Ultrafast zero balance of the oscillator-strength sum rule in graphene.

Oscillator-strength sum rule in light-induced transitions is one general form of quantum-mechanical identities. Although this sum rule is well established in equilibrium photo-physics, an experimental corroboration for the validation of the sum rule in a nonequilibrium regime has been a long-standing unexplored question. The simple band structure of graphene is an ideal system for investigating this question due to the linear Dirac-like energy dispersion. Here, we employed both ultrafast terahertz and optical spectroscopy to directly monitor the transient oscillator-strength balancing between quasi-free low-energy oscillators and high-energy Fermi-edge ones. Upon photo-excitation of hot Dirac fermions, we observed that the ultrafast depletion of high-energy oscillators precisely complements the increased terahertz absorption oscillators. Our results may provide an experimental priori to understand, for example, the intrinsic free-carrier dynamics to the high-energy photo-excitation, responsible for optoelectronic operation such as graphene-based phototransistor or solar-energy harvesting devices.

Probing graphene grain boundaries with optical microscopy.

Grain boundaries in graphene are formed by the joining of islands during the initial growth stage, and these boundaries govern transport properties and related device performance. Although information on the atomic rearrangement at graphene grain boundaries can be obtained using transmission electron microscopy and scanning tunnelling microscopy, large-scale information regarding the distribution of graphene grain boundaries is not easily accessible. Here we use optical microscopy to observe the grain boundaries of large-area graphene (grown on copper foil) directly, without transfer of the graphene. This imaging technique was realized by selectively oxidizing the underlying copper foil through graphene grain boundaries functionalized with O and OH radicals generated by ultraviolet irradiation under moisture-rich ambient conditions: selective diffusion of oxygen radicals through OH-functionalized defect sites was demonstrated by density functional calculations. The sheet resistance of large-area graphene decreased as the graphene grain sizes increased, but no strong correlation with the grain size of the copper was revealed, in contrast to a previous report. Furthermore, the influence of graphene grain boundaries on crack propagation (initialized by bending) and termination was clearly visualized using our technique. Our approach can be used as a simple protocol for evaluating the grain boundaries of other two-dimensional layered structures, such as boron nitride and exfoliated clays.

Diffusion mechanism of lithium ion through basal plane of layered graphene.

Coexistence of both edge plane and basal plane in graphite often hinders the understanding of lithium ion diffusion mechanism. In this report, two types of graphene samples were prepared by chemical vapor deposition (CVD): (i) well-defined basal plane graphene grown on Cu foil and (ii) edge plane-enriched graphene layers grown on Ni film. Electrochemical performance of the graphene electrode can be split into two regimes depending on the number of graphene layers: (i) the corrosion-dominant regime and (ii) the lithiation-dominant regime. Li ion diffusion perpendicular to the basal plane of graphene is facilitated by defects, whereas diffusion parallel to the plane is limited by the steric hindrance that originates from aggregated Li ions adsorbed on the abundant defect sites. The critical layer thickness (l(c)) to effectively prohibit substrate reaction using CVD-grown graphene layers was predicted to be ∼6 layers, independent of defect population. Our density functional theory calculations demonstrate that divacancies and higher order defects have reasonable diffusion barrier heights allowing lithium diffusion through the basal plane but neither monovacancies nor Stone-Wales defect.

Gate-controlled nonlinear conductivity of Dirac fermion in graphene field-effect transistors measured by terahertz time-domain spectroscopy.

We present terahertz spectroscopic measurements of Dirac fermion dynamics from a large-scale graphene that was grown by chemical vapor deposition and on which carrier density was modulated by electrostatic and chemical doping. The measured frequency-dependent optical sheet conductivity of graphene shows electron-density-dependence characteristics, which can be understood by a simple Drude model. In a low carrier density regime, the optical sheet conductivity of graphene is constant regardless of the applied gate voltage, but in a high carrier density regime, it has nonlinear behavior with respect to the applied gate voltage. Chemical doping using viologen was found to be efficient in controlling the equilibrium Fermi level without sacrificing the unique carrier dynamics of graphene.

