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A Method to detect triplet exciton transfer from singlet fission into silicon solar cells: comparing different surface treatments
No full textSinglet fission is one of the most promising routes to overcome the single-junction efficiency limit for solar cells. Singlet fission-enhanced silicon solar cells are the most desirable implementation, but transfer of triplet excitons, the product of singlet fission, into silicon solar cells has proved to be very challenging. Here, we report on an all optical measurement technique for the detection of triplet exciton quenching at semiconductor interfaces, a necessary requirement for triplet exciton or charge transfer. The method relies on the growth of individual, single-crystal islands of the singlet fission material on the silicon surface. The islands have different heights, and we correlate these heights to the quenching efficiency of triplet excitons. The quenching efficiency is measured by spatially resolved delayed fluorescence and compared to a diffusion–quenching model. Using silicon capped with a blocking thermal oxide and aromatic monolayers, we demonstrate that this technique can quickly screen different silicon surface treatments for triplet exciton quenching
Models for spatial organization of microtubules and cell polarization
Get PDFThe main actors in this PhD thesis are microtubules, dynamic polymers built from protein subunits that play many important roles in cells of all higher organisms. We study a number of their functions using mathematical modelling and computer simulations. First, we consider the role of microtubules in establishing the ordered cortical array, a structure that plays a key role in the plant cell elongation. We show theoretically that the observed effect that new microtubules are nucleated from pre-existing microtubules strongly enhances the order of the array and makes it more robust against variations in the conditions. Next, we turn to the role the shape of the cell plays in the spatial organization of microtubules. We show that depending on how the microtubules interact with the cell boundary, either the long or the short axis of the cell determines the majority direction of the microtubules. Additionally, we formulate a model that predicts the positioning of the mitotic spindle, which is the cellular structure that segregates the duplicated chromosomes during eukaryotic cell division. We analyze how the speed, precision and final direction of the spindle orientation process depends on cell shape and the parameters that describe the microtubules. Finally, we turn to the potential role of microtubules in establishing cell polarity, the non-uniform distribution of cellular constituents, which is crucial for many developmental processes. Based solely on the propensity of microtubules to bind and transport proteins to the cell membrane, we set up a feasible and robust cell polarization mechanism, which could potentially be used to set up polarity in a minimal cell-like environment using a biochemical reconstitution approach. We study this model in its pure form in a perfectly spherical cell, in order to establish proof-of-principle, but also show that the effect remains in a more realistic non-spherical cell
Mycorrhizal Fungi Respond to Resource Inequalityby Moving Phosphorus from Rich to PoorPatches across Networks
Get PDFThe world’s ecosystems are characterized by an unequal distribution of resources [1
]. Trade partnerships between organisms of different species—mutualisms—can help individuals cope with such resource inequality [2
, 3
, 4
]. Trade allows individuals to exchange commodities they can provide at low cost for resources that are otherwise impossible or more difficult to access [5
, 6
]. However, as resources become increasingly patchy in time or space, it is unknown how organisms alter their trading strategies [7
, 8
]. Here, we show how a symbiotic fungus mediates trade with a host root in response to different levels of resource inequality across its network. We developed a quantum-dot-tracking technique to quantify phosphorus-trading strategies of arbuscular mycorrhizal fungi simultaneously exposed to rich and poor resource patches. By following fluorescent nanoparticles of different colors across fungal networks, we determined where phosphorus was hoarded, relocated, and transferred to plant hosts. We found that increasing exposure to inequality stimulated trade. Fungi responded to high resource variation by (1) increasing the total amount of phosphorus distributed to host roots, (2) decreasing allocation to storage, and (3) differentially moving resources within the network from rich to poor patches. Using single-particle tracking and high-resolution video, we show how dynamic resource movement may help the fungus capitalize on value differences across the trade network, physically moving resources to areas of high demand to gain better returns. Such translocation strategies can help symbiotic organisms cope with exposure to resource inequality
Nanoscale spatial limitations of large-area substrate conformal imprint lithography
No full textWe demonstrate a soft-imprint nanofabrication technique offering nanometer resolution over an area as large as a 150 mm diameter wafer. It makes use of a composite imprint stamp composed of a quaternary siloxane-modified poly-di-methyl-siloxane patterned rubber layer with a relatively high Young's modulus that is laminated on a thin glass support. The in-plane stiffness of the stamp avoids pattern deformation over large areas, while out-of-plane flexibility allows conformal contact to be made over the entire substrate area. The stamp is used in conjunction with a novel tetra-methyl-ortho-siloxane/methyl-tri-methoxy-siloxane sol-gel imprint resist material developed to replicate nanoscale features in rigid silica at room temperature. We demonstrate better than 10 nm resolution in imprinted line gratings and individual pillars with aspect ratio as high as 5:1. Gaps as small as 6 nm can be reproduced. The patterns can be used as an etch mask to pattern 150 mm diameter silicon and quartz substrates while maintaining sub-10 nm resolution
