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Evidence for Phase Formation in Potassium Intercalated 1, 2; 8, 9-Dibenzopentacene

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Evidence for Phase Formation in Potassium Intercalated 1, 2; 8, 9-Dibenzopentacene Evidence for phase formation in potassium intercalated 1,2;8,9-dibenzopentacene Friedrich Roth,1 Andreas K¨onig,1 Benjamin Mahns,1 Bernd B¨uchner,1 and Martin Knupfer1 1IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany (Dated: June 30, 2021) We have prepared potassium intercalated 1,2;8,9-dibenzopentacene films under vacuum condi- tions. The evolution of the electronic excitation spectra upon potassium addition as measured using electron energy-loss spectroscopy clearly indicate the formation of particular doped phases with compositions Kxdibenzopentacene (x = 1,2,3). Moreover, the stability of these phases as a function of temperature has been explored. Finally, the electronic excitation spectra also give insight into the electronic ground state of the potassium doped 1,2;8,9-dibenzopentacene films. I. INTRODUCTION Apart from the introduction of charge carriers, the ad- dition of potassium to dibenzopentacene can also lead to Molecular crystals|built from π conjugated mole- stable phases with particular stoichiometries. A very im- cules|are in the focus of research for a number of rea- portant prerequisite for detailed studies as well as the un- sons. Within this class of materials, almost every ground derstanding of physical properties is the knowledge about state can be realized at will, spanning from insulators such phases and their existence and stability regions. For to semiconductors, metals, superconductors or magnets. instance, the physical properties and the conclusive anal- Due to their relatively open crystal structure their elec- ysis of experimental data of alkali metal fullerenes have been demonstrated to be strongly dependent on the ex- tronic properties can be easily tuned by the addition 4,6,7,17{25 of electron acceptors and donors. In some cases, this isting phases and their characterization . resulted in intriguing and unexpected physical proper- In this contribution we report on an investigation of ties. For instance, carbon based superconductors have a the structural and electronic properties of potassium long history dating back to 1965, when superconductivity doped dibenzopentacene using electron energy-loss spec- was found in alkali-metal doped graphite, with transition troscopy (EELS) in transmission. EELS studies of other 1 undoped and doped molecular materials in the past temperatures of Tc < 1 . Recently, the Tc was increased up to 11.5 K for calcium intercalated graphite2. have provided useful insight into their electronic proper- ties26{28. We discuss the changes that are induced in the However, more than 25 years later the discovery electronic excitation spectrum as a function of doping, of a superconducting phase in the alkali metal doped and we provide clear evidence for the formation of three fullerides3,4 represented a breakthrough in the field of doped phases with K C H ,K C H and K C H superconductivity and attracted a lot of attention, also 1 30 18 2 30 18 3 30 18 composition. Moreover, temperature dependent investi- because of rather high transition temperatures up to gations also allowed first insight into the stability regions 40 K5{8. In this context, further interesting phenomena of those phases, and the electronic excitation spectra sug- were observed in alkali metal doped molecular materials gest insulating as well as metallic ground states. such as the observation of an insulator-metal-insulator transition in alkali doped phthalocyanines9, a transition from a Luttinger to a Fermi liquid in potassium doped carbon nanotubes10, or the formation of a Mott state in II. EXPERIMENTAL potassium intercalated pentacene11. In the case of organic superconductors, transition tem- 1,2;8,9-dibenzopentacene (C30H18) is a molecule peratures similar to those of the fullerides could not be formed by seven benzene rings as depicted in Fig. 1. It observed in other molecular crystals until 2010, when looks like a pentacene molecule with one snapped off ben- superconductivity has been reported for another alkali zene ring on both ends. Up to now, no details of the metal doped molecular solid, K-picene, with a Tc up crystal structure are published. to 18 K12. Furthermore, after this discovery supercon- Thin films of dibenzopentacene (BGB Analytik AG, 4461 arXiv:1206.0526v1 [cond-mat.supr-con] 4 Jun 2012 ductivity was also reported in other alkali metal inter- Boeckten, Switzerland) were prepared by thermal evap- calated polycyclic aromatic hydrocarbons, such as K- oration under high vacuum conditions (base pressure 13,14 15 −8 phenanthrene (Tc = 5 K) , K-coronene (Tc = 15 K) lower than 10 mbar) onto single crystalline KBr sub- 16 and K-(1,2;8,9-dibenzopentacene) (Tc = 33 K) . Espe- strates kept at room temperature with a deposition rate cially in the latter case, the