Novel structures of Gallenene intercalated in epitaxial Graphene

TL;DR

Atomically thin gallium has been realized via confinement epitaxy by intercalating Ga under epitaxial graphene on SiC using MBE deposition and sequential UHV annealing. The study reveals coexistence of multiple Ga phases under graphene, including b010-gallenene and a Ga(III)–like phase, and uncovers distinct moiré superstructures such as 12-by-12 and 5-by-1 patterns arising from graphene–gallenene interactions. A thermal- and defect‑mediated intercalation pathway is demonstrated, including intercalation under the buffer layer and a model that explains phase stability and transitions. The results establish a tunable, scalable platform protected by graphene for exploring superconductivity, moiré physics, and metal–insulator transitions, with potential extension to other elements and compounds.

Abstract

The creation of atomically thin layers of non-exfoliable materials remains a crucial challenge, requiring the development of innovative techniques. Here, confinement epitaxy is exploited to realize two-dimensional gallium via intercalation in epitaxial graphene grown on silicon carbide. Novel superstructures arising from the interaction of gallenene (a monolayer of gallium) with graphene and the silicon carbide substrate are investigated. The coexistence of different gallenene phases, including b010-gallenene and the elusive high-pressure Ga(III) phase, is identified. This work sheds new light on the formation of two-dimensional gallium and provides a platform for investigating the exotic electronic and optical properties of confined gallenene.

Figures (14)

• Figure 1: (a) STM image (0.4 V, 0.8 nA) of 2D-gallium intercalated over large area. 2L-Ga@MLG is represented in yellow, pristine MLG in blue. (b) STM image (0.1 V, 0.7 nA) of an area in which both layers of an intercalated island are visible. The upper level (2L-Ga@MLG) is represented in yellow, the lower (1L-Ga@MLG) in orange, and MLG in blue. (c) Height profile acquired along the red line in (b) which shows that both steps are about 0.2 nm high. (d) STM image (1.0 V, 1.0 nA) of an intercalated area originating from a SiC step edge (indicated by the light blue dashed line). Scale bars in (a), (b), and (d) are 100 nm, 50 nm, and 50 nm, respectively. Right: color scale for (a), (b), and (d).
• Figure 2: (a) STM image (3.0 V, 0.8 nA) of 2L-Ga@MLG in which the striped pattern is imaged. 2L*-Ga@MLG regions are highlighted by the red dashed lines. (b) Close-up of (a) (corresponding to the green square) for a better visualization of the striped pattern. (c) FFT of (a). The observed hexagonal pattern is given by three pairs of spots, one per each orientation of the stripes (each orientation is highlighted by a different color). (d) Atomically resolved STM image (0.24 V, 0.6 nA) in which the stripes are visible, the graphene lattice, and the centered rectangular lattice of Ga(III). Sketches of the graphene and Ga lattices are superimposed to the image. (e) Left: FFT of (d). Graphene spots are highlighted by the grey hexagon and spots from gallium by the green rectangle. The spots from the striped moiré (only one direction is visible here) are circled in orange. Right: scheme of the spots in which unit vectors of Ga (b$_1$, b$_2$) and graphene (a$_1$, a$_2$) are indicated as well as the angle between them. (f) Left: schematic representation of the observed centered rectangular lattice of Ga(III). Right: unit cell of bulk Ga(III) where the (110) plane is highlighted in blue. Scale bars in (a) and (d) are 20 nm and 2 nm, respectively.
• Figure 3: (a) STM measurement (0.4 V, 0.8 nA) in a region mainly of 1L-Ga@MLG (represented in orange). Both the SiC-$6 \times 6$ moiré of pristine MLG and the graphene-$12 \times 12$ moiré on 1L-Ga@MLG are observed. (b) FFT of (a). The SiC-$6\times6$ and the graphene-$12 \times 12$ spots are highlighted in green and yellow, respectively. (c) LEED measurement performed on the same sample. The SiC-$6 \times 6$ and the graphene-$12 \times 12$ spots are highlighted with the same colors as in (b). (d) STM image ($-0.55$ V, $-0.31$ nA) acquired on 1L-Ga@MLG. A sketch of b010-gallenene is superimposed onto the measured lattice. (e) STM image (0.46 V, 0.12 nA) of the graphene-$12 \times 12$ moiré on 1L-Ga@MLG. (f) Simulated moiré given by the overlap of b010-Ga and graphene. The scale bars in (a), (d), (e), and (f) are 20 nm, 1 nm, 5 nm, and 5 nm, respectively.
• Figure 4: (a) STM image ($-2.3$ V, $-0.62$ nA) of Ga intercalated on BL. The red dashed line indicates the edge between BL (on the left) and MLG (on the right). All three intercalated layers of Ga are imaged. (b) High magnification STM image (2.0 V, 0.5 nA) of the striped pattern observed on 2L-Ga@BL. (c) STM image ($-0.64$ V, $-0.24$ nA) acquired on 1L-Ga@BL exhibiting both the graphene lattice (unit cell represented in red) and the graphene-$2 \times 2$ reconstruction due to Ga (unit cell represented in light blue). The scale bars in (a), (b), and (c) indicate 20 nm, 5 nm, and 2.5 nm, respectively.
• Figure 5: Sketch of the proposed model for the intercalation of gallium in MLG (top) and BL (bottom).