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Friction has been recognized and studied for a long time due to its fundamental significance in the processes of human manufacturing and production. However, friction behavior at the nanoscale sometimes violates the conventional classical friction theory. Thickness-dependent friction behavior has been reported in nonpolar van der Waals materials but only within the few-layer range. Here, we report a wide thickness range of thickness-dependent friction behavior in functional van der Waals copper-deficient CuInP2S6 (Cu0.2In1.26P2S6), which exhibits spontaneous phase separation of ferroelectric and paraelectric phases. The friction gradually decreases with the increase of the thickness within the range of 20–230 nm. A higher adhesion force and phononic damping collectively contribute to higher friction in thinner nanoflakes.
Wang et al. (Wed,) studied this question.