Large-Area Growth of Uniform Single-Layer MoS2 Thin Films by Chemical Vapor Deposition
© Baek et al. 2015
Received: 17 August 2015
Accepted: 27 September 2015
Published: 6 October 2015
We report the largest-size thin films of uniform single-layer MoS2 on sapphire substrates grown by chemical vapor deposition based on the reaction of gaseous MoO3 and S evaporated from solid sources. The as-grown thin films of single-layer MoS2 were continuous and uniform in thickness for more than 4 cm without the existence of triangular-shaped MoS2 clusters. Compared to mechanically exfoliated crystals, the as-grown single-layer MoS2 thin films possessed consistent chemical valence states and crystal structure along with strong photoluminescence emission and optical absorbance at high energy. These results demonstrate that it is possible to scale up the growth of uniform single-layer MoS2 thin films, providing potentially important implications on realizing high-performance MoS2 devices.
KeywordsMoS2 Single layer Thin films Chemical vapor deposition
PACS81.05.Hd 81.10.Bk 81.15.Gh
Two-dimensional transition metal dichalcogenides (TMDs) have received great attention because of their interesting electronic, optical, and chemical properties. Among various TMDs, molybdenum disulfide (MoS2) has been most extensively investigated for the applications of thin-film transistors (TFTs), photodetectors, and energy storage [1–3]. TFTs based on single- or multilayer MoS2 exhibit intriguing transistor performance including high on/off current ratio (~107), high mobility at room temperature (~100 cm2 V−1 s−1), and low subthreshold swing (~70 mV decade−1) [4, 5]. Moreover, photodetectors based on single- or multilayer MoS2 show high photoresponsivity (300–800 A W−1) exceeding that of silicon-based ones [6, 7]. However, the aforementioned examples have been demonstrated using mechanically exfoliated MoS2 flakes, which are typically micrometer-scale in size. Hence, the growth of large-area MoS2 is one of the critical challenges to realize its promising potential.
So far, a variety of synthesis approaches have been reported to grow large-area MoS2 including liquid exfoliation (sonication in solvents) , two-step chemical vapor deposition (CVD, sulfurization or decomposition of pre-deposited Mo-based thin films) [9–11], one-step CVD (reaction of gaseous Mo and S precursors) [12–14], and physical vapor deposition (sputtering and pulsed laser deposition) [15, 16]. Special emphasis has been put on one-step CVD as it shows greater potential for growing uniform large-grain thin films of single-layer MoS2. The most common one-step CVD is based on the reaction of gaseous MoO3 and S evaporated from solid sources due to the simplicity of processing and the easy availability of solid sources . When the optimized CVD process conditions with MoO3 and S precursors are combined with the use of mica or substrate treatment, the formation of single-layer MoS2 thin films can be obtained up to about a centimeter [17–19]. However, CVD processes based on MoO3 and S powders typically result in triangular-shaped discontinuous clusters of either single-layer MoS2 or mixtures of single- and few-layer MoS2 . Therefore, more work is needed to establish a CVD process that can reproducibly provide continuous large-area thin films of uniform single-layer MoS2.
Here, we investigate CVD methods based on MoO3 and S powders to grow continuous thin films of single-layer MoS2 for more than 4 cm on sapphire substrates. Our MoS2 thin films are the largest in size grown by CVD methods based on MoO3 and S powders. The large-area deposition, thickness uniformity, and crystallinity of single-layer MoS2 thin films are confirmed by scanning electron microscopy (SEM), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, photoluminescence (PL) spectroscopy, ultraviolet (UV)-visible spectroscopy, and transmission electron microscopy (TEM).
MoS2 films were deposited on (0001)-oriented sapphire substrates in a two-zone tube furnace. MoO3 (99.98 %, Sigma-Aldrich) and S (99.98 %, Sigma-Aldrich) powders in two separate Al2O3 boats were used as precursors. MoO3 powder (15 mg) was placed upstream at zone 1 (700 °C), and S powder (1 g) was placed at the upstream entry of the furnace. The substrates were placed downstream at zone 2 (600 °C). MoO3 powder was heated up to 700 °C at a rate of 15 °C min−1, and the substrates were heated up to 600 °C at 38 °C min−1. After 30-min deposition, the furnace was slowly cooled down to room temperature. Ar flow of 100 sccm and a pressure of ~0.5 Torr were maintained during deposition.
The surface morphology of deposited thin films was observed by SEM (JEOL JSM-7610F) and AFM (Park Systems XE-100). Elemental composition was analyzed using XPS (PHI X-tool). The thickness and uniformity of deposited thin films were measured by Raman and PL spectra (Horiba LabRAM Aramis) using a laser of 532 nm in wavelength. Optical absorbance was measured by UV-visible spectroscopy (PerkinElmer Lambda 35). Crystal structure was analyzed by TEM (FEI Titan 80–300) at 300 kV.
Results and Discussion
The thickness of MoS2 thin films is further confirmed by PL spectra. While the indirect bandgap of multilayer MoS2 does not allow PL emission, the direct bandgap of single-layer MoS2 allows PL emission [23, 24]. Figure 2b shows the PL spectra of the same four MoS2 samples—CVD MoS2 thin films on sapphire substrates, CVD MoS2 thin films transferred on SiO2/Si substrates, mechanically exfoliated single-layer MoS2 flakes on SiO2/Si substrates, and bulk MoS2 single crystals. While the emission intensity completely disappears for bulk MoS2, our single-layer MoS2 thin films on sapphire show a PL emission peak at 1.88 eV confirming they are single-layer MoS2. The PL spectrum of our single-layer MoS2 thin films on sapphire is different from that of mechanically exfoliated single-layer MoS2 flakes on SiO2/Si. Two emission peaks are observed for mechanically exfoliated single-layer MoS2 flakes on SiO2/Si at 1.85 and 2.00 eV known as A and B direct excitonic transitions [23, 24]. The shift of emission peak A and the absence of emission peak B in our single-layer MoS2 thin films on sapphire are due to the effect of the underlying substrate . The consistent PL emission spectrum from transferred single-layer MoS2 on SiO2/Si with that of mechanically exfoliated single-layer MoS2 flakes on SiO2/Si supports this.
The UV-visible absorption spectrum in Fig. 2c shows A and B absorption due to excitonic transitions along with C and D absorption associated with van Hove singularity [25, 26]. As the existence of van Hove singularity can enhance light-matter interactions, single-layer MoS2 thin films may be suitable for photovoltaic cells and photodetectors due to enhanced photon absorption and electron-hole creation.
In summary, we synthesized uniform large-area thin films of single-layer MoS2 on sapphire substrates by CVD based on MoO3 and S precursors. The as-grown thin films were composed of single-layer MoS2 and continuous for more than 4 cm without triangular-shaped clusters of MoS2. The chemical configuration, thickness, thickness uniformity, and crystalline quality of MoS2 thin films were confirmed by XPS, AFM, Raman and PL spectra, and TEM analysis. The optical absorbance measurement further suggested the existence of van Hove singularity at high energy. These results will help scale up the growth of two-dimensional TMDs, providing potentially important implications on realizing the promising potential of high-performance MoS2 devices such as thin-film transistors, sensors, and photodetectors.
This work was supported by the National Research Foundation of Korea (Grant NRF-2013R1A1A2008191, NRF-2013K1A4A3055679, and NRF-2013K1A3A1A32035549) and Industrial Strategic Technology Development Program (Grant 10045145).
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