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The presented thesis summarizes the results from four and a half years of intense lithography development on (Cd,Hg)Te/HgTe/(Cd,Hg)Te quantum well structures. The effort was motivated by the unique properties of this topological insulator. Previous work from Molenkamp at al.\ has proven that the transport through such a 2D TI is carried by electrons with opposite spin, counter-propagating in 1D channels along the sample edge. However, up to this thesis, the length of quantized spin Hall channels has never been reported to exceed 4 µm. Therefore, the main focus was put on a reproducible and easy-to-handle fabrication process that reveals the intrinsic material parameters.
Every single lithography step in macro as well as microscopic sample fabrication has been re-evaluated. In the Development, the process changes have been presented along SEM pictures, microgaphs and, whenever possible, measurement responses.
We have proven the conventional ion milling etch method to damage the remaining mesa and result in drastically lower electron mobilities in samples of microscopic size.
The novel KI:I2:HBr wet etch method for macro and microstructure mesa fabrication has been shown to leave the crystalline structure intact and result in unprecedented mobilities, as high as in macroscopic characterization Hall bars. Difficulties, such as an irregular etch start and slower etching of the conductive QW have been overcome by concentration, design and etch flow adaptations. In consideration of the diffusive regime, a frame around the EBL write field electrically decouples the structure mesa from the outside wafer. As the smallest structure, the frame is etched first and guarantees a non-different etching of the conductive layer during the redox reaction. A tube-pump method assures reproducible etch results with mesa heights below 300 nm. The PMMA etch mask is easy to strip and leaves a clean mesa with no redeposition. From the very first attempts, to the final etch process, the reader has been provided with the characteristics and design requirements necessary to enable the fabrication of nearly any mesa shape within an EBL write field of 200 µm.
Magneto resistance measurement of feed-back samples have been presented along the development chronology of wet etch method and subsequent lithography steps. With increasing feature quality, more and more physics has been revealed enabling detailed evaluation of smallest disturbances. The following lithography improvements have been implemented. They represent a tool-box for high quality macro and microstructure fabrication on (CdHg)Te/HgTe of almost any kind.
The optical positive resist ECI 3027 can be used as wet and as dry etch mask for structure sizes larger than 1 µm. It serves to etch mesa structures larger than the EBL write field.
The double layer PMMA is used for ohmic contact fabrication within the EBL write field. Its thickness allows to first dry etch the (Cd,Hg)Te cap layer and then evaporate the AuGe contact, in situ and self-aligned. Because of an undercut, up to 300 nm can be metalized without any sidewalls after the lift-off. An edge channel mismatch within the contact leads can be avoided, if the ohmic contacts are designed to reach close to the sample and beneath the later gate electrode.
The MIBK cleaning step prior to the gate application removes PMMA residuals and thereby improves gate and potential homogeneity.
The novel low HfO2-ALD process enables insulator growth into optical and EBL lift-off masks of any resolvable shape. Directly metalized after the insulator growth, the self-aligned method results in thin and homogeneous gate electrode reproducibly withholding gate voltages to +-10 V.
The optical negative resist ARN 4340 exhibits an undercut when developed. Usable as dry etch mask and lift-off resist, it enables an in-situ application of ohmic contacts first etching close to the QW, then metalizing AuGe. Up to 500 nm thickness, the undercut guarantees an a clean lift-off with no sidewalls.
The undertaken efforts have led to micro Hall bar measurements with Hall plateaus and SdH-oszillations in up to now unseen levels of detail.
The gap resistance of several micro Hall bars with a clear QSH signal have been presented in Quantum Spin Hall. The first to exhibit longitudinal resistances close to the expected h/2e2 since years, they reveal unprecedented details in features and characteristics. It has been shown that their protection against backscattering through time reversal symmetry is not as rigid as previously claimed. Values below and above 12.9 kΩ been explained, introducing backscattering within the Landauer-Büttiker formalism of edge channel transport. Possible reasons have been discussed. Kondo, interaction and Rashba-backscattering arising from density inhomogeneities close to the edge are most plausible to explain features on and deviations from a quantized value. Interaction, tunneling and dephasing mechanisms as well as puddle size, density of states and Rashba Fields are gate voltage dependent. Therefore, features in the QSH signal are fingerprints of the characteristic potential landscape.