Tc is higher than in any other of 0.3 nm/min and an evaporation temperature of about organic superconductor besides the alkali-metal doped 530 K. The film thickness was about 100 nm. These di- fullerides. Now, a thorough investigation of the physical benzopentacene films were floated off in destilled water, properties of 1,2;8,9-dibenzopentacene in the undoped as mounted onto standard electron microscopy grids and well as in the doped state is required in order to de- transferred into the spectrometer. Prior to the EELS velop an understanding of the superconducting and nor- measurements the films were characterized in situ using mal state properties. electron diffraction. The diffraction spectra show no sig- 2 III. RESULTS AND DISCUSSION KxC30H18 x = 0 x = 1 x = 2 x = 3 Intensity (arb. units) FIG. 1. Schematic representation of the molecular structure of 1,2;8,9-dibenzopentacene (C30H18). 275 280 285 290 295 300 305 nificant pronounced texture which leads to the conclusion Energy (eV) that our films are essentially polycrystalline. All measurements were carried out using the 172 keV FIG. 2. C 1s and K 2p core level excitations of Kx(1,2;8,9- spectrometer described in detail elsewhere29. We note dibenzopentacene) for x = 0,1,2,3. that at this high primary beam energy only singlet exci- tations are possible. The energy and momentum resolu- The amount of potassium in our doped dibenzopenta- tion were chosen to be 85 meV and 0.03 A˚−1, respectively. cene films was determined using core level excitation We have measured the loss function Im[-1/(q;!)] for a spectra. In Fig. 2 we show C 1s and K 2p core level exci- momentum transfer q parallel to the film surface, which tations of undoped and potassium doped dibenzopenta- probes the electronic excitations of the films [(q;!) is cene. These data can be used to analyze the doping in- the dielectric function]. In addition, the C 1s and K 2p duced changes of the potassium doped films. Moreover, core level excitations were measured with an energy res- the C 1s excitations represent transitions into empty olution of about 200 meV and a momentum resolution C 2p-derived levels, and thus allow to probe the projected of 0.03 A.˚ In order to obtain a direction independent unoccupied electronic density of states of carbon-based core level excitation information, we have determined the materials31{34. All spectra were normalized at the step- core level data for three different momentum directions like structure in the region between 291 eV and 293 eV, such that the sum of these spectra represent an aver- i. e. to the σ∗ derived intensity, which is proportional to aged polycrystalline sample30. The core excitation spec- the number of carbon atoms. For the undoped case (red tra have been corrected for a linear background, which line), we can clearly identify a sharp and strong feature has been determined by a linear fit of the data 10 eV in the range between 283 - 286 eV, which can be assigned below the excitation threshold. Since molecular crys- to transitions into π∗ states representing the unoccupied tals often are damaged by fast electrons, we repeatedly electronic states. The step-like structure above 291 eV checked our samples for any sign of degradation. In par- corresponds to the onset of transitions into σ∗-derived ticular, degradation was followed by watching an increas- unoccupied levels. ing amorphous-like background in the electron diffrac- By doping the sample with potassium the spectrum is tion spectra and a broadening of the spectral features still dominated by a sharp excitation feature right after in the loss function. It turned out that under our mea- the excitation onset at 283 eV and, in addition, by K 2p surement conditions the spectra remained unchanged for core excitations, which can be observed at 297.2 eV and about 14 h at 20 K and 8 h at room temperature. Samples 299.8 eV, and which can be seen as a first evidence of the that showed any signature of degradation were not con- successful doping of the sample. Importantly, a reduction sidered further but replaced by newly prepared thin films. of the spectral weight of the first C 1s excitation feature The results from the different films have been shown to is observed in Fig. 2 upon doping, which can be seen as a be reproducible. further signal of succesfully doping because it represents Potassium was added in several steps by evapora- the filling of the conduction band. tion from a commercial SAES (SAES GETTERS S.p.A., The stoichiometry analysis can be substantiated by Italy) getter source under ultra-high vacuum conditions comparing the K 2p and C 1s core excitation intensities (base pressure lower than 10−10 mbar). In each doping in comparison to other doped molecular films with well 35 step, the sample was exposed to potassium for several known stoichiometry, such as K6C60 . Details of this minutes, the current through the SAES getter source was procedure can be found in previous publications31,36.
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