(Invited) Atomic Resolution, Coordinated Electron Microscopy Characterization of 2D Layers Synthesized By Confinement Heteroepitaxy (CHet) Technique
Authors: ElSherif Hesham , Briggs Natalie , Bersch Brian , Rajabpour Siavash , Robinson Joshua , Bassim Nabil 
Published: May 2020

Since the discovery of graphene in 2004 [1] there has been a flurry of activity in investigating two dimensional (2D) materials, because of their superlative properties: spin-orbit coupling, massless Dirac fermions, mechanical strength, thermal and chemical stability [2]. Remarkably, the in-plane hexagonal symmetry of graphene is largely responsible for its behavior. However, the zero-band gap of graphene presents hurdles to its use as a semiconductor. This has spurred interest in synthesis other van der Waals 2D structures, such as h-BN [3] and more recently 2D Gallium Nitride (2D GaN) [4].

The synthesis of 2D Nitrides was first reported in 2016 by applying Confinement Heteroepitaxy (CHet) process to form a 2D Gallium Nitride (GaN) layer with 1-2 atom thickness. In this technique, a silicon carbide (SiC) substrate is graphitized and deliberately damaged using oxygen plasma to provide a two-step intercalation path. First a metal (like Gallium, or other group V metal) through evaporation is intercalated between the graphene and SiC substrate. Then, to complete the half reaction, nitrogen is introduced by ammonia annealing at 700 °C to form a 1-layer thick III-nitride.

In this study, the growth method is extended from 2D GaN to variety of other 2D metals (Indium), Nitrides (InN) and alloys (InGa alloys). We employ a coordinated approach to understand the surface in both plan and cross-section view – linking through a variety of electron microscopy techniques, including auger electron spectroscopy (AES), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Bright-field, Dark-field, aberration-corrected STEM-HAADF imaging, and atomic-scale electron energy loss spectroscopy (EELS) mapping using a Gatan K2 SI direct electron detector and other techniques mapping were all applied to characterize the SiC/2D-layer/graphene interface. These electron microscopy techniques allow a precise, local measurement of the atomic structure of these new materials, including such growth parameters as the effect of graphene on the 2D layer stability, the effect of the SiC step edges on the layers structure, and the effect of the plasma type and source used in damaging the graphene on the 2D layer chemistry. Plasmon mapping also describes the unique bonding in the materials and their electronic structure. Ultimately, the measurements presented here characterize a whole new class of nanomaterials and provide insight on the fundamentals of the intercalation growth process.

1. Novoselov, K.S., et al., Electric field effect in atomically thin carbon films. Science, 2004. 306(5696): p. 666-669.

2. Soldano, C., A. Mahmood, and E. Dujardin, Production, properties and potential of graphene. Carbon, 2010. 48(8): p. 2127-2150.

3. Watanabe, K., T. Taniguchi, and H. Kanda, Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal. Nature Materials, 2004. 3(6): p. 404-409

4. Al Balushi, Z.Y., et al., Two-dimensional gallium nitride realized via graphene encapsulation. Nature Materials, 2016. 15(11): p. 1166-1171.

Figure 1