Bulk and Surface Nucleation Processes in Ag2S Conductance Switches

We studied metallic Ag formation inside and on the surface of Ag2S thin films, induced by the electric field created with a STM tip. Two clear regimes were observed: cluster formation on the surface at low bias voltages, and full conductance switching at higher bias voltages (V > 70mV). The bias voltage at which this transition is observed is in agreement with the known threshold voltage for conductance switching at room temperature. We propose a model for the cluster formation at low bias voltage. Scaling of the measured data with the proposed model indicates that the process takes place near steady state, but depends on the STM tip geometry. The growth of the clusters is confirmed by tip retraction measurements and topography scans. This study provides improved understanding of the physical mechanisms that drive conductance switching in solid electrolyte memristive devices.Comment: In press for PR

Resistive switching in mixed conductors : Ag2S as a model system

Resistive switching memories have gained an increased interest due to the possibilities for downscaling of memory devices down to a few nanometers. These memories consist of a resistive material sandwiched between two metal electrodes, and applying a voltage between them induces resistance switching. In this thesis we study the specific case when switching is due to the reversible formation of a conductive path that connects and disconnects the electrodes. We investigate the electrical conductance properties and transport mechanisms in solid electrolyte memory devices, to gain a fundamental understanding of conductance switching. Our model system consists of Ag2S thin films with a Ag bottom electrode and a Pt AFM or STM tip as top electrode. We present a quantitative analysis of the steady state transport that leads up to resistance switching. We discuss the relation between stoichiometry and resistance changes in the material, and the necessity of Ag supersaturation prior to nucleation and switching. We also discuss the possible presence of two distinct switching mechanisms in Ag2S. Our findings could be extended to other semiconductor materials with mobile donors or acceptors.LEI Universiteit LeidenFOMQuantum Matter and Optic

Determining fresh fruit and vegetable program (FFVP) implementation correlates with fruit and vegetable outcomes in K-2nd graders

Background: The positive health effects of fruits and vegetables (FV) are well known. School-based FV interventions have been used as preventative child health strategies and may include nutrition education, gardening, or FV distribution. The Fresh Fruit and Vegetable Program (FFVP) supports low-income schools in providing FV as snacks. The objectives of this research were to evaluate available K-2nd grade nutrition curricula related to FV, to determine differences in FV outcomes in an FFVP and non-FFVP school, determine the impact of FFVP on FV preferences, and evaluate the FFVP statewide in Illinois. Methods: Web of Science, EBSCO, and PubMed were searched for articles with named curricula and curricula impact for K-2nd grade. Publications listed (n=5,498) were reviewed for relevancy (grade, curricula named, impact). To evaluate differences in an FFVP and a non-FFVP school, a FV Preference survey was developed for K-2nd graders (12 fruits/12 vegetables). Data were collected from K-2nd graders (n=435, FFVP school n=235 with 12 teachers, non-FFVP school n=200, 10 teachers). Fruits (F=28) and vegetables (V=29) were distributed twice/week over 35 weeks at the participating FFVP school. Preference ratings were analyzed over time. FFVP surveys were developed and distributed to Illinois schools to assess implementation of the program statewide. Additionally, a scoring index was created to classify schools as low/high implementers of the FFVP. Results: Twelve nutrition curricula were found within 11 publications. Most had control groups (n=9); were part of multi-component studies (n=11); and curricula included food model use, healthy eating, and food groups. Regarding preferences, there were significant differences in mean preference scores, with higher fruit scores at the FFVP school (P50%) reported coordinating FV from FFVP with school-wide NE activities, and almost 80% reported having a committee in place for the FFVP. A low percentage of teachers and FFVP coordinators noted receiving training for the FFVP (4.4 to 44.8%). A school categorized as a high implementer, according to the FFVP teacher survey, was a significant predictive variable for the amount of FV consumed by children. However, more teachers and coordinators noted that children consumed all or most of the fruits compared to vegetables. Conclusions: The 12 curricula found had minimal research supporting impact. For differences between the FFVP and non-FFVP school, results suggest the students at the FFVP school had better FV identification. At the FFVP school, it was found that, overall, preference ratings were negatively impacted by time, grade level, and vegetables served. Being exposed to a variety of FV, generally, did not improve ratings for vegetables. Finally, FFVP surveys distributed to schools statewide in Illinois revealed differences between preferences and consumption patterns of children for FV according to teacher and coordinator surveys. High implementer schools may be predictive of children’s consumption of FV. More research is needed to determine factors of implementation that are particularly impactful and methods of improving implementation of the FFVP

