[Paper Review] Rotational domains of graphene on Ir(111)

Elena Loginova, Shu Nie+3 more|arXiv (Cornell University)|Apr 8, 2009

Graphene research and applications3 citations

TL;DR

This study identifies three new rotational domains of graphene on Ir(111), with misorientation angles of approximately 14°, 18.5°, and 30°, using LEEM, LEED, and STM. The 30°-rotated structure is further analyzed via STM, and atomic models are proposed; linear defects forming during cooling are identified as graphene ridges due to local delamination from substrate contraction.

ABSTRACT

We use low-energy electron microscopy (LEEM), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) to study four different orientations of single graphene sheets on Ir(111). The most-abundant orientation has been previously characterized in the literature. As measured using selective-area LEED, we find the graphene sheets in the other three variants to be rotated by approximately 14, 18.5 and 30 degrees, respectively. The 30-degree-rotated structure is also studied by STM. We propose atomic models for the new variants. The moire structures can be classified using simple geometric rules involving the different periodic and quasiperiodic structural motifs. In addition, LEEM reveals that linear defects form in the graphene sheets during cooling from the synthesis temperature. STM shows that the defects are ridges where the graphene sheets locally delaminate as the Ir substrate contracts.

Motivation & Objective

• To identify and characterize previously unreported rotational orientations of single-layer graphene on Ir(111).
• To understand the structural motifs and moiré patterns formed by graphene-substrate lattice mismatch.
• To investigate the origin and nature of linear defects observed in graphene during cooling from synthesis temperature.
• To develop atomic models for the newly identified rotational variants based on geometric and diffraction analysis.

Proposed method

• Employed low-energy electron microscopy (LEEM) to image large-area graphene domains and detect linear defects.
• Applied selective-area low-energy electron diffraction (LEED) to measure rotational angles between graphene and Ir(111) substrate.
• Used scanning tunneling microscopy (STM) to resolve atomic-scale structure of the 30°-rotated graphene variant.
• Analyzed moiré patterns using geometric rules relating periodic and quasiperiodic structural motifs.
• Correlated structural evolution during cooling with defect formation, particularly ridge-like features.

Experimental results

Research questions

• RQ1 What are the rotational misorientations of graphene relative to Ir(111) in the three newly observed domains?
• RQ2 How do the moiré superstructures in these rotated domains relate to the underlying lattice periodicities?
• RQ3 What causes the formation of linear defects in graphene during cooling from synthesis temperature?
• RQ4 Can atomic models be constructed to explain the observed rotational variants and their interface structures?
• RQ5 How does thermal contraction of the Ir substrate contribute to graphene delamination and defect morphology?

Key findings

• Three new rotational domains of graphene on Ir(111) were identified, with misorientation angles of approximately 14°, 18.5°, and 30° relative to the substrate.
• The 30°-rotated graphene structure was imaged with STM, revealing a complex moiré pattern consistent with the proposed atomic model.
• LEED analysis confirmed the rotational angles with high precision, supporting the classification of the new variants.
• LEEM revealed the presence of linear defects that form during cooling, which are later confirmed by STM to be ridges from local graphene delamination.
• The defects are attributed to differential thermal contraction between Ir(111) and graphene, causing local lifting and ridge formation at the interface.
• Moiré structures were successfully classified using geometric rules based on periodic and quasiperiodic lattice matching.