The major goals of the thesis are design and characterization of functional self-assembled monolayers (SAMs) in context of electrostatic interfacial engineering and molecular electronics as well as a study of their thermal stability. The issue of electrostatic engineering can be addressed using custom-designed SAMs with either terminal dipolar groups or dipolar groups embedded into the molecular backbone. As for the first task, the novel concept of embedded dipole was successfully applied to the oxide substrates, which are highly important for photovoltaic applications. A variation of the work function of indium tin oxide (ITO) by 0.5 eV as compared to the reference non-polar functionalization was achieved at the invariable character of the SAM-ambient interface, allowing, thus, to decouple electrostatic engineering from the interface chemistry. The extremely low work function value for one of the tested monolayers expands a rather limited selection of SAMs capable of significantly lowering the work function of ITO. As a further task, electrostatic effects in charge transport across monomolecular films were studied, which is currently one of the most intensely discussed topics in molecular electronics. The tuning of the electrostatic properties was achieved by the fabrication of binary SAMs of biphenylthiolates (BPT) on Au(111), namely by mixing of BPT with fluorine-substituted-BPT (F-BPT) and 4-methyl-4′-BPT (CH3-BPT) with 4-trifluoromethyl-4′-BPT (CF3-BPT). The charge tunneling rate across the binary SAMs was found to vary progressively with their composition between the values for the single-component monolayers, and could, consequently, be fine-tuned and correlated with the work function. The observed behavior was tentatively explained by the appearance of an internal electrostatic field in the SAMs, leading to a change of the energy-level alignments within the junction upon contact of the SAMs to the top eutectic GaIn electrode. The height of the respective injection barrier is, however, unaffected by such a field, corresponding to the values of the transition voltage, which do not change notably with the SAM composition. Analysis of the presented and literature data suggests that the position of a dipolar group in SAM-forming molecules has significant impact on the charge transport behavior of the respective SAMs in the context of molecular electronics. As the next sub-project in the latter context, custom-designed SAMs of ferrocene/ruthenocene-substituted biphenylthiolates and fluorenethiolates on Au(111) were studied. The novel element of these SAMs was the fully conjugated molecular backbone, in contrast to the previous studies utilizing alkyl linkers as elements of molecular diodes. The designed SAMs exhibited a highly exceptional charge transport behavior showing conductance switching triggered by the applied bias. The extent of this switching, described by a maximum rectification ratio (RR) higher than 1000, was comparable to the best performing molecular diodes but in contrast to these “devices” was maintained at very low bias, close to zero volts. The observed behavior could be tentatively explained by a non-reversible redox process affecting the electronic structure of the molecules and their coupling to the top electrode. The above results are particularly promising to create novel molecular devices for potential applications in electronic circuits, molecular memory, or as an electrochemical sensor. Finally, the issue of thermal stability of functional SAMs on coinage metal and oxide substrates was addressed. This issue is of a crucial importance for applications, defining the temperature range of SAM-based devices and framing the preparation routes involving high temperature steps. Several representative SAMs with thiol anchoring group on Au(111) substrates and phosphonic acid (PA) anchoring group on Al2O3 substrates were studied by high resolution X-ray photoelectron spectroscopy chosen as the most suitable experimental tool. The range of the thermal stability and the degradation pathways were found to depend on the chemical composition of the SAM-forming molecules and the character of the substrates, with such crucial parameters as the strength of substrate-anchoring group bond and the presence of a backbone-specific “weak links”. In general, PA monolayers on oxide substrates were found to have higher robustness and better thermal stability compared to thiolates SAMs on coinage metal substrates. My results show, however, that is always advisable to test thermal stability of a specifically designed functional SAM in context of possible “weak links” as far as this stability is important for a particular application.
