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.
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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- Lembke D, Bertolazzi S, Kis A (2015) Single-layer MoS2 electronics. Acc Chem Res 48:100–110View ArticleGoogle Scholar
- Xia F, Wang H, Xiao D, Dubey M, Ramasubramaniam A (2014) Two-dimensional material nanophotonics. Nat Photon 8:899–907View ArticleGoogle Scholar
- Bonaccorso F, Colombo L, Yu G, Stoller M, Tozzini V, Ferrari AC, Ruoff RS, Pellegrini V (2015) Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage. Science 347:1246501View ArticleGoogle Scholar
- Radisavljevic B, Radenovic A, Brivio J, Giacometti V, Kis A (2011) Single-layer MoS2 transistors. Nat Nanotechnol 6:147–50View ArticleGoogle Scholar
- Kim S, Konar A, Hwang WS, Lee JH, Lee J, Yang J, Jung C, Kim H, Yoo JB, Choi JY, Jin YW, Lee SY, Jena D, Choi W, Kim K (2012) High-mobility and low-power thin-film transistors based on multilayer MoS2 crystals. Nat Commun 3:1011View ArticleGoogle Scholar
- Lopez-Sanchez O, Lembke D, Kayci M, Radenovic A, Kis A (2013) Ultrasensitive photodetectors based on monolayer MoS2. Nat Nanotech 8:497–501View ArticleGoogle Scholar
- Kwon J, Hong YK, Han G, Omkaram I, Choi W, Kim S, Yoon Y (2015) Giant photoamplification in indirect-bandgap multilayer MoS2 phototransistors with local bottom-gate structures. Adv Mater 27:2224–30View ArticleGoogle Scholar
- Coleman JN, Lotya M, O’Neill A, Bergin SD, King PJ, Khan U, Young K, Gaucher A, De S, Smith RJ, Shvets IV, Arora SK, Stanton G, Kim H-Y, Lee K, Kim GT, Duesberg GS, Hallam T, Boland JJ, Wang JJ, Donegan JF, Grunlan JC, Moriarty G, Shmeliov A, Nicholls RJ, Perkins JM, Grieveson EM, Theuwissen K, McComb DW, Nellist PD, Nicolosi V (2011) Two-dimensional nanosheets produced by liquid exfoliation of layered materials. Science 331:568–71View ArticleGoogle Scholar
- Liu KK, Zhang W, Lee Y-H, Lin Y-C, Chang M-T, Su C-Y, Chang C-S, Li H, Shi Y, Zhang H, Lai C-S, Li L-J (2012) Growth of large-area and highly crystalline MoS2 thin layers on insulating substrates. Nano Lett 12:1538–44View ArticleGoogle Scholar
- Zhan Y, Liu Z, Najmaei S, Ajayan PM, Lou J (2012) Large-area vapor-phase growth and characterization of MoS2 atomic layers on a SiO2 Substrate. Small 8:966–71View ArticleGoogle Scholar
- Lin Y-C, Zhang W, Huang J-K, Liu K-K, Lee Y-H, Liang C-T, Chu C-W, Li L-J (2012) Wafer-scale MoS2 thin layers prepared by MoO3 sulfurization. Nanoscale 4:6637–41View ArticleGoogle Scholar
- Lee Y-H, Zhang X-Q, Zhang W, Chang M-T, Lin C-T, Chang K-D, Yu Y-C, Wang JT-W, Chang C-S, Li L-J, Lin T-W (2012) Synthesis of large-area MoS2 atomic layers with chemical vapor deposition. Adv Mater 24:2320–25View ArticleGoogle Scholar
- Yu Y, Li C, Liu Y, Su L, Zhang Y, Cao L (2013) Controlled scalable synthesis of uniform, high-quality monolayer and few-layer MoS2 films. Sci Rep 3:1866Google Scholar
- Kang K, Xie S, Huang L, Han Y, Huang PY, Mak KF, Kim C-J, Muller D, Park J (2015) High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity. Nature 520:656–60View ArticleGoogle Scholar
- Serrao CR, Diamond AM, Hsu S-L, You L, Gadgil S, Clarkson J, Carraro C, Maboudian R, Hu C, Salahuddin S (2015) Highly crystalline MoS2 thin films grown by pulsed laser deposition. Appl Phys Lett 106:052101View ArticleGoogle Scholar
- Ling ZP, Yang R, Chai JW, Wang SJ, Leong WS, Tong Y, Lei D, Zhou Q, Gong X, Chi DZ, Ang K-W (2015) Large-scale two-dimensional MoS2 photodetectors by magnetron sputtering. Opt Exp 23:13580–86View ArticleGoogle Scholar
- Ji Q, Zhang Y, Gao T, Zhang Y, Ma D, Liu M, Chen Y, Qiao X, Tan P-H, Kan M, Feng J, Sun Q, Liu Z (2013) Epitaxial monolayer MoS2 on mica with novel photoluminescence. Nano Lett 13:3870–77View ArticleGoogle Scholar
- Dumcenco D, Ovchinnikov D, Marinov K, Lazic P, Gibertini M, Marzari N, Lopez-Sanchez O, Kung Y-C, Krasnozhon D, Chen M-W, Bertolazzi S, Gillet P, Fontcuberta i Morral A, Radenovic A, Kis A (2015) Large-area epitaxial monolayer MoS2. ACS Nano 9:4611–20View ArticleGoogle Scholar
- Jeon J, Jang SK, Jeon SM, Yoo G, Jang YH, Park J-H, Lee S (2015) Layer-controlled CVD growth of large-area two-dimensional MoS2 films. Nanoscale 7:1688–95View ArticleGoogle Scholar
- Ji Q, Zhang Y, Zhang Y, Liu Z (2015) Chemical vapour deposition of group-VIB metal dichalcogenide monolayers: engineered substrates from amorphous to single crystalline. Chem Soc Rev 44:2587–602View ArticleGoogle Scholar
- Lee C, Yan H, Brus LE, Heinz TF, Hone J, Ryu S (2010) Anomalous lattice vibrations of single and few-layer MoS2. ACS Nano 4:2695–700View ArticleGoogle Scholar
- Buscema M, Steele GA, van der Zant HSJ, Castellanos-Gomez A (2014) The effect of the substrate on the Raman and photoluminescence emission of single-layer MoS2. Nano Res 7:561–71View ArticleGoogle Scholar
- Splendiani A, Sun L, Zhang Y, Li T, Kim J, Chim C, Galli G, Wang F (2010) Emerging photoluminescence in monolayer MoS2. Nano Lett 10:1271–5View ArticleGoogle Scholar
- Mak KF, Lee C, Hone J, Shan J, Heinz TF (2010) Atomically thin MoS2: a new direct-gap semiconductor. Phys Rev Lett 105:136805View ArticleGoogle Scholar
- Britnell L, Ribeiro RM, Eckmann A, Jalil R, Belle BD, Mishchenko A, Kim Y-J, Gorbachev RV, Georgiou T, Morozov SV, Grigorenko AN, Geim AK, Casiraghi C, Castro Neto AH, Novoselov KS (2013) Strong light-matter interactions in heterostructures of atomically thin films. Science 340:1311–4View ArticleGoogle Scholar
- Klots AR, Newaz AKM, Wang B, Prasai D, Krzyzanowska H, Lin J, Caudel D, Ghimire NJ, Yan J, Ivanov BL, Velizhanin KA, Burger A, Mandrus DG, Tolk NH, Pantelides ST, Bolotin KI (2014) Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy. Sci Rep 4:6608View ArticleGoogle Scholar