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Energy-Momentum Cathodoluminescence Spectroscopy of Dielectric Nanostructures
Get PDFPrecise knowledge of the local density of optical states (LDOS) is fundamental to understanding nanophotonic systems and devices. Complete LDOS mapping requires resolution in energy, momentum, and space, and hence a versatile measurement approach capable of providing simultaneous access to the LDOS components is highly desirable. Here, we explore a modality of cathodoluminescence spectroscopy able to resolve, in single acquisitions, the dispersion in energy and momentum of the radiative LDOS. We perform measurements on a titanium nitride diffraction grating, bulk molybdenum disulfide, and silicon to demonstrate that the technique can probe and disentangle the dispersion of coherent and incoherent cathodoluminescence signals. The approach presented raises cathodoluminescence spectroscopy to a versatile tool for subwavelength design and optimization of nanophotonic devices in the reciprocal space.
Keywords: cathodoluminescence spectroscopy; dielectrics; energy−momentum spectroscopy; Fourier imaging; local density of optical state
Schaatsen over water
Get PDFHet feit dat ijs glad is zorgt er niet alleen voor dat gletsjers naar beneden glijden, maar ook dat je erop kunt schaatsen, terwijl dat niet kan op glas of beton. Maar wat maakt ijs glad? We weten dat dit komt door het bijzondere oppervlak van ijs, maar hoe dat oppervlak er op moleculair niveau uitziet is nog niet goed bekend. Door de vibraties van watermoleculen te bestuderen is het tijdens mijn promotieonderzoek gelukt om nieuw inzicht te krijgen in het ijsoppervlak
A Caenorhabditis elegans Zinc Finger Transcription Factor, ztf-6, Required for the Specification of a Dopamine Neuron-Producing Lineage Maria
Get PDFInvertebrate and vertebrate nervous systems generate different types of dopaminergic neurons in distinct parts of the brain. We have taken a genetic approach to understand how the four functionally related, but lineally unrelated, classes of dopaminergic neurons of the nematode Caenorhabditis elegans, located in distinct parts of its nervous system, are specified. We have identified several genes involved in the generation of a specific dopaminergic neuron type that is generated from the so-called postdeirid lineage, called PDE. Apart from classic proneural genes and components of the mediator complex, we identified a novel, previously uncharacterized zinc finger transcription factor, ztf-6. Loss of ztf-6 has distinct effects in different dopamine neuron-producing neuronal lineages. In the postdeirid lineage, ztf-6 is required for proper cell division patterns and the proper distribution of a critical cell fate determinant, the POP-1/TCF-like transcription factor
Reduced Near-Resonant Vibrational Coupling at the Surfaces of Liquid Water and Ice
Get PDFWe study the resonant interaction of the OH stretch vibrations of water molecules at the surfaces of liquid water and ice using heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy. By studying different isotopic mixtures of H2O and D2O, we vary the strength of the interaction, and we monitor the resulting effect on the HD-SFG spectrum of the OH stretch vibrations. We observe that the near-resonant coupling effects are weaker at the surface than in the bulk, both for water and ice, indicating that for both phases of water the OH vibrations are less strongly delocalized at the surface than in the bulk. incl. Support Informatio
General Considerations for Improving Photovoltage in Metal-Insulator-Semicondutor Photoanodes
Get PDFMetal-insulator-semiconductor (MIS) photoelectrodes offer a simple yet efficient alternative to the traditional semiconductor-liquid junction and the conventional p-n junction electrode. Highly efficient MIS photoanodes often require interfacial surface passivating oxides and high workfunction metals to produce a high photovoltage. Herein we investigate and analyze the effect of interfacial oxides and metal workfunctions to the semiconductor barrier height and the photovoltage of the photoanode. We use two metal components in bimetal contact configuration and observe modulation of the effective barrier height in the presence of the secondary outer metal, and thus the resulting photovoltage. The photovoltage indicates a strong linear dependence with increasing inner metal workfunction, with the highest photovoltage achieved by MIS photoanode using a platinum inner metal. We found that coupling a thin aluminium oxide with an interfacial silicon oxide and controlling the oxide thickness can significantly improve the photovoltage of a MIS junction photoanode
Programeerbare metamaterialen
Get PDFDe meeste natuurlijke materialen om ons heen hebben vaste
eigenschappen. Metaal is bijvoorbeeld erg stijf, hout is buigzaam en
rubber kan meerdere malen zijn eigen lengte uitrekken zonder te breken.
