Effect of Rotational Degrees of Freedom on Molecular Mobility

M Jafary-Zadeh and CD Reddy and YW Zhang, JOURNAL OF PHYSICAL CHEMISTRY C, 117, 6800-6806 (2013).

DOI: 10.1021/jp312438u

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 C-60 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.

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