Natural two-dimensional pyrophyllite: Nanoscale lubricant, electrical insulator and easily-machinable material

Abstract

Pyrophyllite, with the chemical formula Al2Si4O10(OH)2, is a naturally occurring and abundant van der Waals mineral belonging to the group of phyllosilicates. It is very soft, layered crystal used for sculpting and an excellent electrical and thermal insulator aimed for the operation at high pressure and temperature. Here, for the first time, two-dimensional (2D) pyrophyllite obtained by both mechanical and liquid phase exfoliation is presented and investigated at the nanoscale. The layered structure provides low friction coefficient of around 0.1 as measured by friction force microscopy. The wear properties, studied by atomic force microscope (AFM) based scratching, are distinctly different from graphene. Since the wear is initiated at low normal forces, 2D pyrophyllite can be routinely carved by the AFM tip and it is suitable for scratching based nanolithography. According to our optical measurements, 2D pyrophillite is an insulator with a band gap of ∼5.2eV. Local current measurements by conductive AFM reveal that 2D pyrophyllite flakes behave as efficient electrical insulators with a breakdown voltage of around 6MV/cm. Therefore, the obtained results indicate possible applications of 2D pyrophyllite as a low-cost electric insulator and lubricant, as well as an easily-machinable material at the nanoscale.