Maar zou het niet interessant zijn als we een materiaal zouden kunnen
maken dat zijn eigenschappen drastisch kan veranderen, van bijvoorbeeld
zacht naar stijf of van kleur verschietend? In dit artikel laten we zien
hoe we op basis van origami nieuwe materiaalconcepten maken die
door aanpassing van hun microstructuur hun eigenschappen kunnen
veranderen en op deze manier ‘geprogrammeerd’ kunnen worden om het
door ons gewenste gedrag te laten zien
Optimizing Nonbonded Interactions of the OPLS Force Field for Aqueous Solutions of Carbohydrates: How to Capture Both Thermodynamics and Dynamics
Get PDFKnowledge on thermodynamic and transport properties of aqueous solutions of carbohydrates is of great interest for process and product design in the food, pharmaceutical, and biotechnological industries. Molecular simulation is a powerful tool to calculate these properties, but current classical force fields cannot provide accurate estimates for all properties of interest. The poor performance of the force fields is mainly observed for concentrated solutions, where solute–solute interactions are overestimated. In this study, we propose a method to refine force fields, such that solute–solute interactions are more accurately described. The OPLS force field combined with the SPC/Fw water model is used as a basis. We scale the nonbonded interaction parameters of sucrose, a disaccharide. The scaling factors are chosen in such a way that experimental thermodynamic and transport properties of aqueous solutions of sucrose are accurately reproduced. Using a scaling factor of 0.8 for Lennard-Jones energy parameters (ϵ) and a scaling factor of 0.95 for partial atomic charges (q), we find excellent agreement between experiments and computed liquid densities, thermodynamic factors, shear viscosities, self-diffusion coefficients, and Fick (mutual) diffusion coefficients. The transferability of these optimum scaling factors to other carbohydrates is verified by computing thermodynamic and transport properties of aqueous solutions of d-glucose, a monosaccharide. The good agreement between computed properties and experiments suggests that the scaled interaction parameters are transferable to other carbohydrates, especially for concentrated solutions
Predicting the Kinetics of Ice Recrystallization in Aqueous Sugar Solutions
Get PDFThe quality of stored frozen products such as foods and biomaterials generally degrades in time due to the growth of large ice crystals by recrystallization. While there is ample experimental evidence that recrystallization within such products (or model systems thereof) is often dominated by diffusion-limited Ostwald ripening, the application of Ostwald-ripening theories to predict measured recrystallization rates has only met with limited success. For a model system of polycrystalline ice within an aqueous solution of sugars, we here show recrystallization rates can be predicted on the basis of Ostwald ripening theory, provided (1) the theory accounts for the fact the solution can be nonideal, nondilute and of different density than the crystals, (2) the effect of ice-phase volume fraction on the diffusional flux of water between crystals is accurately described, and (3) all relevant material properties (involving binary Fick diffusion coefficients, the thermodynamic factor of the solution, and the surface energy of ice) are carefully estimated. To enable calculation of material properties, we derive an alternative formulation of Ostwald ripening in terms of the MaxwellStefan instead of the Fick approach to diffusion. First, this leads to a cancellation of the thermodynamic factor (a measure for the nonideality of a solution), which is a notoriously difficult property to obtain. Second, we show that MaxwellStefan diffusion coefficients can to a reasonable approximation be related to self-diffusion coefficients, which are relatively easy to measure or predict in comparison to Fick diffusion coefficients. Our approach is validated for a binary system of water and sucrose, for which we show predicted recrystallization rates of ice compare well to experimental results, with relative deviations of at most a factor of 2
The Potential of Singlet Fission Photon Multipliers as an Alternative to Silicon-based Tandem Solar Cells
Get PDFSinglet fission, an exciton multiplication process in organic semiconductors which converts one singlet exciton into two triplet excitons is a promising way to reduce thermalization losses in conventional solar cells. One way to harvest triplet excitons is to transfer their energy into quantum dots, which then emit photons into an underlying solar cell. We simulate the performance potential of such a singlet fission photon multiplier combined with a silicon base cell and compare it to a silicon-based tandem solar cell. We calculate the influence of various loss-mechanisms on the performance potential under real-world operation conditions using a variety of silicon base cells with different efficiencies. We find that the photon multiplier is more stable against changes in the solar spectrum than two-terminal tandem solar cells. We furthermore find that, as the efficiency of the silicon base cell increases, the efficiency of the photon multiplier increases at a higher rate than the tandem solar cell. For current record silicon solar cells, the photon multiplier has the potential to increase the efficiency by up to 4.2% absolute
A Plausible Microtubule-Based Mechanism for Cell Division Orientation in Plant Embryogenesis
Get PDFOriented cell divisions are significant in plant morphogenesis because plant cells are embedded in cell walls and cannot relocate. Cell divisions follow various regular orientations, but the underlying mechanisms have not been clarified. We show that cell-shape dependent self organisation of cortical microtubule arrays is crucial for determining planes of early tissue-
generating divisions and forms the basis forrobust control of cell division orientation
in the embryo. To achieve this, we simulate microtubules on actual cell surface shapes from which we derive a minimal set of three rules for proper array
orientation. The first rule captures the effects of cell shape alone on microtubule organisation
, the second rule describes the regulation of microtubule stability at cell edges
and the third rule includes the differential effect of auxin on local microtubule stability. These rules explain early embryonic division plane orientations
and offer a framework for understanding patterned cell divisions in plant
morphogenesis