Evidence for metallic states in potassium-intercalated picene film on graphite

We report the results of photoemission spectroscopy on the potassium concentration ($x$) dependence of K${}_x$ picene films on graphite. The results show a systematic shift of a whole valence band and the appearance of a clear Fermi edge for a certain value of $x$, with additional structures at higher binding energies. Observed spectral evolutions are reminiscent to those of doped C${}_{60}$, implying that the K${}_x$ picene film has multiple phases depending on the K concentration. Appearance of a clear Fermi edge, in contrast to multilayer K${}_x$ picene films on Au, SiO${}_2$, and Ag(111) substrates, where no clear Fermi edges are observed, suggests that the emergence of the metallic states for K${}_x$ picene films depends on the specific molecular orientations in the films on different substrates.

Figure 1

(a) Evolution of the valence band photoemission spectra of a picene film on a HOPG substrate upon K doping measured with He$$i. (b) The magnified spectra around $E_{\mathrm{F}}$ in (a).

Figure 2

Evolution of the valence band spectra of a picene film on a HOPG substrate upon K doping measured with Xe$$i$\alpha$.

Figure 3

(a) Comparisons of the spectra with the same K-deposition times (20, 40, 60, and 80 s) in Figs. 1 and 2. The gray bars around $E_{\mathrm{F}}$ are the assumed base lines for the data measured with He$$i for the intensity estimation at $E_{\mathrm{F}}$ in (b). The bar for a K-deposition time of 80 s is slightly inclined because of the overlap of the K 3$p$ signals excited by He$$ii$\alpha$. (b) Intensity variations at $E_{\mathrm{F}}$ in (a).