Full length articleSynthesis of two-dimensional MoS2/graphene heterostructure by atomic layer deposition using MoF6 precursor
Graphical abstract
Introduction
Two-dimensional (2D) transition metal dichalcogenides (TMDs; e.g., MoS2, WS2, and WSe2) have received considerable attention as post‑silicon materials with their own extraordinary electrical, optical, and mechanical properties [1,2]. Semiconducting 2D TMD have high on/off ratio (~108) with optical bandgaps in the rage of 1–2 eV, making their potential for used in optoelectronics even in atomically thin thickness [3,4]. Recently, atomically thin 2D heterostructures (i.e. WSe2/MoS2, MoS2/graphene, and graphene/h-BN) have also been extensively studied because they play an important role in 2D electronic devices and improve the performance [[5], [6], [7], [8]]. Among them, the MoS2/Graphene heterostructure improves the performance of various electronic devices, including field-effect transistors (FETs), due to their low contact resistance with graphene [[9], [10], [11], [12], [13]]. In addition, TMD/TMD heterostructures could be useful for current rectifiers, photodetectors, and solar cells because of their p-n junctions [[14], [15], [16], [17]]. For these reasons, the effective synthesis of 2D heterostructures is essential for their use in electronic devices.
As 2D heterostructures have such applicability, methods for their synthesis have been studied in various ways. Normally, mechanical exfoliation has the advantage of a large grain size with high quality for confirming the characteristics 2D materials [[10], [11], [12]]. However, it is not suitable for practical application owing to its low yield and flake size. Furthermore, transferring a 2D material to other 2D materials caused impurities, defects, and wrinkles between the interface [13]. Therefore, vapor deposition techniques of 2D TMDs are essential for achieving high quality with the desired electrical characteristics. Accordingly, studies have synthesized TMD materials, such as MoS2 and WSe2 by metal organic chemical vapor deposition (CVD) or by sulfurization of a metal-based solution; however, limitations exist regarding the large-area uniformity and layer controllability [[18], [19], [20]].
Therefore, the atomic layer deposition (ALD) process, which provides accurate thickness control on an atomic scale and outstanding uniformity on large-area substrates, must be recognized [21]. Its angstrom level thickness controllability is adequate for synthesizing the 2D materials, compare to other thin film deposition techniques including sputtering, evaporation and chemical vapor deposition [22]. Because of these advantages, the ALD process has been used for synthesizing MoS2 using various molybdenum precursors; however, in most cases, the MoS2 is synthesized in vertical directions or the amorphous state. Moreover, post-treatment at high temperature is also required for crystallinity [[23], [24], [25], [26], [27], [28], [29]]. Previously, chloride-based precursors (MoCl5, WCl6) were used to synthesize 2D TMDs but a few studies have been conducted on the formation of a 2D heterostructure by ALD [30,31].
In this study, we synthesized a layer-controlled 2D TMD by ALD on graphene and another TMD layer using a halide precursor. First, 2D MoS2 was grown directly on the graphene using ALD with MoF6, for the first time, without impurities and the annealing process [26]. Furthermore, the growth characteristics were confirmed by comparing the results with the other halide precursor, MoCl5. From those experiments, the fluoride precursor, MoF6, was adequate to synthesize the 2D TMD on graphene, whereas the chloride precursor was inappropriate. However, the chloride precursor was rather suitable for forming the TMD/TMD heterostructure, such as WSe2/MoS2, while the MoF6 was inadequate to synthesize the MoS2 on WSe2 due to the precursor etching effect to the substrate. Optical microscopy (OM), Raman spectroscopy, photoluminescence (PL), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) were used to analyze the 2D layers. The electrical and optical characteristics were also measured. From those measurements, we present a suitable ALD precursor for synthesizing the 2D heterostructure and its selectivity to the type of substrate for the first time.
Section snippets
Synthesis of ALD MoS2 nanosheets
Prior to the ALD process, the SiO2 (285 nm)/Si substrates were ultrasonically cleaned in acetone, isopropyl alcohol (IPA), and deionized water for 5 min each. The growth of the 2D MoS2 atomic layers was conducted in a tube furnace. MoF6, MoCl5, Mo(CO)6 and H2S were chosen as the Mo and S precursors, respectively. The MoF6, Mo(CO)6 and H2S were kept at room temperature (25 °C) while MoCl5 was kept at 80 °C. The vapor pressure of MoF6 was finely controlled by a metering valve and H2S by a mass
Results and discussions
The ALD sequence was performed as MoF6 (4 s), Ar purging (5 s), H2S exposure (3 s) and Ar purging (5 s). Because of the volatile characteristics of MoF6, only 1 s precursor exposure is sufficient for the synthesis of MoS2. Furthermore, when the precursor exposure time was increased, the MoS2 layer did not increase, but only saturated to a bilayer in 60 cycles of ALD, as confirmed by the Raman spectra (λexc = 532 nm). This indicates that the MoF6 precursor was adsorbed on the SiO2 substrate and
Conclusion
We achieved the synthesis of layer-controlled 2D MoS2 by the ALD process using MoF6 and compared the growth characteristics with those of MoCl5 and Mo(CO)6. The halide precursor was adequate for synthesizing the 2D structure at high temperature due to its thermal stability. An interesting characteristic was that MoS2 was synthesized on the graphene by MoF6 but not by MoCl5, suggesting that there is precursor selectivity for synthesizing 2D heterostructures. The ALD-grown 1 L MoS2 on graphene by
Acknowledgements
This work was supported by the Materials and Components Technology Development Program of MOTIE/KEIT. [10080527, Development of commercialization technology of highly sensitive gas sensor based on chalcogenide 2D nanomaterial], by the Commercialization Promotion Agency for R&D Outcomes(COMPA) funded by the Ministry of Science and ICT(MSIT). [Development of Plasma-based Synthesis Equipment and Process for Two-Dimensional TMDCs], by Samsung Display CO., LTD., and by the Basic Science Research
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