Model Multilayer Structures Research Articles

Linking Processing Parameters and Morphological Development in Organic Photovoltaics by Energy-Filtered TEM Imaging of Model Multilayer Structures

Linking Processing Parameters and Morphological Development in Organic Photovoltaics by Energy-Filtered TEM Imaging of Model Multilayer Structures

A novel versatile multilayer hybrid stress solid-shell element

This paper presents a versatile multilayer locking free hybrid stress solid-shell element that can be readily employed for a wide range of geometrically linear elastic structural analyses, i.e. from shell-like isotropic structures to multilayer anisotropic composites. This solid-shell element has eight nodes with only displacement degrees of freedom and a few internal parameters that provide the locking free behavior and accurate interlaminar stress resolution through the element thickness. These elements can be stacked on top of each other to model multilayer structures, fulfilling the interlaminar stress continuity at the interlayer surfaces and zero traction conditions on the top and bottom surfaces of composite laminates. The element formulation is based on the modified form of the well-known Fraeijs de Veubeke---Hu---Washizu (FHW) multifield variational principle with enhanced assumed strains (EAS formulation) and assumed natural strains (ANS formulation) to alleviate the different types of locking phenomena in solid-shell elements. The distinct feature of the present formulation is its ability to accurately calculate the interlaminar stress field in multilayer structures, which is achieved by incorporating an assumed stress field in a standard EAS formulation based on the FHW principle. To assess the present formulation's accuracy, a variety of popular numerical benchmark examples related to element patch tests, convergence, mesh distortion, shell and laminated composite analyses are investigated and the results are compared with those available in the literature. This assessment reveals that the proposed solid-shell formulation provides very accurate results for a wide range of structural analyses.

Poly(3-hexylthiophene) and [6,6]-Phenyl-C61-butyric Acid Methyl Ester Mixing in Organic Solar Cells

The mixing behavior of the hole- and electron-transporting materials in bulk heterojunction (BHJ) organic photovoltaic (OPV) blends plays a key role in determining the nanoscale morphology, which is believed to be a decisive factor in determining device performance. We present a systematic investigation of the mixing behavior between poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in model multilayer structures. The bilayer structures are composed of amorphous PCBM that is mechanically laminated to different P3HT layers with varying degrees of crystallinity. We find that mixing is significantly decreased as the crystallinity of P3HT is increased. The mixing behavior can be explained as resulting from (1) nearly complete miscibility of PCBM with amorphous P3HT (based on our results from regiorandom P3HT) and (2) the existence of tie chains between crystalline P3HT domains that restrain the swelling of the P3HT layer by PCBM. We also introduce a unique PCBM–P3HT–PCBM trilayer structure where one of the PCBM layers is highly crystalline. The crystalline PCBM dramatically alters the mixing behavior. Initial mixing of the amorphous PCBM into P3HT is followed by rapid cold crystallization at the crystalline PCBM layer, which depletes the PCBM in the P3HT layer. These bilayer and trilayer experiments illustrate that mixing of P3HT and PCBM is influenced by multiple factors, such as the semicrystalline nature of P3HT (overall crystallinity, characteristics of amorphous chains) and phase (amorphous or crystalline) of the PCBM.

An optimal versatile partial hybrid stress solid-shell element for the analysis of multilayer composites

SUMMARY In the present contribution we propose an optimal low-order versatile partial hybrid stress solid-shell element that can be readily employed for a wide range of geometrically linear elastic structural analyses, that is, from shell-like isotropic structures to multilayer anisotropic composites. This solid-shell element has eight nodes with only displacement degrees of freedom and only a few internal parameters that provide the locking-free behavior and accurate interlaminar shear stress resolution through the element thickness. These elements can be stacked on top of each other to model multilayer composite structures, fulfilling the interlaminar shear stress continuity at the interlayer surfaces and zero traction conditions on the top and bottom surfaces of composite laminates. The element formulation is based on the modified form of the well-known Fraeijs de Veubeke–Hu–Washizu multifield variational principle with enhanced assumed strains formulation and assumed natural strains formulation to alleviate the different types of locking phenomena in solid-shell elements. The distinct feature of the present formulation is its ability to accurately calculate the interlaminar shear stress field in multilayer structures, which is achieved by the introduction of the assumed interlaminar shear stress field in a standard enhanced assumed strains formulation based on the Fraeijs de Veubeke–Hu–Washizu principle. The numerical testing of the present formulation, employing a variety of popular numerical benchmark examples related to element patch test, convergence, mesh distortion, shell and laminated composite analyses, proves its accuracy for a wide range of structural analyses.Copyright © 2012 John Wiley & Sons, Ltd.

