Direct photopatterning and SEM imaging of molecular monolayers on diamond surfaces: mechanistic insights into UV-initiated molecular grafting.

We have used X-ray photoelectron spectroscopy (XPS), infrared reflection-absorption spectroscopy, and field-emission scanning electron microscopy (SEM) to investigate the formation of single- and two-component molecular patterns by direct photochemical grafting of alkenes onto hydrogen-terminated diamond surfaces using sub-band gap 254 nm ultraviolet light. Trifluoroacetamide-protected 1-aminodec-1-ene (TFAAD) and 1-dodecene were used as model systems for grafting. Illumination with sub-band gap light can induce several different kinds of excitations, including creation of mobile electrons and holes in the bulk and creation of radicals at the surface and in the adjacent fluid, which induce grafting of the alkenes to the surface. SEM images of patterned molecular layers on nanocrystalline diamond surfaces reveal sharp transitions between functionalized and nonfunctionalized regions consistent with diffraction-limited excitation. However, identical experiments on type IIb single-crystal diamond yield a significantly more extended transition region in the molecular pattern. These data imply that the spatial resolution of the direct molecular photopatterning is affected by diffusion of charge carriers in the bulk of the diamond samples. The molecular contrast between surfaces with different terminations is consistent with the expected trends in molecular electron affinity. These results provide new mechanistic insights into the direct patterning and imaging of molecular monolayers on surfaces.