Two-photon photoemission from image-potential states of epitaxial graphene

Abstract

Using angle-and time-resolved two-photon photoelectron spectroscopy we observe a single series of image-potential states of graphene on monolayer (MLG) and bilayer graphene (BLG) on SiC(0001). The first image-potential state on MLG (BLG) has a binding energy of 0.93 eV (0.84 eV). Lifetimes of the first three image-potential states of MLG are 9, 44 and 110 fs. On hydrogen-intercalated, quasifreestanding graphene no unoccupied states are observed. We attribute this to the absence of occupied initial states for direct transitions into image-potential states at photon energies below the work function used in two-photon photoemission. The work function varies between 4.14 and 4.79 eV, but the vacuum level stays ∼4.5 eV above the Dirac point for all surfaces studied. This finding suggests that direct excitation of image-potential states cannot be achieved by doping and the electron dynamics for free-standing graphene is not accessible by two-photon photoemission using photon energies below the work function.

Sub‐Monolayer Growth of Titanium, Cobalt, and Palladium on Epitaxial Graphene

“…Hence, the measured height must be influenced by the electronic properties of the system in a way that the measurement grossly overestimates the true island height. Recently, the work function of MLG on SiC has been determined experimentally to be in the range of 4.05 − 4.28 eV, which is considerably lower than the 5.55 eV quoted for Co(0001) . Due to the larger work function of Co one would expect the STM to measure smaller island heights in contrast to the findings in our experiments.…”

Image-potential states and work function of graphene

“…On the other hand, parabolic bands at the center of the Brillouin zone form the band structure in the vicinity of the vacuum level. These image-potential states (IPS) were observed experimentally using scanning tunneling spectroscopy (STS) [18][19][20] and two-photon photoelectron spectroscopy (2PPE) [21][22][23][24][25] for graphene supported by various substrates. Theoretical models find a distinct, mirror-symmetry induced 1 Present address: Department of Chemistry, Columbia University, New York, NY 10027, USA.…”

Ultrafast Multiphoton Thermionic Photoemission from Graphite

“…Another focus has been the image potential states (IPSs) of graphite, which are the only band-structure features at the Γ point to appear in 2PP spectra [ Fig. 1(a)], but the mechanism for their excitation remains controversial because apposite initial states for vertical excitation do not exist [40,42,[49][50][51]. Also at the Γ point, there is a σ-symmetry interlayer band (ILB) with uniquely 3D free-electron character and band minimum at about 4.0 eV above the Fermi level (E F ; all electron energy levels are reported with respect to E F unless otherwise stated).…”