Vesicles consisting of a bilayer membrane of amphiphilic lipid molecules are remarkably flexible surfaces that show an amazing variety of shapes of different symmetry and topology. Owing to the fluidity of the membrane, shape transitions such as budding can be induced by temperature changes or the action of optical tweezers. Thermally excited shape fluctuations are both strong and slow enough to be visible by video microscopy. Depending on the physical conditions, vesicles adhere to and unbind from each other or a substrate. This article describes the systematic physical theory developed to understand the static and dynamic aspects of membrane and vesicle configurations. The preferred shapes arise from a competition between curvature energy, which derives from the bending elasticity of the membrane, geometrical constraints such as fixed surface area and fixed enclosed volume, and a signature of the bilayer aspect. These shapes of lowest energy are arranged into phase diagrams, which separate regi...

Configurations of fluid membranes and vesicles

By focusing electrons on probes with a diameter of 50 pm, aberration-corrected scanning transmission electron microscopy (STEM) is currently crossing the border to probing subatomic details. A major challenge is the measurement of atomic electric fields using differential phase contrast (DPC) microscopy, traditionally exploiting the concept of a field-induced shift of diffraction patterns. Here we present a simplified quantum theoretical interpretation of DPC. This enables us to calculate the momentum transferred to the STEM probe from diffracted intensities recorded on a pixel array instead of conventional segmented bright-field detectors. The methodical development yielding atomic electric field, charge and electron density is performed using simulations for binary GaN as an ideal model system. We then present a detailed experimental study of SrTiO3 yielding atomic electric fields, validated by comprehensive simulations. With this interpretation and upgraded instrumentation, STEM is capable of quantifying atomic electric fields and high-contrast imaging of light atoms.

/pdf/atomic-electric-fields-revealed-by-a-quantum-mechanical-1b0emuc0dc.pdf

Atomic electric fields revealed by a quantum mechanical approach to electron picodiffraction

Flexible electronic devices have attracted a great deal of attention in recent years due to their flexibility, reduced complexity and lightweight. Such devices can conformably attach themselves to any bendable surface and can possess diverse transduction mechanisms. Consequently, with continued emphasis on innovation and development, major technological breakthroughs have been achieved in this area. This review focuses on the advancements of using organic field-effect transistors (OFETs) in flexible electronic applications in the past 10 years. In addition, to the above mentioned features, OFETs have multiple advantages such as low-cost, readout integration, large-area coverage, and power efficiency, which yield synergy. To begin with, we have introduced organic semiconductors (OSCs), followed by their applications in various device configurations and their mechanisms. Later, the use of OFETs in flexible sensor applications is detailed with multiple examples. Special attention is paid to discussing the effects induced on physical parameters of OFETs with respect to variations in external stimuli. The final section provides an outlook on the mechanical aspects of OSCs, activation and revival processes of sensory layers, small area analysis, and pattern recognition techniques for electronic devices.

Organic field-effect transistor-based flexible sensors

Recombination processes in rare-earth metals in semiconductors are a special case due to the localized nature of $f$ electrons. Our work explores in detail the radiative and nonradiative mechanisms of energy transfer for erbium in silicon by investigating the temperature dependence of the intensity and the decay time of the photoluminescence of Er-related centers in Si. We show that nonradiative energy back transfer from the excited Er $4f$ shell causes luminescence quenching below 200 K. We study electroluminescence decay by applying different bias conditions during the decay. In a two-beam experiment the photoluminescence decay is monitored for different background-excitation laser powers. Changes in the decay time are strong evidence of the impurity Auger effect as an efficient luminescence-quenching mechanism for Er in Si. A fast initial luminescence decay component at high pumping powers is related to quenching by excess carriers. The power dependence, the decay-time components, and the two-beam experiment are simulated by a set of rate equations which involve the formation of excitons, a decrease of the pumping efficiency by exciton Auger recombination, and a decrease of radiative efficiency by the impurity Auger effect with free electrons. As a nonradiative deexcitation path competing with spontaneous emission, the impurity Auger effect decreases the excited-state lifetime of Er in Si, and dominates the thermal quenching of luminescence in the temperature range from 4 to 100 K. We find that the decrease of emission intensity above 100 K is caused by an unidentified second back-transfer process.

Electroluminescence of erbium-doped silicon

ZnO single crystals, epilayers, and nanostructures often exhibit a variety of narrow emission lines in the spectral range between 3.33 and 3.35 eV which are commonly attributed to deeply bound excitons ($Y$ lines). In this work, we present a comprehensive study of the properties of the deeply bound excitons with particular focus on the ${Y}_{0}$ transition at 3.333 eV. The electronic and optical properties of these centers are compared to those of the shallow impurity related exciton binding centers ($I$ lines). In contrast to the shallow donors in ZnO, the deeply bound exciton complexes exhibit a large discrepancy between the thermal activation energy and localization energy of the excitons and cannot be described by an effective mass approach. The different properties between the shallow and deeply bound excitons are also reflected by an exceptionally small coupling of the deep centers to the lattice phonons and a small splitting between their two electron satellite transitions. Based on a multitude of different experimental results including magnetophotoluminescence, magnetoabsorption, excitation spectroscopy (PLE), time resolved photoluminescence (TRPL), and uniaxial pressure measurements, a qualitative defect model is developed which explains all $Y$ lines as radiative recombinations of excitons bound to extended structural defect complexes. These defect complexes introduce additional donor states in ZnO. Furthermore, the spatially localized character of the defect centers is visualized in contrast to the homogeneous distribution of shallow impurity centers by monochromatic cathodoluminescence imaging. A possible relation between the defect bound excitons and the green luminescence band in ZnO is discussed. The optical properties of the defect transitions are compared to similar luminescence lines related to defect and dislocation bound excitons in other II--VI and III--V semiconductors.

/pdf/bound-excitons-in-zno-structural-defect-complexes-versus-3rqczj0meo.pdf

Bound excitons in ZnO: Structural defect complexes versus shallow impurity centers

Indexed by Scopus  The series Springer Proceedings in Physics, founded in 1984, is devoted to timely reports of state-of-the-art developments in physics and related sciences. Typically based on material presented at conferences, workshops and similar scientific meetings, volumes published in this series will constitute a comprehensive up-to-date source of reference on a field or subfield of relevance in contemporary physics. Proposals must include the following:  – name, place and date of the scientific meeting  – a link to the committees (local organization, international advisors etc.)  – scientific description of the meeting  – list of invited/plenary speakers  – an estimate of the planned proceedings book parameters (number of pages/articles, requested number of bulk copies, submission deadline).

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