Fig.5(e) shows the dependence of responsivity (R) on light power density. Its formula can be expressed as

(3)R=I_{\mathrm{p}\mathrm{h}}/\left(P\times S\right),

where P and S represent the light power density and the effective photovoltage response area of light irradiation, respectively. The optimized photodetector shows the highest responsivity of 330.14 A·W⁻¹, which realizes 23 times enhancement compared to the photodetector with high V_S density. This is because low V_S density will reduce the number of trapped excitons, thus enhancing the light absorption of the material. Besides, high contact quality of the heterojunction interface will also suppress the recombination of photocarriers. The detectivity (D*) of three self-powered photodetectors was evaluated, as shown in Fig.5(f). The calculation formula of detectivity is

(4)D^{\ast }={\displaystyle \frac{S^{1/2}R}{{\left(2q{I}_{\mathrm{d}\mathrm{a}\mathrm{r}\mathrm{k}}\right)}^{1/2}}},

where R is responsivity, I_dark is the dark current, and q is the unit charge. The p-Si/MoS₂ self-powered photodetectors with the lowest V_S density exhibited the highest detectivity, with a maximum of 1.0×10¹⁰ Jones.

Furthermore, the response speed of the p-Si/MoS₂ self-powered photodetector was evaluated by laser-pulse test system without power supply, as shown in Fig.6(a). Fig.6(b) shows photovoltage response characteristic curves of p-Si/MoS₂ self-powered photodetectors with different V_S densities. The p-Si/MoS₂ photodetector constructed with the lowest V_S density has the strongest self-powered capability, which is consistent with the above photoresponse test results. A single photovoltaic response signal was extracted and normalized, as shown in Fig.6(c). We further discussed the dependence of photovoltage and response time on V_S density, as shown in Fig.6(d). Usually, high photovoltage takes more time to respond. Interestingly, we found that the p-Si/MoS₂ self-powered photodetector with the lowest defect density exhibits the highest photovoltage, together with the fastest response speed. This can be attributed to the high quality of heterojunction interfaces promoting the separation of photocarriers.

To further evaluate the effect of V_S on the photovoltage stability, three photodetectors constructed with different V_S densities were exposed to an environment with 31% humidity treatment for half an hour. Fig.7(a) shows Raman spectra of three photodetectors with different V_S densities after treatment in humidity. In contrast to the {\mathrm{E}}_{2\mathrm{g}}^1 vibration mode, the A_1g vibration mode is sensitive to electrons [39], so we focused on the blue shift in A_1g. When the temperature of S source is 160 °C, the blue shift of A_1g peak positions is the largest (1.9 cm⁻¹). This implies that the largest number of lost electrons from defects is due to the adsorption of water and oxygen [40], and the defect density is the highest. Fig.7(b)–(d) show comparison of photovoltage stability of three p-Si/MoS₂ self-powered photodetectors before and after treatment in humidity environment. As the V_S density increases, the attenuation of the photovoltage for three sets of samples gradually increases, namely: 6.7%, 10.5% and 43.9%, respectively. Thus, the optimized p-Si/MoS₂ self-powered photodetector exhibits the most excellent photovoltage stability. When p-Si/MoS₂ self-powered photodetector is placed in humidity environment, the water and oxygen molecules initially undergo chemisorption at the V_S, and physisorption occurs on the upper layer [41−43]. Chemisorption causes water or oxygen to bond with Mo/S atoms, extracting electrons from MoS₂ and reducing the carrier concentration of MoS₂ [40]. Physisorption causes incident light to be refracted by the surface of water molecules, reducing the absorption of light by the material. Meanwhile, the physisorbed water molecules produce an amount of hydronium ions (H₃O⁺), which reduces the height of the interface barrier [44]. This further weakens the strength of the built-in electric field. The photocarriers cannot be effectively separated and the migration speed of photocarriers is slowed down. Therefore, the photoelectric conversion efficiency of the photodetector is affected by the humidity treatment, which determines the photovoltaic stability.

Above all, compared with the recently reported 2D TMDs-based self-powered photodetectors, our device exhibited high photoresponse and fast response speed (Tab.1).

