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The aim of this dissertation is supporting theoretical transistor material design using a combination of electronic structure theories, transport simulations and local current density analysis. Our main efforts focus on using several carbon based materials including 1D defective zigzag graphene nanoribbons (ZGNRs), 2D fluorinated graphene and 1D fluorinated ZGNRs to design nanowires-based devices. The electronic properties of these materials are foremost studied within periodic density functional theory (DFT) framework. It is found that in defective ZGNRs the degree of defect dilution strongly influences the electronic structures of the system and a considerable bandgap can be introduced by engineering a regular C6 deficiency into the pristine ZGNRs structure. Combining Non-Equilibrium Green���s Function and the Landauer formalism, the global transport properties such as total current-bias voltage dependence of constructed transport models are evaluated. In order to illuminate the local origin of the variation observed in the conductivity, local cur- rent density maps of the devices are investigated via a procedure originally proposed by Evers and co-workers [Walz et al., Phys. Rev. Lett., 2014, 113, 136602.]. The presence of defects in the ZGNRs leads to a concentrated current flow in the middle region close to the defect edges. The degree of defect dilution as well as the width of the nanojunction have strong impact on the local current densities. Inspired by a recent experiment [Yang et al., Adv. Funct. Mater., 2017, 27, 1604096.] which proposed defective graphene materials as biosensor, we construct a transport model composed with a selected defective ZGNRs and a pyrene molecule serving both as adsorbent on the ZGNRs and as linker to the biomolecule. By modifying the distance between the pyrene molecule and the nanojunction plane, the presence of a heavy biomolecule is simulated. The transport calculations reveal a quantitative change in the total current-bias voltage dependence, which correlates to the experimental measurements. The numerical efficiency of the local current analysis procedure is improved by applying sparse matrix storage and spectral filtering techniques while maintaining the resolution standards. The resulting local current density maps qualitatively demonstrate the local variation of the interference between the linker molecule and the nanojunction plane, which elucidates the small quantitative changes found in the global transport properties. For 2D fluorinated graphene materials, on top of DFT, the G0W0 method as well as the G0W0�� method within many-body Green���s function framework are employed. The results show that the fluorination degree has a determining impact on the bandgap value of the system, whereas the fluorination pattern strongly influences the characteristics of their band structures. Including the description of electron-hole interactions, the optical spectra based on Bethe-Salpeter equation (BSE) of various structures are calculated. Depending on the polarization of the applied electromagnetic field, the optical absorption spectra of the same structure could vary significantly. These interesting findings suggest potential optoelectronic applications of partially fluorinated graphene. The study of partially fluorinated ZGNRs is motivated by the tunability of the electronic properties found in 2D fluorinated graphene materials and an experimental publication [Withers et al., Nano letters, 2011, 11, 3912-3916.], where a controlled synthesis of fluorinated graphene via electron beam is reported. The correlation between the conductivity of resulting materials and the width of fluorinated area is revealed in the experimental measurement. In order to understand the detailed transport mechanism, different widths of edge fluorinated ZGNRs with various degrees of fluorination are considered. The total current-bias voltage dependence and the spin resolved local