Browsing by Author "Chen, Chang-Hsiao"

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Band Alignment of 2D Transition Metal Dichalcogenide Heterojunctions
(2017-05-18)
Chiu, Ming-Hui
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Tseng, Wei-Hsuan
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Tang, Hao-Ling
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Chang, Yung-Huang
;
Chen, Chang-Hsiao
It is critically important to characterize the band alignment in semiconductor heterojunctions (HJs) because it controls the electronic and optical properties. However, the well-known Anderson's model usually fails to predict the band alignment in bulk HJ systems due to the presence of charge transfer at the interfacial bonding. Atomically thin 2D transition metal dichalcogenide materials have attracted much attention recently since the ultrathin HJs and devices can be easily built and they are promising for future electronics. The vertical HJs based on 2D materials can be constructed via van der Waals stacking regardless of the lattice mismatch between two materials. Despite the defect-free characteristics of the junction interface, experimental evidence is still lacking on whether the simple Anderson rule can predict the band alignment of HJs. Here, the validity of Anderson's model is verified for the 2D heterojunction systems and the success of Anderson's model is attributed to the absence of dangling bonds (i.e., interface dipoles) at the van der Waal interface. The results from the work set a foundation allowing the use of powerful Anderson's rule to determine the band alignments of 2D HJs, which is beneficial to future electronic, photonic, and optoelectronic devices.
Bandgap tunability at single-layer molybdenum disulphide grain boundaries
(2015-02-01)
Huang, Yu Li
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Chen, Yifeng
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Zhang, Wenjing
;
Quek, Su Ying
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Chen, Chang-Hsiao
Two-dimensional transition metal dichalcogenides have emerged as a new class of semiconductor materials with novel electronic and optical properties of interest to future nanoelectronics technology. Single-layer molybdenum disulphide, which represents a prototype two-dimensional transition metal dichalcogenide, has an electronic bandgap that increases with decreasing layer thickness. Using high-resolution scanning tunnelling microscopy and spectroscopy, we measure the apparent quasiparticle energy gap to be 2.40 +/- 0.05 eV for single-layer, 2.10 +/- 0.05 eV for bilayer and 1.75 +/- 0.05 eV for trilayer molybdenum disulphide, which were directly grown on a graphite substrate by chemical vapour deposition method. More interestingly, we report an unexpected bandgap tunability (as large as 0.85 +/- 0.05 eV) with distance from the grain boundary in single-layer molybdenum disulphide, which also depends on the grain misorientation angle. This work opens up new possibilities for flexible electronic and optoelectronic devices with tunable bandgaps that utilize both the control of two-dimensional layer thickness and the grain boundary engineering.
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
Enhanced Thermoelectric Performance of PEDOT:PSS Flexible Bulky Papers by Treatment with Secondary Dopants
(2015-01-14)
Mengistie, Desalegn A.
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Chen, Chang-Hsiao
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Boopathi, Karunakara M.
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Pranoto, Ferry W.
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Li, Lain-Jong
For inorganic thermoelectric materials, Seebeck coefficient and electrical conductivity are interdependent, and hence optimization of thermoelectric performance is challenging. In this work we show that thermoelectric performance of PEDOT:PSS can be enhanced by greatly improving its electrical conductivity in contrast to inorganic thermoelectric materials. Free-standing flexible and smooth PEDOT:PSS bulky papers were prepared using vacuum-assisted filtration. The electrical conductivity was enhanced to 640, 800, 1300, and 1900 S cm(-1) by treating PEDOT:PSS with ethylene glycol, polyethylene glycol, methanol, and formic acid, respectively. The Seebeck coefficient did not show significant variation with the tremendous conductivity enhancement being 21.4 and 20.6 mu V K-1 for ethylene glycol- and formic acid-treated papers, respectively. This is because secondary dopants, which increase electrical conductivity, do not change oxidation level of PEDOT. A maximum power factor of 80.6 mu W m(-1) K-2 was shown for formic acid-treated samples, while it was only 29.3 mu W m(-1) K-2 for ethylene glycol treatment. Coupled with intrinsically low thermal conductivity of PEDOT:PSS, ZT approximate to 0.32 was measured at room temperature using Harman method. We investigated the reasons behind the greatly enhanced thermoelectric performance.
