Subangstrom Edge Relaxations Probed by Electron Microscopy in Hexagonal Boron Nitride

Nasim Alem, Quentin M. Ramasse, Che R. Seabourne, Oleg V. Yazyev, Kris Erickson, Michael C. Sarahan, Christian Kisielowski, Andrew J. Scott, Steven G. Louie, and A. Zettl

Phys. Rev. Lett. 109, 205502 – Published 16 November 2012

Abstract

Theoretical research on the two-dimensional crystal structure of hexagonal boron nitride ( $h$ -BN) has suggested that the physical properties of $h$ -BN can be tailored for a wealth of applications by controlling the atomic structure of the membrane edges. Unexplored for $h$ -BN, 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 $π^{*}$ 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.

Received 20 July 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.205502

Authors & Affiliations

Nasim Alem^1,2,3, Quentin M. Ramasse^4,*,†, Che R. Seabourne⁵, Oleg V. Yazyev^1,3,6, Kris Erickson^1,3, Michael C. Sarahan⁴, Christian Kisielowski⁷, Andrew J. Scott⁵, Steven G. Louie^1,3, and A. Zettl^1,2,3,*,‡

¹Department of Physics, University of California Berkeley, Berkeley, California 94720, USA
²Center of Integrated Nanomechanical Systems, University of California Berkeley, Berkeley, California 94720, USA
³Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁴SuperSTEM Laboratory, STFC Daresbury, Keckwick Lane, Daresbury WA4 4AD, United Kingdom
⁵Institute for Materials Research, SPEME, University of Leeds, Leeds LS2 9JT, United Kingdom
⁶Institute of Theoretical Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
⁷National Center for Electron Microscopy and Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

^*To whom correspondence should be addressed.
^†qmramasse@superstem.org
^‡azettl@berkeley.edu

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Vol. 109, Iss. 20 — 16 November 2012

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Images

Figure 1
Imaging structural relaxations at the edge of bilayer $h$ -BN. (a) Annular dark field STEM image of a large bilayer $h$ -BN sheet showing a hole with straight zigzag edges. (b) The individual atoms at the magnified $h$ -BN edge are marked with red dots. A distortion pulls the zigzag edge atoms away from the hole creating a curvature at the edge. (c) The intensity at the edge of the bilayer is increased, while no impurities or adatoms were found to reside at this part of the edge.Reuse & Permissions
Figure 2
Image simulations using the calculated structures for closed and open edges. (a) Cross-sectional and plan view schematics of a predicted DFT model showing the formation of interlayer bonds at the zigzag edges of a bilayer $h$ -BN, resulting in local three-dimensional atomic displacements. (b) A simulated ADF-STEM image from the predicted DFT model. (c) Experimental and simulated in-plane atomic coordinates are compared with and without distortion in the zigzag direction. The relaxed edges in the experiment [Fig. 1b] precisely follow the predicted edge distortions from first-principles calculations of closed edges. (d) The relaxed (closed) edge of the simulated structure on the top shows enhanced intensity contrast compared to the pristine (open) edge.Reuse & Permissions
Figure 3
Experimental and simulated EELS data along with annular dark field (ADF) images of bilayer $h$ -BN at the edge. (a) ADF image of bilayer $h$ -BN at the edge along with its EELS spectra at points $1_{e}$ , $2_{e}$ , and $3_{e}$ . Please note that a different image was used to position the beam for spectrum $3_{e}$ . The label $3_{e}$ indicates an “equivalent” atomic column, while the actual beam location image is provided in the Supplemental Material [11]. (b) Cross-sectional and plan view of the pristine and relaxed models of bilayer $h$ -BN edge. The dashed lines on the cross-sectional schematics correspond to the atomic columns $1_{t}$ , $2_{t}$ , and $3_{t}$ for the pristine and relaxed edge models. (c) The experimental and simulated EELS spectra at points 1, 2, and 3 already marked on images (a) and (b). Subscript $e$ stands for experiment and $t$ for theory.Reuse & Permissions
Figure 4
Orbital map of the closed bilayer edge. (a) Depiction of the cross-sectional slice at which unoccupied orbitals have been plotted (b) EELS unoccupied orbital map, illustrating the unoccupied orbitals which have energy windows lying within the $π^{*}$ region for the bulklike position 2. Colors towards the red end of the spectrum indicate greater orbital intensity.Reuse & Permissions
Figure 5
Calculated electronic band structures of three configurations of $h$ -BN. (a) bulk bilayer $h$ -BN and in the presence of (b) open and (c) closed edges calculated from first principles. The structural transformations are schematically shown using the atomistic models.Reuse & Permissions

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