Probing sp and sp3 Carbon Radicals by Iodine Capture

Carbon-centered radicals are important species for on-surface chemical synthesis and bottom-up nanofabrication. Using germaadamantane molecules bound to a Si(100)-2 × 1 surface, long-lived sp and sp3 carbon radicals were generated under ultra-high vacuum (UHV) conditions at 300 K via homolytic carbon-iodine (C-I) bond scission using unpolarized ultraviolet (UV) irradiation. Monitoring the surface-bound molecules via X-ray photoelectron spectroscopy (XPS) revealed that the deiodination efficiency was dependent on the angle between the Si(100) surface and the incident UV light. Molecular iodine produced in vacuo was used to reiodinate the carbon radical sites, and the known C-I binding energy shifts in the I 3d XPS spectra confirmed that the previously cleaved C-I bonds were regenerated. To demonstrate the radical character of the deiodinated carbon atoms, H2 gas was introduced into the vacuum chamber between the de- and reiodination steps. As expected, H2 had no effect on the reiodination efficiency for the less-reactive sp3 carbon-centered radicals, whereas it fully quenched the more-reactive sp carbon-centered radicals, further supporting the radical assignments. Furthermore, the effect of submonolayer molecular coverage on the reiodination efficiency was explored; for both radicals, lower molecular coverage and greater amounts of iodine bonding to the Si surface led to greater reiodination efficiency─up to 100% for the sp3 carbon radicals─while higher coverages and less surface-bound iodine resulted in less efficient reiodination.