Comparison of Raman spectra and vibrational density of states between graphene nanoribbons with different edges

Vibrational properties of graphene nanoribbons are examined with density functional based tight-binding method and non-resonant bond polarization theory. We show that the recently discovered reconstructed zigzag edge can be identified from the emergence of high-energy vibrational mode due to strong triple bonds at the edges. This mode is visible also in the Raman spectrum. Total vibrational density of states of the reconstructed zigzag edge is observed to resemble the vibrational density of states of armchair, rather than zigzag, graphene nanoribbon. Edge-related vibrational states increase in energy which corroborates increased ridigity of the reconstructed zigzag edge. PACS. 61.46.-w Structure of nanoscale materials – 64.70.Nd Structural transitions in nanoscale materials – 63.22-m Phonons or vibrational states in low-dimensional structures and nanoscale materials – 78.30Na Infrared and Raman spectra, fullerenes and related materials In past decades carbon nanomaterials have shown their rich properties in several applications[1,2]. Here graphene nanoribbons are not an exception, and their use in many applications, among which transistors in nanoelectronics, have been investigated considering different edge structures[3, 4]. Low dimensionality and edges make ribbons particularly fascinating both for theory and applications. The precise edge structure in graphene nanoribbons affects many properties like chemical reactivity[5], electronic structure[6] and vibrations[7]. Vibrational properties play a role in structural stability [8], structure identification[9] and ballistic transport through electron-phonon coupling[9]. In structure identification scanning tunneling microscopy[10, 11] can reach near atom resolution but analysis of the full edge structure and properties is often ambiguous. In this case Raman spectroscopy[12,13,9] is a valuable tool. Vibrational properties of graphene nanoribbons have been recently studied for acoustic and optical phonons, symmetries, and Raman activity as a function of ribbon width[12,8]. In this paper we consider recently reported self-passivating edge reconstruction of zigzag ribbon [14], and investigate how the reconstruction affects Raman spectra and vibrational density of states (VDOS). Edge-localized contributions to Raman spectra and vibrational density of states change. It turns out that high-energy modes due to triple-bond vibrations, as well as overall changes in the vibrational density of states and changes in rigidity of the edges, make the reconstruction identifiable and visible in Raman spectra. a [email protected] zigzag57