Ab Initio Study of Phonon-Induced Dephasing of Electronic Excitations in Narrow Graphene Nanoribbons Bradley F. Habenicht, † Oleg N. Kalugin, ‡ and Oleg V. Prezhdo* ,† Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700, USA and Department of Chemistry, KharkiV National UniVersity, KharkiV, 61007, Ukraine Received May 31, 2008; Revised Manuscript Received June 30, 2008 ABSTRACT Vibrational dephasing of the lowest energy electronic excitations in the perfect (16,16) graphene nanoribbon (GNR) and those with the C 2 - bond insertion and rotation defects is studied with ab initio molecular dynamics. Compared to single-walled carbon nanotubes (SWCNTs) of similar size, GNRs shows very different properties. The dephasing in the ideal GNR occurs twice faster than that in the SWCNTs. It is induced primarily by the 1300 cm -1 disorder mode seen in bulk graphite rather than by the 1600 cm -1 C-C stretching mode as in SWCNTs. In contrast to SWCNTs, defects exhibit weaker electron-phonon coupling compared to the ideal system. Therefore, defects should present much less of a practical problem in GNRs compared to SWCNTs. The predicted optical line widths can be tested experimentally. The past two decades have seen a revival in scientific research on carbon-based materials. The primary reason for this renaissance is the identification and emergence of several nanostructured allotropes of carbon. Fullerenes appeared in 1985 1 and were shortly followed in 1991 with carbon nanotubes, 2 which have been heavily studied ever since. A new discovery came in 2004 with the isolation of single sheets of graphite, or graphene, by mechanical exfoliation. 3 This carbon nanostructure, once thought unstable, has once again captured the imagination of the scientific community, and interest in this new form of carbon has already received a great amount of attention. Synthesis methodologies have vastly improved in the past few years from isolation with tape to chemical vapor deposition 4 and, recently, solution processing. 5 The enhanced ability to isolate single, quasi- two-dimensional graphene sheets has provoked a detailed undertaking of the study of its intrinsic properties. Perhaps the greatest driving force for the study of graphene is its potential to replace the channel material in field effect transistors (FETs). 6 Single-walled carbon nanotubes (SWCNTs) were once expected to hold this honor; 7 however the mixture of semiconducting and metallic SWCNTs produced during synthesis is a major roadblock. Although large graphene sheets are zero band gap semimetals, it has been shown theoretically 8–10 and experimentally 11–13 that quantum con- finement effects in narrow graphene nanoribbons (GNR) cause an opening of the band gap. This important verification allows for the patterning of GNRs in FETs without the difficulties associated with SWCNTs. Further, it was recently demonstrated that GNR FETs may be etched lithographically to a size of less than 10 nm. 11 This is an important landmark, since current Si-based FETs are built using lithographic techniques. As GNRs are envisioned as an electronic material of the future, a detailed knowledge of their fundamental properties is of the utmost importance. The electron-phonon interaction influences the electron mean-free-path in nanoribbon FETs and, therefore, deter- mines the length of the FET channel as well as the switching speed of the device. 6 The electron-phonon interaction and phonon-induced electronic dephasing can be expected to play important roles in many other potential applications of GNRs. Charge-phonon scattering will be the main source of energy dissipation and loss in future GNRs electronic devices. Phonon-induced electronic dephasing sets coherence limits on spin 14,15 and charge transport. 16 Electron-phonon interac- tions can create distortions in GNR geometric structure and, therefore, affect many of their mechanical and electronic properties. The present Letter reports the first ab initio study of the vibrationally induced pure-dephasing of the lowest energy electronic excitations in narrow GNRs. Using ab initio molecular dynamics (MD), for the first time, we compute pure-dephasing times in a perfect GNR and the same GNR with two typical defects. Surprisingly, we find that compared to SWCNTs of similar size, GNRs show very different * Corresponding author, prezhdo@u.washington.edu. † University of Washington. ‡ Kharkiv National University. NANO LETTERS 2008 Vol. 8, No. 8 2510-2516 10.1021/nl801556n CCC: $40.75  2008 American Chemical Society Published on Web 07/23/2008