Stable up to 11 K, two distinct but clear power laws have been found in the higher temperature dependence of the QSH in two samples. However, with ΔR = Tα, α = ¼ in one (QC0285) and α = 2 in the other (Q2745), none of the predicted dependencies could be confirmed. Whereas, the gap resistances of QC0285 remains QSH channel dominated up to 3.9 T and thereby confirmed the calculated lifting of the band inversion in magnetic field. The gate-dependent oscillating features in the QSH signal of Q2745 immediately increase in magnetic field. The distinct field dependencies allowed the assumption of two different dominant backscattering mechanisms.
Resulting in undisturbed magneto transport and unprecedented QSH measurements The Novel Micro Hall Bar Process has proven to enable the fabrication of a new generation of microstructures.
Coherent Multidimensional Spectroscopy in Molecular Beams and Liquids Using Incoherent Observables
(2018)
The aim of the present work was to implement an experimental approach that enables coherent two-dimensional (2D) electronic spectroscopy of samples in various states of matter. For samples in the liquid phase, a setup was realized that utilizes the sample fluorescence for the acquisition of 2D spectra. Whereas the liquid-phase approach has been established before, coherent 2D spectroscopy on gaseous samples in a molecular beam as developed in this work is in fact a new method. It employs for the first time cations in a time-of-flight mass spectrometer for signal detection and was used to obtain the first ion-selective 2D spectra of a molecular-beam sample. Additionally, a new acquisition concept was developed in this thesis that significantly decreases measurement times in 2D spectroscopy using optimized sparse sampling and a compressed-sensing reconstruction algorithm.
Characteristic for the variant of 2D spectroscopy presented in this work is the usage of a phase-coherent sequence of four laser pulses in a fully collinear geometry for sample excitation. The pulse sequence was generated by a custom-designed pulse shaper that is capable of rapid scanning by changing the pulse parameters such as time delays and phases with the repetition rate of the laser. The sample's response was detected by monitoring incoherent observables that arise from the final-state population, for instance fluorescence or cations. Phase cycling, i.e., signal acquisition with different combinations of the relative phases of the excitation pulses, was applied to extract nonlinear signal contributions from the full signal during data analysis.
Liquid-phase 2D fluorescence spectroscopy was established with the laser dye cresyl violet as a sample molecule, confirming coherent oscillations previously observed in literature that are originating from vibronic coherences in specific regions of the 2D spectrum.
The data set of this experiment was used subsequently to introduce optimized sparse sampling in 2D spectroscopy. An optimization algorithm was implemented in order to find the best sampling pattern while taking only one quarter of the regular time-domain sampling points, thereby reducing the acquisition time by a factor of four. Signal recovery was based on a new and compact representation of 2D spectra using the von Neumann basis, which required about six times less coefficients than the Fourier basis to retain the relevant information. Successful reconstruction was shown by recovering the coherent oscillations in cresyl violet from a reduced data set.
Finally, molecular-beam coherent 2D spectroscopy was introduced with an investigation of ionization pathways in highly-excited nitrogen dioxide, revealing transitions to discrete auto-ionizing states as the dominant contribution to the ion signal. Furthermore, the advantage of the time-of-flight approach to obtain reactant and product 2D spectra simultaneously enabled the observation of distinct differences in the multiphoton-ionization response functions of the nitrogen dioxide cation and the nitrogen oxide ionic fragment.
The developed experimental techniques of this work will facilitate fast acquisition of 2D spectra for samples in various states of matter and permit reliable direct comparison of results. Therefore, they pave the way to study the properties of quantum coherences during photophysical processes or photochemical reactions in different environments.
