Optical observation of different conformational isomers in rubrene ultra-thin molecular films on epitaxial graphene

Udhardt, Christian; Forker, Roman; Gruenewald, Marco; Watanabe, Yu; Yamada, Takashi; Ueba, Takahiro; Munakata, Toshiaki; Fritz, Torsten

doi:10.1016/j.tsf.2015.12.023

Cited by 15 publications

(22 citation statements)

References 45 publications

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“…The energy positions for these three peaks were nearly independent of T except with a noticeable T -induced broadening for each peak. As listed in Table 1, these IOT peak positions obtained by this work matched well with those absorption peaks obtained for rubrene dissolved in various solvents1820272829303132 and also for rubrene thin films1823303233. The peak at 2.366 eV is assigned to the transition between the ground singlet state (S 0 ) and the first excited singlet state (S 1 ) in a twisted rubrene molecule, and the following two peaks of 2.520 and 2.684 eV are assigned to the vibrational progressions with vibrational energies of around 154 ± 15 and 164 ± 17 meV for the S 1 state1820272829303132.…”

Section: Resultssupporting

confidence: 84%

“…Thus, some of the features observed in a molecular solid can be traced back to the features observed in a free molecule25. The energy levels of a free rubrene molecule18192022 and the electronic band structure of the rubrene crystal326 have been theoretically calculated and experimentally confirmed by optical characterization methods for the cases of rubrene molecules dissolved in various solvents1820272829303132, rubrene thin films1823303233, and rubrene single crystals27282930313334. However, relatively little attention has been paid to the discussion on the role of conformational change in the rubrene molecular solid films fabricated from different growth conditions.…”

mentioning

confidence: 82%

“…For a free rubrene molecule, the total energy of a twisted rubrene is smaller than that of a planar rubrene by about 163~210 meV based on the electronic structures calculation81718. Thus, in vapor phase or in thin amorphous films, rubrene molecules possess the low energy state of twisting conformation; however in highly crystalline films or single crystals, rubrene molecules adopt a planar conformation in which the energy required to planarize the backbone is compensated by the lattice energy81317181920212223. The rubrene molecular solids are formed by weak intermolecular interactions, and hence different growth methods can lead to different polymorphs24.…”

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confidence: 99%

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Role of molecular conformations in rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures

Lin

Wang

Chen

et al. 2017

Sci Rep

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We report on the optical and structural characterization of rubrene polycrystalline films fabricated from vacuum deposition with various substrate temperatures (Tsub). Depending on Tsub, the role of twisted and planar rubrene conformational isomers on the properties of rubrene films is focused. The temperature (T)-dependent inverse optical transmission (IOT) and photoluminescence (PL) spectra were performed on these rubrene films. The origins of these IOT and PL peaks are explained in terms of the features from twisted and planar rubrene molecules and of the band characteristics from rubrene molecular solid films. Here, two rarely reported weak-peaks at 2.431 and 2.605 eV were observed from IOT spectra, which are associated with planar rubrene. Besides, the T-dependence of optical bandgap deduced from IOT spectra is discussed with respect to Tsub. Together with IOT and PL spectra, for Tsub > 170 °C, the changes in surface morphology and unit cell volume were observed for the first time, and are attributed to the isomeric transformation from twisted to planar rubrenes during the deposition processes. Furthermore, a unified schematic diagram in terms of Frenkel exciton recombination is suggested to explain the origins of the dominant PL peaks performed on these rubrene films at 15 K.

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Section: Resultssupporting

confidence: 84%

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confidence: 82%

mentioning

confidence: 99%

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Role of molecular conformations in rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures

Lin

Wang

Chen

et al. 2017

Sci Rep

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“…As the HOMO of rubrene is mainly located at the tetracene backbone it was concluded that in the loose-packed ML the molecular long and short axes of the tetracene backbone are almost parallel to the surface 26 . A flat-lying orientation of rubrene in the contact layer with HOPG and graphene was also recently shown by means of scanning tunneling microscopy (STM) 45,47 . For the close-packed ML the PAD is rather broad with no clear maximum.…”

Section: Resultsmentioning

confidence: 63%

“…In a 3 previous study of monolayer rubrene on HOPG we showed that the main vibrational energy (determined in a single mode analysis of the vibrational fine structure of the UPS HOMO-derived peak) of the twisted conformation is slightly larger than that of the planar conformation 26 . The thin film structure and the conformation of rubrene on various substrates depend critically on the film thickness 37,[43][44][45] . Thus, in order to cover a thickness regime which is more relevant for device application, we explore the hole-phonon coupling beyond monolayer coverage in our combined angle-resolved UPS and metastable atom electron spectroscopy (MAES) study of vacuum-sublimed rubrene on HOPG Experimental section Rubrene (two times purified in high-vacuum) thin films were prepared by vacuum sublimation on clean HOPG surfaces using resistively heated quartz crucibles with deposition rates of about 0.25 Å/min as controlled by a quartz crystal microbalance.…”