Single crystals of organic semiconductors are chemically pristine and exhibit nearly perfect long-range structural order. As such, they provide an ideal platform to investigate intrinsic properties. Vibrational spectroscopy techniques, such as Raman and Fourier-transform infrared spectroscopy (FT-IR), are widely employed techniques for the characterization of organic materials. They are versatile tools that can be used to study molecular packing and polymorphism in crystalline organic semiconductors, albeit with poor spatial resolution. Two fundamentally different scanning probe techniques with infrared spectroscopy and imaging capabilities offer a spatial resolution below 100 nm - atomic force microscopy-infrared spectroscopy (AFM-IR) and scattering-type infrared scanning near-field optical microscopy (IR-SNOM). This thesis compares the AFM-IR and the IR-SNOM with each other and to the conventional FT-IR spectroscopy with regard to their applicability to small-molecule organic semiconductors. To this end, single crystals of TIPS-pentacene, TIPS-tetraazapentacene, rubrene and per uorobutyldicyanoperylene carboxydiimide (PDIF-CN2) are used as the testbed. Significant differences are observed in the spectra of the crystals depending on the technique and polarization of incident light that are associated with the intrinsic molecular structure and packing as well as the different working principles of the applied methods. Furthermore, the imaging mode of the AFM-IR and the IR-SNOM is tested on solution-deposited microcrystals of PDIF-CN2. Micro- and nanostructures of layered organic materials can also be created by liquid-phase exfoliation (LPE), a popular technique used to produce two-dimensional nanosheets from layered inorganic crystals. The orthorhombic and the triclinic polymorphs of rubrene are dispersed in aqueous surfactant solution by ultrasonication. Distinct nanostructures of rubrene, referred to as nanorods and nanobelts, are formed that are isolated via liquid cascade centrifugation. Their crystalline nature is confirmed through electron diffraction measurements and Raman spectroscopy. Absorbance and photoluminescence (PL) of the dispersions are found to be similar to rubrene solutions due to random orientations of the nanostructures, however, their PL lifetimes are comparable to the macroscopic crystals. The likely arrangement of rubrene molecules within the nanorods and the nanobelts is deduced from AFM images, electron diffraction patterns, and IR-SNOM spectra.
Most multi-cellular organisms depend on adhesion mechanisms to provide stability and a pathway for the transduction of information between cells and the extra cellular matrix. Tissue cells can sense and react to the mechanical properties of their local environment which can steer cellular proliferation, migration, as well as differentiation in stem cells. Focal Adhesions are multi-protein assemblies that constitute the mechanosensitive link between ECM and the actin cytoskeleton. Two major FA proteins, talin and vinculin, exhibit an auto-inhibited conformation in the cytoplasm, yet the molecular process that regulates their activation and allows them to unfold their full signalling potential remains widely unknown.
Using extensive molecular dynamics (MD) simulations we reveal a mechanism by which a flexible loop on the talin FERM domain serves as a first contact point with PIP2 lipids in the cellular membrane and subsequently promotes membrane interactions that can compete with the autoinhibitory link to the talin rod domain. We demonstrate that a variety of vinculin binding sites in the talin rod bind to vinculin in a highly force-regulated manner. We describe on an atomistic level a mechanism in which VBS-binding competes with the autoinhibitory link to the vinculin tail, which was corroborated by a collaborator in magnetic tweezers experiments. Lastly, force-probe MD simulations helped to identify the residues involved in the VBS-induced weakening of the vinculin head-tail interface, facilitating protein activation. With this, we propose two novel vinculin mutants that mimic the effect of talin association and show significantly increased interactions with actin -- comparable to VBS-activated wild-type vinculin -- in experiments carried out by collaboration partners.
Summarized, our findings provide novel insights into the molecular underlying of talin and vinculin activation which help to improve our understanding of hierarchical FA maturation and mechanotransduction.
