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Gold at the nanoscale
(2020)
In this cumulative dissertation, I want to present my contributions to the field of plasmonic nanoparticle science. Plasmonic nanoparticles are characterised by resonances of the free electron gas around the spectral range of visible light. In recent years, they have evolved as promising components for light based nanocircuits, light harvesting, nanosensors, cancer therapies, and many more.
This work exhibits the articles I authored or co-authored in my time as PhD student at the University of Potsdam. The main focus lies on the coupling between localised plasmons and excitons in organic dyes. Plasmon–exciton coupling brings light–matter coupling to the nanoscale. This size reduction is accompanied by strong enhancements of the light field which can, among others, be utilised to enhance the spectroscopic footprint of molecules down to single molecule detection, improve the efficiency of solar cells, or establish lasing on the nanoscale. When the coupling exceeds all decay channels, the system enters the strong coupling regime. In this case, hybrid light–matter modes emerge utilisable as optical switches, in quantum networks, or as thresholdless lasers. The present work investigates plasmon–exciton coupling in gold–dye core–shell geometries and contains both fundamental insights and technical novelties. It presents a technique which reveals the anticrossing in coupled systems without manipulating the particles themselves. The method is used to investigate the relation between coupling strength and particle size. Additionally, the work demonstrates that pure extinction measurements can be insufficient when trying to assess the coupling regime. Moreover, the fundamental quantum electrodynamic effect of vacuum induced saturation is introduced. This effect causes the vacuum fluctuations to diminish the polarisability of molecules and has not yet been considered in the plasmonic context.
The work additionally discusses the reaction of gold nanoparticles to optical heating. Such knowledge is of great importance for all potential optical applications utilising plasmonic nanoparticles since optical excitation always generates heat. This heat can induce a change in the optical properties, but also mechanical changes up to melting can occur. Here, the change of spectra in coupled plasmon–exciton particles is discussed and explained with a precise model. Moreover, the work discusses the behaviour of gold nanotriangles exposed to optical heating. In a pump–probe measurement, X-ray probe pulses directly monitored the particles’ breathing modes. In another experiment, the triangles were exposed to cw laser radiation with varying intensities and illumination areas. X-ray diffraction directly measured the particles’ temperature. Particle melting was investigated with surface enhanced Raman spectroscopy and SEM imaging demonstrating that larger illumination areas can cause melting at lower intensities. An elaborate methodological and theoretical introduction precedes the articles. This way, also readers without specialist’s knowledge get a concise and detailed overview of the theory and methods used in the articles. I introduce localised plasmons in metal nanoparticles of different shapes. For this work, the plasmons were mostly coupled to excitons in J-aggregates. Therefore, I discuss these aggregates of organic dyes with sharp and intense resonances and establish an understanding of the coupling between the two systems. For ab initio simulations of the coupled systems, models for the systems’ permittivites are presented, too. Moreover, the route to the sample fabrication – the dye coating of gold nanoparticles, their subsequent deposition on substrates, and the covering with polyelectrolytes – is presented together with the measurement methods that were used for the articles.
The two hallmark features of Brownian motion are the linear growth < x2(t)> = 2Ddt of the mean squared displacement (MSD) with diffusion coefficient D in d spatial dimensions, and the Gaussian distribution of displacements. With the increasing complexity of the studied systems deviations from these two central properties have been unveiled over the years. Recently, a large variety of systems have been reported in which the MSD exhibits the linear growth in time of Brownian (Fickian) transport, however, the distribution of displacements is pronouncedly non-Gaussian (Brownian yet non-Gaussian, BNG). A similar behaviour is also observed for viscoelastic-type motion where an anomalous trend of the MSD, i.e., <x2(t)> ~ ta, is combined with a priori unexpected non-Gaussian distributions (anomalous yet non-Gaussian, ANG). This kind of behaviour observed in BNG and ANG diffusions has been related to the presence of heterogeneities in the systems and a common approach has been established to address it, that is, the random diffusivity approach.
