Browsing by Author "Tite, Teddy"

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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.
Optical second harmonic generation from the twin boundary of ZnO thin films grown on silicon
(2008-03-03)
Lo, Kuang-Yao
;
Lo, Shih-Chieh
;
Yu, Chang-Feng
;
Tite, Teddy
;
Huang, Jung-Y.
The symmetry of the twin boundaries of ZnO epitaxial film was detected with reflective second harmonic generation (RSHG). The twin boundaries exhibit mirror symmetry with a polar configuration across the boundary plane and yield a nonvanishing polar contribution to RSHG. The nonvanishing second-order susceptibility supports the notion that the measured RSHG originates from the planar defect, which depends on the residual stress in the thin film. We analyzed our RSHG result by correlating the macroscopic data from optic probe with the microscopic data from tunneling electron microscope.