Dual character of excited charge carriers in graphene on Ni(111)

The dynamics of excited charge carriers at the graphene/Ni(111) interface has been investigated by means of time-resolved two-photon photoemission spectroscopy, employing femtosecond-XUV pulses with an energy of 39.2 eV produced by high-order-harmonic generation. Due to the interplay of substrate and adsorbate band structures, the dependence of the lifetimes on the energy $(E-E_F)$ of the excited carriers was found to be similar to that of Ni $3d$ electrons measured for clean Ni in the energy range $(E-E_F)<1$ eV, while it resembled that of graphite from 1 eV above $E_F$ onwards. This result is suggested to be the effect of the peculiar electronic structure of the interface, which still possesses features belonging to the pristine graphene layer, such as a residual saddle point.

Figure 1

Photoemission spectrum of the valence band of Gr/Ni(111), acquired with a high-order-harmonic-generation setup producing XUV femtosecond pulses with an energy of 39.2 eV. The dashed blue line represents the spectrum acquired with the XUV probe beam only. The red solid line represents the spectrum acquired with the IR pump and XUV probe at a delay time of $\Delta t=0\mathrm{fs}$.

Figure 2

(a) Difference of the photoemission signal between the two spectra shown in Fig. 1 in the vicinity of the Ni $3d$ peak. The blue points are the experimental data, and the red solid line is a fitting plot resulting from the sum of the three dashed-line peaks. (b) Scheme of the photoemission mechanisms involved in the process (see text for explanation). The IR pump and the XUV probe pulses are represented with red dotted and blue solid arrows, respectively.

Figure 3

Decay rates $\Gamma$ of photoexcited electrons plotted vs the energy of the intermediate states (on double-log scale). Red dots refer to Gr/Ni(111), yellow squares refer to nickel [17], and blue triangles refer to graphite [18]. The values of $n$ refer to the energy dependence of the decay rate $\Gamma$ in the range $E-E_F<1$ eV and $E-E_F>1.5$ eV. The inset shows the pump-probe scan curve (dots) while monitoring electrons photoemitted from states at about 0.98 eV above $E_F$, fitted with a convolution of an exponential decay with the pump-probe cross correlation.

Figure 4

Projected density of states (PDOS) for C atoms calculated (a) for graphite and (b) for the Gr/Ni(111) interface. In each panel a schematic drawing of the model used for the calculations is shown.