This dissertation explores extensively the field of random diffusivity models. Starting from a chronological description of all the main approaches used as an attempt of describing BNG and ANG diffusion, different mathematical methodologies are defined for the resolution and study of these models. The processes that are reported in this work can be classified in three subcategories, i) randomly-scaled Gaussian processes, ii) superstatistical models and iii) diffusing diffusivity models, all belonging to the more general class of random diffusivity models. Eventually, the study focuses more on BNG diffusion, which is by now well-established and relatively well-understood. Nevertheless, many examples are discussed for the description of ANG diffusion, in order to highlight the possible scenarios which are known so far for the study of this class of processes.
The second part of the dissertation deals with the statistical analysis of random diffusivity processes. A general description based on the concept of moment-generating function is initially provided to obtain standard statistical properties of the models. Then, the discussion moves to the study of the power spectral analysis and the first passage statistics for some particular random diffusivity models. A comparison between the results coming from the random diffusivity approach and the ones for standard Brownian motion is discussed. In this way, a deeper physical understanding of the systems described by random diffusivity models is also outlined.
To conclude, a discussion based on the possible origins of the heterogeneity is sketched, with the main goal of inferring which kind of systems can actually be described by the random diffusivity approach.
Towards seasonal prediction: stratosphere-troposphere coupling in the atmospheric model ICON-NWP
(2020)
Stratospheric variability is one of the main potential sources for sub-seasonal to seasonal predictability in mid-latitudes in winter. Stratospheric pathways play an important role for long-range teleconnections between tropical phenomena, such as the quasi-biennial oscillation (QBO) and El Niño-Southern Oscillation (ENSO), and the mid-latitudes on the one hand, and linkages between Arctic climate change and the mid-latitudes on the other hand. In order to move forward in the field of extratropical seasonal predictions, it is essential that an atmospheric model is able to realistically simulate the stratospheric circulation and variability. The numerical weather prediction (NWP) configuration of the ICOsahedral Non-hydrostatic atmosphere model ICON is currently being used by the German Meteorological Service for the regular weather forecast, and is intended to produce seasonal predictions in future. This thesis represents the first extensive evaluation of Northern Hemisphere stratospheric winter circulation in ICON-NWP by analysing a large set of seasonal ensemble experiments.
An ICON control climatology simulated with a default setup is able to reproduce the basic behaviour of the stratospheric polar vortex. However, stratospheric westerlies are significantly too weak and major stratospheric warmings too frequent, especially in January. The weak stratospheric polar vortex in ICON is furthermore connected to a mean sea level pressure (MSLP) bias pattern resembling the negative phase of the Arctic Oscillation (AO). Since a good representation of the drag exerted by gravity waves is crucial for a realistic simulation of the stratosphere, three sensitivity experiments with reduced gravity wave drag are performed. Both a reduction of the non-orographic and orographic gravity wave drag respectively, lead to a strengthening of the stratospheric vortex and thus a bias reduction in winter, in particular in January. However, the effect of the non-orographic gravity wave drag on the stratosphere is stronger. A third experiment, combining a reduced orographic and non-orographic drag, exhibits the largest stratospheric bias reductions. The analysis of stratosphere-troposphere coupling based on an index of the Northern Annular Mode demonstrates that ICON realistically represents downward coupling. This coupling is intensified and more realistic in experiments with a reduced gravity wave drag, in particular with reduced non-orographic drag. Tropospheric circulation is also affected by the reduced gravity wave drag, especially in January, when the strongly improved stratospheric circulation reduces biases in the MSLP patterns. Moreover, a retuning of the subgrid-scale orography parameterisations leads to a significant error reduction in the MSLP in all months. In conclusion, the combination of these adjusted parameterisations is recommended as a current optimal setup for seasonal simulations with ICON.