Uncovering the dynamic precursors to motor-driven contraction of active gels
Get PDFCells and tissues have the remarkable ability to actively generate the forces required to change their shape. This active mechanical behavior is largely mediated by the actin cytoskeleton, a crosslinked network of actin filaments that is contracted by myosin motors. Experiments and active gel theories have established that the length scale over which gel contraction occurs is governed by a balance between molecular motor activity and crosslink density. By contrast, the dynamics that govern the contractile activity of the cytoskeleton remain poorly understood. Here we investigate the microscopic dynamics of reconstituted actin–myosin networks using simultaneous real-space video microscopy and Fourier-space dynamic light scattering. Light scattering reveals different regimes of microscopic dynamics as a function of sample age. We uncover two dynamical precursors that precede macroscopic gel contraction. One is characterized by a progressive acceleration of stress-induced rearrangements, while the other consists of sudden, heterogeneous rearrangements. Intriguingly, our findings suggest a qualitative analogy between self-driven rupture and collapse of active gels and the delayed rupture of passive gels observed in earlier studies of colloidal gels under external loads
Mapping Complex Mode Volumes with Cavity Perturbation Theory
Get PDFMicrocavities and nanoresonators are characterized by their quality factors () and mode volumes (). While is unambiguously defined, there are still questions on and, in particular, on its complex-valued character, whose imaginary part is linked to the non-Hermitian nature of open systems. Helped by cavity perturbation theory and near-field experimental data, we clarify the physics captured by the imaginary part of and show how a mapping of the spatial distribution of both the real and imaginary parts can be directly inferred from perturbation measurements. This result shows that the mathematically abstract complex mode , in fact, is directly observable
Microwave induced mechanical activation of hydrogel dimers
Get PDFWhen grape-sized aqueous dimers are irradiated in a microwave oven, an intense electromagnetic hotspot forms at their point of contact, often igniting a plasma. Here we show that this irradiation can result in the injection of mechanical energy. By examining irradiated hydrogel dimers through high-speed imaging, we find that they repeatedly bounce off of each other while irradiated. We determine that an average of 1 μJ of mechanical energy is injected into the pair during each collision. Furthermore, a characteristic high-pitched audio signal is found to accompany each collision. We show that both the audio signal and the energy injection arise via an interplay between vaporization and elastic deformations in the region of contact, the so-called ‘elastic Liedenfrost effect’. Our results establish a novel, non-contact method of injecting mechanical energy into soft matter systems, suggesting application in fields such as soft robotics
Control of Surface Defects in ZnO Nanorod Arrays with Thermally Deposited Au Nanoparticles for Perovskite Photovoltaics
Get PDFIn this work, we employ vacuum deposited Au nanoparticles (∼4 nm) to control the defect density on the surface of hydrothermally synthesized ZnO nanorod arrays (ZnO-NR), which are of interest for electron-transport layers in perovskite solar cells. Using a combination of photoluminescence spectroscopy, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy, we show that the Au particles reduce the presence of defects in the ZnO-NR. We discuss this in terms of trap filling due to band bending at the ZnO-NR surface. As a proof-of-concept, we apply the Au-decorated ZnO-NR as electron-transport layers in mixed-cation and mixed-halide lead perovskite solar cells (Cs0.15FA0.85PbI2.75Br0.25). Devices prepared with the Au-decorated ZnO-NR electron-transport layers demonstrate higher open-circuit voltages and fill factors compared to solar cells prepared with pristine ZnO-NR, resulting in an increase in the power-conversion efficiency from 11.7 to 13.7%. However, the operational stability of the solar cells is not improved by the Au nanoparticles, indicating that bulk properties of the perovskite may limit device lifetime
Spatial organization of the bacterial cell: in vivo imaging across scales
No full textBacteria are amongst the smallest known living organisms and yet they carry out an incrediblevariety of different functions and are able to respond to a large variety of different externalstimuli. Growing evidence is suggesting that specific proteins and sub-cellular components(e.g. the nucleoid) need to be highly organized both on the membrane and inside the cellularvolume in order to carry out vital tasks like cell replication or chemotaxis and to respond toexternal changes in the environment. In this thesis I investigated experimentally the spatialorganization of different sub-cellular components (nucleoid) and protein species (NAPs andchemoreceptors) and its importance for physiological processes inside living bacterial cells.Most of the experiments presented here are carried out in living cells, and our results thusprovide insights also into the spatio-temporal organization and the dynamics of the responseto controlled perturbations