Wrinkles of graphene on Ir(111): Macroscopic network ordering and internal multi-lobed structure
Marin Petrović, Jerzy T. Sadowski, Antonio Šiber, Marko Kralj
Carbon 94 (2015) 856-563

Some of the main characteristics of epitaxial graphene on metal substrates are its uniformity and high structural quality. However, due to the high synthesis temperatures and very small coefficient of thermal expansion (i.e. contraction) of graphene, cooling to room temperature induces stress. The stress is relaxed in the form of graphene wrinkles which represent deformations of the otherwise planar graphene lattice and as such affect many properties of graphene, e.g. electrical and thermal conductivity, optical transmittance and chemical reactivity. In addition, wrinkles play a major role in graphene intercalation which is often utilized for the creation of hybrid graphene systems. Therefore, a thorough understanding of graphene wrinkles is important for potential applications of graphene.

In the paper published in the journal Carbon (IF = 6.196), M. Petrovć, A. Šiber and M. Kralj from the Institute of Physics in collaboration with J.T. Sadowski from the Brookhaven National Laboratory (USA) present results of micro- and nano-characterization of wrinkles of graphene synthesized on the iridium (111) surface. LEEM (low-energy electron microscope) and STM (scanning tunneling microscope) were used for experimental measurements and a simple analytic model was utilized for the understanding of the wrinkles’ energetics. It is shown that wrinkles, having lengths of the order of micrometers, interconnect in an ordered hexagonal network which is aligned with the substrate (see figure 1). The network can be mathematically described with the aid of Voronoi diagrams, which significantly facilitates its parameterization. Also, a new model is proposed which accounts for the observed changes in the electron reflectivity of graphene and relates it to the local relaxation of the graphene lattice during wrinkle formation.

Moreover, it is determined that structural details of graphene and iridium (e.g. dirt particles and already formed wrinkles) can act as nucleation centers for the formation of new wrinkles. At the nano-scale, individual wrinkles are composed of several lobes (see figure 2) which result from the system frustration which is induced during cooldown from high synthesis temperatures. In terms of energy, the number of lobes is determined by the competition of the van der Waals binding acting between graphene and iridium and the graphene bending energy. Overall, this study provides new insights into graphene wrinkles and their network as a whole, which makes it relevant for future development of devices based on graphene as well as on other 2D materials.

 

Figure 1. (a) LEEM image of graphene’s wrinkle network (field of view: 9.3 μm), (b) Fourier transform of (a) exhibiting hexagonal symmetry, (c) polar plot of radial sums extracted from (b) and (d) illustration of graphene (orange) on Ir(111) (gray balls) with marked directions of wrinkle extension (yellow).

 

Figure 2. STM image of (a) topography and (b) first derivative of topography of graphene wrinkle and (c) wrinkle profile marked by red line in (a). Four lobes constituting the wrinkle can be identified.