Trapping surface electrons on graphene layers and islands

(Received 9 September 2011; revised manuscript received 26 January 2012; published 13 February 2012)We report the use of time- and angle-resolved two-photon photoemission to map the bound, unoccupiedelectronic structure of the weakly coupled graphene/Ir(111) system. The energy, dispersion, and lifetime of thelowest three image-potential states are measured. In addition, the weak interaction between Ir and graphenepermits observation of resonant transitions from an unquenched Shockley-type surface state of the Ir substrateto graphene/Ir image-potential states. The image-potential-state lifetimes are comparable to those of midgapclean metal surfaces. Evidence of localization of the excited electrons on single-atom-layer graphene islands isprovided by coverage-dependent measurements.DOI: 10.1103/PhysRevB.85.081402 PACS number(s): 73