Browsing by Author "Shi, Yumeng"

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Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface
(2015-07-31)
Li, Ming-Yang
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Shi, Yumeng
;
Cheng, Chia-Chin
;
Lu, Li-Syuan
;
Lin, Yung-Chang
Two-dimensional transition metal dichalcogenides (TMDCs) such as molybdenum sulfide MoS2 and tungsten sulfide WSe2 have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. Spatially connected TMDC lateral heterojunctions are key components for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors. However, such structures are not readily prepared via the layer-stacking techniques, and direct growth favors the thermodynamically preferred TMDC alloys. We report the two-step epitaxial growth of lateral WSe2-MoS2 heterojunction, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface.
Gap States at Low-Angle Grain Boundaries in Monolayer Tungsten Diselenide
(2016-06)
Huang, Yu Li
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Ding, Zijing
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Zhang, Wenjing
;
Chang, Yung-Huang
;
Shi, Yumeng
Two-dimensional (2D) transition metal dichalcogenides (TMDs) have revealed many novel properties of interest to future device applications. In particular, the presence of grain boundaries (GBs) can significantly influence the material properties of 2D TMDs. However, direct characterization of the electronic properties of the GB defects at the atomic scale remains extremely challenging. In this study, we employ scanning tunneling microscopy and spectroscopy to investigate the atomic and electronic structure of low-angle GBs of monolayer tungsten diselenide (WSe2) with misorientation angles of 3-6 degrees. Butterfly features are observed along the GBs, with the periodicity depending on the misorientation angle. Density functional theory calculations show that these butterfly; features correspond to gap states that arise in tetragonal dislocation cores and extend to distorted six-membered rings around the dislocation core. Understanding the nature of GB defects and their influence on transport and other device properties highlights the importance of defect engineering in future 2D device fabrication.
Graphene-Au nanoparticle based vertical heterostructures: A novel route towards high-ZT Thermoelectric devices
(2017-08-01)
Juang, Zhen-Yu
;
Tseng, Chien-Chih
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Shi, Yumeng
;
Hsieh, Wen-Pin
;
Ryuzaki, Sou
Monolayer graphene exhibits impressive in-plane thermal conductivity (> 1000 W m(-1) K-1). However, the out-of-plane thermal transport is limited due to the weak van der Waals interaction, indicating the possibility of constructing a vertical thermoelectric (TE) device. Here, we propose a cross-plane TE device based on the vertical heterostructures of few-layer graphene and gold nanoparticles (AuNPs) on Si substrates, where the incorporation of AuNPs further inhibits the phonon transport and enhances the electrical conductivity along vertical direction. A measurable Seebeck voltage is produced vertically between top graphene and bottom Si when the device is put on a hot surface and the figure of merit ZT is estimated as 1 at room temperature from the transient Harman method. The polarity of the output voltage is determined by the carrier polarity of the substrate. The device concept is also applicable to a flexible and transparent substrate as demonstrated.
Growth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating Substrates
(2012-03-01)
Liu, Keng-Ku
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Zhang, Wenjing
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Lee, Yi-Hsien
;
Lin, Yu-Chuan
;
Chang, Mu-Tung
The two-dimensional layer of molybdenum disulfide (MoS2) has recently attracted much interest due to its direct-gap property and potential applications in optoelectronics and energy harvesting. However, the synthetic approach to obtain high-quality and large-area MoS2 atomic thin layers is still rare. Here we report that the high-temperature annealing of a thermally decomposed ammonium thiomolybdate layer in the presence of sulfur can produce large-area MoS2 thin layers with superior electrical performance on insulating substrates. Spectroscopic and microscopic results reveal that the synthesized MoS2 sheets are highly crystalline. The electron mobility of the bottom-gate transistor devices made of the synthesized MoS2 layer is comparable with those of the micromechanically exfoliated thin sheets from MoS2 crystals. This synthetic approach is simple, scalable, and applicable to other transition metal dichalcogenides. Meanwhile, the obtained MoS2 films are transferable to arbitrary substrates, providing great opportunities to make layered composites by stacking various atomically thin layers.
