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MATERIALS SYNTHESIS
Two-dimensional gallium nitride
Graphene is used as a capping sheet to synthesize 2D gallium nitride by means of migration-enhanced
encapsulation growth. This technique may allow the stabilization of 2D materials that are not amenable to
synthesis by traditional methods.
Nikhil A. Koratkar
T
he groundbreaking discovery 1 that
graphite could be cleaved into its 2D
building block, graphene, has led
to similar efforts directed towards other
layered 3D solids. In fact, a wide variety
of 2D materials have been discovered over
the past decade: phosphorene, transition
metal dichalcogenides and MXenes
(transition metal carbides, nitrides or
carbonitrides) to name just a few 2,3. Most
such 2D materials are derived from 3D
van der Waals solids (layered materials
comprising 2D sheets stacked on each other
and held together by weak van der Waals
forces)3. Such van der Waals bonded
layered compounds can be exfoliated by
mechanical, chemical or electrochemical
methods to isolate their 2D form. But there
is no clear path so far to extract 2D forms of
3D crystals that lack this layered structure.
For example, tetrahedrally coordinated bulk
crystals such as wurtzite gallium nitride
(GaN) cannot be easily cleaved, owing to
unsaturated dangling bonds on the surface.
a
gallium metal islands act as the favoured
sites for gallium to penetrate through the
graphene. Once gallium atoms intercalate
into the graphene/SiC interface, they appear
to be stabilized as a bilayer of gallium,
probably because of the highly passivated
nature of the hydrogenated SiC/graphene
interface. In the final growth steps, atomic
nitrogen (created from decomposition
of ammonia) also penetrates through the
defective graphene capping sheet (Fig. 1b)
and reacts with the intercalated gallium
to form a patchwork of 2D GaN islands
(Fig. 1c,d). The presence of the graphene
bilayer is indispensable for successful
GaN growth in its 2D form; in fact, the
researchers showed that GaN grown on the
SiC substrate without the graphene capping
sheet resulted only in 3D islands.
Aberration-corrected scanning
transmission electron microscopy indicates
that the 2D GaN is in a buckled state owing
to the passivation of surface dangling bonds.
Further, the stoichiometry of 2D GaN is
b
Bilayer graphene
capping sheet
SiC
substrate
c
If one could passivate such dangling bonds,
then synthesis of 2D forms of GaN could
in fact be possible. Writing in Nature
Materials, Al Balushi and co-workers4
report that they have done just this through
a migration-enhanced encapsulation
growth (MEEG) technique that uses
graphene as a capping or stabilizing layer
during synthesis.
The unique aspect of MEEG is that
the GaN growth occurs in between
the growth substrate (silicon carbide,
SiC) and a graphene capping bilayer
(Fig. 1a–c). This graphene capping sheet
is formed by sublimation of Si from the
SiC substrate followed by hydrogenation.
Trimethylgallium is decomposed on the
graphene/SiC surface, causing gallium atoms
to penetrate through the graphene into the
interstitial space between the SiC substrate
and the graphene bilayer as illustrated
schematically in Fig. 1a. Al Balushi and
co-workers4 show that defects (such as
carbon vacancies), wrinkles (tears) and large
H
Si
C
d
2D GaN bonded
to SiC substrate
2D GaN
3D GaN
island
Figure 1 | Schematic of 2D GaN synthesis. a,b, Diffusion of gallium (pink; a) and nitrogen (blue; b) atoms through defects and wrinkles in a bilayer graphene
capping sheet. c, The buckled 2D GaN structure sandwiched between SiC and graphene. d, Patchwork of 2D GaN islands intermingled with a few 3D regions.
Figure adapted from ref. 4, Nature Publishing Group.
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
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not 1:1 as in the bulk version. Interestingly,
the 2D GaN is covalently bonded to the SiC
substrate while preserving a van der Waals
spatial gap with the top graphene layer
(Fig. 1c). The covalent connection of GaN to
the substrate may be aided by displacement
of the hydrogen terminal groups on SiC by
nitrogen atoms, which then bond with the
GaN layer and passivate it. Because of the
weak coupling between graphene and GaN,
the graphene layers can be easily etched
away while preserving the ordered structure
of GaN on SiC. After graphene removal, the
2D GaN layer is exposed to the ambient.
In spite of this, there is no evidence of
oxidation or change in the chemistry of the
2D GaN.
Al Balushi and co-workers also probe
the electronic and optoelectronic properties
of 2D GaN using both experimental and
theoretical means4. They report that buckled
2
structures of 2D GaN possess a direct
bandgap of ~5 eV, which is much larger than
for bulk GaN (3.42 eV), probably owing to
quantum confinement in two dimensions.
Experimental realization of 2D GaN on
technologically relevant substrates such
as SiC opens up the possibility of device
applications ranging from power electronics
to topological insulators and single-photon
emitters. Before reaching broad applications,
however, surface coverage of the 2D GaN
on the SiC substrate must be further
improved. For this, it may be possible to
defect-engineer 5 the graphene capping
sheet so as to induce better intercalation
of Ga, possibly leading to more uniformly
distributed and larger 2D GaN islands,
with the ultimate objective of coalescing
the patchwork of islands into a continuous
film. It will also be exciting to extend this
approach for the experimental realization
of other 2D nitrides and other hitherto
unexplored compounds. It is conceivable
that the MEEG approach could prove
to be a powerful engine for future 2D
materials discovery.
❐
Nikhil A. Koratkar is in the Department of
Mechanical, Aerospace and Nuclear Engineering
and the Department of Materials Science and
Engineering at the Rensselaer Polytechnic Institute,
Troy, New York 12180, USA.
e-mail: [email protected]
References
Novoselov, K. S. et al. Science 306, 666–669 (2004).
Naguib, M. et al. Adv. Mater. 23, 4248–4253 (2011).
Jariwala, D. et al. ACS Nano 8, 1102–1120 (2014).
Al Balushi, Z. Y. et al. Nat. Mater. http://dx.doi.org/10.1038/
nmat4742 (2016).
5. Zandiatashbar, A. et al. Nat. Commun. 5, 3186 (2014).
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Published online: 29 August 2016
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
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