Raman Spectrum of Graphene and Graphene Layers A. C. Ferrari, 1, * J. C. Meyer, 2 V. Scardaci, 1 C. Casiraghi, 1 M. Lazzeri, 3 F. Mauri, 3 S. Piscanec, 1 D. Jiang, 4 K. S. Novoselov, 4 S. Roth, 2 and A. K. Geim 4 1 Cambridge University, Engineering Department, JJ Thompson Avenue, Cambridge CB3 0FA, United Kingdom 2 Max Planck Institute for Solid State Research, Stuttgart 70569, Germany 3 IMPMC, Universite ´s Paris 6 et 7, CNRS, IPGP, 140 rue de Lourmel, 75015 Paris, France 4 Department of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, United Kingdom (Received 9 June 2006; published 30 October 2006) Graphene is the two-dimensional building block for carbon allotropes of every other dimensionality. We show that its electronic structure is captured in its Raman spectrum that clearly evolves with the number of layers. The D peak second order changes in shape, width, and position for an increasing number of layers, reflecting the change in the electron bands via a double resonant Raman process. The G peak slightly down-shifts. This allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area. DOI: 10.1103/PhysRevLett.97.187401 PACS numbers: 78.67.Bf, 63.20.Dj, 63.20.Kr, 78.30.j The current interest in graphene can be attributed to three main reasons. First, its electron transport is described by the Dirac equation and this allows access to quantum electrodynamics in a simple condensed matter experiment [1– 5]. Second, the scalability of graphene devices to nano- dimensions [6 –10] makes it a promising candidate for applications, because of its ballistic transport at room temperature combined with chemical and mechanical stability. Remarkable properties extend to bilayer and few-layers graphene [4 – 6,8,11]. Third, various forms of graphite, nanotubes, buckyballs, and others can all be viewed as derivatives of graphene and, not surprisingly, this basic material has been intensively investigated theo- retically for the past 60 years [12]. The recent discovery of graphene [1] at last allows us to probe it experimentally, which paves the way to better understanding the other allotropes and to resolve controversies. Graphene can be obtained using the procedure of Ref. [1], i.e., micromechanical cleavage of graphite. Alternative procedures, such as exfoliation and growth, so far only produced multilayers [6,8,13], but it is hoped that in the near future efficient growth methods will be developed, as happened for nanotubes. Despite the wide use of the micromechanical cleavage, the identification and counting of graphene layers is a major hurdle. Monolayers are a great minority amongst accompanying thicker flakes. They cannot be seen in an optical microscope on most substrates. They only become visible when deposited on oxidized Si substrates with a finely tuned thickness of the oxide layer (typically, 300 nm SiO 2 ) since, in this case, even a monolayer adds to the optical path of reflected light to change the interference color with respect to the empty substrate [1,4]. Atomic force microscopy (AFM) has been so far the only method to identify single and few layers, but it is low throughput. Moreover, due to the chemical con- trast between graphene and the substrate (which results in an apparent chemical thickness of 0.5–1 nm, much bigger of what expected from the interlayer graphite spacing [1,4]), in practice, it is only possible to distinguish between one and two layers by AFM if films contain folds or wrinkles [1,4]. This poses a major limitation to the range of substrates and is a setback for the widespread utilization of this material. Here, we show that graphene’s electronic structure is uniquely captured in its Raman spectrum. Raman fingerprints for single layers, bilayers, and few layers reflect changes in the electron bands and allow unambiguous, high-throughput, nondestructive identifica- tion of graphene layers, which is critically lacking in this emerging research area. Here the samples are prepared by micromechanical cleavage [1]. To provide the most definitive identification of single and bilayers (beyond the AFM counting proce- dure) we perform transmission electron microscopy (TEM) on some of the samples to be measured by Raman spec- troscopy. Samples for TEM are prepared following a simi- lar process to that previously used to make freestanding and TEM-compatible nanotube devices [14]. In addition, this allows us to have freestanding layers on a grid easily seen in an optical microscope, facilitating their location during Raman measurements, Fig. 1(a). Electron diffrac- tion is done in a Zeiss 912  microscope at a voltage of 60 kV, and high-resolution images are obtained with a Philips CM200 microscope at 120 kV. A high resolution- TEM analysis of foldings at the edges or within the free- hanging sheets gives the number of layers by direct visual- ization, since at a folding the sheet is locally parallel to the beam, Figs. 1(b) –1(e). Edges and foldings of one or two layers are dominated by one or two dark lines, respectively. The number of layers is also obtained by a diffraction analysis of the freely suspended sheets for varying inci- dence angles, and confirms the number of layers from the foldings, Figs. 1(d) and 1(e). In particular, the diffraction analysis of the bilayer shows that it is A-B stacked (the intensity of the 11–20 diffraction spots (outer hexagon) is roughly twice that of the 1–100 (inner hexagon), Fig. 1(h), in agreement with diffraction simulations obtained by a PRL 97, 187401 (2006) PHYSICAL REVIEW LETTERS week ending 3 NOVEMBER 2006 0031-9007= 06=97(18)=187401(4) 187401-1  2006 The American Physical Society