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Density functional theory calculations support the possibility that Li10Si4 and Li6Si5 clusters can be combined to assemble structures, opening opportunities for silicon–lithium cluster assembled materials.

Here, it is shown that the M

The stability, electronic, and transfer properties of a photo-sensitizer donor-( π -bridge)-acceptor (D- π -A) zinc-porphyrin dye adsorbed on anatase TiO2(101) and TiO2(001) surfaces are studied by periodic density functional theory calculations, including the screened hybrid functional. We also study the dye adsorption on the TiO2(101) surface with an oxygen vacancy. Our results show that the deprotonated dye attaches strongly on surface Ti 5 c atoms forming both bidentate bridging and chelating linkage modes. The protonated dye is found stable on pristine TiO2(101), forming the monodentate linkage mode. However, on TiO2(001) and defective TiO2(101), the monodentate mode stabilizes after a spontaneous dye deprotonation. The electronic band structure of the TiO2-dye assemblies shows the dye lowest-unoccupied molecular-orbital resonant with TiO2 conduction bands, exhibiting a strong hybridization, where the electron injection potentials are calculated around 1 eV for all systems under study. This high value, together with the favorable electronic coupling, suggest a fast electron injection from the dye into the semiconductor as experimentally reported. Moreover, the electronic properties of TiO2-dye assemblies show similarities independent of the adsorption modes or the surface characteristic, suggesting that the high efficiency of anatase TiO2-based dye sensitized solar cell resides largely in its favorable electron injection properties.

68-Atom gold nanoparticles (Au68) were adsorbed on fractured carbon nanotubes by acid treatment to obtain an active catalyst (Au68–CNT) for the oxygen reduction reaction (ORR). TEM images show the importance of acid treatment to create defects on the walls of the CNT, where Au68 would then adsorb. X-Ray photoelectron spectroscopy confirmed that the Au68 surface is exclusively composed of metallic gold when at the surface of CNT. FTIR characterization showed the presence of 4-mercapto-benzoic acid residues, which are necessary for the synthesis and are then removed through electrochemical reduction. The electrocatalytic studies were conducted in an alkaline medium, where Au68–CNT proved to be an efficient catalyst for the ORR with the transfer of 3.9 electrons for direct water production. In acidic medium, poor performance and stability were observed. Durability tests were conducted by performing 5000 accelerated cyclic voltammograms, during which the half-wave potential of the polarization curve for the ORR decreased only by 20 mV. The DFT calculations were performed to simulate the interactions between O2 and Au68–CNT, and energy barriers, using the NEB method for O2 dissociation and contrasting values with Au (111), Au (110), Au (100), and Pt68–CNT. Large energy barriers indicating low ORR activity were obtained at surfaces different from Au68–CNT and Pt68–CNT. O2 would bind to four Au atoms in Au68–CNT with a binding energy of −0.56 eV. The reactivity of Au68–CNT for ORR would be associated with the irregular geometry of the Au68 surface.

The oxygen reduction reaction (ORR) is a highly important reaction in electrochemistry. The following short review details recent advances in novel non-precious metal catalysts containing transition metal macrocycles for use in the ORR. Unbound, many of these electrodes were found to generate high levels of side products such as O2− and H2O2 via 2-electron processes, and for this reason, it is aimed to create systems which would favor a 4-electron process which would completely convert oxygen to H2O. The 4-electron reduction of O2 releases the most energy in a fuel cell. Novel catalytic materials containing metal macrocycles were created mimicking the structure of enzyme metal centers, the metal in the macrocycle bound to a fifth axial ligand. These structures were observed to exhibit improved catalytic activity, and in the case of cobalt, phthalocyanine systems were observed to move away from the inefficient 2-electron process towards the more complete 4-electron process in alkaline media. Both experimental results (XPS, EPR, cyclic voltammetry and polarization curves) and theoretical models were gathered for various pentacoordinate systems and various electronic effects of the axial ligand on metal center were proposed. Penta-coordinate macrocycles are an important tool for further manipulating and tuning the electronic behavior of transition metal centers for catalysis of the ORR.

11 pags., 4 figs., 2 tabs., Fe(III) 5,10,15,20-(tetraphenyl)porphyrin chloride (FeTPP) and Co(III) 5,10,15,20-(tetraphenyl)porphyrin chloride (CoTPP) were adsorbed on carbon Vulcan and studied as electrocatalysts for the oxygen reduction reaction (ORR) before and after pyrolysis. The pyrolysis process was also simulated through ab initio molecular dynamic simulations and the minimum energy path for the O dissociation after the interaction with the metal center of the FeTPP and CoTPP were calculated. After the pyrolysis the FeTPP showed the best performances reducing O completely to HO with increased limiting current and lower overpotential. Tafel slops for the various catalysts did not change after the pyrolytic process suggesting that the mechanism for the ORR is not affected by the heat treatment. TEM images, X-ray diffraction, XPS spectroscopy, Fe Mössbauer, and DFT simulations, suggest that there is no breakdown of the macrocyclic complex at elevated temperatures, and that the macro cyclic geometry is preserved. Small variations in the Metal-O (M-O) binding energies and the M–N bond length were observed which is attributed to the dispersive interaction between the macrocycles and the irregular surface of the Vulcan substrate induced by the heat treatment and causing better interaction with the O molecule. The theoretical strategy herein applied well simulate and explain the nature of the M–N–C active sites and the performances towards the ORR., We thank fnancial support from FONDECYT Projects No . 1181840, 1220988 1170480 Dicyt-USACH publicaciones and DICYT POSTDOC 20211768. Te computational resources used in this work were provided by Fenix HPC (UNAB).

