Determination of the highest occupied molecular orbital energy of pentacene single crystals by ultraviolet photoelectron and photoelectron yield spectroscopies

Abstract

The electronic structures of pentacene single crystals (SCs) were elucidated by ultraviolet photoelectron spectroscopy (UPS) and photoelectron yield spectroscopy (PYS). An asymmetric HOMO peak profile of the pentacene SCs obtained by UPS exhibits a close similarity to the k-projected density-of-states of the valence band that has been predicted by a theoretical calculation [H. Yoshida and N. Sato, Phys. Rev. B 77, 235205 (2008)]. The ionization energy of the pentacene SCs is successfully determined to be 4.95 (+ 0.03) eV which is evidently greater than that of the bulk films of pentacene [4.90 (+ 0.02) eV].

Electronic Structures of a Well-Defined Organic Hetero-Interface: C₆₀on Pentacene Single Crystal

“…Accordingly, the ionization energies of the Pen-SC and C 60 overlayer are settled to be 4.99 eV and 6.41 eV, respectively. These values agree well with our previous results [6,23].…”

“…2(b). Note that the measurements were conducted in an "angle-integrated" condition and thus momentum-integrated density-of-states are represented in the obtained spectra [11,23]. The highest occupied molecular orbital (HOMO)-derived peak of pentacene appeared in the range of BE = 0.5 − 1.3 eV.…”

“…The highest occupied molecular orbital (HOMO)-derived peak of pentacene appeared in the range of BE = 0.5 − 1.3 eV. This feature can be separated into two spectral components (H1 and H2) of dissimilar height and width which phenomenologically approximate the upper and lower valence bands, respectively, of the Pen-SC [23,27]. Here we only discuss the low BE component (H1) which predominantly play a role of a hole conduction state.…”

“…After production, the crystals were bounded onto gold-coated Si plates with silver paste in air for XPS and UPS experiments, while they were fixed by a tiny piece of conductive carbon tape onto bare Si substrates for PYS. Detailed description on the sample preparation scheme can be found elsewhere [22,23]. It should be noted that the Pen-SC samples were used without any surface treatment and thus the surface may not be the "clean surface" but a few percent of oxidized species existed at the surface [10].…”

“…XPS and UPS measurements were carried out at BL-13A [24] or 13B of Photon Factory (PF), KEK, by means of concentric hemispherical analyzer SES-200 of Gammadata Scienta, while PYS was conducted by a home-built system equipped with an energy-tunable UV light source, hemispherical total electron collector, and precise ammeter (Keithley 6430) to measure the photoelectron emission amount as a function of excitation photon energy [23]. XPS and UPS spectra were obtained at the photon energy (hν) of 820 and 30 eV where the electron pass energy through the analyzer was set at 75 and 10 eV, respectively.…”