Elsevier

Applied Surface Science

Volume 470, 15 March 2019, Pages 129-134
Applied Surface Science

Full Length Article
Low-temperature wafer-scale growth of MoS2-graphene heterostructures

https://doi.org/10.1016/j.apsusc.2018.11.126Get rights and content

Highlight

  • Practical growth for 2D MoS2-graphene heterostructure (MGH) was introduced.

  • Low-temperature sulfurization of Mo thin film was realized by H2S plasma.

  • As-grown MoS2 film on graphene naturally contains large number of active sites.

  • The MGH was shown enhanced electrocatalytic performance.

Abstract

In this study, we successfully demonstrate the fabrication of a MoS2-graphene heterostructure (MGH) on a 4 inch wafer at 300 °C by depositing a thin Mo film seed layer on graphene followed by sulfurization using H2S plasma. By utilizing Raman spectroscopy and high-resolution transmission electron microscopy, we have confirmed that 5–6 MoS2 layers with a large density of sulfur vacancies are grown uniformly on the entire substrate. The chemical composition of MoS2 on graphene was evaluated by X-ray photoelectron spectroscopy, which confirmed the atomic ratio of Mo to S to be 1:1.78, which is much lower than the stoichiometric value of 2 from standard MoS2. To exploit the properties of the nanocrystalline and defective MGH film obtained in our process, we have utilized it as a catalyst for hydrodesulfurization and as an electrocatalyst for the hydrogen evolution reaction. Compared to MoS2 grown on an amorphous SiO2 substrate, the MGH has smaller onset potential and Tafel slope, indicating its enhanced catalytic performance. Our practical growth approach can be applied to other two-dimensional crystals, which are potentially used in a wide range of applications such as electronic devices and catalysis.

Introduction

Realization of two-dimensional (2D) heterostructures has been intensively studied in view of their unique chemical, physical, and electrical properties [1], [2], [3], [4]. Thus far, the main strategy for the preparation of a 2D heterostructure has been based on the sequential stacking of the layered materials using wet or dry transfer methods [5], [6]. Ideally, this method allows for a conceptually new class of flexible and transparent films, with applications in batteries, electronic devices, and electrochemical cells [7], [8], [9]. However, these methods require time-consuming and complicated transfer processes, which also generate defects or residues at the interface of the 2D heterostructure [10]. The conventional thermal chemical vapor deposition (CVD) method using thermal decomposition of feedstocks, is considered a practical approach for the manufacture of 2D materials, whereby high-quality atomic scale heterostructures over large area can be obtained [11]. In particular, graphene, which is flexible, transparent, and highly conductive, is an ideal template to synthesize various transition metal dichalcogenide (TMDC) materials [12], [13], [14]. However, CVD reactions need relatively high growth temperatures (600–1000 °C) [15], [16], [17], which is incompatible with the complementary metal-oxidesemiconductor (CMOS) process and can therefore increase the total thermal budget in device fabrication [18], [19], [20]. To overcome this limitation, plasma-enhanced CVD (PECVD)-based synthesis technique has been introduced for 2D materials [21], [22]. Although initial installation cost of PECVD system is higher than that of typical CVD system, in the presence of accelerated energetic electrons, excited molecules, free radicals, photons, and other active species in the plasma, the controlled growth of 2D materials can be realized at a relatively lower temperature. Our group recently demonstrated the low-temperature growth of uniform MoS2 on a flexible plastic substrate [23], [24].

In view of its tunable band gap and relatively high carrier mobility, MoS2 has been investigated for applications in future electronic devices [25]. In addition, defects and edges of MoS2 on the basal plane can act as active catalyst for hydro-desulfurization, due to which MoS2 is considered as a strong candidate to replace currently used noble Pt catalyst [4], [26]. Since Pt, which is considered as the best catalyst for HER, is low in natural abundance and high in cost, alternative catalysts such as metal alloys, TMDs, and composite with TMDs were developed [9], [27]. In particular, the combination of MoS2 with graphene opens up new possibilities in electronic applications and also shows great potential as a catalyst [12], [13], [28], [29], [30], [31], [32]. To the best of our knowledge, however, the direct wafer-scale growth of MoS2 on graphene at temperatures compatible with CMOS technology has not yet been reported. In this study, we demonstrate the fabrication of a MoS2-graphene heterostructure (MGH) on a 4 inch wafer at 300 °C. A thin Mo film, which functions as a seed layer, was deposited on transferred graphene using e-beam evaporation after which, sulfurization was carried out using H2S plasma. Raman spectroscopy and high-resolution transmission electron microscopy (HR-TEM) were used to confirm that MoS2 layers are uniformly grown on the entire substrate. The chemical composition of MoS2 on graphene was determined by X-ray photoelectron spectroscopy (XPS); observed XPS data confirmed that the atomic ratio of Mo to S was 1:1.78, which is far beyond the stoichiometric value of the standard MoS2. To exploit the unique properties of the nanocrystalline MGH film with high density of sulfur vacancy, we tested it as a catalyst for hydrodesulfurization and as an electro-catalyst for the hydrogen evolution reaction (HER) [33].

Section snippets

Growth and transfer of monolayer graphene.

Graphene was synthesized on Cu foils (25 μm-thick, Alpha Aesar, 99.99% purity) using a conventional CVD process. Cu foils were placed in a 4 inch diameter quartz tube and after evacuation, H2 (8 sccm, 99.999%) was introduced into the chamber and the Cu foil was annealed at 1040 °C for 2 h to remove residual impurities. Next, graphene growth was carried out on the Cu foil for 1 h by injecting 30 sccm of CH4 and 50 sccm of H2. Finally, the furnace was rapidly cooled to room temperature under H2

Results and discussion

A photograph of the 100-cm2-area MGH (dark blue rectangular region) shows highly uniform growth of MGH on the 4-inch SiO2/Si wafer in Fig. 2(a). HR-TEM was conducted to determine both the number of layers and the crystallinity of the MGH. Cross sectional HR-TEM image of MGH and its corresponding electron energy loss spectroscopy elemental mapping images show that top MoS2 layers of total thickness 6–7 nm are uniformly and continuously grown on the underlying graphene (Fig. 2(b) and (c)). The d

Conclusion

In conclusion, we have successfully synthesized a 4 inch sized MoS2-on-graphene heterostructure through a H2S sulfurization process. The Raman and HR-TEM results reveal that 5–6 MoS2 layers with a large density of sulfur vacancies and grain boundaries are uniformly grown on the entire substrate. To exploit the nanocrystalline MGH film containing a high sulfur vacancy concentration, we investigated the MGH as a catalyst for hydro-desulfurization and as an electro-catalyst for the HER. When

Acknowledgment

This work was supported by the Presidential Postdoctoral Fellowship Program of the Ministry of Education, through the NRF (2014R1A6A3A04058169) and NRF-2017R1A2B3011222. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2018R1D1A1B07040292).

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