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- ItemAnalysis of electronic properties frommagnetotransport measurements on Ba(Fe1-xNix)2As2 thin films(Basel : MDPI AG, 2020) Shipulin, I.; Richter, S.; Thomas, A.A.; Nielsch, K.; Hühne, R.; Martovitsky, V.We performed a detailed structural, magnetotransport, and superconducting analysis of thin epitaxial Ba(Fe1-xNix)2As2 films with Ni doping of x = 0.05 and 0.08, as prepared by pulsed laser deposition. X-ray diffraction studies demonstrate the high crystalline perfection of the films, which have a similar quality to single crystals. Furthermore, magnetotransport measurements of the films were performed in magnetic fields up to 9 T. The results we used to estimate the density of electronic states at the Fermi level, the coefficient of electronic heat capacity, and other electronic parameters for this compound, in their dependence on the dopant concentration within the framework of the Ginzburg-Landau-Abrikosov-Gorkov theory. The comparison of the determined parameters with measurement data on comparable Ba(Fe1-xNix)2As2 single crystals shows good agreement, which confirms the high quality of the obtained films.
- ItemEffects of PNDIT2 end groups on aggregation, thin film structure, alignment and electron transport in field-effect transistors(London [u.a.] : RSC, 2016) Matsidik, Rukiya; Luzio, Alessandro; Hameury, Sophie; Komber, Hartmut; McNeill, Christopher R.; Caironi, Mario; Sommer, MichaelTo develop greener protocols toward the sustainable production of conjugated polymers, we combine the advantages of atom-economic direct arylation polycondensation (DAP) with those of the green solvent 2-methyltetrahydrofuran (MeTHF). The n-type copolymer PNDIT2 is synthesized from unsubstituted bithiophene (T2) and 2,6-dibromonapthalene diimide (NDIBr2) under simple DAP conditions in MeTHF. Extensive optimization is required to suppress nucleophilic substitution of NDIBr end groups, which severely limits molar mass. Different carboxylic acids, bases, palladium precursors and ligands are successfully screened to enable quantitative yield and satisfyingly high molar masses up to Mn,SEC ∼ 20 kDa. In contrast to PNDIT2 made via DAP in toluene with tolyl-chain termini, nucleophilic substitution of NDIBr chain ends in MeTHF finally leads to NDI-OH termination. The influence of different chain termini on the optical, thermal, structural and electronic properties of PNDIT2 is investigated. For samples with identical molecular weight, OH-termination leads to slightly reduced aggregation in solution and bulk crystallinity, a decreased degree of alignment in directionally deposited films, and a consequently reduced, but not compromised, electron mobility with promising values still close to 0.9 cm2 V−1 s−1.
- ItemInfluence of chemical interactions on the electronic properties of BiOI/organic semiconductor heterojunctions for application in solution-processed electronics(London [u.a.] : RSC, 2023) Lapalikar, Vaidehi; Dacha, Preetam; Hambsch, Mike; Hofstetter, Yvonne J.; Vaynzof, Yana; Mannsfeld, Stefan C. B.; Ruck, MichaelBismuth oxide iodide (BiOI) has been viewed as a suitable environmentally-friendly alternative to lead-halide perovskites for low-cost (opto-)electronic applications such as photodetectors, phototransistors and sensors. To enable its incorporation in these devices in a convenient, scalable, and economical way, BiOI thin films were investigated as part of heterojunctions with various p-type organic semiconductors (OSCs) and tested in a field-effect transistor (FET) configuration. The hybrid heterojunctions, which combine the respective functionalities of BiOI and the OSCs were processed from solution under ambient atmosphere. The characteristics of each of these hybrid systems were correlated with the physical and chemical properties of the respective materials using a concept based on heteropolar chemical interactions at the interface. Systems suitable for application in lateral transport devices were identified and it was demonstrated how materials in the hybrids interact to provide improved and synergistic properties. These indentified heterojunction FETs are a first instance of successful incorporation of solution-processed BiOI thin films in a three-terminal device. They show a significant threshold voltage shift and retained carrier mobility compared to pristine OSC devices and open up possibilities for future optoelectronic applications.
