Defects of graphene on Ir(111): rotational domains and ridges

TL;DR

This study investigates the structural defects and rotational variants of graphene on Ir(111) using microscopy techniques, revealing four moire structures and local delamination caused by substrate contraction.

Contribution

It identifies three previously uncharacterized rotational variants of graphene on Ir(111) and links defect formation to substrate contraction during cooling.

Findings

Four moire structures identified with specific rotational angles.

Linear defects are ridges caused by local delamination.

Substrate contraction during cooling influences defect formation.

Abstract

We use low-energy electron microscopy (LEEM), low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) to study different orientations of single-layer graphene sheets on Ir(111). The most-abundant orientation has previously been characterized in the literature. Using selective-area LEED we find three other variants, which are rotated 14, 18.5 and 30 degrees with respect to the most common variant. The about 30-degree-rotated structure is also studied by STM. We propose that all 4 variants are moire structures that can be classified using simple geometric rules involving periodic and quasi-periodic structural motifs. In addition, LEEM reveals that linear defects form in the graphene sheets during cooling from the synthesis temperature. STM shows that these defects are ridges, suggesting that the graphene sheets delaminate locally as the Ir substrate contracts.