Section: Introductionmentioning

confidence: 99%

Transient Monolayer Structure of Rubrene on Graphite: Impact on Hole–Phonon Coupling

Bussolotti

Xin

et al. 2016

J. Phys. Chem. C

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Charge transport in molecular thin films is often dominated by incoherent hopping processes and charge-carrier phonon coupling plays a major role in defining mobilities. Our high resolution angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) study reveals the influence of molecular configuration and packing on the hole-phonon coupling in vacuum-sublimed thin films of rubrene on graphite and allows determining charge reorganization energies. In the contact layer to the substrate rubrene is well-ordered with, for low coverages, the tetracene backbone being almost parallel to the graphite surface forming a loose-packed monolayer. Increasing the coverage leads to an orientational transition and a more tilted orientation of rubrene in a close-packed monolayer. This transition results in dramatic changes of spectral features in ARUPS including the photoelectron angular distribution of the highest occupied molecular orbital derived intensity. The charge reorganization energy, however, only changes slightly. The transient monolayer structure of rubrene on graphite allows thus to demonstrate that hole-phonon coupling in organic thin films does not depend very critically on the packing structure. IntroductionLow charge-carrier mobilities are often limiting the performance of organic (opto-)electronic devices as charge transport in organic semiconductor thin films is often dominated by incoherent hopping processes 1-4 . Hopping mobilities depend critically on the charge-carrier phonon coupling 5-8 , which determines the charge reorganization energy λ. In most molecular thin films intermolecular interactions are weak and λ is consequently dominated by intramolecular vibrations and is often taken from gas-phase calculations 5,9-11 or gas-phase ultraviolet photoelectron spectroscopy (UPS) data 9,12-14 . However, the relevant quantities for device application are the solid state values, which depend on the molecular surrounding, i.e. the interaction with neighboring molecules and/or a substrate 7,15 . Due to the similarities of ionization (followed by neutralization) during localized hopping transport and photoionization, UPS is the method of choice to measure charge reorganization energies of organic thin films by an analysis of the vibrational fine structure of the highest occupied molecular orbital (HOMO)-derived peak 7,8,[15][16][17][18] . The relevant vibrational energies (hν i ) of a molecule are often centered closely and in good approximation the analysis can be done by a single mode fit with one main vibrational energy (hν) 19 . Moreover, in most molecular thin films the relaxation energies for ionization and neutralization of a molecule are rather similar and the charge reorganization energy is thus simply given by λ=2Shν with S being the Huang-Rhys factor, which is determined by the relative intensity contribution to the vibrational progressions 7,13 .

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Influence of Dispersion Interactions on the Thermal Desorption of Nonplanar Polycyclic Aromatic Hydrocarbons on HOPG

Weippert

Huber

Schulz

et al. 2019

Physica Rapid Research Ltrs

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A combination of low energy ion beam deposition and mass resolved thermal desorption spectroscopy is applied to analyze the binding behavior of two nonplanar polycyclic aromatic hydrocarbons (PAHs) to highly oriented pyrolytic graphite (HOPG) surfaces—also concerning their lateral dispersion interactions. In particular, the fullerene precursor C60H30 (FPC) and rubrene C42H28 are studied. Due to their smaller contact areas, both molecules exhibit significantly weaker binding energies to the HOPG surface compared to planar PAHs of similar size: C60H30 is bound to the surface by 3.04 eV, which is 0.6 eV lower than for a fully planar homologue. For rubrene, an isolated molecule–substrate binding energy of 1.59 eV is found, which is about 1 eV less than that of the corresponding planar homologue hexabenzocoronene C42H18. In contrast to FPC, rubrene shows a significant (intermolecular) lateral dispersion contribution to the binding energy as the submonolayer coverage increases.

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Optical observation of different conformational isomers in rubrene ultra-thin molecular films on epitaxial graphene

Cited by 15 publications

References 45 publications

Role of molecular conformations in rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures

Role of molecular conformations in rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures

Transient Monolayer Structure of Rubrene on Graphite: Impact on Hole–Phonon Coupling

Influence of Dispersion Interactions on the Thermal Desorption of Nonplanar Polycyclic Aromatic Hydrocarbons on HOPG

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