4 Conclusion

In summary, we proposed a method to gradually regulate the reaction concentration of precursors during CVD growth to suppress the generation of vacancy defects in TMDs materials. Different from previous reports, we found that the suitable concentration of S precursor contributed to the growth of MoS₂ nanosheets with low density of V_S. As a result, the MoS₂ grown at the S source region 170 °C has the lowest V_S density that can be confirmed by Raman, PL, and XRD. Moreover, the optimized MoS₂ nanosheet was applied to the high-performance self-powered photodetector, which realized high responsivity (330.14 A·W⁻¹), fast response speed (40 μs/133 μs), and excellent photovoltage stability after being exposed to 31% humidity treatment. This study provides an avenue for defect regulation and material optimization of other TMDs, which is helpful for application in high-performance photodetectors.

References

[1] R. Yang, J. Fan, and M. Sun, Transition metal dichalcogenides (TMDCs) heterostructures: Optoelectric properties, Front. Phys. 17(4), 43202 (2022)

[2] C. L. Tan, Z. C. Lai, and H. Zhang, Ultrathin two-dimensional multinary layered metal chalcogenide nanomaterials, Adv. Mater. 29(37), 1701392 (2017)

[3] Y. Luo, J. Zhao, A. Fieramosca, Q. Guo, H. Kang, X. Liu, T. C. H. Liew, D. Sanvitto, Z. An, S. Ghosh, Z. Wang, H. Xu, and Q. Xiong, Strong light-matter coupling in van der Waals materials, Light Sci. Appl. 13(1), 203 (2024)

[4] X. Fan, R. Xin, L. Li, B. Zhang, C. Li, X. Zhou, H. Chen, H. Zhang, F. OuYang, and Y. Zhou, Progress in the preparation and physical properties of two-dimensional Cr-based chalcogenide materials and heterojunctions, Front. Phys. 19(2), 23401 (2024)

[5] Z. Wang, N. Ali, T. D. Ngo, H. Shin, S. Lee, and W. J. Yoo, Achieving ultrahigh electron mobility in PdSe₂ field-effect transistors via semimetal antimony as contacts, Adv. Funct. Mater. 33(28), 2301651 (2023)

[6] Y. Chen, S. Yu, T. Jiang, X. Liu, X. Cheng, and D. Huang, Optical two-dimensional coherent spectroscopy of excitons in transition-metal dichalcogenides, Front. Phys. 19(2), 23301 (2024)

[7] A. Dodda, D. Jayachandran, A. Pannone, N. Trainor, S. P. Stepanoff, M. A. Steves, S. S. Radhakrishnan, S. Bachu, C. W. Ordonez, J. R. Shallenberger, J. M. Redwing, K. L. Knappenberger, D. E. Wolfe, and S. Das, Active pixel sensor matrix based on monolayer MoS₂ phototransistor array, Nat. Mater. 21(12), 1379 (2022)

[8] K. Shi, J. Li, Y. Xiao, L. Guo, X. Chu, Y. Zhai, B. Zhang, D. Lu, and F. Rosei, High-response, ultrafast-speed, and self-powered photodetection achieved in InP@ZnS-MoS₂ phototransistors with interdigitated Pt electrodes, ACS Appl. Mater. Interfaces 12(28), 31382 (2020)

[9] J. Shan, J. Li, X. Chu, M. Xu, F. Jin, X. Fang, Z. Wei, and X. Wang, Enhanced photoresponse characteristics of transistors using CVD-grown MoS₂/WS₂ heterostructures, Appl. Surf. Sci. 443, 31 (2018)

[10] S. Li, X. Liu, H. Yang, H. Zhu, and X. Fang, Two-dimensional perovskite oxide as a photoactive high-κ gate dielectric, Nat. Electron. 7(3), 216 (2024)

[11] K. N. Nazif, F. U. Nitta, A. Daus, K. C. Saraswat, and E. Pop, Efficiency limit of transition metal dichalcogenide solar cells, Commun. Phys. 6(1), 367 (2023)

[12] T. Abhijith, R. Suthar, and S. Karak, Synergistic plasmonic responses of multi-shaped Au nanostructures hybridized with few-layer WS₂ nanosheets for organic solar cells, ACS Appl. Nano Mater. 6(13), 11737 (2023)

[13] H. Yang, R. Luo, K. Shi, J. Li, M. Xu, X. Chu, Y. Zhai, G. Qu, and X. Fang, Pollution-free interface of 2D-MoS₂/1D-CuO vdWs heterojunction for high-performance photodetector, Nanotechnology 35(10), 105202 (2024)

[14] J. Yan, K. Ye, Z. Jia, Z. Zhang, P. Li, L. Liu, C. Mu, H. Huang, Y. Cheng, A. Nie, J. Xiang, S. Wang, and Z. Liu, High-performance broadband image sensing photodetector based on MnTe/WS₂ van der Waals epitaxial heterostructures, ACS Appl. Mater. Interfaces 16(15), 19112 (2024)