current density maps of selected systems suggest that specific fluorination pattern and fluorination degrees have determining impact on the conductivity and the electron flux migration pathway within the nanojunction. In addition, the well-known edge effect of the ZGNRs can be observed via spin-resolved local current analysis, which is especially promising for their potential spintronic application. Finally, we apply a similar procedure on a device model composed by the 1D triarylmethyls polymers. Depending on the orientation of the local magnetic moments on the central �� carbons, either spin paired or antiferromagnetic electronic solution can be achieved. Different functional groups on the aryl rings can modify the conductivity of the system drastically. The local current analysis verifies the influence of the electronic solution of the system on its electron flux migration pathway., Das Ziel dieser Dissertation ist die theoretische Unterst��tzung des Designs von Transitor- Materialien durch eine Kombination von Elektronenstrukturmethoden, Transportsimulationen und lokaler Stromdichteanalyse. Wir konzentrieren uns auf die Verwendung verschiedener Kohlenstoff-basierter Materialien, darunter 1D-Zickzack-Graphen- Nanob��nder (ZGNRs) mit Defekten, 2D-fluoriertes Graphen und 1D-fluorierte ZGNRs, um auf Nanodr��hte basierende elektronische Bauelemente zu entwickeln. Die elektronischen Eigenschaften dieser Materialien werden haupts��chlich im Rahmen der periodischen Dichtefunktionaltheorie (DFT) untersucht. Es wurde festgestellt, dass in ZGNRs mit regelm����igen C6- Defekten der Defektdichtegrad die elektronischen Strukturen des Systems stark beeinflusst und eine erhebliche Bandl��cke durch die Defekten in die urspr��ngliche ZGNRs-Struktur eingef��hrt werden kann. Durch die Kombination der Non-Equilibrium Green���s Funktion und des Landauer-Formalismus werden die globalen Transporteigenschaften wie die Gesamtstrom-Spannungsabh��ngigkeit konstruierter Transportmodelle berechnet. Um den lokalen Ursprung der beobachteten Anderungen der Leitf��higkeit zu beleuchten, werden die lokalen Stromdichte-Verteilungen der Bauelemente mit einem Verfahren untersucht, das urspr��nglich von Evers und Mitarbeitern [Walz et al., Phys. Rev. Lett., 2014, 113, 136602.] vorgeschlagen wurde. Das Vorhandensein von Defekten in den ZGNRs f��hrt zu einem konzentrierten Stromfluss im mittleren Bereich nahe den Defektr��ndern. Der Defektdictegrade sowie die Breite des Graphenbandes haben starken Einfluss auf die lokalen Stromdichte. Inspiriert durch ein aktuelles Experiment [Yang et al., Adv. Funct. Mater., 2017, 27, 1604096.], in dem Graphen-Materialen mit Defekten als Biosensoren eingesetzt wurden, konstruieren wir ein Transportmodell, das aus ausgew��hlten Graphenb��nder mit Defekten und Pyren besteht. Das Pyren dient sowohl als Adsorbens an den ZGNRs als auch als Linker-Molek��l zum Biomolek��l. Durch Modifizieren des Abstands zwischen dem Linker-Molek��l (Pyren) und der Graphenband Ebene wird die Anwesenheit eines schweren Biomolek��ls simuliert. Die Transportrechnungen zeigen eine quantitative ��nderung der Gesamtstrom-Spannungs-abh��ngigkeit, die mit den experimentellen Messungen korreliert. Die numerische Effizienz des Verfahrens der lokalen Stromanalyse wird durch Anwendung von Sparse Matrix Speicher- und Spektral Filter Techniken verbessert, w��hrend die Aufl��sungs- standards beibehalten werden. Die resultierenden lokalen Stromdichte Verteilungen zeigen qualitativ die lokale Variation der Interferenz zwischen dem Linker-Molek��l und dem Graphenband, was die kleinen quantitativen ��nderungen der globalen Transporteigenschaften verdeutlicht. F��r 2D-fluorierte Graphen-Materialien werden zus��tzlich zur DFT die G0W0-Methode sowie die G0W0��-Methode innerhalb des Vielteilchen-Green-Funktion-Methode verwendet. Die Ergebnisse zeigen, dass der Fluorierungsgrad einen bestimmten Einfluss auf den Bandl��cke-Wert des Systems hat, w��hrend das Fluorierungsmuster die Lage der