Graphene vacancies are engineered for novel functionalities, however, the charge state of these defects, the key parameter that is vital to charge transfer during chemical reactions and carrier scattering, is generally unknown. Here, we carried out atomic resolution imaging of graphene vacancy defects created by Ar plasma using noncontact atomic force microscopy, and made the first determination of their charge state by local contact potential difference measurements. Combined with density functional theory calculations, we show that graphene vacancies are typically positively charged, with size-dependent charge states that are not necessarily integer-valued. These findings provide new insights into carrier scattering by vacancy defects in graphene, as well as its functionalization for chemical sensing and catalysis, and underline the tunability of these functions by controlling the size of vacancy defect.

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Abstract:Epitaxial GeMnN 2 thin films are synthesized by plasma-assisted molecular beam epitaxy.Transmission electron microscopy and x-ray diffraction measurements confirm that it is the orthorhombic variant, consistent with the predictions of first-principles calculations. The magnetic properties of the films are related to defects, with samples grown under Ge-rich conditions exhibiting a net magnetic moment above room temperature. These results are explained by first-principles calculations, indicating that the preferential substitution of one magnetic sublattice of GeMnN 2 by impurities and/or intrinsic defects such as Ge antisites produces a net magnetic moment in an antiferromagnetic background and also introduces spinpolarized carriers near the Fermi level.

We report the direct imaging of standing waves on a GaN(0001)-pseudo (1 × 1) metallic surface, which consists of two atomic Ga layers with the top layer incommensurate. Two types of periodic oscillation are observed by scanning tunneling microscopy at room temperature. The longer wavelength standing waves are due to electron scattering by dislocation-induced steps and two-dimensional InN islands. The localized shorter wavelength waves are attributed to a structural transition of the incommensurate Ga bilayer to a tetrahedral Ga bilayer after the growth of the InN islands.

The surface morphology and atomic structure of InN grown on the Ga-rich GaN(0001)-pseudo (1 × 1) structure is studied by scanning tunneling microscopy. Spirals are formed as a result of screw dislocations emerging at the surface to relieve the strain from the lattice mismatch. Two additional types of strain relaxation mechanisms are also found, both due to the incorporation of excess Ga atoms from the starting pseudo (1 × 1) surface into the growing films. For films below 8 nm where the Ga concentration is larger than 7%, the formation of stacking faults at the InN/GaN interface produces a triangular network on the surface. The density of the stacking faults is found to decrease with film thickness and with the gradual consumption of the Ga atoms, and the network is therefore no longer observable above a critical thickness that varies from 8 to 10 nm. Instead, vacancy islands, one atomic layer deep, are formed to relieve the stain near the surface region. These results provide atomic scale insights into the interplay between the surface morphology and strain relaxation during the epitaxial growth of highly lattice mismatched InN/GaN heterostructures.