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Aliphatic isocyanurates are nowadays used routinely in the development of advanced materials like polyurethane nanocomposites and 3D-printed components, due to their versatile reactivity and the good mechanical and optical properties they confer to the final material. In these applications, a control of the properties at the micro- and nanoscale is desired, therefore a deep understanding of intermolecular interactions is required. Using ab-initio calculations and molecular dynamics simulations, the intermolecular interactions of aliphatic isocyanurates are investigated in detail. The presence of an isocyanate-isocyanurate interaction is demonstrated, and the strong dispersion character of isocyanurate-based interactions is revealed. Calculations of the free energy of binding of the different interactions in gas and liquid phase are provided. The microscopic structure of aliphatic functional and non-functional isocyanurates is analysed and related to the different interaction types, and finally their possible relationship with macroscopic dynamic variables is discussed, This project has received funding from the European Union’s Horizon 2020 - Research and Innovation Framework Programme under the Marie Skłodowska-Curie Grant Agreement no. 642890 (http://thelink-project.eu/) and it was partially supported by the Portuguese Science and Technology Foundation (FCT) in the framework of the Strategic Funding UID/FIS/04650/2013, by the FCT grant SFRH/BD/128666/2017 and by the project “ SearchON2: Revitalization of HPC infrastructure of Uminho” (NORTE07-0162-FEDER-0000869), under the National Strategic Reference Framework, through the European Regional Development Fund. The authors also acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing HPC resources that have contributed to the research results reported within this paper

During the last few decades, the interest over chalcopyrite and related photovoltaics has been growing due the outstanding structural and electrical properties of the thin-film Cu(In,Ga)Se2 photoabsorber. More recently, thin film deposition through solution processing has gained increasing attention from the industry, due to the potential low-cost and high-throughput production. To this end, the elimination of the selenization procedure in the synthesis of Cu(In,Ga)Se2 nanoparticles with following dispersion into ink formulations for printing/coating deposition processes are of high relevance. However, most of the reported syntheses procedures give access to tetragonal chalcopyrite Cu(In,Ga)Se2 nanoparticles, whereas methods to obtain other structures are scarce. Herein, we report a large-scale synthesis of high-quality Cu(In,Ga)Se2 nanoparticles with wurtzite hexagonal structure, with sizes of 10–70 nm, wide absorption in visible to near-infrared regions, and [Cu]/[In + Ga] ≈ 0.8 and [Ga]/[Ga + In] ≈ 0.3 metal ratios. The inclusion of the synthesized NPs into a water-based ink formulation for screen printing deposition results in thin films with homogenous thickness of ≈4.5 µm, paving the way towards environmentally friendly roll-to-roll production of photovoltaic systems., This research was funded by the Portuguese Foundation for Science and Technology (PTDC/CTM-ENE/5387/2014, PTDC/NAN-MAT/28745/2017, UID/FIS/04650/2020, UID/QUI/ 0686/2020, PTDC/FIS-MAC/28157/2017, POCI-01-0145-FEDER-028108, SFRH/BD/121780/2016); the Basque Government Industry Department (ELKARTEK, HAZITEK); the National Science Foundation (DMR-2003783 grant); the Search-ON2: revitalization of HPC infrastructure of UMinho, (NORTE07-0162-FEDER-000086), co-funded by the North Portugal Regional Operational Programme (ON.2-O Novo Norte), under the National Strategic Reference Framework (NSRF), through the European Regional Development Fund (ERDF). The use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.

Colloidal synthesis is harnessed for the gram-scale preparation of hexagonal-shaped plate-like Bi2Te2.7Se0.3 particles, yielding nearly 5 g of the product in one experiment. The resultant textured particles are highly crystalline, phase-pure, chemically uniform, and can serve as a starting material for the preparation of bulk thermoelectrics for room temperature applications. The consolidation occurs via spark plasma sintering, which affords nanostructured n-type Bi2Te2.7Se0.3 material exhibiting a high figure of merit ZT ≈ 1 at 373 K with an average ZT ≈ 0.93 (300-473 K). Our experimental and theoretical studies indicate that the high thermoelectric performance is attributed to a favorable combination of the resultant transport properties. Specifically, bottom-up formation of the plate-like particles results in the substantial reduction of thermal conductivity by nanostructuring as observed experimentally and can be ascribed to phonon scattering at grain boundaries and suppressed bipolar conduction. When coupled with high electrical conductivity, which is preserved at the bulk scale as confirmed by ab initio calculations, these factors boost the thermoelectric performance of the as-synthesized n-type Bi2Te2.7Se0.3 bulk nanostructured alloy to the state-of-the-art level. The combination of a newly developed scalable colloidal synthesis with optimized spark plasma sintering constitutes a convenient route to nanostructured bulk thermoelectrics, which is an interesting pathway for the preparation of simple and complex thermoelectric chalcogenides., This work was supported by Portuguese National Funding Agency for Science, Research and Technology (FCT) under the UT-BORN-PT project (UTAP-EXPL/CTE/0050/2017). B. A. K. acknowledges funding of this work by the Robert A. Welch Foundation (grant no. F-1464).

Nanogels, due to their unique properties, have potential for promising applications in several fields. There is a growing interest in understanding which are the mechanisms controlling their properties, as those depend not only on composition but also on internal structure. In this work, dissipative particle dynamics simulations were applied to simulate the formation of nanogels from monodisperse end-functionalised polymer chains, with the cross-linking reaction modelled using a distance-based algorithm, including a reversible intermediate step. Nanogel properties are studied by calculating their swelling transition curves and structure properties, such as network defects and entanglement state. Various chain lengths, reaction schemes, nanogel in silico synthesis methods and solvent ratios are compared and discussed, showing how these factors influence the nanogel swelling behaviour and structure. Our results show that, in order to be compared with experiments, nanogel simulations should carefully consider processing parameters., This project has received funding from the European Union’s Horizon 2020-Research and Innovation Framework Programme under the Marie Skłodowska-Curie Grant Agreement no. 642890 (http://thelink-project.eu/) and it was partially supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2020-2023, by the FCT grant SFRH/BD/128666/2017. The authors acknowledge FCT and the National Advanced Computing Network (RNCA) for providing HPC resources from cluster Bob at Minho Advanced Computing Center (MACC).

