No abstract

We use annular dark-field scanning transmission electron microscopy (ADF-STEM) to study how solutiondeposited molecules bind to the edges and surface regions around nanopores in MoS 2 monolayers. Nanopores with clean atomically flat edges and controllable mean diameter were generated by time-dependent large-area electron beam exposure during an in situ heating process, ready for subsequent molecular attachment. An organic molecule was designed to have a dithiolane end group that binds to Mo-terminated sites and a ligand structure that incorporates a single transition metal atom (Pt) marker for ADF-STEM detection. Pt atoms were used to track molecular binding around zigzag edges of MoS 2 and to predict the orientations and conformations of molecules upon binding. We found that the molecules preferred to reside on the surface of the MoS 2 , pointing inward when attaching to the edge, rather than dangling out from the edge into free space, which is attributed to van der Waals interactions between the aromatic core of the molecule and the MoS 2 basal planes. These results help us understand the way solution-deposited single molecules attach to free-standing edges of 2D crystals and the influence of van der Waals forces in guiding molecular binding.

We present a transition-metal complex containing a Pt center, large aromatic ligand, and Mo-binding terminal group, designed to maximize the dispersion of individual molecules across the 2D MoS2 substrate. The bulky organic ligand structure shows higher thermal stability than typical halides in inorganic Pt salt, such as H2PtCl6, and thus provides a large number of isolated single Pt atoms on 2D surfaces even after thermal annealing up to 900 °C. Epitaxially aligned Pt nanocrystals that are 3–5 nm in diameter are also formed upon the thermal treatment, displaying different shapes (i.e., 2D vs 3D clusters), resulting from the different atomic diffusion mechanism based on each Pt precursor. The successful generation of abundant single Pt atom sites on MoS2 surface using a large complex precursor indicates the significance of precursor design for reducing the noble metal loading in various catalysis materials.