Kavitäts-Exziton-Polaritonen (Polaritonen) sind hybride Quasiteilchen, die sich aufgrund starker Kopplung von Halbleiter-Exzitonen mit Kavitätsphotonen ausbilden. Diese Quasiteilchen weisen eine Reihe interessanter Eigenschaften auf, was sie einerseits für die Grundlagenforschung, andererseits auch für die Entwicklung neuartiger Bauteile sehr vielversprechend macht. Bei Erreichen einer ausreichend großen Teilchendichte geht das System in den Exziton-Polariton-Kondensationszustand über, was zur Emission von laserartigem Licht führt. Organische Halbleiter als aktives Emittermaterial zeigen in diesem Kontext großes Potential, da deren Exzitonen neben großen Oszillatorstärken auch hohe Bindungsenergien aufweisen. Deshalb ist es möglich, unter Verwendung organischer Halbleiter selbst bei Umgebungsbedingungen äußerst stabile Polaritonen zu erzeugen. Eine wichtige Voraussetzung zur Umsetzung von integrierten opto-elektronischen Bauteilen basierend auf Polaritonen ist der kontrollierte räumliche Einschluss sowie die Realisierung von frei konfigurierbaren Potentiallandschaften. Diese Arbeit beschäftigt sich mit der Entwicklung und der Untersuchung geeigneter Plattformen zur Erzeugung von Exziton-Polaritonen und Polaritonkondensaten in hemisphärischen Mikrokavitäten, in die organische Halbleiter eingebettet sind.
This Thesis investigates the interplay of a central degree of freedom with an environment. Thereby, the environment is prepared in a localized phase of matter.
The long-term aim of this setup is to store quantum information on the central degree of freedom while exploiting the advantages of localized systems.
These many-body localized systems fail to equilibrate under the description of thermodynamics, mostly due to disorder. Doing so, they form the most prominent phase of matter that violates the eigenstate thermalization hypothesis. Thus, many-body localized systems preserve information about an initial state until infinite times without the necessity to isolate the system.
This unique feature clearly suggests to store quantum information within localized environments, whenever isolation is impracticable.
After an introduction to the relevant concepts, this Thesis examines to which extent a localized phase of matter may exist at all if a central degree of freedom dismantles the notion of locality in the first place. To this end, a central spin is coupled to the disordered Heisenberg spin chain, which shows many-body localization. Furthermore, a noninteracting analog describing free fermions is discussed. Therein, an impurity is coupled to an Anderson localized environment.
It is found that in both cases, the presence of the central degree of freedom manifests in many properties of the localized environment. However, for a sufficiently weak coupling, quantum chaos, and thus, thermalization is absent. In fact, it is shown that the critical disorder, at which the metal-insulator transition of its environment occurs in the absence of the central degree of freedom, is modified by the coupling strength of the central degree of freedom. To demonstrate this, a phase diagram is derived.
Within the localized phase, logarithmic growth of entanglement entropy, a typical signature of many-body localized systems, is increased by the coupling to the central spin. This property is traced back to resonantly coupling spins within the localized Heisenberg chain and analytically derived in the absence of interactions. Thus, the studied model of free fermions is the first model without interactions that mimics the logarithmic spreading of entanglement entropy known from many-body localized systems.
Eventually, it is demonstrated that observables regarding the central spin significantly break the eigenstate thermalization hypothesis within the localized phase. Therefore, it is demonstrated how a central spin can be employed as a detector of many-body localization.
