Fabrication of Two-dimensional MoS2 based Heterostructures for Photodetector and Gas sensing Applications

Abstract

In the past decade, two-dimensional (2D) layered materials have emerged as a promising area of research, with significant efforts devoted to their fundamental studies and practical device applications. Due to their unique layered structure and atomic-scale thickness, these materials possess exclusive electrical and optical properties, which make them attractive for various applications. Among these materials, molybdenum disulfide (MoS2) is the most widely studied transition metal dichalcogenide. Its direct and tunable bandgap, high carrier mobility, large current on/off ratio, thermal and chemical stability, and other unique characteristics make it an ideal choice for applications such as photodetectors, solar cells, sensors, transistors, etc. Recent research has shown that vertically aligned MoS2 (VA-MoS2) flakes exhibit enhanced electrical, optical, and catalytic activities, compared to in-plane MoS2, due to their high aspect ratio, large surface area, and more exposed edges. As a result, these materials have become an attractive choice for photodetection and gas-sensing applications. However, large-area controlled growth of VA-MoS2 flakes is necessary to realize their potential in commercial 2D technology. The fabrication of photodetectors using in-plane MoS2 has revealed limited optical absorbance, which significantly hinders its photodetection capability. Furthermore, pristine MoS2 photodetectors exhibit slow response times due to the high carrier recombination rate in bare MoS2 and the absence of a high-quality junction. In addition, the active wavelength detection range of pristine MoS2-based devices is confined to its bandgap, which severely restricts their potential for broadband photodetection. To overcome these limitations, integrating MoS2 with other potential semiconductor materials has proven to be a viable option for high-performance photodetector fabrication. Despite considerable research in this field, the carrier dynamic at the interface remains a critical factor affecting device performance. Therefore, details studies are required to fully harness the potential of heterostructures for practical applications by optimizing their interface properties. Additionally, the choice of materials is also important to ensure that they complement each other and contribute to the overall performance of the device. In the present thesis work, we addressed the challenges related to the controlled growth of large-area VA-MoS2 flakes, as well as the fabrication of high-performance broadband photodetectors and gas sensors using MoS2 heterostructures. To overcome these challenges, we employed a facile and efficient magnetron sputtering technique, which enabled the growth of large-area MoS2 flakes with a controlled morphology ranging from in-plane to VA-MoS2,through precise control of the growth parameters. We carried out a comprehensive analysis of the morphology, structure, and spectroscopic properties of the MoS2 film, which revealed the transition from in-plane to VA-MoS2 flakes. Furthermore, we fabricated multiple heterostructures of MoS2, including MoS2/ReS2, MoS2/Ga2O3, and Pt@MoS2, for broadband photodetection, and performed a detailed investigation of their photodetector performance. The results demonstrate that the fabricated heterostructures exhibited high responsivity, high detectivity, and fast optical switching when compared to pristine MoS2. In order to gain a deeper understanding of the device mechanism, we utilized advanced techniques such as photoelectron spectroscopy and kelvin probe force microscopy to investigate the energy band alignment and charge carrier dynamics at the heterointerface. Finally, we explored the interaction of gas molecules and VA-MoS2. We fabricated Pt nanoparticles decorated VA-MoS2 (Pt@MoS2) sensor that showed selective hydrogen sensing at room temperature. We systematically investigated the sensing performance of the Pt@MoS2 sensor and discuss the possible sensing mechanism. Our fabricated sensor demonstrates excellent sensitivity and low detection ability (>1 ppm), highlighting, its potential for use in hydrogen vehicles and other related technologies in near future.