Delivering quantum dots to lubricants: Current status and prospect

Very recently, two-dimensional quantum dots (2D QDs) have been pioneeringly investigated as lubricant additives, which exhibit superior friction-reducing and wear resistance. Compared with 2D nanoparticles, 2D QDs possess small size (~10 nm) and abundant active groups. These distinguished advantages enable them to quickly disperse into common lube mediums and maintain long-term storage stability. The good dispersion stability of 2D QDs not only effectively improves their embedding capacity, but also enables continuous supplements of lubricants during the sliding process. Therefore, 2D QDs are attracting increasing research interest as efficient lubricants with desirable service life. In this review, we focus on the latest studies of 2D QDs as liquid lubricant additives (both in polar and nonpolar mediums), self-lubricating solid coatings and gels, etc. Various advanced strategies for synthesis and modification of 2D QDs are summarized. A comprehensive insight into the tribological behavior of a variety of 2D QDs together with the associated mechanism is reviewed in detail. The superior lubricating performances of 2D QDs are attributed to various mechanisms, including rolling effect, self-mending performance, polishing effect, tribofilm formation, nanostructure transfer and synergistic effects, etc. Strategies for friction modulation of 2D QDs, including internal factors (surface modification, elemental doping) and extrinsic factors (counter surfaces, test conditions) are discussed, special attentions for achieving intelligent tribology toward superlubricity and bio-engineering, are also included. Finally, the future challenges and research directions regarding QDs as lubricants conforming to the concept of "green tribology" toward a sustainable society are discussed.