Ultrahard carbon film from epitaxial two-layer graphene (1801.00520v1)

Abstract: Atomically thin graphene exhibits fascinating mechanical properties, although its hardness and transverse stiffness are inferior to those of diamond. To date, there hasn't been any practical demonstration of the transformation of multi-layer graphene into diamond-like ultra-hard structures. Here we show that at room temperature and after nano-indentation, two-layer graphene on SiC(0001) exhibits a transverse stiffness and hardness comparable to diamond, resisting to perforation with a diamond indenter, and showing a reversible drop in electrical conductivity upon indentation. Density functional theory calculations suggest that upon compression, the two-layer graphene film transforms into a diamond-like film, producing both elastic deformations and sp2-to-sp3 chemical changes. Experiments and calculations show that this reversible phase change is not observed for a single buffer layer on SiC or graphene films thicker than 3 to 5 layers. Indeed, calculations show that whereas in two-layer graphene layer-stacking configuration controls the conformation of the diamond-like film, in a multilayer film it hinders the phase transformation.

Insights into the Ultra-Hard Carbon Film Derived from Epitaxial Two-Layer Graphene

This paper explores the mechanical properties of atomically thin two-layer epitaxial graphene films on SiC(0001) and presents a compelling experimental and theoretical case that these films exhibit mechanical properties comparable to those of diamond under specific conditions. In particular, it reports that two-layer graphene can transform into a diamond-like structure when subjected to nano-indentation, a transformation that is reversible and characterized by changes in the film's transverse mechanical modulus and conductivity.

The paper leverages a range of sophisticated techniques to investigate the properties of graphene films. Sub-angstrom resolution modulated nano-indentation (MoNI), atomic force microscopy (AFM), micro-hardness measurements, and density functional theory (DFT) calculations were employed to measure mechanical properties and evaluate the phase transition of the graphene films. Results indicate that two-layer epitaxial graphene exhibits a transverse stiffness exceeding 400 GPa post-indentation, corroborating the diamond-like transformation under pressure. This behavior contrasts with single layer or multilayer (3-5 layers) graphene, where no transformation is observed, likely due to impediments in layer stacking configurations for thicker films.

The DFT calculations elaborate on the intrinsic nature of this transformation, identifying that the structural change involves an sp-to-sp hybridization, turning the two-layer graphene into a diamond-like form that is structurally and chemically distinct. The paper further elucidates that the mechanical transformation is not influenced by extrinsic factors like adsorbates for stabilization, thus reinforcing the spontaneous nature of the conversion.

These findings present two-layer epitaxial graphene as a potential candidate for pressure-activated ultra-hard coatings, with applications in areas requiring adaptive and highly resilient materials. They also introduce a new dimensional avenue in carbon phase transition research, particularly regarding room temperature graphite-to-diamond transitions.

Practically, this research suggests pathways to utilizing pressure-induced phase transitions to synthesize and pattern diamond-like carbon films at the nanoscale. Future developments could explore combining these phase transitions with variables such as temperature, chemical functionalization, or local pressure manipulation to stabilize the diamond-like state. This work could significantly influence the design of advanced materials in nanoelectronics and spintronics, further enhancing our ability to tailor material properties at the atomic scale.

Overall, the paper significantly advances the understanding of graphene's mechanical properties and their tunability, offering empirical depth and theoretical breadth on transforming graphene films into ultra-hard diamond-like phases. The implications reach beyond fundamental materials science, opening doors for practical applications where mechanical robustness and adaptability are crucial.