Browsing by Author "Liu, Keng-Ku"

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Direct Formation of Wafer Scale Graphene Thin Layers on Insulating Substrates by Chemical Vapor Deposition
(2011-09-01)
Su, Ching-Yuan
;
Lu, Ang-Yu
;
Wu, Chih-Yu
;
Li, Yi-Te
;
Liu, Keng-Ku
Direct formation of high-quality and wafer scale graphene thin layers on insulating gate dielectrics such as SiO(2) is emergent for graphene electronics using Si-wafer compatible fabrication. Here, we report that in a chemical vapor deposition process the carbon species dissociated on Cu surfaces not only result in graphene layers on top of the catalytic Cu thin films but also diffuse through Cu grain boundaries to the interface between Cu and underlying dielectrics. Optimization of the process parameters leads to a continuous and large-area graphene thin layers directly formed on top of the dielectrics. The bottom-gated transistor characteristics for the graphene films have shown quite comparable carrier mobility compared to the top-layer graphene. The proposed method allows us to achieve wafer-sized graphene on versatile insulating substrates without the need of graphene transfer.
Efficient reduction of graphene oxide catalyzed by copper
(2012)
Lin, Yu-Chuan
;
Liu, Keng-Ku
;
Wu, Chih-Yu
;
Chu, Chih-Wei
;
Wang, Jacob Tse-Wei
We report that copper thin films deposited on top of graphene oxide (GO) serve as an effective catalyst to reduce GO sheets in a diluted hydrogen environment at high temperature. The reduced GO (rGO) sheets exhibit higher effective field-effect hole mobility, up to 80 cm(2) V-1 s(-1), and lower sheet resistance (13 k Omega square(-1)) compared with those reduced by reported methods such as hydrazine and thermal annealing. Raman and XPS characterizations are addressed to study the reduction mechanism on graphene oxide underneath copper thin films. The level of reduction in rGO sheets is examined by Raman spectroscopy and it is well correlated with hole mobility values. The conductivity enhancement is attributed to the growth of the graphitic domain size. This method is not only suitable for reduction of single GO sheets but also applicable to lower the sheet resistance of Langmuir-Blodgett assembled GO films.
Electrical Probing of Submicroliter Liquid Using Graphene Strip Transistors Built on a Nanopipette
(2012-01-09)
Chen, Chang-Hsiao
;
Lin, Cheng-Te
;
Lee, Yi-Hsien
;
Liu, Keng-Ku
;
Su, Ching-Yuan
Few-Layer MoS2 with High Broadband Photogain and Fast Optical Switching for Use in Harsh Environments
(2013-05-01)
Tsai, Dung-Sheng
;
Liu, Keng-Ku
;
Lien, Der-Hsien
;
Tsai, Meng-Lin
;
Kang, Chen-Fang
Few-layered MoS2 as Schottky metal-semiconductor-metal photodetectors (MSM PDs) for use In harsh environments makes its debut as two-dimensional (2D) optoelectronics with high broadband gain (up to 13.3), high detectivity (up to similar to 10(10) cm Hz(1/2)/W), fast photoresponse (rise time of similar to 70 mu s and fall time of similar to 110 mu s), and high thermal stability (at a working temperature of up to 200 degrees C). Ultrahigh responsivity (0.57 A/W) of few-layer MoS2 at 532 nm is due to the high optical absorption (similar to 10% despite being less than 2 nm in thickness) and a high photogain, which sets up a new record that was not achievable in 2D nanomaterials previously. This study opens avenues to develop 2D nanomaterial-based optoelectronics for harsh environments in imaging techniques and light-wave communications as well as in future memory storage and optoelectronic circuits.
Growth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating Substrates
(2012-03-01)
Liu, Keng-Ku
;
Zhang, Wenjing
;
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
;
Liu, Keng-Ku
;
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.
Label-Free Electrical Detection of DNA Hybridization on Graphene using Hall Effect Measurements: Revisiting the Sensing Mechanism
(2013-05-13)
Lin, Cheng-Te
;
Phan Thi Kim Loan
;
Chen, Tzu-Yin
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Liu, Keng-Ku
;
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.
Opening an Electrical Band Gap of Bilayer Graphene with Molecular Doping
(2011-09-01)
Zhang, Wenjing
;
Lin, Cheng-Te
;
Liu, Keng-Ku
;
Tite, Teddy
;
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.
Wafer-scale MoS2 thin layers prepared by MoO3 sulfurization
(2012)
Lin, Yu-Chuan
;
Zhang, Wenjing
;
Huang, Jing-Kai
;
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.