Photophysics and Dynamics of Surface Plasmon Polaritons-Mediated Energy Transfer in the Presence of an Applied Electric Field

The possibility to transfer energy between molecular excitons across a metal film up to 150 nm thick represents a very attractive solution to control and improve the performances of thin optoeletronic devices. This process involves the presence of coupled surface plasmon polaritons (SPPs) at the two dielectric-metal interfaces, capable of mediating the interactions between donor and acceptor, located on opposite sides of the metal film. In this Article, the photophysics and the dynamics of an efficient SPP-mediated energy transfer between a suitable dye and a conjugated polymer is characterized by means of steady-state and time-resolved photoluminescence techniques. The process is studied in model multilayer structures (donor/metal/acceptor) as well as in electrically pumped heterostructures (donor/metal cathode/acceptor/anode), to verify the effects of applied electric fields on the efficiency and the dynamics of SPP-mediated energy transfer. A striking enhancement of the overall luminescence was recorded in a particular range of applied bias, suggesting the presence of cooperative effects between optical and electrical stimulations.

Molecular depth profiling of model biological films using low energy monoatomic ions

As time-of-flight secondary ion mass spectrometry (ToF-SIMS) is increasingly used to characterize biological samples, fundamental studies on model samples are needed to better understand the interactions between primary ions and biological matter, and to help interpreting the complex ToF-SIMS data. In this work, model biological films were prepared by thermal evaporation and were characterized by XPS and ToF-SIMS. Eight biomolecules were carefully chosen to cover important physical and chemical properties such as aromaticity and oxygen content. The model films allowed studying systematically the sputter rates, ion yields and amenability to molecular depth profiling when using low energy Xe+ and Cs+ etching ions. The results show that aliphatic molecules such as cysteine and aspartic acid can be easily depth profiled using low energy monoatomic primary ions. However, aromatic samples such as phenylalanine could only be depth profiled using reactive Cs+ ions. The systematic study also revealed that the presence of oxygen strongly reduces the deprotonated ion signal during cesium ion bombardment. Finally, model multilayer structures were depth profiled using 500eV Xe+ and Cs+ primary ions with depth resolutions lower than 10nm. Isotopic variations of the same biomolecule were used for the first time to create multilayers structures with constant ion yields and etching rates.

A mixed solid-shell element for the analysis of laminated composites

SUMMARY This paper presents a versatile low order locking-free mixed solid-shell element that can be readily employed for a wide range of linear elastic structural analyses, that is, from thick isotropic structures to multilayer anisotropic composites. This solid-shell element has eight nodes with only displacement degrees of freedom and few assumed stress parameters that provide very accurate interlaminar stress calculations through the element thickness. These elements can be stacked on top of each other to model multilayer structures, fulfilling the interlaminar stress continuity at the interlayer surfaces and zero traction conditions on the top and bottom surfaces of the laminate. The element formulation is based on the well-known Fraeijs de Veubeke–Hu–Washizu mixed variational principle with enhanced assumed strains formulation and assumed natural strains formulation to alleviate the different types of locking phenomena in solid-shell elements. The distinct feature of the present formulation is its ability to accurately calculate the interlaminar stress field in multilayer structures, which is achieved by the introduction of a constraint equation on the interlaminar stresses in the Fraeijs de Veubeke–Hu–Washizu principle-based enhanced assumed strains formulation. The intelligent computer coding of the present formulation makes the present element appropriate for a wide range of structural analyses. To assess the present formulation's accuracy, a variety of popular numerical benchmark examples related to element convergence, mesh distortion, and shell and laminated composite analyses are investigated and the results are compared with those available in the literature. These benchmark examples reveal that the proposed formulation provides very good results for the structural analysis of shells and multilayer composites. Copyright © 2011 John Wiley & Sons, Ltd.