[15] J. You, Z. Jin, Y. Li, T. Kang, K. Zhang, W. Wang, M. Xu, Z. Gao, J. Wang, J. Kim, and Z. Luo, Epitaxial growth of 1D Te/2D MoSe₂ mixed-dimensional heterostructures for high-efficient self-powered photodetector, Adv. Funct. Mater. 34(10), 2311134 (2024)

[16] Z. He, H. Guan, X. Liang, J. Chen, M. Xie, K. Luo, R. An, L. Ma, F. Ma, T. Yang, and H. Lu, Broadband, polarization-sensitive, and self-powered high-performance photodetection of hetero-integrated MoS₂ on lithium niobate, Research 6, 0199 (2023)

[17] F. Lan, R. Yang, K. Sun, Z. Wang, Y. Zhang, Y. Wang, and H. Cheng, Growth of wafer scale continuous monolayer WS₂ film with millimeter grain size, Vacuum 201, 111091 (2022)

[18] J. Li, S. Wang, L. Li, Z. Wei, Q. Wang, H. Sun, J. Tian, Y. Guo, J. Liu, H. Yu, N. Li, G. Long, X. Bai, W. Yang, R. Yang, D. Shi, and G. Zhang, Chemical vapor deposition of 4 inch wafer-scale monolayer MoSe₂, Small Sci. 2(11), 2200062 (2022)

[19] H. Zeng, Y. Wen, L. Yin, R. Cheng, H. Wang, C. Liu, and J. He, Recent developments in CVD growth and applications of 2D transition metal dichalcogenides, Front. Phys. 18(5), 53603 (2023)

[20] R. Addou, L. Colombo, and R. M. Wallace, Surface defects on natural MoS₂, ACS Appl. Mater. Interfaces 7(22), 11921 (2015)

[21] I. Lee, M. Kang, S. Park, C. Park, H. Lee, S. Bae, H. Lim, S. Kim, W. Hong, and S. Choi, Healing donor defect states in CVD-grown MoS₂ field-effect transistors using oxygen plasma with a channel-protecting barrier, Small 20(2), 2305143 (2024)

[22] L. Gao, X. Zhang, H. Yu, M. Hong, X. Wei, Z. Chen, Q. Zhang, Q. Liao, Z. Zhang, and Y. Zhang, Deciphering vacancy defect evolution of 2D MoS₂ for reliable transistors, ACS Appl. Mater. Interfaces 15(32), 38603 (2023)

[23] A. Schwarz, H. Alon-Yehezkel, A. Levi, R. K. Yadav, K. Majhi, Y. Tzuriel, L. Hoang, C. S. Bailey, T. Brumme, A. J. Mannix, H. Cohen, E. Yalon, T. Heine, E. Pop, O. Cheshnovsky, and D. Naveh, Thiol-based defect healing of WSe₂ and WS₂, npj 2D Mater. Appl. 7, 59 (2023)

[24] T. Grünleitner, A. Henning, M. Bissolo, M. Zengerle, L. Gregoratti, M. Amati, P. Zeller, J. Eichhorn, A. V. Stier, A. W. Holleitner, J. J. Finley, and I. D. Sharp, Real-time investigation of sulfur vacancy generation and passivation in monolayer molybdenum disulfide via in situ X-ray photoelectron spectromicroscopy, ACS Nano 16(12), 20364 (2022)

[25] S. Zheng, D. Yin, S. Zhang, Y. Wang, J. Li, Z. Wang, Y. Yuan, H. S. Tsai, and J. Hao, Tailoring selenium vacancies in MoSe₂ through oxygen passivation for room-temperature NO₂ sensing enhancement, J. Mater. Chem. A 11(35), 18755 (2023)

[26] J. Li, J. Bai, M. Meng, C. Hu, H. Yuan, Y. Zhang, and L. Sun, Improved temporal response of MoS₂ photodetectors by mild oxygen plasma treatment, Nanomaterials (Basel) 12(8), 1365 (2022)

[27] W. Zhou, X. Zou, S. Najmaei, Z. Liu, Y. Shi, J. Kong, J. Lou, P. M. Ajayan, B. I. Yakobson, and J. C. Idrobo, Intrinsic structural defects in monolayer molybdenum disulfide, Nano Lett. 13(6), 2615 (2013)