Additionally, this thesis discusses further possible influences on the stratospheric polar vortex, including the influence of tropical phenomena, such as QBO and ENSO, as well as the influence of a rapidly warming Arctic. ICON does not simulate the quasi-oscillatory behaviour of the QBO and favours weak easterlies in the tropical stratosphere. A comparison with a reanalysis composite of the easterly QBO phase reveals, that the shift towards the easterly QBO in ICON further weakens the stratospheric polar vortex. On the other hand, the stratospheric reaction to ENSO events in ICON is realistic. ICON and the reanalysis exhibit a weakened stratospheric vortex in warm ENSO years. Furthermore, in particular in winter, warm ENSO events favour the negative phase of the Arctic Oscillation, whereas cold events favour the positive phase. The ICON simulations also suggest a significant effect of ENSO on the Atlantic-European sector in late winter. To investigate the influence of Arctic climate change on mid-latitude circulation changes, two differing approaches with transient and fixed sea ice conditions are chosen. Neither ICON approach exhibits the mid-latitude tropospheric negative Arctic Oscillation circulation response to amplified Arctic warming, as it is discussed on the basis of observational evidence. Nevertheless, adding a new model to the current and active discussion on Arctic-midlatitude linkages, further contributes to the understanding of divergent conclusions between model and observational studies.
The current thesis is focused on the properties of graphene supported by metallic substrates and specifically on the behaviour of electrons in such systems. Methods of scanning tunneling microscopy, electron diffraction and photoemission spectroscopy were applied to study the structural and electronic properties of graphene. The purpose of the first part of this work is to introduce the most relevant aspects of graphene physics and the methodical background of experimental techniques used in the current thesis.
The scientific part of this work starts with the extensive study by means of scanning tunneling microscopy of the nanostructures that appear in Au intercalated graphene on Ni(111). This study was aimed to explore the possible structural explanations of the Rashba-type spin splitting of ~100 meV experimentally observed in this system — much larger than predicted by theory. It was demonstrated that gold can be intercalated under graphene not only as a dense monolayer, but also in the form of well-periodic arrays of nanoclusters, a structure previously not reported. Such nanocluster arrays are able to decouple graphene from the strongly interacting Ni substrate and render it quasi-free-standing, as demonstrated by our DFT study. At the same time calculations confirm strong enhancement of the proximity-induced SOI in graphene supported by such nanoclusters in comparison to monolayer gold. This effect, attributed to the reduced graphene-Au distance in the case of clusters, provides a large Rashba-type spin splitting of ~60 meV.
The obtained results not only provide a possible mechanism of SOI enhancement in this particular system, but they can be also generalized for graphene on other strongly interacting substrates intercalated by nanostructures of heavy noble d metals.
Even more intriguing is the proximity of graphene to heavy sp-metals that were predicted to induce an intrinsic SOI and realize a spin Hall effect in graphene. Bismuth is the heaviest stable sp-metal and its compounds demonstrate a plethora of exciting physical phenomena. This was the motivation behind the next part of the current thesis, where structural and electronic properties of a previously unreported phase of Bi-intercalated graphene on Ir(111) were studied by means of scanning tunneling microscopy, spin- and angle-resolved photoemission spectroscopy and electron diffraction. Photoemission experiments revealed a remarkable, nearly ideal graphene band structure with strongly suppressed signatures of interaction between graphene and the Ir(111) substrate, moreover, the characteristic moiré pattern observed in graphene on Ir(111) by electron diffraction and scanning tunneling microscopy was strongly suppressed after intercalation. The whole set of experimental data evidences that Bi forms a dense intercalated layer that efficiently decouples graphene from the substrate. The interaction manifests itself only in the n-type charge doping (~0.4 eV) and a relatively small band gap at the Dirac point (~190 meV). The origin of this minor band gap is quite intriguing and in this work it was possible to exclude a wide range of mechanisms that could be responsible for it, such as induced intrinsic spin-orbit interaction, hybridization with the substrate states and corrugation of the graphene lattice. The main origin of the band gap was attributed to the A-B symmetry breaking and this conclusion found support in the careful analysis of the interference effects in photoemission that provided the band gap estimate of ~140 meV.
