Superlubricity of Borophene: Tribological Properties in Comparison to hBN

TL;DR

The study experimentally demonstrates superlubricity in borophene and directly contrasts its tribological performance with hBN using lateral borophene–hBN heterostructures on Ir(111). Through STM, nc-AFM, KPFM, friction measurements, and complementary Prandtl–Tomlinson modeling and ab initio PES calculations, it shows a much lower interfacial friction for borophene (COF around 1.2×10^-3) than for hBN, which is attributed to weaker tip–surface interactions and differences in moiré corrugation and energy dissipation. The PES and modeling reveal smaller sliding barriers for borophene (≈0.03 eV/atom) versus hBN (≈0.12 eV/atom), with borophene displaying a flatter, more rigid surface under tip interaction. Overall, the work validates borophene as a promising low-friction 2D lubricant and highlights lateral heterostructures as a powerful platform for directly comparing tribological properties across 2D materials.

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 moiré 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.

Figures (4)

• Figure 1: Large-scale borophene-hBN interface. a) STM topography. b) nc-AFM topography and corresponding c) dissipation in meV per oscillation cycle. d) CPD and corresponding profile. e) High resolution STM topography. f) High resolution nc-AFM topography and corresponding g) dissipation (meV per oscillation cycle) and h) torsional frequency shift. Parameters: a) $I = 200$ pA, $U = -0.3$ V. b,c) $f_0 = 164$ kHz, $A = 4$ nm $\Delta f = -17$ Hz. e) $I = 90$ pA, $U = 1$ V. f,g,h) $f_0 = 169$ kHz, $A = 2$ nm $\Delta f = -130$ Hz, $f_t = 1.53$ MHz, $A_t = 80$ pm. Scale bars a,b,c) $50$ nm. d) $500$ nm. e,f,g,h) $3$ nm.
• Figure 2: Borophene friction properties. a) Lateral force map of the borophene hBN layers on Ir(111) at an applied load of 4 nN. b) Mean friction force of the borophene layer. c) Lateral force traces (left) and corresponding images (right) for applied loads of 4, 72 and 104 nN respectively. $\nu$ = $49$ nm/s. Scale bar a) 40 nm, image are $10$ nm wide in c).
• Figure 3: Friction properties of the (borophene-hBN)/Ir(111) lateral heterostructure. a) Friction force versus load for borophene and hBN. Dots are from experiments, lines from calculations. b) Scheme of the PT model used for the calculations. c) Lateral force image of the lateral borophene-hBN interface with the corresponding scheme and lateral force traces. Parameters: c). Scale bar $10$ nm
• Figure 4: Schematic representation of the a) Ir-borophene-tip and b) Ir-hBN-tip geometric models used in the ab initio calculations. The golden, green, white and blue sphere represent the position of the Ir, B, N and Si atoms, respectively. c) Potential energy surface (PES) thermal map of borophene on Ir(111) (left) and hBN on Ir(111) (right).