[28] R. Tomar, B. Hsu, A. Perez, M. Stroscio, and M. Dutta, Probing sulfur vacancies in CVD-grown monolayer MoS₂ on SiO₂/Si in the temperature range 750–900 °C, J. Electron. Mater. 52(8), 5513 (2023)

[29] I. H. Abidi,S. P. Giridhar,J. O. Tollerud,J. Limb,A. Mazumder,E. L. H. Mayes,B. J. Murdoch,C. Xu,A. Bhoriya,A. Ranjan,T. Ahmed,Y. Li, J. A. Davis,C. L. Bentley,S. P. Russo, E. D. Gaspera,S. Walia, Intrinsic defect engineering of CVD grown monolayer MoS₂ for tuneable functional nanodevices, arXiv: 2311.07984 (2023)

[30] L. P. L. Mawlong, A. T. Hoang, J. Chintalapalli, S. Ji, K. Lee, K. Kim, and J. H. Ahn, Reduced defect density in MOCVD-grown MoS₂ by manipulating the precursor phase, ACS Appl. Mater. Interfaces 15(40), 47359 (2023)

[31] S. Feng, J. Tan, S. Zhao, S. Zhang, U. Khan, L. Tang, X. Zou, J. Lin, H. M. Cheng, and B. Liu, Synthesis of ultrahigh-quality monolayer molybdenum disulfide through in situ defect healing with thiol molecules, Small 16(35), 2003357 (2020)

[32] S. Wang, Y. Rong, Y. Fan, M. Pacios, H. Bhaskaran, K. He, and J. H. Warner, Shape evolution of monolayer MoS₂ crystals grown by chemical vapor deposition, Chem. Mater. 26(22), 6371 (2014)

[33] G. Plechinger, S. Heydrich, J. Eroms, D. Weiss, C. Schüller, and T. Korn, Raman spectroscopy of the interlayer shear mode in few-layer MoS₂ flakes, Appl. Phys. Lett. 101(10), 101906 (2012)

[34] F. Chen and W. Su, The effect of the experimental parameters on the growth of MoS₂ flakes, CrystEngComm 20(33), 4823 (2018)

[35] H. Bretscher, Z. Li, J. Xiao, D. Y. Qiu, S. Refaely-Abramson, J. A. Alexander-Webber, A. Tanoh, Y. Fan, G. Delport, C. A. Williams, S. D. Stranks, S. Hofmann, J. B. Neaton, S. G. Louie, and A. Rao, Rational passivation of sulfur vacancy defects in two-dimensional transition metal dichalcogenides, ACS Nano 15(5), 8780 (2021)

[36] X. Zhang, S. Wang, C. K. Lee, C. M. Cheng, J. C. Lan, X. Li, J. Qiao, and X. Tao, Unravelling the effect of sulfur vacancies on the electronic structure of the MoS₂ crystal, Phys. Chem. Chem. Phys. 22(38), 21776 (2020)

[37] W. Song, J. Chen, Z. Li, and X. Fang, Self-powered MXene/GaN van der Waals heterojunction ultraviolet photodiodes with superhigh efficiency and stable current outputs, Adv. Mater. 33(27), 2101059 (2021)

[38] Z. Liu, D. Zhao, T. Min, J. Wang, G. Chen, and H. X. Wang, Photovoltaic three-dimensional diamond UV photodetector with low dark current and fast response speed fabricated by bottom–up method, IEEE Electron Device Lett. 40(7), 1186 (2019)

[39] B. Chakraborty, A. Bera, D. V. S. Muthu, S. Bhowmick, U. V. Waghmare, and A. K. Sood, Symmetry-dependent phonon renormalization in monolayer MoS₂ transistor, Phys. Rev. B 85(16), 161403 (2012)

[40] A. Abnavi, R. Ahmadi, A. Hasani, M. Fawzy, M. R. Mohammadzadeh, T. De Silva, N. Yu, and M. M. Adachi, Free-standing multilayer molybdenum disulfide memristor for brain-inspired neuromorphic applications, ACS Appl. Mater. Interfaces 13(38), 45843 (2021)

[41] A. S. Pawbake, S. R. Jadkar, and D. J. Late, High performance humidity sensor and photodetector based on SnSe nanorods, Mater. Res. Express 3(10), 105038 (2016)

[42] Q. Li, S. Zheng, J. Pu, W. Wang, L. Li, and L. Wang, Revealing the failure mechanism and designing protection approach for MoS₂ in humid environment by first-principles investigation, Appl. Surf. Sci. 487, 1121 (2019)