Fast Visible-Light Phototransistor Using CVD-Synthesized Large-Area Bilayer WSe2
(2014)
Liu, Pang-Shiuan
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Chen, Chang-Hsiao
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Hsu, Wei-Ting
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Lin, Chih-Pin
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Lin, Tzu-Ping
P-channel transition metal dichalcogenide ultrathin-body phototransistor (UTB-PT) with a response time as fast as 100 mu s has been demonstrated for the first time using the CVD-synthesized large-area bilayer WSe2. Because of its excellent compatibility with mass production, the application of WSe2 UTB-PT for high-speed proximity interactive display has been proposed.
Graphene/MoS2 Heterostructures for Ultrasensitive Detection of DNA Hybridisation
(2014-07-23)
Phan Thi Kim Loan
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Zhang, Wenjing
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Lin, Cheng-Te
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Wei, Kung-Hwa
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Li, Lain-Jong
The photoluminescence signals of a graphene/MoS2 heterostructural stacking film are sensitive to environmental charges, which allows the single-base sequence-selective detection of DNA hybridization with sensitivity to the level of aM. [GRAPHICS] .
Hybrid Si/TMD 2D Electronic Double Channels Fabricated Using Solid CVD Few-Layer-MoS2 Stacking for V-th Matching and CMOS-Compatible 3DFETs
(2014)
Chen, Min-Cheng
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Lin, Chia-Yi
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Li, Kai-Hsin
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Li, Lain-Jong
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Chen, Chang-Hsiao
Stackable 3DFETs such as FinFET using hybrid Si/MoS 2 channels were developed using a fully CMOS-compatible process. Adding several molecular layers (3-16 layers) of the transition-metal dichalcogenide (TMD), MoS2 to Si fin and nanowire resulted in improved (+25%) I-on,I-n of the FinFET and nanowire FET (NWFET). The PFETs also operated effectively and the N/P device V-th are low and matched perfectly. The proposed heterogeneous Si/TMD 3DFETs can be useful in future electronics.
Label-Free Electrical Detection of DNA Hybridization on Graphene using Hall Effect Measurements: Revisiting the Sensing Mechanism
(2013-05-13)
Lin, Cheng-Te
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Phan Thi Kim Loan
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Chen, Tzu-Yin
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Liu, Keng-Ku
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Chen, Chang-Hsiao
There is broad interest in using graphene or graphene oxide sheets as a transducer for label-free and selective electrical detection of biomolecules such as DNA. However, it is still not well explored how the DNA molecules interact with and influence the properties of graphene during the detection. Here, Hall effect measurements based on the Van der Pauw method are used to perform single-base sequence selective detection of DNA on graphene sheets, which are prepared by chemical vapor deposition. The sheet resistance increases and the mobility decreases with the addition of either complementary or one-base mismatched DNA to the graphene device. The hole carrier concentration of the graphene devices increases significantly with the addition of complementary DNA but it is less affected by the one-base mismatched DNA. It is concluded that the increase in hole carrier density, indicating p-doping to graphene, is better correlated with the DNA hybridization compared to the commonly used parameters such as conductivity change. The different electrical observations of p-doping from Hall effect measurements and n-doping from electrolyte-gated transistors can be explained by the characteristic morphology of partially hybridized DNA on graphene and the mismatch between DNA chain length and Debye length in electrolytes.
