Determining concentration depth profiles in fluorinated networks by means of electric force microscopy

5 páginas, 6 figuras.-- et al.By means of electric force microscopy, composition depth profiles were measured with nanometric resolution for a series of fluorinated networks. By mapping the dielectric permittivity along a line going from the surface to the bulk, we were able to experimentally access to the fluorine concentration profile. Obtained data show composition gradient lengths ranging from 30 nm to 80 nm in the near surface area for samples containing from 0.5 to 5 wt. % F, respectively. In contrast, no gradients of concentration were detected in bulk. This method has several advantages over other techniques because it allows profiling directly on a sectional cut of the sample. By combining the obtained results with x-ray photoelectron spectroscopy measurements, we were also able to quantify F/C ratio as a function of depth with nanoscale resolution.L.A.M., M.M.K., G.A.S., A.A., and J.C. acknowledge the financial support provided by the Basque Country Government (Grant No. IT-436-07), the Spanish Ministry of Science and Innovation (Grant Nos. MAT 2007-63681 and CSD 2006- 00053). The Donostia International Physics Center (DIPC) financial support is also acknowledged.Peer reviewe

Interplay between steps and oxygen vacancies on curved TiO2(110)

et al.A vicinal rutile TiO(110) crystal with a smooth variation of atomic steps parallel to the [1-10] direction was analyzed locally with STM and ARPES. The step edge morphology changes across the samples, from [1-11] zigzag faceting to straight [1-10] steps. A step-bunching phase is attributed to an optimal (110) terrace width, where all bridge-bonded O atom vacancies (O vacs) vanish. The [1-10] steps terminate with a pair of 2-fold coordinated O atoms, which give rise to bright, triangular protrusions (S) in STM. The intensity of the Ti 3d-derived gap state correlates with the sum of O vacs plus S protrusions at steps, suggesting that both O vacs and steps contribute a similar effective charge to sample doping. The binding energy of the gap state shifts when going from the flat (110) surface toward densely stepped planes, pointing to differences in the Ti polaron near steps and at terraces.We acknowledge financial support from the Spanish Ministry of Economy (Grants MAT2013-46593-C6-4-P and MAT2013-46593-C6-2-P) and the Basque Government (Grant IT621-13 and IT756-13). M.S. and U.D. acknowledge support from the ERC Advanced Grant “OxideSurfaces”. D.S.P. and M.M. acknowledge support from the Marie Curie ITN “THINFACE” and financial support by the Deutsche Forschungsgemeinschaft. through SFB 1083 “Structure and Dynamics of Internal Interfaces”.Peer Reviewe

Strain dependent light-off temperature in catalysis revealed by planar laser-induced fluorescence

Understanding how specific atom sites on metal surfaces lower the energy barrier for chemical reactions is vital in catalysis. Studies on simplified model systems have shown that atoms arranged as steps on the surface play an important role in catalytic reactions, but a direct comparison of how the light-off temperature is affected by the atom orientation on the step has not yet been possible due to methodological constraints. Here we report in situ spatially resolved measurements of the CO production over a cylindrical-shaped Pd catalyst and show that the light-off temperature at different parts of the crystal depends on the step orientation of the two types of steps (named A and B). Our finding is supported by density functional theory calculations, revealing that the steps, in contrast to what has been previously reported in the literature, are not directly involved in the reaction onset but have the role of releasing stress.The authors thank the Knut and Alice Wallenberg Foundation, the Swedish Research Council, the Swedish Foundation for Strategic Research, and the Crafoord Foundation. Support by the MAX IV staff is gratefully acknowledged. The calculations were performed at C3SE through a SNIC grant. J.E.O. acknowledges support from the Spanish Ministry of Economy (MAT2013-46593-C6-4-P) and the Basque Government (IT621-13).Peer Reviewe

Interplay between steps and oxygen vacancies on curved TiO2(110)

Trabajo presentado al Symposium on Surface Science (3S), celebrado en St. Christoph am Arlberg (Austria) del 21 al 27 de febrero de 2016.We acknowledge financial support from the Spanish Ministry of Economy (grant MAT2013-46593-C6-4-P and MAT2013-46593-C6-2-P) and the Basque Government (grant IT621-13 and IT756-13), the ERC Advanced Grant “OxideSurfaces”., and the Marie Curie ITN “THINFACE”.Peer reviewe

Study of relaxation and transport processes by means of AFM based dielectric spectroscopy

Since its birth a few years ago, dielectric spectroscopy studies based on atomic force microscopy (AFM) have gained a growing interest. Not only the frequency and temperature ranges have become broader since then but also the kind of processes that can be studied by means of this approach. In this work we analyze the most adequate experimental setup for the study of several dielectric processes with a spatial resolution of a few nanometers by using force mode AFM based dielectric spectroscopy. Proof of concept experiments were performed on PS/PVAc blends and PMMA homopolymer films, for temperatures ranging from 300 to 400 K. Charge transport processes were also studied by this approach. The obtained results were analyzed in terms of cantilever stray contribution, film thickness and relaxation strength. We found that the method sensitivity is strongly coupled with the film thickness and the relaxation strength, and that it is possible to control it by using an adequate experimental setup.Peer Reviewe

Mapping chemical structure–glass transition temperature relationship through artificial intelligence

