Abstract
Structural rearrangements govern the various properties of disordered systems and visualization of these dynamical processes can provide critical information on structural deformation and phase transformation of the systems. However, direct imaging of individual atoms or molecules in a disordered state is quite challenging. Here, we prepare a model molecular system of C70 molecules on graphene and directly visualize the structural and dynamical evolution using aberration-corrected transmission electron microscopy. E-beam irradiation stimulates dynamics of fullerene molecules, which results in the first-order like structural transformation from the molecular crystal to molecular liquid. The real-time tracking of individual molecules using an automatic molecular identification process elucidates the relaxation behavior of a stretched exponential functional form. Moreover, the directly observed heterogeneous dynamics bear similarity to the dynamical heterogeneity in supercooled liquids near the glass transition. Fullerenes on graphene can serve as a new model system, which allows investigation of molecular dynamics in disordered phases.
Highlights
The main experimental limitation in directly imaging atoms and molecules in disordered states can be overcome by modifying the sample geometry, to an atomically thin two-dimensional (2D) glassy system
Our study demonstrates that fullerenes on graphene can serve as a new model system for investigation of super-cooled liquid and glass at molecular resolution
The atomically flat surface of graphene facilitates the dynamical behavior of the molecular liquid under imaging conditions, allowing systematic study of its molecular structure and dynamics
Summary
The main experimental limitation in directly imaging atoms and molecules in disordered states can be overcome by modifying the sample geometry, to an atomically thin two-dimensional (2D) glassy system. Direct imaging of atomic structure has been successfully performed in these atomic disordered systems, in-depth observation and analysis of both structure and dynamics at the atomic or molecular resolution are still mainly lacking. We prepare a C70 molecular system on graphene, and directly visualize both the structural and dynamical evolution of the system at molecular resolution. Our computerized method precisely identifies molecular positions in the disordered state, and the pair correlation functions of molecules clearly show the short-range liquid-like ordering. Time-dependent relaxation behaviors of the molecular structure are studied in-depth by van Hove correlation functions, which clearly shows the relationship between the local structure ordering and the dynamical behavior of the system. Our study demonstrates that fullerenes on graphene can serve as a new model system for investigation of super-cooled liquid and glass at molecular resolution
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