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electron microscopy. While nitrogen centres
or nitrogen–vacancy centres have been
imaged in graphene
, such structures
would be masked if they are embedded in a
thick three-dimensional sample that possibly
also has an amorphous cover layer. Hence,
going from the extended to the point
defects still entails some formidable
challenges lying ahead — for example, in
sample preparation and radiation damage
prevention. Uncovering the vast zoo of
defects with unknown structures would
open new doors for tailoring optical and
electronic properties of diamond. The work
of Olivier and colleagues is a remarkable step
forward for characterizing this challenging
material, and it also highlights a promising
avenue to study defects in diamond where
the identification of the platelet structure
can be seen as a starting point. ❐
Physics of Nanostructured Materials Group,
University of Vienna, Vienna, Austria.
Published online: 21 February 2018
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(Springer, Berlin, 2001).
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5. Barry, J., Bursill, L. & Hutchison, J. Phil. Mag. A 51, 15–49 (1985).
6. Korneychuk, S., Turner, S., Abakumov, A. & Verbeeck, J. in Proc.
European Microscopy Congress 331–332 (2016).
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8. Kirkland, E. J. Ultramicroscopy 111, 1523–1530 (2011).
9. Kiawi, I. & Bruley, J. Diam. Relat. Mater. 9, 87–93 (2000).
10. Meyer, J. C. et al. Nat. Mater. 10, 209–215 (2011).
11. Lin, Y.-C. et al. Nano Lett. 15, 7408–7413 (2015).
Fig. 2 | Comparison between theoretical simulation and experimental result of the platelet structure
viewed along the
direction. a, Simulated image of the zigzag model structure. b, Experimental
STEM image. The red arrows indicate two atomic columns with a separation of only 0.89 Å. The blue
arrows indicate the location of the platelet, where in this particular view a set of weak features can
be observed that allows one to distinguish the zigzag model from the other candidates. Scale bar, 1 Å.
Credit: adapted from ref.
, Macmillan Publishers Ltd.
NATURE MATERIALS | VOL 17 | MARCH 2018 | 210–220 | www.nature.com/naturematerials
© 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
VAN DER WAALS MATERIALS
Illuminating interlayer interactions
A synchrotron X-ray diraction experiment demonstrates an unexpected accumulation of electron density in the
interlayer region of TiS
, and provides a benchmark for theoretical models of weak interlayer bonding.
Xiaohui Qiu and Wei Ji
an der Waals (vdW) layered
materials, such as transition metal
dichalcogenides (TMDs), were
brought into the spotlight soon after the
discovery of graphene. In contrast to
graphene, whose monolayer is a single sheet
of carbon atoms, the elementary layer of
TMDs is a sandwich structure consisting of
three atomic sheets: the top and bottom ones
are chalcogen atoms in a triangular lattice
structure, and the middle one is formed of
metal atoms each covalently bonded to three
chalcogen atoms in the top sheet and three
in the bottom. This structural configuration
ensures that each layer of TMDs has fully
saturated chemical bonds on the surface,
and therefore their interlayer interactions
are widely believed to be dominated by vdW
forces. Because of these flawless surface
structures, which are free of dangling bonds
and trap states, monolayer and few-layer
TMDs exhibit extraordinary electronic
and optical properties. Also thanks to vdW
forces, highly disparate sheet materials
can be integrated layer by layer via this
weak interaction without the constraints of
crystal lattice matching, holding promise
for building novel heterostructures with
functions that were not previously possible
Van der Waals forces are non-covalent
interactions. They contain repulsive and
attractive components that respectively
originate from Pauli and Coulomb
repulsions, electrostatic attraction between
permanent charges, electric multipoles
induced by permanent charges and
instantaneous electric multipoles. Unlike
covalent bonds, vdW interactions do not
involve the sharing of electrons between
interacting atoms. As a result, for a period
of time, the community of two-dimensional
layered materials was prone to take it for
granted that the weak vdW interactions
dominate the interlayer interactions of
TMDs. This led to the interlayer coupling
being overlooked as vdW interactions
usually have little effect on electronic
hybridization, and would not influence
the electric and optical properties. Now,
writing in Nature Materials, Bo B. Iversen
report the use of a
state-of-the-art X-ray diffraction technique
to experimentally determine the electron
density in an archetypical vdW TMD
. Charge sharing was observed
in the interlayer region, suggesting stronger
chemical bonding than predicted by theory