Photo-Induced Force Microscopy-Based Spectroscopy and Chemical Mapping of Grafted Electrochemically Reduced Aryl Diazonium Salts

Aryl diazonium salts are well-known for electrochemically grafting conductive surfaces. The substrate surface chemistry, the solvent used, and the salt's variable functional group tamper with the grafted layer density, packing, and morphology. Standard surface microscopy methods are unable to determine the chemical structure of a modified surface at the nanoscale, which prevents associating nanometric features to chemical identities. Photoinduced force microscopy (PiFM) is a vibrational nanospectroscopy technique capable of simultaneously measuring topography by atomic force microscopy and surface chemistry by infrared spectroscopy with an ∼5 nm spatial resolution. Compared to other localized infrared surface vibrational techniques, PiFM has a higher signal-to-noise ratio, enabling monolayer sensitivity and making it useful for the investigation of thin layers, like reduced aryl diazonium salts. We used PiFM to study grafted samples morphologically and spectroscopically, providing insights into chemical structure. Lastly, a micrometer area was functionalized using scanning electrochemical cell microscopy (SECCM) and then chemically mapped with the PiFM.