The production, processing, and application of aliphatic isocyanate (NCO)-based thermosets such as polyurethane coatings and adhesives are generally limited by the surprisingly high viscosity of tri-functionality and higher-functionality isocyanurates. These compounds are essential crosslinking additives for network formation. However, the mechanism by which these high viscosities are caused is not yet understood. In this work, model aliphatic isocyanurates were synthesized and isolated in high purity (>99%), and their viscosities were accurately determined. It was shown that the presence of the NCO group has a strong influence on the viscosity of the system. From density functional theory calculations, a novel and significant bimolecular binding potential of −8.7 kJ/mol was identified between NCO groups and isocyanurate rings, confirming the important role of the NCO group. This NCO-to-ring interaction was proposed to be the root cause for the high viscosities observed for NCO-functional isocyanurate systems. Molecular dynamics simulations carried out to further confirm this influence also suggest that the NCO-to-ring interaction causes a significant additional contribution to viscosity. Finally, model functional isocyanurates were further reacted into densely crosslinked polyisocyanurate networks which showed interesting material properties

Aliphatic diisocyanates and their derivatives are key liquid components in the industrial processing of polyurethane materials. In particular, for the synthesis of crosslinked polyurethane materials, the higher functionality molecules obtained by reacting three -or more- diisocyanates are of interest. However, despite their widespread application, the relation between molecular structure and macroscopic physical properties, in particular viscosity, is poorly understood in these systems. In this work, we introduce a new force field parameter set, GAFF-IC, based on the widely-used and versatile GAFF force field, meant for accurate predictions of physical properties of isocyanate-based molecular liquids. The new parameters allow to predict the vaporization enthalpies and densities of several isocyanate-based molecules, which are found in excellent agreement with the available experimental data. The effectiveness and transferability of the improved parameters is verified by calculating the viscosities of several isocyanates, isocyanate dimers (uretdiones) and isocyanate trimers (isocyanurates), resulting in accurate viscosity predictions in excellent agreement with experimental values., This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sk lodowska-Curie Grant Agreement no. 642890 (http://thelink-project.eu/) and it was partially supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2013, and by the project ”Search-ON2: Revitalization of HPC infrastructure of Uminho” (NORTE-07-0162-FEDER-0000869), under the National Strategic Reference Framework, through the European Regional Development Fund

The recently-observed self-assembly of certain salphen-based compounds into neuron-like networks of microrings interconnected with nano-thin strings may suggest a new highly-potent tool for nanoscale patterning. However, the mechanism behind such phenomena needs to be clarified before they can be applied in materials design. Here we show that, in contrast with what was initially presumed, the emergence of a “rings-and-rods” pattern is unlikely to be explained by merging, collapse and piercing of vesicles as in previously reported cases of nanorings self-assembly via non-bonding interactions. We propose an alternative explanation: the compounds under study form a 1D coordination polymer, the fibres of which are elastic enough to fold into toroidal globules upon solvent evaporation, while being able to link separate chains into extended networks. This becomes possible because the structure of the compound's scaffold is found to adopt a very different conformation from that inferred in the original work. Based on ab initio and molecular dynamics calculations we propose a step-by-step description of self-assembly process of a supramolecular structure which explains all the observed phenomena in a simple and clear way. The individual roles of the compound' s scaffold structure, coordination centres, functional groups and solvent effects are also explained, opening a route to control the morphology of self-assembled networks and to synthesize new compounds exhibiting similar behaviour., Spanish MINECO, project CTQ-201460419-R. NORTE-07-0162-FEDER-000086. FP7-PEOPLE-ITN2008-238363, info:eu-repo/semantics/publishedVersion

Polymeric composite materials reinforced by curved cylindrical inclusions of very high aspect ratio are studied for their elastic stiffness. The waviness and curliness of the fibres are described by sinusoidal and helical models with different amplitudes and spiral radii, respectively. Effects are investigated in detail by finite element-based homogenisation methods, analytical models and molecular dynamics simulations. Within the finite element models two types of discretisation are considered for the fibres, using continuum solid and beam elements, respectively. Periodic boundary conditions or a special set of mixed boundary conditions are applied for approximating the effective elastic properties. The analytical investigations use a mean-field approximation in which inhomogeneities are split into unconnected segments of appropriate orientation, the elasticity tensor being calculated with a Mori–Tanaka method. It is shown that both curved fibre geometries, sinusoidal and helical, significantly reduce the longitudinal elastic stiffness of the composite. Beam element-based fibre models and analytical solutions give low and high estimates, respectively, for the elastic constants. The continuum mechanical results are found to be in good agreement with the molecular dynamics predictions.

Organic and hybrid perovskite based solar cells have a huge potential to significantly contribute to a clean electricity supply of the future. However, so far they exhibit complex and hierarchical degradation paths and their understanding can only be acquired through the application of complementary chemical and physical characterization techniques. This limited device stability is the main hurdle for a successful and large scale market introduction of these emerging solar cell technologies. Our StableNextSol Action has created a highly interdisciplinary network of laboratories, as well as corresponding industry, overall more than 120 partners, with complementary analytical techniques for the study and understanding of the degradation mechanisms occurring in state-of-the-art devices. Our Action integrates and generates fundamental knowledge and expertise to foster disruptive innovations targeted to mitigate device failure and to propose and develop new concepts for more stable solar cells. Value is added to the entire value chain of photovoltaic research at European and international level, as well as variety decision makers in the public sector by supporting specialisation policy and standards still lacking in this research field. The outcome of the Action will contribute to resolve the global challenges facing the industry and this COST Action initiative has brought together all these expertises and resources to promote the cooperation between different sectors, academia, public authorities and industry.

