Showing papers by "Cinzia Casiraghi published in 2009"

Raman Spectroscopy of Graphene Edges

Abstract: 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.

Doping dependence of the Raman peaks intensity of graphene close to the Dirac point

Abstract: Here we use pristine graphene samples in order to analyze how the Raman peaks intensity, measured at 2.41 and 1.96 eV excitation energy, changes with the amount of doping. The use of pristine graphene allows investigating the intensity dependence close to the Dirac point. We show that the $G$ peak intensity is independent on the doping, while the 2D peak intensity strongly decreases for increasing doping. Analyzing this dependence in the framework of a fully resonant process, we found that the total electron-phonon scattering rate is $\ensuremath{\sim}40\text{ }\text{meV}$ (60 ${\text{ps}}^{\ensuremath{-}1}$) at 2.41 eV.

Probing disorder and charged impurities in graphene by Raman spectroscopy

Abstract: Disorder and doping can strongly affect the properties of graphene. Here we analyze these effects on several samples by Raman spectroscopy. In particular, we show that pristine and unprocessed graphene samples deposited on silicon, covered with a thin silicon oxide layer, show strong variations in their Raman spectra, even in absence of disorder. The variation in the Raman parameters is assigned to charged impurities. This shows that as-deposited graphene is unintentionally doped, reaching charge concentrations up to 10(13) cm(-2) under ambient conditions. The doping varies from sample to sample and the charges are inhomogeneously distributed on a submicron scale. (c) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Quantitative composition of a single-walled carbon nanotube sample: Raman scattering versus photoluminescence

Abstract: We investigated the spectral data of three carbon nanotube (CNT) species obtained by Raman spectroscopy and photoluminescence (PL) measurements. The corresponding relative signal intensities without further corrections yielded significantly different relative distributions of the CNT species. Theoretical calculations of optical transition probabilities and electron-phonon coupling were included, providing simple models in order to estimate the relative distribution of the three species within the sample. We proposed the product of PL and PL excitation intensities to be a candidate for quantitative analysis of CNT species. Applying the models, we confirmed that both spectroscopic methods agree on one nanotube species dominating the distribution.