Metallic nanostructures possess the ability to support resonances in the visible wavelength regime which are related to localized surface plasmons. These create highly enhanced electric fields in the immediate vicinity of metal surfaces. Nanoparticles with dipolar resonance also radiate efficiently into the far-field and hence serve as antennas for light. Such optical antennas have been explored during the last two decades, however, mainly as standalone units illuminated by external laser beams and more recently as electrically driven point sources, yet merely with basic antenna properties. This work advances the state of the art of locally driven optical antenna systems. As a first instance, the electric driving scheme including inelastic electron tunneling over a nanometer gap is merged with Yagi-Uda theory. The resulting antenna system consists of a suitably wired feed antenna, incorporating a tunnel junction, as well as several nearby parasitic elements whose geometry is optimized using analytical and numerical methods. Experimental evidence of unprecedented directionality of light emission from a nanoantenna is provided. Parallels in the performance between radiofrequency and optical Yagi-Uda arrays are drawn. Secondly, a pair of electrically connected antennas with dissimilar resonances is harnessed as electrodes in an organic light emitting nanodiode prototype. The organic material zinc phthalocyanine, exhibiting asymmetric injection barriers for electrons and holes, in conjunction with the electrode resonances, allows switching and controlling the emitted peak wavelength and directionality as the polarity of the applied voltage is inverted. In a final study, the near-field based transmission-line driving of rod antenna systems is thoroughly explored. Perfect impedance matching, corresponding to zero back-reflection, is achieved when the antenna acts as a generalized coherent perfect absorber at a specific frequency. It thus collects all guided, surface-plasmon mediated input power and transduces it to other nonradiative and radiative dissipation channels. The coherent interplay of losses and interference effects turns out to be of paramount importance for this delicate scenario, which is systematically obtained for various antenna resonances. By means of the here developed semi-analytical toolbox, even more complex nanorod chains, supporting topologically nontrivial localized edge states, are studied. The results presented in this work facilitate the design of complex locally driven antenna systems for optical wireless on-chip communication, subwavelength pixels, and loss-compensated integrated plasmonic nanocircuitry which extends to the realm of topological plasmonics.
Metallic nano-optical systems allow to confine and guide light at the nanoscale,
a fascinating ability which has motivated a wide range of fundamental as well
as applied research over the last two decades. While optical antennas provide
a link between visible radiation and localized energy, plasmonic waveguides
route light in predefined pathways. So far, however, most experimental demonstrations
are limited to purely optical excitations, i.e. isolated structures are
illuminated by external lasers. Driving such systems electrically and generating
light at the nanoscale, would greatly reduce the device footprint and pave the
road for integrated optical nanocircuitry. Yet, the light emission mechanism as
well as connecting delicate nanostructures to external electrodes pose key challenges
and require sophisticated fabrication techniques. This work presents various
electrically connected nano-optical systems and outlines a comprehensive
production line, thus significantly advancing the state of the art. Importantly,
the electrical connection is not just used to generate light, but also offers new
strategies for device assembly. In a first example, nanoelectrodes are selectively
functionalized with self-assembled monolayers by charging a specific electrode.
This allows to tailor the surface properties of nanoscale objects, introducing an
additional degree of freedom to the development of metal-organic nanodevices.
In addition, the electrical connection enables the bottom-up fabrication of tunnel
junctions by feedback-controlled dielectrophoresis. The resulting tunnel barriers
are then used to generate light in different nano-optical systems via inelastic
electron tunneling. Two structures are discussed in particular: optical Yagi-Uda
antennas and plasmonic waveguides. Their refined geometries, accurately fabricated
via focused ion beam milling of single-crystalline gold platelets, determine
the properties of the emitted light. It is shown experimentally, that Yagi-Uda
antennas radiate light in a specific direction with unprecedented directionality,
while plasmonic waveguides allow to switch between the excitation of two
propagating modes with orthogonal near-field symmetry. The presented devices
nicely demonstrate the potential of electrically connected nano-optical systems,
and the fabrication scheme including dielectrophoresis as well as site-selective
functionalization will inspire more research in the field of nano-optoelectronics.
In this context, different future experiments are discussed, ranging from the
control of molecular machinery to optical antenna communication.