In this work, we report on the production of Ge nanoparticles (NPs) in an inert Ar gas atmosphere by pulsed laser deposition (PLD) at room temperature (RT). The direct deposition of energetic particles/droplets resulting from the ablation process of the target material has been avoided by using an original and customized off-axis shadow mask (shadowed off-axis) deposition set-up where the NPs deposition on the substrate takes place by means of scattering between the NPs formed in the vapor phase and the background Ar atoms. It is found that the Ar gas pressure parameter has a relevant role in the crystallization process, with better crystallinity obtained as the background Ar pressure is raised for the given experimental conditions. © Springer-Verlag 2012

During the last years it has been clear that it is importance to understand and control the nanostructure of the active polymer layer used in optoelectronic devices, like polymer diodes, solar cells or field effect transistors. Several experimental studies have shown that the nanostructure of polymer thin films used in these optoelectronic devices depends on the conditions used to deposit the polymer layer between the electrodes. As a result, in solid state conjugated polymer chains tend to be stacked and aligned relative to the electrodes creating domains with different sizes that influence the performance of these devices. To understand how the spatial arrangement of polymer chains affects the functioning of optoelectronic devices, we performed computer simulations using our mesoscopic model based on a generalized dynamic Monte Carlo method. We focus our study on the influence of the nanomorphology on the electric properties of polymer light emitting diodes. Our results show that for a pristine polymer layer and in the presence of ohmic contacts between the electrodes and the polymer layer, the electric properties of the device, namely current density, charge density, internal electric field and the number of charges that undergo recombination strongly depends on the polymer morphology at nanoscale., Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BD/22143/2005, Fundo Europeu de Desenvolvimento Regional (FEDER) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001, CONC-REEQ/443/EEI/2005

The actual interest on polymer light emitting diodes (PLEDs) is based on the fact that they are easy to process, which reduces the cost of fabrication and thus opening a new branch in the electronic market the low-cost electronics. However, these devices present a limited efficiency compared to their inorganic counterparts mainly due to the unbalanced charge injection, which reduces the fluorescence emission. One of the first strategies to improve PLEDs efficiency was using a bilayer structure composed by two polymers to improve charge injection and transport, and at the same time tune charge recombination zone to reduce the effect of the electrodes on exciton quenching. Although this is a very ingenious device architecture some of these bilayer devices showed a lower efficiency than it was expected. The reason for that is attributed to the dissolution of the first polymer layer by the solvent used for the deposition of the second polymer layer, which do not allow to create a define polymer/polymer interface. Although cross-linking the first polymer layer can solve this problem, there is not a clear understanding why the presence of a graded interface between both polymer layers can lead to a change on PLED efficiency. In order to clarify the effect of a graded polymer/polymer interface as compared to a sharp one on the functioning of a PLED, we performed computer experiments using a mesoscopic model of a bilayer PLED developed by us that considers the morphology of both polymers at nanoscale and their properties at molecular scale. The results present in this work show clearly a significant change on the charge recombination profile within the polymer device depending on the type of interface formed between the two polymers, which can be a plausible explanation for the loss of efficiency in the bilayer 7-CN-PPV/PPV LED., Fundação para a Ciência e a Tecnologia (FCT) - POCTI/CTM/41574/2001, CONC-REEQ/443/EEI/2005, SFRH/BD/22143/2005

Semiconductor polymers are successfully implemented in a broad range of applications such as light emitting diodes, field effect transistors and photovoltaic devices. Most of the achievements reached in the development of these devices were obtained at experimental level, being difficult to identify individually the influence of each factor that limits and controls these devices efficiency. One of the factors that strongly influence the performance of polymer-based devices is the presence of chemical defects in the polymer strands that change their molecular properties. As a result, these polymer strands can work like traps or deep energetic states for charge transport, leading, for instance, to a decrease on charge mobility. At experimental level it is a difficult task to isolate the influence of each type of chemical defects individually on the molecular properties of the polymer strands. It is in this context that theoretical modelling seems to be the most suitable approach to get a deep understanding of the influence of chemical defects on the molecular properties of semiconductor polymers. By performing quantum molecular dynamics calculations we study the influence of chemical defects on the molecular properties of poly(para-phenylenevinylene) (PPV). Our results show clearly a significant difference on the electronic properties of defective polymer strands (e.g. charge carrier localization, ionization potential, electron affinity and electric-field threshold for charge carrier mobility along the polymer backbone) as compared with defect-free strands., Fundação para a Ciência e a Tecnologia (FCT) - CONC-REEQ/443/EEI/2005, SFRH/BD/22143/2005

We have performed first principles density functional theory calculations to study the formation energy of point defects in TiC, TiO and TiCO compounds. The formation energy of isolated vacancies were obtained for different equilibrium conditions. For binary compounds, we have also calculated the formation energy of antisite defects. It was found that the defect formation energies strongly depend on the chemical environment. Our results show that C vacancies are easily formed in TiC and TiCO. For the TiO compound, Ti vacancies are highly probable to occur and O vacancies are also easily formed under titanium rich atmosphere., Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – CONC-REEQ/443/EEI/2005, POCTI/CTM/69362/2006

Conjugated polymers show interesting properties that make them appropriated for nanoelectronics. Several studies of poly(p-phenylene vinylene) (PPV) have suggested that each polymer chain consists of several planar segments, with conjugation length of nanoscale dimension, linked by twists or kinks. A pronounced twist between two planar segments in a PPV chain not only causes loss of main-chain conjugation but it may also alter electron and hole mobility along the chain, which has further implications for the percolation of charge through the polymer film. We used self-consistent quantum molecular dynamics calculations to provide information on the electric field needed to move the injected charges (either electrons or holes) along the planar segments of PPV and to cross the twist between two planar segments perpendicular to each other. Field-dependent charge mobility was also estimated for conjugated segments of various lengths. Our results suggest that electrons can cross the twist between adjacent planar segments for lower applied electric fields than holes if there is no more than one electronic charge (electron or hole) on the PPV chain, otherwise similar fields are needed., Fundação para a Ciência e a Tecnologia FCT) Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001, CONC-REEQ/443/EEI/2001 e SFRH/BD/11231/2002

We have used a self-consistent quantum molecular dynamics approach to calculate the mobility of both positive and negative polaron-type carriers on solated chains of poly(p-phenylene vinylene) (PPV) and some of its derivatives and the dependence of their mobility on the applied electric field. Our results suggest that polaron-type mobility along most of these polymer chains has a clear dependence on the electric field which is quite different from the result derived for bulk PPV-based materials., Fundação para a Ciência e a Tecnologia (FCT) Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001, CONC-REEQ/443/EEI/2001 e SFRH/BD/11231/2002

We describe a new approach to model the dynamical structural modifications of ferroelectric polymer molecules induced by uniaxial stretching and by the application of electric fields of various strengths and orientations. Our approach combines a self-consistent quantum mechanical method with molecular dynamics, removing the need to use inter-atomic potentials in the dynamic calculations. Here we present results obtained for individual molecules of poly(vinylidene fluoride) in its alpha and beta form. The effects of structural disorder due to the presence of inverted monomer units in a “head-to-head” and “tail-to-tail” position are also discussed.

