Chun-Chiang Kuo

Band Gap Engineering of Chemical Vapor Deposited Graphene by in-situ BN Doping

Band gap opening and engineering is one of the high priority goals in the development of graphene electronics. Here, we report on the opening and scaling of band gap in BN doped graphene (BNG) films grown by low-pressure chemical vapor deposition method. High resolution transmission electron microscopy is employed to resolve the graphene and h-BN domain formation in great detail. X-ray photoelectron, micro-Raman, and UV-vis spectroscopy studies revealed a distinct structural and phase evolution in BNG films at low BN concentration. Synchrotron radiation based XAS-XES measurements concluded a gap opening in BNG films, which is also confirmed by field effect transistor measurements. For the first time, a significant band gap as high as 600 meV is observed for low BN concentrations and is attributed to the opening of the π-π* band gap of graphene due to isoelectronic BN doping. As-grown films exhibit structural evolution from homogeneously dispersed small BN clusters to large sized BN domains with embedded diminutive graphene domains. The evolution is described in terms of competitive growth among h-BN and graphene domains with increasing BN concentration. The present results pave way for the development of band gap engineered BN doped graphene-based devices.

Correlating defect density with carrier mobility in large-scaled graphene films: Raman spectral signatures for the estimation of defect density

We report a correlation between carrier mobility and defect density in large-scaled graphene films prepared by chemical vapor deposition (CVD). Raman spectroscopy is used for investigating the layer number and the crystal quality of graphene films, and the defect density is estimated by the intensity ratios of the D and G peaks. By carefully controlling the growth parameters, especially the H(2)/CH(4) ratios during growth, and employing H(2) during cooling, monolayer-dominant graphene films can be obtained with different D peak intensities in Raman spectra, which show good correspondence with their carrier mobility obtained by Hall measurements. Also, a progressive shift of neutrality points to a more negative gate voltage is observed with the increase in defect density. Both the connections of carrier mobility and the shift of neutrality points to a negative direction in relation to the defect density in graphene are observed for the first time in CVD-grown graphene films. With the best growth conditions, a cm-scaled graphene film with carrier mobility of ∼ 1350 cm(2) V(-1) s(-1) (p-type in air) can be obtained.

Functionalized GaN nanowire-based electrode for direct label-free voltammetric detection of DNA hybridization

This study demonstrates the utility of functionalized GaN nanowires (GaNNWs) for electrochemical detection of nucleic acids, in aqueous solution, using cyclic voltammetry. In order to link probe DNA to the NW surface, we employed an organosulfur compound, 3-mercaptopropyl trimethoxysilane (MPTS), to functionalize the GaNNW surface. Interestingly, the MPTS-modified GaNNWs exhibited a potential window of 4.5 V, the widest reported to date, with very low background current, which provides an advantage for sensing DNA immobilization/hybridization, down to sub-pM concentration, via monitoring adenine and guanine oxidation. The oxidation of guanine was characterized by its peak potential and peak current, where the former serves as a fingerprint for DNA hybridization and the latter as a parameter for the extent of hybridization. Moreover, the GaNNW-based sensor exhibited excellent consistency in hybridization-dehybridization-rehybridization cycles.

Land-surface deformation corresponding to seasonal ground-water fluctuation, determining by SAR interferometry in the SW Taiwan

In this study we implement the InSAR technique for identifying the seasonal surface deformation in the SW Taiwan. The focus of our investigation is on the Pingtung plain, a tectonic valley with a high water-pumping rate. Our preliminary results show that the land subsidence occurs specifically in the dry season and even ceases during the wet season. Whereas local land subsidence is usually caused by dewatering of sediments, we compare therefore our InSAR observations with the groundwater data of this area. This comparison shows that the subsidence rate is associated with the descending trend of groundwater level. Based on our research results, we infer that the dewatering during the dry season removes the buoyant support from the sediments and the effect of sediment compaction simultaneously results in local land subsidence. This successful test exhibits that with suitable images, the InSAR is a useful high-resolution tool for monitoring the earth surface deformation and can provide significant information for the issue of natural hazard mitigation.

Optical properties of functionalized GaN nanowires

The evolution of the optical properties of GaN nanowires (NWs) with respect to a sequence of surface functionalization processes is reported; from pristine hydroxylated to finally, 3-mercaptopropyltrimethoxysilane (MPTMS) functionalized GaN NWs. Photoluminescence, Raman, stationary, and time-resolved photoluminescence measurements were applied to investigate the GaN NWs with different surface conditions. A documented surface passivation effect of the GaN NWs induced by the MPTMS functionalization is determined based on our characterization results. A hypothesis associated with the surface band bending and the defect levels near the band edges is proposed to explain the observed experimental results.

Using Optical Anisotropy as a Quality Factor To Rapidly Characterize Structural Qualities of Large-Area Graphene Films

In this study, we find that the optical anisotropy of graphene films could be used as an alternative quality factor for the rapid characterization of large-area graphene films prepared through chemical vapor deposition. We develop an angle-variable spectroscopic method to rapidly determine the optical anisotropy of graphene films. Unlike approaches using Raman scattering spectroscopy, this optical anisotropy method allows ready characterization of the structural quality of large-area graphene samples without the application of high-intensity laser irradiation or complicated optical setups. Measurements of optical anisotropy also allow us to distinguish graphene samples with different extents of structural imperfections; the results are consistent with those obtained from using Raman scattering spectroscopy. In addition, we also study the properties of graphene-based transparent conductive films at wide incident angles because of the advantage of the optical anisotropic properties of graphene. The transmittance of graphene is much higher than that of indium tin oxide films, especially at large incident angles.

THz-bandwidth coherent phonon emission by supported monolayer graphene in the out-of-plane direction

In this report, it is demonstrated that the generation of THz band-width longitudinal coherent acoustic phonons (CAPs) that propagate perpendicularly to graphenes basal plane can be achieved by photo-excitation of supported monolayer graphene. Through exclusively exciting graphene with femtosecond lasers, the observation of backward Brillouin oscillations showed that after graphene absorbed the 800nm light, longitudinal CAPs with momentum in the out-of-plane direction were generated. With the assist of a piezoelectric InGaN quantum well buried inside a wurtzite GaN substrate, the full spectrum of the CAPs was revealed. Experimental results showed that the longitudinal CAP pulse had a bipolar shape and its spectrum had reached up to 1THz. In attempt to understand the generation mechanism of the longitudinal CAP pulse, we proposed a hypothesis based on graphenes carrier dynamics.

InGaN/GaN MQW LEDs with Si treatment

Y. P. Hsu, S. J. Chang, Y. K. Su, J. M. Tsai S. C. Chen, W. C. Lai, C. H. Kuo and C. S. Chang Institute of Microelectronics & Department of Electrical Engineering National Cheng Kung University, Tainan, 70101, TAIWAN 1 South Epitaxy Corporation, Hsin-Shi 744, TAIWAN 2Department of Electronic Engineering National Yunlin University of Science and Technology Touliu, 640, TAIWAN changsj@mail.ncku.edu.tw