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Frictional traces of (a) unlubricated Cu-on-Cu contact, mechanically exfoliated NbSe2 micro/particles. (b) NbSe2 micro/nanoparticles by ultrasonic-assisted exfoliation without and with ageing. ME: mechanically exfoliated; UE: ultrasonic-assisted exfoliation without ageing; UEA: ultrasonic-assisted exfoliation with ageing.
