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 present near-field Raman spectroscopy and imaging of single isolated single-walled carbon nanotubes with a spatial resolution of 25 nm. The near-field origin of the image contrast is confirmed by the measured dependence of the Raman scattering signal on tip-sample distance and the unique polarization properties. The method is used to study local variations in the Raman spectrum along a single single-walled carbon nanotube. DOI: 10.1103/PhysRevLett.90.095503 PACS numbers: 61.46.+w, 07.79.-v, 78.30.Na, 78.67.Ch Recent rapid advances in nanotechnology and nanoscience are largely due to our newly acquired ability to measure and manipulate individual structures on the nanoscale. Among the new methods are scanning probe techniques, optical tweezers, and high-resolution electron microscopes. Recently, a near-field optical technique has been demonstrated which allows spectroscopic measurements with 20 nm spatial resolution [1]. The method makes use of the strongly enhanced electric field close to a sharp metal tip under laser illumination and relies on the detection of two-photon excited fluorescence. However, fluorescence imaging requires a high fluorescence quantum yield of the system studied or artificial labeling with fluorophores. Furthermore, fluorescence quenching by the metal tip competes with the local field enhancement effect and therefore limits the general applicability. On the other hand, Raman scattering probes the unique vibrational spectrum of the sample and directly reflects its chemical composition and molecular structure. A main drawback of Raman scattering is the extremely low scattering cross section which is typically 14 orders of magnitude smaller than the cross section of fluorescence. Surface enhanced Raman scattering (SERS), induced by nanometer-sized metal structures, has been shown to provide enormous enhancement factors of up to 10 15 allowing for Raman spectroscopy even on the single molecule level [2,3]. Controlling SERS with a sharp metal tip which is raster scanned over a sample surface has been proposed [1,4], and near-field Raman enhancement has been experimentally demonstrated [5][6][7][8][9]. Here, we show the chemical specificity of this near-field technique and demonstrate an unprecedented spatial resolution.Single-walled carbon nanotubes (SWNTs) have been the focus of intense interest due to a large variety of potential nanotechnological applications. The unique properties of SWNTs arise from their particular onedimensional structure which is directly linked to the characteristic Raman bands. Raman scattering on SWNTs has been studied intensively in the literature (see, e.g., Refs. [10 -13]) and Raman enhancements of up to 10 12 have been reported for tubes in contact with fractal silver colloidal clusters [14]. In this Letter, nearfield Raman imaging of SWNTs is demonstrated using a sharp silver tip as a probe. We show, for the first time, that single isolated SWNTs can be detected optically with a spatial resolution better than 30 nm. This high-resolution capability is ap...

We investigate graphene and graphene layers on different substrates by monochromatic and white-light confocal Rayleigh scattering microscopy. The image contrast depends sensitively on the dielectric properties of the sample as well as the substrate geometry and can be described quantitatively using the complex refractive index of bulk graphite. For a few layers (<6), the monochromatic contrast increases linearly with thickness. The data can be adequately understood by considering the samples behaving as a superposition of single sheets that act as independent two-dimensional electron gases. Thus, Rayleigh imaging is a general, simple, and quick tool to identify graphene layers, which is readily combined with Raman scattering, that provides structural identification.

We show that strong photoluminescence (PL) can be induced in single-layer graphene using an oxygen plasma treatment. The PL is spatially uniform across the flakes and connected to elastic scattering spectra distinctly different from those of gapless pristine graphene. Oxygen plasma can be used to selectively convert the topmost layer when multilayer samples are treated.

The field near a sharp metal tip can be strongly enhanced if irradiated with an optical field polarized along the tip axis. We demonstrate that the enhanced field gives rise to local second-harmonic (SH) generation at the tip surface thereby creating a highly confined photon source. A theoretical model for the excitation and emission of SH radiation at the tip is developed and it is found that this source can be represented by a single on-axis oscillating dipole. The model is experimentally verified by imaging the spatial field distribution of strongly focused laser modes. [3][4][5]. The enhancement originates from a combination of the electrostatic lightning-rod effect, which is due to the geometric singularity of sharply pointed structures, and localized surface plasmon resonances which depend sensitively on the excitation wavelength. In most studies, the field enhancement factor is either deduced from theoretical calculations or from ensemble averaged measurements. However, recent spatially resolved SERS measurements on single molecules have shown that the field enhancement may vary by many orders of magnitude between different sites of a colloidal metal surface [6,7]. In order to understand which structures render particularly strong enhancements it is necessary to perform measurements on a single particle level. In this Letter, we demonstrate that local SH generation provides a direct means to determine the field enhancement near a nanoscale structure such as a sharp metal tip.SH generation in the context of near-field optical microscopy has been discussed by various authors [8][9][10][11] but the strong dependence on local field enhancement has not been analyzed. Here, we show that the field enhancement effect localizes SH generation to the very end of a metal tip thereby creating a highly confined photon source. We experimentally demonstrate the strong light confinement and investigate the radiation properties of the SH light. We also show that the tip acts as a local probe for fields polarized along the tip axis (longitudinal fields). This unique property can be used for the mapping of field distributions in optical fields as demonstrated recently with trapped ions [12] and single molecules [13,14].

