How Chassis Structure and Substrate Crystalline Direction Affect the Mobility of Thermally Driven p-Carborane-Wheeled Nanocars

SMH Lavasani and HN Pishkenari and A Meghdari, JOURNAL OF PHYSICAL CHEMISTRY C, 123, 4805-4824 (2019).

DOI: 10.1021/acs.jpcc.8b10779

In recent years, various nanocars have been synthesized in order to provide controlled mechanical function, transport other nanoparticles, or enable bottom-up assembly capability. There have even been racing competitions among well-known nanocars in which the wheels play an influential role. In this paper, the motion of thermally driven nanocars equipped with p-carborane wheels on Au(111) and Au(001) substrates is investigated. For the sake of comparison, classical all-atom molecular dynamics (MD) and rigid-body MD (RBMD) have been used to study the motion threshold as well as to analyze the effect of temperature, substrate crystalline direction, and chassis shape on the diffusive motion of a nanocooper, trimer, nanocaterpillar, and angled nanocar. It was observed that the motion regime of the nanocars on a gold substrate is a function of temperature and translational diffusion as well as the rotational diffusion coefficient, which shows non-Arrhenius behavior. Nanocar motion has three main regimes, trapped in the crystal structure, short-range fluctuations, and continuous motion for different temperatures from 50 to 600 K. Nanocar structure and crystalline direction may have a significant influence on the translational or rotational diffusion as well as on the surface temperatures in which the motion regime switch occurs. Fluctuations of the nanocars at temperatures below 450 K do not lead to considerable displacement; in this regard, there is a suitable consistency with experimental observations. The simulation results indicate that RBMD overestimates the diffusion coefficient by at least 10 times more than classic MD and predict less adsorption energy on gold, both of which have been reported in previous studies. Rotational motion of nanocars around an axis perpendicular to the gold surface initiates at higher temperatures relative to their pure translational motion; as a result, carborane- wheeled nanocars have less tendency to rotate and rather perform translational motion. In addition, p-carborane wheels have the tendency to slide toward an adjacent adsorption site rather than to roll, and their rotations occur completely independent from each other. These findings can be used to predict the behavior of other variants of carborane-based nanocars, such as motorized or nonmotorized, and to assist designers to increase their nanocar structure-based controllability on the substrate.

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