Rubidium intercalation in epitaxial monolayer graphene

TL;DR

This study addresses how rubidium intercalates under epitaxial monolayer graphene on SiC(0001) and whether ordered intercalated phases can form. Using a multi-technique approach combining LEED, STM, LEEM, μ-LEED, and DFT, the authors map phase formation as a function of Rb density and temperature, uncovering two ordered single-layer intercalation phases, $2\times 2$ and $(\sqrt{3}\times\sqrt{3})R30^\circ$, between graphene and the buffer layer and observing strong $n$-type doping of graphene; diffusion and desorption enable reversible intercalation up to approximately $600^{\circ}\mathrm{C}$. The work provides structural and electronic insights into alkali-metal intercalation in graphene/SiC systems, informing design principles for graphene-based electronics, energy storage, and catalysis and advancing understanding of 2D intercalation chemistry.

Abstract

Alkali metal intercalation of graphene layers has been of particular interest due to potential applications in electronics, energy storage, and catalysis. Rubidium (Rb) is one of the largest alkali metals and the one less investigated as intercalant. Here, we report a systematic investigation, with a multi-technique approach, of the phase formation of Rb under epitaxial monolayer graphene on SiC(0001). We explore a wide phase space with two control parameters: the Rb density (i.e., deposition time) and sample temperature (i.e., room- and low-temperature). We reveal the emergence of $(2 \times 2)$ and $(\sqrt{3} \times \sqrt{3})$R30° structures formed by a single alkali metal layer intercalated between monolayer graphene and the interfacial C-rich reconstructed surface, also known as buffer layer. Rb intercalation also results in a strong n-type doping of the graphene layer. Progressively annealing to high temperatures, we first reveal diffusion of Rb atoms which results in the enlargement of intercalated areas. As desorption sets in, intercalated regions progressively shrink and fragment. Eventually, at approximately 600°C the initial surface is retrieved, indicating the reversibility of the intercalation process.

Figures (8)

• Figure 1: Schematic representation, in top view, of the AM distribution (a) in the $(2 \times 2)$ superstructure, and (b) in the $(\sqrt{3} \times \sqrt{3})$R30° superstructure. Black circles are C atoms, while red circles are AM atoms. The graphene unit cell is highlighted by a green rhombus while the unit cells of the superstructures are highlighted in yellow.
• Figure 2: Evolution of the LEED pattern upon Rb deposition on epitaxial graphene on SiC(0001) at room temperature. LEED pattern obtained from (a) the pristine graphene sample, and after depositing Rb for (b) 1 min, (c) 2 min, and (d) 3 min. The graphene, SiC, and $6\sqrt{3}$ structures are indicated by peach, blue, and pink arrows, respectively. The Rb$(2\times2)$ and Rb$(\sqrt{3}\times \sqrt{3})$R30° structures are highlighted by yellow and green arrows, respectively. Beam energy: (a) 60 eV, (b) 75 eV, (c) 60 eV, and (d) 65 eV.
• Figure 3: (a) Overview STM scan of the graphene surface obtained after Rb deposition for 3 min at RT showing a wrinkles network. Inset to (a): Cross-section taken across a wrinkle, along the blue line in (a). (b) Close-up view of monolayer graphene showing the wrinkles network and Rb intercalated regions, labeled as RbG. Inset to (b): Cross-section taken across RbG regions, along the blue line in (b). Scan size: (a) $(3~\mu\text{m}\times3~\mu\text{m})$ and (b) $(500\text{~nm}\times500\text{~nm})$. Scale bar: (a) 500 nm and (b) 100 nm.
• Figure 4: STM topographic images of Rb-intercalated regions in monolayer graphene showing a Rb$(2\times2)$ ordering. (a) STM scan showing the $6\sqrt{3}$-moirè modulation and the Rb$(2\times2)$ ordering highlighted in the corresponding FFT shown in the inset by white and red hexagons, respectively. (b) Atomically resolved STM scan showing the Rb$(2\times2)$ arrangement together with the graphene lattice, highlighted by black and blue rhombi, respectively. Scan size: (a) $(28\text{~nm}\times28\text{~nm})$ and (b) $(10\text{~nm}\times10\text{~nm})$. Scale bar: (a) 5 nm, inset to (a) 2 nm$^{-1}$, and (b) 2 nm.
• Figure 5: Charge density difference (CDD) and optimized geometry of the Rb intercalated $(2\times 2)$ structure in monolayer graphene on SiC(0001) obtained from DFT analysis. Due to Rb intercalation, the graphene-buffer layer separation increases to 5.77 Å, which is in good agreement with results we obtained by STM for the Rb-$(2\times 2)$ reconstruction.