Nanomaterials play an important role in the flourishing field of nanoscience. Size reduction of materials results in a broad range of outstanding physical and chemical properties as well as a wealth of potential applications. A particularly interesting class of low-dimensional nanostructures are two-dimensional (2D) materials, i.e. individual layers of so-called van der Waals crystals. The research was triggered in 2014 by Geim and Novoselov through the isolation and characterization of graphene, a single layer of two-dimensionally arranged sp2 hybridised carbon atoms. 2D nanomaterials can be obtained by various methods including bottom-up approaches such as chemical vapour deposition and top-down approaches such as liquid phase exfoliation (LPE) and mechanical exfoliation. In recent years, LPE has gained increasing attention due to the high production rates and broad applicability to a range of structures beyond graphene including transition metal dichalcogenides (TMDs), hexagonal boron nitride, metal phosphorus trisulfides and many more. In LPE, high energy and shear forces (e.g. through sonication) are applied to reduce the dimensions of the crystal and the resulting nanosheets are stabilized in the liquid medium through appropriate solvents and surfactant systems. The resultant nanosheets are extremely polydisperse in lateral size and thickness so that LPE is typically coupled with size selection, for example through centrifugation. Due to this additional processing step, it is difficult to assess the impact of the stabilizer on for example the optical properties of the nanosheets which will be a function of both size and stabilizer. In addition, the number of pure organic solvents suitable to prevent reaggregation is very limited which is a bottleneck for further processing and deposition. The goal of the work conducted within the scope of this thesis is to establish protocols to make high quality 2D nanosheets from LPE accessible in a range of liquid media and to achieve a deeper understanding of the impact of the stabilizer on the optical properties of the nanomaterial. To this end, tungsten disulphide (WS2), a semiconducting transition metal dichalcogenide was chosen as model substance due to unique optical fingerprints of the monolayers (e.g. narrow linewidth photoluminescence from exciton only in WS2 monolayers). Throughout this thesis, monolayer-rich dispersions of WS2 nanosheets were prepared by sonication-assisted LPE in a common detergent solution in combination with liquid cascade centrifugation for size selection. In the first part, a protocol was developed to transfer these nanosheets into a range of additive/solvent systems. The advantage over a direct exfoliation in this systems is that dispersions containing nanosheets of the same size/thickness can be compared. This allowed to assess the impact of various chemical environments on the optical properties and to study effects associated with the dielectric screening of excitons (e.g. changes in exciton energy and width). With this foundation established, the nanosheets were transferred into a range of common pure organic solvents using a modified protocol. This is more challenging due to aggregation taking place. Nonetheless, this broad screening made it possible to relate the changes in exciton response to physical parameters such as refractive index and dielectric constant. Importantly, it was confirmed that monolayers can be stable in solvents that are not suitable for the exfoliation itself greatly expanding the choice of solvent for further processing. The third part focuses on precise deposition of the nanosheets on substrates using spin coating. Experimental difficulties such as aggregation and restacking of nanosheets in solvents are addressed in detail together with solutions to improve the colloidal stability of the nanomaterials. In the optimized samples, monolayer properties, such as exciton photoluminescence, are retained after deposition. At last, a new route for transferring nanosheets from water-based WS2 dispersions into different media is introduced which greatly facilitates deposition. In this approach, water-insoluble polymers are added to the aqueous surfactant solution prior to sonication. Through hydrophpobic interaction, the polymer is driven to the interface between the hydrophobic part of the detergent and the nanomaterial. This polymer coating on the nanomaterial reduces aggregation after transfer to hydrophobic organic solvents, suitable for thin-film processing. Such techniques for nanomaterial processing are highly demanded for the integration of these materials into functional devices.