This PhD thesis addresses the photophysics of selected small organic molecules with the purpose of using them for efficient and even novel light sources. In particular, the studies presented focused on revealing the underlying exciton dynamics and determining the transition rates between different molecular states. It was shown how the specific properties and mechanisms of light emission in fluorescent molecules, molecules with phosphorescence or thermally activated delayed fluorescence (TADF), biradicals, and multichromophores can be utilized to build novel light-emitting devices. The main tool employed here was the analysis of the emitters’ photon statistics, i.e. the analysis of the temporal distribution of emitted photons, during electrical or optical excitation. In the introduction of this work, the working principle of an organic light-emitting diode (OLED) was introduced, while Chapter 2 provided the physical background of the relevant properties of organic molecules and their interaction with light. In particular, the occurrence of discrete energy levels in organic semiconductors and the process of spontaneous light emission were discussed. Furthermore, in this chapter a mathematical formalism was elaborated with the goal to find out what kind of information about the studied molecule can be obtained by analyzing its photon statistics. It was deduced that the intensity correlation function g (2)(t) contains information about the first two factorial moments of the photon statistics and that higher order factorial moments do not contain any additional information about the system under study if the system is always in the same state after the emission of a photon. To conclude the introductory part, Chapter 3 introduced the utilized characterization methods including confocal microscopy of single molecules, time correlated single photon counting and temperature dependent photoluminescence measurements. To provide the background necessary for an understanding of for the following result chapters, in Section 4.1 a closer look was taken at the phenomenon of blinking and photobleaching of individual molecules. For a squaraine-based fluorescent emitter rapid switching between a bright and dark state was observed during photoexcitation. Using literature transition rates between the molecular states, a consistent model was developed that is able to explain the distribution of the residence times of the molecule in the bright and dark states. In particular, an exponential and a power-law probability distribution was measured for the time the molecule resides in tis bright and dark state, respectively. This behavior as well as the change in photoluminescence intensity between the two states was conclusively explained by diffusion of residual oxygen within the sample, which had been prepared in a nitrogen-filled glovebox. For subsequent samples of this work, thin strips of atomic aluminum were deposited on the matrices to serve as oxygen getter material. This not only suppressed the efficiency of photobleaching, but also noticeably prolonged the time prior to photobleaching, which made many of the following investigations possible in the first place. For emitters used in displays, emission properties such as narrow-band luminescence and short fluorescence lifetimes are desired. These properties can be influenced not only by the emitter molecule itself, but also by the interaction with the chosen environment. Therefore, before focusing on the photophysics of individual small organic molecules, Section 4.2 highlighted the interaction of a perylene bisimide-based molecular species with its local environment in a disordered polymethyl methacrylate matrix. In a statistical approach, individual photophysical properties were measured for 32 single molecules and correlations in the variation of the properties were analyzed. This revealed how the local polarity of the molecules’ environment influences their photophysics. In particular, it was shown how an increase in local polarity leads to a red-shifted emission, narrower emission lines, broader vibronic splitting between different emission lines in combination with a smaller Huang-Rhys parameter, and a longer fluorescence lifetime. In the future, these results may help to embed individual chromophores into larger macromolecules to provide the chromophore with the optimal local environment to exhibit the desired emission properties. The next two sections focused on a novel and promising class of chromophores, namely linear coordinated copper complexes, synthesized in the group of Dr. Andreas Steffen at the Institute of Inorganic Chemistry at the University of Würzburg. In copper atoms, the d-orbitals are fully occupied, which prevents undesirable metal-centered d-d⋆ states, which tend to lie low in energy and recombine non-radiatively. Simultaneously, the copper atom provides a flexible coordination geometry, while complexes in their linear form are expected to exhibit the least amount of excited state distortions. Depending on the chosen ligands, these copper complexes can exhibit phosphorescence as well as temperature activated delayed fluorescence. In Section 4.3, a phosphorescent copper complex with a chlorine atom and a 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethyl-2-pyrrolidine-ylidene- ligand was tested for its suitability as an optically active material in an OLED. For this purpose, an OLED with a polyspirobifluorene-based copolymer matrix and the dopant at a concentration of 20 wt% was electrically excited. Deconvolution of the emission spectrum in contributions from the matrix and the dopant revealed that 60 % of the OLEDs emission was due to the copper complex. It was also shown that the shape of the emission spectrum of the copper complex