The equilibrium geometries and thermodynamic properties of anion vacancies in cadmium telluride, as predicted by density functional theory, are revisited using semilocal and hybrid density functionals. We find that stable configurations in different charge states can only be found after a systematic search considering several starting geometries. The stable charge states, 0 and 2+, display closed-shell electronic configurations, without deep bandgap levels. The 2+ charge state has a Td symmetry with an outward relaxation, while the neutral state is a mixture of configurations with C2v and C3v symmetries, both with the same energy and a negligible energy barrier. Therefore, the neutral charge state presents an effective Td symmetry. Configurations with different symmetries, e.g., D2d, can exist as metastable states. We show that certain configurations may seem falsely stable due to several facts: the bandgap error of generalized gradient approximation, the k-point sampling used in small supercells, or the use of a restricted set of starting geometries. We believe that the HSE06 hybrid functional allows to obtain accurate formation energies and geometries. We analyze the effect of the spin-orbit coupling and GW quasiparticle corrections to the HSE06 results, and find no qualitative differences. The spin-orbit coupling and GW corrections to the HSE06 energies partially cancel each other. Finally, we investigate the divacancy VCdVTe. The obtained formation energies suggest that isolated tellurium vacancies in neutral charge state can be found only in Te-poor growth conditions, coexisting with divacancies.

Extensive explorations of their potential energy surfaces, combined with high-level quantum chemical computations, strikingly show that the lowest energy structures of the (Li6 Si5 )2-5 systems consist of 2-5 Si5 6- aromatic units, surrounded by Li+ counterions, respectively. These viable gas-phase compounds are the pioneering reported cases of oligomers made by planar aromatic silicon rings. Based on the key evidence that these oligomers are energetically favored, and that their silicon rings aromaticity is thoroughly preserved, the Li6 Si5 cluster is proposed as a potential assembly unit to build silicon-lithium nanostructures, thus opening new paths to design Zintl compounds at the nanoscale level.

Changes in cellular identity (also known as histologic transformation or lineage plasticity) can drive malignant progression and resistance to therapy in many cancers, including lung adenocarcinoma (LUAD). The lineage-specifying transcription factors FoxA1 and FoxA2 (FoxA1/2) control identity in NKX2-1/TTF1-negative LUAD. However, their role in NKX2-1-positive LUAD has not been systematically investigated. We find that Foxa1/2 knockout severely impairs tumorigenesis in KRAS-driven genetically engineered mouse models and human cell lines. Loss of FoxA1/2 leads to the collapse of a dual-identity state, marked by co-expression of pulmonary and gastrointestinal transcriptional programs, which has been implicated in LUAD progression. Mechanistically, FoxA1/2 loss leads to aberrant NKX2-1 activity and genomic localization, which in turn actively inhibits tumorigenesis and drives alternative cellular identity programs that are associated with non-proliferative states. This work demonstrates that FoxA1/2 expression is a lineage-specific vulnerability in NKX2-1-positive LUAD and identifies mechanisms of response and resistance to targeting FoxA1/2 in this disease.

Change in cancer cell identity is well characterized as a mechanism of cancer progression and acquired resistance to targeted therapies. Lung adenocarcinoma (LUAD) exhibits significant heterogeneity in cell identity and differentiation state; these characteristics correlate directly with prognosis, response to available therapies, and acquisition of drug resistance. In previous work, we have shown that FoxA1 and FoxA2 (FoxA1/2) activate a gastric differentiation program in NKX2-1-negative LUAD. Here we investigate the role of FoxA1/2 in NKX2-1-positive LUAD. We find that FoxA1/2 are consistently expressed in NKX2-1-positive human LUAD. Foxa1/2 deletion severely impairs proliferation and significantly prolongs overall survival in a genetically engineered mouse model of KRAS-driven LUAD. FoxA1/2 activate expression of a mixed-lineage transcriptional program characterized by co-expression of pulmonary (Alveolar Type II) and gastrointestinal marker genes. Loss of FoxA1/2 causes a lineage switch, activating gene expression programs associated with Alveolar Type I cells and maturing squamous epithelial cells. Inhibition of NKX2-1 partially rescues the antiproliferative impact of FoxA1/2 loss, showing that NKX2-1 retains some degree of activity in FoxA1/2-null cells, despite its inability to activate canonical target genes. In summary, this study identifies FoxA1/2 expression as a novel vulnerability in NKX2-1 positive LUAD and shows that FoxA1/2 actively regulate cellular identity in this disease.

Iron(II) 2,3,9,10,16,17,23,24-octa(cyano)phthalocyanine (OCNFePc), was tested as a catalyst for the oxygen reduction reaction (ORR) adsorbed on carbon nanotubes. The composite was analyzed spectroscopically and electrochemically characterized at pH 13 and pH 1. The composite showed close to 4 electron processes at pH 13. Computational analysis indicates that the O2 molecule binds end-on to the metal center and that the dioxygen molecule is dissociated on both the Fe metal center and the corral ring. An Analysis of the molecular electrostatic potential confirms the behavior of cyano residues as electron-withdrawing moieties in OCNFePc. As a result of its catalytic behavior and theoretical analysis of its O2 binding energy, OCNFePc was placed in a high position on a volcano correlation of similar phthalocyanine composites.

In this work we propose a two-dimensional carbon-nitrogen allotrope composed of pentagons and octagons in a structure of carbon chains bound by nitrogen dimers. Our first-principles calculations show the system behaves as a small-gap semiconductor which is thermodynamically stable at room temperature confirmed by molecular dynamic simulations. By cutting the sheet along different crystallographic directions, a variety of metallic or semiconductor one-dimensional ribbons can be obtained. We explore the topological properties of these nanoribbons finding that for a particular boundary geometry nontrivial topological phases can be obtained, depending on the ribbon termination. We investigate the emergence of symmetry protected interface topological states in heterojunctions and topological-induced edge states in finite ribbons.

