Unusual Thickness-Dependent Friction on CuInP_{2}S_{6} Originating from Work-Function Regulation

Thickness dependence is a unique trait for the friction of two-dimensional materials, where thinner samples are typically found to exhibit higher friction than thicker ones. In this Letter, we show that friction on two-dimensional CuInP_{2}S_{6} nanosheets transferred on silicon substrates with predominantly upward polarization can behave in a manner completely opposite to the conventional trend, showing enhanced friction with increasing thicknesses. Assisted by Kelvin Probe Force Microscopy measurements and first-principles calculations, this unusual behavior is attributed to the work-function regulated friction mechanism. Specifically, as the sample thickness increases, the work function decreases for the ferroelectric material, which results in a more pronounced electron transfer effect and thereby stronger interactions across the intimate contact interface. This work sheds light on the physical origins of friction for ferroelectric materials and suggests an effective strategy to actively regulate friction via work-function engineering.