While the previous chapters were focused on adjusting the properties of graphene by proximity to heavy metals, graphene on its own is a great object to study various physical effects at crystal surfaces. The final part of this work is devoted to a study of surface scattering resonances by means of photoemission spectroscopy, where this effect manifests itself as a distinct modulation of photoemission intensity. Though scattering resonances were widely studied in the past by means of electron diffraction, studies about their observation in photoemission experiments started to appear only recently and they are very scarce.
For a comprehensive study of scattering resonances graphene was selected as a versatile model system with adjustable properties. After the theoretical and historical introduction to the topic of scattering resonances follows a detailed description of the unusual features observed in the photoemission spectra obtained in this work and finally the equivalence between these features and scattering resonances is proven. The obtained photoemission results are in a good qualitative agreement with the existing theory, as verified by our calculations in the framework of the interference model. This simple model gives a suitable explanation for the general experimental observations.
The possibilities of engineering the scattering resonances were also explored. A systematic study of graphene on a wide range of substrates revealed that the energy position of the resonances is in a direct relation to the magnitude of charge transfer between graphene and the substrate. Moreover, it was demonstrated that the scattering resonances in graphene on Ir(111) can be suppressed by nanopatterning either by a superlattice of Ir nanoclusters or by atomic hydrogen. These effects were attributed to strong local variations of tork function and/or destruction of long-range order of thephene lattice. The tunability of scattering resonances can be applied for optoelectronic devices based on graphene. Moreover, the results of this study expand the general understanding of the phenomenon of scattering resonances and are applicable to many other materials besides graphene.
Lattice dynamics
(2020)
In this thesis I summarize my contribution to the research field of ultrafast structural dynamics in condensend matter. It consists of 17 publications that cover the complex interplay between electron, magnon, and phonon subsystems in solid materials and the resulting lattice dynamics after ultrafast photoexcitation. The investigation of such dynamics is necessary for the physical understanding of the processes in materials that might become important in the future as functional materials for technological applications, for example in data storage applications, information processing, sensors, or energy harvesting.
In this work I present ultrafast x-ray diffraction (UXRD) experiments based on the optical pump – x-ray probe technique revealing the time-resolved lattice strain. To study these dynamics the samples (mainly thin film heterostructures) are excited by femtosecond near-infrared or visible light pulses. The induced strain dynamics caused by stresses of the excited subsystems are measured in a pump-probe scheme with x-ray diffraction (XRD) as a probe. The UXRD setups used during my thesis are a laser-driven table-top x-ray source and large-scale synchrotron facilities with dedicated time-resolved diffraction setups. The UXRD experiments provide quantitative access to heat reservoirs in nanometric layers and monitor the transient responses of these layers with coupled electron, magnon, and phonon subsystems. In contrast to optical probes, UXRD allows accessing the material-specific information, which is unavailable for optical light due to the detection of multiple indistinguishable layers in the range of the penetration depth.
In addition, UXRD facilitates a layer-specific probe for layers buried opaque heterostructures to study the energy flow. I extended this UXRD technique to obtain the driving stress profile by measuring the strain dynamics in the unexcited buried layer after excitation of the adjacent absorbing layers with femtosecond laser pulses. This enables the study of negative thermal expansion (NTE) in magnetic materials, which occurs due to the loss of the magnetic order. Part of this work is the investigation of stress profiles which are the source of coherent acoustic phonon wave packets (hypersound waves). The spatiotemporal shape of these stress profiles depends on the energy distribution profile and the ability of the involved subsystems to produce stress. The evaluation of the UXRD data of rare-earth metals yields a stress profile that closely matches the optical penetration profile: In the paramagnetic (PM) phase the photoexcitation results in a quasi-instantaneous expansive stress of the metallic layer whereas in the antiferromagnetic (AFM) phase a quasi-instantaneous contractive stress and a second contractive stress contribution rising on a 10 ps time scale adds to the PM contribution. These two time scales are characteristic for the magnetic contribution and are in agreement with related studies of the magnetization dynamics of rare-earth materials.