Observing “Quantized” Conductance Steps in Silver Sulfide: Two Paralell Resistive Switching Mechanisms

014302Quantum Matter and Optic

Towards a quantitative description of solid electrolyte conductance switches

Quantum Matter and Optic

Single-Source Pulsed Laser Deposited Perovskite Solar Cells with > 19% Efficiency

Single-source vapor deposition of metal halide perovskites has, to date, remained challenging due to the dissimilar volatilities of the perovskite precursors, limiting the controlled transfer of multiple elements at once. This Chapter demonstrates that pulsed laser deposition (PLD) addresses the rate-control challenges of single-source evaporation, enabling solar cells with power conversion efficiencies (PCE) above 19%. For this, we combined dry mechanochemical synthesis and PLD to fabricate MA1-xFAxPbI3 and Cl-passivated MA1-xFAxPbI3 films from a single-source target. The films are grown onto hole-selective self-assembled monolayers (SAMs-2PACz), where first a thin PbI2-rich layer forms, leading to full perovskite conversion as confirmed by grazing-incidence wide-angle X-ray scattering. Onto the perovskite, an oleylammonium iodide (OAmI) post-treatment is then applied to passivate its top surface by forming a 2D perovskite film. This was followed via in-situ PL monitoring during the 2D application. Further, we found that when incorporating PbCl2 in the target and OAmI-based 2D passivation, a remarkable 19.7% PCE for p–i–n perovskite solar cells is achieved with enhanced device stability. These findings emphasize the importance of interface and passivation strategies to improve the performance of PSC-containing vapor-deposited absorbers. Further, these results represent one of the highest PCEs achieved within the state-of-the-art single-source vapor deposition methods, as far as our knowledge extends. Consequently, this study highlights the appeal of PLD to fully unlock the potential of single-source vapor-deposited perovskite towards low-cost and efficient photovoltaics

Toward Annealing Stable Molybdenum Oxide Based Hole Selective Contacts For Silicon Photovoltaics

Molybdenum oxide MoOX combines a high work function with broadband optical transparency. Sandwiched between a hydrogenated intrinsic amorphous silicon passivation layer and a transparent conductive oxide, this material allows a highly efficient hole selective front contact stack for crystalline silicon solar cells. However, hole extraction from the Si wafer and transport through this stack degrades upon annealing at 190 C, which is needed to cure the screen printed Ag metallization applied to typical Si solar cells. Here, we show that effusion of hydrogen from the adjacent layers is a likely cause for this degradation, highlighting the need for hydrogen lean passivation layers when using such metal oxide based carrier selective contacts. Pre MoOX deposition annealing of the passivating a Si H layer is shown to be a straightforward approach to manufacturing MoOX based devices with high fill factors using screen printed metallization cured at 190

Single-Source, Solvent-Free, Room Temperature Deposition of Black γ-CsSnI₃ Films

The presence of a non-optically active polymorph (yellow-phase) competing with the optically active polymorph (black $\gamma$-phase) at room temperature in CsSnI3 and the susceptibility of Sn to oxidation, represent two of the biggest obstacles for the exploitation of CsSnI3 in optoelectronic devices. Here room-temperature single-source in vacuum deposition of smooth black $\gamma$ - CsSnI3 thin films is reported. This has been done by fabricating a solid target by completely solvent-free mixing of CsI and SnI2 powders and isostatic pressing. By controlled laser ablation of the solid target on an arbitrary substrate at room temperature, the formation of CsSnI3 thin films with optimal optical properties is demonstrated. The films present a band gap of 1.32 eV, a sharp absorption edge and near-infrared photoluminescence emission. These properties and X-ray diffraction of the thin films confirmed the formation of the orthorhombic (B-$\gamma$) perovskite phase. The thermal stability of the phase was ensured by applying in situ an Al2O$_3$ capping layer. This work demonstrates the potential of pulsed laser deposition as a volatility-insensitive single-source growth technique of halide perovskites and represents a critical step forward in the development and future scalability of inorganic lead-free halide perovskites.Comment: Accepted by Advanced Materials Interfaces, 16 pages, 4 figures, and supplemen

Commentaries on ‘Hand washing for preventing diarrhoea’, with a response from the review authors

944-950These are commentaries on a Cochrane review, published in this issue of EBCH, first published as: Ejemot RI, Ehiri JE, Meremikwu MM, Critchley JA. Hand washing for preventing diarrhoea. Cochrane Database of Systematic Reviews 2008, Issue 1. Art. No.: CD004265. DOI: 10.1002/14651858.CD004265.pub2