This SHEA white paper identifies knowledge gaps and challenges in healthcare epidemiology research related to coronavirus disease 2019 (COVID-19) with a focus on core principles of healthcare epidemiology. These gaps, revealed during the worst phases of the COVID-19 pandemic, are described in 10 sections: epidemiology, outbreak investigation, surveillance, isolation precaution practices, personal protective equipment (PPE), environmental contamination and disinfection, drug and supply shortages, antimicrobial stewardship, healthcare personnel (HCP) occupational safety, and return to work policies. Each section highlights three critical healthcare epidemiology research questions with detailed description provided in supplementary materials. This research agenda calls for translational studies from laboratory-based basic science research to well-designed, large-scale studies and health outcomes research. Research gaps and challenges related to nursing homes and social disparities are included. Collaborations across various disciplines, expertise and across diverse geographic locations will be critical.

We present the adaptive optics assisted, near-infrared VLTI instrument GRAVITY for precision narrow-angle astrometry and interferometric phase referenced imaging of faint objects. With its two fibers per telescope beam, its internal wavefront sensors and fringe tracker, and a novel metrology concept, GRAVITY will not only push the sensitivity far beyond what is offered today, but will also advance the astrometric accuracy for UTs to 10 μas. GRAVITY is designed to work with four telescopes, thus providing phase referenced imaging and astrometry for 6 baselines simultaneously. Its unique capabilities and sensitivity will open a new window for the observation of a wide range of objects, and — amongst others — will allow the study of motion within a few times the event horizon size of the Galactic Center black hole.

Higher open circuit voltages of the microcrystalline silicon bottom cell have a direct impact on the efficiency of the micromorph (μc-Si:H/a-Si:H) tandem cell. In this paper it is shown that open circuit voltages over 500 mV can be achieved leading to gc-Si:H cell efficiencies of 8.5 %. The behaviour of such cells is characterised both by the illuminated and the dark I-V characteristics in function of cell temperature. Microcrystalline cells with Voc-values higher than 500 mV and micromorph tandems possess in general a lower value of the temperature coefficient of the fill factor and thus of the efficiency, when compared to c-Si. Temperature-dependent dark I-V measurements suggest that the dominant recombination mechanism in lgc-Si:H cells is different from that prevailing in a-Si:H solar cells.

In the past, microcrystalline silicon (μc-Si:H) has been successfully used as active semiconductor in entirely μc-Si:H p-i-n solar cells and a new type of tandem solar cell, called the “micromorph” cell, was introduced [1]. Micromorph cells consist of an amorphous silicon top cell and a microcrystalline bottom cell. In the paper a micromorph cell with a stable efficiency of 10.7 % (confirmed by ISE Freiburg) is reported.

Among sofar existing crystalline silicon-based solar cell manufacturing techniques, the application of microcrystalline silicon is a new promising way towards implementing thin-film silicon solar cells with a low temperature deposition. Microcrystalline silicon can, indeed, be deposited at temperatures as low as 220°C; hence, the way is here open to use cheap substrates as, e.g. plastic or glass. In the present paper, the development of single and tandem cells containing microcrystalline silicon is reviewed. As stated in previous publications, microcrystalline silicon technique has at present a severe drawback that has yet to be overcome: Its deposition rate for solar-grade material is about 2Å/s; in a more recent case 4.3 Å/s [2] could be obtained. In the present paper, using suitable mixtures of silane, hydrogen and argon, deposition rates of 9.4 Å/s are presented. Thereby the dominating plasma mechanism and the basic properties of resulting layers are described in detail. A first entirely microcrystalline cell deposited at 8.7 Å/s has an efficiency of 3.15%.

We performed plasma impedance measurements at room temperature for a hydrogen plasma using an impedance analyser. The plasma excitation frequency range spans from 40 to 70 MHz. Both the real and imaginary part of the impedance decrease monotonously with increasing frequency. These measurements are in agreement with prior experimental observations [1], that the required peak-to-peak voltage between the electrodes is reduced at higher excitation frequencies. Using a simple equivalent circuit for the plasma this effect can be mainly attributed to the increased sheath capacitance. Furthermore, by modelling the sheath with a simple parallel plate capacitor, its thickness could be estimated: it decreases from 2 MM at 40 MHz to about 1.4 MM at 70 MHz plasma excitation frequency. Finally, a possible link between the decreasing sheath thickness on the increase of deposition rate is discussed.

Structural, optical and electrical properties of a-Si:H films prepared with the VHF Glow Discharge system were studied as function of deposition temperature, particularly in the range below 100°C. Usually, this material has poor optoelectronic properties. The a-Si:H material discussed in this work shows a surprisingly low hydrogen content, a low dihydride to monohydride ratio, good optical and electrical properties.

We have grown two series, ‘SLOW’ and ‘FAST’, of high quality a-Si:H films by the VHF-GD (Very-High-Frequency Glow-Discharge) technique, the first at a low deposition rate of R∼3.6 Å/sec and the other at a high rate of R˜17 Å/sec. Both series show the same bulk deep defect density Nb≈4×1015/cm3 in the annealed state ‘A’ as measured by PDS. From lightinduced degradation experiments (Staebler-Wronski effect, SWE) the following interesting features are observed : a) The SWE is a bulk effect. To demonstrate this result we propose the use of PDS in combination with variable film thicknesses as a powerful and unambiguous method, b) The increase of the deep bulk defect density ΔNb up to the surated state of the SWE (state ‘B’) is the same for series ‘SLOW and ‘FAST’, c) However, the kinetics of the SWE differ considerably.

These results are interpreted with the help of experimentally observed continuous variation of the Urbach energy E0 of the valence bandtail accross the film. The E0 profile is, in turn, related to the deposition rate R. The presence of this inhomofgenei yis also discussed in context with.the recently developed equilibration theories relating deep and shallow (bandtail) states. Preliminary results in this study seem also to support the model involving conversion of weak Si-Si bonds into dangling bond upon light soaking. Further the influence of internal film stress as well as the effect of impurities on the degradation behaviour are addressed.

We have performed a coordinated experimental and theoretical investigation of the structure and stability of Ti/C multilayers. The samples were prepared by conventional techniques and the lattice structure was characterized by X-ray diffraction. Electron Energy Loss and Auger spectroscopies (EELS,AS), in conjunction with Rutherford Backscattering (RBS) were employed to characterize the chemical composition of the samples. It was observed that the Ti/C interface is isolated by a thin titanium oxide layer. The center of the Ti layer gives a typical EELS spectrum of Ti metal. The carbon shows a graphitic character. We have used total energy calculations to study the crystallographic structure and electronic behavior at the interface in Ti-C superlattices. This involved examining a series of structures in the Ti-C system proceeding from the ground state NaCl structure to superlattices. In particular we performed total energy calculations for an arrangement of atoms designed to allow direct investigation of the competition between Ti-C bonds and C-C bonds. We conclude that the equilibrium structure is dominated by C-C bonding and so we find that the interface has a graphite like atomic arrangement rather than a carbide like arrangement. We than further explored the interactions in this graphitic arrangement and found that the interface does not have a significant Ti-C bonding and that the interface most likely consists of simply a layer of graphite adjoining a titanium surface. This carries important implications for the diffusion and bonding at the interface.