Probing the Functionalized Black Phosphorus Surface through Advanced Microscopy

Surface functionalization of two-dimensional materials by solution phase methods is a key step towards scalable tuning of their physical and chemical properties, yet experimental strategies to directly probe and image the resulting modified surfaces are scarce. Semiconducting few layer black phosphorus (bP) is of particular interest among two dimensional materials due to its thickness dependent bandgap and highly reactive surface making it amenable for functionalization. We have shown in past work that the surface of bP can be functionalized with discrete molecular fragments, such as organic groups and Lewis acids, altering its physical and electronic properties. While bulk measurements, such as vibrational spectroscopy and X-ray photoelectron spectroscopy (XPS), report on success of functionalization and structure of the functional group on bP, the exact location and surface density of these fragments have yet to be determined. Herein, we introduce a facile solution phase protocol to modify the surface of bP with discrete organometallic fragments and directly image them using photoinduced force microscopy (PiFM) and electron microscopy methods. PiFM is used to track organometallic fragments by measuring site specific vibrational frequencies on bP flakes. This not only provides a map of the fragments on each flake, but also reveals information on the local binding environment surrounding the metal atom. Along with this scanning probe method, high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) coupled with annular dark field (ADF) and energy dispersive X-ray spectroscopy (EDS) detection is employed to image the organometallic fragments attached to the bP surface revealing both single metal atom sites and small metallic clusters along the basal plane and edges. This solution phase protocol and imaging characterization opens a new dimension to tune the bP surface and therefore its electronic properties.