- ItemRhodamine 6G and 800 intermolecular heteroaggregates embedded in PMMA for near-infrared wavelength shifting(London [u.a.] : RSC, 2022) Castillo-Seoane, Javier; Gonzalez-Garcia, Lola; Obrero-Perez, José M.; Aparicio, Francisco J.; Borrás, Ana; González-Elipe, Agustín R.; Barranco, Ángel; Sanchez-Valencia, Juan R.The opto-electronic properties of small-molecules and functional dyes usually differ when incorporated into solid matrices with respect to their isolated form due to an aggregation phenomenon that alters their optical and fluorescent properties. These spectroscopic modifications are studied in the framework of the exciton theory of aggregates, which has been extensively applied in the literature for the study of molecular aggregates of the same type of molecules (homoaggregation). Despite the demonstrated potential of the control of the heteroaggregation process (aggregation of different types of molecules), most of the reported works are devoted to intramolecular aggregates, complex molecules formed by several chromophores attached by organic linkers. The intramolecular aggregates are specifically designed to hold a certain molecular structure that, on the basis of the exciton theory, modifies their optical and fluorescent properties with respect to the isolated chromophores that form the molecule. The present article describes in detail the incorporation of Rhodamine 6G (Rh6G) and 800 (Rh800) into polymeric matrices of poly-(methyl methacrylate), PMMA. The simultaneous incorporation of both dyes results in an enhanced fluorescent emission in the near-infrared (NIR), originating from the formation of ground-state Rh6G–Rh800 intermolecular heteroaggregates. The systematic control of the concentration of both rhodamines provides a model system for the elucidation of the heteroaggregate formation. The efficient energy transfer between Rh6G and Rh800 molecules can be used as wavelength shifters to convert effectively the light from visible to NIR, a very convenient wavelength range for many practical applications which make use of inexpensive commercial detectors and systems.
- ItemStep-flow growth in homoepitaxy of β-Ga2O3 (100)—The influence of the miscut direction and faceting(Melville, NY : AIP Publ., 2019) Schewski, R.; Lion, K.; Fiedler, A.; Wouters, C.; Popp, K.; Levchenko, S.V.; Schulz, T.; Schmidbauer, M.; Bin Anooz, S.; Grüneberg, R.; Galazka, Z.; Wagner, G.; Irmscher, K.; Scheffler, M.; Draxl, C.; Albrecht, M.We present a systematic study on the influence of the miscut orientation on structural and electronic properties in the homoepitaxial growth on off-oriented β-Ga2O3 (100) substrates by metalorganic chemical vapour phase epitaxy. Layers grown on (100) substrates with 6° miscut toward the [001⎯⎯] direction show high electron mobilities of about 90 cm2 V−1 s−1 at electron concentrations in the range of 1–2 × 1018 cm−3, while layers grown under identical conditions but with 6° miscut toward the [001] direction exhibit low electron mobilities of around 10 cm2 V−1 s−1. By using high-resolution scanning transmission electron microscopy and atomic force microscopy, we find significant differences in the surface morphologies of the substrates after annealing and of the layers in dependence on their miscut direction. While substrates with miscuts toward [001⎯⎯] exhibit monolayer steps terminated by (2⎯⎯01) facets, mainly bilayer steps are found for miscuts toward [001]. Epitaxial growth on both substrates occurs in step-flow mode. However, while layers on substrates with a miscut toward [001⎯⎯] are free of structural defects, those on substrates with a miscut toward [001] are completely twinned with respect to the substrate and show stacking mismatch boundaries. This twinning is promoted at step edges by transformation of the (001)-B facets into (2⎯⎯01) facets. Density functional theory calculations of stoichiometric low index surfaces show that the (2⎯⎯01) facet has the lowest surface energy following the (100) surface. We conclude that facet transformation at the step edges is driven by surface energy minimization for the two kinds of crystallographically inequivalent miscut orientations in the monoclinic lattice of β-Ga2O3.
- ItemCharge transfer from and to manganese phthalocyanine: bulk materials and interfaces(Frankfurt, M. : Beilstein-Institut zur Förderung der Chemischen Wissenschaften, 2017-8-4) Rückerl, Florian; Waas, Daniel; Büchner, Bernd; Knupfer, Martin; Zahn, Dietrich R. T.; Haidu, Francisc; Hahn, Torsten; Kortus, JensManganese phthalocyanine (MnPc) is a member of the family of transition-metal phthalocyanines, which combines interesting electronic behavior in the fields of organic and molecular electronics with local magnetic moments. MnPc is characterized by hybrid states between the Mn 3d orbitals and the π orbitals of the ligand very close to the Fermi level. This causes particular physical properties, different from those of the other phthalocyanines, such as a rather small ionization potential, a small band gap and a large electron affinity. These can be exploited to prepare particular compounds and interfaces with appropriate partners, which are characterized by a charge transfer from or to MnPc. We summarize recent spectroscopic and theoretical results that have been achieved in this regard.