Browsing by Author "Lee, Yi-Hsien"

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A gate-free monolayer WSe2 pn diode
(2018-08-07)
Chen, Jhih-Wei
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Lo, Shun-Tsung
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Ho, Sheng-Chin
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Wong, Sheng-Shong
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Vu, Thi-Hai-Yen
Interest in bringing p- and n-type monolayer semiconducting transition metal dichalcogenides (TMD) into contact to form rectifying pn diode has thrived since it is crucial to control the electrical properties in two-dimensional (2D) electronic and optoelectronic devices. Usually this involves vertically stacking different TMDs with pn heterojunction or, laterally manipulating carrier density by gate biasing. Here, by utilizing a locally reversed ferroelectric polarization, we laterally manipulate the carrier density and created a WSe2 pn homojunction on the supporting ferroelectric BiFeO3 substrate. This non-volatile WSe2 pn homojunction is demonstrated with optical and scanning probe methods and scanning photoelectron microspectroscopy. A homo-interface is a direct manifestation of our WSe2 pn diode, which can be quantitatively understood as a clear rectifying behavior. The non-volatile confinement of carriers and associated gate-free pn homojunction can be an addition to the 2D electron-photon toolbox and pave the way to develop laterally 2D electronics and photonics.
Efficient Defect Healing of Transition Metal Dichalcogenides by Metallophthalocyanine
(2018-08-29)
Ahn, Hyeyoung
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Huang, Yu-Chiao
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Lin, Chang-Wei
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Chiu, Yi-Lun
;
Lin, Erh-Chen
Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted great attention as alternatives to graphene with semiconducting band gaps. Mono- or few-layer TMDCs can be prepared by various methods, but regardless of the fabrication methods [such as mechanical exfoliation and chemical vapor deposition (CVD)], TMDCs contain many structural defects, which significantly affect their physical properties and limit their performance in applications. Metallophthalocyanines (MPcs) are organic semiconductors, and as dopants, they are capable of modulating the optical and electrical properties of other semiconducting materials. Here, we report that besides the ability to modulate the optoelectronic properties of 2D TMDCs, MPc molecules can be used to heal defects and improve the physicochemical properties of TMDCs. Doping of planar MPc molecules to TMDCs is achieved by a simple solution dip-coating method and results in a significant improvement in the optical properties and thermal responses of CVD-grown TMDCs, even comparable to those of mechanically exfoliated counterparts. Study of carrier dynamics shows that the adsorption of MPc on the TMDC surface leads to the complete suppression of the mid-gap defect-induced absorption in TMDCs. Furthermore, MPc molecules with a large lateral size are found to effectively reduce the point defects in mechanically exfoliated TMDCs introduced during the preparation process. Our results not only clarify the optoelectronic modulation mechanism of chemical doping but also offer a simple method to control the nanosized defects in 2D TMDCs.
Electrical Probing of Submicroliter Liquid Using Graphene Strip Transistors Built on a Nanopipette
(2012-01-09)
Chen, Chang-Hsiao
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Lin, Cheng-Te
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Lee, Yi-Hsien
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Liu, Keng-Ku
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Su, Ching-Yuan
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
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Lin, Yu-Chuan
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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
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Wu, Chih-Yu
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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.
Highly Efficient Electrocatalytic Hydrogen Production by MoSx Grown on Graphene-Protected 3D Ni Foams
(2013-02-06)
Chang, Yung-Huang
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Lin, Cheng-Te
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Chen, Tzu-Yin
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Hsu, Chang-Lung
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Lee, Yi-Hsien
Label-free detection of DNA hybridization using transistors based on CVD grown graphene
(2013-03-15)
Chen, Tzu-Yin
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Phan Thi Kim Loan
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Hsu, Chang-Lung
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Lee, Yi-Hsien
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Wang, Jacob Tse-Wei
The high transconductance and low noise of graphene-based field-effect transistors based on large-area monolayer graphene produced by chemical vapor deposition are used for label-free electrical detection of DNA hybridization. The gate materials, buffer concentration and surface condition of graphene have been optimized to achieve the DNA detection sensitivity as low as 1 pM (10(-12) M), which is more sensitive than the existing report based on few-layer graphene. The graphene films obtained using conventional PMMA-assisted transfer technique exhibits PMMA residues, which degrade the sensing performance of graphene. We have demonstrated that the sensing performance of the graphene samples prepared by gold-transfer is largely enhanced (by 125%). (C) 2012 Elsevier B.V. All rights reserved.
Monolayer Semiconductor Surface-Emitting Lasers Using 2D Dark Plasmonic Cavities
(2018-01-01)
Wang, Chun-Yuan
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Shi, Jinwei
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Raja, Soniya S.
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Chen, Chun-An
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Zhang, Xin-Quan
We demonstrated a low-threshold lasing from a monolayer gain medium which is made possible due to 2D feedback mechanism via strong exciton-SPP coupling in both real space and a wide range of momentum space.