Semiconducting, single-walled carbon nanotubes (SWCNTs) have mechanical and electronic properties that render them a promising material for solution-processable, stretchable and flexible electronics. However, their strong tendency to form aggregates in dispersion constitutes a large obstacle to realize the film uniformity necessary for the transition of devices from laboratory to commercial scale. The resulting inhomogeneities in film morphology lead to an undesired spread in device performance. Based on the tailored formulation of colloidal inks via suitable solvents and additives the first part of this thesis presents a simple yet effective method to slow down aggregation of polymer-wrapped SWCNTs in organic solvents. This effect on aggregation by 1,10- phenanthroline as a stabilizing additive can be monitored with time-dependent absorption spectroscopy. The improved homogeneity of the SWCNT networks deposited from stabilized dispersions after several days of ink storage lead to higher charge carrier mobilities with strongly reduced device-to-device variations compared to inks without additive. The intrinsic ambipolarity of SWCNTs is a great disadvantage for their use in electronic circuits as it leads to large power dissipation. While pure hole conduction can be achieved relatively easily by doping with, for example, ambient oxygen, facilitating exclusive electron conduction represents a large challenge. A solution-processable n-dopant from the family of guanidino-functionalized aromatics (GFAs) is introduced to overcome this limitation. The resulting SWCNT network field-effect transistors (FETs) exhibit pure electron transport with high mobility while hole transport is fully suppressed, excellent switching behavior and good operational stability. Their application potential (combined with a doped p-type FET) is highlighted by complementary inverters with very low power dissipation. This modification of the charge transport behavior is applied to another promising solution-processable semiconductor, i.e., donor-acceptor-polymers. Doping of these polymers with two GFA compounds under various processing conditions improves electron injection and transport while hole transport is suppressed. Again, these transistors display good environmental stability under operating conditions. The extended applicability of the newly introduced GFA dopants to different semiconductors emphasizes their potential for transistors based on solution-processable semiconductor
The development of innovative technologies often requires fundamental new materials properties of individual components. In the present work, metastable metal fibers with unique mechanical and thermodynamic properties as well as crystallographic characteristics were linked to form mechanically stable, elastic 3-dimensional networks without changing the geometry, physical and thermodynamic properties of the fibers. The metal fiber networks have been used in lithium-ion battery technology as 3-dimensional current collectors, fundamentally improving battery technology. Worldwide there is an urgent need for improved battery technologies which are more ecological and economical. The metal fibers produced by a Melt spinning process are based on a copper silicon alloy. The basic physical and thermodynamic properties as well as the crystallographic state of the metal fibers were first quantified. The metal fibers had a length of several centimeters, a width of 10-100 micrometers, and a thickness of 2-10 micrometers. The mechanical properties were quantified as a function of metal fiber dimension, crystallographic and thermodynamic state. In particular, the thermodynamic metastable phase was shown to be instrumental for linking the metal fibers. During the metal fiber fabrication process, the molten metal is cooled and solidified within a few milliseconds. As a result, the crystalline structure at room temperature is not in a thermodynamic equilibrium, the stored energy of which was determined. A permanent linkage of the metal fibers was successfully implemented by cold and warm sintering of the fibers without affecting the basic geometry and partly the physical as well as thermodynamic properties of the fibers. Low activation of atomic diffusion by means of mechanically built-up pressure or temperature resulted in a linkage preferentially at the contact points between fibers. The reason for this is the crystallographic energy stored in the fibers during quenching of the molten metal. The 3-dimensional metal fiber networks were investigated in terms of their suitability as electrical current conductors in lithium-ion batteries. In particular, this work focused on improving the mechanical and electrical properties of lithium-ion batteries, which are the basis for a variety of processes in batteries. In principle, the 3-dimensional metal fiber networks enabled the fabrication of functional super-thick battery electrodes, which have a significantly increased surface capacitance of greater than 8 mAh cm-1.
Quantum chemical simulations of molecular properties are crucial to obtain in-depth insight into a multitude of chemical and biological phenomena. In particular for investigating light-driven systems, modeling of electronic excitations by computational means is indispensable for supporting, complementing, and extending experimental findings. The complexity in terms of electronic structure, intermolecular interactions, and dynamics of the involved molecular systems, however, pushes the limits of computational feasibility. Hybrid quantum-classical environment schemes tackle this complexity by splitting the system into a quantum region and its environment. Thus, they retain the high-level quantum chemical description for the part of interest without neglecting the pivotal effects of the environment.
