Synthesis of quasi-free-standing bilayer graphene nanoribbons on SiC surfaces

Abstract

Scaling graphene down to nanoribbons is a promising route for the implementation of this material into devices. Quantum confinement of charge carriers in such nanostructures, combined with the electric field-induced break of symmetry in AB-stacked bilayer graphene, leads to a band gap wider than that obtained solely by this symmetry breaking. Consequently, the possibility of fabricating AB-stacked bilayer graphene nanoribbons with high precision is very attractive for the purposes of applied and basic science. Here we show a method, which includes a straightforward air annealing, for the preparation of quasi-free-standing AB-bilayer nanoribbons with different widths on SiC(0001). Furthermore, the experiments reveal that the degree of disorder at the edges increases with the width, indicating that the narrower nanoribbons are more ordered in their edge termination. In general, the reported approach is a viable route towards the large-scale fabrication of bilayer graphene nanostructures with tailored dimensions and properties for specific applications.

Figure 1. Process for the synthesis of quasi-free-standing bilayer GNRs on SiC(0001).

(a–c) Different stages of the layer-by-layer graphene formation initiating on a surface step of a SiC(0001) surface. (a,b) Evolution of the growth of the (6√3 × 6√3)R30° BL over the SiC(0001). (c) Monolayer GNRs can be formed at the step edge region by controlling the growth process. (d) Changes promoted by air annealing: decoupling of the BL underneath the monolayer GNR from the substrate due to the oxidation of the SiC surface and etching of the bare BL. These two effects lead to the formation of quasi-free-standing bilayer GNRs exclusively at step edge regions. Note that the illustrations are not to scale.

Figure 2. AFM and TEM results for bilayer GNRs.

(a) AFM phase contrast image taken from a sample containing 320 nm wide bilayer GNRs (growth at 1,500 °C for 15 min followed by air annealing). Scale bar, 1 μm. The inset shows a close-up view of a single GNR. Scale bar, 0.5 μm. (b) Height profile taken from the same area (yellow dashed line in (a)). (c) Phase contrast image taken from a single bilayer GNR (growth at 1,470 °C for 15 min followed by air annealing). Scale bar, 0.5 μm. (d) Phase contrast image of a step edge without a GNR. Scale bar, 0.5 μm. (e) Phase shift profiles taken from images (c) (red dashed line) and (d) (yellow dashed line). (f) Cross-sectional phase-contrast TEM image of a bilayer GNR that extends along the surface terrace (growth at 1,450 °C for 15 min followed by air annealing). Scale bar, 20 nm. (g) A close-up view of the same region revealing the existence of few-layer graphene at the step facet. Scale bar, 5 nm.

Figure 3. Raman results for monolayer and bilayer GNRs on SiC(0001).

(a) Raman spectra of pristine monolayer GNRs (on top of the BL) grown on SiC(0001) for 15 min at different temperatures. Spectra of a bare BL (bottom spectrum) as well as from a monolayer graphene (on top of the BL, top spectrum) are plotted for comparison. (b) Raman spectra of quasi-free-standing bilayer GNRs created after thermally treating monolayer GNRs in air. (c,d) Raman 2D peaks measured for a GNR grown on a SiC surface step edge at 1,450 °C for 15 min (c) before and (d) after annealing in air. (e) Raman mapping (8 × 13 μm²) of the G peak intensity taken from a quasi-free-standing bilayer GNR synthesized at 1,470 °C for 15 min followed by air annealing. (f) Splitting of the Raman G peak into two components for the same bilayer GNR. (g) Dependence of the D peak intensity (I_D) on the angle between the polarization of the linearly polarized laser and the GNR edge for a bilayer GNR prepared on surface steps perpendicular to the [1–100] direction (growth at 1,490 °C for 15 min followed by air annealing). The solid red line represents the best fitting obtained using the function I_min+(I_max–I_min)cos⁴(θ) (ref. 42). (h) Dependence of the I_min/I_max ratio on the ribbon width. The Raman spectra were recorded with spatial resolution of about 1 μm. Error bars in (g) and (h) represent s.d.