Several publications in this thesis demonstrate the scientific progress in the field of active strain control to drive a second excitation or engineer an ultrafast switch. These applications of ultrafast dynamics are necessary to enable control of functional material properties via strain on ultrafast time scales.
For this thesis I implemented upgrades of the existing laser-driven table-top UXRD setup in order to achieve an enhancement of x-ray flux to resolve single digit nanometer thick layers. Furthermore, I developed and built a new in-situ time-resolved magneto-optic Kerr effect (MOKE) and optical reflectivity setup at the laser-driven table-top UXRD setup to measure the dynamics of lattice, electrons and magnons under the same excitation conditions.
Galaxies are gravitationally bound systems of stars, gas, dust and - probably - dark matter. They are the building blocks of the Universe. The morphology of galaxies is diverse: some galaxies have structures such as spirals, bulges, bars, rings, lenses or inner disks, among others. The main processes that characterise galaxy evolution can be separated into fast violent events that dominated evolution at earlier times and slower processes, which constitute a phase called secular evolution, that became dominant at later times. Internal processes of secular evolution include the gradual rearrangement of matter and angular momentum, the build-up and dissolution of substructures or the feeding of supermassive black holes and their feedback. Galaxy bulges – bright central components in disc galaxies –, on one hand, are relics of galaxy formation and evolution. For instance, the presence of a classical bulge suggests a relatively violent history. In contrast, the presence of a disc-like bulge instead indicates the occurrence of secular evolution processes in the main disc. Galaxy bars – elongated central stellar structures –, on the other hand, are the engines of secular evolution. Studying internal properties of both bars and bulges is key to comprehending some of the processes through which secular evolution takes place. The main objectives of this thesis are (1) to improve the classification of bulges by combining photometric and spectroscopic approaches for a large sample of galaxies, (2) to quantify star formation in bars and verify dependencies on galaxy properties and (3) to analyse stellar populations in bars to aid in understanding the formation and evolution of bars. Integral field spectroscopy is fundamental to the work presented in this thesis, which consists of three different projects as part of three different galaxy surveys: the CALIFA survey, the CARS survey and the TIMER project.
The first part of this thesis constitutes an investigation of the nature of bulges in disc galaxies. We analyse 45 galaxies from the integral-field spectroscopic survey CALIFA by performing 2D image decompositions, growth curve measurements and spectral template fitting to derive stellar kinematics from CALIFA data cubes. From the obtained results, we present a recipe to classify bulges that combines four different parameters from photometry and kinematics: The bulge Sersic index nb, the concentration index C20;50, the Kormendy relation and the inner slope of the radial velocity dispersion profile ∇σ. The results of the different approaches are in good agreement and allow a safe classification for approximately 95% of the galaxies. We also find that our new ‘inner’ concentration index performs considerably better than the traditionally used C50;90 and, in combination with the Kormendy relation, provides a very robust indication of the physical nature of the bulge. In the second part, we study star formation within bars using VLT/MUSE observations for 16 nearby (0.01 < z < 0.06) barred active-galactic-nuclei (AGN)-host galaxies from the CARS survey. We derive spatially-resolved star formation rates (SFR) from Hα emission line fluxes and perform a detailed multi-component photometric decomposition on images derived from the data cubes. We find a clear separation into eight star-forming (SF) and eight non-SF bars, which we interpret as indication of a fast quenching process. We further report a correlation between the SFR in the bar and the shape of the bar surface brightness profile: only the flattest bars (nbar < 0.4) are SF. Both parameters are found to be uncorrelated with Hubble type. Additionally, owing to the high spatial resolution of the MUSE data cubes, for the first time, we are able to dissect the SFR within the bar and analyse trends parallel and perpendicular to the bar major axis. Star formation is 1.75 times stronger on the leading edge of a rotating bar than on the trailing edge and is radially decreasing. Moreover, from testing an AGN feeding scenario, we report that the SFR of the bar is uncorrelated with AGN luminosity. Lastly, we present a detailed analysis of star formation histories and chemical enrichment of stellar populations (SP) in galaxy bars. We use MUSE observations of nine very nearby barred galaxies from the TIMER project to derive spatially resolved maps of stellar ages and metallicities, [α/Fe] abundances, star formation histories, as well as Hα as tracer of star formation. Using these maps, we explore in detail variations of SP perpendicular to the bar major axes. We find observational evidence for a separation of SP, supposedly caused by an evolving bar. Specifically, intermediate-age stars (∼ 2-6 Gyr) get trapped on more elongated orbits forming a thinner bar, while old stars (> 8 Gyr) form a rounder and thicker bar. This evidence is further strengthened by very similar results obtained from barred galaxies in the cosmological zoom-in simulations from the Auriga project. In addition, we find imprints of typical star formation patterns in barred galaxies on the youngest populations (< 2 Gyr), which continuously become more dominant from the major axis towards the sides of the bar. The effect is slightly stronger on the leading side. Furthermore, we find that bars are on average more metal-rich and less α-enhanced than the inner parts of the discs that surrounds them. We interpret this result as an indication of a more prolonged or continuous formation of stars that shape the bar as compared to shorter formation episodes in the disc within the bar region.
