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Large-Area Synthesis of High-Quality Uniform Few-Layer MoTe2 Lin Zhou, Kai Xu, Ahmad Zubair, Albert Liao, Wenjing Fang, Fangping Ouyang, Yi-Hsien Lee, Keiji Ueno, Riichiro Saito, Tomas Palacios, Jing Kong, and Mildred S. Dresselhaus J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b07452 • Publication Date (Web): 25 Aug 2015 Downloaded from http://pubs.acs.org on August 27, 2015
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Journal of the American Chemical Society
Large-Area Synthesis of High-Quality Uniform Few-Layer MoTe2 Lin Zhou1, Kai Xu1,2, Ahmad Zubair1, Albert Liao1, Wenjing Fang1, Fangping Ouyang1,3, Yi-Hsien Lee4, Keiji Ueno5, Riichiro Saito6, Tomás Palacios1, Jing Kong*1, and Mildred S. Dresselhaus*1,7 1
Department of Electrical Engineering and Computer Sciences, Massachusetts Institute of Technology, Cambridge, 2 Massachusetts 02139, United States; State Key Laboratory of Heavy Oil Processing, China University of Petroleum, 3 Beijing 102249, China; School of Physics Science and Technology, Central South University, Changsha 410083, Chi4 5 na; Material Sciences and Engineering, National Tsing-Hua University, Hsinchu 30013, Taiwan; Department of 6 Chemistry, Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan; Department 7 of Physics, Tohoku University, Sendai 980-8578, Japan; Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
Supporting Information Placeholder ABSTRACT: The controlled synthesis of large-area, atomically thin molybdenum ditelluride (MoTe2) crystals is crucial for its various applications based on the attractive properties of this emerging material. Here, we develop a chemical vapor deposition (CVD) synthesis to produce large-area, uniform, and highly crystalline few-layer 2H and 1T′ MoTe2 films. It was found that these two different phases of MoTe2 can be grown based on the choice of Mo precursor. Due to the high crystalline structure, the as-grown few-layer 2H MoTe2 films display electronic properties that are comparable with those of mechanically exfoliated MoTe2 flakes. Our growth method paves the way for large-scale application of MoTe2 in high performance nanoelectronics and optoelectronics.
Two-dimensional transition metal dichalcogenides (TMDs) have been attracting increasing interest owing to their unique structures and remarkable properties, which make them promising materials for a wide range of applica1-3 4-6 tions related to, e.g., electronics, optoeletronics, valley7 8 9,10 tronics, spintronics, and catalysis. As a member of the TMD family, MoTe2 possesses interesting features. Bulk MoTe2 has an indirect bandgap of ~ 1.0 eV, and single-layer MoTe2 is a direct-gap semiconductor with an optical 11 bandgap of 1.1 eV. Because of the smaller bandgap compared with other group VI TMDs, single- and few-layer MoTe2 holds promise for use in easily controllable ambipolar fieldeffect transistors and extends the operating range of TMD optoelectronic devices from the visible to the near-infrared 11-13 range. In particular, the bandgap which is quite close to that of Si (~ 1.1 eV), the strong absorption throughout solar 14 spectrum, as well as the strong spin-orbit coupling, suggest that MoTe2 is a highly attractive material for use in electronic devices, photovoltaic devices, spintronic and valley11,12 optoeletronic devices.
A crucial step toward the practical application of MoTe2 in electronics and optoelectronics is the controlled production of high-quality, large-area, and atomically thin MoTe2 films. Thus far, single- and few-layer MoTe2 have only been 12,15 achieved using “top-down” exfoliation methods. However, exfoliation produces only small MoTe2 flakes of arbitrary shapes that are randomly distributed on a surface, thus preventing production for large-scale applications. Liquid exfoliation is a promising method for mass production of atomi15 cally thin MoTe2, but the low quality of MoTe2 using this technique cannot satisfy the requirements for electronic and optoelectronic applications. Therefore, a technology for the mass-production of high-quality, large-area, and atomically thin MoTe2 films is highly desirable. Compared with other group VI TMD materials, stoichiometric MoTe2 films are more difficult to achieve. The electronegativity difference between Te and Mo is much smaller (0.3 eV) among these materials. Therefore, the bonding energy of Mo-Te bonds is quite small, which translates to less tendency for the material formation and difficulty to obtain stoichiometric MoTe2. Moreover, at high temperatures, instead of evaporating as a compound, MoTe2 decomposes and loses Te as vapor. These properties make it challenging to directly obtain atomically thin MoTe2 film by physical vapor deposition, and there is very often Te deficiency in asprepared MoTe2. Considering the synthesis of MoTe2, another unique feature distinguishing MoTe2 from other TMDs is its small energy difference (4.8×10 cm (Figure 4d). Unlike other group VI TMDs (MoS2, MoSe2, 1,18,22 WS2) which usually behave as n-type semiconductors, the p-type semiconductor MoTe2 is essential for several applications, such as pn junctions and complementary logic circuits. Based on the equation for the carrier mobility µ = (dIds/dVbg)(L/W)(1/VdsCg), where L, W, and Cg stand for the channel length, width, and the gate capacitance per unit area, we can estimate the field-effect hole mobility of this 2 −1 −1 MoTe2 FET to be about 1 cm V s , which is comparable with that of back-gated FETs made from mechanically exfoliated 2 −1 −1 12,13 MoTe2 flakes (0.3−10 cm V s ). Since these are only two terminal devices and the effect of Schottky contact are present, it is anticipated that our few-layer MoTe2 should have better intrinsic transport behavior (e.g., mobility).
Figure 4. Electrical properties of devices made from a 2H MoTe2 film. (a) A typical optical image of a MoTe2 device on 90-nm SiO2/Si. The darker region is MoTe2. In the following, the channel length and width of this FET are ~6.5 and 20.0 µm, respectively. (b) Source-drain current (Ids) vs. voltage (Vds) characteristics of our MoTe2 FET device at various values of gate voltage. (c-d) Source-drain current (Ids) vs. gate voltage curve for the same devices at various value of Vds on a linear scale (c) and a logarithmic scale (d), respectively. The chemical composition of Mo precursors is found to be crucial for the CVD growth of MoTe2. The resulting MoTe2 phase and the efficiency of the tellurization are both strongly dependent on the oxidation state of the Mo precursor. When using Mo (instead of MoO3) as a precursor, a homogenous 1T′ MoTe2 film (Figure 5a) can also be grown under the same growth conditions mentioned above. Raman spectrum of the -1 -1 -1 MoTe2 film shows peaks at 108 cm , 127 cm , 161 cm , -1 -1 17,23 189 cm and 257 cm (Figure 5b). These peak frequencies 12,13 are consistent with the Raman spectra of bulk 1T′ MoTe2. Moreover, the rectangle shape of SEAD pattern (Figure5c) further verifies that the as-synthesized material is a 1T′ MoTe2 film. The high-resolution XPS spectra for Mo 3d and Te 3d further identify the resulting film after tellurization as being a MoTe2 film. The peaks at 228 and 231.1 eV are as-
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signed to the Mo 3d5/2 and Mo 3d3/2 peaks of the Mo-Te bonds (figures 5d). The peaks at 572.6 and 583 eV in figures 5e correspond to the Te 3d5/2 and Te 3d3/2 peaks of MoTe2. Moreover, the atomic ratio between Mo and Te elements is around 1:2, indicating that the 1T′ MoTe2 phase also has good stoichiometry. We found that MoO3 reacts more easily with Te and forms 2H MoTe2 under our synthesis conditions. In contrast, Mo and MoOx (x