Enhanced light output power of near UV light emitting diodes with graphene / indium tin oxide nanodot nodes for transparent and current spreading electrode.

We report GaN-based near ultraviolet (UV) light emitting diode (LED) that combines indium tin oxide (ITO) nanodot nodes with two-dimensional graphene film as a UV-transparent current spreading electrode (TCSE) to give rise to excellent UV emission efficiency. The light output power of 380 nm emitting UV-LEDs with graphene film on ITO nanodot nodes as TCSE was enhanced remarkably compared to conventional TCSE. The increase of the light output power is attributed to high UV transmittance of graphene, effective current spreading and injection, and texturing effect by ITO nanodots.

Influence of copper morphology in forming nucleation seeds for graphene growth.

We report that highly crystalline graphene can be obtained from well-controlled surface morphology of the copper substrate. Flat copper surface was prepared by using a chemical mechanical polishing method. At early growth stage, the density of graphene nucleation seeds from polished Cu film was much lower and the domain sizes of graphene flakes were larger than those from unpolished Cu film. At later growth stage, these domains were stitched together to form monolayer graphene, where the orientation of each domain crystal was unexpectedly not much different from each other. We also found that grain boundaries and intentionally formed scratched area play an important role for nucleation seeds. Although the best monolayer graphene was grown from polished Cu with a low sheet resistance of 260 Ω/sq, a small portion of multilayers were also formed near the impurity particles or locally protruded parts.

Graphene/carbon nanotube hybrid-based transparent 2D optical array.

Graphene/carbon nanotube (CNT) hybrid structures are fabricated for use as optical arrays. Vertically aligned CNTs are directly synthesized on a graphene/quartz substrate using plasma-enhanced chemical vapor deposition (PECVD). Graphene preserves the transparency and resistance during CNT growth. Highly aligned single-walled CNTs show a better performance for the diffraction intensity.

Controlled growth of semiconducting nanowire, nanowall, and hybrid nanostructures on graphene for piezoelectric nanogenerators.

Precise control of morphologies of one- or two-dimensional nanostructures during growth has not been easy, usually degrading device performance and therefore limiting applications to various advanced nanoscale electronics and optoelectronics. Graphene could be a platform to serve as a substrate for both morphology control and direct use of electrodes due to its ideal monolayer flatness with π electrons. Here, we report that, by using graphene directly as a substrate, vertically well-aligned zinc oxide (ZnO) nanowires and nanowalls were obtained systematically by controlling gold (Au) catalyst thickness and growth time without inflicting significant thermal damage on the graphene layer during thermal chemical vapor deposition of ZnO at high temperature of about 900 °C. We clarify Au nanoparticle positions at graphene-ZnO heterojunctions that are very important in realizing advanced nanoscale electronic and optoelectronic applications of such nanostructures. Further, we demonstrate a piezoelectric nanogenerator that was fabricated from the vertically aligned nanowire-nanowall ZnO hybrid/graphene structure generates a new type of direct current through the specific electron dynamics in the nanowire-nanowall hybrid.

Thermionic field emission transport in carbon nanotube transistors.

With experimental and analytical analysis, we demonstrate a relationship between the metal contact work function and the electrical transport properties saturation current (Isat) and differential conductance (σsd=∂Isd/∂Vsd) in ambient exposed carbon nanotubes (CNT). A single chemical vapor deposition (CVD) grown 6 mm long semiconducting single-walled CNT is electrically contacted with a statistically significant number of Hf, Cr, Ti, Pd, and Au electrodes, respectively. The observed exponentially increasing relationship of Isat and σsd with metal contact work function is explained by a theoretical model derived from thermionic field emission. Statistical analysis and spread of the data suggest that the conduction variability in same CNT devices results from differences in local surface potential of the metal contact. Based on the theoretical model and methodology, an improved CNT-based gas sensing device layout is suggested. A method to experimentally determine gas-induced work function changes in metals is also examined.