This thesis is focused on a better understanding of the formation mechanism of bulk birefringence gratings (BBG) and a surface relief gratings (SRG) in photo-sensitive polymer films. A new set-up is developed enabling the in situ investigation how the polymer film is being structured during irradiation with modulated light. The new aspect of the equipment is that it combines several techniques such as a diffraction efficiency (DE) set-up, an atomic force microscope (AFM) and an optical set-up for controlled illumination of the sample. This enables the simultaneous acquiring and differentiation of both gratings (BBG and SRG), while changing the irradiation conditions in desired way.
The dissertation is based on five publications. The first publication (I) is focused on the description of the set-up and interpretation of the measured data. A fine structure within the 1st-order diffraction spot is observed, which is a result of the inhomogeneity of the inscribed gratings.
In the second publication (II) the interplay of BBG and SRG in the DE is discussed. It has been found, that, dependent on the polarization of a weak probe beam, the diffraction components of the SRG and BBG either interfere constructively or destructively in the DE, altering the appearance of the intensity distribution within the diffracted spot.
The third (III) and fourth (IV) publications describe the light-induced reconfiguration of surface structures. Special attention is payed to conditions influencing the erasure of topography and bulk gratings. This can be achieved via thermal treatment or illumination of the polymer film. Using the translation of the interference pattern (IP) in a controlled way, the optical erase speed is significantly increased. Additionally, a dynamic reconfigurable surface is generated, which could move surface attached objects by the continuous translation of the interference pattern during irradiation of the polymer films.
The fifth publication (V) deals with the understanding of polymer deformation under irradiation with SP-IP, which is the only IP generating a half-period topography grating (compared to the period of the IP) on the photo-sensitive polymer film. This mechanism is used, e.g. to generate a SRG below the diffraction limit of light. It also represents an easy way of changing the period of the surface grating just by a small change in polarization angle of the interfering beams without adjusting the optical pass of the two beams. Additionally, complex surface gratings formed in mixed polarization- and intensity interference patterns are shown.
I J. Jelken, C. Henkel and S. Santer, Applied Physics B, 125 (2019), 218
II J. Jelken, C. Henkel and S. Santer, Appl. Phys. Lett., 116 (2020), 051601
III J. Jelken and S. Santer, RSC Advances, 9 (2019), 20295
IV J. Jelken, M. Brinkjans, C. Henkel and S. Santer, SPIE Proceedings, 11367 (2020), 1136710
V J. Jelken, C. Henkel and S. Santer, Formation of Half-Period Surface Relief Gratings in Azobenzene Containing Polymer Films (submitted to Applied Physics B)
Perovskite solar cells have become one of the most studied systems in the quest for new, cheap and efficient solar cell materials. Within a decade device efficiencies have risen to >25% in single-junction and >29% in tandem devices on top of silicon. This rapid improvement was in many ways fortunate, as e. g. the energy levels of commonly used halide perovskites are compatible with already existing materials from other photovoltaic technologies such as dye-sensitized or organic solar cells. Despite this rapid success, fundamental working principles must be understood to allow concerted further improvements. This thesis focuses on a comprehensive understanding of recombination processes in functioning devices.
