Scalable Characterization of 2D Gallium-Intercalated Epitaxial Graphene

Abstract

Scalable synthesis of two-dimensional gallium (2D-Ga) covered by graphene layers was recently realized through confinement heteroepitaxy using silicon carbide substrates. However, the thickness, uniformity, and area coverage of the 2D-Ga heterostructures have not previously been studied with high-spatial resolution techniques. In this work, we resolve and measure the 2D-Ga heterostructure thicknesses using scanning electron microscopy (SEM). Utilizing multiple correlative methods, we find that SEM image contrast is directly related to the presence of uniform bilayer Ga at the interface and a variation of the number of graphene layers. We also investigate the origin of SEM contrast using both experimental measurements and theoretical calculations of the surface potentials. We find that a carbon buffer layer is detached due to the gallium intercalation, which increases the surface potential as an indication of the 2D-Ga presence. We then scale up the heterostructure characterization over a few-square millimeter area by segmenting SEM images, each acquired with nanometer-scale in-plane resolution. This work leverages the spectroscopic imaging capabilities of SEM that allows high-spatial resolution imaging for tracking intercalants, identifying relative surface potentials, determining the number of 2D layers, and further characterizing scalability and uniformity of low-dimensional materials.

Nanoscale characterization of the heterogeneous interfacial oxidation layer of graphene/Cu based on a SEM electron beam induced reduction effect

“…Confocal laser scanning Raman microscopy can realize identification of graphene and copper oxides with the spatial resolution of B1 mm; however, Raman microscopy is not applicable for the thin interfacial oxide layer of Gr/Cu with thickness down to a few nanometers, and it may be time intensive for mapping measurements though it offers fast characterization for single point measurement. Focused ion beam-transmission electron microscopy (FIB-TEM) and high resolution transmission electron microscopy (HRTEM) can be used to characterize the thickness and morphology of interfacial oxide layers 30,31 at nanoand even atomic-scale; however, statistically reliable information can only be acquired by imaging at multiple regions for heterogeneous interfaces.…”

Correlative Microscopy Reveals Air-Stable 2D Gallium-Intercalated Monolayer Epitaxial Graphene

“…The AES maps were acquired using JEOL JAMP-9500F at 10 kV, 10−20 nA, and <10 −6 mbar where the Auger electrons are analyzed using a hemispherical analyzer equipped with a seven-channel detector. Here we utilize correlative microscopy workflows to reveal that the 2D-Ga regions has an oxygen signal but locates above the metal in cross-section viewwithout actual oxidation of the 2D-Ga. By preparing a site-specific cross-section from the 2D-Ga surface [2], we could employ aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) investigations to understand the role of bilayer graphene in protecting the 2D-Ga.…”

“…2a,b. Identifying this site and its contents is possible using the SEM contrast as previously studied [2]. A double-corrected FEI Titan STEM (operated at 200 keV and 50-100 nA screen current) is used to acquire high annular angle dark field (HAADF) images from both the peeled and unpeeled regions in the cross-section view.…”

“…1a). To enable large area intercalation, the EG surface is deliberately damaged before the intercalation using plasma (3:1 O 2 /He mixture at 50 Watts for 60 seconds), which facilities scalable fabrication of 2D metals and refractory compounds over millimeterscale [2]. The 2D CHet layers exhibit novel properties such as superconductivity [3], enormous second harmonic generation [4], and epsilon-near-zero behavior [5].…”

Observation of 2D Si-Vacancies Filled by Gallium Intercalation of Epitaxial Graphene

“…Although the EG is deliberately damaged using plasma to open intercalation holes prior to the CHet process, the EG is found to be healed after the metal intercalation due to a metal catalytic effect. The CHet process facilitates scalable and environmentally air-stable 2D metals and alloys over millimeter-scale [2,3]. CHet metals and alloys exhibit novel properties, such as enormous second harmonic generation [4], superconductivity [1], and epsilonnear-zero behavior [5].…”