Strain Relaxation of Graphene Layers by Cu Surface Roughening

Abstract

The surface morphology of copper (Cu) often changes after the synthesis of graphene by chemical vapor deposition (CVD) on a Cu foil, which affects the electrical properties of graphene, as the Cu step bunches induce the periodic ripples on graphene that significantly disturb electrical conduction. However, the origin of the Cu surface reconstruction has not been completely understood yet. Here, we show that the compressive strain on graphene induced by the mismatch of thermal expansion coefficient with Cu surface can be released by forming periodic Cu step bunching that depends on graphene layers. Atomic force microscopy (AFM) images and the Raman analysis show the noticeably longer and higher step bunching of Cu surface under multilayer graphene and the weaker biaxial compressive strain on multilayer graphene compared to monolayer. We found that the surface areas of Cu step bunches under multilayer and monolayer graphene are increased by ∼1.41% and ∼0.77% compared to a flat surface, respectively, indicating that the compressive strain on multilayer graphene can be more effectively released by forming the Cu step bunching with larger area and longer periodicity. We believe that our finding on the strain relaxation of graphene layers by Cu step bunching formation would provide a crucial idea to enhance the electrical performance of graphene electrodes by controlling the ripple density of graphene.

Epitaxial Growth of 6 in. Single‐Crystalline Graphene on a Cu/Ni (111) Film at 750 °C via Chemical Vapor Deposition

“…In contrast, on the graphene grown on Cu/Ni (111) film at higher temperature (say, 1000 °C), corrugations and steps can be observed, as shown in AFM image (Figure b) and SEM image (Figure e). The corrugations and steps normally form at the process of cooling from high temperature and could easily induce winkle and fold in graphene after transfer . In addition, low growth temperature can decrease the contamination of silica particles.…”

Anisotropic Strain Relaxation of Graphene by Corrugation on Copper Crystal Surfaces

“…This means two sides of a coin: Cu substrate is under tensile stress exerted by the thermal expansion of graphene, and graphene is under compressive stress exerted by the thermal contraction of Cu substrate. Accordingly, two correlative effects coexist: Cu undergoes surface roughening, and graphene bulks to form wrinkles .…”

“…Other low‐index crystal faces of Cu(100) and Cu(110), and high‐index faces undergo surface premelting with varied initiated temperature and thickness of premelting layer . During cooling in the CVD process, the thermal mismatch between Cu and graphene induces strong tensile strain on Cu surfaces . As a result, the Cu surface underwent surface roughening by solidifying the premelting layer.…”

“…The distinct surface reconstruction behaviors among Cu crystal faces may affect the remaining strain of graphene layers . We quantitatively investigated the correlation between Cu surface roughness and remaining strain of graphene by combining morphology imaging and vibrational analysis.…”

“…As indicated in Figure a, the flat Cu domain and rough Cu domain were separated by a deep Cu grain boundary groove . The as‐grown graphene was monolayer to exclude the effect of layers on the surface roughening and strain relaxation (Figure S10, Supporting Information). Figure b illustrated the height profiles on the flat and rough Cu domain.…”