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Miguel Robles

Publications and source records attributed to Miguel Robles.

6 recordsLinked to original sources
arXiv2017

Understanding bond formation and its impact on the capacitive properties of SiW12 polyoxometalates adsorbed on functionalized Carbon Nanotubes

In the last decade, a large number of studies at the experimental level in electrochemical systems for energy storage devices have been performed. However, theoretical approaches are highly desirable to understand the physicochemical properties giving rise to energy storage phenomena. This work was intended to provide insights into the $in$ $silico$ design of novel nanocomposite materials formed by the Keggin polyoxometalate SiW12 anchored to an organic functional group $φ-X$ (with $X = -NH2, -OH, -COH$ and $-COOH$), linked to a carbon nanotube. In these systems, the Density of States around the Fermi level is enhanced, giving the composite material the capacity of facile electron transport that may be determinant at the charge/discharge cycling performed in energy storage devices. Charge transfer at the composite materials under study is greatest for the $φ-COOH$ functional group, yielding an attraction with the SiW12 cluster of the same order of magnitude as that of covalent nature. The rest of the functional groups induce a non-covalent interaction of the electrostatic type, mediated by a van der Waals attraction. Our proposed methodology may represent a tool to develop novel electrode materials that may improve the performance on energy storage devices, such as supercapacitors or Li$-$ion batteries.

cond-mat.mtrl-sci↗
arXiv2017

Electronic structure and non-linear optical properties of organic photovoltaic systems with potential applications on solar cell devices: A DFT approach

The use of eco-friendly materials for the environment has been addressed as a critical issue in the development of systems for renewable energy applications. In this regard, the investigation of organic photovoltaic (OPV) molecules for the implementation in solar cells, has become a subject of intense research in the last years. The present work is a systematic study at the B3LYP level of theory performed for a series of 50 OPV materials. Full geometry optimizations revealed that those systems with a twisted geometry are the most energetically stable. Nuclear independent Chemical shifts (NICS) values show a strong aromatic character along the series, indicating a possible polymerization in solid-state, via a π-π stacking, which may be relevant in the design of a solar cell device. The absorption spectra in the series was also computed using Time Dependent DFT at the same level of theory, indicating that all spectra are red-shifted along the series. This is a promissory property that may be directly implemented in a photovoltaic material, since it is possible to absorb a larger range of visible light. The computed HOMO-LUMO gaps as a measurement of the band gap in semiconductors, show a reasonable agreement with those found in experiment, predicting candidate materials that may be directly used in photovoltaic applications. Non-linear optical (NLO) properties were also estimated with the aid of a PCBM molecule as a model of an acceptor, and a final set of optimal systems was identified as potential candidates to be implemented as photovoltaic materials. The methodological approach presented in this work may aid in the in silico assisted-design of OPV materials.

physics.chem-ph↗
arXiv2014

Note: Equation of state and the freezing point in the hard-sphere model

The merits of different analytical equations of state for the hard-sphere system with respect to the recently computed high-accuracy value of the freezing-point packing fraction are assessed. It is found that the Carnahan-Starling-Kolafa and the branch-point approximant equations of state yield the best performance.

cond-mat.soft↗
arXiv2011

On the relation between virial coefficients and the close-packing of hard disks and hard spheres

The question of whether the known virial coefficients are enough to determine the packing fraction $η_\infty$ at which the fluid equation of state of a hard-sphere fluid diverges is addressed. It is found that the information derived from the direct Padé approximants to the compressibility factor constructed with the virial coefficients is inconclusive. An alternative approach is proposed which makes use of the same virial coefficients and of the equation of state in a form where the packing fraction is explicitly given as a function of the pressure. The results of this approach both for hard-disk and hard-sphere fluids, which can straightforwardly accommodate higher virial coefficients when available, lends support to the conjecture that $η_\infty$ is equal to the maximum packing fraction corresponding to an ordered crystalline structure.

cond-mat.stat-mech↗
arXiv2008

Virial series for fluids of hard hyperspheres in odd dimensions

A recently derived method [R. D. Rohrmann and A. Santos, Phys. Rev. E. {\bf 76}, 051202 (2007)] to obtain the exact solution of the Percus-Yevick equation for a fluid of hard spheres in (odd) $d$ dimensions is used to investigate the convergence properties of the resulting virial series. This is done both for the virial and compressibility routes, in which the virial coefficients $B_j$ are expressed in terms of the solution of a set of $(d-1)/2$ coupled algebraic equations which become nonlinear for $d \geq 5$. Results have been derived up to $d=13$. A confirmation of the alternating character of the series for $d\geq 5$, due to the existence of a branch point on the negative real axis, is found and the radius of convergence is explicitly determined for each dimension. The resulting scaled density per dimension $2 η^{1/d}$, where $η$ is the packing fraction, is wholly consistent with the limiting value of 1 for $d \to \infty$. Finally, the values for $B_j$ predicted by the virial and compressibility routes in the Percus-Yevick approximation are compared with the known exact values [N. Clisby and B. M. McCoy, J. Stat. Phys. {\bf 122}, 15 (2006)]

cond-mat.soft↗