Structure and electronic transport in graphene wrinkles

Wrinkling is a ubiquitous phenomenon in two-dimensional membranes. In particular, in the large-scale growth of graphene on metallic substrates, high densities of wrinkles are commonly observed. Despite their prevalence and potential impact on large-scale graphene electronics, relatively little is known about their structural morphology and electronic properties. Surveying the graphene landscape using atomic force microscopy, we found that wrinkles reach a certain maximum height before folding over. Calculations of the energetics explain the morphological transition, and indicate that the tall ripples are collapsed into narrow standing wrinkles by van der Waals forces, analogous to large-diameter nanotubes. Quantum transport calculations show that conductance through these collapsed wrinkle structures is limited mainly by a density-of-states bottleneck and by interlayer tunneling across the collapsed bilayer region. Also through systematic measurements across large numbers of devices with wide folded wrinkles, we find a distinct anisotropy in their electrical resistivity, consistent with our transport simulations. These results highlight the coupling between morphology and electronic properties, which has important practical implications for large-scale high-speed graphene electronics.Comment: 5 figures supplemental information in separated fil

An algorithm for calculating the Lorentz angle in silicon detectors

Future experiments will use silicon sensors in the harsh radiation environment of the LHC (Large Hadron Collider) and high magnetic fields. The drift direction of the charge carriers is affected by the Lorentz force due to the high magnetic field. Also the resulting radiation damage changes the properties of the drift. In this paper measurements of the Lorentz angle of electrons and holes before and after irradiation are reviewed and compared with a simple algorithm to compute the Lorentz angle.Comment: 13 pages, 7 figures, final version accepted by NIMA. Mainly clarifications included and slightly shortene

Relating 3D Geometry and Photoelectrochemical Activity of WO3-loaded n-Si Nanowires: Design Rules for Photoelectrodes

Nanostructured electrodes for photoelectrochemical (PEC) applications, such as water splitting, have rather low photocurrent density regarding their highly enlarged surface area compared to plain electrodes. This demands for further understanding of the relation between the 3D geometry and the PEC activity. To this end, we fabricate WO3/Si nanowire array photoanodes with various nanowire lengths (1.3 µm, 2.7 µm, 3.2 µm and 3.8 µm) and different WO3 thicknesses (10 nm, 30 nm and 50 nm) using wet chemical etching for nanostructuring of Si and atomic layer deposition for the deposition of WO3. It is found that by increasing the etching time, the nanowires become longer and the top surface area decreases. The photocurrent density first increases and then decreases with increasing Si etching time. This behaviour can be explained by different and opposite effects regarding absorption, geometry and materials specific properties. Particularly, the decrease of the photocurrent density can be due to: First, the longer the nanowires the heavier the recombination of the photogenerated carriers. Second, the long-time Si etching results in a loss of top part of the nanowire arrays. Because of shadowing, the WO3 located at the top part of the nanowires is more effective than that at the bottom part for the WO3/Si nanowire arrays and therefore the photocurrent is decreased. It reveals a trade-off between the top part surface area and the length of the nanowires. This study contributes to a better understand of the relation between the geometry of nanostructures and the performance of PEC electrodes.</p

Diamond thin Film Detectors for Beam Monitoring Devices

Diamonds offer radiation hard sensors, which can be used directly in primary beams. Here we report on the use of a polycrystalline CVD diamond strip sensor as beam monitor of heavy ion beams with up to 2.10^9 lead ions per bunch. The strips allow for a determination of the transverse beam profile to a fraction of the pitch of the strips, while the timing information yields the longitudinal bunch length with a resolution of the order of a few mm.Comment: 6 pages, 7 figures, to appear in the Proceedings of the Hasselt Diamond Workshop (Hasselt, Belgium, Feb. 2006), v4: accidentally submitted figure, appearing at end, remove

Preferences for different nitrogen forms by co-existing plant species and soil microbes: reply

The growing awareness that plants might use a variety of nitrogen (N) forms, both organic and inorganic, has raised questions about the role of resource partitioning in plant communities. It has been proposed that coexisting plant species might be able to partition a limited N pool, thereby avoiding competition for resources, through the uptake of different chemical forms of N. In this study, we used in situ stable isotope labeling techniques to assess whether coexisting plant species of a temperate grassland (England, UK) display preferences for different chemical forms of N, including inorganic N and a range of amino acids of varying complexity. We also tested whether plants and soil microbes differ in their preference for different N forms, thereby relaxing competition for this limiting resource. We examined preferential uptake of a range of 13C15N-labeled amino acids (glycine, serine, and phenylalanine) and 15N-labeled inorganic N by coexisting grass species and soil microbes in the field. Our data show that while coexisting plant species simultaneously take up a variety of N forms, including inorganic N and amino acids, they all showed a preference for inorganic N over organic N and for simple over the more complex amino acids. Soil microbes outcompeted plants for added N after 50 hours, but in the long term (33 days) the proportion of added 15N contained in the plant pool increased for all N forms except for phenylalanine, while the proportion in the microbial biomass declined relative to the first harvest. These findings suggest that in the longer term plants become more effective competitors for added 15N. This might be due to microbial turnover releasing 15N back into the plant–soil system or to the mineralization and subsequent plant uptake of 15N transferred initially to the organic matter pool. We found no evidence that soil microbes preferentially utilize any of the N forms added, despite previous studies showing that microbial preferences for N forms vary over time. Our data suggest that coexisting plants can outcompete microbes for a variety of N forms, but that such plant species show similar preferences for inorganic over organic N

ALD-grown two-dimensional TiSx metal contacts for MoS2 field-effect transistors

Metal contacts to MoS2 field-effect transistors (FETs) play a determinant role in the device electrical characteristics and need to be chosen carefully. Because of the Schottky barrier (SB) and the Fermi level pinning (FLP) effects that occur at the contact/MoS2 interface, MoS2 FETs often suffer from high contact resistance (Rc). One way to overcome this issue is to replace the conventional 3D bulk metal contacts with 2D counterparts. Herein, we investigate 2D metallic TiSx (x ∼ 1.8) as top contacts for MoS2 FETs. We employ atomic layer deposition (ALD) for the synthesis of both the MoS2 channels as well as the TiSx contacts and assess the electrical performance of the fabricated devices. Various thicknesses of TiSx are grown on MoS2, and the resultant devices are electrically compared to the ones with the conventional Ti metal contacts. Our findings show that the replacement of 5 nm Ti bulk contacts with only ∼1.2 nm of 2D TiSx is beneficial in improving the overall device metrics. With such ultrathin TiSx contacts, the ON-state current (ION) triples and increases to ∼35 μA μm−1. Rc also reduces by a factor of four and reaches ∼5 MΩ μm. Such performance enhancements were observed despite the SB formed at the TiSx/MoS2 interface is believed to be higher than the SB formed at the Ti/MoS2 interface. These device metric improvements could therefore be mainly associated with an increased level of electrostatic doping in MoS2, as a result of using 2D TiSx for contacting the 2D MoS2. Our findings are also well supported by TCAD device simulations.<br/

Effects of Motivational Prompts on Motivation, Effort, and Performance on a Low-Stakes Standardized Test

Increased demands for accountability have placed an emphasis on assessment of student learning outcomes. At the post-secondary level, many of the assessments are considered low-stakes, as student performance is linked to few, if any, individual consequences. Given the prevalence of low-stakes assessment of student learning, research that investigates the relationship between student motivation, effort, and performance on low-stakes tests is warranted as these tests are increasingly being used to make judgments about the quality of student learning. This quasi-experimental study was conducted at a public mid-sized university with 87 undergraduate students enrolled in four 100-level general education courses. The researchers examined the effects of motivational prompts on student motivation, effort, and performance on a low-stakes test. Results indicated that motivational condition had a significant effect on students\u27 performance as measured by total mean scores on a low-stakes standardized test. Students in the personal motivational condition outperformed students in the other conditions. However, motivational prompts were not found to affect students\u27 critical thinking subscores or self-reported effort and importance scores

CONTRACTING-OUT OF PRIMARY HEALTH CARE SERVICES IN CONFLICT-AFFECTED SETTINGS: THE CASE OF SOUTH SUDAN

South Sudan introduced a nation-wide geographically focused contracting approach with funding from 3 main donors, to rapidly scale-up access to health services starting mid-2012. The overall aim of the thesis is to assesses its effect and impact on maternal and child health. In the first paper, I provide extensive background on the history of health services delivery in South Sudan prior to assessing progress in increasing health facility utilization among women and children under-five, using routine health facility data. I thus compare the period (2011-2013) to the policy period (2013-2015) at national and sub-national level using Prais-Winsten and Cochrane-Orcutt regression and find significant increases in health facility utilization. Given the overall increase in health facility utilization, the second paper assesses impact of the policy on child mortality using a novel approach to construct a synthetic South Sudan from a panel of lower- and middle-income countries (LMICs), and using data from the World Bank Developmental Indicators (WDI) database. The analysis shows that on average, contracting out had a negative effect on the rate of decline of U5MR during the policy period. These findings suggest limitations in the approach implemented in South Sudan. The third paper further evaluates the contracting intervention by evaluating three contracting approaches in Jonglei and Upper Nile States; contracting-out (C-O) using non-governmental organizations (NGOs), contracting-in (C-I) using county health departments (CHD) and performance-based contracting (PBC) in select counties. Using difference-in difference (DD) and DD with propensity score matching. I hypothesized that contracting-out to NGOs leads to higher performance relative to contracting-in with CHDs, that performance-based contracting incentivizes health workers, hence PBC counties have higher utilization relative to non-PBC counties. Results are not as straightforward; there are no significant differences in the double differences for ANC 4th visits, health facility deliveries, DPT 1, DPT 3 and diarrhoea. Additionally, PBC had no effect on utilization relative to counties where there was no PBC incentive