In this thesis, I develop methods for modeling molecular properties in complex environments. The first half of the thesis is dedicated to new combined approaches of the polarizable embedding (PE) model and the algebraic-diagrammatic construction (ADC) scheme for the polarization propagator for computational spectroscopy simulations. I derive and implement two PE-ADC coupling schemes: The first scheme – pt-PE-ADC – uses a self-consistent PE reference state with a canonical ADC procedure and is suited for computation of electronic excitation energies including a posteriori perturbative corrections. The second scheme – LR-PE-ADC – includes direct coupling to the polarizable environment in an iterative manner, making it suitable for excited electronic states and higher-order response properties. Furthermore, I derive working equations to evaluate analytic nuclear gradients using PE-ADC. To advance the availability of the PE model in general, I implement a standalone, open-source, and hybrid Python/C++ library, called CPPE, and interface it to several freely available quantum chemical host programs. The PE-ADC schemes are implemented with adcc, a toolkit for development of ADC-based methods and combinable with several Python-driven host programs. The simple and clean design of both libraries allows for extension of existing workflows and rapid prototyping. Moreover, I implement response properties using ADC and the intermediate state representation (ISR) in a new Python library, called respondo. The synergy of all three libraries enables the user to implement new features in a straightforward manner, while maintaining usability and efficiency for practical calculations. I test the individual approaches in several benchmark calculations and case studies. For example, I find that excitation energy errors using pt-PE-ADC for microsolvated p-nitroaniline are much smaller than the intrinsic error of ADC itself. Furthermore, I investigate the charge transfer (CT) state involved in the photoprotection mechanism of the flavoprotein dodecin. In addition, I conduct the first computations of higher-order response properties with ADC and a polarizable model. In these studies, I observe that LR-PE-ADC greatly improves the accuracy of the property compared to simpler coupling schemes. I further show that corrections for electron spill-out artifacts and the physically sound evaluation of PE-ADC intensities are decisive when benchmarking against supersystem calculations. With my theoretical derivations and open-source implementations, I provide, to the best of my knowledge, the most complete and unique feature set of polarizable models combined with ADC to date.
The second half of the thesis first contains a general performance improvement of PE models. I implement a PE scheme where the classical electric fields in the environment are evaluated using the fast multipole method (FMM) instead of direct summation. Consequently, the electric field evaluations as rate-limiting step of the classical part exhibit an asymptotic linear scaling in the PE-FMM scheme, making it suitable for efficient simulations of environments with over a million polarizable sites. Next, I show algorithm details for numerically stable solution of response equations in the ADC/ISR framework, and I analyze convergence behavior of different solver algorithms. These algorithms are beneficial for efficient evaluation of PE-ADC response properties, too. I present derivations and numerical case studies of complex excited state polarizabilities which extend the ADC/ISR framework beyond ground state response properties. Then, I investigate the distortion of molecules under external forces. I develop a new electronic structure method to apply hydrostatic pressure in standard quantum chemical simulations via Gaussian potentials, called GOSTSHYP. This implicit embedding scheme directly exerts pressure on a molecule via compression of the electron density, such that it becomes possible to treat atoms and molecules and to run geometry optimizations and dynamics simulations at a pre-defined pressure. This feature set is not found in any other comparable method. I use steered molecular dynamics (SMD) simulations with quantum chemical strain analysis tools to elucidate the rupture process of rubredoxin. I prove that the extremely low rupture force does not result from hydrogen bond networks to the protein as assumed so far in the literature, but that its origin is likely more intricate. Finally, I present the design of novel photocages based on fluorene derivatives. Using an efficient computational screening protocol, I propose cyclopenta-dithiophene as scaffold, leading to the next generation of fluorene-based photocages with desirable absorption and uncaging properties.
Although cells represent the smallest building blocks of life, they are already exhibiting a high level of complexity. It is therefore not surprising that researchers tend to utilize simplified systems for investigating, but also reconstituting, cellular processes in artificially constructed cells. The conception of those artificial cells can be done using bottom-up biological approaches. Currently, the reconstitution of cellular process is achieved using natural or nature-derived components like the actomyosin-complex for the reconstitution of cellular motility. Although a reduction of complexity can be accomplished in this way, natural systems still require specialized conditions and sophisticated buffers. Ultimately, the reduction of complexity can only be realized by the reconstitution of cellular processes using completely synthetic materials. In this thesis, a completely artificial cytoskeleton based on thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) to trigger motility of synthetic cells was established. To this end, a PNIPAM-based composite material containing gold nanorods was generated and its physiochemical behavior was characterized. The composite material was introduced and assembled inside water-in-oil emulsion droplets that have been stabilized by either a custom-made PNIPAM-containing surfactant (PNS) or a commercially available fluorosurfactant (CS). Besides the reversibility of the PNIPAM phase transition in bulk and droplet-based synthetic cells, the on-demand inducible deformation of droplets due to this phase transition was shown. Additionally, droplet migration was triggered using asymmetric PNIPAM-containing droplets in combination with fluorophilic-coated surfaces. The production of those cell polarization-mimicking droplets was achieved via targeted fusion of stable and unstable droplets during polymerization. Following the assessment of PNIPAM-mediated droplet motility, for the first time, the artificial cytoskeleton material was combined with the natural cytoskeleton protein actin by a sequential pico-injection approach thus elucidating the biocompatibility of the presented system. Herein, a versatile PNIPAM-based artificial cytoskeleton for synthetic cell applications was designed and implemented. The control over the PNIPAM volume transition processes allowed to achieve the motility of the droplet-based synthetic cells by their dynamic deformation. The biocompatibility of the system by combining natural and synthetic cytoskeleton components could be shown. The tolerance of the PNIPAM system also towards specialized buffer systems, might expand the bandwidth of the bottom-up synthetic biology tool kit, enhancing the assembly of hybrid cellular modules. Therefore, the developed approach represents an important milestone in the development of bottom-up synthetic biology applications.