The energetics and diffusion of water molecules and hydrated ions (Na+, Cl−) passing through nanopores in graphene are addressed by dispersion-corrected density functional theory calculations and ab initio molecular dynamics (MD) simulations. Pores of about 0.8 nm in diameter with different pore-edge passivations, with (H) and (O, H) atoms, were considered. Our MD simulations show a water flux through the hydroxylated pores of about one H2O molecule every three picoseconds, in close agreement with recent experiments that estimated a water flux of three molecules per picosecond through pores of ∼1 nm. We also find that both pores are effective in blocking hydrated Na+ and Cl− ions with large energy barriers, ranging from 12 to 15 eV. In addition, pore passivation with O atoms would increase the water transport through hydroxylated pores, due to the formation of hydrogen bonds with nearby water molecules, which is not observed in the hydrogenated pores.

Iron macrocyclic complexes (MN4) are promising catalysts for replacing platinum (the industrial standard) in electrocatalysis. In particular, FeN4 complexes have shown lower overpotential than Pt for the oxygen reduction reaction (ORR) in alkaline media. To predict the electrochemical activity of metal electrodes and molecular catalysts towards the ORR, reactivity descriptors with typical volcano correlation have been demonstrated. The most important are M–O2 binding energy and M(n)+/M(n−1)+ redox potentials for the complexes. We studied a new Fe complex, which possesses powerful electron-withdrawing fluorine residues at the periphery of the phthalocyanine ring. Fe hexadecafluorophthalocyanine (16(F)FePc) was characterized by electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS) in the presence and in absence of O2. Experimental and calculated O2–Fe binding energies, as well as electrochemical characterization confirms the excellent activity of this complex for the ORR placing this complex at the top of the MN4 volcano correlation.

Defect energetics, charge transition levels, and electronic band structures of several Cl-related complexes in CdTe are studied using density-functional theory calculations. We investigate substitutional chlorine (ClTe and ClCd) and complexes formed by ClTe with the cadmium vacancy (ClTe-VCd and 2ClTe-VCd) and the TeCd antisite (ClTe-TeCd). Our calculations show that none of the complexes studied induce deep levels in the CdTe band gap. Moreover, we find that ClTe-VCd and ClTe are the most stable Cl-related centers in n-type and p-type CdTe, under Te-rich growth conditions, showing shallow donor and acceptor properties, respectively. This result suggests that the experimentally-observed Fermi level pinning near midgap would be originated in self-compensation. We also find that the formation of the ClTe-TeCd complex passivates the deep level associated to the Te antisite in neutral charge state.

The oxygen reduction reaction (ORR) is a fundamental chemical reaction in industrial processes as well as in living systems, being O2, the final electron (e-) acceptor of many reactions. Since then its importance in the fuel cell technology where energy would be stored in the form of H2 to be oxidized in conjunction with the reduction of the cheap and abundant O2. However, the ORR is a complicated reaction which involves the transfer of 4 electrons and 4 protons and for its nature, it proceeds through the consequent formation of more than one intermediate accounting for the so-called “scaling correlations” [1]. The complexity of the reaction determines the slow kinetics and the difficulties into finding an adequate catalyst for it to proceed without energy losses. The “industrial standard” catalysts for this reaction are based on the very expensive and rare Pt metal. An uncountable number of non-precious metal catalysts (NPMC) have been studied with the purpose to replace Pt. In our research group, we are adopting various strategies to increase the activity and the stability of metal phthalocyanines like the substitution of planar neutral residues with more electron-negative ones, and the addition of axial back ligands [2-5]. In this research work we studied the electrocatalytic activity towards the ORR of a new compound i.e. the iron 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadeca(fluoro) phthalocyanine (16(F)FePc) in the absence and in the presence of a fifth pyridine axial ligand (FeN5). Interesting the 16(F)FePc appears to be the most active among all the FeN4 studied to the moment. The very high redox potential of the active Fe(III)/Fe(II) redox couple (-0.075 V and 0.44 V vs. SCE at pH 13 and at pH 1 respectively) confirms the correlation between the activity of the catalyst vs. redox potential of the active redox couple of the metal in the catalyst. 16(F)FePc was so active towards the ORR that the production of H2O2 measured at rotating ring electrodes was nil at pH 13 and very low at pH 1. In the presence of pyridine axial ligand, the redox potential increased of almost 60 mV and catalytic currents and stability of the catalyst also drastically increased. The catalysts were characterized with electrochemical techniques and by EPR and XPS spectroscopy in the presence and in the absence of O2. We could therefore experimentally evaluate the binding energy of O2 to the Fe metal centre. Ab initio calculations, confirm the experimental data and the importance of the pyridine axial ligand to lower the binding energy of O2 to the Fe metal centre because of decoupling of the Fe from the carbon support and changes in the geometrical and electronic structure. [1] Reactivity Descriptors for the Activity of Molecular MN4 Catalysts for the Oxygen Reduction Reaction. Angew. Chem. Int. Ed., 55 (2016) 14510-14521. J.H. Zagal, M.T.M. Koper. [2] Adsorption of 4,4 ́-dithiodipyridine axially coordinated to Iron(II) phthalocyanine on Au(111) as a new strategy for oxygen reduction electrocatalysis. Chemphyschem, (2018), 19(13), 1599-1604.S. Herrera, F. J. Williams, E. J. Calvo, F. Tasca. [3] Biomimicking vitamin B12. A Co phthalocyanine pyridine axial ligand coordinated catalyst for the oxygen reduction reaction. Electrochimica Acta. (2018), 265, 547-555. J. Riquelme, K. Neira, W. Orellana, P. Hermosilla, J. F. Marco, J. H. Zagal, F. Tasca. [4] Comparison of the Catalytic Activity for the Reduction of O2 of Fe and Co MN4 Complexes Adsorbed on Edge Plane Graphite Electrodes and on Carbon Nanotubes. Physical Chemistry Chemical Physics. (2017), 19(31), 20441-20450. R. Venegas, F. J. Recio, J. Riquelme, K. Neira, J. F. Marco, I. Ponce, J. H. Zagal, F. Tasca. [5] Biomimetic reduction of O2 in acid medium on iron phthalocyanines axially coordinated to pyridine anchored on carbon nanotubes”. Journal of Material Chemistry A. (2017), 5(24), 12054-12059. R. Venegas, F. J. Recio, J. Riquelme, K. Neira, J. F. Marco, I. Ponce, J. H. Zagal, F. Tasca.