Opening an Electrical Band Gap of Bilayer Graphene with Molecular Doping
(2011-09-01)
Zhang, Wenjing
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Lin, Cheng-Te
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Liu, Keng-Ku
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Tite, Teddy
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Su, Ching-Yuan
The opening of an electrical band l gap in graphene Is crucial for its application for logic circuits. Recent studies have shown that an energy gap in Bernal-stacked bilayer graphene can be generated by applying an electric displacement field. Molecular doping has also been proposed to open the electrical gap of bilayer graphene by breaking either in-plane symmetry or inversion symmetry; however, no direct observation of an electrical gap has been reported. Here we discover that the organic molecule triazine is able to form a uniform thin coating on the top surface of a bilayer graphene, which efficiently blocks the accessible doping sites and prevents ambient p-doping on the top layer. The charge distribution asymmetry between the top and bottom layers can then be enhanced simply by increasing the p-doping from oxygen/moisture to the bottom layer. The on/off current ratio for a bottom-gated bilayer transistor operated in ambient condition Is improved by at least 1 order of magnitude. The estimated electrical band gap is up to similar to 111 meV at room temperature. The observed electrical band gap dependence on the hole-carrier density increase agrees well with the recent density-functional theory calculations. This research provides a simple method to obtain a graphene bilayer transistor with a moderate on/off current ratio, which can be stably operated in air without the need to use an additional top gate.
Phase-driven magneto-electrical characteristics of single-layer MoS2
(2016)
Yang, Chao-Yao
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Chiu, Kuan-Chang
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Chang, Shu-Jui
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Zhang, Xin-Quan
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Liang, Jaw-Yeu
Magnetism of the MoS2 semiconducting atomic layer was highlighted for its great potential in the applications of spintronics and valleytronics. In this study, we demonstrate an evolution of magneto-electrical properties of single layer MoS2 with the modulation of defect configurations and formation of a partial 1T phase. With Ar treatment, sulfur was depleted within the MoS2 flake leading to a 2H (low-spin) -> partial 1T (high-spin) phase transition. The phase transition was accompanied by the development of a ferro-magnetic phase. Alternatively, the phase transition could be driven by the desorption of S atoms at the edge of MoS2 via O-2 treatment while with a different ordering magnitude in magnetism. The edge-sensitive magnetism of the single-layer MoS2 was monitored by magnetic force microscopy and validated by a first-principle calculation with graded-Vs (sulfur vacancy) terminals set at the edge, where band-splitting appeared more prominent with increasing Vs. Treatment with Ar and O-2 enabled a dual electrical characteristic of the field effect transistor (FET) that featured linear and saturated responses of different magnitudes in the I-ds-V-ds curves, whereas the pristine MoS2 FET displayed only a linear electrical dependency. The correlation and tuning of the Vs-1T phase transition would provide a playground for tailoring the phase-driven properties of MoS2 semiconducting atomic layers in spintronic applications.
Plasmonic enhancement and control of optical nonlinearity in monolayer WS2
(2018-01-01)
Liang, Wei-Yun
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Ahn, Hyeyoung
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Sang, Yungang
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Wang, Yanrong
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Shi, Jinwei
A giant SHG enhancement is achieved from monolayer-WS2 incorporated onto 2D plasmonic metasurfaces (Ag-nanogroove-grating) in resonance with the excitonic resonance of monolayer-WS2. An optical encoding technique is demonstrated based on plasmonic manipulation of SHG polarization.
Plasmonic Enhancement and Manipulation of Optical Nonlinearity in Monolayer Tungsten Disulfide
(2018-10-01)
Shi, Jinwei
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Liang, Wei-Yun
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Raja, Soniya S.
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Sang, Yungang
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Zhang, Xin-Quan
Monolayer transition metal dichalcogenides (TMDCs) have large second-order optical nonlinearity owing to broken inversion symmetry in two-dimensional (2D) crystals. However, despite the strong light-matter coupling in monolayer TMDCs, their nonlinear responses are ultimately limited by subnanometer thickness. Here, a dramatic enhancement of the second-harmonic generation (SHG) is achieved from monolayer tungsten disulfide (WS2) incorporated onto a 2D silver (Ag) nanogroove grating with subwavelength pitch. By tuning surface plasmon mode and second-harmonic frequency in resonance with the C exciton in WS2, a large SHG enhancement factor (approximate to 400) and a large conversion efficiency (approximate to 2.0 x 10(-5)) can be obtained. Furthermore, the azimuthal angle dependence of polarized SHG from monolayer WS2 can be manipulated by the nanogroove plasmonic mode. Based on this property, a polarization-modulated optical encoding technique is demonstrated. The results suggest that 2D TMDC-plasmonic hybrid metasurface structures can provide an ideal integration platform for on-chip nonlinear photonics and plasmonics.
Wafer-scale MoS2 thin layers prepared by MoO3 sulfurization
(2012)
Lin, Yu-Chuan
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Zhang, Wenjing
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Huang, Jing-Kai
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Liu, Keng-Ku
;
Lee, Yi-Hsien
Atomically thin molybdenum disulfide (MoS2) layers have attracted great interest due to their direct-gap property and potential applications in optoelectronics and energy harvesting. Meanwhile, they are extremely bendable, promising for applications in flexible electronics. However, the synthetic approach to obtain large-area MoS2 atomic thin layers is still lacking. Here we report that wafer-scale MoS2 thin layers can be obtained using MoO3 thin films as a starting material followed by a two-step thermal process, reduction of MoO3 at 500 degrees C in hydrogen and sulfurization at 1000 degrees C in the presence of sulfur. Spectroscopic, optical and electrical characterizations reveal that these films are polycrystalline and with semiconductor properties. The obtained MoS2 films are uniform in thickness and easily transferable to arbitrary substrates, which make such films suitable for flexible electronics or optoelectronics.