Abstract
Direct observation of atomic migration both on and below surfaces is a long-standing but important challenge in materials science as diffusion is one of the most elementary processes essential to many vital material behaviors. Probing the kinetic pathways, including metastable or even transition states involved down to atomic scale, holds the key to the underlying physical mechanisms. Here, we applied aberration-corrected transmission electron microscopy (TEM) to demonstrate direct atomic-scale imaging and quasi-real-time tracking of diffusion of Mo adatoms and vacancies in monolayer MoS2, an important two-dimensional transition metal dichalcogenide (TMD) system. Preferred kinetic pathways and the migration potential-energy landscape are determined experimentally and confirmed theoretically. The resulting three-dimensional knowledge of the atomic configuration evolution reveals the different microscopic mechanisms responsible for the contrasting intrinsic diffusion rates for Mo adatoms and vacancies. The new insight will benefit our understanding of material processes such as phase transformation and heterogeneous catalysis.
Highlights
Diffusion phenomena on surfaces and inside solids are important material processes closely associated with solid-state phase transformation,[1] nanomaterials growth,[2−4] single-atom doping,[1] and heterogeneous catalysis.[5]
We report a direct observation of the dynamics of bulk-like in-plane diffusion of Mo vacancies and surface migration of Mo adatoms in monolayer MoS2, by atomic-scale tracking using chemically sensitive scanning transmission electron microscopy (STEM)
Ab initio density function theory (DFT) calculation further provides detailed quantum mechanical description of the atomic processes along such kinetic pathways, accounting for the different diffusion rates measured for adatoms and vacancies
Summary
Ab initio calculation confirms that the experimentally mostfavored kinetic pathway for Mo adatom migration follows a TMo1 → H → TMo2 transition (red arrows shown in Figure 4i and observed in SI movie file 5) with an initial energy barrier of 0.62 eV. Compared with Mo vacancy diffusion, the reduced energy barriers for surface adatom migration are related to the smaller structural relaxation (Figure S11) with breaking of at most only one Mo−S bond Both MoS2 and graphene surfaces are considered to be inert, the kinetic pathways for the adatoms are very different and depend on both the surface and the adatoms, indicating strong chemical adatom−surface interaction in both cases. One may note the slight atomic distortion of the lattice Mo below the migrating Mo in the vacancy hopping, which can be interpreted as a temporary displacement resulting from the electron beam radiation on the shallow potential landscape shown above. During the review of this work, we noticed another work focused on the migration of single Pt atoms in MoS2 monolayer.[56]
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