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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.


By imaging emission from graphene's π-band at the reciprocal space K point, photoemission microscopy enabled a direct comparison of the local density of states (DOS) of flat and buckled graphene. The low intensity observed in the latter suggests the disruption of the Dirac cones (see Fig. 2). In relation to the experiment, theory revealed that the metallic-like character of the buckled phase does not originate from strain or rippling, but rather from the chemisorption to the substrate. The novel aspect highlighted in our study is that the change in the graphene DOS at the Dirac point is induced by the chemisorption of just a small fraction of the atoms in the unit cell.
A striking feature of Graphene/Ir(100) is that the buckled phase can be reversibly transformed into flat physisorbed graphene by varying the sample temperature. This observation suggests that the film changes morphology and structure in order to relieve the strain resulting from the different thermal contraction of the substrate and the anchored graphene island. At variance with the (111) substrates of Ir and Pt, where narrow and tall wrinkles are formed, here corrugated graphene organizes in microscopically extended domains. Regions of flat unstrained, physisorbed graphene alternate with the denser, buckled phase. The relative proportion of the two phases is dictated by their density difference and the thermal contraction of the substrate upon cooling below growth temperature. Further, the domain orientation of the buckled phase, about 70° with respect to the substrate main direction, is determined by the anisotropic difference in density between BG and FG.

 

Figure 2:  (a) microprobe-ARPES pattern of graphene/Ir(100) at room temperature; (b) cross section through one of the Dirac cones along a plane normal to Γ-K, as indicated in (a); (c) the intensity profile along the vertical red line in (b) shows the Fermi level (d) dark-field XPEEM image at the K point at the Fermi energy; the image intensity is proportional to the local DOS; (e) normal emission XPEEM image at Γ.Reprinted with permission from A. Locatelli et al., ACS Nano, 7, 6955–6963 (2013); Copyright (2013) American Chemical Society.

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.


This research was conducted by the following research team:

  • A. Locatelli, T.O. Menteş, Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
  • C. Wang, N. Stojić, N. Binggeli, Abdus Salam International Centre for Theoretical Physics, Trieste, Italy and IOM-CNR Democritos, Trieste, Italy
  • C. Africh, G. Comelli, IOM-CNR Laboratorio TASC, Trieste, Italy and Physics Department, University of Trieste, Trieste, Italy and Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy


Contact person:
Andrea Locatelli: andrea.locatelli@elettra.eu

Reference

A. Locatelli, C.Wang, C. Africh , N. Stojić, T.O. Menteş, G. Comelli, N. Binggeli, “Temperature Driven Reversible Rippling and Bonding of a Graphene Superlattice”, ACS Nano, 7, 6955 (2013); DOI: 10.1021/nn402178u
 
 
Last Updated on Wednesday, 16 October 2013 09:05