nano-onions. (a) A group of MoS2 nano-onions before friction test. (b) The selected nano-onion (marked with arrow in the previous image), 30 nm in diameter, shifted for 35 nm toward the left during the second scanning. (c) The same group of nano-onions after the friction test with shifted position of the selected nano-onion for 50 nm toward the left and rotation for 10° around the axis perpendicular to the substrate. (d) Friction as a function of load with two slopes revealing COF of 0.077 ± 0.004 and 0.12 ± 0.003, respectively.