Temperature dependent optical properties of pentacene films on zinc oxide

Abstract

The optical transitions of pentacene films deposited on ZnO have been studied by absorption spectroscopy as a function of temperature in the range of room temperature down to 10 K. The pentacene films were prepared with thicknesses of 10 nm, 20 nm, and 100 nm on the ZnO-O(000-1) surface by molecular beam deposition. A unique temperature dependence has been observed for the two Davydov components of the excitons for different film thicknesses. At room temperature, the energetic positions of the respective absorption bands are the same for all films, whereas the positions differ more than 20 meV at 10 K caused by the very different expansion coefficients of pentacene and ZnO. Although the pentacene is just bonded via van der Waals interaction to the ZnO substrate, the very first pentacene monolayer (adlayer) is forced to keep the initial position on the ZnO surface and suffering, therefore, a substantial tensile strain. For all the subsequent pentacene monolayers, the strain is reduced step by step resulting electronically in a strong potential gradient at the interface.

Structural and excited-state properties of oligoacene crystals from first principles

“…For the larger acenes, singlet energies change by at most 0.15 eV, while triplet energies change by even less, when varying the volume by ∼3% (similar to thermal expansion at room temperature). This agrees with the experimentally observed absence of significant temperature dependence of low-lying excitation energies [152,153].…”

Optical and structural properties of the pentacene/quartz (0001) interface

“…For the 7-ML-thick film, ε 2 exhibits the same spectral characteristics as for the first monolayer but is redshifted to lower energies. Actually, the ε 2 of the 7-ML-thick film closely matches the spectrum of bulk pentacene condensed in the thin-film phase [22][23][24]. Due to the resonant intermolecular interaction and the existence of two translationally nonequivalent molecules in the unit cell, the lowest electronic transition splits into two bands in the crystal due to the so-called Davydov splitting [10].…”

“…In fact, an increase of the Davydov splitting of pentacene thin films due to thermal contraction at low temperature has been reported by Faltermeier et al [22]. In a recent study, Helzel et al have revealed a change of the Davydov splitting of the interfacial pentacene layer due to the strain induced by the ZnO substrate at low temperature [23]. The distinct optical properties for the first monolayer can thus be attributed to the presence of the interface.…”

“…3 can thus be assigned to the lower and the upper Davydov components of the charge-transfer exciton, respectively [24]. Features c and d at higher energies are attributed to the highest occupied and lowest unoccupied molecular orbital band gap and its first vibronic replica, respectively [22][23][24]. Accompanying the redshift of the lower Davydov component a discussed in Fig.…”

Synthesis and Solid‐State Structures of 6,13‐Bis(trifluoromethyl)‐ and 6,13‐Dialkoxypentacene

“… UV/Vis spectra of pentacene, 9 , and 14 in CH 2 Cl 2 (50 μ M ) and for solid films evaporated onto KCl substrates. For comparison, data are also shown for PEN dissolved in CH 2 Cl 2 18a and for crystalline films grown on ZnO (0001) 18b…”