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Akira Fujimoto1 Shoji Maeda1 Ryota Isu1 Kei-ichi Sakamoto1 Kentaro Matano1 Masatoshi Koyama1 Yoshiyuki Harada1 Kazuto Koike1 Shigehiko Sasa1 Mitsuaki Yano1

1, Osaka Institute of Technology, Osaka, , Japan

Graphene and 2-dimensional (2D) materials such as molybdenum disulfide (MoS2) have attracted much attention in a wide range of unique electrical and optical properties. These high-quality 2D materials are obtained by means of mechanical or chemical exfoliation from natural crystals. But they are basically very small flakes and difficult to control thickness. In comparison with the exfoliation method, chemical vapor deposition has a better potential for fabricating large area 2D materials, but the synthesis is still a challenge. Furthermore, MoS2 films have been prepared by dip-coating ammonium tetrathiomolybdates ((NH4)2MoS4) followed by the annealing process in the atmosphere of Ar and S in order to make up for S deficiencies [1]. The purpose of our study is to synthesize large-area high-quality MoS2 on SiO2/Si substrate by thermolysis of (NH4)2MoS4. By means of employing thioacetamide (CH3CSNH2), we are trying to compensate for deficiency of S in MoS2 thin films and to improve the crystallinity. Moreover, we are performing Raman scattering measurements in order to explore the molar concentration dependence of the chemical precursors used for the growth of MoS2.
Aiming for chemically synthesizing MoS2 we employed the aqueous thermolysis method. (NH4)2MoS4 and CH3CSNH2 with the same molar concentration were mixed in deionized water and the mixture was heated at 95 degree C for 5 hours. The solution containing the synthesized MoS2 was spin-coated onto the 90-nm SiO2/Si substrates. We confirmed that for the thin films synthesized with the molar concentration less than or equal to 10 mM, Raman double peaks E12g and A1g appeared at about 383 cm-1 and 407 cm-1, respectively. The E12g and A1g peak positions shift to a lower wavenumber with increasing the molar concentration. The Raman peak energy difference can be used to identify the number of MoS2 layers. We found the thickness of our MoS2 is as thick as that of MoS2 natural crystal. On the other hand, for the films synthesized with the larger molar concentration E12g and A1g peaks did not appear and another two Raman peaks strongly appeared at around 810 and 1000 cm-1 originating in MoO3 [2]. We found a synthesis of MoS2 using aqueous thermolysis method should be performed under the condition of a lower molar concentration of the chemical precursors.
Furthermore, as for the sample synthesized with the lower molar concentration of (NH4)2MoS4 and CH3CSNH2 we performed Raman scattering measurement again after a week. As the result, Raman peak strongly appeared at around 810 cm-1 originating in MoO3. It is conceivable that oxidation of Mo is caused by a reaction with the residue of water after the synthesis, therefore we consider the residue of water should be evaporated completely to prevent the oxidation of MoS2. For this purpose, we coated the solution directly on a SiO2/Si substrate and heated at 200 degree C for 3 hours. Even in a week after the synthesis by means of this improved process, we confirmed E12g and A1g double peaks appeared and Raman peaks derived from MoO3 did not. Now we are evaluating the electrical properties of these MoS2 thin films. Moreover, we will stack the MoS2 thin films and graphene for the application of gas sensors, which transduces gas-molecules adsorption on the surface of these 2D materials and at the MoS2/graphene interface into a change of the resistance.
[1] K-K. Liu et al., Nano Lett. 12 1538-1544 (2012).
[2] K. Koike, et al., Jpn. J. Appl. Phys. 53, 05FJ02 (2014).

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