Materials synthesis: Two-dimensional gallium...
2
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials 1 news & views 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. 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-workers 4 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. e 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). is 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-workers 4 show that defects (such as carbon vacancies), wrinkles (tears) and large 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). e 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 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 2D GaN bonded to SiC substrate Bilayer graphene capping sheet 3D GaN island 2D GaN a b c d SiC substrate H Si C 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. ©2016MacmillanPublishersLimited,partofSpringerNature.Allrightsreserved.
Transcript of Materials synthesis: Two-dimensional gallium...
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials 1
news & views
The groundbreaking discovery1 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 few2,3. 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.
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
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
MATERIALS SYNTHESIS
Two-dimensional gallium nitrideGraphene 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
2D GaN bonded to SiC substrate
Bilayer graphene capping sheet
3D GaNisland
2D GaN
a b
c d
SiC substrate
HSiC
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.
© 2016
Macmillan
Publishers
Limited,
part
of
Springer
Nature.
All
rights
reserved.
2 NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
news & views
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
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-engineer5 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]
References1. Novoselov, K. S. et al. Science 306, 666–669 (2004).2. Naguib, M. et al. Adv. Mater. 23, 4248–4253 (2011).3. Jariwala, D. et al. ACS Nano 8, 1102–1120 (2014).4. 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).
Published online: 29 August 2016
© 2016
Macmillan
Publishers
Limited,
part
of
Springer
Nature.
All
rights
reserved.
