Tuning the Structural and Optoelectronic Properties of Cs2AgBiBr6 Double-Perovskite Single Crystals through Alkali-Metal Substitution

Lead-free double perovskites have great potential as stable and nontoxic optoelectronic materials. Recently, Cs₂AgBiBr₆ has emerged as a promising material, with suboptimal photon-to-charge carrier conversion efficiency, yet well suited for high-energy photon-detection applications. Here, the optoelectronic and structural properties of pure Cs₂AgBiBr₆ and alkali-metal-substituted (Cs_1−xY_x)₂AgBiBr₆ (Y: Rb⁺, K⁺, Na⁺; x = 0.02) single crystals are investigated. Strikingly, alkali-substitution entails a tunability to the material system in its response to X-rays and structural properties that is most strongly revealed in Rb-substituted compounds whose X-ray sensitivity outperforms other double-perovskite-based devices reported. While the fundamental nature and magnitude of the bandgap remains unchanged, the alkali-substituted materials exhibit a threefold boost in their fundamental carrier recombination lifetime at room temperature. Moreover, an enhanced electron–acoustic phonon scattering is found compared to Cs₂AgBiBr₆. The study thus paves the way for employing cation substitution to tune the properties of double perovskites toward a new material platform for optoelectronics.     

Lessons learned from the design and evaluation of visual information-seeking systems

Designing information-seeking systems has become an increasingly complex task as today’s information spaces are rapidly growing in quantity, heterogeneity, and dimensionality. The challenge is to provide user interfaces that have a satisfying usability and user experience even for novice users. Although information visualization and interaction design offer solutions, many information-seeking systems such as online catalogs for libraries or web search engines continue to use outdated user-interface concepts developed decades ago. In this paper, we will present four principles that we identified as crucial for the successful design of a modern visual information-seeking system. These are (1) to support various ways of formulating an information need, (2) to integrate analytical and browsing-oriented ways of exploration, (3) to provide views on different dimensions of the information space, and (4) to make search a pleasurable experience. These design principles are based on our experience over a long period in the user-centered design and evaluation of visual information-seeking systems. Accordingly, we will showcase individual designs from our own work of the past 10 years to illustrate each principle and hence narrow the gap between the scientific discussion and the designing practitioner that has often hindered research ideas from becoming reality. However, most of the times search is only one part of a higher level user activity (e.g. writing a paper). Thus future research should focus on the challenges when regarding search in such a broader context. We will use the final two chapters to point out some of these challenges and outline our vision of an integrated and consistent digital work environment named Zoomable Object-oriented Information Landscape.     

Second Harmonic Generation Investigation of Symmetry Breaking and Flexoelectricity Induced by Nanoindentations in SrTiO3

Strain gradients induced by nanoindentations in bulk undoped strontium titanate (SrTiO₃) are investigated. After indenting a 850 nm deep pattern on the SrTiO₃ surface with a 8 GPa hardness and an 83 GPa indentation modulus by a Berkovich tip, the resulting deformation patterns are imaged by Raman spectroscopy and by second harmonic generation microscopy. Cross‐shaped compressively stressed regions along the cubic SrTiO₃ primary slip planes are observed. These zones relax toward the unstrained bulk material through localized strain gradients on a length scale of several microns. Second harmonic tomography reveals the reduction of the crystal's centrosymmetry in the vicinity of the crosslike features due to a strain gradient. These results indicate the appearance of flexoelectricity following the nanoindentation process due to the creation of an inhomogeneous strain.     

Modulation doping and energy filtering as effective ways to improve the thermoelectric power factor