Using a mesoscopic modelling approach, the authors performed computer experiments to study the influence of polymer density on bipolar charge evolution through thin layers of polydiacetylene (PDA) exhibiting specific microstructures. We found that the competition between charge transport, trapping and recombination within the polymer layer leads to several general trends, some of them being non-intuitive, as one varies polymer density. Our results show that polymer density mainly affects current and recombination efficiencies in the absence of defects or impurity states. The overall trends depend both on chain orientation relative to the electrodes and on the strength of the external applied electric field. These results suggest that adequate modelling of charge transport in electronic and optoelectronic devices based on conducting and semiconducting polymers, such as PDA, must include their structure and related key factors at mesoscopic scale. Such models provide the necessary knowledge-base to optimize the polymer film structure for electronic applications., Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001 e SFRH/BD/11231/2002, Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)

We present and discuss a set of boundary conditions (BCs) to use in three-dimensional, mesoscopic, finite element models of mid-infrared pulsed laser ablation of brittle materials. These models allow the study of the transient displacement and stress fields generated at micrometer scales during and after one laser pulse, where using conventional BCs may lead to some results without physical significance that can be considered an artefact of the calculations. The proposed BCs are tested and applied to a micrometer-scale continuous model of human dental enamel under CO2 radiation (10.6 mm, 0.35 ms pulse, sub-ablative fluence), giving rise to the following results: the highest stress is obtained at the irradiated surface of the model, at the end of the laser pulse, but afterwards it decreases rapidly until it becomes significantly lower than the stress in a region 2.5 mm deep in the model; a thermally induced vibration in the material is predicted. This non-intuitive dynamics in stress and displacement distribution cannot be neglected and has to be considered in dynamic laser ablation models, since it may have serious implications in the mechanisms of ablation., Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência, Tecnologia, Inovação” - POCTI/ESP/37944/2001, SFRH/BD/4725/2001., Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER).

We present an analytical model for a ceramic material (hydroxyapatite) containing nanometre-scale water pores, and use it to estimate the pressure at the pore as a function of temperature at the end of a single 0.35 µs laser pulse by Er:YAG (2.94 µm) and CO2 (10.6 µm) lasers. Our results suggest that the pressure at the pore is directly related to pore temperature, and that very high pressures can be generated simply by the thermal expansion of liquid water. Since the temperature reached at the pores at the end of the laser pulse is a strong function of pore size for Er:YAG lasers, but is independent of pore size for CO2 lasers, our present results provide a possible explanation for the fact that the enamel threshold ablation fluences are more variable for Er:YAG lasers than for CO2 lasers, and suggest that experimentalists should analyse their results accounting for factors (like age or type of tooth) that may change the pore size distribution in their samples., Comunidade Europeia (CE) - Fundo Comunitário Europeu (FEDER)., Fundação para a Ciência e a Tecnologia (FCT).

Poling processes play an important role in the design and preparation of many ferroelectric materials for practical uses in the field of sensors and actuators. Particularly, the processing of piezoelectric beta-poly(vinylidene fluoride) (PPVDF) involves mechanical stretching in order to transform the extruded a-phase into the P-phase and poling of this later material in order to optimize the piezoelectric response. This poling process affects the orientation of the dipolar moments of the beta-chains and improves the alpha to beta transformation. Poling processes in general and in PVDF in particular are still quite empirical because a firm understanding of the physical processes involved in poling has not been fully established. In the present work we use a self-consistent quantum molecular dynamics method to study the effect of the electric field on both alpha and beta chains of PVDF. The orientation of the dipolar moments in each chain as a function of an electric field and the accompanying structural modifications due to these reorientations will be the main parameters discussed. The theoretical results will be used to better understand the changes at a molecular level due to the poling process, as observed in FTIR experiments, performed in poled and non-poled beta-PVDF from the same batch., Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER), Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência, Tecnologia, Inovação POCTI/CTM/41574/2001, SFRH/BD/11231/2002, Grant POCTI/CTM/33501/99.

Introducing methoxy electron donor groups into a poly(para-phenylene vinylene) (PPV) chain will lead to the appearance of unique electronic properties at the molecular scale which should affect the overall properties of light-emitting diodes based on these polymers. Self-consistent quantum molecular dynamics calculations have been used to provide information on intra-molecular properties of poly(2,5-dimethoxy-para-phenylene vinylene) (DMeOPPV), which are relevant for the modelling and characterization of polymer light-emitting diodes at nanometric length scale. We focus our attention on those properties that have been somewhat neglected in previous literature: the charge distribution associated with positive and negative charge carriers and their intra-molecular mobility when an electric field is applied., Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência, Tecnologia, Inovação” - POCTI/CTM/41574/2001, SFRH/BD/11231/2002., Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)

The performance of a single layer polymer light-emitting diode depends on several interdependent factors, although recombination between electrons and holes within the polymer layer is believed to play an important role. Our aim is to carry out computer experiments in which bipolar charge carriers are injected in polymer networks made of poly(p-phenylene vinylene) chains randomly oriented. In these simulations, we follow the charge evolution in time from some initial state to the steady state. The intra-molecular properties of the polymer molecules obtained from self-consistent quantum molecular dynamics calculations are used in the mesoscopic model. The purpose of the present work is to clarify the effects of intra-molecular charge mobility and energy disorder on recombination efficiency. In particular, we find that charge mobility along the polymer chains has a serious influence on recombination within the polymer layer. Our results also show that energy disorder due to differences in ionization potential and electron affinity of neighbouring molecules affects mainly recombinations that occur near the electrodes at polymer chains parallel to them., Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência, Tecnologia, Inovação” - POCTI/CTM/41574/2001, Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)

Using a finite element approach the authors model the influence of enamel's microstructure and water distribution on the temperature and stress at the centre of the laser spot, for a CO2 laser working at 10.6 μm, with 0.35 μs pulse duration and subablative intensity. The authors found that the distribution of water in enamel significantly influences the stress generated at the end of one laser pulse: much lower (two orders of magnitude) stress values occur in models with homogeneously distributed water than in models with 0.27 vol.% water located in pores or 4 vol.% in layers. The amount of water in enamel has a strong influence on the stress distribution, but not on the maximum stress values reached. However, different water contents do not influence the temperature distribution in enamel. These results suggest that adequate modelling of the ablation mechanisms in enamel, as in other highly inhomogeneous materials, must include their structure at the mesoscopic scale., Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER) - Programa Operacional "Ciência, Tecnologia, Inovação (POCTI) - POCTI/ESP/37944/2001., Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BD/4725/2001.

