Quantum Influences in the Diffusive Motion of Pyrrole on Cu(111)

Abstract

The adsorption of aromatic molecules on surfaces is of broad interest in the fields of self-assembled networks and selforganized growth. [1,2] During self-assembly, adsorbates diffuse and adjust their orientation to form regular superstructures. The result is a delicate balance between supramolecular chemistry (dipole-dipole interactions, hydrogen bonding, and van der Waals forces) and molecule-surface interactions. These factors determine the structure, energy, site-specificity, and two-dimensional molecular translational and rotational diffusion. [2][3][4][5] Differently substituted polyaromatic rings form key building blocks, [6] defining the structure of chemically functionalized surfaces. It is crucial to understand the motion of single molecular precursors to gain insight into the complex processes underlying self-assembly. Aromatic adsorbates are extended particles of large mass that are assumed to obey classical mechanics for temperatures where self-assembly occurs. We will see, however, that quantum effects can nevertheless play a crucial role in the dynamics of surface diffusion even when the quantized degrees of freedom are not associated directly with translational motion.Here, we study pyrrole, C 4 H 4 NH, an important precursor for many chemically and technologically relevant materials. In recent years, functionalized pyrroles and related molecules have attracted increasing attention as self-assembling adsorbates. [7] Pyrrole has been studied on several metal surfaces, including Pt(111), [8] Pd (111), [9] Rh(111), [10] and Cu(100). [11] Whilst little is known about potential energy landscapes and interactions between molecules, pyrrole is usually found to adsorb in a flat-lying geometry at low coverages, often moving to a tilted geometry at higher coverages.Herein we describe a helium spin-echo (HeSE) and density functional theory (DFT) study of the dynamics of this elementary building block on Cu(111) to investigate interadsorbate interactions, the effect of friction, and the role of quantum modes in a sub-monolayer regime. We will show that the behavior is dominated by the quantum contribution to the total energy of pyrroles vibrational ground state. Surprisingly, the important vibrations are not those associated with motion of the center of mass but are internal modes.The HeSE method is new and uniquely sensitive to atomic scale motion on pico-to nanosecond timescales. [12] Figure 1 illustrates the principle, showing how surface motion occurring over a time difference, t SE , reduces coherent scattering and gives a correlation function that typically decays with time as f(t) = a exp(Àat SE ) + c. [12] The characteristic timescale of the motion is then given by the decay rate, a, which depends on the angle of scattering. Motion, in real space, is deduced by comparing the dependence on scattering angle, expressed as a momentum transfer, DK, with Langevin molecular dynamics (MD) simulations. [13] We have performed experiments at (121-170) K along two crystal directions. The resulting a(DK) depend...

Mass Transport in Surface Diffusion of van der Waals Bonded Systems: Boosted by Rotations?

“…At low momentum transfers, <0.5 Å –1 , the experimental data lies above the analytic model and shows a local maximum, which is particularly evident in the ⟨11̅0⟩ direction. Such features are a signature of repulsive interactions between neighboring benzene molecules and have been seen with other aromatic adsorbates …”

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“…Helium atom scattering provides known sensitivity to organic adsorbates, ,, and here we determine the benzene–substrate energy landscape from helium spin–echo (HeSE) measurements of the thermal motion of adsorbed benzene. The principle of the technique is illustrated in Figure a, while a detailed description has been given elsewhere. , In brief, helium atoms scatter from mobile adsorbates on the surface, providing a measurement of the correlation in their positions with time.…”

Nanoscopic diffusion of water on a topological insulator

“…Since the DFT calculations are static (essentially corresponding to an ideal state at a temperature of 0 K), more evolved theoretical studies such as ab-initio molecular dynamics simulations could address phonons and molecular vibration/rotation effects at finite temperatures but they are computationally extremely demanding for the current system. Furthermore, in most weakly-bound supramolecular systems, zero-point energy effects do not affect substantially the magnitude of the barrier height and are usually ignored since the weak frustrated translation and out-of-plane bending modes are extremely difficult to accurately calculate 61,62 . Hence while the DFT calculations provide a good measure for the energetics of adsorption sites and configurations, for further dynamic properties we rely on other computationally less expensive theoretical approaches as explained below.…”

Combined Diffraction and Density Functional Theory Calculations of Halogen-Bonded Cocrystal Monolayers

“…The quality of the TS corrections for surface calculations and supramolecular self-assembly calculations is generally robust. 13,27,28 Dispersion force corrections, rather than nonlocal functionals, were used because in the plane-wave DFT code employed in this work (CASTEP), cell structure optimizations cannot be performed with fully nonlocal functionals. This is because there is no known analytical expression for the stress tensor components in the functional formalism.…”