Electronic Structures of the Highest Occupied Molecular Orbital Bands of a Pentacene Ultrathin Film

We report the energy dispersions of the highest occupied molecular orbitals (HOMO)-derived bands of a pentacene (Pn) thin film, whose in-plane structure resembles closely that of the $ab$ plane of a low-density bulk Pn phase. Our present photoemission result indicates that the overlap of the $\pi$-orbitals of adjacent Pn molecules is larger than what was expected from theoretical calculations. Further, of the two HOMO-derived bands, the large dispersion width of the band with higher binding energy suggests that this one mainly contributes to the bandlike charge transport in a Pn crystal.

Figure 1

The $ab$ plane and $bc$ plane of a Pn crystal. ${\theta }_{\mathrm{Pn}}$ is the angle between the molecular planes of the two inequivalent molecules in the unit cell (the herringbone angle).

Figure 2

(a) Pn deposition time-dependent secondary electron cutoff measured with the sample biased at $-15.0\mathrm{V}$. (b) RHEED pattern of the 1 ML Pn film obtained with a 15 keV electron beam along the $[11\overline{2}]$ direction. (a) and (b) are obtained at room temperature. The adsorption sites of Pn on Bi(001) obtained by the RHEED pattern are displayed in (c). (d) Surface Brillouin zones (SBZs) of the Bi(001) surface and the Pn surface parallel to the $ab$ plane. ${\overline{K}}_{\mathrm{Bi}}$ and ${\overline{M}}_{\mathrm{Bi}}$ are the symmetry point of the Bi(001) SBZ, and $\overline{X}$, $\overline{Y}$, and $\overline{M}$ are those of the Pn SBZ.

Figure 3

(a) ARPES spectra of the Pn 1 ML on Bi(001) measured at 140 K along the $\overline{\Gamma }\mathrm{\text{−}}\overline{Y}$ direction. The uppermost spectrum shows the decomposition of the Pn induced structure at ${\theta }_e=13°$. (b) Band dispersions of the two HOMO-derived bands of the 1 ML Pn film adsorbed on Bi(001). The theoretical dispersions of the HOMO-derived bands reported in Ref. 19 are indicated by dashed lines. Solid curves are the dispersion obtained by analyzing the higher $E_B$ HOMO-derived band using a simple tight-binding model.