The softness of conjugated polymers leads to strong coupling between polymer's electrons and lattice vibrations. Therefore, it is necessary to perform quantum molecular dynamics computer simulations in order to study their electronic and optical properties at molecular level. We have used self-consistent molecular dynamics calculations with inter-atomic forces evaluated from quantum mechanical calculations at the complete neglect of differential overlap level to discuss some of the issues relating to the electronic processes involved in polydiacetylene and poly(p-phenylene vinylene). Specifically addressed are the charge induced structural changes of the polymer chains and the intra-molecular charge mobility. The change in the chemical potential of individual polymer strands at zero temperature is also discussed. Our results suggest a geometrical distortion in the bond length distribution relative to the uncharged chains which is accompanied by changes in atomic charges at the distortion site. The charge carrier mobility is predicted to depend on the strength of the electric field, in accordance with experiments.

We present generalized Monte Carlo calculations to assess the effects of texture and related key factors on the properties of polymer-based light emitting diodes. We, describe one class of mesoscopic model giving specific realizations of the polymer network. The model, with simple physically based rules, shows the effects of polymer structural order on current flow, trapping and radiative and non-radiative charge recombination within the polymer layer. Interactions between charges are included explicitly, as are image interactions with the electrodes. It is important that these Coulomb interactions are not simplified to an averaged space charge, since the local interactions can lead to effective trapping of charge, even in the absence of defective chains or impurity trapping. There proves to be an important role for trapping, in which charges are localized for times long compared with transit times. The competition between current flow, trapping and radiative and non-radiative charge recombination means that some of the trends are not intuitively obvious. For example, if radiative recombination occurs only on short polymer chains, as is the case for certain polymer systems, the internal efficiency appears to saturate for a concentration of these shorter luminescent chains of about 20-30%. As the proportion of shorter chains increases, trapping increases, whereas current efficiency decreases. Our approach provides a natural link between atomistic models of individual polymer molecules and the macroscopic descriptions of device modelling. Such mesoscopic models provide a means to design better film structures, and hence to optimize the effectiveness of new organic materials in a range of applications., Engineering and Physical Sciences Research Council (EPSRC).

It is common to look at the atomic processes of removal of atoms or ions from surfaces. At this microscopic scale, one has to understand which surface ions are involved, which excited states are created, how electrons are transferred and scattered, and how the excitation leads to ion removal. It is even more common to look at continuum models of energy deposition in solids, and at the subsequent heat transfer In these macroscopic analyses, thermal conduction is combined with empirical assumptions about surface binding. Both these pictures are useful, and both pictures have weaknesses. The atomistic pictures concentrate on relatively few atoms, and do not recognize structural features or the energy and carrier fluxes on larger scales. The continuum macroscopic models leave out crystallographic information and the interplay of the processes with high nonequilibrium at smaller scales. Fortunately, there is a middle way: mesoscopic modeling, which both models the key microstructural features and provides a link between microscopic and macroscopic. In a mesoscopic model, the length scale is determined by the system; often this scale is similar to the grain size. Microstructural features like grain boundaries or dislocations are considered explicitly. The time scale in a mesoscopic model is determined by the ablation process (such as the pulse length:) rather than the short time limitations of molecular dynamics, yet the highly nonequilibrium behavior is adequately represented. Mesoscopic models are especially important when key process rates vary on a short length scale. Some microstructural feature (like those in dentine or dental enamel) may absorb light much more than others; other features (like grain boundaries) may capture carriers readily, or allow easier evaporation, or capture and retain charge (like grain boundaries); it is these processes which need a mesoscopic analysis. The results described will be taken largely from the work on MgO of Ribeiro, Ramos, and Stoneham for ablation by sub-band Sap light., Fundação para a Ciência e a Tecnologia (FCT) – PBICT/FIS/2151/95., Engineering and Physical Sciences Research Council (EPSRC) - GR/L02678, GR/L70615.

Conducting polymers have attracted much attention concerning the possibility of their use as active components of electronic and optoelectronic devices. We use a molecular dynamics method with semi-empirical quantum chemistry at CNDO (complete Neglect of differential overlap) level to study the chemical interactions between aluminium atoms and trans-polyacetylene during the interface formation. Our results suggest that aluminium dimer (Al2) bound to a polymer chain is energetically more favourable than the adsorption of isolated aluminium atoms. In both cases, the compound formation is accompanied by charge transfer between metal and polymer. As a result charge rearrangement among the polyacetylene atoms is induced. We shall describe the charge-induced structural relaxation of the trans-polyacetylene backbone which is accompanied by a local change in the electronic structure of the polymer, commonly called defects.

This paper presents some preliminary results on simulations of the absorption of high fluence laser radiation by a target surface. We show the role of absorbing defects and their concentration as well as the influence on target roughness of places that absorb radiation more strongly than others. We then develop a mesoscopic model, and find that the grain boundary width (including any atmosphere of defects or space charge region) is an important parameter., Junta Nacional de Investigação, Ciência e Tecnologia (JNICT) - PBIC/C/FIS/2151/95

The study of the average distance that singlet excitons travel during their lifetime in conjugated polymers has attracted considerable attention during the past decade, because of its importance in the functioning of many polymer-based optoelectronic devices, like solar cells and photodetectors. Intriguingly, different values of exciton diffusion length have been extracted from experiments on seemingly identical conjugated polymers. Here we use computer simulations to show that the observed discrepancies in the reported values of the exciton diffusion length may arise from differences in the orientation of conjugated polymer strands relative to the substrate surface, a factor which has been mostly overlooked. Our results show that, on pristine polymer nanodomains with conjugated strands perpendicular to the substrate surface, exciton migration length is approximately 30% and 40% lower than on those with parallel and random strand orientation relative to that surface, respectively, resulting from the different contents of physical traps present in nanodomains with different strand orientation. This work underlines the importance of molecular arrangement on exciton migration, and provides a novel theoretical framework for estimating the dependence of the exciton diffusion length with the orientation of conjugated polymers strands within the nanodomains, as well as helping the design of more efficient polymer-based optical and optoelectronic devices, such as optical sensors, photodiodes, photovoltaic cells and white light-emitting diodes., This work was supported by FEDER through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/UI607/2011, and under the projects CONC-REEQ/443/EEI/2005 and PEst-C-FIS/UI607/2011-2012. Two of us (H.M.G.C. and H. M. C. B.) are also indebted to FCT and POPH for financial support the post-doctoral grants SFRH/BPD/64554/2009 and SFRH/BPD/80561/2011.

