No abstract

Carbon(C) doped hexagonal boron nitride(hBN) has been experimentally reported in recent years to be a possible catalytic host to oxygen reduction reaction(ORR), as well as a possible ferromagnet at room temperature. Substitution by C in hBN has been also reported to form islands of graphene. In this work, we explore from first principles, the connection between these different aspects of C doped hBN. We find formation of graphene islands covering unequal number of B and N sites in hBN to be energetically plausible. They posses a net non-zero magnetic moment and are also found to be substantially more chemically active than their non-magnetic counterparts covering equal number of B and N sites. On-site Coulomb repulsion between electrons, known to be responsible for magnetism in bipartite lattices like graphene and hBN, is also found to play a central role in chemical activation of not only the C atoms at the zigzag interface of magnetic graphene islands and hBN, but also of boron(B) sites in the immediate hBN neighborhood. However, such activated B or C due to substitution at B site, which is energetically more favorable than at N site, has been reported to be unfavorable for ORR. Advantageously, we find that the activation of C at B sites moderates systematically with increasing size of graphene islands, paving the way for abundance of efficient catalytic sites at the edges of magnetic graphene islands covering more B sites than N sites. Accordingly, as an alternate to precious metals for electrodes, we propose a class of graphene-hBN hybrids with lattices of magnetic graphene islands embedded in hBN, which can be metallic. arXiv:1803.06998v2 [cond-mat.mtrl-sci]

Grain boundaries (GBs) are defects originating in multi-crystalline silicon during crystal growth for device Si solar cell fabrication. The presence of GBs changes the coordination of Si, making it advantageous for charge carriers to recombine, which brings a significant reduction of carrier lifetimes. Therefore, GBs can be highly detrimental for device performances. Furthermore, GBs easily form vacancies with deep defect electronic states and are also preferential segregation sites for various impurity species, such as C, N, and O. We studied from first principles the correlation between structural, energetics, and electronic properties of the Σ3{111} Si GB with and without vacancies, and the segregation of C, N, and O atoms. C and O atoms strongly increase their ability to segregate when vacancies are present. However, the electronic properties of the Σ3{111} Si GB are not affected by the presence of O, while they can strongly change in the case of C. For N atoms, it is not possible to find a clear trend in the energetics and electronic properties both with and without vacancies in the GB. In fact, as N is not isovalent with Si, as C and O, it is more flexible in finding new chemical arrangements in the GB structure. This implies a stronger difficulty in controlling the properties of the material in the presence of N impurity atoms compared to C and O impurities.

Carbon(C) doped hexagonal boron nitride(hBN) has been experimentally reported to be ferromagnetic at room temperature. Substitution by C in hBN has been also reported to form islands of graphene. In this work we derive a mechanistic understanding of ferromagnetism with graphene islands in hBN from first principles and mean-field Hubbard model. We find a general property, that in bipartite lattices where the sublattices differ in on-site energies, as in hBN, the ordering between local magnetic moments can be substantial and predominantly anti-ferromagnetic(AFM) if they are embedded in the same sublattice, unless dominated by Mott like inter-sublattice spin separation due to strong localization. The dominant AFM order is rooted at spin resolved spatial separation of lone pairs of nitrogen(N) and back transferred electrons on boron(B) due to Coulomb repulsion thus essentially implying a super-exchange pathway. Subsequently we propose a class of ferri-magnetically ordered inter-penetrating super-lattices of magnetic graphene islands in hBN, which can be chosen to be a ferromagnetic semiconductor or a half-metal, and retain a net non-zero magnetic moment at room temperature. arXiv:1809.08270v3 [cond-mat.mes-hall]

The interaction of grain boundaries (GBs) with inherent defects and/or impurity elements in multicrystalline silicon plays a decisive role in their electrical behavior. Strain, depending on the types of GBs and defects, plays an important role in these systems. Herein, the correlation between the structural and electronic properties of Σ3{111} Si‐GB in the presence of interstitial oxygen impurities is studied from the first‐principles framework, considering the global and local model of strain. It is observed that the distribution of strain along with the number of impurity atoms modifies the energetics of the material. However, the electronic properties of the considered Si‐GBs are not particularly affected by the strain and by the oxygen impurities, unless a very high local distortion induces additional structural defects.