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

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. A local transition induced by the force of the tip is accompanied by local rehybridization from an sp$^2$-bonded to an sp$^3$-bonded local structure. Density functional theory (DFT) predicts that a repulsive threshold of $\approx13$\,nN, followed by a force reduction by $\approx4$\,nN is overcome when inducing the graphite-diamond transition. The experimental observation of the third harmonic with a magnitude of about 200\,fm fits well to overcoming a force barrier of $F_{barrier}\approx 5$\,nN, followed by a force reduction by $-F_{barrier}$ and an upswing by $F_{barrier}$ for decreasing distances. Experimental evidence for this transition is provided by the emergence of third harmonics in the cantilever oscillation when the laterally flexible CO terminated tip exerts a large repulsive force. Probing the sample with rigid CuOx tips in the strong repulisive regime shows a strong difference in the yielding of the A versus B sites to the pressure of the tip. The large repulsive overall force of $\approx 10$\,nN is only compatible with the experimental data if one assumes that the repulsive force acting on the tip when inducing the transition is compensated by a heavily increased van-der-Waals attraction of the tip due to form fitting of tip and sample by local indentation. The experiment also shows that atomic force microscopy allows to perform high pressure physics on the atomic scale.