In the past few years, tribocorrosion has become a focus of research because of its relevance in terms of the future in-service degradation mechanisms of materials. In the particular case of decorative coatings, tribocorrosion is certainly one of the most important issues, and sweat corrosion and human contact wear are two other factors that may act as material selection tools. Thus, the current study aimed to investigate the tribocorrosion behavior of a new class of thin films, the Ti–C–O–N system, which is being developed to be used as a surface decorative material due to its relatively dark appearance. The films were prepared by reactive magnetron sputtering. The influence of the structural features on the tribocorrosion behavior is discussed., This research is supported by FEDER funds through COMPETE-Programa Operacional Factores de Competitividade and by national funds through the Fundacao para a Ciencia e a Tecnologia, project PTDC/CTM/69362/2006 and contract SFRH/BD/27569/2006.

et al., The process of accurately gauging lifetime improvements in organic photovoltaics (OPVs) or other similar emerging technologies, such as perovskites solar cells is still a major challenge. The presented work is part of a larger effort of developing a worldwide database of lifetimes that can help establishing reference baselines of stability performance for OPVs and other emerging PV technologies, which can then be utilized for pass-fail testing standards and predicting tools. The study constitutes scanning of literature articles related to stability data of OPVs, reported until mid-2015 and collecting the reported data into a database. A generic lifetime marker is utilized for rating the stability of various reported devices. The collected data is combined with an earlier developed and reported database, which was based on articles reported until mid-2013. The extended database is utilized for establishing the baselines of lifetime for OPVs tested under different conditions. The work also provides the recent progress in stability of unencapsulated OPVs with different architectures, as well as presents the updated diagram of the reported record lifetimes of OPVs. The presented work is another step forward towards the development of pass-fail testing standards and lifetime prediction tools for emerging PV technologies., This work has been supported by the European Commission's StableNextSol COST Action MP1307. The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 609788 (CHEETAH) and n° 290022 (ESTABLIS). ICN2 acknowledges the support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant Nos. SEV-2013-0295 and ENE2013-48816-C5-4-R. Thanks goes to Agencia de Gestio d'Ajuts Universitaris i de Recerca for the support through the Xarxa de Referencia en Materials Avançats per a l'Energia (XaRMAE) and consolidated research group No. 2014SGR-1212. This work has been carried out under the Materials Science Ph.D. Degree for A.M. of the Universitat Autònoma de Barcelona. R.R. and H.H. are grateful for financial support from BMBF (grant number 03EK3502) and ESF. F.P. received support from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 659747. L.S. and M.R. received supported from the Portuguese Foundation for Science and Technology in the framework of the Strategic Program UID/FIS/04650/2013.

In this work, the influence of air pressure during the annealing of Ge quantum dot (QD) lattices embedded in an amorphous Al 2O 3 matrix on the structural, morphological and compositional properties of the film is studied. The formation of a regularly ordered void lattice after performing a thermal annealing process is explored. Our results show that both the Ge desorption from the film and the regular ordering of the QDs are very sensitive to the annealing parameters. The conditions for the formation of a void lattice, a crystalline Ge QD lattice and a disordered QD lattice are presented. The observed effects are explained in terms of oxygen interaction with the Ge present in the film. © 2012 IOP Publishing Ltd., This study has been partially funded through the projects (i) FEDER funds through the COMPETE program ‘Programa Operacional Factores de Competitividade’ and by Portuguese funds through the Portuguese foundation for Science and Technology (FCT) in the frame of the project PTDC/FIS/70194/2006 and PEst-C-FIS/UI607/2011–2012; (ii) ELETTRA Synchrotron Radiation Center through the European Community’s Seventh Framework Programme (FP7/2007–2013) under grant agreement no 226716. SRCP is grateful for financial support through the FCT grant SFRH/BPD/73548/2010 and JMS is grateful for financial support through the Spanish CSIC JAE-DOC program. MB acknowledges support from the Croatian Ministry of Science, Higher Education and Sport (project number 098-0982886-2859).

In this work, we report on the synthesis of Ge nanocrystals (NCs) by pulsed laser deposition (PLD) at room temperature (RT) in an argon atmosphere without any further annealing process. Our results show that functional thin films of crystalline Ge nanoparticles with spherical shapes can be obtained by PLD directly on alumina layers deposited on n-doped Si (100) substrates. In addition, we also demonstrate that a uniform size distribution of NCs with an average diameter of about 3 nm and a density of 2.3x10^11 cm^-2 can be obtained by optimizing a shadow mask set-up, where a solid disk is introduced between the target and the substrate. Charge/discharge effects in Ge NCs deposited on a high-k amorphous alumina layer are also evidenced by conductive atomic force microscopy, which makes them suitable for memory applications., This work has been fully funded through the project PTDC/FIS/70194/2006 financed by the Portuguese Foundation for Science and Technology (FCT), European COST MP0901-NanoTP and COMPETE. SRCP, EMFV, and JMS are grateful for financial support through FCT grants SFRH/BD/29657/2006, SFRH/BD/45410/2008, and SFRH/BPD/64850/2009, respectively. We would like to thank David J. Barber, Augusto Lopes, Ana J. Martins, and Jose Santos for scientific discussion and technical support., European COST MP0901-NanoTP, COMPETE

The optimization of polymer-based optoelectronic devices such as light-emitting diodes (LEDs), photodetectors and photovoltaic cells requires the understanding how molecular properties and the spatial arrangement of the conjugated strands affect the electronic processes underlying the functioning of these devices. Since some of the important features are determined largely by the individual molecular strands and other features depend strongly on the nanostructure, a multi-scale modelling of materials and device properties is needed. In this work we discuss the atomistic and nanoscale modelling of charge injection, transport and trapping single-carrier diode based on poly(p-phenylene venylene) (PPV), which also applies to other optoelectronic devices., This work is part of the research projects POCTI/CTM/41574/2001 and CONC-REEQ/443/EEI/2005, approved by the Portuguese Foundation for Science and Technology (FCT) and support by the European Community Fund FEDER. One of us (H.M.G.C.) is also indebted to FCT for financial support under the post-doctoral grant no. SFRH/BPD/64554/2009.

