Charged defects in graphene and the ionicity of hexagonal boron nitride in direct images

Abstract

We report on the detection and charge distribution analysis for nitrogen substitutional dopants in single layer graphene membranes by aberration-corrected high-resolution transmission electron microscopy (HRTEM). Further, we show that the ionicity of single-layer hexagonal boron nitride can be confirmed from direct images. For the first time, we demonstrate by a combination of HRTEM experiments and firstprinciples electronic structure calculations that adjustments to the atomic potentials due to chemical bonding can be discerned in HRTEM images. Our experiments open a way to investigate electronic configurations in point defects or other non-periodic arrangements or nanoscale objects that can not be studied by an electron or x-ray diffraction experiment.

Optimized conditions for imaging the effects of bonding charge density in electron microscopy

“…The R 1 value computed from the DFT and IAM curves in Figure 4 of [29] is 0.9%, which is consistent with the maximum simulated value of 1.2% presented herein, and represents the spatially averaged contrast contribution over the entire image rather than a localized maximum.…”

“…However, for a single monolayer of BN, the maximum R 1 value for the bond contribution to contrast without regard to noise was only 1.2%, which compares unfavorably to the maximum value of 32.9% for Forsterite. Despite this experimental challenge, local bonding effects have recently been resolved for monolayer BN in HREM images [29] wherein bonding contributions were confirmed both experimentally and computationally to contribute as much as 10% to the single pixel contrast at the nitrogen site.…”

Grain boundary loops in graphene

“…Our present study reveals that this structure is a low-energy rotational grain boundary formed in the high temperature growth, identical to the flower defect discovered by transmission electron microscopy on CVD graphene. 21,22 Simulated STM topographs of this rotational grain boundary are in excellent agreement with experimental STM images. Scanning tunneling spectroscopy (STS) measurements show that this defect has an electronic state at 0.5 eV above the charge neutrality point (Dirac point) and DFT calculations of the density of electron states show a localized resonance in the same energy range.…”

“…We label the smallest member of this family the "flower" defect based on STM observations 4,20 and on our demonstration in this work of its equivalence to the flower defect observed in CVD grown graphene. 21,22 In this family, the central 6 (m+n) 2 in ideal graphene, 108° in a regular pentagon, and 128.6° in a regular heptagon. The flower rotational grain boundary is stable with respect to out-of-plane distortions and is calculated to have an energy of +7.0 eV above ideal graphene; higher than the +4 eV calculated energy of a Stone-Wales defect, 27,34 but smaller than the +10 eV energy of an (unreconstructed) carbon vacancy (see Table 1).…”

“…A theoretical study of the energies of rotational grain boundaries, originating from paired five-and seven-membered ring disclinations, shows that the previously unidentified "flower" defect in STM topographs is the smallest member of a family of rotational grain boundary defects with C 6 symmetry. The calculated energy per 5-7 pair (dislocation core) is the smallest for any known topological defect, suggesting that this defect is likely to form under conditions where mobile dislocations exist (during the preparation of this manuscript, the flower defect was identified independently in CVD-grown graphene on Ni 21,22 ).…”