Organic molecules are attracting much interest to use for a variety of electronic applications. Pentacene (Pn) is one of such molecules. The charge transport in Pn crystal has been reported to have a band-like nature at low temperature [1,2], and the band-like charge transport is expected to play a major part at room temperature as well though charge carriers are scattered by lattice vibration [3]. One of the origins of the band-like transport is the adequate overlap of the pi-orbitals of adjacent molecules, and the overlap of orbitals produces orbital-derived electronic bands whose dispersion behaviors correlate closely with the charge transport mechanism. This means that a proper understanding on the electronic band structures is an essential input to fully comprehend the charge transport mechanism of a Pn crystal. In this paper, we present the dispersions of the highest occupied molecular orbital (HOMO)-derived bands of a single crystal Pn monolayer film grown on a Bi(001) surface [4,5] along the three symmetrical directions of the ab plane. Two HOMO-derived bands, which could not be separated in the former studies, were clearly observed in the ARPES spectra. Of these two bands, the one with higher binding energy shows dispersion in all the three directions, while the one with lower binding energy hardly disperses. The dispersion widths of the higher binding energy band were 330+-40 meV, 210+-40 meV and 220+-40 meV in the three directions. Our present result indicates that the overlap of the pi-orbitals of adjacent Pn molecules is larger than what was expected from theoretical calculations, and the observed dispersions suggest that the higher binding energy HOMO-derived band mainly contributes to the band-like charge transport mechanism of a Pn crystal. We will also provide evidences of the weak interaction between a Pn molecule and the Bi surface that was expected in Refs. [4,5], and discuss the hole mobility of the Pn monolayer film.
[1] O. D. Jurchescu, J. Baas, and T. T. M. Palstra, Appl. Phys. Lett. 84, 3061 (2004).
[2] O. Ostroverkhova et al., Phys. Rev. B 71, 035204 (2005).
[3] N. Koch et al., Phys. Rev. Lett. 96, 156803 (2006).
[4] J. T. Sadowski et al., Appl. Phys. Lett. 86, 073109 (2005)
[5] G. E. Thayer et al., Phys. Rev. Lett. 95, 256106 (2005). |