What Drives Metal-Surface Step Bunching in Graphene Chemical Vapor Deposition?

Ding Yi, Da Luo, Zhu-Jun Wang, Jichen Dong, Xu Zhang, Marc-Georg Willinger, Rodney S. Ruoff, and Feng Ding

Phys. Rev. Lett. 120, 246101 – Published 12 June 2018

Abstract

Compressive strain relaxation of a chemical vapor deposition (CVD) grown graphene overlayer has been considered to be the main driving force behind metal surface step bunching (SB) in CVD graphene growth. Here, by combining theoretical studies with experimental observations, we prove that the SB can occur even in the absence of a compressive strain, is enabled by the rapid diffusion of metal adatoms beneath the graphene and is driven by the release of the bending energy of the graphene overlayer in the vicinity of steps. Based on this new understanding, we explain a number of experimental observations such as the temperature dependence of SB, and how SB depends on the thickness of the graphene film. This study also shows that SB is a general phenomenon that can occur in all substrates covered by films of two-dimensional (2D) materials.

Received 1 January 2018

DOI:https://doi.org/10.1103/PhysRevLett.120.246101

Physics Subject Headings (PhySH)

Growth Nucleation on surfaces Roughness Surface diffusion Surface reconstruction

Graphene

Chemical vapor deposition

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Ding Yi¹, Da Luo¹, Zhu-Jun Wang², Jichen Dong¹, Xu Zhang¹, Marc-Georg Willinger^2,*, Rodney S. Ruoff^1,3,4,†, and Feng Ding^1,3,‡

¹Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea
²Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Berlin-Dahlem D-14195, Germany
³School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
⁴Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

^*willinger@fhi-berlin.mpg.de
^†ruofflab@gmail.com
^‡f.ding@unist.ac.kr

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Issue

Vol. 120, Iss. 24 — 15 June 2018

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Images

Figure 1
(a)–(c) In situ scanning electron microscopy images of graphene-covered Cu surfaces taken at $\sim 600 ° C$ during cooling following graphene growth. $A$ , $B$ , $C$ , and $D$ in (a) mark four areas covered with graphene, but where no step bunches appear. (d) Schematic illustrations of a model to describe metal surface SB beneath graphene.
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Figure 2
(a)–(e) Atomic force microscope (AFM) topographic images of transferred graphene on single crystal Cu(111) foils after thermal annealing at $200 ° C$ (a), $300 ° C$ (b), $500 ° C$ (c), and $600 ° C$ (d),(e) followed by slow cooling (a)–(d) and fast cooling (e), while images of 400 and $700 ° C$ are shown in Fig. S2 in the Supplemental Material [34]. (f) Raman spectra of the pristine and annealed graphene samples on single crystal Cu(111) foils. (g) Plot of step heights versus the annealing temperatures under both slow cooling (blue) and fast cooling (red) conditions. (h) AFM height profiles of the samples shown in (a)–(e) and in Fig. S2 [34]. In (f) and (h), SC and FC represent slow cooling and fast cooling, respectively.
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Figure 3
(a),(b) Side views of a Cu atom on a Cu(111) surface with and without a graphene overlayer, top views of atom diffusion, and schematic plots showing formation energies and diffusion barriers in eV. Gray, red, and orange balls represent C atoms, Cu adatom, and Cu atoms in the substrate, respectively. (c) Flux of Cu atoms on bare Cu and graphene-covered Cu surfaces as a function of temperature.
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Figure 4
(a) The process of the Cu(111) surface SB beneath a graphene layer. A series of images from top to bottom showing the increase in height of the highest step in each configuration, marked by black arrows with $H = 1$ , 2, 4, 8, and 16. (b) The relative energy variation of bare Cu (black line) and SLG-covered Cu (red line) during the SB process (data for BLG and TLG-covered Cu are shown in Fig. S6 in the Supplemental Material [34]). (c) Schematic illustrations showing the different ways of bending of the graphene overlayer during the SB process. (d) Formation energies of bare Cu (black line) and Cu covered by SLG (red line), BLG (blue line), and TLG (green line), as a function of the step height.
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Figure 5
Cu surface SB beneath the adlayer graphene (a)–(c) and the dissociation of the bunched steps (d),(e) after graphene removal observed in a MD simulation.
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Figure 6
AFM images and step height distributions of Cu(111) surfaces covered with SLG and BLG. AFM images, height profiles, and statistics of step height distributions of (a)–(c) SLG and (d)–(f) BLG.
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