Graphene edges are of particular interest since their orientation determines the electronic properties. Here we present a detailed Raman investigation of graphene flakes with edges oriented at different crystallographic directions. We also develop a real space theory for Raman scattering to analyze the general case of disordered edges. The position, width, and intensity of G and D peaks are studied as a function of the incident light polarization. The D-band is strongest for polarization parallel to the edge and minimum for perpendicular. Raman mapping shows that the D peak is localized in proximity of the edge. For ideal edges, the D peak is zero for zigzag orientation and large for armchair, allowing in principle the use of Raman spectroscopy as a sensitive tool for edge orientation. However, for real samples, the D to G ratio does not always show a significant dependence on edge orientation. Thus, even though edges can appear macroscopically smooth and oriented at well-defined angles, they are not necessarily microscopically ordered.

We studied the exciton energy transfer in pairs of semiconducting nanotubes using high-resolution optical microscopy and spectroscopy on the nanoscale. Photoluminescence from large band gap nanotubes within bundles is observed with spatially varying intensities due to distancedependent internanotube transfer. The range of efficient energy transfer is found to be limited to a few nanometers because of competing fast nonradiative relaxation responsible for low photoluminescence quantum yield.Single-walled carbon nanotubes (SWNTs) feature unique electronic properties, making them ideal candidates for ultrahigh density devices in electronics, photonics, and optoelectronics.1-3 At nanoscale distances, energy transfer from large to small band gap nanotubes is expected to occur, facilitating novel architectures including crossbars and 3D arrays but also imposing design restrictions. Photoluminescence (PL) in semiconducting nanotubes results from exciton recombination [4][5][6] and is found to be quenched in bundles. 7-10Very recently, resonant exciton energy transfer between semiconducting nanotubes has been observed for SWNTs in micelles suspensions for the first time and was explained by near-field coupling corresponding to fluorescence resonance energy transfer (FRET) well-known for molecular systems.11 In ref 12, spectroscopic signatures of internanotube transfer were observed, and it was suggested that efficient coupling results from carrier migration requiring direct physical contact. In ensemble measurements, however, the identification of donor and acceptor spectral signatures is complicated by overlapping contributions from different nanotube species, including phonon-assisted absorption and possible emission from lower lying defect-associated states. [13][14][15] Up to now, the range of exciton transfer and its efficiencies are unknown.We used tip-enhanced near-field optical microscopy (TEN-OM [16][17][18] ) as a tool to visualize energy transfer in pairs of semiconducting nanotubes forming bundles and crossings on a nanometer length scale. Near-field PL and topography images of a single nanotube bundle reveal the presence of two semiconducting nanotubes with different chiralities having an internanotube spacing ranging from 1 to 4 nm. Photoluminescence from large band gap nanotubes was observed with unexpectedly high intensities although varying spatially along the nanotubes due to distance-dependent internanotube energy transfer. Efficient transfer is found to be limited to a few nanometers because of competing fast nonradiative relaxation and can be explained in terms of electromagnetic near-field coupling. From the experimental data, we estimate transfer efficiencies and time scales.The setup used for near-field optical microscopy and spectroscopy is based on an inverted confocal optical microscope that is combined with a scan head for shearforce detection providing tip-sample distance control. [16][17][18] The signal is detected either by a combination of a spectrograph and a cooled charged coupl...