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Large-scale growth of mostly monolayer molybdenum disulfide (MoS2) on quartz, sapphire, SiO2/Si, and waveguide substrates is demonstrated by chemical vapor deposition with the same growth parameters. Centimeter-scale areas with large flakes and films of MoS2 on all the growth substrates are observed. The atomic force microscopy and Raman measurements indicate the synthesized MoS2 is monolayer with high quality and uniformity. The MoS2 field effect transistors based on the as-grown MoS2 exhibit carrier mobility of 1–2 cm2V−1s−1 and On/Off ratio of ~104 while showing large photoresponse. Our results provide a simple approach to realize MoS2 on various substrates for electronics and optoelectronics applications.

The band alignment in AlGaN/GaN HEMT devices was investigated using scanning photocurrent microscopy (SPCM) with UV and visible light sources. The polarity of the SPCM on the conduction channel exhibited a switching behavior from p-type to n-type response as the gate bias was increased. The flat-band voltage, which was higher than the DC turn-on voltage, indicates that an ohmic metallic contact was formed for electron transport. We obtained the band offset between the conduction band and the metal Fermi level, which yielded a value of 2.1 eV on average.

Electrical characteristics of ion gels prepared by loading different amounts of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) in Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) are investigated and compared with those of ion liquid, [EMIM][TFSI] for possible application as a gate stack for flexible electronic devices. Capacitance and impedance as a function of frequency are measured, which can be well accounted for by a simple circuit model identifying the local device components. The operation of a flexible field effect transistor based on graphene and the ion gel as a top gate stack is also demonstrated.

We report on the effect of dielectric substrate on the resonance shift of metallic metamaterials operating in the terahertz frequency region. The resonance frequencies for various metamaterials obtained by time-domain spectroscopy agree well with calculations based on the finite-difference time-domain method. The dependencies of the resonance frequency to substrate index are studied, and are systematically determined by introducing an effective substrate index. The relative contributions of the substrate index for various metamaterials are obtained by fitting the numerically obtained effective refractive indices as a function of substrate index, which is found to be ∼0.63 regardless of the metamaterial geometry.

We report the characterization of individual carbon nanotube and Si nanowire field-effect transistors through high-speed scanning photocurrent microscopy with a scanning speed of 1 frame/s and a photocurrent sensitivity of less than 1 pA. This enables us to record photocurrent images that are free from hysteresis effects that modify the field configurations applied by the gate bias voltage. We can clearly resolve the photocurrent signals with polarity inversion near the metallic contacts under gate bias conditions which cause severe hysteresis effects in the nanowire devices. We also studied the dynamics of the hysteresis effects for different gate bias configurations. This high-speed photocurrent imaging technique is particularly useful for obtaining two-dimensional, localized optoelectronic characteristics and their correlation with overall device performance without encountering undesired dynamic responses.

We report on terahertz electromagnetic field transmission through metallic slot antennas supported on various dielectric substrates. The substrate effect strongly modifies the transmission resonance compared to the free-standing case, while the resonance can be determined systematically by introducing an effective refractive index. The relative contribution of the substrate index to the effective index has been measured for various substrates, and shows a good agreement with theoretical calculations based on a coupled mode method. In addition, the slot antenna arrays embedded in a flexible substrate show a transmission resonance governed by a substrate refractive index.