Although semiconducting polymers are very attractive to be used in optoelectronic devices due to their molecular structure, they are not pristine semiconductors. After deposition it is possible to find out several structural and chemical defects, with different origins, that strongly influence exciton dynamics since they create deep energetic sites, where excitons can migrate leading to their quenching or reducing exciton diffusion length. By using a self-consistent quantum molecular dynamics method we performed a computational study to understand the influence of well-known polymer defects on excitons dynamics. Our results show that these defects influences mainly intramolecular exciton localization and exciton energy., We are indebted to the Portuguese Foundation for Science and Technology (FCT) for financial support under the contract CONC-REEQ/443/EEI/2005. One of us (H.M.G.C.) is also indebted to FCT for financial support under the post-doctoral grant no. SFRH/BPD/64554/2009.

The charge transfer through single-stranded and double-stranded DNA polymer molecules has been the subject of numerous experimental and theoretical studies concerning their applications in molecular electronics. However, the underlying mechanisms responsible for their different electrical conductivity observed in the experiments are poorly understood. Here we use a self-consistent quantum molecular dynamics method to study the effect of an applied electric field along the molecular axis on charge transfer through single-stranded and double-stranded DNA polymer molecules with an injected electron or hole and assess the consequences for electronic applications. Charge transfer through both single-stranded and double-stranded DNA polymer molecules is predicted, regardless of the sign of the injected charge, the molecular structure and the base sequence. The amount of charge transfer through a double-stranded DNA polymer molecule is slightly lower than through the corresponding two isolated single-strands as a result of the lower charge transport through the purine-pyrimidine base-stacking as compared with through DNA nucleobase-stacking. These results suggest that each DNA polymer strand can act as a molecular wire with both the sugar-phosphate backbone and the bases playing an important role in charge transfer, which opens new perspectives for molecular electronics applications., In this work, the calculations were performed on SeARCH (Services & Advanced Computing with HTC/HPC) funded FEDER funds through COMPETE program and by the Portuguese Foundation for Science and Technology (FCT) funds, under contract CONC-REEQ/443/EEI/2005,. One of us (H.M.G.C.) is also indebted to FCT for financial support under the post-doctoral grant no. SFRH/BPD/64554/2009.

One of the factors that limit the efficiency of polymer-based optoelectronic devices, such as photovoltaic solar cells and light emitting diodes, is the exciton diffusion within the polymeric network. Due to the amorphous nature the of polymeric materials, the diffusion of excitons is limited by the energetic and spatial disorder in such systems, which is a consequence not only of the chemical structure of the polymer but also from its orphology at nanoscale. To get a deep understanding on how such effects influence exciton dynamics we performed a quantum molecular dynamics simulations to determine the energetic disorder within the polymer system, and Monte Carlo simulations to study exciton diffusion in three-dimensional (3D) polymer networks that present both spatial and energetic disorder at nanometre scale. Our results show clearly that exciton diffusion in poly(p-phenylenevenylene) (PPV) occurs preferentially in the direction parallel to the electrodes surface for a polymer-based optoelectronic devices with the orientation of the conjugated strands similar to those obtained by the spin-coating technique and the decay of such excitons occurs preferentially in longer strands which allow us to get insight on exciton behaviour in polymeric systems that are not possible to be obtained directly from the experiments., Fundação para a Ciência e a Tecnologia (FCT) – POCTI/CTM/41574/2001, CONC-REEQ/443/EEI/2005, SFRH/BD/22143/2005, Fundação Calouste Gulbenkian

The electric behaviour of polymer diodes has the influence of several factors such as the electrodes work function, the experimental conditions used to deposit the active component or the chemistry of the polymer. Although experimentally it is possible to study the effect of some of these factors on the device performance, for instance by changing the chemical structure of the polymer used or the type of electrodes, it is impossible to study individually each one of these effects because changing one of them can influence the others. Quantum mechanical calculations have shown that depending on the chemical structure of the polymer, its intramolecular properties (e.g. ionization potential, electron affinity or intramolecular charge mobility) can be changed. To understand the effect of the intramolecular properties in the performance of polymer diodes we use a generalized dynamical Monte Carlo method that considers the nanostructure of the polymer layer and the main electronic processes involved in diode functioning. Our results show that the influence of the intramolecular properties on the electric behaviour of pristine polymer-diodes with ohmic contacts depends on the morphology of the polymer layer at nanoscale that can alter not only hole and electron current density for the same applied electric field but also charge density and charge distribution inside the polymer layer., Fundação para a Ciência e a Tecnologia (FCT) – POCTI/CTM/41574/2001; CONC-REEQ/443/EEI/2005; SFRH/BD/22143/2005, FEDER

We have studied the electronic, structural, and elastic properties of Ti 1− x − y Si x Al y N metastable phase, using first principles calculations based on the density functional theory. These calculations provide the lattice parameter, density of states, cohesive energy, formation energy and elastic constants, when Si and Al atoms replace Ti in the TiN lattice. The calculated values of lattice parameters and elastic constants are generally in good agreement with experiments and compare well with other theoretical results. We show that the trend followed by cohesive energy, formation energy, elastic constants is related to the electronic properties and bonding characteristics of these compounds.

In polymer light emitting diodes (PLEDs) each semiconducting polymer chain consists of a large number of conjugated segments linked by kinks or twists and each one of them behaves like a separated straight strand. The length and orientation of the conjugated strands relative to the electrodes surface depend on the deposition conditions used. Atomistic results have shown that the molecular properties of the conjugated strands depend on their length, which can affect the electronic processes involved in PLEDs. The aim of this work is to study the influence of the average conjugation length within the polymer layer on charge injection, trapping and recombination in PLEDs for all polymer strand orientations relative to the electrodes surface obtained experimentally by different techniques. For that purpose we use a mesoscopic model that considers the morphology and the molecular properties of the polymer. Our results show that by increasing the average conjugation length of the active polymer layer the amount of charge injected into the device increases and the recombination probability occurs preferentially in segments longer than the average conjugation length, both effects having implications on the performance of polymer LEDs., Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001, CONC-REEQ/443/EEI/2005 e SFRH/BD/22143/2005, European Community Fund FEDER

