arXiv:1006.5738v1 [cond-mat.mes-hall] 29 Jun 2010 Determination of Edge Purity in Bilayer Graphene Using μ-Raman Spectroscopy Milan Begliarbekov 1 , Onejae Sul 2 , Sokratis Kalliakos 1 , Eui-Hyeok Yang 2 , Stefan Strauf 1∗ 1 Department of Physics & Engineering Physics, Stevens Institute of Technology, Hoboken NJ, USA and 2 Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken NJ, USA Polarization resolved μ-Raman spectroscopy was carried out at the edges of bilayer graphene. We find strong dependence of the intensity of the G band on the incident laser polarization, with its intensity dependence being 90 ◦ out of phase for the armchair and zigzag case, in accordance with theoretical predictions. For the case of mixed-state edges we demonstrate that the polarization contrast reflects the fractional composition of armchair and zigzag edges, providing a monitor of edge purity, which is an important parameter for the development of efficient nanoelectronic devices. The recent discovery of graphene [1], a two-dimensional crystal comprised of a single layer of carbon atoms, trig- gered intensive research efforts in the physics and materi- als science communities. The high degree of crystallinity and outstanding electronic and thermal properties make graphene a promising candidate for nanoelectronic devices [2–4]. The addition of a second layer forms bilayer graphene with a largely changed electronic band structure resulting in field-tunable electronic band gaps [5] and strongly sup- pressed electronic noise [6]. Of particular importance for device applications are the underlying edge chiralities of bi- layer graphene and graphene nanoribbons (GNRs), since the atomic edge composition influences the electronic structure and thus transport properties [7, 8] as well as chemical re- activity [9]. As a nondestructive technique, Raman spec- troscopy has been widely utilized to determine the number of graphitic layers [10, 11]. Furthermore, since the chirality of graphitic edges and the orientation of the crystalline axis have a strong impact on phonon modes localized at the edges, Raman spectroscopy can also be utilized for edge state char- acterization [12–15]. Although previous experiments have addressed the issue of edge state identification by Raman spectroscopy using the D band around 1350 cm −1 [13], a detailed analysis and methodology to determine edge purity in the case of mixed edges has not yet been presented. Un- like the D band, the G band around 1580 cm −1 was recently predicted to show a strong polarization sensitivity with re- spect to armchair and zigzag edges, with Raman scattering amplitudes 90 degrees out of phase [15]. Here, we report on polarization-resolved μ-Raman experi- ments performed at the edges of bilayer graphene flakes. We find a strong dependence of the Raman intensity of the G- band on the polarization of incident laser light with respect to various edge orientations and we confirm that amplitudes of armchair and zigzag edges are 90 ◦ out of phase. Further- more, we demonstrate that the varying polarization contrast of the G band is a useful monitor to characterize edges with mixed armchair/zigzag boundaries. In these experiments, graphene flakes were mechanically exfoliated from a highly ordered pyrolized graphite (HOPG) block and deposited onto pre-patterned p ++ silicon wafer with a thermally grown 300 nm silicon oxide. Room tem- perature μ-Raman spectra were obtained using a 2.33 eV ∗ Electronic address: strauf@stevens.edu Figure 1: a) The scattering mechanisms that give rise to the var- ious Raman modes, here solid horizontal lines show phonon scat- tering, whereas dashed horizontal lines show defect scattering; b) abridged Raman spectrum labeling the bands identified in a; c) G’ band Raman spectra obtained from several different flakes (offset for clarity) showing the dependence of the G’ band on the number of graphitic layers. The solid red line is the sum of the Lorentzian sub-components. laser diode with a spot size of about 2 μm. Half wave plates were used to rotate the plane of polarization with respect to the sample in the laser excitation path and to rotate the plane of polarization in the collection path back to its origi- nal configuration in order to eliminate any errors introduced by the dependence of the spectrometer’s grating and other optical components on the polarization of light. The prominent spectral bands of graphene are shown in the Raman spectrum in Fig. 1b., while Fig. 1a shows the physi- cal mechanisms that give rise to these bands. Each band can be used as a tool to probe different material characteristics. The G’ band (sometimes referred to as the 2D band) provides unambiguous information about the number of constituent graphene layers. This phonon band (2700 cm −1 ) originates from inter-valley scattering of two in-plane transverse optical (iTO) phonons at the K and K’ points at the edges of the Brillouin zone [16, 17]. The impact of the number of layers on the G’ band is shown in Fig 1c. In single-layer graphene, the G’ band can be approximated by a single Lorentzian func- tion (Fig. 1c, lower panel), whereas several Lorentzian func- tions are required in the case of multilayer graphene (Fig.