Bulk versus nanoscale WS2: Finite size effects and solid state lubrication

Abstract

Metal dichalcogenide nanostructures have recently attracted attention due to their unique closed-cage structures, hierarchy of length scales, and outstanding solid state lubrication behavior. To understand the finite size effects and tribological properties in these nanoscale materials, we measured the low temperature specific heat of layered and nanoparticle WS2. Below 9 K, the specific heat of the nanoparticles deviates from that of the bulk. Further, the thermal response of the nested nanoparticles deviates from the usual T3 dependence below 4 K because of both finite size effects and inter-particle-motion entropy. This separation of nanoscale effects from T3 dependence can be modeled by assuming that the phonon density of states is flexible, changing with size and shape of the nanoparticle. We also invoke relationships between the low temperature T3 phonon term, Young’s modulus, and friction coefficient to assess the difference in the tribological properties. Based on this analysis, we conclude that the improved lubrication properties of the nanoparticles are extrinsic in origin.