Competing Pathways in N-Allylurea Adsorption on Si(111)-(7 x 7)

Functionalization of silicon surfaces with N-allylurea (CH2=CH-CNH-CO-NH2) represents a valuable strategy to obtain covalently bonded Si-C interfaces with amino and/or carbonyl termination. In this work, we studied N-allylurea adsorption on the Si(111)-(7× 7) surface by combining X-ray and ultraviolet photoemission spectroscopy (XPS and UPS) with high resolution energy loss spectroscopy (HREELS) measurements. XPS core level analysis provides information on the molecular attachment process. Si-C covalent bonding is evidenced by the presence of a C Is component at 284.8 eV, while interaction through N-Si bonding is proved by the presence of a N 1s component at 397.8 eV. Three different adsorption mechanisms are envisaged: (I) [2 + 2]-like cycloaddition occurring at the rest atom-adatom dimer through cleavage of the vinyl group, (II) Si-N bonding at adatom sites upon cleavage of NH2 and rearrangement of the ureic group to form an imidol species (-N=C-OH), with release of a H atom, and (III) hydrosilylation at adatom sites, through cleavage of the vinyl group and involvement of H atoms provided by reaction II.