B��nder zueinander stark beeinflusst. Die optischen Spektren werden mit Ber��cksichtigung der Elektron-Loch-Wechselwirkungen mit der Bethe-Salpeter-Gleichung (BSE) f��r verschiedenen Strukturen berechnet. Abh��ngig von der Polarisation des angelegten elektromagnetischen Feldes k��nnen die optischen Absorptionsspektren derselben Struktur erheblich variieren. Diese interessanten Erkenntnisse legen die potentiellen optoelektronischen Anwendungen von teilfluoriertem Graphen nahe. Motiviert durch die Ergebnisse zur elektronischen Strukturen von 2D-fluorierten Graphen-Materialien und einer experimentellen Publikation [Withers et al., Nano letters, 2011, 11, 3912-3916.], die von einer r��umlich kontrollierten Synthese berichtet, untersuchen wir fluorierte ZGNRs. In Messungen wurde eine Korrelation zwischen Leitf��higkeit und der Breite der fluorierten Bereiche gefunden. Um den detaillierten Transportmechanismus zu verstehen, werden verschiedene Breiten von Fluorkanten-ges��ttigten ZGNRs mit unterschiedlichen Fluorierungsgraden betrachtet. Die Gesamtstrom- Spannungsabh��ngigkeit und die spinaufgel��sten lokalen Stromdichte Verteilungen ausgew��hlter Systeme legen nahe, dass spezifische Fluorierungsmuster und Fluorierungsgrade einen Einfluss auf die Leitf��higkeit und den Elektronenfluss-Pfad innerhalb des Graphenbandes haben. Dar��ber hinaus kann der bekannte Kanteneffekt der ZGNRs ��ber die lokale Stromanalyse beobachtet werden, sodass diese Materialien vielversprechende Kandidaten f��r die Anwendung im Bereich Spintronic sind. Schlie��lich wenden wir ein��hnliches Verfahren auf ein molekulares Bauelement an, das aus den 1D-Triarylmethylpolymeren besteht. Abh��ngig von der Orientierungen der lokalen magnetischen Momente an den zentralen ��-Kohlenstoffen kann entweder Spin-gepaarte oder eine antiferromagentische L��sung erhalten werden. Verschiedene funktionelle Gruppen der Aryl-Ringe k��nnen die Leitf��higkeit des Systems drastisch ver��ndern. Die lokale Stromdichte-Analyse verifiziert den Einfluss der elektronischen L��sung des Systems auf ihre Elektromigration.

The chiral induced spin selectivity (CISS) effect in which selective transport of electron spins through helical chiral molecules occurs, has attracted a lot of attention in recent years. This effect was used to magnetize ferromagnetic (FM) samples by utilizing adsorbed chiral molecules. The electron transfer through the molecules was generated optically or electrically. In the optical configuration, circularly polarized light induced efficient magnetization by spin torque transfer (STT), using a hybrid of quantum dots (QDs) and chiral molecule self-assembled monolayer (SAM). Here, we use X-ray magnetic chiral dichroism (XMCD) spectroscopy in order to probe the optically induced magnetization on thin FM films. The results show differences in the FM magnetization depending on the optical circular polarization, matching previous non-local Hall probe measurements.

Molecules that contain one or more unpaired electrons delocalized within a ?-conjugated backbone are promising candidates for applications in spin electronics or simply 'spintronics'. Our group develops functional organic materials based on ?-conjugated hydrocarbon molecules, where the electrons are unpaired either in the ground state or in the excited state that is low in energy and can be populated thermally. We aim to learn how to introduce and control a multitude of properties, namely, optical, chiroptical, magnetic, and conductive, in a bulk material made of these molecules, by manipulating spin interactions between the unpaired electrons. The first model system that was developed in our group is a hydrocarbon named cethrene, which has a diradicaloid singlet ground state and a low-lying triplet excited state. In this article, the structural parameters and their impact on the properties and reactivity of cethrene are discussed within the realm of the three C's that symbolize cethrene's C-shape, chirality, and chameleonic reactivity.

