F. Plentz

Probing the electronic structure of bilayer graphene by Raman scattering

The electronic structure of bilayer graphene is investigated from a resonant Raman study using different laser excitation energies. The values of the parameters of the Slonczewski-Weiss-McClure model for graphite are measured experimentally and some of them differ significantly from those reported previously for graphite, specially that associated with the difference of the effective mass of electrons and holes. The splitting of the two TO phonon branches in bilayer graphene is also obtained from the experimental data. Our results have implications for bilayer graphene electronic devices.

Direct experimental evidence of exciton-phonon bound states in carbon nanotubes

We present direct experimental observation of exciton-phonon bound states in the photoluminescence excitation spectra of isolated single-walled carbon nanotubes (SWNT) in aqueous suspension. The photoluminescence excitation spectra from several distinct SWNTs show the presence of at least one sideband related to the tangential modes, lying 0.2 eV above the main absorption or emission peak. Both the energy position and line shapes of the sidebands are in excellent agreement with recent calculations [Phys. Rev. Lett. 94, 027402 (2005)] that predict the existence of exciton-phonon bound states, a sizable spectral weight transfer to these exciton-phonon complexes, and that the amount of this transfer depends on the specific nanotube structure and diameter.

The use of a Ga+ focused ion beam to modify graphene for device applications

In this work, we clarify the features of the lateral damage of line defects in single layer graphene. The line defects were produced through well-controlled etching of graphene using a Ga(+) focused ion beam. The lateral damage length was obtained from both the integrated intensity of the disorder induced Raman D band and the minimum ion fluence. Also, the line defects were characterized by polarized Raman spectroscopy. It was found that graphene is resilient under the etching conditions since the intensity of the defect induced Raman D peak exhibits a dependence on the direction of the lines relative to the crystalline lattice and also on the direction of the laser polarization relative to the lines. In addition, electrical measurements of the modified graphene were performed. Different ion fluences were used in order to obtain a completely insulating defect line in graphene, which was determined experimentally by means of charge injection and electric force microscopy measurements. These studies demonstrate that a Ga+ ion column combined with Raman spectroscopy is a powerful technique to produce and understand well-defined periodic arrays of defects in graphene, opening possibilities for better control of nanocarbon devices.

Hysteresis in the resistance of a graphene device induced by charge modulation in the substrate

We have fabricated graphene devices on lightly doped Si substrates and show that pronounced changes in resistance versus gate voltage, R(Vg), characteristics of these devices at 77 K are induced by the variation in the charge distribution in substrate with both gate voltage and illumination. The R(Vg) of the graphene devices in the dark shows remarkable changes as the carriers in the underlying substrate go through accumulation, depletion, and inversion regimes. We demonstrate the possibility of using a graphene device as an optical-latch.

Characterizing intrinsic charges in top gated bilayer graphene device by Raman spectroscopy

D. L. Mafra, P. Gava, L. M. Malard, R. S. Borges, G. G. Silva, J. A. Leon, F. Plentz, F. Mauri, M. A. Pimenta Departamento de F́ısica, Universidade Federal de Minas Gerais, 30123-970, Belo Horizonte, Brazil. IMPMC, Universit Paris 6 et 7, CNRS, IPGP, Paris, France. Departamento de Qúımica, Universidade Federal de Minas Gerais, 30123-970, Belo Horizonte, Brazil. ∗ These authors contributed equally to this work.

Measuring the electronic properties of single-walled carbon nanotubes with adsorbed porphyrins using optical transitions

The dielectric constants of diverse media surrounding single-walled carbon nanotubes (SWCNTs) were probed using photoluminescence (PL) excitation maps of porphyrin/SWCNT aqueous suspensions. The excitation and emission maxima of the nanotubes in these maps were used to probe the dielectric constant variation and doping originated from the porphyrin molecules. The net dielectric constant was calculated for the surrounding medium for each nanotube index and porphyrin isomer. The spread of the dielectric constant values calculated from the data for each (n, m) nanotube chiral index is interpreted on the basis of selective adsorption by each (n, m) nanotube, for each porphyrin isomer. Ultraviolet (UV) Raman spectroscopy corroborates the doping process through the shift of a G band around 1608 cm-1

Atomic size-limited intercalation into single wall carbon nanotubes

Intercalation of single wall carbon nanotubes (SWNTs) provides an important tool to modify their electronic band structure. Using multiple excitation wavelength Raman spectroscopy, we demonstrate that intercalation into SWNT interiors can be limited by intercalant size resulting in an unusual material comprising SWNTs with varying charge density. In the particular case of iodine intercalation, larger SWNTs with iodine-filled interiors were found to carry significantly higher charge density as compared to smaller empty ones. This difference was used to separate the intercalated SWNT material into fractions with homogeneous charge density.

Magneto-optical properties of stacked self-assembled InAs quantum dots

In this article, we report magneto-photoluminescence measurements on stacked self-assembled InAs quantum dots. By applying a magnetic field parallel to the growth direction, we determined the exciton reduced mass and exciton radius from the photoluminescence (PL) peak energy. We observed an asymmetric increase of the full width at half maximum of the quantum dots PL peak to the high-energy side that we associate to the size selectivity of the oscillator strength of the ground state transitions. The observed increase of the integrated intensity of the quantum dots line is explained in terms of the reabsorption of the photons emitted by the GaAs substrate and the InAs wetting layer. These effects are related to the multilayer structure of the sample.

Spin-dependent photoluminescence at ν=1 filling factor

Abstract We perform high-resolution, polarized photoluminescence (PL) experiments on a high-mobility GaAs/AlGaAs single quantum well near ν=1. Upon entering the ν=1 quantum-Hall region the PL shows a remarkable polarization dependence. For RCP polarization the PL exhibits the usual reduction of the peak intensity, and a cusp in the spectral energy of the peak. For LCP polarization we observe unanticipated novel behavior: a major change in the PL line shape, and the development of two new peaks, one above and one below the main emission peak. This large asymmetry between RCP and LCP polarizations can be modeled by considering the initial spin-configuration prior to the recombination of the e–h pair. Among the models consistent with our observations are a “shake-up” process in which finite-momentum spin waves are generated in the electron system upon recombination in the LCP polarization.

Thionine Self-Assembled Structures on Graphene: Formation, Organization, and Doping

The association of organic molecules with two-dimensional (2D) materials, creating hybrid systems with mutual influences, constitutes an important testbed for both basic science self-assembly studies and perspective applications. Following this concept, in this work, we show a rich phenomenology that is involved in the interaction of thionine with graphene, leading to a hybrid material formed by well-organized self-assembled structures atop graphene. This composite system is investigated by atomic force microscopy, electric transport measurements, Raman spectroscopy, and first principles calculations, which show (1) an interesting time evolution of thionine self-assembled structures atop graphene; (2) a highly oriented final molecular assembly (in accordance with the underlying graphene surface symmetry); and (3) a strong n-type doping effect introduced in graphene by thionine. The nature of the thionine-substrate interaction is further analyzed in experiments using mica as a polar substrate. The present results may help pave the way to achieve tailored 2D material hybrid devices via properly chosen molecular self-assembly processes.