Image-potential states and work function of graphene

Abstract

Image-potential states of graphene on various substrates have been investigated by two-photon photoemission and scanning tunneling spectroscopy. They are used as a probe for the graphene-substrate interaction and resulting changes in the (local) work function. The latter is driven by the work function difference between graphene and the substrate. This results in a charge transfer which also contributes to core-level shifts in x-ray photoemission. In this review article, we give an overview over the theoretical models and the experimental data for image-potential states and work function of graphene on various substrates.

Formation of image-potential states at the graphene/metal interface

“…Although graphene might possess only a weak interaction with SiC, our results generally show that the shape of the image-potential states is very sensitive to the substrate-induced symmetry break even for large separations due to their large spatial extent. In another 2PPE experiment on graphene/SiC(0001), Shearer et al [42] indeed observed a single series of sharp symmetric peaks of image-potential states in agreement with data shown in [13]. Therefore, it seems rather unlikely that the experimentally found splitting of the first image-potential state on graphene/SiC [8,10] can be interpreted as a remnant of the mirror symmetry of freestanding graphene with a small energy separation between these two states.…”

“…It has been argued that these states are remnants of the first even and odd state of freestanding graphene even if its mirror symmetry is in fact broken due to the presence of the substrate. In contrast to these findings, only a single series of image-potential states has been observed on weakly interacting graphene/metal systems [11][12][13][14]. For other metals, like Ni, Pd, Rh, and Ru, the interaction between graphene and the metal can be much stronger.…”

Modification of electronic surface states by graphene islands on Cu(111)

“…Several recent experiments have taken advantage of these exposed surface states of crystalline metal substrates for such studies. [4][5][6][7] On Cu(111), an occupied Shockley surface state forms in the projected band gap leading to a quasi-2D electron gas, 8 which can act as a sensitive probe of changes in the surface electronic potential. Additional unoccupied surface states form due to the potential well created by an electron and its image charge.…”

Mechanical dissipation via image potential states on a topological insulator surface

“…Image potential states (IPS) on metallic surfaces [6][7][8][9][10][11][12] resembling Rydberg series were observed on several topological insulators [13][14][15][16][17], with the energy states lying slightly below the vacuum level. Angle-resolved two-photon photoemission (2PPE) studies of Bi 2 Te 2 Se surfaces reported on the first IPS to be at E = 4.5eV above Fermi level [15].…”

“…IPS are weakly coupled to the bulk in comparison to the other surface states. The image potential states of TIs have relatively long lifetimes in the order of fs, comparable to metallic surfaces [12]. In Scanning Tunneling Spectroscopy (STS), IPS are detected as Gundlach oscillations, which is a phenomenon of field emission resonance through IPS in the tip-sample gap [18].…”