Artificial neural networks (ANNs) have been successfully used in the past to predict different properties of polymers based on their chemical structure and to localize and quantify the intramonomer contributions to these properties. In this work, we propose to move forward in order to use the mathematical framework of the ANN for embedding the chemical structure of monomers into a high-dimensional abstract space. This approach allows us not only to accurately predict the glass transition temperature (Tg) of polymers but, even more important, also to encode their chemical structure as m-dimensional vectors in a mathematical space. For this aim, we employed a fully connected neural network trained with a set of more than 200 atactic acrylates that provide the coordinates of the vectorized chemical structures into the m-dimensional space. These data points were then treated with a hierarchical nonparametric clusterization method in order to automatically group similar chemical structures into clusters with alike properties. These clusters were then projected into a human-readable three-dimensional space using principal component analysis. This approach allows us to deal with chemical structures as if they were mathematical entities and therefore to perform quantitative operations, so far hardly imaginable, being essential for both the design of new materials and the understanding of the structure–property relationships.We gratefully acknowledge the financial support from the Spanish Government “Ministerio de Ciencia e Innovación” (PID2019-104650GB-C21) and the Basque Government (IT-1175-19).Peer reviewe

From chemical structure to quantitative polymer properties prediction through convolutional neural networks

In this work convolutional-fully connected neural networks were designed and trained to predict the glass transition temperature of polymers based only on their chemical structure. This approach has shown to successfully predict the Tg of unknown polymers with average relative errors as low as 6%. Several networks with different architecture or hiperparameters were successfully trained using a previously studied glass transition temperatures dataset for validation, and then the same method was employed for an extended dataset, with larger Tg dispersion and polymer´s structure variability. This approach has shown to be accurate and reliable, and does not require any time consuming or expensive measurements and calculations as inputs. Furthermore, it is expected that this method can be easily extended to predict other properties. The possibility of predicting the properties of polymers not even synthesized will save time and resources for industrial development as well as accelerate the scientific understanding of structure-properties relationships in polymer science.Fil: Miccio, Luis Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Schwartz, Gustavo A.. Consejo Superior de Investigaciones Científicas; Españ

Localizing and quantifying the intra-monomer contributions to the glass transition temperature using artificial neural networks

We used fully connected artificial neural networks (ANN) to localize and quantify, based on the monomer structure of several polymers, the specific features responsible for their observed glass transition temperatures (Tg). The use of ANNs allows us not only to successfully predict the Tg of the polymers but, even more important, to understand what parts of the monomer are mainly contributing to it. For this task, we used the weights of a trained ANN as obtained after fitting the input data (monomer structure) to the corresponding Tg value. The study was performed for a set of more than 200 atactic acrylates for which typical Tg defining features were identified. Thus, the ANN is able to recognize the relevance of the backbone stiffness, the length of pending groups or the presence of methyl groups on the value of the glass transition temperature. This approach can be easily extended to many other interesting properties of polymers and it is worth noting that only the monomer chemical structure is needed as input. This method is potentially useful for identifying orthogonal ways of tuning polymer properties during the design and development of new materials and it is expected that it will contribute to a better understanding of the polymer's behavior.Fil: Miccio, Luis Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Schwartz, Gustavo A.. Consejo Superior de Investigaciones Científicas; Españ

Dielectric spectroscopy at the nanoscale by atomic force microscopy: A simple model linking materials properties and experimental response

The use of an atomic force microscope for studying molecular dynamics through dielectric spectroscopy with spatial resolution in the nanometer scale is a recently developed approach. However, difficulties in the quantitative connection of the obtained data and the material dielectric properties, namely, frequency dependent dielectric permittivity, have limited its application. In this work, we develop a simple electrical model based on physically meaningful parameters to connect the atomic force microscopy (AFM) based dielectric spectroscopy experimental results with the material dielectric properties. We have tested the accuracy of the model and analyzed the relevance of the forces arising from the electrical interaction with the AFM probe cantilever. In this way, by using this model, it is now possible to obtain quantitative information of the local dielectric material properties in a broad frequency range. Furthermore, it is also possible to determine the experimental setup providing the best sensitivity in the detected signal. © 2014 AIP Publishing LLC.We acknowledge the financial support provided by the Basque Country Government (IT-654-13) and the Spanish Ministry of Science and Innovation (MAT2012-31088). Financial support from EU-funded “European soft matter infrastructure” (reference 262348 ESMI) is also acknowledged.Peer Reviewe

AFM based dielectric spectroscopy: Extended frequency range through excitation of cantilever higher eigenmodes

In the last years, a new AFM based dielectric spectroscopy approach has been developed for measuring the dielectric relaxation of materials at the nanoscale, the so called nanoDielectric Spectroscopy (nDS). In spite of the effort done so far, some experimental aspects of this technique remain still unclear. In particular, one of these aspects is the possibility of extending the experimental frequency range, to date limited at high frequencies by the resonance frequency of the AFM cantilever as a main factor. In order to overcome this limitation, the electrical excitation of cantilever higher eigenmodes for measuring the dielectric relaxation is here explored. Thus, in this work we present a detailed experimental analysis of the electrical excitation of the cantilever second eigenmode. Based on this analysis we show that the experimental frequency range of the AFM based dielectric spectroscopy can be extended by nearly two decades with a good signal-to-noise ratio. By using the combination of first and second cantilever eigenmodes we study dissipation processes on well known PVAc based polymeric samples. Both, relaxation spectra and images with molecular dynamics contrast were thus obtained over this broader frequency range.We acknowledge the financial support provided by the Basque Country Government (IT-654-13) and the Spanish Ministry of Science and Innovation (MAT2012-31088). Financial support from EU-funded ‘European Soft Matter Infrastructure’ (Reference 262348 ESMI) is also acknowledged.Peer Reviewe