Synthesis and characterization of two dimensional MoS2 flakes and study their gas sensing and photodetector applications

Abstract

Atomically thin monolayer two-dimensional transition-metal dichalcogenides (TMDCs), especially MoS2 has unique electronic, optical, physical and chemical properties in contrast to their bulk counterpart and other two-dimensional (2D) materials. MoS2 monolayer has a direct band gap of 1.8 eV which opens up its applications in optoelectronics, solar cells, gas sensors, water disinfection and FETs for next generation nano-electronic device applications. Vertically aligned MoS2 flakes have attracted immense attention among research community over inert inplane MoS2, owing to its superior electrical and optical properties. Therefore, exposing more edges of MoS2 flakes by controlling its nanostructure allows the formation of high performance optical and electronic devices. However, thermodynamics favors the presence of in-plane growth, limiting the number of active sites at the MoS2 surface. Therefore, controlled growth with variable density of vertically aligned MoS2 flakes and understanding their growth mechanism would be one step closer to realize their great potential in optoelectronics and in gas sensor development. In addition, scalability, uniformity and reproducible growth remains great challenges, hindering further progress. MoS2 has a large specific surface area. The large surface area provides maximum favorable adsorption sites for the adsorption of gas molecules and to enhance the surface perturbation in the presence of the gas molecules. MoS2 is very sensitive and amenable to be used in gas sensing devices. In this context, MoS2 established as the promising chemical sensing material. The gas molecules adsorption in MoS2 is position dependent. Thus, engineering the morphology could be a feasible approach to develop high performance gas sensors. Nevertheless, present gas sensors based on bare MoS2 flakes suffer from low gas sensing performance. Thus, rigorous approaches are needed to develop high performance gas sensors based on bare MoS2 working at room temperature. Moreover, the in-plane MoS2 film is of only a few nanometres and hence the absorption of incident light is significantly less. The performance of MoS2 based photodetector suffered from the absence of high-quality p-n junctions. So, the charge carrier recombination rate is high in bare MoS2 which compromises the photodetector performance of MoS2. Integrating MoS2 with other high absorption materials proves to be a viable choice for high-performance PD fabrication. However, no significant research efforts have been put out in this area. Therefore, the field is still open to fully realize the promising potential of hybrid heterojunctions for practical applications and comprehensive understanding of the charge transfer which is substantially crucial for the devices with multiple functionalities. Therefore, we explored these challenging issues such as controlled growth of vertical aligned MoS2 flakes, high performance gas sensors and broadband photodetectors. To tackle down these challenges, the modified tube in tube atmospheric chemical vapor deposition technique (APCVD) is adopted to grow MoS2 flakes. The density of vertically aligned MoS2 flakes is controlled by the carrier gas flow rate. We have optimized the critical APCVD growth parameters. These experimental finding revealed that gas flow rate is an important parameter to control the growth of MoS2 flakes. Various characterization techniques are used to analyze and confirm the structural, morphological and optical properties of synthesized MoS2 flakes. The surface morphology, chemical composition, elemental bonding and structural properties are determined by using scanning electron microscopy (SEM), atomic force microscope (AFM), X-ray diffraction (XRD), energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS). The optical properties are investigated using Raman, photoluminescence (PL) and UV-VIS-NIR spectroscopy. Based on the structural, morphological and spectroscopic analysis, a detailed growth mechanism is proposed where in-plane MoS2 is found to work as a seed layer for the initial growth of vertically aligned MoS2 flakes that finally leads to the growth of interconnected 3-D network of vertically aligned MoS2 flakes. We fabricated and developed fast responsive and recoverable sensitive H2 and NO2 gas sensors. We experimentally demonstrated that the gas adsorption in MoS2 is position-dependent and understanding of favorable adsorption sites is necessary to develop the highly sensitive, fast recover RT gas sensors. Considering this, we synthesized morphology driven H2 and NO2 gas sensors. We synthesized 3-D network of vertically aligned MoS2 and monolayer pyramid MoS2 structures for H2 gas sensing. We developed NO2 gas sensor based on mixed MoS2 flakes and demonstrated photo-activated NO2 sensor at room temperature. The mixed MoS2 based sensor shows complete recovery and fast response in photoactive mode at room temperature and thermally active mode at moderate temperature, respectively. Lastly, we studied the interaction of vertically aligned MoS2 flakes with light. We developed the high responsive, broadband, fast photodetector by forming the heterostructures of MoS2. We observed that a change in the orientation of MoS2 flakes from in-plane MoS2 to vertical MoS2 flakes not only improved the absorption of gas molecules but also helpful in developing the broadband PDs.