Synthesis of Monolayer MoS2 by CVD Approach

The monolayer MoS2 thin film was deposited directly on quartz substrate by chemical vapor deposition (CVD) approach using MoO3 and sulfur powders as reactants. Raman spectroscopic and photoluminescence (PL) spectroscopic analyses were conducted to evaluate the structural and optical property of the grown MoS2 thin films. The Raman characteristic peaks in 385cm (E2g) and 405cm (A1g) prove the grown film is monolayer MoS2. In comparision, we also found in some areas of the grown fim is bulk MoS2 film, the Raman characteristic peaks of which are 384 cm (E2g) and 409 cm (A1g). And two pronounced emission peaks at 620 and 670 nm were observed in photoluminescence spectrum of monolayer MoS2. The results suggest that we synthesize monolayer MoS2 with optical bandgap of 1.85eV. Introduction Recently, researchers have been focusing on graphene-like 2D materials, aiming at overcoming the shortage of graphene and broadening its range of applications. Monolayer molybdenum disulphide (MoS2) is a typical example from the layered transition-metal dichalcogenide (TMD) family of materials with a direct bandgap of 1.8eV . Due to its attractive electronic, optoelectronic, and mechanical properties , great effort has been devoted to the study of MoS2. Bulk MoS2 is known to have an indirect bandgap of ~1.2 eV and widely used as lubricants at first, whereas single-layer MoS2 is a direct gap semiconductor [1, 5, 6] with a bandgap of 1.8 eV. The direct bandgap also results in photoluminescence from monolayer MoS2, which opens the possibility of many optoelectronic applications . In addition, bulk (or multilayer) MoS2 exhibits relatively high in-plane carrier mobility comparable to that of crystalline silicon as well as robust mechanical and chemical properties, which makes it an attractive material for making flexible electronic devices with high performance and long lifetime. The current approaches for fabricating thin layers of MoS2 are as follows, mechanical and liquid exfoliation [10, 11] of bulk materials, physical vapor deposition [12] and chemical vapor deposition [13, . CVD is a direct bottom-up method for producing large-area uniform poly crystalline thin films. In this article, we describe the approach for preparing monolayer MoS2 films on quartz by CVD. We used MoO3 powder and sulfur powder is as reactants. Furthermore, structural, spectral, and morphology of the monolayer MoS2 films were studied. Experimental Procedure The quartz substrate was treated according to the standard Radio Corporation of America (RCA) cleaning procedure. As illustrated in Figure 1, monolayer MoS2 film was synthesized in a horizontal quartz tube furnace with sulfur and MoO3 powders as reactants. The MoO3 (0.1 g, 99.99%) was placed downstream in a ceramic boat and loaded into the central uniform-temperature zone of the furnace. The quartz substrate was faced down and mounted on the top of boat. Another ceramic boat with sulfur powder (0.8g, 99.9%) was placed upstream in a low-temperature zone. The center of the furnace was heated from room temperature to 550°C in 30min at a rate of approximately 20°C per minute. As the temperature approached 550°C, the sulfur slowly evaporated, and then the chamber was heated to 850°C at a slower pace of ~5°C per minute. After the temperature was kept at 850 °C for 10 min, at such a high temperature, MoO3 powder evaporated and reacts with sulfur vapor to 2nd Annual International Conference on Advanced Material Engineering (AME 2016) © 2016. The authors Published by Atlantis Press 1034 form volatile suboxide MoO3–x. [13] these suboxide compounds diffused to the substrate and further reacted with sulfur vapor to grow MoS2 films . Finally, the furnace was cooled down to room temperature naturally. Ultra-high purity N2 gas was flowed with the flowing rate of 100 sccm during the whole growth process. Optical microscopy (OM) images, Raman spectroscopic and photoluminescence (PL) spectroscopic analyses were conducted to observe the surface morphology of the films, the structural and optical property of the grown MoS2 thin films. Figure 1 Schematic diagram of the CVD system used for the growth of MoS2. It has been reported that MoS2 formed as the result of the stepwise reaction of substitution and reduction which lead to an intermediate product of MoO3-x. Although the growth dynamics of MoO3-x and sulfur are still not fully understood, there are two possible channels for the surface growth that can be expected: (1) MoO3-x species adsorb and diffuse on the substrate, reacting with S to form MoS2; (2) MoO3-x and sulfur react directly in the gas phase, and the resulting MoS2 clusters adsorb, nucleate, and grow on the substrate. In our experiment, the reaction process is described as follows. Firstly, a precursor MoO3 powder and the quartz substrate are placed in downstream location in furnace. When the temperature in the central furnace rise up to 550°C, the MoO3 powder begins evaporating. Meanwhile, the sulfur powder also evaporates, and is conveyed by N2 gas to downstream in the furnace. The MoO3 then react with sulfur vapour to form volatile MoO3-x species, which subsequently deposit on the quartz substrate to form MoO3-x microplates. Finally, the oxygen atoms in the well-ordered packing of MoO3-x are replaced with sulfur atoms to form MoS2 films on the substrate. Considering the fact that MoO2, the most common intermediate product, is one of the most stable oxides of molybdenum, we can assum the x=1. A possible stepwise reaction process of MoO3 and S is given in equations (1) and (2) . 2 2 3 2 1 2 1 SO MoO S MoO + → + (1) 2 2 2 3 oO SO MoS S M + → + (2) Results and discussion Optical microscopy images of the MoS2 layers Figure 2 shows the OM image of the layered MoS2 films grown on quartz substrate. It indicates that we fabricated MoS2 films on the quartz substrate. The largest area of the films is about 20×20μm, indicating uniform MoS2 film. Furthermore, as the optical image shows, the shape of the MoS2 is rhombus. This can be explained that the intermediate product MoO2 films was nucleated on quartz substrates and grew up to rhomboidal microplates, which is in consistent with the work of Xinsheng Wang etc. They thermally evaporated MoO2 powder on the SiO2/Si substrate, and it grew up to rhomboidal microplates. D. O. Scanlon etc. theoretically proved that the shape of MoO2 films to be closely related to rutile, which further confirms our assumption.