The purpose of this study is to show that H2 is easily dissociated on lithium edge decorated carbon systems to form strong C–H and Li–H bonds. This mechanism has not been considered in previous studies where these kinds of systems have been proposed as good candidates to serve as hydrogen storage materials. The reactivity of molecular hydrogen (H2) on three representative lithium edge-decorated carbon systems (on the clusters C5Li7+ (1) and C6Li6 (2), and on lithium edge-decorated zig-zag graphene nanoribbon (GNR-Li) (3)) have been studied using ab initio calculations based on the density functional theory with dispersion-corrected van der Waals exchange correlation functional. Our calculations show, on the one hand, that heterolytic hydrogen dissociation can precede with relatively low reaction barriers (0.60, 0.45 and 0.56 eV for systems 1, 2 and 3, respectively) along the minimum energy path and, on the other hand, that chemisorption energies are highly stabilizing (in the range of 1.15–1.54 eV). It is important to note that the highest activation barrier is found for the unique system, characterized as global minimum, on its corresponding potential energy surface (PES), which is system 1. These findings suggest that reversibility of the hydrogen absorption/desorption reactions, required in promising hydrogen storage materials, does not apply in these systems.

The oxygen reduction reaction (ORR) is a crucial chemical process, which contributes significantly to the energy efficiency of fuel cells and metal-air batteries. Although Pt and its alloys are known as the most efficient catalysts for ORR, its high costs and scarcity limits their applications. Alternatively, Fe–N–C catalysts have attracted the most interest owing to their low-cost and facile preparation methods. Metal macrocycles, such as iron phthalocyanines (FePc), are the precursors most widely used to prepare the ORR catalysts in fuel cells and metal-air batteries. However, limited progress has been made in improving their electrocatalytic activity and durability. In particular, FePc has demonstrated to suffer from severe degradation in the acidic fuel cell environments, commonly attributed to demetalation and/or degradation by ORR intermediates. The typical life cycle for FePc catalysts is less than 100 cycles in either acidic or basic media. In this work we use ab initio molecular dynamic (MD) simulations to investigate the degradation of graphene-supported FePc in acidic and basic media at operational conditions. The acidic media includes hydrochloric, acetic, sulfuric and phosphoric acids, whereas the basic media includes sodium hydroxide. The solution concentration in all cases is 1M. The MD simulations were performed by dispersion-corrected DFT calculations, using periodic boundary conditions, at 80°C through a simulation time of 2 ps. Our results show that demetalation is not the cause of the FePc degradation, instead we find that for acetic and hydrochloric acids, the acid anion binds to the Fe center, with binding energies of -1.1 and -6.5 eV, respectively. The formation of this ligand would hinder the O2 adsorption on the metal center, limiting the FePc catalytic activity. On the contrary, for phosphoric and sulfuric acids, as well as for basic media, the ion-Fe binding is energetically unlikely, enabling the O2-metal interaction. Therefore, our results suggest that the catalytic activity and degradation of FePc are connected to the acidic media involved. This work was supported by ANID/FONDECYT under Grant No. 1170480

Iron(II) 2,3,9,10,16,17,23,24-octa(cyano)phthalocyanine (OCNFePc), was tested as a catalyst for the oxygen reduction reaction (ORR) adsorbed on carbon nanotubes. The composite was analyzed spectroscopically and electrochemically characterized at pH 13 and pH 1. The composite showed close to 4 electron processes at pH 13. Computational analysis indicates that the O2 molecule binds end-on to the metal center and that the dioxygen molecule is dissociated on both the Fe metal center and the corral ring. An Analysis of the molecular electrostatic potential confirms the behavior of cyano residues as electron-withdrawing moieties in OCNFePc. As a result of its catalytic behavior and theoretical analysis of its O2 binding energy, OCNFePc was placed in a high position on a volcano correlation of similar phthalocyanine composites.

In this work, cadmium telluride (CdTe) thin films were doped with Sn using a low-cost co-sublimation technique by a homemade close-spaced sublimation (CSS) system. Sn amount was calculated to obtain CdTe film doped with a percentage content lower than 0.4 at%. Chemical, structural, morphological, and optical properties of the as-deposited CdTe (u-CdTe) and Sn-doped CdTe (t-CdTe) films were measured and compared. Raman and XPS characterization showed the presence of Sn in t-CdTe thin films and XRD analysis revealed a strong signal related to CdTe planes (220) and (311) and in a lesser extent a signal related to SnTe. Irregular shaped grains with similar size related to Sn incorporation in the films were found with FE-SEM technique. Photoluminescence spectrum indicates an energy band at 0.7 eV, generating a band inside of CdTe bandgap, due to defects induced by the Sn in CdTe lattice, matching with a theoretical study. The changes observed in Sn doped CdTe film suggest that it can be used for optoelectronics applications.

