DE-WETTING OF Fe OXIDE FROM CLEAN AND SULFUR-COVERED Fe METAL SUBSTRATES

摘要

Studies performed in UHV on Fe/Fe-oxide interfaces suggest a mechanism for S-induced thermal instability of the interface. In the absence of sulfur, oxide debonding is initiated by the diffusion of oxygen into the bulk from an Fe-O interfacial "glue" layer. Once diffusion occurs, the polar oxide dewets from the nonpolar metal substrate. In the presence of S, the formation of the FeO layer is inhibited due to site competition between S and O atoms at the interface. Upon heating, S reacts with existing interfacial oxide sites to produce SO_2, and destroying interfacial oxide-metal bonds at a lower temperature (640 K) than that at which oxygen diffusion from the FeO layer occurs (～900 K). At this point there is no interfacial bonding between the polar oxide and the non-polar S-modified substrate, and oxide dewetting occurs. Due to lower Fe coordination at the surface, Fe-S bonds are largely covalent, and therefore the S-modified surface is still nonpolar.