Growth selectivity of hexagonal-boron nitride layers on Ni with various crystal orientations
(2012)
Lee, Yi-Hsien
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Liu, Keng-Ku
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Lu, Ang-Yu
;
Wu, Chih-Yu
;
Lin, Cheng-Te
Layered hexagonal-boron nitride (h-BN) films were synthesized by chemical vapor deposition (CVD) on Ni foils using ammonia borane as a precursor. Confocal Raman spectroscopy and electron backscatter diffraction (EBSD) were used to probe the effect of underlying Ni crystals with various orientations on growth behaviors of h-BN layers. The growth of the h-BN layers strongly depends on the Ni crystal orientations, where the growth rate of h-BN is larger on Ni(100)-like crystal surfaces but the growth on Ni(111)-like surfaces is not detectable, suggesting that Ni (100)-like facets are likely to promote the growth of h-BN compared with Ni (111)-like surfaces. The observation is in clear contrast to the reported growth of h-BN on Ni(111) in an ultrahigh vacuum environment. The as-grown CVD h-BN films on Ni exhibit a layered structure as revealed by atomic force microscopy (AFM). Thin h-BN layers are found on the Ni domain with a low growth rate. The observation of h-BN growth on various Ni grains may provide insights for the control of thickness, size and morphology of CVD h-BN films.
Photoluminescence Enhancement and Structure Repairing of Monolayer MoSe2 by Hydrohalic Acid Treatment
(2016-01-01)
Han, Hau-Vei
;
Lu, Ang-Yu
;
Lu, Li-Syuan
;
Huang, Jing-Kai
;
Li, Henan
Atomically thin two-dimensional transition metal dichalcogenides (TMDCs) have attracted much attention recently due to their unique electronic and optical properties for future optoelectronic devices. The chemical vapor deposition (CVD) method is able to generate TMDCs layers with a scalable size and a controllable thickness. However, the TMDC monolayers grown by CVD may incorporate structural defects, and it is fundamentally important to understand the relation between photoluminescence and structural defects. In this report, point defects (Se vacancies) and oxidized Se defects in CVD-grown MoSe2 monolayers are identified by transmission electron microscopy and X-ray photoelectron spectroscopy. These defects can significantly trap free charge carriers and localize excitons, leading to the smearing of free band-to-band exciton emission. Here, we report that the simple hydrohalic acid treatment (such as HBr) is able to efficiently suppress the trap state emission and promote the neutral exciton and trion emission in defective MoSe2 monolayers through the p-doping process, where the overall photoluminescence intensity at room temperature can be enhanced by a factor of 30. We show that HBr treatment is able to activate distinctive trion and free exciton emissions even from highly defective MoSe2 layers. Our results suggest that the HBr treatment not only reduces the n-doping in MoSe2 but also reduces the structural defects. The results provide further insights of the control and tailoring the exciton emission from CVD-grown monolayer TMDCs.
Photoluminescence Enhancement and Structure Repairing of Monolayer MoSe2 by Hydrohalic Acid Treatment
(2016-01)
Han, Hau-Vei
;
Lu, Ang-Yu
;
Lu, Li-Syuan
;
Huang, Jing-Kai
;
Li, Henan
Atomically thin two-dimensional transition metal dichalcogenides (TMDCs) have attracted much attention recently due to their unique electronic and optical properties for future optoelectronic devices. The chemical vapor deposition (CVD) method is able to generate TMDCs layers with a scalable size and a controllable thickness. However, the TMDC monolayers grown by CVD may incorporate structural defects, and it is fundamentally important to understand the relation between photoluminescence and structural defects. In this report, point defects (Se vacancies) and oxidized Se defects in CVD-grown MoSe2 monolayers are identified by transmission electron microscopy and X-ray photoelectron spectroscopy. These defects can significantly trap free charge carriers and localize excitons, leading to the smearing of free band-to-band exciton emission. Here, we report that the simple hydrohalic acid treatment (such as HBr) is able to efficiently suppress the trap state emission and promote the neutral exciton and trion emission in defective MoSe2 monolayers through the p-doping process, where the overall photoluminescence intensity at room temperature can be enhanced by a factor of 30. We show that HBr treatment is able to activate distinctive trion and free exciton emissions even from highly defective MoSe2 layers. Our results suggest that the HBr treatment not only reduces the n-doping in MoSe2 but also reduces the structural defects. The results provide further insights of the control and tailoring the exciton emission from CVD-grown monolayer TMDCs.
