Study of the Charge-Transfer Dynamics at the Interface of Two-Dimensional Materials

In this thesis the charge-transfer dynamics in two-dimensional transition metal dichalco- genide (TMD) heterostructures (HS) were studied. In order to enable efficient study of TMD heterostructures a re-imagined experimental technique was employed: optical second-harmonic (SH) imaging microscopy. The newly developed setup allows for the study of µm small samples, giving access to the crystal orientation as well as enabling time-resolved measurements with a time-resolution of about 10fs. After designing und building the experiment it was deployed to measure and determine the lifetimes of excitons in mono- and multilayers of MoS_2 . This proof-of-principle experiment found lifetimes for the monolayer of τ_1 = 1.9 ± 0.7ps and τ_2 = 48.5 ± 2.1ps as well as τ_1 = 3.2 ± 0.9ps for the multilayer, well in accordance with current literature. Furthermore it allowed for association of the pump-induced decrease in the nonlinear susceptibility with the generation of intralayer excitons, giving direct access to their lifetimes. After validation of the viability of the experiment for the further study of hetero- structure charge-carrier dynamics we used the setup to measure a MoSe_2 /WSe_2 hetero- structure, which was misaligned under a stacking angle of 32.3°. We discovered that it was possible to measure selectively the time-resolved changes of the individual mono- layers inside the heterostructure via probe polarization dependent measurements. This was achieved by carefully tuning the probe polarization to suppress either of the monolayers and extract the lifetime from other. This unique characteristic of the setup in tandem with energy dependent measurements revealed a bi-directional charge transfer. Following the resonant excitation of one layer a delayed decrease in the op- tical second-harmonic signal becomes visible when sensitive to the other layer of the heterostructure. This delayed filling is absent when the probe polarization is tuned to the pumped material further corroborating the identification as a charge-transfer process in which interlayer excitons are formed. The hole transfer times found were τ_MoSe_2 →WSe_2 =210±60fs from MoSe_2 into WSe_2 following excitation at 1.80eV, as well as τ_WSe_2 →MoSe_2 =610±150fs following excitation at 2.09eV. The next step to deepen our understanding of the charge-transfer was a system- atic study of samples consisting of MoS_2 and WSe_2 with different stacking angles. By applying the insight gained from the previous study we identified an electron trans- fer from WSe_2 into MoS_2 after selectively exciting the WSe_2 . Overall three stacking configuration were measured and clear differences were identified. The transfer times were found to vary significantly for different stacking angles in a range from 12 ± 4fs up to 85 ± 9fs. These disparities were accredited to an enhanced spatial wavefunc- tion overlap between states in the interfacial plane. Furthermore, the importance of hybridized states for the charge-recombination dynamics was illustrated. Therefore the pump-photon energy was tuned to 1.85eV. The larger excitation energy opened previously non-active radiative recombination channels via states around the Γ-point, which led to an equalization of the decay times for all samples to ≈ 200ps. Lastly in an exploratory study the aptitude of a new material class in the form of chiral metal halides for application in more complex heterostructures was inves- tigated. The results consolidated the interest in the material showing promise for future applications in more complex Van der Waals coupled heterostructures

Social Network Services: Competition and Privacy

Social Network Services (SNS) business models highly depend on the gathering and analyzation of user data to obtain an advantage in competition for advertising clients. Nevertheless, an extensive collection and analysis of this data poses a threat to users’ privacy. Based on an economic perspective it seems rational for Social Network Operators (SNO) to ignore the users’ desire for privacy. However, privacy-friendly services might have the potential to earn users’ trust, leading to an increased revelation of personal data. Addressing these issues, we examine the existing privacy problem in SNS in the context of competition between SNO to investigate whether competition tend to enhance user privacy or whether it is the root of its violation. Therefore, this paper investigates the interconnectedness of the market structure and privacy problems in SNS. After analyzing the users’ and the advertisers’ side of SNS, their competitiveness and its influence on user privacy are examined

Job loss in a group of older Canadian workers: Challenges in the sustainable labour market reintegration process

In Western countries, the loss of jobs among older workers is a highly worrisome situation, since it can be synonymous with long-term employment precariousness and definitive exclusion from the labour market. This precariousness is occurring while the labour force in these countries is aging, and governments are looking to extend people’s working lives. It is therefore particularly relevant to study different labour market reintegration processes and to understand their sustainability from a psychological perspective. The present article is examining these processes using a longitudinal study over an 18-month period with 61 older Canadian workers. Time 1 and Final Time were documented with semi-structured individual interviews. These data allowed us to qualitatively construct three reintegration processes (blocked, downgrading, and sustainable) that describe a large spectrum of workers’ experiences regarding occupational repositioning. Quantitative analyses likewise suggest moderate statistical links between the reintegration process and changes in subjective variables associated with the relationship to work and identity representations. Altogether, the results underline the importance of returning to the labour market in qualified, decent, sustainable work that allows people to have a decent and meaningful personal life. The results also suggest, in keeping with the psychology of sustainability, that interventions should promote occupational and personal enrichment, both at the individual and organizational levels