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Nanoscale inhomogeneities and impurity clustering are often found to drastically affect the magnetic and transport properties in disordered/diluted systems, giving rise to rich and complex phenomena. However, the physics of these systems still remains to be explored in more details as can be seen from the scarce literature available. We present a detailed theoretical analysis of the effects of nanoscale inhomogeneities on the spin excitation spectrum in diluted magnetic systems. The calculations are performed on relatively large systems (up to $N$=$66^3$). It is found that even low concentrations of inhomogeneities have drastic effects on both the magnon density of states and magnon excitations. These effects become even more pronounced in the case of short ranged magnetic interactions between the impurities. In contrast to the increase of critical temperatures $T_C$, reported in previous studies, the spin-stiffness $D$ is systematically suppressed in the presence of nanoscale inhomogeneities. Moreover $D$ is found to strongly depend on the inhomogeneities' concentration, the cluster size, as well as the range of the magnetic interactions. The findings are discussed in the prospect of potential spintronics applications. We believe that this detailed numerical work could initiate future experimental studies to probe this rich physics with the most appropriate tool, Inelastic Neutron Scattering (INS)., One new figure added, text revised and some figures modified to match the published version

Electrical injection and detection of spin-polarized electrons is demonstrated for the first time in a single wafer, all-semiconductor, GaAs-based lateral spintronic device, employing p+-(Ga,Mn)As/n+-GaAs ferromagnetic Esaki diodes as spin aligning contacts. The conversion of spin-polarized holes into spin-polarized electrons via Esaki tunnelling leaves its mark in a bias dependence of the spin-injection efficiency, which at maximum reaches the relatively high value of 50%., 11 pages, 3 figures, sent to PRL

In this work, magnetic semiconductors were investigated. The work on magnetic semiconductors is motivated by the great interest in the field of spinelectronics in recent years. Of particular interest are magnetic semiconductors which are ferromagnetic with Curie-temperatures well above room temperature. As ferromagnetic compound semiconductor, EuS was investigated as thin film system. Particular attention was paid to the influence of the substrate temperature during growth on the sample properties. Changes in the growth temperature lead to a change in stoichiometry of the films with Eu rich compositions for low groth temperatures. Hence, the carrier concentrations increase for decreasing growth temperatures. Therefore, the Curie-temperatures decrease and the resistances increase with increasing growth temperatures. The increase in the Curie-temperature as well as the characteristic transport properties are caused by the strong f-d interaction between conduction band and localized magnetic moments. The samples grown and investigated here show an anomalous Hall effect. This has been observed for the first time in EuS, bulk samples do not show this effect. As diluted magnetic semiconductors (DMS), GaN films magnetically doped with Mn or Gd were investigated. In both material systems, ferromagnetism far above room temperature was demonstrated.While GaGdN shows a homogeneous magnetism, all ferromagnetic GaMnN samples show small clusters (phase separations). In addition, measurements of the optical absorption and the magnetic circular dichroism (MCD) were performed on the GaMnN samples. In the optical measurements, the Mn was identified as deep acceptor. At low temperatures, both the magneto-absorption and the MCD show a Zeeman-shift of an absorption band for all doping levels. The exchange energies between valence band and localized magnetic moments can be extracted from fits of the Brillouin function to the Zeeman shifts. This yields values between 1.4eV and 1.7eV for the lowest doping levels and between 1.7eV and 2.1eV for the highest doped sample. This fits into the general trend observed in other DMS, where the exchange constants show a pronounced dependence on the lattice constant. At low temperatures, no ferromagnetic behavior was found in the magneto-optical experiments. The signal is dominated by the localized Mn spin system. At room temperature, the MCD shows a ferromagnetic signature. At elevated temperatures, the signal from the Mn spin system is small enough that the influence of the clusters on the band structure of the host lattice becomes visible. Contrary to the GaMnN samples, the GaGdN films show a homogeneous magnetism. Because of the strong localization of the magnetic 4f states of the Gd and the high resistance of the films, direct-, double- and superexchange as well as carrier mediated ferromagnetism can be ruled out as exchange mechanisms. A possible exchange path is an intra-atomic f-d exchange between the localized magnetic 4f and the band states analog to the Euchalcogenides. This is consistent with the increase of the Curie-temperature upon increase of free carriers by Si co-doping. The magnetically doped GaN thin film systems investigated here exhibit different behavior. The exchange mechanism proposed for GaGdN leads to a homogeneous ferromagnetism and is similar to the magnetic exchange in the Eu chalcogenides. The interplay between this intra-atomic f-d exchange and the magnetic end electric properties was demonstrated for the EuS samples. The behavior of the GaMnN samples is contrary to the aforementioned samples. In the ferromagnetic samples, the extrinsic and intrinsic magnetic properties were separated and the magnitude of the exchange between localized magnetic states and the valence band was determined. The samples investigated here do not show the homogeneous carrier mediated ferromagnetism theoretically predicted.