Trapping surface electrons on graphene layers and islands

We report the use of time- and angle-resolved two-photon photoemission to map the bound, unoccupied electronic structure of the weakly coupled graphene/Ir(111) system. The energy, dispersion, and lifetime of the lowest three image-potential states are measured. In addition, the weak interaction between Ir and graphene permits observation of resonant transitions from an unquenched Shockley-type surface state of the Ir substrate to graphene/Ir image-potential states. The image-potential-state lifetimes are comparable to those of midgap clean metal surfaces. Evidence of localization of the excited electrons on single-atom-layer graphene islands is provided by coverage-dependent measurements.

Figure 1

Arrows indicate 2PPE transitions between surface and image-potential states. The experimental results (dots) are compared to calculations (lines). The projected bulk-band structure of Ir(111) along the $\Gamma$$K$ direction is shaded according to the total and $sp$ density of states (DOS) at the right and left, respectively.

Figure 2

(a) Intensity map of the 2PPE signal recorded with photon energy $h\nu =1.59$ eV for 1-ML graphene on Ir(111). Points represent the intensity of the lowest $n=1$ band. (b) ARPES map showing initial states for $h\nu =55$ eV.

Figure 3

(a) Time-resolved measurements of the image-potential-state series at $k_{\parallel }=0$. (b) Lifetimes as a function of binding energy compared to various power-law dependencies (see text).

Figure 4

Sample work function (open symbols) and image-potential states $n=1$, 2, and 3 binding energies (solid symbols) as a function of graphene coverage. The dashed line represents a linear fit for the work-function change.