In situ observation of stress relaxation in epitaxial graphene

TL;DR

This study uses in situ microscopy techniques to observe stress-induced wrinkle formation in epitaxial graphene on metal substrates, revealing hysteresis and stress relaxation mechanisms during temperature cycling.

Contribution

It provides the first in situ visualization and analysis of wrinkle formation and stress relaxation in epitaxial graphene on Pt(111) and Ir(111).

Findings

Wrinkles are stripes of partially delaminated graphene.

Hysteresis observed in wrinkle appearance/disappearance.

Stress relaxation linked to thermal expansion mismatch.

Abstract

Upon cooling, branched line defects develop in epitaxial graphene grown at high temperature on Pt(111) and Ir(111). Using atomically resolved scanning tunneling microscopy we demonstrate that these defects are wrinkles in the graphene layer, i.e. stripes of partially delaminated graphene. With low energy electron microscopy (LEEM) we investigate the wrinkling phenomenon in situ. Upon temperature cycling we observe hysteresis in the appearance and disappearance of the wrinkles. Simultaneously with wrinkle formation a change in bright field imaging intensity of adjacent areas and a shift in the moire spot positions for micro diffraction of such areas takes place. The stress relieved by wrinkle formation results from the mismatch in thermal expansion coefficients of graphene and the substrate. A simple one-dimensional model taking into account the energies related to strain, delamination and bending of graphene is in qualitative agreement with our observations.