Abstract: Several structures and electronic states of the C2N and CN2 molecules have been studied using complete active space SCF (CASSCF), multireference configuration interaction (MRCI), and coupled cluster (CCSD(T)) methods. Both molecules are very stable. Our best computed total atomization energies SIGMAD(e) are 288.6 +/- 2 kcal/mol for CN2, and 294.1 +/- 2 kcal/mol for C2N. The CNC and CCN structures for C2N are nearly isoenergetic. CNN(3PI) lies about 30 kcal/mol above NCN(3PI(g)), but has a high barrier towards interconversion and is therefore observed experimentally. Computed harmonic frequencies for CNN are sensitive to the correlation treatment: they are reproduced well using multireference methods as well as the CCSD(T) method. High spin contamination has a detrimental effect on computed harmonic frequencies at the CCSD(T) level.

Abstract: Gold nanocluster properties exhibit unique size-dependence. In this contribution, we employ reactive molecular dynamics simulations to calculate the size- and temperature-dependent surface energies, strain energies and atomic displacements for icosahedral, cuboctahedral, truncated octahedral and decahedral Au-nanoclusters. The calculations demonstrate that the surface energy decreases with increasing cluster size at 0 K but increases with size at higher temperatures. The calculated melting curves as a function of cluster size demonstrate the Gibbs-Thomson effect. Atomic displacements and strain are found to strongly depend on the cluster size and both are found to increase with increasing cluster size. These results are of importance for understanding the size-and temperature-dependent surface processes on gold nanoclusters.

Abstract: The thermodynamic aspects of various 2D materials are explored using Density Functional Theory (DFT). Various metal chalcogenides (MX2, M = metal, chalcogen X = S, Se, Te) are investigated with respect to their interaction and stability under different ambient conditions met in the integration process of a transistor device. Their interaction with high- dielectrics is also addressed, in order to assess their possible integration in Complementary Metal Oxide Semiconductor (CMOS) field effect transistors. 2D materials show promise for high performance nanoelectronic devices, but the presence of defects (vacancies, grain boundaries,...) can significantly impact their electronic properties. To assess the impact of defects, their enthalpies of formation and their signature levels in the density of states have been studied. We find, consistently with literature reports, that chalcogen vacancies are the most likely source of defects. It is shown that while pristine 2D materials are in general stable whenever set in contact with different ambient atmospheres, the presence of defective sites affects the electronic properties of the 2D materials to varying degrees. We observe that all the 2D materials studied in the present work show strong reactivity towards radical oxygen plasma treatments while reactivity towards other common gas phase chemical such as O-2 and H2O and groups present at the high- surface varies significantly between species. While energy band-gaps, effective masses and contact resistivities are key criteria in selection of 2D materials for scaled CMOS and tunneling based devices, the phase and ambient stabilities might also play a very important role in the development of reliable nanoelectronic applications.

Abstract: Nitrogen-doped graphene was recently synthesized and was reported to be a catalyst for hydrogen dissociative adsorption under a perpendicular applied electric field (F). In this work, the diffusion of H atoms on N-doped graphene, in the presence and absence of an applied perpendicular electric field, is studied using density functional theory. We demonstrate that the applied field can significantly facilitate the binding of hydrogen molecules on N-doped graphene through dissociative adsorption and diffusion on the surface. By removing the applied field the absorbed H atoms can be released efficiently. Our theoretical calculation indicates that N-doped graphene is a promising hydrogen storage material with reversible hydrogen adsorption/desorption where the applied electric field can act as a switch for the uptake/release processes.

Abstract: ZnM2O4 (M = Co, Rh, Ir) spinels are considered as a class of potential p-type transparent conducting oxides (TCOs). We report the formation energy of acceptor-like defects using first principles calculations with an advanced hybrid exchange-correlation functional (HSE06) within density functional theory (DFT). Due to the discrepancies between the theoretically obtained band gaps with this hybrid functional and the – scattered – experimental results, we also perform GW calculations to support the validity of the description of these spinels with the HSE06 functional. The considered defects are the cation vacancy and antisite defects, which are supposed to be the leading source of disorder in the spinel structures. We also discuss the band alignments in these spinels. The calculated formation energies indicate that the antisite defects ZnM (Zn replacing M, M = Co, Rh, Ir) and VZn act as shallow acceptors in ZnCo2O4, ZnRh2O4 and ZnIr2O4, which explains the experimentally observed p-type conductivity in those systems. Moreover, our systematic study indicates that the ZnIr antisite defect has the lowest formation energy in the group and it corroborates the highest p-type conductivity reported for ZnIr2O4 among the group of ZnM2O4 spinels. To gain further insight into factors affecting the p-type conductivity, we have also investigated the formation of localized small polarons by calculating the self-trapping energy of the holes.

Abstract: Motivated by a recent experiment that reported the synthesis of a new 2D material nitrogenated holey graphene (C2N) [Mahmood et al., Nat. Commun., 2015, 6, 6486], the electronic, magnetic, and mechanical properties of nitrogenated (C2N), phosphorated (C2P) and arsenicated (C2As) monolayer holey graphene structures are investigated using first-principles calculations. Our total energy calculations indicate that, similar to the C2N monolayer, the formation of the other two holey structures are also energetically feasible. Calculated cohesive energies for each monolayer show a decreasing trend going from the C2N to C2As structure. Remarkably, all the holey monolayers considered are direct band gap semiconductors. Regarding the mechanical properties (in-plane stiffness and Poisson ratio), we find that C2N has the highest in-plane stiffness and the largest Poisson ratio among the three monolayers. In addition, our calculations reveal that for the C2N, C2P and C2As monolayers, creation of N and P defects changes the semiconducting behavior to a metallic ground state while the inclusion of double H impurities in all holey structures results in magnetic ground states. As an alternative to the experimentally synthesized C2N, C2P and C2As are mechanically stable and flexible semiconductors which are important for potential applications in optoelectronics.

Abstract: Perovskite oxides are active room-temperature bifunctional oxygen electrocatalysts in alkaline media, capable of performing the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with lower combined overpotentials relative to their precious metal counterparts. However, their semiconducting nature necessitates the use of activated carbons as conductive supports to generate applicably relevant current densities. In efforts to advance the performance and theory of oxide electrocatalysts, the chemical and physical properties of the oxide material often take precedence over contributions from the conductive additive. In this work, we find that carbon plays an important synergistic role in improving the performance of La1-xSrxCoO3- (0 x 1) electrocatalysts through the activation of O-2 and spillover of radical oxygen intermediates, HO2- and O-2(-), which is further reduced through chemical decomposition of HO2- on the perovskite surface. Through a combination of thin-film rotating disk electrochemical characterization of the hydrogen peroxide intermediate reactions (hydrogen peroxide reduction reaction (HPRR), hydrogen peroxide oxidation reaction (HPOR)) and oxygen reduction reaction (ORR), surface chemical analysis, HR-TEM, and microkinetic modeling on La1-xSrxCoO3- (0 x 1)/carbon (with nitrogen and non-nitrogen doped carbons) composite electrocatalysts, we deconvolute the mechanistic aspects and contributions to reactivity of the oxide and carbon support.

Abstract: Cold atmospheric plasma in contact with solutions has many applications, but its chemistry contains many unknowns such as the undescribed reactions with solutes. By combining experiments and modelling, we report the first direct demonstration of the reaction of chloride with oxygen atoms in aqueous solutions exposed to cold plasma.