Single-molecule fluorescence spectroscopy was used to determine the electronic properties of individual single-walled carbon nanotubes. Carbon nanotube structure was determined simultaneously from Raman spectroscopy. Fluorescence spectra from individual nanotubes with identical structures have different emission energies and linewidths that likely arise from defects or the local environment. Unlike most other molecules studied to date, the fluorescence intensity or spectrum from a single nanotube unexpectedly did not fluctuate.

Organic bulk heterojunctions combining electron donor and acceptor phases are of great interest for designing organic photovoltaic devices. [1] While impressive advances have been achieved with these systems, so far a deterministic control of their nanoscale morphology has been elusive. It would be a major breakthrough to be able to create model systems with periodic, interpenetrating networks of electron donor and acceptor phases providing maximum control over all structural and electronic features.Herein we report a significant step towards this goal on the basis of the recently discovered class of crystalline covalent organic frameworks (COFs) which are created by condensation of molecular building blocks. [2][3][4][5] Specifically, the stacked layers of two-dimensional COFs permit charge migration through the framework, [6] and several semiconducting structures [7] with high carrier mobilities [8][9][10] have been described. We have created a COF containing stacked thieno[2,3-b]thiophene-based building blocks serving as electron donors (TT-COF), with high surface area and a 3 nm open pore system. This open framework takes up the wellknown fullerene electron acceptor [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM), thus forming a novel structurally ordered donor-acceptor network. Spectroscopic results demonstrate light-induced charge transfer from the photoconductive TT-COF donor network to the encapsulated PCBM phase in the pore system. Moreover, we have created the first working COF-based photovoltaic device with the above components. The organization of the molecular building blocks into a crystalline framework with defined conduction paths provides a promising model system for ordered and interpenetrated networks of donors and acceptors at the nanoscale.The most prominent hole-conducting material used in organic solar cells is poly(3-hexylthiophene) (P3HT), a thiophene-containing polymer with high charge-carrier mobilities. The soluble fullerene derivative PCBM is often used as an electron acceptor in organic photovoltaics. [11] Because of the lack of structural order in the respective bulk heterojunctions it is very difficult to assess the impact of molecular building blocks, bonding motifs, and energy levels on the microscopic processes involving light-induced exciton formation, charge separation, and transport in such systems. Hence ordered charge-transporting networks with a periodicity of several nanometers are of great interest to understand the mechanistic details of the light-induced processes and ultimately to obtain design rules for the creation of efficient and stable organic photovoltaic devices. [12,13] The new TT-COF was synthesized under solvothermal conditions by co-condensation of thieno[3,2-b]thiophene-2,5diyldiboronic acid (TTBA) and the polyol 2,3,6,7,10,11hexahydroxytriphenylene (HHTP; Figure 1 a). Reaction parameters are described in the Supporting Information.As described in the following, the thienothiophene-based COF forms stacks in an AA arrangement, as confirmed by N 2 sorpti...

Covalent organic frameworks (COFs) offer a strategy to position molecular semiconductors within a rigid network in a highly controlled and predictable manner. The π-stacked columns of layered two-dimensional COFs enable electronic interactions between the COF sheets, thereby providing a path for exciton and charge carrier migration. Frameworks comprising two electronically separated subunits can form highly defined interdigitated donor–acceptor heterojunctions, which can drive the photogeneration of free charge carriers. Here we report the first example of a photovoltaic device that utilizes exclusively a crystalline organic framework with an inherent type II heterojunction as the active layer. The newly developed triphenylene–porphyrin COF was grown as an oriented thin film with the donor and acceptor units forming one-dimensional stacks that extend along the substrate normal, thus providing an optimal geometry for charge carrier transport. As a result of the degree of morphological precision that can be achieved with COFs and the enormous diversity of functional molecular building blocks that can be used to construct the frameworks, these materials show great potential as model systems for organic heterojunctions and might ultimately provide an alternative to the current disordered bulk heterojunctions.