Diese Arbeit beschäftigt sich mit der Metall-Ligand-Wechselwirkung in Ln(III), An(III) und An(IV)-Komplexverbindungen mit O-, N,O- und N-Donorliganden. Hauptaugenmerk liegt hierbei auf der NMR-Spektroskopie, die durch Methoden wie die Electron Spray Ionization Massenspektrometrie (ESI-MS), die zeitaufgelöste Laserfluoreszenzspektroskopie (TRLFS) und quantenmechanische Modellrechnungen punktuell ergänzt wird. Der erste Teil der Arbeit konzentriert sich auf die NMR-spektroskopische Untersuchung der Komplexierung von Ln(III) und Am(III) mit dem O-Donorliganden N,N,N'N' Tetraethyldigylcolamid (TEDGA). Daraus geht hervor, dass die Ln(III)-O und Am(III)-O Bindung im [M(TEDGA)3]3+ Komplex (M = Ln, Am) ähnliche Bindungseigenschaften besitzen. Die Wechselwirkung zwischen Ligand und Am(III) bzw. Ln(III) ist daher von vergleichbarer elektrostatischer Natur. Im zweiten Teil wird die Komplexierung von Ln(III) und Am(III) mit dem N,O-Donorliganden N,N,N',N'-Tetraethyl-2,6-carboxamidopyridin (Et-Pic) untersucht. Die NMR-Analyse von [M(Et-Pic)3]3+ (M = Ln, Am) legt ähnliche Bindungseigenschaften von Ln(III)-O und Am(III)-O-Bindung nahe. Im Gegensatz dazu zeigen die Ln(III)-N und Am(III)-N-Bindung unterschiedliche Eigenschaften. Der partiell höhere kovalente Bindungsanteil der An(III)-N-Bindung kann mittels TRLFS bestätigt werden, da Cm(III) um eine Größenordnung stabilere 1:3-Komplexe mit Et-Pic bildet als Eu(III). Der dritte Teil der Arbeit beschäftigt sich mit der NMR-spektroskopischen Studie von [Th(nPr-BTP)3]4+ (nPr-BTP = 2,6-Bis(dipropyl-1,2,4-triazin-3-yl)pyridin). Abhängig von der Wahl des Lösungsmittels zeigen sich für [Th(nPr-BTP)3]4+ unterschiedliche Komplexspezies. Eine asymmetrische Spezies wird in polar-protischen Lösungsmitteln gebildet, wohingegen eine symmetrische Spezies in polar-aprotischen Lösungsmitteln auftritt. Die symmetrische Spezies ist dabei isostrukturell zu den bereits untersuchten Komplexen [M(nPr-BTP)3]3+ (M = Ln, Am). Untersuchungen von [Th(nPr-BTP)3]4+ in einer Vielzahl deuterierter Lösungsmittel und binären Lösungsmittelgemischen zeigen, dass die Ligandkonfiguration von der Fähigkeit des Lösungsmittels abhängig ist, aktiv H-Brückenbindungen auszubilden. Ursache hierfür ist eine stärkere Wechselwirkung zwischen dem Komplex und dem Lösungsmittel. Unterstützt werden die experimentellen Befunde durch theoretische Rechnungen, die die Struktur der asymmetrischen Spezies untersuchen.
Singlet fission (SF) is a photophysical reaction where a singlet excited organic molecule shares its energy with a neighbor ground state organic molecule, generating two spin-triplet states. In this work, the role of chemical substitutions and geometrical changes were investigated on SF in thin films, in direct-linked heterodimers, and in bridge-linked heterodimers.
