Evidence for temporary and local transition of sp2 graphite-type to sp3 diamond-type bonding induced by the tip of an atomic force microscope

TL;DR

This study demonstrates that an atomic force microscope tip can induce a temporary, localized transition from sp2 graphite bonding to sp3 diamond bonding in graphene, revealing atomic-scale high-pressure effects.

Contribution

It provides experimental and theoretical evidence of a local graphite-to-diamond transition induced by AFM tips, a novel approach to high-pressure physics at the atomic scale.

Findings

Observation of third harmonics indicating transition

Force measurements consistent with bond transformation

Evidence of local sp2 to sp3 bonding change

Abstract

Artificial diamond is created by exposing graphite to pressures on the order of 10\,GPa and temperatures of about 2000\,K. Here, we provide evidence that the pressure exerted by the tip of an atomic force microscope onto graphene over the carbon buffer layer of silicon carbide can lead to a temporary transition of graphite to diamond on the atomic scale. We perform atomic force microscopy with CO terminated tips and copper oxide (CuOx) tips to image graphene and to induce the structural transition. For a local transition, DFT predicts that a repulsive barrier of $\approx13$\,nN, followed by a force reduction by $\approx4$\,nN is overcome when inducing the graphite-diamond transition. Experimental evidence for this transition is provided by the observation of third harmonics in the cantilever oscillation for relative flexible CO terminated tips and a kink in the force versus distance curve for rigid CuOx tips. The experimental observation of the third harmonic with a magnitude of about 200\,fm fits to a force with an amplitude of $\pm 3$\,nN. The large repulsive overall force of $\approx 10$\,nN is only compatible with the experiment if one assumes that the repulsive force acting on the tip when inducing the transition is compensated by an increased van-der-Waals attraction of the tip due to form fitting of tip and sample by local indentation. The transition changes flat sp$^2$ bonds to corrugated sp$^3$ bonds, resulting in a different height of the two basis atoms in the elementary cell of graphene. Both tip types show a strong asysmmetry between the two basis atoms of the lattice when using large repulsive tip forces that induce the transition. Experimental data of tunneling current, frequency shift and dissipation are consistent with the proposed transition. The experiment also shows that atomic force microscopy allows to perform high pressure physics on the atomic scale.