Real-time observation of epitaxial graphene domain reorientation

Abstract

Graphene films grown by vapour deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, that is, 'ripening'; domain boundary motion within islands; and continuous lattice rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1 meV per C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moiré corrugation of the graphene sheet due to local variations in the graphene-substrate interaction reproduces the results. This work suggests new strategies for improving the van der Waals epitaxy of 2D materials.

Figure 1. Three domain realignment mechanisms of graphene on Ir(111).

(a) LEEM image of a sub-monolayer graphene film on Ir(111) at 1,050 °C (75 μm field of view (FOV)). The spatial extent of individual domains of a given orientation can be deduced from differences in the reflectivity within the graphene islands. Rotation angle is denoted by the number following the letter R, as determined by selected-area low-energy electron diffraction (LEED). (b) Evolution of graphene islands after annealing at 1,050 °C for 155 min. (c) Image stacks of a and b after removing the background shows how the islands evolved. With the initial frame (false-colored purple) on top, exposed areas show growth, reversing the image order exposes areas that dissolved. (d) LEEM image after annealing for 257 min; circles in b and d mark a section of the R12 domain that continuously transformed to the R4 orientation. (e) Island reflectivity monitored with time for the R12 domain and the R12 to R4 transition region. (f) Selected-area LEED taken before and after the reflectivity change. Arrows: graphene (green) and Ir(111) (black) first-order LEED spots. (g) Domain boundary motion between R11 and R12. Middle frame highlights the radius of curvature of the boundary as it moved (red arrow). Time, from left to right: 55, 102 and 165 min. Scale bar, 10 μm.

Figure 2. Orientation-dependent chemical potential as determined from simultaneous growth and dissolution experiments.

(a) Interpretation of the simultaneous growth and dissolution of graphene domains, RX and RY, respectively, for a spatially uniform C adatom concentration. (b) Relative chemical potential of graphene rotational variants. Multiple rotations on one line indicate that these rotation couples have not been observed together in the same experiment and cannot be further distinguished.

Figure 3. Temperature-controlled ripening of graphene domains.

(a) LEEM images (50 μm FOV) of graphene islands at 500 s (left) and 3,200 s (right) during temperature-controlled ripening. (b) R14 and R8 domain areas measured every 10th frame (every 100 s). At 997 °C (200–800 s), all domains grew; increasing the temperature to 1,005 °C (800–1,000 s) resulted in the R14 domain dissolving, while the R8 domain continued to grow. Increasing the temperature further ended the simultaneous dissolution and growth between the R14 and R8 domains: at 1,030 °C, the R8 domain ceased growing (2,800–3,200 s). Substrate temperature (green line) was adjusted in steps of ∼5 K. (c) Standalone R0 island area was measured every frame (every 10 s) and continued to grow the entire time. (d) Quantitative differences in chemical potential relative to R0. Individual experiments are colour-coded; R8 data were offset in the x axis for clarity. Error bars are from the uncertainty in the velocity measurement of R0 islands used to determine the difference in chemical potential.

Figure 4. Moiré corrugation model.

(a) Distance of C atoms from the Ir(111) substrate obtained from DFT calculations for graphene rotated by 0, 15 and 30 degrees relative to the Ir(111) lattice. Scale bar is in angstroms. (b) Moiré corrugation model (MCM) results. Green line: the preferred separation distance of C atoms from the substrate (MCM-PSD) taken along the close-packed zigzag direction (representative white lines in a), where filled circles represent C atoms. Due to the bending rigidity of the graphene sheet, the C atoms cannot follow the short-wavelength corrugations and the resulting sheet corrugation from the MCM is given by the red line. Corrugation profiles taken from the DFT results are shown in blue for comparison. Scale bar applies to all three profiles for all four rotations. Note: the MCM-PSD profile is multiplied by ½ for clarity. (c) Energy per C atom relative to the R0 orientation as a function of rotation angle as determined from the MCM.