Self-assembly of hierarchical MoSx/CNT nanocomposites (2 < x < 3): towards high performance anode materials for lithium ion batteries
(2013-07-09)
Shi, Yumeng
;
Wang, Ye
;
Wong, Jen It
;
Tan, Alex Yuan Sheng
;
Hsu, Chang-Lung
Two dimension (2D) layered molybdenum disulfide (MoS2) has emerged as a promising candidate for the anode material in lithium ion batteries (LIBs). Herein, 2D MoSx (2 <= x <= 3) nanosheet-coated 1D multiwall carbon nanotubes (MWNTs) nanocomposites with hierarchical architecture were synthesized via a high-throughput solvent thermal method under low temperature at 2006 degrees C. The unique hierarchical nanostructures with MWNTs backbone and nanosheets of MoSx have significantly promoted the electrode performance in LIBs. Every single MoSx nanosheet interconnect to MWNTs centers with maximized exposed electrochemical active sites, which significantly enhance ion diffusion efficiency and accommodate volume expansion during the electrochemical reaction. A remarkably high specific capacity (i.e., >1000 mAh/g) was achieved at the current density of 50 mA g(-1), which is much higher than theoretical numbers for either MWNTs or MoS2 along (similar to 372 and similar to 670 mAh/g, respectively). We anticipate 2D nanosheets/1D MWNTs nanocomposites will be promising materials in new generation practical LIBs.
Self-assembly of hierarchical MoSx/CNT nanocomposites (2<x<3): towards high performance anode materials for lithium ion batteries
(2013-07-09)
Shi, Yumeng
;
Wang, Ye
;
Wong, Jen It
;
Tan, Alex Yuan Sheng
;
Hsu, Chang-Lung
Two dimension (2D) layered molybdenum disulfide (MoS2) has emerged as a promising candidate for the anode material in lithium ion batteries (LIBs). Herein, 2D MoSx (2 <= x <= 3) nanosheet-coated 1D multiwall carbon nanotubes (MWNTs) nanocomposites with hierarchical architecture were synthesized via a high-throughput solvent thermal method under low temperature at 2006 degrees C. The unique hierarchical nanostructures with MWNTs backbone and nanosheets of MoSx have significantly promoted the electrode performance in LIBs. Every single MoSx nanosheet interconnect to MWNTs centers with maximized exposed electrochemical active sites, which significantly enhance ion diffusion efficiency and accommodate volume expansion during the electrochemical reaction. A remarkably high specific capacity (i.e., >1000 mAh/g) was achieved at the current density of 50 mA g(-1), which is much higher than theoretical numbers for either MWNTs or MoS2 along (similar to 372 and similar to 670 mAh/g, respectively). We anticipate 2D nanosheets/1D MWNTs nanocomposites will be promising materials in new generation practical LIBs.
Synthesis and structure of two-dimensional transition-metal dichalcogenides
(2015-07-01)
Shi, Yumeng
;
Zhang, Hua
;
Chang, Wen-Hao
;
Shin, Hyeon Suk
;
Li, Lain-Jong
Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) exhibit unique electrical, optical, thermal, and mechanical properties, which enable them to be used as building blocks in compact and lightweight integrated electronic systems. The controllable and reliable synthesis of atomically thin TMDCs is essential for their practical application. Recent progress in large-area synthesis of monolayer TMDCs paves the way for practical production of various 2D TMDC layers. The intrinsic optical and electrical properties of monolayer TMDCs can be defined by stoichiometry during synthesis. By manipulating the lattice structure or layer stacking manner, it is possible to create atomically thin van der Waals materials with unique and unexplored physical properties. In this article, we review recent developments in the synthesis of TMDC monolayers, alloys, and heterostructures, which shine light on the design of novel TMDCs with desired functional properties.