Extensive explorations of their potential energy surfaces, combined with high-level quantum chemical computations, strikingly show that the lowest energy structures of the (Li

Indexación: Scopus. This paper was supported by the Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT, Chile) under Grants No. 1170480 (W.O.) and No. 1171807 (M.A.F. and E.M-P.). Powered@NLHPC: This research was partially supported by the supercomputing infrastructure of the NLHPC (ECM-02). Hybrid functionals, which mix a fraction of Hartree-Fock exchange with local or semilocal exchange, have become increasingly popular in quantum chemistry and computational materials science. Here, we assess the accuracy of the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional to describe many-electron interactions and charge localization in semiconductors. We perform diffusion quantum Monte Carlo calculations to obtain the accurate ground-state spin densities of the negatively charged (SiV)- and the neutral (SiV)0 silicon-vacancy center in diamond and of the cubic silicon carbide (3C-SiC) with an extra electron. We compare our diffusion quantum Monte Carlo results with those obtained with the HSE functional and find a good agreement between the two methods for (SiV)- and (SiV)0, whereas the correct description of 3C-SiC with an extra electron crucially depends on the amount of Hartree-Fock exchange included in the functional. Also, we examine the case of the neutral Cd vacancy in CdTe, for which we assess the performance of HSE versus the many-body GW approximation for the description of the position of the defect states in the band gap. © 2018 American Physical Society. https://journals.aps.org/prb/abstract/10.1103/PhysRevB.98.155131

CoN4 complexes like Co phthalocyanines (CoPc) have been extensively studied as electrocatalysts for the oxygen reduction reaction (ORR) but they only promote the 2-electron reduction of O to give peroxide. In contrast, vitamin B a Co macrocyclic naturally occurring molecule has attracted the attention of the scientific community because instead of catalysing the 2-electron reduction of O to HO like the other Co macrocycles it promotes the 4-electron reduction to HO. Vitamin B possesses an axial back ligand and this seems to be the reason for its higher activity and selectivity for the 4-electron reduction of O. To test this hypothesis, we synthetized a CoPc axially coordinated to pyridine anchored to carbon nano-tubes (CoPc-Py-CNT). The Co centre is therefore coordinated to 5 N as in vitamin B. The modified CoPc containing catalytic material was characterized by EPR and XPS spectroscopy. Ab initio calculations, Koutecky– Levich extrapolation and Tafel plots well describe the similarities between the 2 complexes and reveal insights into the mechanism of action of Co penta-coordinated complexes. According to our results the pyridine back ligand increases the Co-O binding energy and making it more similar to that of Vitamin B, favouring the splitting of the O-O bond. The back ligand then plays a crucial role in modifying Co-O binding energy which is a well know reactivity descriptor.

The structural and electronic properties of pristine and H-passivated C-terminated β-SiC(0 0 1)-c(2 × 2) surface are addressed by ab initio calculations. Here, we verify the formation of C chains composed by double-bonded dimers rows (C C), separated by triple-bonded bridged dimers (C C). The surface states near the bandgap are confined along the C C dimer rows, with no electronic contribution from the C C bridged dimers. After hydrogenation, the C-chains are strongly modified, forming subsurface voids or nanotunnel (NT) structures. By considering a plausible set of energy release steps for increasing hydrogenation, we obtain a C-rich NT ruled by the C C dimer rows. Somewhat similar to that recently reported on the Si-rich termination, but 0.8 eV lower in energy. The electronic band structures of both Si-rich and C-rich NTs have been examined within the hybrid HSE06 functional, which are compared with those previously reported using a semilocal functional.

The quantum states and thermodynamical properties of the Te vacancy in CdTe are addressed by first principles calculations, including the supercell size and quasiparticle corrections. It is shown that the 64-atoms supercell calculation is not suitable to model the band structure of the isolated Te vacancy. This problem can be solved with a larger 216-atoms supercell, where the band structure of the defect seems to be a perturbation of that of the perfect crystal. It is interesting to note that the Te-vacancy formation energy calculated with both supercell sizes are close in energy, which is attributed to error cancelation. We also show that the interplay between supercell size effects and the band gap underestimation of the generalized gradient approximation strongly influences the predicted symmetry of some charge states.

We investigate the self-compensation mechanism in phosphorus-doped CdTe. The formation energies, charge transition levels, and defects states of several P-related point defects susceptible to cause self-compensation are addressed by first-principles calculations. Moreover, we assess the in uence of the spin-orbit coupling and supercell-size effects on the stability of AX centers donors, which are believed to be responsible for most of the self-compensation. We report an improved result for the lowest-energy configuration of the P interstitial (P$_\text{i}$) and find that the self-compensation mechanism is not due to the formation of AX centers. Under Te-rich growth conditions, (P$_\text{i}$) exhibits a formation energy lower than the substitutional acceptor (P$_\text{Te}$) when the Fermi level is near the valence band, acting as compensating donor. While, for Cd-rich growth conditions, our results suggest that p-type doping is limited by the formation of (P$_\text{Te}$-V$_\text{Te}$) complexes., Comment: 5 pages

The stability, electronic and optical properties of single-walled carbon nanotubes (CNTs) and graphene noncovalently functionalized with free-base tetraphenylporphyrin (TPP) molecules is addressed by density functional theory calculations, including corrections to dispersive interactions. We study the TPP physisorption on 42 CNT species, particularly those with chiral indices ($n$,$m$), where $5 \leq n \leq 12$ and $0\leq m\leq n$. Our results show a quite strong $\pi$-$\pi$ interaction between TPP and the CNT surface, with binding energies ranging from 1.1 to 1.8 eV, where higher energies can be associated with increasing CNT diameters. We also find that the TPP optical absorptions would not be affected by the CNT diameter or chirality. Results for the TPP physisorption on graphene show a remarkable stability with binding energy of 3.2 eV, inducing a small redshift on the $\pi$-stacked TPP absorption bands. The strong graphene-TPP interaction also induces a charge transfer from TPP to graphene, indicating a $n$-type doping mechanism without compromising the graphene structure., Comment: Accepted for publication in Journal of Materials Science

