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Has files: YesSubject: search.filters.subject.transition metal dichalcogenides

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    Spectroscopic Signatures for Interlayer Coupling in MoS2-WSe2 van der Waals Stacking
    (2014-09-01)
    Chiu, Ming-Hui
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    Li, Ming-Yang
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    Zhang, Wengjing
    ;
    Hsu, Wei-Ting
    ;
    Chang, Wen-Hao
    Stacking of MoS2 and WSe2 monolayers is conducted by transferring triangular MoS2 monolayers on top of WSe2 monolayers, all grown by chemical vapor deposition (CVD). Raman spectroscopy and photoluminescence (PL) studies reveal that these mechanically stacked monolayers are not closely coupled, but after a thermal treatment at 300 degrees C, it is possible to produce van der Waals solids consisting of two interacting transition metal dichalcogenide (TMD) monolayers. The layer-number sensitive Raman out-of-plane mode A(1g)(2) for WSe2 (309 cm(-1)) is found sensitive to the coupling between two TMD monolayers. The presence of interlayer excitonic emissions and the changes in other intrinsic Raman modes such as E \'\' for MoS2 at 286 cm(-1) and A(1g)(2) for MoS2 at around 463 cm(-1) confirm the enhancement of the interlayer coupling.
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    Monolayer MoSe2 Grown by Chemical Vapor Deposition for Fast Photodetection
    (2014-08-01)
    Chang, Yung-Huang
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    Zhang, Wenjing
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    Zhu, Yihan
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    Han, Yu
    ;
    Pu, Jiang
    Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the electrical and optoelectronic properties of monolayer MoS2 devices. Here, highly crystalline molybdenum diselenide (MoSe2) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS2 and MoSe2 monolayers reveals that the MoSe2 monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe2 presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS2 monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS2. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe2 and MoS2 and is useful for guiding future designs in 2D material-based optoelectronic devices.
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    Strong Enhancement of Raman Scattering from a Bulk-Inactive Vibrational Mode in Few-Layer MoTe2
    (2014-04-01)
    Yamamoto, Mahito
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    Wang, Sheng Tsung
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    Ni, Meiyan
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    Lin, Yen-Fu
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    Li, Song-Lin
    Two-dimensional layered crystals could show phonon properties that are markedly distinct from those of their bulk counterparts, because of the loss of periodicities along the c-axis directions. Here we investigate the phonon properties of bulk and atomically thin alpha-MoTe2 using Raman spectroscopy. The Raman spectrum of alpha-MoTe2 shows a prominent peak of the in-plane E-2g(1) mode, with its frequency upshifting with decreasing thickness down to the atomic scale, similar to other dichalcogenides. Furthermore, we find large enhancement of the Raman scattering from the out-of-plane B-2g(1) mode in the atomically thin layers. The B-2g(1) models Raman inactive in the bulk, but is observed to become active in the few-layer films. The intensity ratio of the B-2g(1) to E-2g(1) peaks evolves significantly with decreasing thickness, in contrast with other dichalcogenides. Our observations point to strong effects of dimensionality on the phonon properties of MoTe2.
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    Large-Area Synthesis of Highly Crystalline WSe2 Mono layers and Device Applications
    (2014-01-01)
    Huang, Jing-Kai
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    Pu, Jiang
    ;
    Hsu, Chang-Lung
    ;
    Chiu, Ming-Hui
    ;
    Juang, Zhen-Yu
    The monolayer transition metal dichalcogenides have recently attracted much attention owing to their potential in valleytronics, flexible and low-power electronics, and optoelectronic devices. Recent reports have demonstrated the growth of large-size two-dimensional MoS2 layers by the sulfurization of molybdenum oxides. However, the growth of a transition metal selenide monolayer has still been a challenge. Here we report that the introduction of hydrogen in the reaction chamber helps to activate the selenization of WO3, where large size WSe2 monolayer flakes or thin films can be successfully grown. The top-gated field-effect transistors based on WSe2 monolayers using ionic gels as the dielectrics exhibit ambipolar characteristics, where the hole and electron mobility values are up to 90 and 7 cm(2)/Vs, respectively. These films can be transferred onto arbitrary substrates, which may inspire research efforts to explore their properties and applications. The resistor-loaded inverter based on a WSe2 film, with a gain of similar to 13, further demonstrates its applicability for logic-circuit integrations.