First the impact the energy level alignment between the perovskite and the electron transport layer based on fullerenes is investigated. This controversial topic is comprehensively addressed and recombination is mitigated through reducing the energy difference between the perovskite conduction band minimum and the LUMO of the fullerene. Additionally, an insulating blocking layer is introduced, which is even more effective in reducing this recombination, without compromising carrier collection and thus efficiency. With the rapid efficiency development (certified efficiencies have broken through the 20% ceiling) and thousands of researchers working on perovskite-based optoelectronic devices, reliable protocols on how to reach these efficiencies are lacking. Having established robust methods for >20% devices, while keeping track of possible pitfalls, a detailed description of the fabrication of perovskite solar cells at the highest efficiency level (>20%) is provided. The fabrication of low-temperature p-i-n structured devices is described, commenting on important factors such as practical experience, processing atmosphere & temperature, material purity and solution age. Analogous to reliable fabrication methods, a method to identify recombination losses is needed to further improve efficiencies. Thus, absolute photoluminescence is identified as a direct way to quantify the Quasi-Fermi level splitting of the perovskite absorber (1.21eV) and interfacial recombination losses the transport layers impose, reducing the latter to ~1.1eV. Implementing very thin interlayers at both the p- and n-interface (PFN-P2 and LiF, respectively), these losses are suppressed, enabling a VOC of up to 1.17eV. Optimizing the device dimensions and the bandgap, 20% devices with 1cm2 active area are demonstrated. Another important consideration is the solar cells’ stability if subjected to field-relevant stressors during operation. In particular these are heat, light, bias or a combination thereof. Perovskite layers – especially those incorporating organic cations – have been shown to degrade if subjected to these stressors. Keeping in mind that several interlayers have been successfully used to mitigate recombination losses, a family of perfluorinated self-assembled monolayers (X-PFCn, where X denotes I/Br and n = 7-12) are introduced as interlayers at the n-interface. Indeed, they reduce interfacial recombination losses enabling device efficiencies up to 21.3%. Even more importantly they improve the stability of the devices. The solar cells with IPFC10 are stable over 3000h stored in the ambient and withstand a harsh 250h of MPP at 85◦C without appreciable efficiency losses. To advance further and improve device efficiencies, a sound understanding of the photophysics of a device is imperative. Many experimental observations in recent years have however drawn an inconclusive picture, often suffering from technical of physical impediments, disguising e. g. capacitive discharge as recombination dynamics. To circumvent these obstacles, fully operational, highly efficient perovskites solar cells are investigated by a combination of multiple optical and optoelectronic probes, allowing to draw a conclusive picture of the recombination dynamics in operation. Supported by drift-diffusion simulations, the device recombination dynamics can be fully described by a combination of first-, second- and third-order recombination and JV curves as well as luminescence efficiencies over multiple illumination intensities are well described within the model. On this basis steady state carrier densities, effective recombination constants, densities-of-states and effective masses are calculated, putting the devices at the brink of the radiative regime. Moreover, a comprehensive review of recombination in state-of-the-art devices is given, highlighting the importance of interfaces in nonradiative recombination. Different strategies to assess these are discussed, before emphasizing successful strategies to reduce interfacial recombination and pointing towards the necessary steps to further improve device efficiency and stability. Overall, the main findings represent an advancement in understanding loss mechanisms in highly efficient solar cells. Different reliable optoelectronic techniques are used and interfacial losses are found to be of grave importance for both efficiency and stability. Addressing the interfaces, several interlayers are introduced, which mitigate recombination losses and degradation.