The oxygen reduction reaction (ORR) is a fundamental chemical reaction in industrial processes as well as in living systems, being O2, the final electron (e-) acceptor of many reactions. Since then its importance in the fuel cell technology where energy would be stored in the form of H2 to be oxidized in conjunction with the reduction of the cheap and abundant O2. However, the ORR is a complicated reaction which involves the transfer of 4 electrons and 4 protons and for its nature, it proceeds through the consequent formation of more than one intermediate accounting for the so-called “scaling correlations” [1]. The complexity of the reaction determines the slow kinetics and the difficulties into finding an adequate catalyst for it to proceed without energy losses. The “industrial standard” catalysts for this reaction are based on the very expensive and rare Pt metal. An uncountable number of non-precious metal catalysts (NPMC) have been studied with the purpose to replace Pt. In our research group, we are adopting various strategies to increase the activity and the stability of metal phthalocyanines like the substitution of planar neutral residues with more electron-negative ones, and the addition of axial back ligands [2-5]. In this research work we studied the electrocatalytic activity towards the ORR of a new compound i.e. the iron 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadeca(fluoro) phthalocyanine (16(F)FePc) in the absence and in the presence of a fifth pyridine axial ligand (FeN5). Interesting the 16(F)FePc appears to be the most active among all the FeN4 studied to the moment. The very high redox potential of the active Fe(III)/Fe(II) redox couple (-0.075 V and 0.44 V vs. SCE at pH 13 and at pH 1 respectively) confirms the correlation between the activity of the catalyst vs. redox potential of the active redox couple of the metal in the catalyst. 16(F)FePc was so active towards the ORR that the production of H2O2 measured at rotating ring electrodes was nil at pH 13 and very low at pH 1. In the presence of pyridine axial ligand, the redox potential increased of almost 60 mV and catalytic currents and stability of the catalyst also drastically increased. The catalysts were characterized with electrochemical techniques and by EPR and XPS spectroscopy in the presence and in the absence of O2. We could therefore experimentally evaluate the binding energy of O2 to the Fe metal centre. Ab initio calculations, confirm the experimental data and the importance of the pyridine axial ligand to lower the binding energy of O2 to the Fe metal centre because of decoupling of the Fe from the carbon support and changes in the geometrical and electronic structure. [1] Reactivity Descriptors for the Activity of Molecular MN4 Catalysts for the Oxygen Reduction Reaction. Angew. Chem. Int. Ed., 55 (2016) 14510-14521. J.H. Zagal, M.T.M. Koper. [2] Adsorption of 4,4 ́-dithiodipyridine axially coordinated to Iron(II) phthalocyanine on Au(111) as a new strategy for oxygen reduction electrocatalysis. Chemphyschem, (2018), 19(13), 1599-1604.S. Herrera, F. J. Williams, E. J. Calvo, F. Tasca. [3] Biomimicking vitamin B12. A Co phthalocyanine pyridine axial ligand coordinated catalyst for the oxygen reduction reaction. Electrochimica Acta. (2018), 265, 547-555. J. Riquelme, K. Neira, W. Orellana, P. Hermosilla, J. F. Marco, J. H. Zagal, F. Tasca. [4] Comparison of the Catalytic Activity for the Reduction of O2 of Fe and Co MN4 Complexes Adsorbed on Edge Plane Graphite Electrodes and on Carbon Nanotubes. Physical Chemistry Chemical Physics. (2017), 19(31), 20441-20450. R. Venegas, F. J. Recio, J. Riquelme, K. Neira, J. F. Marco, I. Ponce, J. H. Zagal, F. Tasca. [5] Biomimetic reduction of O2 in acid medium on iron phthalocyanines axially coordinated to pyridine anchored on carbon nanotubes”. Journal of Material Chemistry A. (2017), 5(24), 12054-12059. R. Venegas, F. J. Recio, J. Riquelme, K. Neira, J. F. Marco, I. Ponce, J. H. Zagal, F. Tasca. Acknowledgments F.T thanks Fondeyct project 1181840. J.H.Z thanks the Millennium Project RC120001, Fondecyt 1181037. W.O. thanks Fondecyt 1170480.

The O2 dissociation after the chemisorption on the metal center of M–N4 moieties in graphene (with M = Mn, Fe, and Co) is addressed by density functional theory calculations. Both minimum energy paths and saddle points for the oxygen reduction reaction (ORR) in the allowed spin states have been identified. Our calculations indicate that ORR can evolve through different spin states, those where the M–O2 adducts are stable. We find that Mn–N4 and Fe–N4 centers in graphene exhibit the lowest O2 dissociation energies of ∼0.7 and 1.1 eV, respectively, over three spin channels, while for Co–N4 we find two spin channels with the same dissociation energy of ∼1.6 eV. The O2 dissociation barriers on the Mn–N4 and Fe–N4 centers are comparable to that found on Pt(111), suggesting similar ORR catalytic activity, in agreement with experimental results.

