Benjamin Mahns

Absence of photoemission from the Fermi level in potassium intercalated picene and coronene films: Structure, polaron, or correlation physics?

The electronic structure of potassium intercalated picene and coronene films has been studied using photoemission spectroscopy. Picene has additionally been intercalated using sodium. Upon alkali metal addition core level as well as valence band photoemission data signal a filling of previously unoccupied states of the two molecular materials due to charge transfer from potassium. In contrast to the observation of superconductivity in K(x)picene and K(x)coronene (x ~ 3), none of the films studied shows emission from the Fermi level, i.e., we find no indication for a metallic ground state. Several reasons for this observation are discussed.

Electronic properties of molecular solids: the peculiar case of solid picene

Recently, a new organic superconductor, K-intercalated picene, with high transition temperatures Tc (up to 18K) has been discovered. We have investigated the electronic properties of an undoped relative of this super- conductor, solid picene, using a combination of experimental and theoretical methods. Our results provide deep insights into the occupied and unoccupied electronic states.

Dynamic response and electronic structure of potassium-doped picene investigated by electron energy-loss spectroscopy

We performed electron energy-loss spectroscopy studies in order to get a deeper insight into the electronic properties of potassium intercalated picene, a recently discovered superconductor. A comparison of the loss function of the undoped and doped compound shows the appearance of a new peak in the optical gap, which we attribute to the charge carrier plasmon. We find a dramatic increase for the value of the background dielectric constant

Electronic structure of undoped and potassium-doped coronene investigated by electron energy-loss spectroscopy

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Phthalocyanine dimers in a blend: Spectroscopic and theoretical studies of MnPcδ +/F16CoPcδ −

upon doping. Our core level excitation data clearly signal filling of the conduction bands with electrons upon potassium addition.

Exciton character in picene molecular solids

We performed electron energy-loss spectroscopy studies in transmission in order to obtain insight into the electronic properties of potassium intercalated coronene, a recently discovered superconductor with a rather high transition temperature of about 15\,K. A comparison of the loss function of undoped and potassium intercalated coronene shows the appearance of several new peaks in the optical gap upon potassium addition. Furthermore, our core level excitation data clearly signal filling of the conduction bands with electrons.

Electronic states and the influence of oxygen addition on the optical absorption behaviour of manganese phthalocyanine.

We have prepared mixed phthalocyanine films out of MnPc and F(16)CoPc, which were characterized by means of photoemission spectroscopy and electron energy-loss spectroscopy. Our data reveal the formation of MnPc/F(16)CoPc charge transfer dimers in analogy to the related heterojunction. The electronic excitation spectrum of these blends is characterized by a new low energy excitation at 0.6 eV. Density functional theory calculations show that the new signal is caused by a strong absorption between the states of the interface induced two level system.

Loss spectroscopy of molecular solids: combining experiment and theory

We have studied the low-energy electronic excitations of solid picene at 20 K using momentum dependent electron energy-loss spectroscopy. Our results demonstrate the presence of five excitonic features below the transport energy gap of picene, which all are characterized by a negligible dispersion. One of these excitons has not been observed in the optical absorption spectrum of picene molecules in solution and thus is assigned to a (solid-state induced) charge transfer exciton. This conclusion is supported by the momentum dependent intensity variation of this exciton which clearly signals a significant dipole forbidden contribution, in contrast to the other low energy excitations.

Exciton properties of selected aromatic hydrocarbon systems

The effects of electronic states and air exposure on the spectroscopic properties of manganese phthalocyanine (MnPc) have been examined. The observed features of the Q-band in the absorption spectra can be explained by intrinsic electronic properties of MnPc, i.e., the formation of singly charged molecules by charge transfer excitations. However, the reaction of MnPc with atmospheric molecular oxygen leads to deviations in peak intensities but does not change the fundamental characteristics of the spectra. Nevertheless, the reaction with oxygen changes the spin state from S = 3/2 to S = 1/2. X-ray diffraction measurements also indicate a slow diffusion process of the oxygen into the MnPc crystal. We discuss both influences to explain the behaviour of MnPc in various spectroscopic methods (EELS, ellipsometry, PES). Furthermore, we support the experimental investigations by detailed ab-initio calculations of spectroscopic properties using methods of the density functional theory framework.

Influence of film thickness and air exposure on the transport gap of manganese phthalocyanine

The nature of the lowest-energy electronic excitations in prototyp- ical molecular solids is studied here in detail by combining electron energy loss spectroscopy (EELS) experiments and state-of-the-art many-body calcula- tions based on the Bethe-Salpeter equation. From a detailed comparison of the spectra in picene, coronene and tetracene we generally find a good agreement 8 Authors to whom any correspondence should be addressed. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.