Temperature Driven Reversible Rippling and Bonding of a Graphene Superlattice

Graphene on Ir(100), a support with square symmetry, provides a remarkable model for investigating the intriguing physics of the metal-graphene interface. In our study on this system, we discovered distinct flat and buckled graphene phases on that coexist at room temperature, forming stripe-shaped domains which relieve the strain accumulated after cooling the film below growth temperature. In the buckled phase, a small fraction of the carbon atoms chemisorbs to the substrate, originating a textured structure with exceptionally large one-dimensional ripples of nm periodicity. Our results unravel the complex interplay between film and support, disentangling the effects of the film configuration and substrate interaction on the quasi-particle dispersion.
In order to overcome the challenges imposed by the complexity of this system, we employed advanced experimental and theoretical methods. Spectroscopic photoemission and low energy electron microscopy (SPELEEM) measurements were carried out at the Nanospectroscopy beamline of Elettra, and were complemented by scanning tunneling microscopy (STM) for characterization at the atomic scale.  The experimental results were corroborated by density functional theory (DFT) calculations.

Figure 1: (a-c) bright and dark-field LEEM images of flat (FG) and buckled (BG) graphene; (d) corresponding microprobe-LEED pattern; the coincidence spots are due to BG; the FG first order spots are indicated by the red arrows; blue circles mark the position of the Ir first-order diffraction; (e-f) STM images of FG and BG; (g) atomic resolution image of BG; (h) two contiguous BG unit cells; (i) side view over BG, illustrating its large corrugation; (j) simulated STM image of BG. Reprinted with permission from A. Locatelli et al., ACS Nano, 7, 6955–6963 (2013); Copyright (2013) American Chemical Society.
 

The micrographs in Fig. 1a-d illustrate the structure of a graphene island on Ir(100) at ambient temperature, revealing the coexistence of two distinct phases; the minority phase forms stripes that extend over micron lengths, embedded into the other phase. STM data show that these two regions correspond to graphene phases characterized by buckled (BG) and flat (FG) morphology, respectively, with the buckled phase exhibiting perfectly regular stripes separated by just 2.1 nm. The honeycomb lattice is continuous across the stripes and passing from flat to buckled regions, see Fig. 1e-g. The unit cell of the buckled phase was identified by STM and microprobe low energy electron diffraction (LEED). The equilibrium atomic positions were determined theoretically (see Fig. 1h). The side view of the cell is shown Fig. 1i. By using the DFT-D approach, taking into account Van der Waals interactions, the minimum and maximum separation between graphene and Ir were found to be 2.1 Å and 3.8 Å,respectively. The buckling of 1.7 Å is significantly larger than any buckling previously found for graphene. The analysis of the calculated spatial distribution of the charge density, used to generate the image in Fig. 1i, indicates that only 11% of the C atoms are chemisorbed to Iridium.

We expect that one-dimensional ripples showing high degree of order might be observed in a multitude of sp²-bonded layers supported on square or rectangular symmetry surfaces. In these regards, we highlight the potential of buckled graphene as candidate substrate for synthesizing one-dimensional supported nanostructures characterized by fascinating physical and chemical properties.