Ultrahard carbon film from epitaxial two-layer graphene

TL;DR

This study demonstrates that two-layer graphene on SiC can be transformed into a diamond-like ultra-hard structure at room temperature, exhibiting properties comparable to diamond, through a reversible phase change induced by compression.

Contribution

It provides the first practical evidence of transforming two-layer graphene into a diamond-like ultra-hard material via a reversible phase change.

Findings

Two-layer graphene exhibits diamond-like hardness after nano-indentation.

The phase change involves sp2-to-sp3 chemical transformation.

Thicker graphene layers do not undergo the same phase transformation.

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.