Control and Characterization of Individual Grains and Grain Boundaries in Graphene Grown by Chemical Vapor Deposition

Abstract: The strong interest in graphene has motivated the scalable production of high quality graphene and graphene devices. Since large-scale graphene films synthesized to date are typically polycrystalline, it is important to characterize and control grain boundaries, generally believed to degrade graphene quality. Here we study single-crystal graphene grains synthesized by ambient CVD on polycrystalline Cu, and show how individual boundaries between coalescing grains affect graphene's electronic properties. The graphene grains show no definite epitaxial relationship with the Cu substrate, and can cross Cu grain boundaries. The edges of these grains are found to be predominantly parallel to zigzag directions. We show that grain boundaries give a significant Raman "D" peak, impede electrical transport, and induce prominent weak localization indicative of intervalley scattering in graphene. Finally, we demonstrate an approach using pre-patterned growth seeds to control graphene nucleation, opening a route towards scalable fabrication of single-crystal graphene devices without grain boundaries.

• The paper demonstrates controlled growth of hexagonally shaped, single-crystal graphene grains using ambient CVD, challenging prior epitaxial assumptions.
• It reveals that graphene can seamlessly grow across Cu grain boundaries, contradicting the need for single-crystal substrates in high-quality synthesis.
• The study introduces a pre-patterning method for precise nucleation control, offering a scalable route to defect-free graphene films for advanced electronics.

Control and Characterization of Individual Grains and Grain Boundaries in Graphene Grown by Chemical Vapor Deposition

The paper investigates the control and characterization of individual grains and grain boundaries in graphene grown via chemical vapor deposition (CVD) on polycrystalline copper (Cu) substrates. Graphene, a two-dimensional allotrope of carbon, exhibits remarkable electronic, thermal, and mechanical properties, which has driven extensive research and development efforts aimed at scalable production methods for high-quality graphene suitable for device applications.

Key Findings

The research highlights the growth of hexagonally shaped, single-crystal graphene grains using ambient CVD on polycrystalline Cu. The findings suggest that these grains predominantly orient along zigzag directions, and this orientation is not definitively correlated with the Cu substrate's crystallography, underscoring the weak epitaxial interaction between graphene and Cu. Such insights contradict prior assumptions of epitaxial relationships being crucial for orientational control and are indicative of the weak van der Waals forces at play.

Significant findings of this study include:

• Growth Across Cu Grain Boundaries: Graphene grains could grow continuously across Cu grain boundaries, maintaining their hexagonal shape. This observation challenges the traditional view of substrate crystallinity as a limiting factor in single-crystal growth.
• Grain Boundary Impact on Electronic Properties: Grain boundaries were identified as regions of higher resistivity and enhanced intervalley scattering, evidenced by a significant Raman "D" peak and prominent weak localization effects.
• Nucleation Control: The study introduces a method to control graphene nucleation using pre-patterned growth seeds, suggesting a route towards large-scale manufacturing of defect-free, single-crystal graphene.

Through a combination of transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and Raman spectroscopy, the researchers verified the single-crystalline nature of the grains and provided a comprehensive spatial and structural analysis of the grain boundaries and orientations.

Practical and Theoretical Implications

This paper advances the understanding of how to control and manipulate graphene growth at the grain level on polycrystalline substrates. The demonstration that single-crystalline graphene can grow across Cu grain boundaries questions the long-held belief in the necessity of single-crystal substrates for high-quality graphene synthesis. The technique of pre-patterning growth seeds represents a substantive stride towards scalable production of uniform graphene films, free of electronic degrading grain boundaries. Such advancements hold significant implications for the development of graphene-based electronic devices and could accelerate the integration of graphene into semiconductor technology.

From a theoretical perspective, this work prompts a reevaluation of substrate-grain interaction dynamics and suggests that the growth of single-crystalline graphene is more influenced by kinetic factors than crystallographic alignment. This could lead to further exploration of growth conditions that exploit these factors to enhance grain size and quality.

Future Prospects

Future research will likely explore optimizing the pre-patterning process, enhancing seed crystal quality, and exploring alternative substrates to refine the growth process further. The continuation of such studies could address remaining challenges associated with random nucleation and defects introduced during the synthesis and transfer of graphene, thereby advancing the material's application in high-performance, consistent graphene-based electronics. Moreover, extending these methodologies to other two-dimensional materials could broaden the impact of these findings, offering valuable insights into the scalable synthesis of a wide array of nanomaterials.