Image potential states in monolayer, bilayer, and trilayer epitaxial graphene studied with time- and angle-resolved two-photon photoemission spectroscopy

Image potential states (IPSs) on monolayer, bilayer, and trilayer graphene epitaxially grown on SiC(0001) have been studied by time- and angle-resolved two-photon photoemission (2PPE) spectroscopy. The free-electron-like dispersions of even and odd symmetry IPSs with a quantum number of $n$ = 1${}^{+}$, 1${}^{\ensuremath{-}}$, 2, 3 were observed. All observed IPSs showed the dispersions with effective masses of ${m}^{*}=1.0\ifmmode\pm\else\textpm\fi{}0.1{m}_{e}$. The 2PPE intensity of the lowest IPS ($n$ = 1${}^{+}$) was attenuated with an increasing number of graphene layers. The time-resolved 2PPE measurements revealed that these IPSs have significantly shorter lifetimes, suggesting a coupling of IPSs with electronic states in the buffer layer and the SiC substrate.