remains unchanged upon incorporation into the OLED, but is red-shifted by about 233 meV. In Section 4.4, a second copper complex exhibiting thermally activated delayed fluorescence was analyzed. This complex comprised a carbazolate as well as a 2-(2,6- diisopropyl)-phenyl-1,1-diphenyl-isoindol-2-ium-3-ide ligand and was examined in the solid state and at the single-molecule level, where single photon emission was recorded up to an intensity of 78’000 counts per second. The evaluation of the second-order autocorrelation function of the emitted light proved an efficient transition between singlet and triplet excited states on the picosecond time scale. In the solid state, the temperature- dependent fluorescence decay of the complex was analyzed after pulsed photoexcitation in the temperature range between 300 K and 5 K. From these measurements, a small singlet-triplet energy gap of only 65 meV and a triplet sublevel splitting of 3.0 meV were derived. The transition rates between molecular states could also be determined. Here, the fast singlet decay time of τS1 = 9.8ns proved the efficient thermally activated delayed fluorescence process, which was demonstrated for the first time for this new class of copper(I) complexes thus. While the use of thermally activated delayed fluorescence is a potential way to harness otherwise long-living dark triplet states, radicals completely avoid dark triplet states. However, this usually comes with the huge drawback of the molecules being chemically unstable. Therefore, two chemically stable biradical species were synthesized in the framework of the DFG research training school GRK 2112 on Molecular biradicals: structure, properties and reactivity, by Yohei Hattori in the group of Prof. Dr. Christoph Lambert and Rodger Rausch in the group of Prof. Dr. Frank Würthner at the Institute of Organic Chemistry at the University of Würzburg, respectively. In Section 4.5, it was investigated how these molecules can be used in OLEDs. In the first isoindigo based biradical (6,6’-bis(3,5-di-tert-butyl-4-phenoxyl)-1,1’-bis(2- ethylhexyl)-[3,3’-biindolinyl-idene]-2,2’-dione) two tert-butyl moieties kinetically block chemical reactions at the place of the lone electrons and an electron-withdrawing core shifts the electron density into the center of the chromophore. With these properties, it was possible to realize a poly(p-phenylene vinylene) copolymer based OLED doped with the biradical and to observe luminescence during optical as well as electrical excitation. Analyzing shapes of the photo- and electroluminescence spectra at different doping concentrations, Förster resonance energy transfer was determined to be the dominant transition mechanism for excitons from the matrix to the biradical dopants. Likewise, OLEDs could be realized with the second diphenylmethylpyridine based birad- ical (4-(5-(bis(2,4,6-trichlorophenyl)methyl)-4,6-dichloropyridin-2-yl)-N-(4-(5-(bis(2,4,6- -trichlorophenyl)methyl)-4,6-dichloropyridin-2-yl)phenyl)-N-(4-methoxyphenyl)aniline) as dopant. In this biradical, chlorinated diphenylmethyl groups protect the two unpaired electrons. Photo- and electroluminescence spectra showed an emission in the near in- frared spectral range between 750 nm and 1000 nm. Also, Förster resonance energy trans- fer was the dominant energy transfer mechanism with an transfer efficiency close to 100 % even at doping concentrations of only 5 wt%. In addition to demonstrating the working OLEDs based in biradicals, the detection of luminescence of the two biradical species in devices also constitutes an important step toward making use of experimental techniques such as optically detected electron spin resonance, which could provide information about the electronic states of the emitter and their spin manifold during OLED operation. Another class of emitters studied are molecules in which several chromophores are co- valently linked to form a macrocyclic system. The properties of these multichromophores were highlighted in Section 4.6. Here, it was analyzed how the photophysical behavior of the molecules is affected by the covalent linking, which determines the interaction be- tween the chromophores. The first multichromophore, 2,2’-ditetracene, was synthesized by Lena Ross in the group of Prof. Dr. Anke Krüger at the Institute of Organic Chemistry at the University of Würzburg and was analyzed in this work both at the single-molecule level and in its aggregated crystalline form. While the single crystals were purified and grown in a vertical sublimation oven, the samples for the single molecule studies were prepared in matrices of amorphous polymethyl methacrylate and crystalline anthracene. Tetracene was analyzed concurrently to evaluate the effects of covalent linking. In samples where the distance between two molecules is sufficiently large, tetracene and 2,2’-ditracene show matching emission profiles with the only difference in the Franck-Condon factors and a de- creased photoluminescence decay time constant from 14 ns for tetracene to 5 ns for 2,2’- ditracene, which can be attributed to the increased density of the vibrational modes in 2,2’-ditracene. Evaluation of the photon statistics of individual 2,2’-ditracene molecules however showed that the system does not behave as two individual chromophores but as a collective state, preserving the spectral properties of the two tetracene chromophores. Complementary calculations performed by Marian Deutsch in the group of Prof. Dr. Bernd Engels at the Institute of Physical and Theoretical Chemistry at the University of Würzburg helped to understand the processes in the materials and could show that the electronic and vibronic modes of 2,2’-ditracene are superpositions of the modes occurring in tetracene. In contrast, single-crystalline 