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Sci Rep ; 9(1): 11061, 2019 Jul 30.
Artigo em Inglês | MEDLINE | ID: mdl-31363140


The recent reaching of 20% of conversion efficiency by solar cells based on metal hybrid perovskites (MHP), e.g., the methylammonium (MA) lead iodide, CH3NH3PbI3 (MAPbI3), has excited the scientific community devoted to the photovoltaic materials. However, the toxicity of Pb is a hindrance for large scale commercial of MHP and motivates the search of another congener eco-friendly metal. Here, we employed first-principles calculations via density functional theory combined with the generalized quasichemical approximation to investigate the structural, thermodynamic, and ordering properties of MAPb1-xSixI3, MAPb1-xGexI3, and MAPb1-xSnxI3 alloys as pseudo-cubic structures. The inclusion of a smaller second metal, as Si and Ge, strongly affects the structural properties, reducing the cavity volume occupied by the organic cation and limitating the free orientation under high temperature effects. Unstable and metaestable phases are observed at room temperature for MAPb1-xSixI3, whereas MAPb1-xGexI3 is energetically favored for Pb-rich in ordered phases even at very low temperatures. Conversely, the high miscibility of Pb and Sn into MAPb1-xSnxI3 yields an alloy energetically favored as a pseudo-cubic random alloy with tunable properties at room temperature.

J Phys Chem Lett ; 10(15): 4245-4251, 2019 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-31304757


We overcome the great theoretical computational challenge of mixed perovskites, providing a rigorous and efficient model by including quasiparticle, spin-orbit coupling, and disorder effects. As a benchmark, we consider the mixed MAPb1-xSnxI3 perovskites. The calculations are based on the generalized quasichemical approach and the DFT-1/2 approximated quasiparticle correction. Both cubic and tetragonal structures are investigated. By mapping the entire range of compositions, we correctly describe the bowing-like behavior for the energy gaps with 1.24 eV as the minimum value at x = 0.70, in very good agreement with the experimental data. Furthermore, while the tetragonal alloy reaches the maximum absorbance with a limit for the red shift at x = 1.0, the cubic alloy sets a maximum absorbance/red shift for the optimal composition at x = 0.70.

J Phys Chem Lett ; 10(3): 685-692, 2019 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-30681851


Gold nanoclusters have been the focus of numerous computational studies, but an atomistic understanding of their structural and dynamical properties at finite temperature is far from satisfactory. To address this deficiency, we investigate gold nanoclusters via ab initio molecular dynamics, in a range of sizes where a core-shell morphology is observed. We analyze their structure and dynamics using state-of-the-art techniques, including unsupervised machine-learning nonlinear dimensionality reduction (sketch-map) for describing the similarities and differences among the range of sampled configurations. In the examined temperature range between 300 and 600 K, we find that whereas the gold nanoclusters exhibit continuous structural rearrangement, they are not amorphous. Instead, they clearly show persistent motifs: a cationic core of 1-5 atoms is loosely bound to a shell which typically displays a substructure resulting from the competition between locally spherical versus planar fragments. Besides illuminating the properties of core-shell gold nanoclusters, the present study proposes a set of useful tools for understanding their nature in operando.

J Chem Phys ; 146(16): 164304, 2017 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-28456198


Although several studies have been reported for Pt55 and Au55 nanoclusters, our atomistic understanding of the interplay between the adsorbate-surface interactions and the mechanisms that lead to the formation of the distorted reduced core (DRC) structures, instead of the icosahedron (ICO) structure in gas phase, is still far from satisfactory. Here, we report a density functional theory (DFT) investigation of the role of the adsorption effects of PH3 (one lone pair of electrons) and SH2 (two lone pairs) on the relative stability of the Pt55 and Au55 nanoclusters. In gas phase, we found that the DRC structures with 7 and 9 atoms in the core region are about 5.34 eV (Pt55) and 2.20 eV (Au55) lower in energy than the ICO model with Ih symmetry and 13 atoms in the core region. However, the stability of the ICO structure increases by increasing the number of adsorbed molecules from 1 to 18, in which both DRC and ICO structures are nearly degenerate in energy at the limit of 18 ligands, which can be explained as follows. In gas phase, there is a strong compression of the cationic core region by the anionic surface atoms induced by the attractive Coulomb interactions (core+-surface-), and hence, the strain release is obtained by reducing the number of atoms in the cationic core region, which leads to the 55 atoms distorted reduced core structures. Thus, the Coulomb interactions between the core+ and surface- contribute to break the symmetry in the ICO55 structure. On the other hand, the addition of ligands on the anionic surface reduces the charge transfer between the core and surface, which contributes to decrease the Coulomb interactions and the strain on the core region of the ICO structure, and hence, it stabilizes a compact ICO structure. The same conclusion is obtained by adding van der Waals corrections to the plain DFT calculations. Similar results are obtained by the addition of steric effects, which are considered through the adsorption of triphenylphosphine (PPh3) molecules on Au55, in which the relative stability between ICO and DRC is the same as for PH3 and SH2. However, for Pt55, we found an inversion of stability due to the PPh3 ligand effects, where ICO has higher stability than DRC by 2.40 eV. Our insights are supported by several structural, electronic, and energetic analyses.

J Phys Chem A ; 119(47): 11565-73, 2015 Nov 25.
Artigo em Inglês | MEDLINE | ID: mdl-26524466


We report a density functional theory investigation of the adsorption properties of CO, NO, and OH on the Cu13, Pt7Cu6, and Pt13 clusters in the cationic, neutral, and anionic states with the aim to improve our atomistic understanding of the adsorption properties on bimetallic clusters compared with monometallic clusters. The adsorption energy of CO and NO are substantially stronger on Pt13 than on Cu13, and hence, CO and NO bind preferentially on Pt sites on Pt7Cu6. Thus, it can contribute to drive the migration of the Pt atoms from the core to the surface region in large PtCu nanoalloys. The CO and NO adsorption energies on the bimetallic cluster are enhanced by a few percent compared with the energies of the monometallic clusters, which shows that the Pt-Cu interaction can contribute to an increase in the adsorption energy. In contrast with CO and NO trends, the OH adsorption energies on Cu13, Pt7Cu6, and Pt13 deviates only up to 0.31 eV, and hence, there is no clear preference for Cu or Pt sites on Pt7Cu6 or an enhancement of the adsorption energy on the bimetallic systems. We found a reduction of the CO and NO vibrational frequencies upon adsorption, which indicates a weakening of the CO and NO binding energies, and it is supported by a slight increase in the bond lengths. However, the OH vibrational frequency increases upon adsorption, which indicates an enhancement of the OH binding energy, which is supported by a slight decrease in the bond length by about 0.01 Å. It can be explained by the large charge transfer from the clusters to the O atom, which enhances the electrostatic interaction in the O-H bonding.