Influence of Vacancies and Grain Boundaries on the Diffusive Motion of Surface Rolling Molecules

Molecular machines
and surface rolling molecules show great potential
to accomplish different tasks in several fields, such as bottom-up
assembly and nanomanipulation. Many researchers have investigated
molecular machines, most of which was on a flat single-crystal substrate.
In this paper, we studied the influence of vacancies in different
sizes on the motion of a nanocar, a nanotruck, and C60 on
a gold substrate at different temperatures by employing classical
all-atom molecular dynamics. At the temperature of 200 K, a hole or
vacancy appears as a repellent obstacle in the path of C60, and at higher temperatures, C60 can enter this hole.
Although C60 has enough energy to escape single-atom vacancies
at 400 K or higher temperatures, larger holes become permanent traps
for C60. We also studied the nanocar motion at the temperature
range of 400–600 K. A nanocar has a flexible chassis, which
provides short-range freedom for fullerene wheels. The impact of a
hole on nanocar motion is almost similar to C60. The nanocar
is capable of releasing itself from a 1-atom hole; however, it cannot
escape larger holes. Becoming trapped inside holes at temperatures
of 500 and 600 K disrupts the diffusive motion of the nanocar. A nanotruck
has a relatively rigid chassis, which limits the motion of the wheels.
As a result, a 9-atom or larger hole appears as an impenetrable obstacle
blocking the nanotruck path. We can conclude that irregularities and
vacancies in grain boundaries can drastically affect the surface rolling
molecules. In certain conditions, C60 can pass over the
grain boundary. However, holes in the grain interface will most likely
prevent the free movement of nanocars by either repelling or trapping
them. Therefore, a single-crystal substrate is essential for the uninterrupted
motion of nanocars, C60, and other similar surface rolling
molecules. The repellent effect of holes can be utilized to direct
the diffusive motion of nanocars and guide them in a desired path
in certain conditions. Since nanocars and C60 get trapped
inside holes in some situations, we can employ this effect to build
a two-dimensional (2D) pattern of C60 molecules and nanocars
or fix them in place for more precise experimental investigations.