Magnetic Properties of Molecular and Nanoscale Magnets

The idea of miniaturizing devices down to the nanoscale where quantum ffeffects become relevant demands a detailed understanding of the interplay between classical and quantum properties. Therefore, characterization of newly produced nanoscale materials is a very important part of the research in this fifield. Studying structural and magnetic properties of nano- and molecular magnets and the interplay between these properties reveals new interesting effects and suggests ways to control and optimize the respective material. The main task of this thesis is investigating the magnetic properties of molecular magnetic clusters and magnetic nanoparticles recently synthesized by several collaborating groups. This thesis contains two main parts focusing on each of these two topics. In the first part the fundamental studies on novel metal-organic molecular complexes is presented. Several newly synthesized magnetic complexes were investigated by means of different experimental techniques, in particular, by electron spin resonance spectroscopy. Chapter 1 in this part provides the theoretical background which is necessary for the interpretation of the effects observed in single molecular magnetic clusters. Chapter 2 introduces the experimental techniques applied in the studies. Chapter 3 contains the experimental results and their discussion. Firstly, the magnetic properties of two Ni-based complexes are presented. The complexes possess different ligand structures and arrangements of the Ni-ions in the metal cores. This difffference dramatically affffects the magnetic properties of the molecules such as the ground state and the magnetic anisotropy. Secondly, a detailed study of the Mn2Ni3 single molecular magnet is described. The complex has a bistable magnetic ground state with a high spin value of S = 7 and shows slow relaxation and quantum tunnelling of the magnetization. The third section concentrates on a Mn(III)-based single chain magnet showing ferromagnetic ordering of the Mn-spins and a strong magnetic anisotropy which leads to a hysteretic behavior of the magnetization. The last section describes a detailed study of the static and dynamic magnetic properties of three Mn-dimer molecular complexes by means of static magnetization, continuous wave and pulse electron spin resonance measurements. The results indicate a systematic dependence of the magnetic properties on the nearest ligands surrounding of the Mn ions. The second part of the thesis addresses magnetic properties of nano-scaled magnets such as carbon nanotubes fifilled with magnetic materials and carbon-coated magnetic nanoparticles. These studies are eventually aiming at the possible application of these particles as agents for magnetic hyperthermia. In this respect, their behavior in static and alternating magnetic fifields is investigated and discussed. Moreover, two possible hyperthermia applications of the studied magnetic nanoparticles are presented, which are the combination of a hyperthermia agents with an anticancer drug and the possibility to spatially localize the hyperthermia effffect by applying specially designed static magnetic fifields

Exciton dispersion in para-quaterphenyl: Significant molecular interactions beyond Coulomb coupling

We have experimentally determined the momentum dependence of the electronic excitation spectra of para-quaterphenyl single crystals. The parallel arrangement of para-quaterphenyl molecules results in a strong Coulomb coupling of the molecular excitons. Such crystals have been considered to be a very good realization of the Frenkel exciton model, including the formation of H-type aggregates. Our data reveal an unexpected exciton dispersion of the upper Davydov component, which cannot be rationalized in terms of inter-molecular Coulomb coupling of the excitons. A significant reduction of the nearest neighbor coupling due to additional charge-transfer processes is able to provide an explanation of the data. Furthermore, the spectral onset of the excitation spectrum, which represents a heavy exciton resulting from exciton-phonon coupling, also shows a clear dispersion, which had been unknown so far. Finally, an optically forbidden excitation about 1 eV above the excitation onset is observed. © 2021 Author(s)

Tuning the charge transfer in Fx-TCNQ/rubrene single-crystal interfaces

Interfaces formed by two different organic semiconductors often exhibit a large conductivity, originating from transfer of charge between the constituent materials. The precise mechanisms driving charge transfer and determining its magnitude remain vastly unexplored, and are not understood microscopically. To start addressing this issue, we have performed a systematic study of highly reproducible single-crystal interfaces based on rubrene and Fx-TCNQ, a family of molecules whose electron affinity can be tuned by increasing the fluorine content. The combined analysis of transport and scanning Kelvin probe measurements reveals that the interfacial charge carrier density, resistivity, and activation energy correlate with the electron affinity of Fx-TCNQ crystals, with a higher affinity resulting in larger charge transfer. Although the transport properties can be described consistently and quantitatively using a mobility-edge model, we find that a quantitative analysis of charge transfer in terms of single-particle band diagrams reveals a discrepancy ~ 100 meV in the interfacial energy level alignment. We attribute the discrepancy to phenomena known to affect the energetics of organic semiconductors, which are neglected by a single-particle description, such as molecular relaxation and band-gap renormalization due to screening. The systematic behavior of the Fx-TCNQ/rubrene interfaces opens the possibility to investigate these phenomena experimentally, under controlled conditions

