Scanning Electron Microscopy for Imaging Photopatterned Self-Assembled Monolayers on Gold

Scanning electron microscopy (SEM) can be used to detect images resulting from irradiation of self-assembled monolayers of bis(undecanyl 4-azidobenzoate) disulfide, I, on Au. The resulting surface-confined aryl azide has previously been shown to be photosensitive and yields primary photoproducts which are derivatives of 3H-azepine and hydrazine when irradiated with UV light in the presence of amines. Irradiation of the monolayer, Au-I, through a Cr-on-glass mask and a thin film of N-(2,2,2-trifluoroethyl)-N-(2-ferrocenylethyl)amine, II, results in the surface attachment of 3 × 10⁻¹⁰ mol/cm², or approximately one monolayer, of ferrocenyl centers as demonstrated by cyclic voltammetry. Maps of F and molecular fragments of ferrocene obtained by secondary ion mass spectrometry (SIMS) confirm that II is selectively attached to the surface only where irradiated. Contrast observed in the SEM images is a function of surface composition and length of exposure to energetic electrons. The critical finding is that SEM reveals the same image that is established by the SIMS technique. SEM, therefore, can be used as a routine and convenient method to obtain high lateral resolution images of photochemically patterned monolayers.