Abstract
Fabrication of high-performance heterostructure devices requires fundamental understanding of the diffusion dynamics of metal species on 2D materials. Here, we investigate the room-temperature diffusion of Ag, Au, Cu, Pd, Pt, and Ru adatoms on graphene using ab initio and classical molecular dynamics simulations. We find that Ag, Au, Cu, and Pd follow Lévy walks, in which adatoms move continuously within ∼1–4 nm2 domains during ∼0.04 ns timeframes, and they occasionally perform ∼2–4 nm flights across multiple surface adsorption sites. This anomalous diffusion pattern is associated with a flat (<50 meV) potential energy landscape (PEL), which renders surface vibrations important for adatom migration. The latter is not the case for Pt and Ru, which encounter a significantly rougher PEL (>100 meV) and, hence, migrate via conventional random walks. Thus, adatom anomalous diffusion is a potentially important aspect for modeling growth of metal films and nanostructures on 2D materials.
Highlights
Fabrication of high-performance heterostructure devices requires fundamental understanding of the diffusion dynamics of metal species on 2D materials
The temperature-dependent jump rate ν is typically approximated by the Arrhenius equation, in which the static (i.e., 0 K) surface migration barrier is determined by ab initio or classical computational methods,[6−16] while the jump attempt frequency is estimated viaharmonic transition-state theory approaches.[17,18]
At temperatures significantly higher than those used for thinfilm synthesis experiments, the above-described notion of random walk can be inadequate[20−25] because ⟨x2(t)⟩ may not vary as ∼t
Summary
We use AIMD simulations to study atomistic diffusion mechanisms with density functional theory (DFT) accuracy. AIMD runs are complemented by CMD simulations of Ag adatom diffusion on a graphene sheet of 680 atoms. These simulations are performed using LAMMPS.[64] The C−C and Ag−C interactions are described using AIREBO65 and Lennard-Jones[66] potentials, respectively. The AIREBO potential is well-established in the literature for simulating carbonbased materials[67−70] and, together with Lennard-Jones, is used to study the diffusion of noble metals (including Ag) clusters on graphite surfaces.[71] More information about CMD validation can be found in the Supporting Information. Comparison of 0.02 ns long AIMD trajectories (S1); calculation of scaling exponent α (S2); high-accuracy calculation of static (0 K) adsorption energies (S3); entropy effects on diffusion dynamics (S4); potential energy landscape using classical interatomic potentials and CMD simulation analysis (S5) (PDF).
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