Second Harmonic Generation from Artificially Stacked Transition Metal Dichalcogenide Twisted Bilayers
(2014-03-01)
Hsu, Wei-Ting
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Zhao, Zi-Ang
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Li, Lain-Jong
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Chen, Chang-Hsiao
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Chiu, Ming-Hui
Optical second harmonic generation (SHG) is known as a sensitive probe to the crystalline symmetry of few-layer transition metal dichalcogenides (TMDs). Layer-number dependent and polarization resolved SHG have been observed for the special case of Bernal stacked few-layer TMDs, but it remains largely unexplored for structures deviated from this ideal stacking order. Here we report on the SHG from homo- and heterostructural TMD bilayers formed by artificial stacking with an arbitrary stacking angle. The SHG from the twisted bilayers is a coherent superposition of the SH fields from the individual layers, with a phase difference depending on the stacking angle. Such an interference effect is insensitive to the constituent layered materials and thus applicable to hetero-stacked bilayers. A proof-of-concept demonstration of using the SHG to probe the domain boundary and crystal polarity of mirror twins formed in chemically grown TMDs is also presented. We show here that the SHG is an efficient, sensitive, and nondestructive characterization for the stacking orientation, crystal polarity, and domain boundary of van der Waals heterostructures made of noncentrosymmetric layered materials.
Three- Dimensional Molybdenum Sulfi de Sponges for Electrocatalytic Water Splitting
(2014-03-01)
Chang, Yung-Huang
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Wu, Feng-Yu
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Chen, Tzu-Yin
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Hsu, Chang-Lung
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Chen, Chang-Hsiao
Ultrafast Multi-Level Logic Gates with Spin-Valley Coupled Polarization Anisotropy in Monolayer MoS2
(2015-02-06)
Wang, Yu-Ting
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Luo, Chih-Wei
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Yabushita, Atsushi
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Wu, Kaung-Hsiung
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Kobayashi, Takayoshi
The inherent valley-contrasting optical selection rules for interband transitions at the K and K' valleys in monolayer MoS2 have attracted extensive interest. Carriers in these two valleys can be selectively excited by circularly polarized optical fields. The comprehensive dynamics of spin valley coupled polarization and polarized exciton are completely resolved in this work. Here, we present a systematic study of the ultrafast dynamics of monolayer MoS2 including spin randomization, exciton dissociation, free carrier relaxation, and electron-hole recombination by helicity- and photon energy-resolved transient spectroscopy. The time constants for these processes are 60 fs, 1 ps, 25 ps, and, similar to 300 ps, respectively. The ultrafast dynamics of spin polarization, valley population, and exciton dissociation provides the desired information about the mechanism of radiationless transitions in various applications of 2D transition metal dichalcogenides. For example, spin valley coupled polarization provides a promising way to build optically selective-driven ultrafast valleytronics at room temperature. Therefore, a full understanding of the ultrafast dynamics in MoS2 is expected to provide important fundamental and technological perspectives.
Ultrahigh-Gain Photodetectors Based on Atomically Thin Graphene-MoS2 Heterostructures
(2014-01-23)
Zhang, Wenjing
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Chuu, Chih-Piao
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Huang, Jing-Kai
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Chen, Chang-Hsiao
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Tsai, Meng-Lin
Due to its high carrier mobility, broadband absorption, and fast response time, the semi-metallic graphene is attractive for optoelectronics. Another two-dimensional semiconducting material molybdenum disulfide (MoS2) is also known as light-sensitive. Here we show that a large-area and continuous MoS2 monolayer is achievable using a CVD method and graphene is transferable onto MoS2. We demonstrate that a photodetector based on the graphene/MoS2 heterostructure is able to provide a high photogain greater than 108. Our experiments show that the electron-hole pairs are produced in the MoS2 layer after light absorption and subsequently separated across the layers. Contradictory to the expectation based on the conventional built-in electric field model for metal-semiconductor contacts, photoelectrons are injected into the graphene layer rather than trapped in MoS2 due to the presence of a perpendicular effective electric field caused by the combination of the built-in electric field, the applied electrostatic field, and charged impurities or adsorbates, resulting in a tuneable photoresponsivity.