Band-like Electron Transport with Record-High Mobility in the TCNQ family

In highest quality organic single-crystal field-effect transistors, electron transport occurs in the band-like regime, with the carrier mobility increasing upon lowering temperature. Neither the microscopic nature of this regime, nor why it occurs only in a small number of materials is currently understood. Here, comparative studies of closely related materials, exhibiting high-quality reproducible transport properties are needed. We performed a study of electron transport in single-crystals of different TCNQ (tetracyanoquinodimethane) molecules, combined with band structure calculations. We show that F2-TCNQ devices exhibit very high electron mobility and an unprecedented increase in mobility upon cooling, whereas in TCNQ and F4-TCNQ the mobility is substantially lower and decreases upon cooling. We analyze the crystal and electronic structures of these materials and find that F2-TCNQ crystals are indeed ideal to achieve outstanding transport properties. Our analysis also shows that to understand the difference between the three materials, studying their band structure is not sufficient, and that the electron-phonon coupling needs to be investigated as well. Besides the outstanding transport properties of F2-TCNQ, a key result of our work is the identification of the Fx-TCNQ family as a paradigm to investigate the most fundamental aspects of electronic transport in organic crystals

Directed exciton transport highways in organic semiconductors

Exciton bandwidths and exciton transport are difficult to control by material design. We showcase the intriguing excitonic properties in an organic semiconductor material with specifically tailored functional groups, in which extremely broad exciton bands in the near-infrared-visible part of the electromagnetic spectrum are observed by electron energy loss spectroscopy and theoretically explained by a close contact between tightly packing molecules and by their strong interactions. This is induced by the donor–acceptor type molecular structure and its resulting crystal packing, which induces a remarkable anisotropy that should lead to a strongly directed transport of excitons. The observations and detailed understanding of the results yield blueprints for the design of molecular structures in which similar molecular features might be used to further explore the tunability of excitonic bands and pave a way for organic materials with strongly enhanced transport and built-in control of the propagation direction

Charge Carrier Mobility Improvement in Diketopyrrolopyrrole Block-Copolymers by Shear Coating

Shear coating is a promising deposition method for upscaling device fabrication and enabling high throughput, and is furthermore suitable for translating to roll-to-roll processing. Although common polymer semiconductors (PSCs) are solution processible, they are still prone to mechanical failure upon stretching, limiting applications in e.g., electronic skin and health monitoring. Progress made towards mechanically compliant PSCs, e.g., the incorporation of soft segments into the polymer backbone, could not only allow such applications, but also benefit advanced fabrication methods, like roll-to-roll printing on flexible substrates, to produce the targeted devices. Tri-block copolymers (TBCs), consisting of an inner rigid semiconducting poly-diketo-pyrrolopyrrole-thienothiophene (PDPP-TT) block flanked by two soft elastomeric poly(dimethylsiloxane) (PDMS) chains, maintain good charge transport properties, while being mechanically soft and flexible. Potentially aiming at the fabrication of TBC-based wearable electronics by means of cost-efficient and scalable deposition methods (e.g., blade-coating), a tolerance of the electrical performance of the TBCs to the shear speed was investigated. Herein, we demonstrate that such TBCs can be deposited at high shear speeds (film formation up to a speed of 10 mm s−1). While such high speeds result in increased film thickness, no degradation of the electrical performance was observed, as was frequently reported for polymer−based OFETs. Instead, high shear speeds even led to a small improvement in the electrical performance: mobility increased from 0.06 cm2 V−1 s−1 at 0.5 mm s−1 to 0.16 cm2 V−1 s−1 at 7 mm s−1 for the TBC with 24 wt% PDMS, and for the TBC containing 37 wt% PDMS from 0.05 cm2 V−1 s−1 at 0.5 mm s−1 to 0.13 cm2 V−1 s−1 at 7 mm s−1. Interestingly, the improvement of mobility is not accompanied by any significant changes in morphology

Optical Anisotropy and Momentum-Dependent Excitons in Dibenzopentacene Single Crystals

High-quality single crystals of the organic semiconductor (1,2;8,9)-dibenzopentacene were grown via physical vapor transport. The crystal structure─unknown before─was determined by single-crystal X-ray diffraction; polarization-dependent optical absorption measurements display a large anisotropy in the ac plane of the crystals. The overall Davydov splitting is ∼110 meV, which is slightly lower than that in the close relative pentacene (120 meV). Momentum-dependent electron energy-loss spectroscopy measurements show a clear exciton dispersion of the Davydov components. An analysis of the dispersion using a simple 1D model indicates smaller electron- and hole-transfer integrals in dibenzopentacene as compared to pentacene. The spectral weight distribution of the excitation spectra is strongly momentum-dependent and demonstrates a strong momentum-dependent admixture of Frenkel excitons, charge-transfer excitons, and vibrational modes