Role of anions in the AuCl3-doping of carbon nanotubes.

The doping/dedoping mechanism of carbon nanotubes (CNTs) with AuCl(3) has been investigated with regard to the roles of cations and anions. Contrary to the general belief that CNTs are p-doped through the reduction of cationic Au(3+) to Au(0), we observed that chlorine anions play a more important role than Au cations in doping. To estimate the effects of Cl and Au on CNTs, the CNT film was dedoped as a function of the annealing temperature (100-700 °C) under an Ar ambient and was confirmed by the sheet resistance change and the presence of a G-band in the Raman spectra. The X-ray photoelectron spectroscopy (XPS) analysis revealed that the doping level of the CNT film was strongly related to the amount of adsorbed chlorine atoms. Annealing at temperatures up to 200 °C did not change the amount of adsorbed Cl atoms on the CNTs, and the CNT film was stable under ambient conditions. Alternatively, Cl atoms started to dissociate from CNTs at 300 °C, and the stability of the film was degraded. Furthermore, the change in the amount of Cl atoms in CNTs was inversely proportional to the change in the sheet resistance. Our observations of the Cl adsorption, either directly or mediated by an Au precursor on the CNT surface, are congruent with the previous theoretical prediction.

Laser thinning for monolayer graphene formation: heat sink and interference effect.

Despite the availability of large-area graphene synthesized by chemical vapor deposition (CVD), the control of a uniform monolayer graphene remained challenging. Here, we report a method of acquiring monolayer graphene by laser irradiation. The accumulation of heat on graphene by absorbing light, followed by oxidative burning of upper graphene layers, which strongly relies on the wavelength of light and optical parameters of the substrate, was in situ measured by the G-band shift in Raman spectroscopy. The substrate plays a crucial role as a heat sink for the bottom monolayer graphene, resulting in no burning or etching. Oscillatory thinning behavior dependent on the substrate oxide thickness was evaluated by adopting a simple Fresnel's equation. This paves the way for future research in utilizing monolayer graphene for high-speed electronic devices.

Layer-by-layer doping of few-layer graphene film.

We propose a new method of layer-by-layer (LbL) doping of thin graphene films. Large area monolayer graphene was synthesized on Cu foil by using the chemical vapor deposition method. Each layer was transferred on a polyethylene terephthalate substrate followed by a salt-solution casting, where the whole process was repeated several times to get LbL-doped thin layers. With this method, sheet resistance was significantly decreased up to approximately 80% with little sacrifice in transmittance. Unlike samples fabricated by topmost layer doping, our sample shows better environmental stability due to the presence of dominant neutral Au atoms on the surface which was confirmed by angle-resolved X-ray photoelectron spectroscopy. The sheet resistance of the LbL-doped four-layer graphene (11 x 11 cm(2)) was 54 Omega/sq at 85% transmittance, which meets the technical target for industrial applications.

Anomalous Schottky barriers and contact band-to-band tunneling in carbon nanotube transistors.

Devices incorporating nanoscale materials, particularly carbon nanotubes (CNTs), offer exceptional electrical performance. Absent, however, is an experimentally backed model explaining contact-metal work function, device layout, and environment effects. To fill the void, this report introduces a surface-inversion channel model based on low temperature and electrical measurements of a distinct single-walled semiconducting CNT contacted by Hf, Cr, Ti, and Pd electrodes. Anomalous barrier heights and metal-contact dependent band-to-band tunneling phenomena are utilized to show that, dependent upon contact work function and gate field, transport occurs either directly between the metal and CNT channel or indirectly via injection of carriers from the metal-covered CNT region to the CNT channel. The model is consistent with previously contradictory experimental results, and the methodology is simple enough to apply in other contact-dominant systems.