Effect of Rotational Degrees of Freedom on Molecular Mobility

Abstract

A molecule is a basic building block in bottom-up approach to develop nanoscale machinery and electromechanical systems. Because of finite size and rotational degrees of freedom (DOFs), its diffusion behavior is generally more complex than its atomic counterpart. Understanding the effect of rotational DOFs on molecular diffusion is not only important for controlling molecular motion but also essential for many device applications. Here, we use molecular dynamics simulations to study the effect of rotational DOFs on the surface diffusion of C60 on graphene substrate. We show that there is an intermediate temperature range in which the rotational DOFs play a pronounced role in enhancing the molecular mobility in the system, which is in contrast to both the low- and high-temperature regimes in which the rotational DOFs show little influence. We further analyze the underlying mechanistic origin for the enhancement and find that in this regime the rotational DOFs provide alternative routes for the admolecule to overcome the energy barriers of the system, leading to a quasi-continuous Brownian motion on the surface. Our results provide new insights into molecular surface diffusion.

Effect of temperature on kinetic nanofriction of a Brownian adparticle

“…At low temperatures (quasi-continues Brownian regime), the C 60 admolecule has to overcome the shallow potential energy corrugation of the system. In contrast, at high temperatures (ballisticlike Brownian regime), the admolecule has to overcome the physical roughness of the surface due to thermal corrugation and outof-plane motion of the graphene atoms, imposing higher energy barriers against its motion [37,38]. Hence, the energy barrier of the system PES and the activation energies of the Arrhenius fit, E a , are not necessarily equal.…”

Molecular mobility on graphene nanoroads

“…To compare the surface diffusion of C 60 on GNRDs with that of C 60 on infinite graphene, a series of simulations were performed for the C 60 /graphene system in the same way as the previous works 7 8 9 . At the beginning of each simulation, we performed energy minimization using Polak-Ribiere conjugate gradient (CG) method as implemented in LAMMPS package.…”

“…Graphene is electrically and thermally conductive, chemically inert, and mechanically robust. Besides, the kinetic friction of physisorbed molecules on graphene is significantly low, which is desirable for efficient and high speed molecular transportation 7 8 9 . Hence, graphene-based materials are promising for constructing mass conveyer systems by applying a temperature gradient or external electric field 10 11 12 13 .…”

Molecular mobility on graphene nanoribbons

“…where, Dr J is the component of admolecule COM displacement parallel to GNR edge, t is the time, h.i denotes ensemble or time averaging [36][37][38] as employed in the studies. [18][19][20][21] admolecule. Indeed, our further analysis shows that the diffusive behavior of the admolecule on a GNR mainly depends on the nanoribbon width rather than the edge type (such as terminated zigzag or armchair edges, or chemically functionalized edges).…”

“…Recently, several issues related to the surface diffusion of physisorbed admolecules on the infinite graphene-based substrates (pristine and functionalized) have been addressed. [17][18][19][20][21][22] It is known that due to thermal fluctuations, the adsorbates on graphene may undergo spontaneous Brownian motion (BM) even below room temperature, which is different from the traditional picture of surface diffusion based on the hopping mechanism. 19,23 Nevertheless, to use finite size GNRs as mass conveyor highways, certain fundamental issues must be taken into account.…”