Two-photon photoemission from image-potential states of epitaxial graphene. (13th October 2015)

• Record Type: Journal Article
• Authors: Gugel, Dieter; Niesner, Daniel; Eickhoff, Christian; Wagner, Stefanie; Weinelt, Martin; Fauster, Thomas
• Is Part Of: 2D materials. Volume 2:Number 4(2015)
• Publication Date: 2015-10-13
• Subjects: graphene -- work function -- image-potential states -- silicon carbide -- two-photon photoemission; 73.22.Pr -- 73.20.-r -- 79.20.Ws; Graphene -- Periodicals; Materials science -- Periodicals; Nanostructured materials -- Periodicals; 620.115
• DOI: 10.1088/2053-1583/2/4/045001
• Languages: English
• ISSNs: 2053-1583

Abstract

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, quasi-freestanding 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.