Formation energies, charge transitions levels, and quasiparticle defect states of the tellurium antisite ( Te Cd ) in CdTe are addressed within the DFT + GW formalism. We find that ( Te Cd ) induces a (+2/0) deep level at 0.99 eV above the valence band maximum, exhibiting a negative-U effect. Moreover, the calculated zero-phonon line for the excited state of ( Te Cd ) 0 corresponds closely with the ∼1.1 eV band, visible in both luminescence and absorption experiments. Our results differ from previous theoretical studies, mainly due to the well-known band gap error and the incorrect position of the band edges predicted by standard DFT calculations.

The role of oxygen doping in CdTe is addressed by first-principles calculations. Formation energies, charge transition levels, and quasiparticle defect states are calculated within the $\text{DFT}+GW$ formalism. The formation of a new defect is identified, the $({\text{O}}_{\text{Te}}{\text{-Te}}_{\text{Cd}})$ complex. This complex is energetically favored over both isovalent $({\mathrm{O}}_{\text{Te}})$ and interstitial oxygen $({\mathrm{O}}_{\text{i}})$, in the Te-rich limit. We find that the incorporation of oxygen passivates the harmful deep energy levels associated with $({\mathrm{Te}}_{\text{Cd}})$, suggesting an improvement in the efficiency of CdTe based solar cells. Substitutional $({\text{O}}_{\text{Cd}})$ is only stable in the neutral charge state and undergoes a Jahn--Teller distortion. We also investigate the diffusion profiles of interstitial oxygen and find a low-energy diffusion barrier of only 0.14 eV between two structurally distinct interstitial sites.

The covalent functionalization of metallic single-walled carbon nanotubes (CNTs) with transition metal phthalocyanines (MPc, with M = Mn, Fe and Co) are addressed by density functional calculations. The CNT–MPc catalytic activity toward the oxygen reduction reaction (ORR) is investigated through the O2 stretching frequency adsorbed on the phthalocyanine metal center. We find better reduction abilities when the CNT functionalization occurs through sp 2 -like bonds. Multiple stable-spin states for the M–O2 adduct are also found for M = Mn and Fe, suggesting higher ORR rates. The CNT–MPc complexes show metallic characteristics, suggesting favorable conditions to work as ORR cathode catalysts in fuel cells.

The energetic, thermal stability and dynamical properties of the ternary layered ceramic Ti3SiC2(0001) surface are addressed by density-functional theory calculations and molecular dynamic (MD) simulations. The equilibrium surface energy at 0 K of all terminations is contrasted with thermal stability at high temperatures, which are investigated by ab initio MD simulations in the range of 800 to 1400 °C. We find that the toplayer (sublayer) surface configurations: Si(Ti2) and Ti2(Si) show the lowest surface energies with reconstruction features for Si(Ti2). However, at high temperatures they are unstable, forming disordered structures. On the contrary, Ti1(C) and Ti2(C) despite their higher surface energies, show a remarkable thermal stability at high temperatures preserving the crystalline structures up to 1400 °C. The less stable surfaces are those terminated in C atoms, C(Ti1) and C(Ti2), which at high temperatures show surface dissociation forming amorphous TiCx structures. Two possible atomic scale mechanisms involved in the thermal stability of Ti3SiC2(0001) are discussed.

Objetivo: Conocer los antibióticos de mayor prescripción en los casos de infecciones respiratorias agudas (IRAS) en el paciente pediátrico de un centro de atención primaria de la región Callao. Estudio: Estudio de utilización de medicamentos (EUM) tipo indicación-prescripción de diseño observacional transversal en pacientes de 0 a 9 años del Centro de Salud Santa Fe, Callao- Perú. Materiales y métodos: Selección de historias clínicas de los formularios HIS, recojo de la información en una hoja común de recolección de datos. Principales medidas de resultados: Cálculo de frecuencias absolutas y porcentajes mediante el paquete estadístico Excel 2007. Resultados: Los tipos de IRA de mayor incidencia fueron faringitis aguda (46.7%), rinofaringitis aguda (26%) y bronquitis aguda (8%). La amoxicilina se prescribió en el 68%, 44%; y 55% de los casos respectivamente. Cotrimoxazol fue prescrito en el 19%, 22% y 45% de los casos respectivamente. Conclusiones: Amoxicilina y cotrimoxazol fueron los antibióticos más prescritos contra las IRAS más frecuentes.

Indexación: Scopus. We acknowledge support from Chilean funding agency FONDECYT under Grants No. 1170480 (W.O.) and 1171807 (E.M-P.). Powered@NLHPC: This research was partially supported by the supercomputing infrastructure of the NLHPC (ECM-02). The role of interstitial chlorine in the electronic properties of CdTe is addressed by density functional theory calculations including hybrid functionals and large unit cells. The stability and diffusion energy barriers of the impurity are analyzed as a function of the Fermi level position in the band gap. Chlorine is found to be stable in at least five interstitial sites with rather close formation energies, suggesting that they are all probable to be found. In p-type CdTe, the most stable sites are at the center of a CdTe bond and at a split-interstitial configuration, both acting as shallow donors. Whereas in n-type CdTe, it is found at the tetrahedral site surrounded by Cd hosts, acting as a shallow acceptor. We also find that chlorine can induce a deep acceptor level in the bandgap after binding with three Cd host atoms, which can explain the experimentally observed high resistivity in Cl-doped CdTe. The energy barriers for chlorine diffusion in both p-type and n-type CdTe are also discussed. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim https://onlinelibrary.wiley.com/doi/full/10.1002/pssb.201800219

