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.

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.

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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.

Abstract. Urban regions are responsible for emitting significant amounts of fossil fuel carbon dioxide (FFCO2), and emissions at the finer, city scales are more uncertain than those aggregated at the global scale. Carbon-observing satellites may provide independent top-down emission evaluations and compensate for the sparseness of surface CO2 observing networks in urban areas. Although some previous studies have attempted to derive urban CO2 signals from satellite column-averaged CO2 data (XCO2) using simple statistical measures, less work has been carried out to link upwind emission sources to downwind atmospheric columns using atmospheric models. In addition to Eulerian atmospheric models that have been customized for emission estimates over specific cities, the Lagrangian modeling approach – in particular, the Lagrangian particle dispersion model (LPDM) approach – has the potential to efficiently determine the sensitivity of downwind concentration changes to upwind sources. However, when applying LPDMs to interpret satellite XCO2, several issues have yet to be addressed, including quantifying uncertainties in urban XCO2 signals due to receptor configurations and errors in atmospheric transport and background XCO2. In this study, we present a modified version of the Stochastic Time-Inverted Lagrangian Transport (STILT) model, “X-STILT”, for extracting urban XCO2 signals from NASA's Orbiting Carbon Observatory 2 (OCO-2) XCO2 data. X-STILT incorporates satellite profiles and provides comprehensive uncertainty estimates of urban XCO2 enhancements on a per sounding basis. Several methods to initialize receptor/particle setups and determine background XCO2 are presented and discussed via sensitivity analyses and comparisons. To illustrate X-STILT's utilities and applications, we examined five OCO-2 overpasses over Riyadh, Saudi Arabia, during a 2-year time period and performed a simple scaling factor-based inverse analysis. As a result, the model is able to reproduce most observed XCO2 enhancements. Error estimates show that the 68 % confidence limit of XCO2 uncertainties due to transport (horizontal wind plus vertical mixing) and emission uncertainties contribute to ∼33 % and ∼20 % of the mean latitudinally integrated urban signals, respectively, over the five overpasses, using meteorological fields from the Global Data Assimilation System (GDAS). In addition, a sizeable mean difference of −0.55 ppm in background derived from a previous study employing simple statistics (regional daily median) leads to a ∼39 % higher mean observed urban signal and a larger posterior scaling factor. Based on our signal estimates and associated error impacts, we foresee X-STILT serving as a tool for interpreting column measurements, estimating urban enhancement signals, and carrying out inverse modeling to improve quantification of urban emissions.

Urban areas are currently responsible for ∼70% of the global energy-related carbon dioxide (CO 2 ) emissions, and rapid ongoing global urbanization is increasing the number and size of cities. Thus, understanding city-scale CO 2 emissions and how they vary between cities with different urban densities is a critical task. While the relationship between CO 2 emissions and population density has been explored widely in prior studies, their conclusions were sensitive to inconsistent definitions of urban boundaries and the reliance upon CO 2 emission inventories that implicitly assumed population relationships. Here we provide the first independent estimates of direct per capita CO 2 emissions (E pc ) from spaceborne atmospheric CO 2 measurements from the Orbiting Carbon Observatory-2 (OCO-2) for a total 20 cities across multiple continents. The analysis accounts for the influence of meteorology on the satellite observations with an atmospheric model. The resultant upwind source region sampled by the satellite serves as an objective urban extent for aggregating emissions and population densities. Thus, we are able to detect emission 'hotspots' on a per capita basis from a few cities, subject to sampling restrictions from OCO-2. Our results suggest that E pc declines as population densities increase, albeit the decrease in E pc is partially limited by the positive correlation between E pc and per capita gross domestic product. As additional CO 2 -observing satellites are launched in the coming years, our space-based approach to understanding CO 2 emissions from cities has significant potential in tracking and evaluating the future trajectory of urban growth and informing the effects of carbon reduction plans.

The formation of native point defects in cuprous oxide, Cu 2 O, synthesized from solution has been studied by first-principles calculations. Although p-type conduction is obtained in Cu 2 O synthesized from vacuum regardless of copper-rich or oxygen-rich conditions, intrinsically n-type Cu 2 O without doping can be grown in a strong acidic environment from solution. Our calculations show that both n-type and p-type Cu 2 O can be obtained depending on the solution pH value, which are in good agreement with our experimental results.

The concept of "quantum stress (QS)" is introduced and formulated within density functional theory (DFT), to elucidate extrinsic electronic effects on the stress state of solids and thin films in the absence of lattice strain. A formal expression of QS (σ Q ) is derived in relation to deformation potential of electronic states (Ξ) and variation of electron density (∆n), σ Q = Ξ∆n, as a quantum analog of classical Hook's law. Two distinct QS manifestations are demonstrated quantitatively by DFT 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 QS in some physical phenomena are discussed to underlie its importance. 68.35.Md A fundamental property of solid materials is their stress state. At the equilibrium lattice constant, the bulk of a crystalline solid is stress free, but the surface of a solid has intrinsic non-zero stress and stress is commonly induced by any form of lattice distortion [1]. The stress (strain) state of a solid or thin-film material has profound effects on its thermodynamic stability and physical and chemical properties [1][2][3], and has been employed in a wide range of applications such as electromechnical devices [4], mechanochemcial sensors [5] and flexible electronics [6], and even to make new nanotructures [7,8]. Here, we introduce the concept of "quantum stress (QS)", which adds an interesting quantum mechanical aspect to our conventional view of "classical stress (CS)" based on classical mechanics. We mathematically formulate the expression of QS within density function theory (DFT), and use DFT calculations to demonstrate quantitatively two distinct physical manifestations of QS, in the form of bulk stress induced by charge carriers in a homogeneous system of crystalline solids and in the form of surface stress induced by quantum confinement in a heterogeneous system of nanoscale thin films. We will then apply the concept of QS to elucidate a few examples of physical phenomena that underlie the importance and usefulness of QS. Concept of QS.We first introduce the concept of QS in contrast with the CS. Figure 1 illustrates the fundamental difference between the QS and CS using a simple model of a one-dimensional (1D) lattice. Consider a lattice is under compressive (Fig. 1a) or tensile lattice strain (ε), such as in an epitaxial film due to lattice mismatch between the film and substrate [9,10]. The "atomic" lattice deformation energy can be expressed as E = (1/2)Y ε 2 V , where Y is Young's modulus and V is the volume of lattice. By definition, the lattice formation induced lattice stress, which we refer to here as CS, is expressed as σ C = (1/V )(dE/dε) = Y ε , the Hook's law. Now, consider an equilibrium lattice in the absence of strain (ε = 0), but electronically perturbed or excited, such as an electron is kicked out by a photon leaving behind a hole, as shown in Fig. 1b, which redistributes the electron density. The "electronic deformation" energy can be expressed as E...