In this dissertation we introduce a concept of light driven active and passive manipulation of colloids trapped at solid/liquid interface. The motion is induced due to generation of light driven diffusioosmotic flow (LDDO) upon irradiation with light of appropriate wavelength. The origin of the flow is due to osmotic pressure gradient resulting from a concentration gradient at the solid/liquid interface of the photosensitive surfactant present in colloidal dispersion. The photosensitive surfactant consists of a cationic head group and a hydrophobic tail in which azobenzene group is integrated in. The azobenzene is known to undergo reversible photo-isomerization from a stable trans to a meta stable cis state under irradiation with UV light. Exposure to light of larger wavelength results in back photo-isomerization from cis to trans state. The two isomers have different molecular properties, for instance, trans isomer has a rod like structure and low polarity (0 dipole moment), whereas cis one is bent and has a dipole moment of ~3 Debye. Being integrated in the hydrophobic tail of the surfactant molecule, the azobenzene state determines the hydrophobicity of the whole molecule: in the trans state the surfactant is more hydrophobic than in the cis-state. In this way many properties of the surfactant such as the CMC, solubility and the interaction potential with a solid surface can be altered by light. When the solution containing such a surfactant is irradiated with focused light, a concentration gradient of different isomers is formed near the boundary of the irradiated area near the solid surface resulting in osmotic pressure gradient. The generated diffusioosmotic (DO) flow carries the particles passively along.
The local-LDDO flow can be generated around and by each particle when mesoporous silica colloids are dispersed in the surfactant solution. This is because porous particles act as a sink/source which absorbs azobenzene molecule in trans state and expels it when it is in the cis state. The DO flows generated at each particle interact resulting in aggregation or separation depending upon the initial state of surfactant molecules. The kinetic of aggregation and separation can be controlled and manipulated by altering the parameters such as the wavelength and intensity of the applied light, as well as surfactant and particle concentration. Using two wavelengths simultaneously allows for dynamic gathering and separation creating fascinating patterns such as 2D disk of well separated particles or establishing collective complex behaviour of particle ensemble as described in this thesis.
The mechanism of l-LDDO is also used to generate self-propelled motion. This is possible when half of the porous particle is covered by metal layer, basically blocking the pores on one side. The LDDO flow generated on uncapped side pushes the particle forward resulting in a super diffusive motion. The system of porous particle and azobenzene containing surfactant molecule can be utilized for various application such as drug delivery, cargo transportation, self-assembling, micro motors/ machines or micro patterning.
Giant unilamellar vesicles are an important tool in todays experimental efforts to understand the structure and behaviour of biological cells. Their simple structure allows the isolation of the physical elastic properties of the lipid membrane. A central physical
property is the bending energy of the membrane, since the many different shapes of giant vesicles can be obtained by finding the minimum of the bending energy. In the spontaneous curvature model the bending energy is a function of the bending rigidity as well as the mean curvature and an additional parameter called the spontaneous curvature, which describes an internal preference of the lipid-bilayer to bend towards one side or the other. The spontaneous and mean curvature are local properties of the membrane.
Additional constraints arise from the conservation of the membrane surface area and the enclosed volume, which are global properties.
In this thesis the spontaneous curvature model is used to explain the experimental observation of a periodic shape oscillation of a giant unilamellar vesicle that was filled with a protein complex that periodically binds to and unbinds from the membrane.
By assuming that the binding of the proteins to the membrane induces a change in the spontaneous curvature the experimentally observed shapes could successfully be explained. This involves the numerical solution of the differential equations as obtained from the minimization of the bending energy respecting the area and volume constraints, the so called shape equations. Vice versa this approach can be used to estimate the spontaneous curvature from experimentally measurable quantities.
The second topic of this thesis is the analysis of concentration gradients in rigid conic membrane compartments. Gradients of an ideal gas due to gravity and gradients generated by the directed stochastic movement of molecular motors along a microtubulus were considered. It was possible to calculate the free energy and the bending energy analytically for the ideal gas. In the case of the non-equilibrium system with molecular motors, the characteristic length of the density profile, the jam-length, and its dependency on the opening angle of the conic compartment have been calculated in the mean-field limit.
The mean field results agree qualitatively with stochastic particle simulations.