Temperature-Controlled Rotational Epitaxy of Graphene.

Area of Science:

• Materials Science
• Condensed Matter Physics
• Surface Science

Background:

• Moiré superlattices arise from the superposition of periodic structures when graphene is placed on crystalline surfaces.
• The moiré lattice parameter, crucial for electronic, vibrational, and chemical properties, is highly sensitive to small lattice rotations.
• Deterministic transfer allows precise control of graphene's relative orientation, but spontaneous reorientation offers an alternative pathway.

Purpose of the Study:

• To investigate the spontaneous reorientation of graphene on a metallic substrate, specifically Ir(111).
• To identify substrate temperature ranges that induce distinct relative rotational angles.
• To understand the underlying mechanisms, such as strain minimization and thermal expansion mismatch, governing this reorientation.

Main Methods:

• Growth of graphene on Ir(111) at controlled substrate temperatures.
• Analysis of moiré superlattices formed by graphene on the substrate.
• Modeling moiré superlattices as two-dimensional coincidence networks to analyze rotational angles and strain.

Main Results:

• Graphene exhibits spontaneous reorientation on Ir(111) within a specific temperature range (1530-1000 K).
• Distinct relative rotational angles (0°, ± 0.6°, ±1.1°, ±1.7°) were observed.
• These rotations are attributed to favorable low-strain graphene structures and the accommodation of thermal expansion mismatch.

Conclusions:

• Spontaneous reorientation of graphene on Ir(111) is a viable mechanism for controlling moiré superlattice formation.
• Substrate temperature is a critical factor in determining the resulting rotational angles and strain states.
• Understanding these self-organization principles is key to tailoring graphene's properties for various applications.