A partial hybrid stress solid-shell element for the analysis of laminated composites

In this paper we introduce a low order partial hybrid stress solid-shell element based on the composite energy functional for the analysis of laminated composite structures. This solid-shell element has eight nodes with only displacement degrees of freedoms, and three-dimensional constitutive models can be directly employed in the present formulation without any additional treatment. The assumed interlaminar stress field provides very accurate interlaminar stress calculation through the element thickness. These elements can be stacked on top of each other to model multilayer structures, fulfilling the interlaminar stress continuity at the interlayer surfaces and zero traction conditions on the top and bottom surfaces of the laminate. The present solid-shell does not show the transverse shear, trapezoidal and thickness locking phenomenon, and passes both the membrane and the bending patch tests. To assess the present formulation’s accuracy, a variety of popular numerical benchmark examples related to element convergence, mesh distortion, shell and laminated composite analyses are investigated and the results are compared with those available in the literature. The numerical results show the accuracy of the presented solid-shell element for the analysis of laminated composites.

Model multilayer structures for three-dimensional cell imaging

The prospects for SIMS three-dimensional analysis of biological materials were explored using model multilayer structures. The samples were analyzed in a ToF-SIMS spectrometer equipped with a 20 keV buckminsterfullerene (C 60 +) ion source. Molecular depth information was acquired using a C 60 + ion beam to etch through the multilayer structures at specified time intervals. Subsequent to each individual erosion cycle, static SIMS spectra were recorded using a pulsed C 60 + ion probe. Molecular intensities in sequential mass spectra were monitored as a function of primary ion fluence. The resulting depth information was used to characterize C 60 + bombardment of biological materials. Specifically, molecular depth profile studies involving dehydrated dipalmitoyl-phosphatidylcholine (DPPC) organic films indicate that cell membrane lipid materials do not experience significant chemical damage when bombarded with C 60 + ion fluences greater than 10 15 ions/cm 2. Moreover, depth profile analyses of DPPC–sucrose frozen multilayer structures suggest that biomolecule information can be uncovered after the C 60 + sputter removal of a 20 nm overlayer with no appreciable loss of underlying molecular signal. The experimental results support the potential for three-dimensional molecular mapping of biological materials using cluster SIMS.

Alternative model multilayer structures for depth profiling studies

AbstractThere exists a permanent need for new model systems appropriate for the study of depth profiling with different surface analysis techniques, e.g. AES, XPS and SIMS, which involves the ion sputtering process. The development, preparation, transmission electron microscopy (TEM) characterization and AES depth profiling behaviour of the following new multilayer structures are presented: metal/oxide and oxide/oxide multilayers composed of Ni, Cr, NiO and Cr2O3 thin films and metal/semiconductor multilayers composed of Ni, Cr and Si thin films. These multilayer structures were sputter deposited on smooth silicon substrates and were precisely characterized with AES depth profiling during sample rotation and (two of them) with TEM. The measured compositional depth profiles are explained with respect to the influence of cystalline and amorphous structures on depth resolution, and the advantages of each type of multilayer structure are discussed.

AES, XPS and TEM characterization of metal/oxide multilayers

Multilayer structures for application in microelectronics, hard and wear resistant as well as optical coatings are becoming increasingly complex. Therefore, there is a permanent need for new model multilayer structures appropriate for high resolution depth profiling studies which are helpful for the interpretation of everyday analytical results. A new type of metal/oxide multilayer structure composed of alternating Cr, Ni, NiO and Cr 2O 3 layers with a total thickness of about 475 nm was sputter deposited onto a smooth silicon substrate. By TEM investigations we found a columnar crystalline structure of Ni, Cr and NiO layers with a comparable size of crystal grains while the Cr 2O 3 layer showed a partial amorphous structure with a small amount of a crystalline phase. Compositional depth profiles of the model multilayer structure were obtained with Auger and X-ray photoelectron spectroscopy on stationary and rotated samples. The AES and XPS depth profiles are discussed with respect to different instrumental parameters and the influence of crystalline and amorphous structures on depth resolution. The AES depth profiling of stationary samples, using a unidirectional ion beam, shows an evident beneficial influence of the oxide sandwich layers on depth resolution.