Thermoelectric (TE) materials have undergone revolutionary progress over the last 20 years. The thermoelectric figure of merit ZT, which quantifies the ability of a material to convert heat into electricity has more than doubled compared to traditional values of \(ZT\sim 1\), reaching values even beyond \(ZT\sim 2\) in some instances. These improvements are mostly attributed to drastic reductions of the thermal conductivity in nanostructured materials and nanocomposites. However, as thermal conductivities in these structures approach the amorphous limit, any further benefits to ZT must be achieved through the improvement of the thermoelectric power factor. In this work we review two of the most promising avenues to increase the power factor, namely (i) modulation doping and (ii) electron energy filtering, and present a computational framework for analysis of these mechanisms for two example cases: low-dimensional gated Si nanowires (electrostatically achieved doping), and superlattices (energy filtering over potential barriers). In the first case, we show that a material with high charge density, but free of ionized impurities, can provide up to a five-fold thermoelectric power factors increase compared to the power factor of the doped material, which highlights the benefits of modulation doping, or gating of materials. In the second case, we show that optimized construction of energy barriers within a superlattice material geometry can improve the power factor by up to \(\sim 30\,\%\). This paper is intended to be a review of our main findings with regards to efforts to improve the thermoelectric power factor through modulation doping and energy filtering.     

Note: tip enhanced Raman spectroscopy with objective scanner on opaque samples

We report on 14 nm lateral resolution in tip-enhanced Raman spectroscopy mapping of carbon nanotubes with an experimental setup that has been designed for the analysis of opaque samples in confocal side-access through a novel piezo-driven objective scanner. The objective scanner allows for fast and stable laser-to-tip alignment and for the adjustment of the focus position with sub-wavelength precision to optimize the excitation of surface plasmons. It also offers the additional benefit of imaging the near-field generated Raman scattering at the gap between tip and sample as direct control of the tip enhancement.     

Dark chocolate’s compositional effects revealed by oscillatory rheology

In this study, two types of oscillatory shear rheology are applied on dark chocolate with varying volume fraction, particle size distribution, and soy lecithin concentration. The first, a conventional strain sweep, allows for the separation of the elastic and viscous properties during the yielding. The second, a constant strain rate sweep, where the strain rate amplitude is fixed as the frequency is varied, is analyzed to obtain Lissajous curves, dissipated energy, and higher order nonlinear contributions. It is shown that chocolate exhibits complex nonlinear behavior, namely shear thinning, shear thickening, and strain stiffening. The effects on this behavior related to volume fraction, particle size distribution, and lecithin concentration are investigated, and comparison with simple monodisperse hard-sphere suspensions is made.     

Life cycle assessment of village electrification based on straight jatropha oil in Chhattisgarh,India

A decentralized power generation plant fuelled by straight jatropha oil was implemented in 2006 in Ranidhera, Chhattisgarh, India. The goal of this study was to assess the environmental sustainability of that electrification project in order to provide a scientific basis for policy decisions on electrifying remote villages.A full Life Cycle Assessment (LCA) was conducted on jatropha-based rural electrification and then compared with other electrification approaches such as photovoltaic (PV), grid connection and a diesel-fuelled power generator. In summary, the jatropha-based electrification in Ranidhera reduces greenhouse gas emissions over the full life cycle by a factor of 7 compared to a diesel generator or grid connection. The environmental performance is only slightly improved, mainly due to the high air pollution from pre-heating the jatropha seeds. With additional measures oil extraction and overall efficiency could be further improved. However, environmental benefits can only be achieved if jatropha is cultivated on marginal land and land use competition can be excluded. Under these conditions, jatropha-based electricity generation might be a useful alternative to other renewable electrification options, as the technology is very sturdy and can be maintained even in remote and highly under-developed regions.     

Analysis of the Backscattering Coefficient of Salt-Affected Soils Using Modeling and RADARSAT-1 SAR Data

Salt significantly changes the backscattering coefficient of wet soil. This is easily observed on RADARSAT-1 synthetic aperture radar (SAR) images acquired over a salty depression located in the Egyptian desert. The aim of this paper is to use backscattering models to understand the behavior of the backscattering coefficient over salt-affected soils and to evaluate the possibility of monitoring the salt content. Simulations conducted over salt-affected soils show a lower sensitivity of these models to soil moisture compared to nonaffected soils. Besides, there is no model suitable to represent the variation of the backscattering coefficient due to changes in soil salinity. These results are discussed with regard to the magnitude of the two components of the dielectric constant (epsiv' and epsiv'). Based on a series of relationships, we propose a parametric formulation that allows us to determine the salinity acting on RADARSAT-1 SAR images without any use of backscattering models