2,2’-ditetracene behaves significantly different than tetracene, namely exhibiting a red shift in photoluminescence of 150 meV, caused by an altered crys- talline packing that lowers the S1-state energy level. Temperature-dependent photolu- minescence measurements revealed a rich emission pattern from 2,2’-ditetracene single crystals. The mechanisms behind this were unraveled using photoluminescence lifetime density analysis in different spectral regions of the emission spectrum and at different tem- peratures. An excimer state was identified that is located about 5 meV below the S1-state, separated by a 1 meV barrier, and which can decay to the ground state with a time constant of 9 ns. Also, as the S1-state energy level is lowered below the E(S1) ≥ 2 ×E(T1) threshold, singlet fission is suppressed in 2,2’-ditetracene in contrast to tetracene. Therefore, at low temperatures, photoluminescence is enhanced by a factor of 46, which could make 2,2’- ditetracene a useful material for future applications in devices such as OLEDs or lasers. The second multichromophore species, para-xylylene bridged perylene bisimide macrocycles, were synthesized by Peter Spenst in the group of Prof. Dr. Frank Würthner at the Institute of Organic Chemistry at the University of Würzburg, by linking three and four perylene bisimides, respectively. To reveal the exciton dynamics in these macrocycles, highly diluted monomers as well as trimers and tetramers were doped into matrices of polymethyl methacrylate to create thin films in which individual macrocycles could be analyzed. The emission spectra of the macrocycles remained identical to those of the monomers, indicating weak coupling between the chromophores. Single photon emission could be verified for monomers as well as macrocycles, as exciton-exciton annihilation processes suppress the simultaneous emission of two photons from one macrocycle. Nevertheless, the proof of the occurrence of a doubly excited state was obtained by excitation power dependent photon statistics measurements. The formalism developed in the theory part of this thesis for calculating the photon statistics of multichromophore systems was used here to find a theoretical model that matches the experimental results. The main features of this model are a doubly excited state, fast singlet-singlet annihilation, and an efficient transition from the doubly excited state to a dark triplet state. The occurrence of triplet-triplet annihilation was demonstrated in a subsequent experiment in which the macrocycles were excited at a laser intensity well above the saturation intensity of the monomer species. In contrast to the monomers, the trimers and tetramers exhibited neither a complete dark state nor saturation of photoluminescence. Both processes, efficient singlet-singlet and triplet-triplet annihilation make perylene bisimide macrocycles exceptionally bright single photon emitters. These advantages were utilized to realize a room temperature electrically driven fluorescent single photon source. For this purpose, OLEDs were fabricated using polyvinylcarbazole and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol blends as a host material for perylene bisimide trimers. Photon antibunching could be observed in both optically and electrically driven devices, representing the first demonstration of electrically driven single photon sources using fluorescent emitters at room temperature. As expected from the previous optical experiments, the electroluminescence of the molecules was exceptionally bright, emitting about 105 photons per second, which could be seen even by eye under the microscope. Finally, in the last section 4.7 of this thesis, two additional measurement schemes were proposed as an alternative to the measurement of the second-order correlation function g (2)(t) of single molecules, which only provides information about the first two factorial moments of the molecules’ photon statistics. In the first scheme, the g (3)(t) function was measured with three photodiodes, which is a consequential extension of the Hanbury Brown and Twiss measurement with two photodiodes. It was demonstrated how measuring the g (3)(t) function is able to identify interfering emitters with non-Poisson statistics in the experiment. The second setup was designed with an electro-optic modulator that repeatedly gen- erates photoexcitation in the form of a step function. The recording of luminescence transients for different excitation intensities yields the same results as the correspond- ing g (2)-functions measured on single emitters, both in their shape and in their depen- dence on excitation power. To demonstrate this concept, the TADF emitter TXO-TPA (2- [4-(diphenylamino)phenyl]-10,10-dioxide-9H-thioxanthen-9-one) was doped at a concen- tration of 10−4 wt% in a mCP (1,3-Bis(N-carbazolyl)benzene) matrix. This concentration was low enough that TXO-TPA molecules did not interact with each other, but an ensem- ble of molecules was still present in the detection volume. The intramolecular transition rates between singlet and triplet states of TXO-TPA could be derived with an error of at most 5 %. Other experimental techniques designed to obtain this information require ei- ther lengthy measurements on single molecules, where sample preparation is also often a challenge, or temperature-dependent fluorescence lifetime measurements, which require a cryostat, which in turn places constraints on the sample design used. In future, this ap- proach could establish a powerful method to study external factors influencing molecular transition rates. Overall, this thesis has introduced new molecular materials, revealed their photophys- ical properties, and demonstrated how they can be used to fabricate efficient and even novel light sources.