CoN4 macrocyclic complexes like Co phthalocyanines (CoPc) have been extensively studied as electrocatalysts for the ORR since the discovery by Jasinski [1] but they only promote the 2-e reduction of O2 to give peroxide, this, delivering less energy when used in a fuel cell. Further, they lack the long-term stability required for fuel cell performance. However, they can serve as excellent models for establishing reactivity descriptors and activity correlations that can be very useful for design of more active and stable catalysts such as the Pyrolyzed MNx materials. On the other hand, vitamin B12, a naturally occurring CoN4 macrocyclic molecule has attracted the attention of the scientific community because instead of catalysing the 2 electron reduction of O2 to H2O2 like the other Co macrocyclics in alkaline media it promotes the 4-electron reduction to H2O. In alkaline media vitamin B12 possesses an axial back ligand and this seems to be the reason for its higher activity and selectivity for the 4-e reduction of O2. This is also true for another natural occurring catalyst: FeN4 cytochrome c [2]. To test this hypothesis, we synthesized a CoPc axially coordinated to pyridine anchored to carbon nanotubes (Co-Py-CNT). The Co center becomes then coordinated to 5 nitrogen atoms as in vitamin B12. The modified CoPcPy containing catalytic material was characterized by EPR and XPS spectroscopy. DFT calculations showed that the binding energy of Co to O2 increases dramatically by the presence of the axial ligand (see volcano correlation down below [3]). Koutecky– Levich extrapolation and Tafel plots show the similarities between the 2 complexes (Tafel slopes ca. -RT/F) and reveal insights into the mechanism of action of Co penta-coordinated complexes. According to our results the pyridine back ligand increases the Co-O2 binding energy, making it more similar to VitB12, favouring the splitting of the O-O bond. The back ligand then plays a crucial role in modifying Co-O2 binding energy which is a well know reactivity descriptor [3]. A further evidence eof this dramatic change in reactivity of CoPc by the presence of the "Py" back ligand is that it moves from the weak binding leg of a volcano correlation to the strong leg of the volcano, as illustrated in the Figure down below. This might explain why nature uses a back ligand in cytochrome -c to catalyze the ORR. ACKOWLEDGEMENTS: F.T. thanks for financial support the Fondecyt Iniciación Project 11130167, and Proyecto Basal. J.H.Z thanks the financial support of Millenium Project RC120001, Project Anillo ACT 1412 and Dicyt-USACH. Fondecyt 1140199. REFERENCES: 1. R. Jasinski, Nature, 201 (1964) 1212-1213 2. R. Boulatov, N4 Macrocyclic Metal Complexes (J.H. Zagal, F. Bedioui, J.P. Dodelet Eds.) Springer, Berlin,2006, pp.1-60 3. J.H. Zagal, M.T.M. Koper, Angewandte Chemie Int. Ed., 55 (2016) 14510-14521. Figure 1

We report a comparative study of the magnetic properties of free-standing PdN clusters (2 ≤ N ≤ 21) obtained through two different theoretical approaches that are extensively employed in electronic structure calculations: a semi-empirical Tight-Binding (TB) model and an ab-initio DFT pseudopotential model. Conclusions are drawn about the reliability of the TB model for the investigation of the electronic structure and magnetic properties of such complex 4d Transition Metals (TM) systems and we compare the results with previous systematic DFT calculations and comment on some available experiments in the literature. PACS. 75.75.+a Magnetic properties of nanostructures - 36.40.Cg Electronic and magnetic properties of clusters - 75.50.-y Studies of specific magnetic materials

The optoelectronic properties of single- and double-wall carbon nanotubes (CNTs) noncovalently functionalized with tetraphenylporphyrins (TPPs) are addressed by dispersion-corrected ab initio calculations. Five CNT species with different chiralities were considered. We find that the most stable configurations are those where the CNTs are fully covered by TPPs, exhibiting binding energy of about 2 eV/TPP. The semiconducting CNT–TPP compounds show optical response characterized by a strong absorption associated to the TPP bands, with increasing intensity with the TPP concentration. In addition, molecular dynamic simulations show that the compounds would be stable at temperatures as high as 100 °C.

La subasta es el mecanismo fundamental por medio del cual el Banco Central de Bolivia (BCB) ejecuta sus operaciones de mercado abierto, instrumento principal de su política monetaria. Este mecanismo es utilizado también en la colocación de valores para financiamiento fiscal. Luego de varios años de utilizar una subasta a sobre cerrado y precio discriminante, el año 2005 el BCB implementó una subasta electrónica interactiva, cuya modalidad es conocida usualmente como inglesa abierta o de precios ascendentes. La teoría de subastas sostiene que el cambio de formato en éstas conlleva modificaciones importantes en las estrategias de los participantes y, por ende, en los resultados obtenidos. En síntesis, se esperaría que la aplicación de la nueva modalidad permita: a) la reducción de la llamada "maldición del ganador", b) una mayor participación en subasta, y c) la caída de los costos de financiamiento para el emisor. Con el fin de evaluar y verificar estos postulados teóricos, el presente estudio realiza una comparación de los resultados generados por ambos formatos de subasta, mediante un análisis estadístico descriptivo y la estimación econométrica de dos ecuaciones reducidas, las cuales pretenden representar el comportamiento de los agentes en subasta.

The structural and electronic properties of a metallic single-walled carbon nanotube (CNT), adsorbed on hydrogenated Si(0 0 1) surfaces, have been investigated by first-principles calculations. We find that the electronic properties of the adsorbed CNT can be ruled by the H concentration along the CNT–H/Si(0 0 1) contact region. (i) On the fully hydrogenated Si(0 0 1), the CNT is physisorbed, preserving almost unchanged its metallic character. (ii) Removing half the H atoms along the adsorption site, we find an enhancement on the metallicity of the adsorbed CNT. (iii) When all the H atoms along the adsorption site are removed, the adsorbed CNT becomes semiconducting, exhibiting an energy gap. These results suggest that metallic CNTs adsorbed on H/Si(0 0 1) could be transformed into metal–semiconductor junctions by grading the H concentration along the CNT–surface interface.