Changes in alpine plant growth under future climate conditions

Alpine shrub- and grasslands are shaped by extreme climatic conditions such as a long-lasting snow cover and a short vegetation period. Such ecosystems are expected to be highly sensitive to global environmental change. Prolonged growing seasons and shifts in temperature and precipitation are likely to affect plant phenology and growth. In a unique experiment, climatology and plant growth was monitored for almost a decade at 17 snow meteorological stations in different alpine regions along the Swiss Alps. Regression analyses revealed highly significant correlations between mean air temperature in May/June and snow melt out, onset of plant growth, and plant height. These correlations were used to project plant growth phenology for future climate conditions based on the gridded output of a set of regional climate models runs. Melt out and onset of growth were projected to occur on average 17 days earlier by the end of the century than in the control period from 1971–2000 under the future climate conditions of the low resolution climate model ensemble. Plant height and biomass production were expected to increase by 77% and 45%, respectively. The earlier melt out and onset of growth will probably cause a considerable shift towards higher growing plants and thus increased biomass. Our results represent the first quantitative and spatially explicit estimates of climate change impacts on future growing season length and the respective productivity of alpine plant communities in the Swiss Alps

Changes in alpine plant growth under future climate conditions

Alpine shrub- and grasslands are shaped by extreme climatic conditions such as a long-lasting snow cover and a short vegetation period. Such ecosystems are expected to be highly sensitive to global environmental change. Prolonged growing seasons and shifts in temperature and precipitation are likely to affect plant phenology and growth. In a unique experiment, climatology and plant growth was monitored for almost a decade at 17 snow meteorological stations in different alpine regions along the Swiss Alps. Regression analyses revealed highly significant correlations between mean air temperature in May/June and snow melt out, onset of plant growth, and plant height. These correlations were used to project plant growth phenology for future climate conditions based on the gridded output of a set of regional climate models runs. Melt out and onset of growth were projected to occur on average 17 days earlier by the end of the century than in the control period from 1971–2000 under the future climate conditions of the low resolution climate model ensemble. Plant height and biomass production were expected to increase by 77% and 45%, respectively. The earlier melt out and onset of growth will probably cause a considerable shift towards higher growing plants and thus increased biomass. Our results represent the first quantitative and spatially explicit estimates of climate change impacts on future growing season length and the respective productivity of alpine plant communities in the Swiss Alps

Cortical control of intraspinal microstimulation to restore motor function after paralysis

Phd ThesisSpinal cord injury (SCI) is a devastating condition affecting the quality of life of many otherwise healthy patients. To date, no cure or therapy is known to restore functional movements of the arm and hand, and despite considerable effort, stem cell based therapies have not been proven effective. As an alternative, nerves or muscles below the injury could be stimulated electrically. While there have been successful demonstrations of restoration of functional movement using muscle stimulation both in humans and non-human primates, intraspinal microstimulation (ISMS) could bear benets over peripheral stimulation. An extensive body of research on spinal stimulation has been accumulated – however, almost exclusively in non-primate species. Importantly, the primate motor system has evolved to be quite different from the frog’s or the cat’s – two commonly studied species –, reecting and enabling changes in how primates use their hands. Because of these functional and anatomical differences, it is fair to assume that also spinal cord stimulation will have different effects in primates. is question – what are the movements elicited by ISMS in the macaque – will be addressed in chapters and . Chronic intraspinal electrode implants so far have been difficult to realise. In chapter we describe a novel use of oating microelectrode arrays (FMAs) as chronic implants in the spinal cord. Compared to implanted microwires or other arrays, these FMAs have the benet of a high electrode density combined with different lengths of electrodes. We were able to maintain these arrays in the cord for months and could elicit movements at low thresholds throughout. If we could build a neural prosthesis stimulating the spinal cord, how would it be controlled? Remarkable progress has been recently achieved in the eld of brain-machine interfaces (BMIs), for example enabling patients to control robotic arms with neural signals recorded from chronically implanted electrodes. Chapter of this thesis examines an approach that combines ISMS with cortical control in a macaque model for upper limb paralysis for the rst time and shows that there is a behavioural improvement. We have devised an experiment in which a monkey trained to perform a grasp-and-pull task receives a temporary cortically induced paralysis of the hand reducing task performance. At the same time, cortical recordings from a different area allow us to control ISMS at sites evoking hand movements – thus partially restoring function. Finally, in appendix A we describe a system we developed in order to introduce automated positive reinforcement training (aPRT) both at the breeding facility and in our animal houses. is system potentially reduces time spent on training animals, adds enrichment to the monkeys’ home environment, and allows for suitability screening of monkeys for behavioural neuroscience experiments

Epilithic diatom communities of selected streams from the Lerma-Chapala Basin, Central Mexico, with the description of two new species

