Lewis acid-base interactions between polysulfides and metal organic framework in lithium sulfur batteries.

Lithium-sulfur (Li-S) battery is one of the most promising energy storage systems because of its high specific capacity of 1675 mAh g(-1) based on sulfur. However, the rapid capacity degradation, mainly caused by polysulfide dissolution, remains a significant challenge prior to practical applications. This work demonstrates that a novel Ni-based metal organic framework (Ni-MOF), Ni6(BTB)4(BP)3 (BTB = benzene-1,3,5-tribenzoate and BP = 4,4'-bipyridyl), can remarkably immobilize polysulfides within the cathode structure through physical and chemical interactions at molecular level. The capacity retention achieves up to 89% after 100 cycles at 0.1 C. The excellent performance is attributed to the synergistic effects of the interwoven mesopores (∼2.8 nm) and micropores (∼1.4 nm) of Ni-MOF, which first provide an ideal matrix to confine polysulfides, and the strong interactions between Lewis acidic Ni(II) center and the polysulfide base, which significantly slow down the migration of soluble polysulfides out of the pores, leading to the excellent cycling performance of Ni-MOF/S composite.

Quantum electronic stress: density-functional-theory formulation and physical manifestation.

The concept of quantum electronic stress (QES) is introduced and formulated within density functional theory to elucidate extrinsic electronic effects on the stress state of solids and thin films in the absence of lattice strain. A formal expression of QES (σ(QE)) is derived in relation to deformation potential of electronic states (Ξ) and variation of electron density (Δn), σ(QE) = ΞΔn as a quantum analog of classical Hooke's law. Two distinct QES manifestations are demonstrated quantitatively by density functional theory calculations: (1) in the form of bulk stress induced by charge carriers and (2) in the form of surface stress induced by quantum confinement. Implications of QES in some physical phenomena are discussed to underlie its importance.

Stabilizing graphitic thin films of wurtzite materials by epitaxial strain.

Recent theory [Phys. Rev. Lett. 96, 066102 (2006)] and experiment [Phys. Rev. Lett. 99, 026102 (2007)] show that (0001) ultrathin films of wurtzite (WZ) materials surprisingly transform into a stable graphitelike structure, but the stability is limited to thicknesses of only a few atomic layers. Using first-principles calculations of both freestanding and substrate-supported thin films, we predict that the thickness range of stable graphitic films depends sensitively on strain and can be substantially extended to much thicker films by epitaxial tensile strain. Moreover, the band gap of the stable strained graphitic films can be tuned over a wide range either above or below that of the bulk WZ phase.

Atomic layers of hybridized boron nitride and graphene domains.

Two-dimensional materials, such as graphene and monolayer hexagonal BN (h-BN), are attractive for demonstrating fundamental physics in materials and potential applications in next-generation electronics. Atomic sheets containing hybridized bonds involving elements B, N and C over wide compositional ranges could result in new materials with properties complementary to those of graphene and h-BN, enabling a rich variety of electronic structures, properties and applications. Here we report the synthesis and characterization of large-area atomic layers of h-BNC material, consisting of hybridized, randomly distributed domains of h-BN and C phases with compositions ranging from pure BN to pure graphene. Our studies reveal that their structural features and bandgap are distinct from those of graphene, doped graphene and h-BN. This new form of hybrid h-BNC material enables the development of bandgap-engineered applications in electronics and optics and properties that are distinct from those of graphene and h-BN.

Dependence of exchange coupling on interfacial conditions in SmCo5/Co system: a first-principles study.

We have performed first-principles calculations to study the interfacial exchange coupling in a SmCo5/Co multilayer model system. The hard phase hcp SmCo5 and the soft phase hcp Co (or Co(1-x)Fe(x)) stacking along (1010) direction are structurally well matched. The atomic structure, including the alignment and the separation between layers, were optimized first. Then the non-collinear magnetic structures were calculated to explore the exchange coupling dependence on the variation of the atomic composition across the interface. We found that the inter-phase exchange coupling strength is strongly dependent on the interface condition between the hard and soft phase by comparing the exchange coupling strengths in different interface conditions. The findings were further confirmed by the calculated site-to-site exchange parameters across the interface.