[43] Y. Wang,Z. He,J. Zhang,H. Liu,X. Lai, B. Liu,Y. Chen,F. Wang,L. Zhang, UV illumination enhanced desorption of oxygen molecules from monolayer MoS₂ surface, Nano Res. 13(2), 358 (2020)

[44] S. Yu, P. Li, H. Ding, and X. Ma, Construction of high-performance g-C₃N₄/MoS₂ heterojunction humidity sensor and investigation of its application, Sens. Actuators B Chem. 419, 136392 (2024)

[45] B. Zhang, Z. Ao, X. Lan, J. Zhong, F. Zhang, S. Zhang, R. Yang, L. Wang, P. Chen, G. Wang, X. Yang, H. Liu, J. Cao, M. Zhong, H. Li, and Z. Zhang, Self-rolled-up WSe₂ one-dimensional/two-dimensional homojunctions: Enabling high-performance self-powered polarization-sensitive photodetectors, Nano Lett. 24(25), 7716 (2024)

[46] J. You, Z. Jin, Y. Li, T. Kang, K. Zhang, W. Wang, M. Xu, Z. Gao, J. Wang, J. K. Kim, and Z. Luo, Epitaxial growth of 1D Te/2D MoSe₂ mixed-dimensional heterostructures for high-efficient self-powered photodetector, Adv. Funct. Mater. 34(10), 2311134 (2024)

[47] A. Saravanan, B. R. Huang, D. Kathiravan, S. Sakalley, and S. C. Chen, Tunable defect-engineered nanohybrid heterostructures: Exfoliated 2D WSe₂–MoS₂ nanohybrid sheet covered on 1D ZnO nanostructures for self-powered UV photodetectors, J. Mater. Chem. C 11(18), 6082 (2023)

[48] M. Hussain,S. H. A. Jaffery,A. Ali,C. D. Nguyen,S. Aftab, M. Riaz,S. Abbas,S. Hussain,Y. Seo,J. Jung, NIR self-powered photodetection and gate tunable rectification behavior in 2D GeSe/MoSe₂ heterojunction diode, Sci. Rep. 11(1), 3688 (2021)

[49] H. Li and Z. Yang, Highly responsive and self-powered photodetector based on PtSe₂/MoS₂ heterostructure, Molecules 29(11), 2553 (2024)

[50] J. J. Lee,D. H. Shin,D. H. Jung,S. D. Oh,H. Lee, A high-performance broadband self-powered photodetector employing an MoS₂/LaVO₃ heterojunction structure, J. Alloys Compd. 937, 168404 (2023)

[51] P. Augustine, K. L. Kumawat, D. K. Singh, S. B. Krupanidhi, and K. K. Nanda, MoS₂/SnO₂ heterojunction-based self-powered photodetector, Appl. Phys. Lett. 120(18), 181106 (2022)

[52] C. Hong, S. G. Jang, Y. J. Yu, and J. H. Kim, Hot electron dynamics in MoS₂/Pt van der Waals electrode interface for self-powered hot electron photodetection, Adv. Mater. Interfaces 10(13), 2300140 (2023)

[53] P. Vashishtha,I. H. Abidi,S. P. Giridhar,A. K. Verma,P. Prajapat,A. Bhoriya,B. J. Murdoch,J. O. Tollerud,C. Xu,J. A. Davis,G. Gupta,S. Walia, CVD-grown monolayer MoS₂ and GaN thin film heterostructure for a self-powered and bidirectional photodetector with an extended active spectrum, ACS Appl. Mater. Interfaces 16(24), 31294 (2024)

[54] J. Hui, Two-dimensional Bi₂S₃/MoS₂ van der Waals heterostructure for self-powered photodetectors, Appl. Phys. Express 16(1), 015507 (2023)

[55] P. Vashishtha, A. Dash, P. Prajapat, P. Goswami, S. Walia, and G. Gupta, Self-powered broadband photodetection of MoS₂/Sb₂Se₃ heterostructure, ACS Appl. Opt. Mater. 1(12), 1952 (2023)

[56] A. Abnavi, R. Ahmadi, H. Ghanbari, M. Fawzy, A. Hasani, T. De Silva, A. M. Askar, M. R. Mohammadzadeh, F. Kabir, M. Whitwick, M. Beaudoin, S. K. O’Leary, and M. M. Adachi, Flexible high-performance photovoltaic devices based on 2D MoS₂ diodes with geometrically asymmetric contact areas, Adv. Funct. Mater. 33(7), 2210619 (2023)

RIGHTS & PERMISSIONS

Higher Education Press