It is well known that the morphology of polymer-based optoelectronic devices can influence their efficiency, since the ways that polymer chains pack inside the active layer can influence not only the charge transport but also the optic properties of the device. By using a mesoscopic model we carried out computer experiments to study the influence of the polymer morphology on the processes of charge injection, transport, recombination and collection by the electrodes opposite to those where the injection of bipolar charge carriers take place. Our results show that for polymer layers where the conjugated segments have perpendicular and random orientation relative to the electrodes surface, the competition between charge collection and charge recombination is affected when the average conjugation length of the polymer strands increase. This effect is more pronounced with the increase of the potential barrier at polymer/electrode interfaces that limit charge injection and increase charge collection. For these molecular arrangements the intra-molecular charge transport plays a major role in device performance, being this effect negligible when the polymer molecules have their axis parallel to the electrodes. Although the polymer morphology modelled in this work is far from real, we believe that our model can give some insights on the role of the microstructure on the functioning of polymer-based devices., European Community Fund (FEDER), Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001, CONC-REEQ/443/EEI/2005 e SFRH/BD/22143/2005

Recently some experimental results have showed that the spatial alignment of conjugated polymer chains on nanometre length scales can influence the behaviour of polymer-based electronic devices, such as light-emitting diodes, field effect transistors, and photovoltaic cells. The effects of chain orientation at electrode-polymer interfaces on the charge injection process and charge mobility through the polymer layer are not well understood. In this work we use a generalized dynamical Monte Carlo method to study the influence of different polymer chain orientation relative to the electrodes surface on the electric behaviour of single-layer polymer diode, namely density current and charge density.

DNA is a material that has the potential to be used in nanoelectronic devices as an active component. However, the electronic properties of DNA responsible for its conducting behaviour remain controversial. Here we use a self-consistent quantum molecular dynamics method to study the effect of DNA structure and base sequence on the energy involved when electrons are added or removed from isolated molecules and the transfer of the injected charge along de molecular axis when an electric field is applied. Our results have shown that the DNA molecules of poly(C)-poly(G) on B-form and poly(A)-poly(T) on A-form have the highest energy released when one electron is added or removed from them and their Z-form has the lowest energy released. Besides, when an electric field is applied to a charged DNA molecule along its axis there is electron transfer through the molecule, regardless of the number and sign of the injected charge, the molecular structure and the base sequence. Results from these simulations provide useful information that is hard to obtain from the experiments and needs to be considered for a further modelling aiming to improve charge transport efficiency in nanoelectronic devices based on DNA., Programa Operacional “Ciência ,Tecnologia, Inovação” – POCTI/CTM/41574/2001, Fundação para a Ciência e a Tecnologia (FCT), REEQ/443/EEI/2005 e SFRH/BD/11231/2002

Transparent conducting oxides are widely used as the transparent electrode in polymer light emitting diodes (PLEDs). The physical properties of these materials and consequently device performance strongly depend on their processing and surface treatment. The injection of charge from the transparent electrode into the polymer layer occurs by tunnelling through a potential barrier from the electrode to molecules close to it. This barrier is influenced by the difference in the relevant energy levels of electrode material and polymer molecules, the external applied potential, the Coulomb potential of the charges present in the polymer layer and the potential of their image charges on the electrodes, and may also be altered by electrode degradation effects. A better understanding of the effect of varying this potential barrier on the functioning of PLED is necessary to achieve further improvements in these applications. Here we present a theoretical study of the influence of changes in the potential barrier at the transparent electrode, on bipolar charge evolution through thin polymer layers, in the absence of defects and impurity states, while the other electrode functions as an ohmic contact. Results of a mesoscopic model provide insight into bipolar charge injection, charge and recombination distribution throughout the polymer layer, and may suggest new materials and processing methods to optimize these optoelectronic devices., FEDER., Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001, CONC-REEQ/443/EEI/2005; SFRH/BD/22143/2005.

The authors have performed self-consistent calculations for molecules adsorbed on a metal substrate in the presence of a scanning tunnelling microscope (STM) tip. These calculations of comprise (i) estimates of the inhomogeneous electric field, (ii) the self-consistent calculation of both electronic structure and molecular geometry using an extended semi-empirical molecular orbital method, and (iii) estimates of the tunnelling current using the resulting wave-functions and geometries. The STM tip causes changes in atomic positions which can complicate the link between current and geometry. In particular, induced molecular reorientation may well explain the lack of observation by STM of adsorbed CO on metals.

We have studied the structural, electronic, and mechanical properties of TiCxOy, using first principles calculations based on the density functional theory and pseudopotential method, within the generalized gradient approximation for the exchange-correlation functional. These calculations provide the lattice parameter, density of states, cohesive energy, elastic constants, and moduli as a function of carbon and oxygen content. The calculated values of lattice parameters and elastic moduli are generally in good agreement with experiments and compare well with other theoretical results.

Substantial progress has been made in fabricating optoelectronic devices using polymers as an active material. In polymer light emitting diodes (PLEDs), a balanced injection of electrons and holes from the electrodes is fundamental to increase their performance. Using a mesoscopic model based on a generalized Monte-Carlo method, we studied the influence of changing zero-field barrier heights at both electrode/polymer interfaces in the performance of a PLED with an active layer of poly paraphenylenevinylene) (PPV). Our results show that by controlling the electrodes work functions it is possible to tune the region inside the device where charge recombination preferentially takes place., This work is part of the research projects POCTI/CTM/41574/2001 and CONC-REEQ/443/EEI/2005, approved by the Portuguese Foundation for Science and Technology (FCT) and POCTI and support by the European Community Fund FEDER. One of us (H.M.C.B.) is also indebted to FCT for financial support under PhD grant Nº SFRH/BD/22143/2005.

The possibility of using infrared (IR) spectroscopy to determine the concentration of inversion monomer defects in polymers depends on the knowledge of the relationship between the spectral properties and the polymer microstructure. This can easily be achieved by performing vibrational analysis. In order to investigate the changes in IR spectra of poly(vinylidene fluoride) resulting from the presence of monomeric units in “head-to-head” and “tail-to-tail” positions, we calculated the frequencies and intensities of IR-active vibrations for individual molecules in alpha and beta form with a defect concentration up to 15% and compared them with the ones obtained for a defect-free molecule., Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001, CONC-REEQ/443/2001 e SFRH/BD/11231/2002., Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER).