TY - JOUR A1 - Lange, Ilja A1 - Reiter, Sina A1 - Paetzel, Michael A1 - Zykov, Anton A1 - Nefedov, Alexei A1 - Hildebrandt, Jana A1 - Hecht, Stefan A1 - Kowarik, Stefan A1 - Woell, Christof A1 - Heimel, Georg A1 - Neher, Dieter T1 - Tuning the work function of polar zinc oxide surfaces using modified phosphonic acid self-assembled monolayers JF - Advanced functional materials N2 - Zinc oxide (ZnO) is regarded as a promising alternative material for transparent conductive electrodes in optoelectronic devices. However, ZnO suffers from poor chemical stability. ZnO also has a moderate work function (WF), which results in substantial charge injection barriers into common (organic) semiconductors that constitute the active layer in a device. Controlling and tuning the ZnO WF is therefore necessary but challenging. Here, a variety of phosphonic acid based self-assembled monolayers (SAMs) deposited on ZnO surfaces are investigated. It is demonstrated that they allow the tuning the WF over a wide range of more than 1.5 eV, thus enabling the use of ZnO as both the hole-injecting and electron-injecting contact. The modified ZnO surfaces are characterized using a number of complementary techniques, demonstrating that the preparation protocol yields dense, well-defined molecular monolayers. KW - ZnO KW - self-assembled monolayers KW - phosphonic acid KW - surface modification KW - electrodes Y1 - 2014 U6 - https://doi.org/10.1002/adfm.201401493 SN - 1616-301X SN - 1616-3028 VL - 24 IS - 44 SP - 7014 EP - 7024 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Kowarik, Stefan A1 - Gerlach, Andreas A1 - Leitenberger, Wolfram A1 - Hu J, Witte A1 - Wöll, Christoph A1 - Schreiber, Frank T1 - Energy-dispersive X-ray reflectivity and GID for real-time growth studies of pentacene thin films N2 - We use energy-dispersive X-ray reflectivity and grazing incidence diffraction (GID) to follow the growth of the crystalline organic semiconductor pentacene on silicon oxide in-situ and in real-time. The technique allows for monitoring Bragg reflections and measuring X-ray growth oscillations with a time resolution of 1 min in a wide q-range in reciprocal space extending over 0.25-0.80 angstrom(-1), i.e. sampling a large number of Fourier components simultaneously. A quantitative analysis of growth oscillations at several q-points yields the evolution of the surface roughness, showing a marked transition from layer-by-layer growth to strong roughening after four monolayers of pentacene have been deposited. (c) 2006 Elsevier B.V. All rights reserved. Y1 - 2007 UR - http://www.sciencedirect.com/science/journal/00406090 U6 - https://doi.org/10.1016/j.tsf.2006.12.020 SN - 0040-6090 ER - TY - JOUR A1 - Liehr, Sascha A1 - Jäger, Lena Ann A1 - Karapanagiotis, Christos A1 - Munzenberger, Sven A1 - Kowarik, Stefan T1 - Real-time dynamic strain sensing in optical fibers using artificial neural networks JF - Optics express : the international electronic journal of optics N2 - We propose to use artificial neural networks (ANNs) for raw measurement data interpolation and signal shift computation and to demonstrate advantages for wavelength-scanning coherent optical time domain reflectometry (WS-COTDR) and dynamic strain distribution measurement along optical fibers. The ANNs are trained with synthetic data to predict signal shifts from wavelength scans. Domain adaptation to measurement data is achieved, and standard correlation algorithms are outperformed. First and foremost, the ANN reduces the data analysis time by more than two orders of magnitude, making it possible for the first time to predict strain in real-time applications using the WS-COTDR approach. Further, strain noise and linearity of the sensor response are improved, resulting in more accurate measurements. ANNs also perform better for low signal-to-noise measurement data, for a reduced length of correlation input (i.e., extended distance range), and for coarser sampling settings (i.e., extended strain scanning range). The general applicability is demonstrated for distributed measurement of ground movement along a dark fiber in a telecom cable. The presented ANN-based techniques can be employed to improve the performance of a wide range of correlation or interpolation problems in fiber sensing data analysis and beyond. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement Y1 - 2019 U6 - https://doi.org/10.1364/OE.27.007405 SN - 1094-4087 VL - 27 IS - 5 SP - 7405 EP - 7425 PB - Optical Society of America CY - Washington ER - TY - JOUR A1 - Horn, Sebastian A1 - Hempel, Stefan A1 - Ristow, Michael A1 - Rillig, Matthias C. A1 - Kowarik, Ingo A1 - Caruso, Tancredi T1 - Plant community assembly at small scales: Spatial vs. environmental factors in a European grassland JF - Acta oecologica : international journal of ecology N2 - Dispersal limitation and environmental conditions are crucial drivers of plant species distribution and establishment. As these factors operate at different spatial scales, we asked: Do the environmental factors known to determine community assembly at broad scales operate at fine scales (few meters)? How much do these factors account for community variation at fine scales? In which way do biotic and abiotic interactions drive changes in species composition? We surveyed the plant community within a dry grassland along a very steep gradient of soil characteristics like pH and nutrients. We used a spatially explicit sampling design, based on three replicated macroplots of 15 x 15, 12 x 12 and 12 x 12 m in extent. Soil samples were taken to quantify several soil properties (carbon, nitrogen, plant available phosphorus, pH, water content and dehydrogenase