Superlubricity of Borophene: Tribological Properties in Comparison to hBN

TL;DR

This study experimentally confirms borophene's superlubricity and compares its tribological properties with hBN, highlighting borophene's ultra-low friction and potential for low-friction applications in 2D materials.

Contribution

It provides the first experimental evidence of borophene's superlubricity and compares its tribological performance directly with hBN using heterostructures.

Findings

Borophene exhibits superlubricity confirmed by experiments.

Borophene has lower friction than hBN due to weaker tip/surface interactions.

Structural and electrical properties of borophene and hBN are characterized and compared.

Abstract

The tribological performance of 2D materials makes them good candidates toward a reduction of friction at the macroscale. Superlubricity has been observed for graphene, MoS\textsubscript{2} and MXenes and hexagonal boron nitride (hBN) is used to reduce or tune friction, but other materials are investigated as potential candidates for low-lubricity applications. Specifically, borophene is predicted to have ultra-low friction. Here, we experimentally investigate frictional properties of borophene and use a borophene-hBN lateral heterostructure to directly compare the tribological properties of the two complementary 2D materials. In particular, we investigate the friction between a sliding tip and (i) the weakly corrugated $\mathcal{X}_6$-borophene layer on Ir(111) or (ii) the hBN/Ir(111) superlattice structures with a strongly corrugated moir\'e reconstruction. Our experimental study performed in ultra-high vacuum at room temperature combined with a Prandtl-Tomlinson (PT) model calculation confirms the superlubricity predicted for borophene, while hBN, which exhibits a higher friction, is nevertheless confirmed as a low friction material. Ab initio calculations show that the lower friction of $\mathcal{X}_6$-borophene with respect to hBN can be rationalized by weaker tip/surface interactions. In addition, we assess structural and electrical properties of borophene and hBN by using scanning probe techniques and compare their dissipation under the oscillating tip to investigate the possible path of energy dissipation occurring during friction. Our study demonstrates the low frictional properties of borophene and the potential of lateral heterostructure investigations to directly compare the properties of these 2D materials.