A combination of scanning transmission electron microscopy, electron energy loss spectroscopy, and ab initio calculations reveal striking electronic structure differences between two distinct single substitutional Si defect geometries in graphene. Optimised acquisition conditions allow for exceptional signal-to-noise levels in the spectroscopic data. The near-edge fine structure can be compared with great accuracy to simulations and reveal either an sp(3)-like configuration for a trivalent Si or a more complicated hybridized structure for a tetravalent Si impurity.

High-temperature superconductivity has a range of applications from sensors to energy distribution. Recent reports of this phenomenon in compounds containing electronically active BiS2 layers have the potential to open a new chapter in the field of superconductivity. Here we report the identification and basic properties of two new ternary Bi-O-S compounds, Bi2OS2 and Bi3O2S3. The former is non-superconducting; the latter likely explains the superconductivity at T(c) = 4.5 K previously reported in "Bi4O4S3". The superconductivity of Bi3O2S3 is found to be sensitive to the number of Bi2OS2-like stacking faults; fewer faults correlate with increases in the Meissner shielding fractions and T(c). Elucidation of the electronic consequences of these stacking faults may be key to the understanding of electronic conductivity and superconductivity which occurs in a nominally valence-precise compound.

A combination of scanning transmission electron microscopy, electron energy loss spectroscopy, and ab initio calculations is used to describe the electronic structure modifications incurred by free6standing graphene through two types of single6atom doping. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 dopant atom shows an unusual broad asymmetric peak which is the result of delocalised π * states away from the B dopant. The asymmetry of the B K towards higher energies is attributed to highly6localised σ* anti6bonding states. These experimental observations are then interpreted as direct fingerprints of the expected p6 and n6type behaviour of graphene doped in this fashion, through careful comparison with density functional theory calculations. graphene, doping, electronic structure, STEM, EELS, ab5initio calculations, DFT

The plane-wave density functional theory code CASTEP was used with the Tkatchenko-Scheffler van der Waals correction scheme and the GGA PBE functional to calculate the binding energy of Au, Cr and Al atoms on the armchair and zigzag edge binding sites of monolayer graphene, and at the high-symmetry adsorption sites of single layer, bilayer and trilayer graphene. All edge site binding energies were found to be substantially higher than the adsorption energies for all metals. The adatom migration activation barriers for the lowest energy migration paths on pristine monolayer, bilayer and trilayer graphene were then calculated and found to be smaller than or within an order of magnitude of at room temperature, implying very high mobility for all adatoms studied. This suggests that metal atoms evaporated onto graphene samples quickly migrate across the lattice and bind to the energetically favourable edge sites before being characterised in the microscope. We then prove this notion for Al and Au on graphene with scanning transmission electron microscopy (STEM) images showing that these atoms are observed exclusively at edge sites, and also hydrocarbon-contaminated regions, where the pristine regions of the lattice are completely devoid of adatoms. Additionally, we review the issue of fixing selected atomic positions during geometry optimisation calculations for graphene/adatom systems and suggest a guiding principle for future studies.

Theoretical research on the two-dimensional crystal structure of hexagonal boron nitride (h-BN) 2 has suggested that the physical properties of hBN can be tailored for a wealth of applications by controlling the atomic structure of the membrane edges. Unexplored for hBN, however, is the possibility that small additional edge-atom distortions could have electronic structure implications critically important to nanoengineering efforts. Here we demonstrate, using a combination of analytical scanning transmission electron microscopy and density functional theory, that covalent interlayer bonds form spontaneously at the edges of a h-BN bilayer, resulting in subangstrom distortions of the edge atomic structure. Orbital maps calculated in 3D around the closed edge reveal that the out-of-plane bonds retain a strong à character. We show that this closed edge reconstruction, strikingly different from the equivalent case for graphene, helps the material recover its bulklike insulating behavior and thus largely negates the predicted metallic character of open edges.