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Sule P.,Institute for Technical Physics and Materials Science | Szendro M.,Eotvos Lorand University
Surface and Interface Analysis | Year: 2014

The accurate molecular dynamics simulation of weakly bound adhesive complexes, such as supported graphene (gr), is challenging because of the lack of an adequate interface potential. Instead of the widely used Lennard-Jones potential for weak and long-range interactions, we use a newly parameterized Tersoff potential for gr/Ru(0001) system. The new interfacial force field provides adequate moire superstructures in accordance with scanning tunneling microscopy images and with density functional theory (DFT) results. In particular, the corrugation of ξ ≈ 1.0 ± 0.2 Å is found that is somewhat smaller than found by DFT approaches (ξ ≈ 1.2 Å) and is close to scanning tunneling microscope measurements (ξ ≈ 0.8 ± 0.3 Å). The new potential could open the way toward large-scale simulations of supported gr with adequate moire supercells in many fields of gr research. Moreover, the new interface potential might provide a new strategy in general for obtaining accurate interaction potentials for weakly bound adhesion in large-scale systems in which atomic dynamics is inaccessible yet by accurate DFT calculations. Copyright © 2013 John Wiley & Sons, Ltd. Source

Fried M.,Institute for Technical Physics and Materials Science | Fried M.,University of Pannonia
Thin Solid Films | Year: 2014

Non-destructive analyzing tools are needed at all stages of thin film photovoltaic (PV) development, and on production lines. In thin film PV, layer thicknesses, micro-structure, composition, layer optical properties, and their uniformity (because each elementary cell is connected electrically in series within a big panel) serve as an important starting point in the evaluation of the performance of the cell or module. An important focus is to express the dielectric functions of each component material in terms of a handful of wavelength independent parameters whose variation can cover all process variants of that material. With the resulting database, spectroscopic ellipsometry coupled with multilayer analysis can be developed for on-line point-by-point mapping and on-line line-by-line imaging. This work tries to review the investigations of different types of PV-layers (anti-reflective coating, transparent-conductive oxide (TCO), multi-diode-structure, absorber and window layers) showing the existing dielectric function databases for the thin film components of CdTe, CuInGaSe2, thin Si, and TCO layers. Off-line point-by-point mapping can be effective for characterization of non-uniformities in full scale PV panels in developing labs but it is slow in the on-line mode when only 15 points can be obtained (within 1 min) as a 120 cm long panel moves by the mapping station. In the last years [M. Fried et al., Thin Solid Films 519, 2730 (2011)], instrumentation was developed that provides a line image of spectroscopic ellipsometry (wl = 350-1000 nm) data. Up to now a single 30 point line image can be collected in 10 s over a 15 cm width of PV material. This year we are building a 30 and a 60 cm width expanded beam ellipsometer the speed of which will be increased by 10 ×. Then 1800 points can be mapped in a 1 min traverse of a 60 ∗ 120 cm PV panel or flexible roll-to-roll substrate. © 2014 Elsevier B.V. All rights reserved. Source

Stolarczyk J.K.,Ludwig Maximilians University of Munich | Deak A.,Institute for Technical Physics and Materials Science | Brougham D.F.,National Institute for Cellular Biotechnology | Brougham D.F.,University College Dublin
Advanced Materials | Year: 2016

The current state of the art in the use of colloidal methods to form nanoparticle assemblies, or clusters (NPCs) is reviewed. The focus is on the two-step approach, which exploits the advantages of bottom-up wet chemical NP synthesis procedures, with subsequent colloidal destabilization to trigger assembly in a controlled manner. Recent successes in the application of functional NPCs with enhanced emergent collective properties for a wide range of applications, including in biomedical detection, surface enhanced Raman scattering (SERS) enhancement, photocatalysis, and light harvesting, are highlighted. The role of the NP–NP interactions in the formation of monodisperse ordered clusters is described and the different assembly processes from a wide range of literature sources are classified according to the nature of the perturbation from the initial equilibrium state (dispersed NPs). Finally, the future for the field and the anticipated role of computational approaches in developing next-generation functional NPCs are briefly discussed. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Source

Tapaszto L.,Institute for Technical Physics and Materials Science | Dumitrica T.,University of Minnesota | Kim S.J.,Korea Research Institute of Standards and Science | Nemes-Incze P.,Institute for Technical Physics and Materials Science | And 2 more authors.
Nature Physics | Year: 2012

Understanding how the mechanical behaviour of materials deviates at the nanoscale from the macroscopically established concepts is a key challenge of particular importance for graphene, given the complex interplay between its nanoscale morphology and electronic properties 1-5. In this work, the (sub)nanometre-wavelength periodic rippling of suspended graphene nanomembranes has been realized by thermal strain engineering and investigated using scanning tunnelling microscopy. This allows us to explore the rippling of a crystalline membrane with wavelengths comparable to its lattice constant. The observed nanorippling mode violates the predictions of the continuum model 6, and evidences the breakdown of the plate idealization of the graphene monolayer. Nevertheless, microscopic simulations based on a quantum mechanical description of the chemical binding accurately describe the observed rippling mode and elucidate the origin of the continuum model breakdown. Spatially resolved tunnelling spectroscopy measurements indicate a substantial influence of the nanoripples on the local electronic structure of graphene and reveal the formation of one-dimensional electronic superlattices. © 2012 Macmillan Publishers Limited. Source

Sule P.,Institute for Technical Physics and Materials Science | Szendro M.,Institute for Technical Physics and Materials Science | Hwang C.,Korea Research Institute of Standards and Science | Tapaszto L.,Institute for Technical Physics and Materials Science
Carbon | Year: 2014

Graphene on copper is a system of high technological relevance, as Cu is one of the most widely used substrates for the CVD growth of graphene. However, very little is known about the details of their interaction. One approach to gain such information is studying the superlattices emerging due to the mismatch of the two crystal lattices. However, graphene on copper is a low-corrugated system making both their experimental and theoretical study highly challenging. Here, we report the observation of a new rotational moiré superlattice of CVD graphene on Cu (1 1 1), characterized by a periodicity of (1.5±0.05) nm and corrugation of (0.15±0.05) Å, as measured by scanning tunneling microscopy (STM). To understand the observed superlattice we have developed a newly parameterized Abell-Tersoff potential for the graphene/Cu (1 1 1) interface fitted to nonlocal van der Waals density functional theory (DFT) calculations. The interfacial force field with time-lapsed classical molecular dynamics (CMD) provides superlattices in good quantitative agreement with the experimental results, for a misorientation angle of (10.4±0.5°), without any further parameter adjustment. Furthermore, the CMD simulations predict the existence of two non-equivalent high-symmetry directions of the moiré pattern that could also be identified in the experimental STM images. © 2014 Elsevier Ltd. All rights reserved. Source

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