Narrowband photoluminescence (PL) in the near-infrared and electrical exciton generation make semiconducting single-walled carbon nanotubes (SWNTs) promising materials for optoelectronic devices. The functionalization of SWNTs with luminescent sp3 defects offers synthetically tunable light emission and enhances their potential for applications such as quantum light sources, bioimaging and sensing. However, the synthetic protocols that are currently used to create these defects are limited to aqueous dispersions of SWNTs, which are compromised by short tube lengths, residual metallic SWNTs and poor solution-processability. Here, the combination of highly selective polymer-sorting and shear force mixing as a mild exfoliation method provides electronically-pure (6,5) SWNTs in toluene with average tube lengths > 1 µm and strategies for their sp3 functionalization are developed based on simple diazonium chemistry. The complexation by an ether crown allows for the solubilization of commercially available aryldiazonium salts in organic solvents and thus enables their reaction with polymer-wrapped SWNTs. The resulting defect-tailored (6,5) SWNTs show a relatively high photoluminescence quantum yield (PLQY) of up to 4 % with 90 % of photons emitted from the sp3 defect. The dependence of the defect-induced PL brightening on the nanotube length indicates that the PLQY of the pristine SWNT may exceed that of the sp3 defect for a sufficiently high nanotube quality. By using custom-synthesized diazonium salts in a modified protocol, stable organic radicals are covalently attached to purified semiconducting SWNTs via luminescent aryl defects. The proximity between the defect-localized exciton and the unpaired electron promotes spin exchange and electron transfer processes, which are identified through time-resolved PL measurements. The results point toward an increased yield of triplet excitons due to radical-enhanced intersystem crossing, which could serve as a general concept to probe triplet states in SWNTs. The dispersion in organic solvents facilitates the integration of defect-tailored, polymer-wrapped SWNTs into optoelectronic devices. A planar dielectric waveguide structure channels the PL emitted from sp3 defects over distances > 1 mm and thus represents a first step toward their interfacing with photonic circuits and photovoltaic devices. Moreover, the demonstration of electroluminescence from sp3 defects in light-emitting field-effect transistors underpins their potential for electrically-driven single-photon sources.
Van der Waals crystals exhibit comparatively strong, typically covalent bonds in two dimensions and comparatively weak, typically non-covalent bonds between the two-dimensional lattice. This enables to separate individual two-dimensional layers of a van der Waals crystal which can be thinned down to atomic thickness in a process called exfoliation. The resulting nanosheets typically exhibit completely different properties compared to their corresponding bulk counterparts which can be exploited for various applications in advanced devices. Different methods have been presented for preparation of two-dimensional nanomaterials each with their respective up- and downsides. While some techniques can provide materials of highest quality, suitable for fundamental studies of inherent material properties, they typically lack scalability. Other methods focus on a high production rate of the nanomaterial, but introduce imperfections to the material due to the harsh conditions required. In recent years, exfoliation in the liquid phase has emerged to a widely used production technique due to the scalability and its wide applicability. While the industrial relevance of two-dimensional nanomaterials is somewhat linked to the quality of the material that can be prepared by high throughput methods, a deeper understanding of underlying fundamentals for the nanosheet preparation is required to improve state-of-the-art techniques. In the case of liquid-exfoliated nanosheets, this can be achieved by statistical studies of the nanomaterial dimensions that can be prepared and isolated by size selection techniques. In this work, sonication-assisted liquid phase exfoliation using different conditions and solvents and subsequent size selection was applied to a total of 17 different van der Waals crystals. The material dimensions of all fractions were quantified through statistical atomic force microscopy. The findings presented in this work demonstrate a fundamental correlation between the nanomaterial lateral size and thickness which is ascribed to equipartition of energy between processes of nanosheet delamination and tearing. This provides an experimental proxy to determine the ratio between the in-plane binding strength and the out-of-plane interlayer attraction. Isolation of different size-fractions of the same material and the knowledge over the nanomaterial dimensions in these fractions enables to study size-dependent changes of material properties in a quantitative manner. Measurements of optical properties on different sizes of dispersed nanosheets reveal systematic changes of the spectra with nanosheet size and enable to de-rive spectroscopic metrics for the size, thickness and concentration for different van der Waals nanomaterials, typically using extinction and absorbance spectroscopy. Structurally and compositionally different materials show similar changes in their optical response with changing material size which can be ascribed to a combination of confinement and dielectric screening effects, as well as changing contributions form scattering and electronically different material edges. Unifying principles across various materials were identified for the changes of the optical spectra with material dimensions. The knowledge of material dimensions and the understanding of the optical spectra enables to study the stability of different nanomaterial systems as function of time using optical spectroscopy such as extinction, absorbance or photoluminescence. A dependence of the speed and degree of the material decomposition on the storage temperature and the water content of the solvent is conveniently accessible for different material dimensions. The results presented within this work provide an advanced understanding of the exfoliation of layered crystals, unifying principles of optical properties as function of nanomaterial dimensions and proof-of-concept experiments for quantification of the material decomposition.