activity as a proxy for overall microbial activity). We performed variance partitioning to assess the effect of these variables on plant composition and statistically controlled for spatial autocorrelation via eigenvector mapping. We also applied null model analysis to test for non-random patterns in species co-occurrence using randomization schemes that account for patterns expected under species interactions. At a fine spatial scale, environmental factors explained 18% of variation when controlling for spatial autocorrelation in the distribution of plant species, whereas purely spatial processes accounted for 14% variation. Null model analysis showed that species spatially segregated in a non-random way and these spatial patterns could be due to a combination of environmental filtering and biotic interactions. Our grassland study suggests that environmental factors found to be directly relevant in broad scale studies are present also at small scales, but are supplemented by spatial processes and more direct interactions like competition. (C) 2015 Elsevier Masson SAS. All rights reserved. KW - Assembly pattern KW - Dispersal limitation KW - Festuca brevipila KW - Niche partitioning KW - Null model KW - Plant community ecology KW - Variance partitioning Y1 - 2015 U6 - https://doi.org/10.1016/j.actao.2015.01.004 SN - 1146-609X SN - 1873-6238 VL - 63 SP - 56 EP - 62 PB - Elsevier CY - Paris ER - TY - INPR A1 - Cierjacks, Arne A1 - Kowarik, Ingo A1 - Joshi, Jasmin Radha A1 - Hempel, Stefan A1 - Ristow, Michael A1 - von der Lippe, Moritz A1 - Weber, Ewald T1 - Biological flora of the british isles: robinia pseudoacacia T2 - The journal of ecology N2 - This account presents information on all aspects of the biology of Robinia pseudoacacia L. that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, and history and conservation.Robinia pseudoacacia, false acacia or black locust, is a deciduous, broad-leaved tree native to North America. The medium-sized, fast-growing tree is armed with spines, and extensively suckering. It has become naturalized in grassland, semi-natural woodlands and urban habitats. The tree is common in the south of the British Isles and in many other regions of Europe.Robinia pseudoacacia is a light-demanding pioneer species, which occurs primarily in disturbed sites on fertile to poor soils. The tree does not tolerate wet or compacted soils. In contrast to its native range, where it rapidly colonizes forest gaps and is replaced after 15-30years by more competitive tree species, populations in the secondary range can persist for a longer time, probably due to release from natural enemies.Robinia pseudoacacia reproduces sexually, and asexually by underground runners. Disturbance favours clonal growth and leads to an increase in the number of ramets. Mechanical stem damage and fires also lead to increased clonal recruitment. The tree benefits from di-nitrogen fixation associated with symbiotic rhizobia in root nodules. Estimated symbiotic nitrogen fixation rates range widely from 23 to 300kgha(-1)year(-1). The nitrogen becomes available to other plants mainly by the rapid decay of nitrogen-rich leaves.Robinia pseudoacacia is host to a wide range of fungi both in the native and introduced ranges. Megaherbivores are of minor significance in Europe but browsing by ungulates occurs in the native range. Among insects, the North American black locust gall midge (Obolodiplosis robiniae) is specific to Robinia and is spreading rapidly throughout Europe. In parts of Europe, Robinia pseudoacacia is considered an invasive non-indigenous plant and the tree is controlled. Negative impacts include shading and changes of soil conditions as a result of nitrogen fixation. KW - climatic limitation KW - ecophysiology KW - geographical and altitudinal distribution KW - germination KW - invasive KW - mycorrhiza KW - nitrogen fixation KW - parasites and diseases KW - reproductive biology KW - soils Y1 - 2013 U6 - https://doi.org/10.1111/1365-2745.12162 SN - 0022-0477 SN - 1365-2745 VL - 101 IS - 6 SP - 1623 EP - 1640 PB - Wiley-Blackwell CY - Hoboken ER - TY - JOUR A1 - Weber, Cornelia A1 - Frank, C. A1 - Bommel, Sebastian A1 - Rukat, Tammo A1 - Leitenberger, Wolfram A1 - Schäfer, Peter A1 - Schreiber, Frank A1 - Kowarik, Stefan T1 - Chain-length dependent growth dynamics of n-alkanes on silica investigated by energy-dispersive x-ray reflectivity in situ and in real-time JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - We compare the growth dynamics of the three n-alkanes C36H74, C40H82, and C44H90 on SiO2 using real-time and in situ energy-dispersive x-ray reflectivity. All molecules investigated align in an upright-standing orientation on the substrate and exhibit a transition from layer-by-layer growth to island growth after about 4 monolayers under the conditions employed. Simultaneous fits of the reflected intensity at five distinct points in reciprocal space show that films formed by longer n-alkanes roughen faster during growth. This behavior can be explained by a chain-length dependent height of the Ehrlich-Schwoebel barrier. Further x-ray diffraction measurements after growth indicate that films consisting of longer n-alkanes also incorporate more lying-down molecules in the top region. While the results reveal behavior typical for chain-like molecules, the findings can also be useful for the optimization of organic field effect transistors where smooth interlayers of n-alkanes without coexistence of two or more molecular orientations are required. Y1 - 2012 U6 - https://doi.org/10.1063/1.4719530 SN - 0021-9606 VL - 136 IS - 20 PB - American Institute of Physics CY - Melville ER -