Spectroscopic ellipsometry was used to characterize the complex refractive index of chemical vapor deposition (CVD) graphene grown on copper foils and transferred to glass substrates. Two ellipsometers, with respective wavelength ranges extending into the ultraviolet and infrared (IR), have been used to characterize the CVD graphene optical functions. The optical absorption follows the same relation to the fine structure constant previously observed in the IR region, and displays the exciton-dominated absorption peak at ∼4.5 eV. The optical functions of CVD graphene show some differences when compared to published values for exfoliated graphene.

Angle-dependent carrier transmission probability in graphene p-n junctions is investigated. Using electrostatic doping from buried gates, p-n junctions are formed along graphene channels that are patterned to form different angles with the junction. A peak in the junction resistance is observed, which becomes pronounced with angle. This angular dependence is observed for junctions made on both exfoliated and CVD-grown graphene and is consistent with the theoretically predicted dependence of transmission probability on incidence angle.

Comparing photoconductivity measurements, using p-n diodes formed along individual single-walled carbon nanotubes (SWNT), with modeling results, allows determination of the quantum efficiency, optical capture cross section, and oscillator strength of the first (E11) and second (E22) excitonic transitions of SWNTs. This is in the infrared region of the spectrum, where little experimental work on SWNT optical absorption has been reported to date. We estimate quantum efficiency (η) ~1-5% and provide a correlation of η, capture cross section, and oscillator strength for E11 and E22 with nanotube diameter. This study uses the spectral weight of the exciton resonances as the determining parameter in optical measurements.

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Realizing basic semiconductor devices such as p-n junctions are necessary for developing thin-film and optoelectronic technologies in emerging planar materials such as MoS2. In this work, electrostatic doping by buried gates is used to study the electronic and optoelectronic properties of p-n junctions in exfoliated MoS2 flakes. Creating a controllable doping gradient across the device leads to the observation of the photovoltaic effect in monolayer and bilayer MoS2 flakes. For thicker flakes, strong ambipolar conduction enables realization of fully reconfigurable p-n junction diodes with rectifying current-voltage characteristics, and diode ideality factors as low as 1.6. The spectral response of the photovoltaic effect shows signatures of the predicted band gap transitions. For the first excitonic transition, a shift of >4kBT is observed between monolayer and bulk devices, indicating a thickness-dependence of the excitonic coulomb interaction. Two-dimensional (2-D) crystalline materials have attracted a significant amount of research efforts since the isolation of graphene by micromechanical exfoliation [1,2,3,4]. They show promise in novel electronic and optoelectronic applications, where the low-dimensionality provides ideal electrostatic control for field-effect transistor devices, or large area-to-volume ratio for sensors and photoelectric devices. Among 2-D crystals, MoS2, a transition metal dichalcogenide (TMDC), has received particular attention as channel material for thin-film or flexible electronics [5,6,7] because its mobility is considerably higher than amorphous or polycrystalline materials, and because it can be used in various heterostructures to enable diverse electronic applications [8,9,10,11,12].The most remarkable attributes of MoS2 lie in its bandstructure, which shows a crossover from an indirect bandgap (~1.3 eV) in bulk to a direct one (~1.9 eV) for a monolayer [13,14]. In the monolayer form, MoS2 has been used in optoelectronics [15,16,17], and has been investigated to enable a new class

The band gap of a semiconductor is one of its most fundamental properties. It is one of the defining parameters for applications, including nanoelectronic and nanophotonic devices. Measuring the band gap, however, has received little attention for quasi-one-dimensional materials, including single-walled carbon nanotubes. Here we show that the current-voltage characteristics of p-n diodes fabricated with semiconducting carbon nanotubes can be used along with the excitonic transitions of the nanotubes to measure both the fundamental (intrinsic) and renormalized nanotube band-gaps.

It is often assumed that single-walled carbon nanotubes grown via catalytic chemical vapor deposition are free of defects, particularly when suspended over a trench. Here we show that semiconducting nanotubes grown in this manner can contain a surprisingly large number of states within the band gap. High-resolution photocurrent spectroscopy is used to probe these states in individual nanotubes. We observe a series of band-gap states in suspended nanotubes, resulting in long trapping and detrapping times. These states significantly alter the exciton spectra, resulting in a manifold of strongly localized exciton states with narrow linewidths (<1 meV at room temperature) within a broader exciton envelope.

Magneto-transport measurements are performed on two-dimensional GaAs electron systems to probe the quantum Hall (QH) effect at low magnetic fields. Oscillations following the Shubnikov-de Haas (SdH) formula are observed in the transition from the insulator to QH liquid when the observed almost temperature-independent Hall slope indicates insignificant interaction correction. Our study shows that the existence of SdH oscillations in such a transition can be understood based on the non-interacting model.