Singlet fission (SF) is a process where two triplet charge-carriers are generated from one photoexcited singlet state. This opens up the possibility to increase the efficiency limit for single-junction solar cells by one third from 33% to 44%. In this work, the long-lasting question of the effect of competing pathways on the efficiency of SF is addressed by time-resolved spectroscopy and a novel global fit approach. This is demonstrated on two examples. First, SF is established in a new class of molecules, the tetraazaperopyrenes. Here, substituent-dependent factors, namely excimer formation as well as vibronic and spin-orbit coupling, are identified to be decisive for SF efficiency. Subsequently, solutions of (hetero-) acenes are investigated, in which comparisons between ambient conditions and deaerated solutions highlight the importance of molecular oxygen for SF: A new, sequential mechanism including atmospheric oxygen as a catalyst is resolved, which allows for a step-wise doubling of triplet states even at low chromophore concentrations. In concentrated solutions, diffusion-controlled SF outcompetes other reaction pathways resulting in triplet yields close to 200%. The absence of any intermediate species emphasises the efficiency of this process.
The extraordinary mechanical and charge transport properties of semiconducting single-walled carbon nanotubes (SWNTs) make them a promising material for solution-processable, flexible and stretchable electronics. Many of these remarkable features are even obtained in randomly-oriented SWNT networks that are compatible with established large-scale thin-film processes based on printing techniques or optical lithography. Given the enormous progress in the purification of solely semiconducting nanotubes as well as in the preparation of SWNT networks with a uniform and defined morphology in recent years, their widespread application as active layers in field-effect transistors (FETs) has become feasible. Likewise, this progress raised subsequent questions of what key parameters determine the charge transport processes across these networks and how they can further be optimized.
This thesis investigates charge transport and its limitations in polymer-sorted semiconducting SWNT networks with a focus on the precise nanotube network composition. The employed FET geometry enabled a reproducible and undistorted analysis of composition- and temperature- dependent transport parameters such as the charge carrier mobility. A comparison between nanotube networks with various selected or even precisely defined SWNT species distributions and average tube diameters reveals that additional energy barriers created at the junctions of adjacent nanotubes with different diameters result in inferior transport properties. While the network charge transport was formerly considered to be solely limited by the charge transfer across these inter-nanotube junctions, the results of this work imply that also the transport within each individual SWNT is important. The specific diameter dependence of this intra-nanotube transport can rationalize the substantially higher carrier mobilities observed for large-diameter networks with a certain SWNT bandgap distribution compared to monochiral networks that contain only a single small-diameter nanotube species. These findings suggest that composition optimizations for SWNT network FETs with maximum carrier mobilities should aim at monochiral large-diameter nanotubes.
Aside from insights into the underlying transport mechanisms, this work demonstrates a novel approach to intentionally modify charge transport in semiconducting SWNT network FETs by adding photochromic spiropyran compounds to the dielectric layer. The strong impact of the spiropyran and its photoinduced isomerization to merocyanine on the charge carrier mobilities give these transistors the properties of basic optical memory devices. Upon UV illumination the carrier mobilities are severely reduced until their recovery is induced by annealing or illumination with visible light. This implemented light responsiveness illustrates the fundamental suitability of SWNT network FETs for multifunctional applications beyond integrated circuits.