The Lerma-Chapala Basin, in Central Mexico, is geologically heterogeneous, climatically diverse and boasts high biodiversity, lying within two Biodiversity Hotspots, namely Mesoamerica and the Madrean Pine–Oak Woodlands. Epilithon and water samples were collected in the basin from 14 sampling sites three times each, two sampling campaigns during the rainy season and one in the dry season. A total of 274 infrageneric taxa in 48 genera were recorded. The taxonomic composition observed was dominated by taxa from the genera Nitzschia, Gomphonema, Pinnularia, Navicula, Sellaphora and Eunotia. About a third of the taxa found could not be identified to the species level. From those unidentified morphodemes, two are described as new species, namely Brachysira altepetlensis and Sellaphora queretana. Furthermore, Eolimna rhombica is transferred to Sellaphora. Canonical Correspondence Analysis (CCA) revealed that specific conductivity and pH were the main environmental factors driving the community composition observed. Three groups of samples were identified after the CCA: 1) characterized by acidic waters and low conductivity; 2) with circumneutral waters, low specific conductivity and high temperature and phosphorous concentrations; and 3) characterized by circumneutral waters, high conductivity and low nitrogen concentrations. The indicator value method (IndVal), based on the relative abundance and relative frequency of the most abundant taxa was calculated based on the groups observed in the CCA, identifying the characteristic taxa for each of the three groups

Concentrations of plankton and detritus in rock- pools decrease during the low tide period.

During a survey between 31.03. and 03.04.2006 on the southern rocky shore of Le Cabellou, Bretagne, we investigated the change in concentration of plankton and detritus in three different rockpools of the mideulittoral during low tide. For this purpose, we took water samples from each rockpool. The immanent planktonic organisms were counted and the combined dried wet weight of plankton and detritus was measured. The generated data show a clear decrease of density of planktonic individuals from the groups Copepoda and Diatomaciea. Similarly, the combined dried wet weight decreased continuously over the low tide. This indicates that, suspension feeders, living in the rockpools consume considerable amounts of the given concentration of plankton and detritus that is carried into the rock pools during each high tide.Bei einer Umfrage vom 31.03. und 03.04.2006 am südlichen felsigen Ufer von Le Cabellou, Bretagne, untersuchten wir die Konzentrationsänderung von Plankton und Detritus in drei verschiedenen Felsbecken des Mideulitorals bei Ebbe. Dazu haben wir aus jedem Felsenpool Wasserproben genommen. Die immanenten planktonischen Organismen wurden gezählt und das kombinierte Trockennaßgewicht von Plankton und Detritus gemessen. Die generierten Daten zeigen eine deutliche Abnahme der Dichte planktonischer Individuen aus den Gruppen Copepoda und Diatomaciea. In ähnlicher Weise nahm das kombinierte Trockennaßgewicht während der Ebbe kontinuierlich ab. Dies deutet darauf hin, dass in den Felsbecken lebende Suspensionsfresser erhebliche Mengen der angegebenen Konzentration an Plankton und Detritus verbrauchen, die bei jeder Flut in die Felsbecken getragen werden.Peer Reviewe

Why not use SDF rather than beta models in performance measurement?

This paper analyzes performance measurement based on stochastic discount factors, compared to beta models traditionally used in computing funds' (Jensen) alphas. From a theoretical point of view, standard alphas suffer from several limitations. Our paper addresses this issue from an empirical point of view using a sample of Swiss mutual funds from 2000 to 2011. Our results suggest that the key for a "fair” comparison between stochastic discount function (SDF) and beta models is the specification of the set of primitive assets used to calibrate the SDF function. Once this is established, the size of (absolute) performance differences considerably decreases between the two model families. However, there are sizeable performance deviations in the cross-section of funds if conditioning information is incorporated in the tests, up to some 20 basis points per month, or about 2.3% per year. In almost all cases, the SDF-alphas are lower than the standard (Jensen) alphas. In absolute terms, the average SDF-based underperformance of the funds is way larger than the average total expense ratio (TER) of the funds, both in a conditional and unconditional setting

Resistive High-Voltage Probe with Frequency Compensation by Planar Compensation Electrode Integrated in Printed Circuit Board Design

Resistive voltage dividers tend to have a highly non-linear transfer function as parasitic and stray capacitances exert an increasing influence with increasing frequency. The non-linear transfer function depends on the topology and resistors used and consists of a low-pass filter with an additional high-pass component in the GHz range. Due to the non-linear transfer function the measured signal differs from the original input signal. Here, we present an improved resistive voltage divider with additional compensation capacities to extend the linear bandwidth. With this new concept, the linear bandwidth is improved from 115 kHz to 88 MHz, while maintaining a DC input impedance of 10 MΩ. For high-voltage insulation and easy manufacturing, surface mounted resistors on a printed circuit board with a compensation electrode on the adjacent side are used. The performance of this resistive voltage divider is demonstrated by measuring a series of high-voltage pulses with an amplitude of 2.5 kVpeak-peak. The measured pulse rise time is about 16 ns, corresponding to an average slew-rate of 150 V/ns. Finally, the developed resistive voltage divider is successfully used to measure fast high-voltage transients required for advanced ion mobility spectrometers with integrated collision induced fragmentation