In this thesis, intermolecular acceptor-acceptor interactions in organic solar cells based on new non-fullerene acceptors are addressed. For this purpose, first the reproducibility of organic electronic devices was tested on a new facility for their fabrication. This was followed by the screening for new acceptor materials. Based on this, three molecular systems were investigated with regard to their acceptor-acceptor interactions and their influence on solar cell efficiency.
This thesis is aimed at establishing modalities of time-resolved photoelectron spectroscopy (tr-PES) conducted at a free-electron laser (FEL) source and at a high harmonic generation (HHG) source for imaging the motion of atoms, charge and energy at photoexcited hybrid organic/inorganic interfaces. Transfer of charge and energy across interfaces lies at the heart of surface science and device physics and involves a complex interplay between the motion of electrons and atoms. At hybrid organic/inorganic interfaces involving planar molecules, such as pentacene and copper(II)-phthalocyanine (CuPc), atomic motions in out-of-plane direction are particularly apparent. Such hybrid interfaces are of importance to, e.g., next-generation functional devices, smart catalytic surfaces and molecular machines. In this work, two hybrid interfaces – pentacene atop Ag(110) and copper(II)-phthalocyanine (CuPc) atop titanium disulfide (1T-TiSe2) – are characterized by means of modalities of tr-PES. The experiments were conducted at a HHG source and at the FEL source FLASH at Deutsches Elektronen-Synchrotron DESY (Hamburg, Germany). Both sources provide photon pulses with temporal widths of ∼ 100 fs and thus allow for resolving the non-equilibrium dynamics at hybrid interfaces involving both electronic and atomic motion on their intrinsic time scales. While the photon energy at this HHG source is limited to the UV-range, photon energies can be tuned from the UV-range to the soft x-ray-range at FLASH. With this increased energy range, not only macroscopic electronic information can be accessed from the sample’s valence and conduction states, but also site-specific structural and chemical information encoded in the core-level signatures becomes accessible. Here, the combined information from the valence band and core-level dynamics is obtained by performing time- and angle-resolved photoelectron spectroscopy (tr-ARPES) in the UV-range and subsequently performing time-resolved x-ray photoelectron spectroscopy (tr-XPS) and time-resolved photoelectron diffraction (tr-XPD) in the soft x-ray regime in the same experimental setup. The sample’s bandstructure in energy-momentum space and time is captured by a time-of-flight momentum microscope with femtosecond temporal and sub-Ångström spatial resolutions. In the investigated systems, out-of-equilibrium dynamics are traced that are connected to the transfer of charge and energy across the hybrid interfaces. While energetic shifts and complementary population dynamics are observed for molecular and substrate states, the shapes of involved molecular orbitals change in energy-momentum space on a subpicosecond time scale. In combination with theory support, these changes are attributed to iiiatomic reorganizations at the interface and transient molecular structures are reconstructed with sub-Ångström precision. Unique to the material combination of CuPc/TiSe2, a structural rearrangement on the macroscopic scale is traced simultaneously: ∼ 60 % of the molecules undergo a concerted, unidirectional in-plane rotation. This surprising observation and its origin are detailed in this thesis and connected to a particularly efficient charge transfer across the CuPc/TiSe2 interface, resulting in a charging of ∼ 45 % of CuPc molecules.